Journal of the Czech Geological Society 46/3ñ4(2001) 215

Remarks on the palaeoecology of agnostid trilobites

Pozn·mky o paleoekologii agnostidnÌch trilobit˘

(3 figs)

JANA SLAVÕ»KOV¡ 1 ñ PETR KRAFT 2

1 Palaeontological Department, National Museum, V·clavskÈ n·m. 68, CZ-115 79 Prague 1, Czech Republic; e-mail: jana.slavickova@nm.cz2 Charles University, Institute of Geology and Palaeontology, Albertov 6, CZ-128 43 Prague 2, Czech Republic; e-mail: kraft@natur.cuni.cz

Several enrolled specimens of agnostid trilobites, partly in an aligned row, from the ä·rka Formation (Barrandian Ordovician), occur in arelationship with an enigmatic fossil. This fossil is a fragment of a branched axis resembling similar ones from the Klabava Formationinterpreted before as possible octocorals. This association provides evidence for a partly epibenthic mode of life for some agnostids.

Key words: Palaeozoic, Ordovician, Trilobita, Agnostida, problematic octocorals, palaeoecology

Bergstrˆm (1973) concluded that at least some agnos-tids were ectoparasitic and the ability of enrolling helpedthem to have a better attachment to their hosts. He alsonoted that some species were probably pelagic whileothers might be benthic.

M¸ller ñ Walossek (1987) found extremely well pre-served phosphatised specimens of Agnostus pisiformiswith appendages. Based on a functional morphologicalstudy of such material, they concluded that agnostidswere completely enrolled in a resting or protective posi-tion. During locomotion and feeding, tests were slightlyopened. These authors suggest a connection with the bot-tom environment which they stated to be not in conflictwith morphological details indicating an ability to swimor float. This idea was recently adopted by Schovsbo(2000) for Cambrian agnostids.

Nielsen (1997) proposed the idea that agnostids ori-ginated as benthic trilobites adapted to dysoxic facieswith a high production of spat, and, even more so bylong-lived spat.

Fortey ñ Owens (1999) suggested planktic, benthicand probably epipelagic modes of life for various agnos-tid trilobites. For more specialized agnostids they alsosupposed a similar mode of life as interpreted for eodis-cids. Based on the possession of a natant hypostome theyare considered to be benthic feeders on minute organicparticles such as algae, detritus, and animalcules. Thesesame authors found the parasitic mode of life (Bergstrˆm1973) improbable. They also criticised the mode of lifesuggested by M¸ller ñ Walossek (1987) while the epi-faunal mode of life on free-floating algal strands theysupposed to be secondary and developed after a putative-ly planktic larval phase. According to them, at least someagnostids could adopt this mode of life, which is com-mon among living ostracods.

Geological setting and material

The studied sample of agnostids and the problematic fos-sil is preserved three-dimensionally in a siliceous nodulewhich was discovered by I. Chlup·Ë in the 1950ís at Pra-ha-Vokovice. The siliceous nodule was collected on the

Introduction

In general, agnostid trilobites are not very common inthe Bohemian Ordovician. They usually occur incompleteas separate cephala or pygidia, and either isolated or inaccumulations with other fauna. The complete specimensoccur predominantly enrolled.

Agnostid trilobites of the ä·rka Formation (Middle Or-dovician) were revised by Pek (1977) in the frameworkof the overall revision of the Bohemian agnostids and, re-cently, their stratigraphical occurrence was summarized byFatka ñ Pek (1999). They mentioned 7 species from theä·rka Formation. For the species studied herein, we fol-low the systematic position proposed by Nielsen (1997).

Prantl (1948), P¯ibyl ñ VanÏk (1976) and Pek (1977)described agnostid trilobites arranged in clusters oraligned rows from the Bohemian Ordovician. They ad-vocated the epifaunal mode of life for agnostids with at-tachment on algal strands.

A sample of several enrolled specimens from the ä·rkaFormation, partly arranged in an aligned row is describedin this paper. It represents an extraordinary find on ac-count of its apparent, at least spatial relationship to anenigmatic fossil occurring in close proximity. Such an ar-rangement of fossils is interpreted as not being an acci-dental case.

The studied sample is deposited in the National Mu-seum in Prague, Czech Republic, No. L 36065.

Supposed mode of life of agnostid trilobites

Various modes of life have been suggested by many au-thors for agnostids: pelagic (Robison 1972, Jago 1973,Lochman-Balk ñ Wilson 1958, etc.), ectoparasitic (Berg-strˆm 1973), benthic (Jaekel 1909), infaunal (Lochman1940), and epifaunal attached to algal strands (P¯ibyl ñVanÏk 1976, Pek 1977). According to Robisonís (1972)suggestion, agnostids spent most of their time enrolled,swimming by clapping of the cephalon and pygidium.Clarkson et al. (1998) found this mode of life not to bein contradiction with their observations but the questionof agnostid life habits was kept open.

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field between Bo¯islavka and »erven˝ vrch. At this local-ity, which does not exist anymore, there occurred nodulesmixed from various stratigraphic levels. However, thosefrom the Didymograