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Taxonomy and Biostratigraphy of the Late Albian Actinoceramus sulcatus Lineage(Early Cretaceous Bivalvia, Inoceramidae)Author(s): James S. Crampton and Andy S. GaleSource: Journal of Paleontology, 83(1):89-109. 2009.Published By: The Paleontological SocietyDOI: http://dx.doi.org/10.1666/08-037R.1URL: http://www.bioone.org/doi/full/10.1666/08-037R.1

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89

J. Paleont., 83(1), 2009, pp. 89–109Copyright � 2009, The Paleontological Society0022-3360/09/0083-89$03.00

TAXONOMY AND BIOSTRATIGRAPHY OF THE LATE ALBIANACTINOCERAMUS SULCATUS LINEAGE

(EARLY CRETACEOUS BIVALVIA, INOCERAMIDAE)JAMES S. CRAMPTON1 AND ANDY S. GALE2

1GNS Science, PO Box 30368, Lower Hutt, New Zealand, �j.crampton@gns.cri.nz�; 2School of Earth and Environmental Sciences,University of Portsmouth, Burnaby Building, Burnaby Road, Portsmouth PO1 3QL, United Kingdom

ABSTRACT—The Actinoceramus sulcatus lineage (Parkinson, 1819) (Bivalvia: Inoceramidae) is a very distinctive and abundant component oflate Albian (Early Cretaceous) molluscan assemblages that is found throughout Europe, Central Asia, Japan and the Far East of Russia, southernand western North America, South Africa, and possibly India, in a range of shallow- to deep-marine facies. The lineage encompasses a wideand continuous range of morphologies that provide evidence of phyletic evolution at varying rates combined with large ecophenotypic plasticitywithin populations. The evolution of A. sulcatus marks the oldest appearance of well-developed radial folds and sulci within the Inoceramidae.The range of morphological variation makes formal taxonomic subdivision of the group problematic. Here we use a combination of formalsuccessional subspecies and informal morphotypes to subdivide the lineage into the following taxa: A. sulcatus forma sulcatus, A. sulcatusforma subsulcatus (Wiltshire, 1869), A. sulcatus forma munsoni (Cragin, 1894), and A. sulcatus biometricus Crampton, 1996. Within these taxaand morphotypes, we synonymise a large number of earlier names that have been applied to variants within the lineage. Each of the formsrecognized has biostratigraphic utility and we describe four new lineage biozones, in ascending order: A. concentricus parabolicus, A. sulcatus,A. sulcatus forma munsoni, and A. sulcatus biometricus biozones. The lowest occurrence of A. sulcatus is approximately coincident with thebase of the upper Albian as currently defined, at least throughout most of Europe, and this datum provides a valuable tool in correlation. Thenature of radial folds within the A. sulcatus lineage poses interesting but still unanswered questions regarding shell morphogenesis in bivalvesand the functional significance (if any) of radial folds in the Inoceramidae.

INTRODUCTION

THE INOCERAMIDS (family Inoceramidae) were cosmopolitanmarine bivalves that dominated the extratropical epibenthic

megafauna during the Late Cretaceous and became extinct at theend of the Cretaceous. They were apparently well adapted togreenhouse climate conditions that prevailed at that time, althoughmany aspects of their paleobiology and paleoecology are not wellunderstood (Harries and Crampton, 1998). Because of their abun-dance, widespread geographic ranges, and rapid evolutionaryrates, inoceramids have great biostratigraphic utility, and a largenumber of taxa have been described—93 genus-level taxa alone.Despite this, taxonomic and phylogenetic relationships within thefamily are relatively poorly resolved because of widespread ho-moplasy, commonly high levels of intraspecific morphologicalvariability, and the paucity of useful morphological characters.

This paper completes a set of studies that have sought to resolveand explain morphological variation, phylogenetic relationships, bio-stratigraphic and paleogeographic distributions, and underlying evo-lutionary modes within a single group of inoceramids: the genusActinoceramus Meek, 1864 (Crampton, 1996a; Kennedy et al., 1999;Crampton and Gale, 2005). Actinoceramus first appeared in the earlyAlbian (late Early Cretaceous) and persisted until at least the middleto ?late Cenomanian (Crampton, 1996a). This interval marked thebeginning of the major diversification of the inoceramids, a diver-sification that apparently coincided with peak global greenhousewarming (e.g., Harries and Crampton, 1998; Schouten et al., 2003;Jenkyns et al., 2004).

The present study is focused on the late Albian species Actin-oceramus sulcatus (Parkinson, 1819) and allied taxa that are dis-tinguished primarily by the presence of large ‘‘radial’’ folds andsulci—here given the general name ‘‘radial elements’’—that af-fect part or all of the shell (see below). Actinoceramus sulcatuswas essentially cosmopolitan in mid- to low paleolatitudes(Crampton, 1996a). It occurs in huge abundance at many locali-ties, both on individual bedding planes and throughout its bio-stratigraphic range, in diverse facies that were deposited at a rangeof paleodepths. In terms of numerical abundance and distribution,A. sulcatus was quite possibly the most ‘‘successful’’ marine bi-valve of the late Albian.

As noted by Crampton and Gale (2005), Actinoceramus sulcatussensu stricto is highly distinctive, has a long history of description,and is one of the earliest named and most consistently identified

inoceramid species (e.g., Sowerby, 1821; Mantell, 1822; Goldfuss,1836; d’Orbigny, 1846; Whiteaves, 1884; and many others). In con-trast, a plethora of diverse forms apparently closely related to A.sulcatus s. s. has, for almost two centuries, presented a perplexingchallenge to inoceramid taxonomists and, indeed, evolutionary mod-els. In response to this challenge, Crampton and Gale (2005) inter-preted the range of morphological forms within the A. sulcatus groupas the result of evolution at widely varying rates within a singlelineage, combined with ecophenotypic polymorphism. Specifically,they identified four major components and underlying causes of mor-phological variation (summarized on Fig. 1):

1. Initial speciation, evolution of potential radial elements, andrelatively rapid increase in frequency of folded morphotypesto dominance in the population.

2. Subsequent, gradual phyletic increase in the average numberof radial elements on individuals.

3. Ecophenotypic variation in the presence, number, and distri-bution of radial elements on individuals.

4. Final, relatively rapid phyletic evolutionary loss of radial el-ements in the lineage. This loss of radial elements was notan exact ‘‘mirror’’ of their earlier evolutionary acquisition;asymmetry is shown by differences in shape and proportionsof particular morphologies at the two transitions.

The present paper is a companion to Crampton and Gale (2005)and aims to revise the taxonomy of Late Albian Actinoceramustaxa with radial elements (henceforth, A. sulcatus sensu lato) andto propose a systematic framework that accommodates the re-markable variability within this lineage.

MATERIAL AND METHODS

This study is based on over 1200 stratigraphically well-con-strained specimens of latest middle to early late Albian Actino-ceramus from three widely spaced sections: Folkestone, Kent,southeastern England; Col de Palluel, Drome, southeasternFrance; and the Fort Worth area, Texas. These key sections aredescribed in Crampton and Gale (2005); details are not repeatedhere. In addition, many hundreds of specimens from other local-ities in England, Switzerland, California, South Africa, CentralAsia and the Far East of Russia have been examined during thecourse of this work; information on some other significant local-ities is noted at relevant points in the text. Important collectionsand specimens are listed in the Appendix.

90 JOURNAL OF PALEONTOLOGY, V. 83, NO. 1, 2009

FIGURE 1—Schematic representation of evolution in the genus Actinoceramus around the middle/upper Albian boundary, showing major components ofevolution and phenotypic plasticity proposed by Crampton and Gale (2005). At the base of the upper Albian, the A. concentricus lineage underwent clado-genesis. The ancestral form persisted with minimal evolution whereas the daughter species, A. sulcatus, experienced rapid and conspicuous evolution. Rep-resentative morphologies are illustrated; formal taxa and informal morphotypes described later in this paper are listed. Also shown are representative ammonitezonations and the Actinoceramus zonation described below.

Morphological terms and dimensions measured are explainedin Fig. 2; morphological terms applied to the ligament area followCrampton (1988).

Commissure plots for specific growth stages were generated bycasting specimens, grinding each cast back to the growth line ofinterest, and ‘‘rolling’’ the cast into a sheet of modeling clay,leaving a planar impression of the commissure. Relative increasesin shell area resulting from the presence of radial elements wereestimated by approximating each valve as a folded cone and cal-culating the increase in linear dimension of the commissure in thefolded shell versus a hypothetical shell in which the commissurefollowed a similar trace but was unfolded; lengths were measuredusing image analysis software.

Shapes of selected groups were compared using Fourier shapeanalysis. The data used comprise 47 digitised outlines taken fromdifferent growth stages of 34 individuals of A. concentricus par-abolicus and 23 outlines taken from 16 individuals of A. sulcatusbiometricus (see Crampton 1996a, appendix 1, for details). Theseoutlines were processed using the computer programs Hangle,Hmatch and Hcurve (see Haines & Crampton, 2000). The analysisis based on the first eight Fourier harmonics, normalised for sizein program Hangle and normalised for orientation using programHmatch. Synthetic mean shapes for the two groups were gener-ated from the averaged Fourier coefficients using program Hcurve.

Multivariate statistical analysis of the Fourier coefficients (dis-criminant function analysis) was undertaken using standard sta-tistical software.

Annotated versions of the synonymy lists that appear in theSystematic Paleontology section are available from the first au-thor. These include extensive notes on the geographic distributionand stratigraphic position of each record, and additional taxonom-ic notes.

A note on terminology relating to radial elements.⎯‘‘Radial’’elements on a bivalve occupy a spectrum between two end-mem-ber states. First are radial folds and sulci sensu stricto, whichradiate in a more-or-less straight line from the umbo to the grow-ing margin and apparently conform to a model of simple, iso-metric growth of specialized, fold-generating mantle segments(Checa and Crampton, 2002). Radial folds are perpendicular tothe shell margin only at the growth axis; elsewhere they intersectthe margin at an angle (Fig. 2). At the other end of the spectrumare ‘‘antimarginal’’ structures (sensu Waller, 1986) that remainperpendicular to the growing margin at all points around the shellthroughout ontogeny. Because of the geometry of shell growth,antimarginal folds are curved on the shell surface, result in adivaricate pattern (Fig. 2), and require (unspecialized?) fold-gen-erating mantle segments to migrate around the margin during

91CRAMPTON AND GALE—TAXONOMY AND BIOSTRATIGRAPHY OF ACTINOCERAMUS SULCATUS

FIGURE 2—Morphological features of Actinoceramus and terms used in thetext.

growth, possibly being constrained to lie within the tracks of ear-lier-formed folds (Checa and Jimenez-Jimenez, 1999). Conditionsbetween these two end-members states are termed ‘‘oblique.’’ Asdetailed in the systematic descriptions, specimens of A. sulcatuspossess radial (s.s.), oblique or antimarginal folds, and all threetypes of element may occur in varying configurations on any in-dividual. Throughout this paper, the term ‘‘radial elements’’ isused in a general sense to encompass all these types of morphol-ogy; where specific meaning is intended, the terms ‘‘radial sensustricto’’ (s.s.), ‘‘antimarginal,’’ and ‘‘oblique’’ are used.

Repository abbreviations.⎯The following repository abbrevi-ations are used throughout the text and prefix all specimen num-bers: BM � Natural History Museum, London, UK; OUM �Oxford University Museum, Oxford, UK; SAM � South AfricanMuseum, Cape Town, South Africa; SM � Sedgwick Museum,Cambridge, UK; USNM � United States National Museum,Washington; WM � World Mollusca Collection, GNS Science,Lower Hutt, New Zealand.

DISCUSSION

Middle to late Albian Actinoceramus: a taxonomic and evolu-tionary overview.⎯Actinoceramus sulcatus and its ancestor, A.concentricus, were described by Parkinson (1819) and includedwithin the catch-all genus Inoceramus Sowerby, 1814. The ho-lotypes of both species were collected from the Gault Clay For-mation at Folkestone, Kent, England. Placement within Actino-ceramus Meek, 1864, and diagnosis of the genus, were discussedin Crampton (1996a).

Actinoceramus concentricus evolved from A. salomoni(d’Orbigny, 1850) close to the early/middle Albian boundary. Ac-tinoceramus concentricus is moderately to highly inequivalve, el-liptical to parabolical in shape, and sculptured with regular toirregular commarginal folds, costae and growth lines. Biometricand evolutionary patterns within this species have been describedby Crampton (1996a) who recognized four middle to late Albianchrono- or successional subspecies (sensu Simpson, 1961), in as-cending order, A. concentricus expandoclunis Crampton, 1996a,A. concentricus concentricus, A. concentricus parabolicus Cramp-ton, 1996a, and A. concentricus gryphaeoides (Sowerby, 1828).Although A. concentricus has been recorded from many parts ofthe world, Crampton (1996a, fig. 16) concluded that it could onlybe identified with confidence from Europe, western Asia, easternSouth America and South Africa.

Actinoceramus sulcatus evolved from A. concentricus para-bolicus at an inferred cladogenetic event at the beginning of thelate Albian (Crampton, 1996a; Crampton and Gale, 2005). Al-though early late Albian inoceramid faunas are dominated by A.sulcatus, the A. concentricus lineage apparently persisted in lowabundance and with little evolutionary change as the subspeciesA. concentricus gryphaeoides (and thus the cladogenetic event isan example of ‘‘budding,’’ sensu Foote [1996]). Both species dis-appear from the fossil record in the middle late Albian. Morpho-logically, the species are generally similar, but most specimens ofA. sulcatus are diagnosed by the presence of large, upstanding,high amplitude, radial elements that affect part or all of the shell.

A minority of workers have used the presence of radial ele-ments to separate the genus Actinoceramus, including A. sulcatus,from ‘‘Inoceramus’’ concentricus and allies (e.g., Aliev, 1958;Khalilov, 1959; Savel’ev, 1962; Ali-zade et al., 1988). This sep-aration is not maintained here. The radial elements and zigzagcommissure of A. sulcatus represent, however, a significant evo-lutionary innovation for the Inoceramidae, although some oldertaxa do possess a single, relatively weakly developed, radial foldand sulcus couplet (e.g., Actinoceramus salomoni [d’Orbigny,1850] and the genus Anopaea Eichwald, 1861). Radial elementssubsequently evolved in several Late Cretaceous inoceramidgroups, including Cladoceramus Heinz, 1932, RhyssomytiloidesHessel, 1988, and Sphenoceramus Bohm, 1915.

Following the description and widespread recognition of Actino-ceramus concentricus and A. sulcatus, Wiltshire (1869) named athird, closely allied taxon, ‘‘Inoceramus’’ subsulcatus. Although list-ed as a species (his p. 188), Wiltshire described this in the text as‘‘a varietal form of Inoceramus sulcatus’’ (p. 190). ‘‘Inoceramus’’subsulcatus is intermediate in form between A. concentricus and A.sulcatus, possessing radial elements on some part of the shell. Thetaxonomic status and precise stratigraphic distribution of ‘‘I.’’ sub-sulcatus have been interpreted variously in the literature.

Confusion surrounding the status of forms apparently inter-mediate between or otherwise allied to A. concentricus and A.sulcatus subsequently has resulted in a plethora of names for var-ious morphotypes: ‘‘Inoceramus’’ munsoni Cragin, 1894; ‘‘I.’’praedigitatus Airaghi, 1904; ‘‘I.’’ (A.) subsulcatiformis Bose,1927; A. perisulcatus Khalilov, 1959; A. pleurosulcatus Khalilov,1959; A. pseudosulcatus Khalilov, 1959; A. sulcatus morpha ae-qualiplicata Savel’ev, 1962; A. sulcatus morpha inaequaliplicataSavel’ev, 1962; A. sulcatus aberratio flexiosocostatus Savel’ev,1962; A. sulcatatoides Savel’ev, 1962; and A. sulcatus biometri-cus Crampton, 1996a. In contrast to this plethora of names, it isworth noting Pokhialainen’s (1985) insightful paper in which hedescribed polymorphic grades within populations of A. ‘‘concen-tricus-sulcatus’’ and the difficulties of distinguishing ‘‘autono-mous’’ species. As indicated by Pokhialainen, it is now clear thatall these named forms are intimately associated, geographically,stratigraphically and phylogenetically, with A. sulcatus s.s.

As outlined in the introduction to the present paper, Crampton and

92 JOURNAL OF PALEONTOLOGY, V. 83, NO. 1, 2009

FIGURE 3—One previous interpretation of the phylogeny and taxonomy ofAlbian ‘‘Inoceramus’’ concentricus and Actinoceramus sulcatus-like forms(redrawn from Khalilov, 1959).

Gale (2005) interpreted the wide range of morphologies in late Al-bian A. sulcatus s.l. as the product of evolution at widely varyingrates combined with ecophenotypic plasticity in the expression anddevelopment of radial elements (summarized on Fig. 1).

Taxonomic concepts.⎯This paper is not the place for lengthydiscussion of species concepts upon which ‘‘more paper has beenconsumed . . . than any other subject in evolutionary and system-atic biology’’ (Wiley, 1978, p. 17; see also de Queiroz, 1998).The case of Actinoceramus sulcatus, however, is a clear exampleof the pitfalls of using a strictly typological species concept andfailing to distinguish phyletic change from the actual multiplica-tion of species.

As discussed in many papers, typological species definitionshave been and continue to be used widely by paleontologists. Inmany cases—perhaps the majority—such definitions serve theneeds of paleontological and evolutionary studies adequately. Inother cases, however, they obscure taxonomic and phylogeneticrelationships and distort estimates of standing diversity and taxicrates. These problems may be acute in fossil groups such as theinoceramids that have rich and well-studied fossil records andexperienced apparently rapid, phyletic evolution. For example,Khalilov (1959) recognized six species of middle to upper AlbianActinoceramus, three of them new, which he depicted with almostentirely overlapping stratigraphic ranges (Fig. 3). These ‘‘species’’were based on very narrow taxonomic concepts, even thoughKhalilov acknowledged the presence of intermediate forms. Incontrast, herein and following Crampton and Gale (2005), weinfer that five of these six ‘‘species’’ represent no more thangrades on a continuum defined by a single population that variedmorphologically through time and/or space and comprised thespecies lineage of A. sulcatus. The sixth ‘‘species’’ was a separateentity—A. concentricus gryphaeoides—and was morphologicallyrelatively conservative. Hence, at any instant during the early lateAlbian, there were just two discrete, coexisting populations ofActinoceramus that defined two lineages through time. We arguethat at no stage during the early late Albian did the diversity ofActinoceramus exceed two species and, therefore, that the tax-onomy should reflect this. Pokhialainen (1985, p. 102), in refer-ence to inoceramid population studies, alluded to this issue whenhe referred to ‘‘many inoceramid species that exist in the literaturebut not in nature.’’

In part, the problem results from the legitimate desire to servethe needs of biostratigraphy through a strict, Linnean binomial no-menclature (e.g., Hughes and Labandeira, 1995). Measurements ofpaleobiodiversity and taxic rates are, however, now key elementsin many macroevolutionary studies and are likely to be major fociof future paleontological research. The example of Actinoceramussulcatus demontrates how easily such measures are influenced bysystematic methodologies: speciation and extinction rates calculatedfrom Khalilov’s (1959) taxonomic scheme will be three-fold higherthan those derived from our taxonomy.

These arguments do not deny the importance of recognizing

and trying to quantify rates of anagenetic evolution within line-ages. We would argue that where possible, however, such evo-lution should be encoded in a way that clearly distinguishes itfrom the process of gene pool splitting and lineage multiplication(i.e., cladogenesis).

Herein, therefore, we have defined Actinoceramus sulcatus asthe phylogenetic lineage that initiated by cladogenesis from A.concentricus at the beginning of the late Albian and, on currentknowledge, persisted without further cladogenesis until the middlelate Albian. Within the species thus defined, distinctive morphol-ogies that have characteristic and largely discrete biostratigraphicdistributions are identified as successional subspecies sensu Simp-son (1961), a usage that is consistent with the expectation ofseparation (either geographic or temporal) between subspecies.(We acknowledge that this pragmatic approach takes no accountof unresolved debates concerning the merits or otherwise of thesubspecies category in taxonomy—e.g., Mayr [1982].) In con-trast, distinctive morphologies that have largely overlapping bio-stratigraphic ranges are recognized as informal morphotypes,identified by the prefix ‘‘forma.’’ This use of morphotypes allowsfor the explicit recognition of unquestionably distinctive fossilsbut draws attention to the polymorphic nature of A. sulcatus pop-ulations. Using both successional subspecies and the changingproportions of morphotypes through time, we are able to quantifyintraspecific evolutionary rates (Crampton and Gale, 2005) anddefine a set of biostratigraphically useful events (see below).

Based on the foregoing considerations, the following formaltaxonomic framework is adopted for A. sulcatus. Our previousstudies (Kennedy et al., 1999; Crampton and Gale, 2005) haveused informal morphotypes to distinguish forms of the species—these morphotype labels are included here in brackets.

1. Actinoceramus sulcatus forma sulcatus (�morphotype A);2. Actinoceramus sulcatus forma subsulcatus (�morphotype B);3. Actinoceramus sulcatus forma munsoni (�morphotype C);4. Actinoceramus sulcatus biometricus (�morphotype D, in

part).

Note that the morphotype D label of earlier publications alsoincluded Actinoceramus concentricus parabolicus and all undif-ferentiated, non-folded specimens in the biostratigraphic range ofA. sulcatus forma sulcatus. The latter specimens represent largelyA. sulcatus biometricus-like end-member ecophenotypes of A. sul-catus; in some cases, specimens of morphotype D are referrableto the A. concentricus parabolicus–A. concentricus gryphaeoideslineage.

Finally, we note that the taxonomic problems outlined here maybe widespread within the Inoceramidae and, indeed, in some othergroups. In cases where anagenetic transitions and other morpho-logical continua involve distinctive or easily characterised traits,then the problems are readily apparent and have attracted com-ment (e.g., Tanabe, 1973). In other cases, the problems may beless readily observed or described, but no less real. Within theInoceramidae, for example, an apparently similar situation in-volving radial elements exists for the Late Cretaceous, nominalgenus-level taxa Cladoceramus Heinz, 1932, Platyceramus Heinz,1932, and Rhyssomytiloides Hessel, 1988 (see discussion in Har-ries et al., 1996), as illustrated by Noda (1983) for the species‘‘Inoceramus (Platyceramus)’’ japonicus (Nagao and Matsumoto,1940) and ‘‘I. (P.)’’ higoensis Noda, 1983.

Paleoceanographic setting during the evolution of A. sulca-tus.⎯It seems likely that the evolution of radial elements andActinoceramus sulcatus were, in some way, linked to environ-mental changes that occurred around the middle/late Albianboundary and had far-reaching biological effects in the marinerealm—the ‘‘Middle-Late Albian Boundary Bio-Event (M/L’Al)’’of Barnes et al. (1995). In North America, the event is markedby short-term extinctions of ammonite genera and species groups

93CRAMPTON AND GALE—TAXONOMY AND BIOSTRATIGRAPHY OF ACTINOCERAMUS SULCATUS

(Kauffman and Hart, 1995). Globally, the base of the upper Al-bian is approximately coincident with an ‘‘explosion in diversityand turnover rates’’ of planktic foraminifera (Bralower et al.,1993, p. 30; Premoli Silva and Sliter, 1999). In the Anglo-Parisbasin of England and northern France, the evolution of A. sulcatusat the middle/upper Albian boundary is coincident with other bio-diversity and faunal changes that mark ‘‘one of the most funda-mental environmental changes within . . . Albian sequencesworld-wide.’’ (Knight, 1997, p. 92). Knight and Morris (1996)even record a marked decrease in inoceramid larval shell size atthis boundary in the Gault at Folkestone, a pattern that would beconsistent, for example, with a rise in surface water temperaturesat the beginning of the late Albian (Lutz and Jablonski, 1978).

Interpretation of the likely sequence and drivers of these bio-logical changes is hampered by the poor integration of biostrati-graphic schemes between regions, environments, provinces andfossil groups. Thus, it is difficult to correlate events between rel-atively shallow-water Boreal successions, subdivided on the basisof ammonites, and deep-water Tethyan and Atlantic successionsthat are zoned using planktic foraminifera and nannofossils. Wedo not currently know, for example, what the base of the upperAlbian as defined by the use of planktic foraminifera means interms of the ammonite succession. Charts that attempt to showthese calibrations are not based on any prima facie evidence, butsolely on inference. This is important because most sea-level in-terpretations utilize shallow-marine successions that are dated us-ing ammonites, which at present cannot be correlated accuratelywith deep water successions dated by planktic foraminifera andnannofossils. Thus, for example, figures showing the relationshipsbetween deep-water anoxic events and sea-level changes (e.g.,Leckie et al., 2002, fig. 1) may have little factual basis in bio-stratigraphic correlation.

Most authors have identified the start of the late Albian asfalling within a period of overall (second-order) transgression,effectively on a global scale (e.g., Jacquin, 1998, fig. 1). BothJacquin et al. (1999) and Immenhauser and Scott (1999) showthird-order sequence boundaries or transgressive surfaces at thebases of the D. cristatum and H. orbignyi/M. pricei ammonitezones, respectively. These interpretations appear to be foundedmost specifically on the work of Amedro (1992), based on ob-servations in the northern Anglo-Paris Basin. Our own interpre-tations of the Folkestone succession are essentially similar. Theerosional break at the base of the D. cristatum Zone, overlain bya nodule bed (the base of Bed VIII, see Fig. 4), represents asequence boundary lying directly beneath a transgressive surface.The higher, main D. cristatum nodule bed (the upper part of BedVIII) is a flooding surface of the same transgressive event. Theseobservations accord with broader stratigraphic patterns in thenorthern Anglo-Paris Basin, where D. cristatum and H. orbignyi/M. pricei zone sediments onlap progressively northwards onto thesouthern London Platform and extend over Paleozoic basement.Thus, the first occurrence of A. sulcatus falls within the earlytransgressive systems tract of the D. cristatum sequence. The suc-ceeding sequence boundary and transgressive surface lie at thenodule bed at the base of the M. pricei Zone (Fig. 4), immediatelybeneath the level at which the ammonite Hysteroceras becomesabundant.

In the central Texas succession, the uppermost Goodland Lime-stone, of earliest D. cristatum Zone age (Kennedy et al., 1999),falls within a carbonate platform facies, containing corals, andoolitic and bioclastic limestones. It is terminated abruptly by aburrowed omission surface, infilled and overlain by sands andclays of the Kiamichi Formation that include storm-depositedsandstones (Fig. 4). A. sulcatus appears 0.7 m above the base ofthe Kiamichi Formation that is largely of M. pricei Zone age. Thedramatic platform drowning event recorded here was thus prob-ably brought about by the basal M. pricei transgression; the D.

cristatum sequence is therefore missing in the hiatus representedby the Goodland/Kiamichi contact.

Literature on Albian stratigraphy published over the last 15years contains numerous references to named events that are di-rectly or indirectly related to dysoxic conditions (oceanic anoxicevents, black shales, excursions in �13C, associated marine faunalchanges) and which are inferred to be global in extent (e.g., Bra-lower et al., 1993; Kauffman and Hart, 1995; Erbacher and Thu-row, 1998; Premoli Silva and Sliter, 1999; Tateo et al., 2000).Very few of these events are well-calibrated biostratigraphicallyand it is not known if any of them are truly global in extent.According to some correlations, the ocean anoxic event (OAE)1c could fall within the range of A. sulcatus (Tateo et al., 2000;Strasser et al., 2001), although it is unclear whether this is reallya single event, or what its precise age and its lateral extent are.As detailed below, we see no evidence to support a link betweenOAE 1c and the evolution of A. sulcatus.

The deep-water succession in the Vocontian Basin, southeasternFrance, provides abundant evidence of periodic sea-floor dysoxiain the form of laminated beds with enhanced organic contentthrough the Aptian to Cenomanian interval. Evidently the basinwas very sensitive to even small changes in oxygenation of thewater column. Many ‘‘anoxic events’’ have been identified andnamed, including the Goguel, Jacob, Kilian, Paquier, Leenhardt,Breistroffer and Thomel levels (Breheret, 1997). In the Col dePalluel succession, the middle Albian is characterised by groupsof decimeter-thick, darker laminated beds containing up to 2%TOC. These become thinner, fewer, and only weakly laminatedin the lower half of the range of A. sulcatus, and disappear entirelythrough the higher part of the range of this species (Fig. 4). Abovethis, all beds are bioturbated and organic-poor up to the Breis-troffer Level, in the upper Upper Albian (above the level shownin Fig. 4). Thus, it is possible to say that limited observationalevidence, from the Vocontian Basin at least, associates the rangeof A. sulcatus with some overall enhancement of deep-water ox-ygenation and not with sea-floor dysoxia. This finding is consis-tent with information from the North German Basin, where anabundance acme of A. sulcatus in the upper part of its range hasbeen interpreted as evidence of increased oxygenation and nutri-ent supply (Prokoph and Thurow, 2000).

In conclusion, therefore, the earliest part of the late Albian andthe evolution of Actinoceramus sulcatus were closely associatedwith transgression in at least two pulses (late D. cristatum Zone,early M. pricei Zone). This transgression resulted in major onlapof early late Albian sediments onto massifs across Europe and, incentral Texas, the later of the two events drowned the underlyingmiddle Albian carbonate platform. The transgressive pulses wereprobably rapid and of greater relative magnitude than indicatedin the literature (e.g., Jacquin et al., 1998, fig. 1). There is limitedevidence to support an increase in deep-water oxygenation overthis interval, in at least the Vocontian and North German basins.Lastly, these environmental changes seem to have coincided ap-proximately with significant turnover in marine faunas globally,a turnover that has been termed the middle-late Albian BoundaryBio-Event by some authors (Barnes et al., 1995).

It is worth noting the similar association of transgression, fau-nal turnover (the ‘‘Coniacian-Santonian Faunal Turnover’’) andevolution of radial elements in the inoceramid genera Sphenocer-amus and Cladoceramus during the early Santonian (Kauffmanand Hart, 1996).

Paleobiological and paleoecological significance of radial el-ements: unanswered questions.⎯Actinoceramus sulcatus was al-most certainly epifaunal and lived either as a cup-shaped recliner,with the commissure horizontal and the more inflated left valvepartially buried in the substrate, or as an edgewise recliner, withthe commissure vertical and the flat anterior face lying on thesubstrate (Seilacher, 1984; Crampton, 1996b, p. 27). It apparentlyfavoured muddy, silty and marly substrates at mid- to outer shelf

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FIGURE 4—Logs for three key sections, showing biostratigraphic distributions of Actinoceramus taxa and forms described here, and ammonite and inoceramidzonations. Thickness scales are in metres. Marked uncertainties on zonal boundaries take no account of obvious uncertainties resulting from sampling gaps.In the log for the Col de Palluel section, 1 � Triplet Calcaires, 2 � Faisceau Silteux, 3 � Petit Verole. Lithostratigraphic data and ammonite correlationsfrom ASG and W. J. Kennedy (unpublished data: Folkestone and Col de Palluel) and Kennedy et al. (1999: Fort Worth).

95CRAMPTON AND GALE—TAXONOMY AND BIOSTRATIGRAPHY OF ACTINOCERAMUS SULCATUS

depths or greater. Another interesting possibility was mooted byTanabe (1973) for the Late Cretaceous species Sphenoceramusnaumanni (Yokoyama, 1890), which has antimarginal folds on theadult shell and is, in some respects, comparable to A. sulcatus.Tanabe suggested that the immature shell lacking radial elementswas pseudoplanktonic, whereas the folded adult assumed a ben-thic mode of life. This argument was supported by biometric anal-yses of shell inflation and thickness. Although such a life habitcannot be discounted for A. sulcatus, no corroborating evidencehas been documented for any Actinoceramus taxa, such as closeand clustered associations of juveniles with potential float objects(e.g., ammonites and logs). The possibility of pseudoplanktoniclife habits in some other inoceramids has been debated in theliterature (e.g., Seilacher, 1982; Kauffman et al., 1992); on currentevidence it seems most likely that the great majority of specieswere entirely benthic in the post-larval stage.

Two interesting questions relating to the paleobiology and pa-leoecology of Actinoceramus sulcatus remain unanswered; wedraw attention to these questions here in the hope of stimulatingfurther research.

First, relatively little is known about shell morphogenesis inbivalves, especially with respect to biological controls of radialto antimarginal fold formation (see introductory comments on ra-dial elements and, e.g., Carter, 1968; Seilacher, 1985; Yoshida,1998; Checa and Jimenez-Jimenez, 1999; Ubukata and Nakaga-wa, 2000). As noted by Crampton and Gale (2005), ancestralActinoceramus concentricus possessed radial color bands of ap-proximately the same form and wavelength as the radial folds ofA. sulcatus (Crampton, 1996a, pl. 3, figs. i, j). Given inferredmorphogenetic links between color and sculpture in bivalves(Savazzi, 1990), the evolution of radial elements in A. sulcatusmay have been facilitated by pre-existing morphogenetic machin-ery. In the case of A. sulcatus, an interpretation of radial elementmorphogenesis would have to account for the following obser-vations (details of radial element patterns are given in the system-atic descriptions):

1. Both antimarginal and strictly radial elements are present inthe lineage and the two types occur together, in a range ofconfigurations, on many individuals (compare Fig. 5.1–5.6).

2. Radial elements can initiate gradually or abruptly at anygrowth stage and can be restricted to any segment of the disk.In some specimens, radial elements terminate during growth;in most of these cases, only strongly antimarginal folds closeto the anterior and posterodorsal margins are affected (Fig.5.6, 5.19; analogous to the ‘‘dorsal suppression’’ of Rudwick,1964, p. 157). In a few specimens, early-formed radial ele-ments terminate during growth across the entire disk so thatthe adult shell lacks radial elements altogether (Fig. 5.18,5.20).

3. Radial folds typically have a simple, non-bifurcating, sharplydefined crest. In some cases, however, they bifurcate (e.g.,Fig. 5.5), intercalate (Fig. 5.15), or have two parallel andclosely spaced crests (Fig. 5.11). In one example, a fold ap-pears to cross an adjacent sulcus during growth and mergewith another fold (Fig. 5.10).

In other words, the form of the radial elements is remarkablyvariable and this suggests that morphogenetic processes werehighly plastic.

Secondly, the possible association between the evolution of Ac-tinoceramus sulcatus and global environmental change, discussedpreviously, raises questions about the functional significance ofradial elements and/or the zigzag commissure. (It is important tonote that radial elements on the shell and the zigzag commissureare potentially distinct functional components and either one mayhave been an inevitable, non-adaptive constructional consequenceor prerequisite of the other. The possibility of such functionalindependence is demonstrated by a few fossil brachiopod species

that have a zigzag commissure in the adult without any radialfolding of the shell, the result of mantle deflection at a late on-togenetic stage [Rudwick, 1964, pl. 27, figs. 1, 2].) As argued byCrampton and Gale (2005, p. 572), the physiological costs asso-ciated with the formation of radial elements in A. sulcatus andthe presence of similar structures in later inoceramid lineages sug-gest that they were not simply non-functional products of pleio-trophy but, instead, conferred some selective advantage in thelineage. Against this, we should balance criticism of the ‘‘adap-tationist programme’’ by Gould and Lewontin (1979), who notedthe tendency to ascribe adaptive functions to structures that mayhave been products of phylogenetic and constructional constraintsrather than functional selection. They (p. 586) singled out thezigzag commissure of bivalves as one example of a structure thathas been ascribed successive functional interpretations as earlierinterpretations apparently ‘‘failed.’’

With this caveat in mind, we note simply that many functionalinterpretations of radial elements in bivalves (and brachiopods)have been proposed or can be envisaged:

1. By increasing the length of the mantle margin, radial ele-ments potentially increased the number of sensory receptorsin contact with the environment.

2. By reducing the size of the gape for a given aperture area,they reduced exposure of soft tissue to predators during feed-ing and respiration, and/or helped exclude larger, non-foodparticles (e.g., irritant crustaceans) (Rudwick, 1964).

3. They served to strengthen the body of the inflated shellagainst crushing, strengthen the margin against breakage, andprevent rotation of the closed valves during predatory attack(Johnson, 1984).

4. They enhanced respiration or feeding by elevating part of thecommissure above the sediment-water interface and/or by in-creasing the aperture area to gape ratio.

5. They enhanced respiration or feeding by exerting a hydro-dynamic effect on the inhalant current or by adding a lateraldimension to this current, resulting in more effective venti-lation of the gills that extended laterally between the valves(Carter, 1968).

6. They increased the area of shell in contact with the sediment,thereby increasing stability and friction (Seilacher, 1984; butsee Alexander, 2001).

7. They increased the area of mantle that performed some novel,unknown function. For example, some living lucinids havehighly folded ‘‘mantle gills’’ that act as secondary respiratorysurfaces and serve to keep oxygenated water separated fromsulphide-containing water that is directed at chemosymbiontshoused in the gill filaments proper (Taylor and Glover, 2000).In the case of Actinoceramus sulcatus, radial elements in-creased the surface area of shell, and therefore mantle, by upto about 18% (Fig. 6).

Although a few of these possible functions are less plausiblethan others (and, indeed, the last is pure conjecture), we are notaware of any evidence that points to a preferred functional inter-pretation of radial elements in A. sulcatus. In part this is a reflec-tion of the poor state of knowledge regarding inoceramid paleo-ecology and paleobiology in general, a subject that has receivedmuch speculation but yielded few firm conclusions.

BIOSTRATIGRAPHY AND CORRELATION

Many authors have documented and/or reviewed the biostrati-graphic ranges of Actinoceramus sulcatus and allied forms in West-ern Europe (e.g., Woods, 1912; Kauffman, 1978; Robaszynski et al.,1980; Gallois and Morter, 1982; Gallois, 1984, 1988; Owen, 1984,1996, 1999; Robaszynski and Amedro, 1986; Lake et al., 1987;Mitchell, 1995; Crampton, 1996a; Knight, 1997; Underwood andMitchell, 1999; Fenner, 2001; Woods et al., 2001; Crampton andGale, 2005; Lehmann et al., 2007), Eastern Europe, Southern Russia

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97CRAMPTON AND GALE—TAXONOMY AND BIOSTRATIGRAPHY OF ACTINOCERAMUS SULCATUS

←

FIGURE 5—Representative specimens of late Albian Actinoceramus sulcatus forma sulcatus (Parkinson, 1819) (5.1–5.16) and A. sulcatus s.l. atypical forms(5.17–5.20). All �1.5 except Fig. 5.16 � �3; all specimens whitened with magnesium oxide. Where relevant or known, heights in measured sections andassignments to ammonite zones are given in Appendix; locality descriptions given below in inverted commas are from old museum labels only. 1, 7, WM12073,left valve and anterior views, respectively; internal mould; locality and age details unknown, East Weir Bay, Folkestone, Kent, England. 2, OUM-K166697b,left valve; inner shell layer; Bed IX, Gault Clay Formation, Copt Point, Folkestone, Kent, England. 3, OUM-K39033, left valve; internal mould with innershell layer partially preserved; Bed VIII, Gault Clay Formation, Copt Point. 4, WM15482e, left valve; internal mould; Bed VIII, Gault Clay Formation, CoptPoint. 5, SM-B31484, left valve; internal mould and inner shell layer; ‘‘Bed IX, Upper Gault Clay Formation, Folkestone.’’ 6, OUM-K39036, left valve;internal mould; locality as for 3. 8, OUM-14480b, left valve; internal mould with inner shell layer partially preserved; Bed IX, Gault Clay Formation, CoptPoint. 9, OUM-14480a, left valve; internal mould with inner shell layer partially preserved; locality as for 8. 10, OUM-K14859e, left valve; internal mould;Bed VIII, Gault Clay Formation, Copt Point. Arrow marks fold crest that apparently crosses between adjacent radial folds. 11, OUM-K16137j, left valve;internal mould; Bed IX, Gault Clay Formation, Copt Point. 12, 16, BM-L5000, anterior view and detail of ligament area, respectively; inner shell layer andpatches of outer shell layer, including ligamentat; locality unknown. 13, WM15484aj, left valve; internal mould with patches of outer shell layer, partiallyflattened; Bed IX, Gault Clay Formation, Copt Point. 14, OUM-K16137a, right valve; internal mould with patches of inner shell layer; locality as for 11. 15,OUM-K14859c, right valve; internal mould; locality as for 10. 17, WM15482s, left valve, atypical specimen showing restriction of radial elements to posteriorpart of disk; internal mould; locality as for 4. 18, OUM-K30503c, right valve, atypical specimen showing loss of early-formed radial elements through growth;internal mould; Bed IX, Gault Clay Formation, Copt Point. 19, OUM-K39035, right valve, oblique view showing loss through growth of early-formed radialelements on anterior; inner shell layer partially preserved; locality as for 3. 20, OUM-KZ22939, right valve, atypical specimen showing loss of early-formedradial elements through growth; internal mould with outer shell layer partially preserved; Marnes Bleues Formation, Col de Palluel, Rosans, SE France.

and Central Asia (e.g., Khalilov, 1959; Savel’ev, 1962; Ianin, 1972,and references therein; Baraboshkin, 1999), the Far East of Russia(Zonova, 2004), and Texas (Kennedy et al., 1999).

In most cases, these studies have correlated the ranges of Ac-tinoceramus to local or regional ammonite zonations. As notedpreviously, however, Albian biostratigraphy is poorly integratedbetween the Tethyan and Boreal provinces and between individualbiostratigraphically important groups. In particular, there is at pre-sent no usable Tethyan ammonite zonation for the entire interval.Rather, the hoplitid zonation based in northwest Europe (Owen,1975) has been extended to a global scale by second- and third-order correlation and interpolation (e.g., Jacquin et al., 1999), al-though for the interval of relevance here, our observations suggestthat the zonation of Amedro (1992) can be applied directly andwith reasonable confidence to Tethyan successions. The problemis further complicated by taxonomic uncertainties involving somekey ammonite lineages (e.g., Amedro, 1992). For these reasons itis difficult to determine precise, reliable, and independent corre-lations of Actinoceramus biostratigraphic events using ammonites.Hence, many of the correlations for A. sulcatus given in the pa-pers listed above are approximate or now known to be erroneous.In reality, for the reasons given below, it may be that the Actin-oceramus events themselves offer the most robust correlation be-tween Boreal and Tethyan provinces for earliest late Albian time,a possibility exploited previously by others (e.g., Fenner, 2001,and references therein).

Figure 7 shows the adopted biostratigraphic ranges of the taxaand forms recognized herein, based on new data together with thesynthesis of a large amount of published information. Herein, wedefine four late Albian Actinoceramus biozones, based on datafrom the Folkestone, Col de Palluel and Fort Worth sections. Theyare essentially lineage zones (sensu Salvador, 1994) based on theevolutionary hypotheses outlined previously. In addition, how-ever, some of them have characteristics of acme zones and in-corporate information about the relative abundance of distinctiveforms, because evolutionary patterns are themselves expressed inthis way. As noted in Salvador (1994, p. 61), lineage zones mayhave a chronostratigraphic status and value that is special com-pared to other types of biostratigraphic zone (assuming, of course,that the defining evolutionary hypotheses are correct). In the pre-sent case, the use of relative abundance information in some ofthe zone definitions introduces an element of stochastic, sampling-related uncertainty. Despite this, we believe that the Actinocera-mus zones described below, in particular the base of the A. sul-catus Zone, may well have high value as approximatelychronostratigraphic markers.

A. concentricus parabolicus Lineage Abundance Zone.⎯Thebase of this zone is defined at the lowest occurrence of Actinocer-amus faunas dominated (�50%) by the successional subspecies A.

concentricus parabolicus. In the sections examined, the lower bound-ary of the zone can be identified readily at the base of an abruptand conspicuous acme of A. concentricus parabolicus. The top ofthe zone is defined by the base of the overlying A. sulcatus Zone.The base of the zone is at 1.0 m height in the Folkestone section(the base of Bed VIII of Owen, 1971, 1975) and the zone is restrictedto a condensed, remanie record. The base is at 23.0 m in the Colde Palluel section (Fig. 4). The zone is clearly represented in otherEnglish sections (e.g., Ford Place in Kent and the Glyndebourneborehole in Sussex; see Crampton, 1996a).

A. sulcatus Lineage Zone.⎯The base of this zone is defined atthe lowest occurrence of Actinoceramus sulcatus s.l. The top ofthe zone is defined by the base of the overlying A. sulcatus formamunsoni Zone. In the Folkestone section, the base of the zone isat about 1.1 m (about 50 mm above the top of the lowest nodulehorizon of Bed VIII of Owen, 1971, 1975); in the Col de Palluelsection the base of the zone is at 26.5 m (Fig. 4). As describedelsewhere in this paper, A. sulcatus is highly distinctive and isdistributed widely in mid- to low paleolatitudes, and it is antici-pated that the sulcatus Zone as defined will have utility through-out this biogeographic range.

In the sections examined here and in many sections throughoutWestern Europe (references given above), the base of the sulcatusZone is placed close to, but probably just above, the base of theDipoloceras cristatum ammonite Zone, which is generally regardedas marking the base of the upper Albian (Figs. 1, 7; e.g., Owen,1999). The great weight of existing evidence suggests, therefore, thatthe lowest occurrence of A. sulcatus s.l. may be regarded as a reliableand readily identified marker that approximates the base of the upperAlbian as currently defined, at least in Europe. In light of this, anupper middle Albian correlation for this datum, inferred in a recentstudy of a single borehole in the Lower Saxony Basin, Germany(Lehmann et al. 2007), seems surprising and argues for careful(re)evaluation of Actinoceramus-ammonite correlations in this andother nearby, well-sampled sections.

A. sulcatus forma munsoni Lineage Abundance Zone.⎯Thebase of this zone is defined at the lowest occurrence of Actino-ceramus faunas comprising one third or more A. sulcatus formamunsoni. The top of the zone is defined by the base of the over-lying A. sulcatus parabolicus Zone. The munsoni Zone has beenrecognized in the three sections detailed here, with its base at 5.3m in the Folkestone section (approximately the base of Bed X ofOwen, 1971, 1975), 74.5 m in the Col de Palluel section, andbetween 2.5 and 6.2 m in the Fort Worth section (uncertaintyresults from small sample sizes within this interval) (Fig. 4).

There is some equivocal evidence from the Bee Creek area,northern California, to suggest that A. sulcatus forma munsonimay have abundance acmes at other levels within the range of A.sulcatus s. l. At Bee Creek, a single collection of Actinoceramus

98 JOURNAL OF PALEONTOLOGY, V. 83, NO. 1, 2009

FIGURE 6—Commissure plots at an axial length of 20 mm for selected left valves of A. sulcatus forma sulcatus; specimens were chosen to encompass arange of radial element morphologies. Left-hand photographs show specimens with the relevant growth lines highlighted. Central line diagrams show, foreach specimen, the ventral view of the commissure at the marked growth line. Right-hand line diagrams show traces of each commissure ‘‘unrolled’’ into theplane of the page (these plots exclude the hingeline, the ends of which are marked by filled circles). Grey lines indicate the hypothetical form of an unfoldedcommissure for each specimen; these lines were used in the calculation of the relative increase in shell area resulting from the presence of radial elements inthese specimens (see Materials and Methods; values given in brackets following specimen catalogue numbers, below). All specimens from the Gault ClayFormation, Folkestone, Kent, England; all except 1 from Bed XIII or Bed IX in the measured section at Copt Point (see Appendix for details). Specimens inphotographs whitened with magnesium oxide. 1, WM12073 (17.5% increase in shell area resulting from presence of radial elements); internal mould. 2,WM15483b (12.8%); internal mould with inner shell layer partially preserved. 3, WM15942� (18.0%); inner shell layer. 4, WM15425a (15.8%); internalmould. 5, WM15483a (16.2%); inner shell layer. 6, WM15941a (13.8%); internal mould.

is composed solely of A. sulcatus forma munsoni and is associatedwith the ammonite D. cristatum, suggesting a correlation with theD. cristatum Subzone (M.A. Murphy, University of California,written. comm. 2001)—i.e., somewhat older than the munsoniZone elsewhere (Fig. 7). At this locality, however, the enclosingmudstone is clearly resedimented and it is possible that the fossilsare of mixed provenance and age (M.A. Murphy, University ofCalifornia, written. comm. 2001).

A. sulcatus biometricus Lineage Abundance Zone.⎯The baseof this zone is defined at the lowest occurrence of Actinoceramusfaunas comprising two thirds or more of the subspecies A. sul-catus biometricus. The top of the zone is defined by the highestoccurrence of the index. In the Folkestone section, the base of thebiometricus Zone is at about 5.5 m (Fig. 4) and the top is pro-visionally placed at about 5.8 m, above which Actinoceramus

specimens have not been examined by us. In the Col de Palluelsection the base of the zone is at 75.8 m and the top is at 76.5m; above this, Actinoceramus is extremely rare and no specimensof A. sulcatus biometricus have been identified (Gale et al., 1996).In the Fort Worth section, the base of the biometricus Zone isplaced at 8.2 m (although there is a 2 m sample gap beneath this)and the top is at 9.2 m; again, there are no records of Actinocer-amus from higher in this section.

SYSTEMATIC PALEONTOLOGY

Order BIVALVIA Linne, 1758Family INOCERAMIDAE Giebel, 1852Genus ACTINOCERAMUS Meek, 1864

Type species.⎯Inoceramus sulcatus Parkinson, 1819, p. 59,plate 1, figure 5, by monotypy (Meek, 1864, p. 32).

99CRAMPTON AND GALE—TAXONOMY AND BIOSTRATIGRAPHY OF ACTINOCERAMUS SULCATUS

FIGURE 7—Summary figure showing the biostratigraphic ranges and varying proportions through time of the taxa and forms described here (data for A.concentricus gryphaeoides and A. concentricus concentricus from Crampton, 1996a). Also indicated are the criteria used to define each of the inoceramidzones. For clarity, the relative thicknesses of the A. concentricus parabolicus and A. sulcatus forma munsoni zones are exaggerated here.

Discussion.⎯For a description and discussion of this genus,see Crampton (1996a).

ACTINOCERAMUS SULCATUS (Parkinson, 1819)Type specimen.⎯Lectotype, designated herein, is BM

LL27504, the specimen figured by Parkinson (1819, pl. 1, fig. 5)from the Gault at Folkestone, Kent, England. In the original de-scription, Parkinson noted that his concept was based on the fig-ured specimen but also on other unfigured material from Mallingand Cambridge. Under Recommendation 73F of the InternationalCode of Zoological Nomenclature, these specimens must all beregarded as syntypes pending designation of a lectotype.

Emended diagnosis.⎯Late Albian Actinoceramus species withweakly to strongly developed radial, oblique or antimarginalfolds, sulci or carinae.

Description.⎯Maximum axial length at least 70 mm. Inequivalve, leftvalve moderately inflated, right valve weakly inflated. Outline shape charac-teristically parabolical, trullate or rhomboidal; moderately prosocline, growthaxis weakly infracrescent to weakly retrocrescent, moderately retrocrescent infew specimens. Anterior margin more-or-less straight; anterior face of leftvalve well differentiated from disc, approximately flat, perpendicular to com-missure to somewhat excavated. Antero-ventral margin weakly convex; pos-terior margin weakly to moderately convex, bending forward to hingeline.Hingeline typically about half shell length; posterior wing small, weakly dif-ferentiated. Umbo narrow, acutely pointed, projecting above hingeline andpiercing plane of commissure in left valve.

Disc affected by one or more radial elements. Arrangement and form of radialelements very highly variable, ranging from numerous, high amplitude folds and

interjacent, deeply excavated sulci that affect entire disc (forma sulcatus), to singlecomparatively weak carina at growth axis (subspecies biometricus). Large varia-tions in number, form and distribution of radial elements expressed within mostpopulations (e.g., Kennedy et al., 1999, fig. 17; Crampton and Gale, 2005, fig.3j-l). Shell also sculptured with weak, low, irregular, rounded commarginal foldsthat may result in irregular swellings on radial elements. Surface sculpture ofclosely and typically regularly spaced growth lines.

Ligament area observed on five specimens, BM-L5000 (Fig. 5.16), BM-L5001 (Fig. 8.4), OUM-KT9122 (Kennedy et al., 1999, fig. 16.7), OUM-KT9254 (Kennedy et al., 1999, fig. 16.9) and SAM-D526a. Area weakly con-cave, with narrow to very narrow, rectangular resilifers that breach and/orweakly crenulate ventral margin of area. Resilifers separated by low, flat-crested ridges of variable width, approximately half width of resilifers in spec-imen OUM KT.9122, approximately twice width of resilifers in specimenOUM KT.9254. Ligament area sculptured by longitudinal growth lines (i.e.,parallel to shell length). Small umbonal septum partially preserved on spec-imen OUM KT.9254.

Shell microstructure of ligamentat unknown; assumed to be similar to A.concentricus (see Crampton, 1996a, fig. 17, pl. 3, fig. K).

Shell thin: 0.4 mm thick on disk of typical adult specimen.Distribution.⎯Actinoceramus sulcatus is known from throughout

Europe, Central Asia, the Far East of Russia and Japan, southernand western North America, South Africa, and possibly India(Crampton, 1996a and herein). It is interesting to note that the ap-parent absence of this species from Australasia appears to be realand not an artefact of poor sampling, given that fossiliferous strataof the appropriate age are well known in both Australia (Hendersonand Kennedy, 2002) and New Zealand (Crampton et al., 2004).

Discussion.⎯The diagnosis given above, although rather

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broad, is sufficient to distinguish this species lineage from allother known Actinoceramus species. The incorporation of an agecriterion, contrary to established practice, is required because itis extremely difficult to diagnose all individuals of A. sulcatusuniquely from earlier species based on morphology alone. Thisproblem is illustrated statistically in Fig. 9. Despite this, however,most specimens of A. sulcatus are highly distinctive and can beidentified readily.

Following the taxonomic protocols explained previously, threemorphotypes and one successional subspecies are recognizedwithin the Actinoceramus sulcatus lineage: A. sulcatus forma sul-catus, A. sulcatus forma subsulcatus, A. sulcatus forma munsoni,and A. sulcatus biometricus. These categories exclude two veryrare, ‘‘aberrant’’ forms with radial elements restricted to the pos-terior part of the shell (observed in a single individual; Fig. 5.17)or restricted to the juvenile/immature part of the shell (well de-veloped in at least four individuals; Fig. 5.18, 5.20).

ACTINOCERAMUS SULCATUS forma SULCATUS (Parkinson, 1819)Figures 5.1–5.16, 6

Inoceramus sulcatus PARKINSON, 1819, p. 59, pl. 1, fig. 5; SOWERBY, 1821,p. 184, pl. 306, figs 1–7; MANTELL, 1822, p. 95–96, pl. 19, fig. 16; MAN-TELL, 1833, p. 169, upper fig. 2; PICTET AND ROUX, 1853, p. ‘‘499–500,’’pl. 1a–c (pages 289–458 incorrectly numbered 389–558); ?PICTET AND

CAMPICHE, 1869, p. 105–107; ?SCHLUTER, 1877, p. 256; WOLLEMANN,1909, p. 273, pl. 6, fig. 10; WOODS, 1911, p. 269–271, pl. 47, figs. 15–20;WOODS, 1912, p. 2–4, figs. 19–21; MORDVILKO ET AL., 1949, p. 152, pl.35, figs. 5, 6 (incorrectly cited as ‘‘Parkinson, 1809’’); MUROMTSEVA AND

IANIN in DRUSHCHITS AND KUDRIAVTSEV, 1960, p. 185, pl. 7, figs. 9, 10;ROBASZYNSKI ET AL., 1980, fig. 10.

Inoceramus sulcatus Sowerby. GOLDFUSS, 1836, p. 112, pl. 110, fig. 1;D’ORBIGNY, 1846, p. 504–505, pl. 403, figs. 3–5.

Not Inoceramus cripsii var. sulcata ROEMER, 1852, p. 56, pl. 7, fig. 2.Inoceramus (Actinoceramus) sulcatus PARKINSON, 1819. WHITEAVES, 1884,

p. 241–242, pl. 32, fig. 3; ZITTEL, 1887, p. 38, fig. 47.Inoceramus praedigitatus AIRAGHI, 1904, p. 183, fig. 1, pl. 4, figs. 13?, 14.Inoceramus concentricus var. subsulcatus WILTSHIRE, 1869. WOODS, 1911,

p. 268–269, pl. 47, figs. 12–14; WOODS, 1912, p. 2–4, fig. 18.Actinoceramus sulcatus (Parkinson, 1819). KHALILOV, 1959, p. 33–34, pl. 2,

fig. 12, pl. 3, fig. 13 (referred to Inoceramus (Actinoceramus) in plate cap-tions); SAVEL’EV, 1962, p. 242–247, pl. 11, figs 1–8; ALI-ZADE ET AL.,1988, p. 247, pl. 3, fig. 8, pl. 4, fig. 1.

Actinoceramus sulcatus Parkinson, 1819 morpha aequaliplicata SAVEL’EV,1962, p. 245, pl. 11, figs. 1–4.

Actinoceramus sulcatus Parkinson, 1819 morpha inaequaliplicata SAVEL’EV,1962, p. 245, pl. 11, figs. 5–7.

Actinoceramus sulcatus Parkinson, 1819 aberratio flexiosocostatus SAVEL’EV,1962, p. 245, pl. 11, fig. 8.

Actinoceramus sulcatoides SAVEL’EV, 1962, p. 247–249, pl. 9, figs. 1–5.Inoceramus (Birostrina) sulcatus PARKINSON, 1819. COX, 1969, p. N315, fig.

C46-2a; MONGIN, 1979, pl. 9-2, fig. 4.Inoceramus (Birostrina) sulcatus sulcatus PARKINSON, 1819. DIMITROVA,

1974, p. 66, pl. 34, fig. 4.Birostrina subsulcatus (WILTSHIRE, 1869). KAUFFMAN, 1978, pl. 1, fig. 21.Inoceramus cf. sulcatus PARKINSON, 1819. ZONOVA, 1978a, p. 79, pl. 7, figs.

1–3.Birostrina sulcata (Parkinson, 1819). OWEN, 1984, p. 339–340; DHONDT AND

DIENI, 1988, p. 24–25, pl. 5, fig. 17; KNIGHT, 1997, fig. A.3.1.b.Actinoceramus pseudosulcatus Khalilov, 1959. ALI-ZADE ET AL., 1988, p.

248, pl. 4, figs. 5, 6 (not I. pseudosulcatus Nagao & Matsumoto, 1940).Actinoceramus sulcatus sulcatus (Parkinson, 1819). CRAMPTON, 1996a, fig.

10; KENNEDY ET AL., 1997, pl. 10, figs. 1–5.Actinoceramus sulcatus (Parkinson, 1819) forma A. KENNEDY et al., 1999,

fig. 16.2.Actinoceramus sulcatus (Parkinson, 1819) morphotype A. CRAMPTON AND

GALE, 2005, figs. 3A, 3B, 3I (upper specimen), 3J, 3O (left-hand speci-men).

Description.⎯Most characters as for species, above. Axial length of mostspecimens �50 mm.

Entire disc affected by large, high amplitude, upstanding radial, oblique orantimarginal folds and interjacent, deeply excavated sulci that initiate, by con-vention, within 5 mm of beak and affect earliest postlarval growth stage. Ataxial length of 10 mm, between three and nine radial folds present (Fig. 10and e.g., compare Fig. 5.8 and 5.9); at axial length of 20 mm, between fiveand 11 folds present. Number of radial elements increases up-section (Fig.10; see also Crampton and Gale, 2005, table 2). Radial elements absent from

anterior face and posterior wing. Arrangement and form of radial elementsvery highly variable within and between individuals. On some specimens, allfolds approximately radial; on others, radial, oblique and antimarginal ele-ments occur in varying configurations. Where present, oblique and antimar-ginal elements found most commonly on posterior part of disc, behind growthaxis; less commonly in front of growth axis. On many specimens, oblique orantimarginal elements diverge, chevron-like and at angles of up to 30�, fromradial fold situated at growth axis. Most folds initiate at beak and have simple,single, sharp crests, but some have paired crests, bifurcate through growth orintercalate relatively late in ontogeny.

Material.⎯See Appendix.Distribution and age.⎯Known with confidence from Europe

(UK, France, Germany, Switzerland, Poland, Bulgaria, Sardinia),Central Asia (Kazakhstan, Azerbaijan), the Far East of Russia,western Canada, South Africa.

Actinoceramus sulcatus forma sulcatus has a lowest occurrenceclose to the base of the D. cristatum Subzone and a highest oc-currence close to the top of the H. orbignyi Subzone. Within thisrange, it dominates most collections, comprising between 60%and 90% of specimens in most samples (but see below). Thelowest occurrence of A. sulcatus forma sulcatus may provide areliable marker that approximates the base of the upper Albian ascurrently defined, at least in Europe (see section on Biostratig-raphy and Correlation).

ACTINOCERAMUS SULCATUS forma SUBSULCATUS

(Wiltshire, 1869)Figure 8.1–8.7

Inoceramus subsulcatus WILTSHIRE, 1869, p. 188, p. 190; ZONOVA, 1978a,p. 79–80, pl. 7, figs. 4–6; ROBASZYNSKI ET AL., 1980, fig. 10.

Inoceramus concentricus var. subsulcatus WILTSHIRE, 1869. WOODS, 1911,p. 268–269, pl. 47, figs. 6–11; WOODS, 1912, p. 2–4, figs 12–17; ?PAS-SENDORFER, 1930, p. 589; ?RENNIE, 1936, p. 314–315; ?DASSARMA AND

SINHA, 1975, p. 24, pl. 3, fig. 2 (incorrectly cited as ‘‘Willshire, 1910’’).Inoceramus (Actinoceramus) subsulcatiformis BOSE, 1927, p. 189, pl. 18, figs.

1–5.Actinoceramus subsulcatiformis BOSE,1928. ADKINS, 1928, p. 96, pl. 7,

fig. 2.Actinoceramus perisulcatus KHALILOV, 1959, p. 32, pl. 2, figs. 9, 10 (referred

to Inoceramus (Actinoceramus) in plate captions). ALI-ZADE ET AL., 1988,p. 246, pl. 3, figs. 5, 6.

Actinoceramus subsulcatus Wiltshire, 1869; KHALILOV, 1959, p. 32–33, pl.2, fig. 11 (incorrectly cited as ‘‘Wiltschire, 1869’’; referred to Inoceramus(Actinoceramus) in plate captions); ALI-ZADE ET AL., 1988, p. 246–247,pl. 3, fig. 7 (incorrectly cited as ‘‘Wiltschire, 1869’’).

Actinoceramus pseudosulcatus ?KHALILOV, 1959, p. 34–35, pl. 3, figs. 17, 18(referred to Inoceramus (Actinoceramus) in plate captions; not I. pseudo-sulcatus Nagao and Matsumoto, 1940).

Inoceramus concentricus subsulcatus Wiltshire, 1869. MATSUMOTO AND

HARADA, 1964, pl. 9, fig. 3.Inoceramus (Birostrina) subsulcatus WILTSHIRE, 1869. COX, 1969, p. N315,

fig. C46-2b; ?CHIPLONKAR AND BADVE, 1976, p. 8.Birostrina subsulcatus (Wiltshire, 1869). KAUFFMAN, 1978, pl. 1, fig. 19.Birostrina sulcata (Parkinson, 1819). KAUFFMAN, 1978, pl. 1, fig. 22.Inoceramus ex gr. sulcatus Parkinson, 1819. ZONOVA, 1978a, p. 80–81, pl.

7, figs. 7–10; ZONOVA, 1978b, pl. 9, fig. 3.Birostrina ‘‘subsulcata’’ Wiltshire, 1869. OWEN, 1984, p. 339–340.Birostrina subsulcata (Wiltshire, 1869). ?Not DHONDT AND DIENI, 1988, p.

24, pl. 5, figs. 14–16; ZONOVA, 2004, pl. 3, figs. 7–9.Actinoceramus sulcatus (Parkinson, 1819) forma B. KENNEDY ET AL., 1999,

p. 1121–1122, figs. 16.3, 16.4, 17.7, 17.8.Birostrina ex gr. sulcata (Parkinson, 1819). ZONOVA, 2004, pl. 3, figs. 10–

12 (authorship incorrectly ascribed to Woods in plate caption).Actinoceramus sulcatus (Parkinson, 1819) morphotype B. CRAMPTON AND

GALE, 1999, fig. 3C, 3D, 3I (lower specimen), 3K, 3M, 3O (right handspecimen).

Description.⎯Most characters as for species, above. Axial length typically�c. 50 mm.

Radial elements affecting much of adult disc but lacking from juvenileshell, initiating (by convention) at axial length of �5 mm. In most specimens,first-formed fold(s) initiate within first 10 mm, but in some folds initiate 25mm or more from umbo (Fig. 11). On any individual, all radial elements mayinitiate at single growth stage or they may initiate over range of growth stages;in latter case, folds closest to anterior margin are typically first to form. Den-sity and number of radial elements highly variable, between one and six at

101CRAMPTON AND GALE—TAXONOMY AND BIOSTRATIGRAPHY OF ACTINOCERAMUS SULCATUS

FIGURE 8—Representative middle to late Albian Actinoceramus and forms from the A. sulcatus lineage. All �1.5 except Fig. 8.4 � �3; all specimenswhitened with magnesium oxide. Where relevant or known, heights in measured sections and assignments to ammonite zones are given in Appendix; localitydescriptions given below in inverted commas are from old museum labels only. 1–7, Actinoceramus sulcatus forma subsulcatus (Wiltshire, 1869). 1, 5, SM-B31485, left valve and anterior views, respectively; internal mould; ‘‘Bed IX, Upper Gault Clay Formation, Folkestone,’’ Kent, England. 2, 4, BM-L5001,left valve and ligament area, respectively; inner shell layer with outer shell layer partially preserved; locality unknown. 3, OUM-K16696c, right valve; internalmould; Bed IX, Gault Clay Formation, Copt Point, Folkestone. 6, SM-B31476, left valve; internal mould with some inner shell layer adhering; ‘‘lowest zoneof the Upper Gault,’’ Folkestone. 7, OUM-K14480d, left valve; internal mould; Bed IX, Gault Clay Formation, Copt Point. 8–11, A. sulcatus forma munsoni(Cragin, 1894). 8, OUM-K14480e, left valve; internal mould; locality as for 7. 9, USNM-32685, syntype(?), left valve; internal mould; Duck Creek Limestone,Duck Creek, Denison, Texas, USA. 10, OUM-KY2198, left valve; internal mould; Koksyirtau section, Mangyshlak Mountains, western Kazakhstan. 11, OUM-K14536a, left valve; internal mould; Bed IX, Gault Clay Formation, Copt Point. 12, A. sulcatus biometricus Crampton, 1996, BM-L17187a, holotype, leftvalve; outer shell layer (silicified); Upper Greensand, Blackdown, Devon, England. 13, A. concentricus parabolicus Crampton, 1996, SM-X24757, paratype,left valve; internal mould; Bed VIII, Gault Clay Formation, Copt Point.

axial length of 10 mm. Other variations in form and arrangement of radialelements as for A. sulcatus forma sulcatus (above).

Material.⎯See Appendix.Distribution and age.⎯Europe (UK, France, Poland), Central

Asia (Kazakhstan, Azerbaijan), the Far East of Russia, Japan, theUnited States of America (Texas), South Africa, ?India.

Actinoceramus sulcatus forma subsulcatus has a lowest occur-rence close to the base of the D. cristatum Subzone and a highestoccurrence close to the top of the H. orbignyi Subzone. It typi-cally comprises a minor proportion of most collections (�20%)

except close to its upper and lower range limits, where it candominate some samples (see section on Biostratigraphy and Cor-relation).

Discussion.⎯In his original description of Inoceramus subsul-catus, Wiltshire (1869, table, p. 188) referred only to the specimenfigured by Pictet and Roux (1853, pl. 1d). Without explicit des-ignation of type material and according to Recommendation 73Fof the International Code of Zoological Nomenclature, this spec-imen should be regarded as a syntype. The specimen in questionis a left valve that bears three radial folds and sulci on the anterior

102 JOURNAL OF PALEONTOLOGY, V. 83, NO. 1, 2009

FIGURE 9—Results of a Fourier shape analysis of A. concentricus para-bolicus and A. sulcatus biometricus, based on the data of Crampton (1996a)(see Methods). The Fourier coefficients have been examined using a discrim-inant function analysis. Results are shown as histograms of the actual outlinesordinated above and below the discriminant function axis (the abscissa). Thepositions of the two group means on the discriminant function are indicatedby dotted lines; the corresponding synthetic mean shapes are shown as filled,shaded outlines. Growth axes are marked within each outline to highlightdifferences in shapes between the two groups. The probability that the groupmeans are the same is 0.000 (F � 7.539, with 21 and 48 degrees of freedom,probability � 4.1 � 10�9).

FIGURE 10—Histograms showing number of radial elements at an axiallength (AL) of 10 mm on specimens of A. sulcatus forma sulcatus. Data shownfor material from the Folkestone and Col de Palluel sections, both by strati-graphic horizon and pooled. The data reveal an increase in the number ofradial elements through time; this increase is statistically significant (for dis-cussion, see Crampton and Gale, 2005). The average number of radial ele-ments at Folkestone is 5.8 (SD � 1.2); the average at Col de Palluel is 6.8(SD � 1.3) (means indicated by arrows). The average for Folkestone is lowerbecause these data do not include specimens from the upper half of the rangeof A. sulcatus forma sulcatus.

part of the disk and it is included here with Actinoceramus sul-catus forma munsoni, described below. Clearly, in a formal tax-onomic context, ‘‘I. munsoni’’ would be included as a junior syn-onym of ‘‘I. subsulcatus.’’ Since 1869, however, a great numberof authors have included within ‘‘I. subsulcatus’’ any specimensintermediate in form between A. concentricus and A. sulcatus for-ma sulcatus, with radial folds on some part of the disk but notextending to the beak. This usage is almost certainly consistentwith Wiltshire’s original intention, since there is nothing to sug-gest that he limited his concept of subsulcatus to those forms withradial elements restricted to the anterior part of the disk. Giventhis precedent, and given the informal taxonomic status accordedto these morphotypes by us, the commonly understood conceptsof A. sulcatus forma subsulcatus and A. sulcatus forma munsoniare maintained here.

ACTINOCERAMUS SULCATUS forma MUNSONI (Cragin, 1894)Figure 8.8–8.11

Inoceramus sulcatus Parkinson, 1819. PICTET AND ROUX, 1853, p. ‘‘499–500,’’ pl. 1d–f (pages 289–458 incorrectly numbered 389–558; see com-ments on specimen of pl. 1d under A. sulcatus forma subsulcatus, above).

Inoceramus munsoni CRAGIN, 1894, p. 55–56.

Inoceramus concentricus var. subsulcatus Wiltshire, 1869. WOODS, 1911, p.268–269, pl. 47, figs. 3–5; WOODS, 1912, p. 2–4, figs. 10, 11.

Inoceramus (Actinoceramus) subsulcatiformis BOSE, 1928. IMLAY, 1937, p.238–240, pl. 17, figs. 9–11, fig. 15.

Actinoceramus pleurosulcatus KHALILOV, 1959, p. 34, pl. 3, figs. 14–16 (re-ferred to Inoceramus (Actinoceramus) in plate captions); ALI-ZADE ET AL.,1988, p. 247, pl. 4, figs. 2–4.

Inoceramus (Actinoceramus) ‘‘subsulcatus’’ Wiltshire, 1869. JONES, 1960, p.158, pl. 1, fig. 6.

Inoceramus subsulcatus Wiltshire, 1869. MUROMTSEVA AND IANIN in DRUSH-CHITS AND KUDRIAVTSEV, 1960, p. 185, pl. 7, figs. 7, 8.

Inoceramus (Birostrina) sulcatus subsulcatus Wiltshire, 1869. ?DIMITROVA,1974, p. 66, pl. 34, fig. 3.

Birostrina subsulcatus (Wiltshire, 1869). KAUFFMAN, 1978, pl. 1, fig. 17.

103CRAMPTON AND GALE—TAXONOMY AND BIOSTRATIGRAPHY OF ACTINOCERAMUS SULCATUS

FIGURE 11—Histogram summarising the axial length (AL) at which thefirst-formed radial element initiated through growth in 87 specimens of A.sulcatus forma subsulcatus from the Folkestone section. By definition, thisdistance cannot be less than 5 mm.

FIGURE 12—Histogram summarising the axial length (AL) at which thefirst-formed radial element initiated through growth in 31 specimens of A.sulcatus forma munsoni from the Fort Worth section. By definition, this dis-tance cannot be less than 5 mm.

Birostrina munsoni (Cragin, 1894), s.l., n. subsp. ?WIEDMANN AND KAUFF-MAN, 1978, p. III.4, pl. 1, fig. 11; ?WIEDMANN, 1979, pl. 1, fig. 11.

Actinoceramus sulcatus biometricus CRAMPTON, 1996a, p. 43–45, pl. 5, fig.C, D.

Actinoceramus sulcatus (Parkinson, 1819) forma C. KENNEDY ET AL., 1999,p. 1122, figs. 16.5–16.9, 17.3–17.6.

Actinoceramus sulcatus (Parkinson, 1819) morphotype C. CRAMPTON AND

GALE, 1999, fig. 3E, 3F, 3L.

Description.⎯Most characters as for species, above. Apparently somewhatlarger than most A. sulcatus forma sulcatus and A. sulcatus forma subsulcatus,axial length up to 70 mm.

Between one and three radial (sensu stricto) fold and sulci couplets restrict-ed to anterior part of disc and initiating at axial lengths of between 5 and 33mm (Fig. 12). Folds relatively broad, subdued and rounded compared to A.sulcatus forma sulcatus and A. sulcatus forma subsulcatus. In addition, a fewspecimens have a single broad, shallow radial sulcus affecting posterior partof disc (Fig. 8.8, 8.9).

Material.⎯See Appendix.Distribution and age.⎯Europe (UK, France, ?Spain, Bulgaria),

Central Asia (Kazakhstan, Azerbaijan), the United States ofAmerica (Oregon, Texas), Mexico, South Africa.

Actinoceramus sulcatus forma munsoni has a lowest occurrenceclose to the base of the D. cristatum Subzone and a highest oc-currence close to the top of the H. orbignyi Subzone. Its rangeextends slightly higher than A. sulcatus forma sulcatus. Through-out most of this interval it comprises a minor component of somecollections. In addition, however, in the Folkestone, Rosans andFort Worth sections it has an abundance acme immediately abovethe uppermost occurrence of A. sulcatus forma sulcatus, where itmay comprise over 40% of some samples (see section on Bio-stratigraphy and Correlation).

Discussion.⎯(See discussion of A. sulcatus forma subsulcatus,above.) In his original description of ‘‘I. munsoni,’’ Cragin (1894)did not identify any types, although he gave measurements fortwo specimens. According to museum labels, Cragin’s ‘‘cotypes’’(i.e., syntypes) are held in the U.S. National Museum, collectionUSNM 32685.

ACTINOCERAMUS SULCATUS BIOMETRICUS Crampton, 1996aFigure 8.12

Inoceramus concentricus Parkinson, 1819. WOODS, 1911, p. 265–268, pl. 46,figs. ?8, 9, 10, pl. 47, figs. 1, 2.

Inoceramus (Actinoceramus) subsulcatiformis Bose, 1928. IMLAY, 1937, p.238–240, pl. 17, figs. 1–8, figs 12–14.

Birostrina concentrica concentrica Parkinson, 1819. ?WIEDMANN AND

KAUFFMAN, 1978, p. III.4, pl. 1, fig. 12.Birostrina concentrica (Parkinson, 1819). ?DHONDT AND DIENI, 1988, p. 22–

24, pl. 5, figs. 8, 13.Birostrina cf. concentrica (Parkinson, 1819) subsp. D of Kauffman. GALLOIS,

1988, pl. 5, fig. 12.Actinoceramus sulcatus biometricus CRAMPTON, 1996a, p. 43–45, pl. 5, figs.

E–J.Actinoceramus sulcatus (Parkinson, 1819) forma D. KENNEDY ET AL., 1999,

p. 1122–1123, fig. 17.1, 17.2.Actinoceramus sulcatus (Parkinson, 1819) morphotype D. CRAMPTON AND

GALE, 2005, fig. 3H, 3N.

Type specimens.⎯Holotype BM L17187a, from the UpperGreensand at Blackdown, Devon, England; probably from Bed 8of Downes (1882).

Paratypes, BM L17192b, BM L17192c, BM L17192m, all fromthe same locality as the holotype.

Emended diagnosis.⎯Actinoceramus sulcatus lacking well-de-fined radial sulci and with poorly defined, rounded umbonal-ven-tral carina that divides disc into subequal parts.

Description.⎯Refer to Crampton (1996a, pp. 43–45).Material.⎯See Appendix.Distribution and age.⎯Europe (UK, France, Spain, ?Sardinia),

Central Asia (Kazakhstan), the United States of America (Texas),Mexico.

Essentially restricted to the uppermost H. orbignyi Subzone andH. varicosum Subzone (see section on Biostratigraphy and Cor-relation). A few Actinoceramus biometricus-like specimens occurthroughout the D. cristatum and M. pricei zones, but they formonly a very minor part of some collections and are regarded asend-member ecophenotypes of A. sulcatus s.l.

Discussion.⎯Actinoceramus sulcatus biometricus is the suc-cessional subspecies that came to dominate populations of A. sul-catus close to the H. orbignyi/H. varicosum subzonal boundary.In some sections there is a clear transition from A. sulcatus formamunsoni to A. sulcatus biometricus, with individual collectionscontaining specimens that span the morphological continuum be-tween the two forms (e.g., the Fort Worth section, see Kennedyet al. 1999, fig. 17).

It is instructive to compare the forms immediately ancestral toand descendant from strongly sulcate A. sulcatus, i.e., A. concen-tricus parabolicus (Fig. 8.13) and A. sulcatus biometricus (Fig.8.12). Some of Crampton’s (1996a) biometric data have been re-worked here using the refined Fourier shape analysis of Haines& Crampton (2000) (see Methods). Figure 9 shows the lateraloutline shapes of A. concentricus parabolicus and A. sulcatusbiometricus ordinated as histograms along the discriminant func-tion axis (DFA) based on the Fourier coefficients. For clarity, thetwo a priori groups have been separated and plotted above andbelow the abscissa. The DFA represents the axis, in multivariatespace, that optimally discriminates between the two populations.Clearly, on the basis of shape, there is little overlap between thetwo and their means are different at k99.9% level of confidence.The outlines reveal, however, that differences are indeed subtleto the eye and, furthermore, some specimens of A. concentricusparabolicus are statistically closest to the mean form of A. sul-catus biometricus and would, on the basis of shape, be misclas-sified. Although not demonstrated here, the two groups cannot bedistinguished on the basis of inflation (Crampton 1996a, table 2)or sculpture.

104 JOURNAL OF PALEONTOLOGY, V. 83, NO. 1, 2009

ACKNOWLEDGMENTS

Many people have contributed helpful discussions and information duringthe long course of this study. In particular, we gratefully acknowledge W. A.Cobban (Denver, Colorado), J. A. Crame (British Antarctic Survey, Cam-bridge, UK), J. E. Eyers (Chiltern Archaeology, UK), H. Hilbrecht (ETH,Zurich, Switzerland), D. Jablonski (University of Chicago), S. R. A. Kelly(CASP, Cambridge, UK), W. J. Kennedy (Oxford University Museum), H.Klinger (South African Museum, Cape Town), R. I. Knight (formerly of Ox-ford University), and M. A. Murphy (University of California, Riverside). Weare also grateful to the many institutions that have given access to or loanedmaterial: the Oxford University Museum (in particular, the Kirby collections),the Sedgwick Museum (Cambridge), the Natural History Museum (London),the South African Museum (Cape Town), and the United States National Mu-seum (Washington). For extremely helpful reviews, we thank Alan Beu (GNSScience, Lower Hutt), Irek Walaszczyk (University of Warsaw, Poland), andChris Wood (Scops Geological Services Limited, Somerset, United Kingdom).

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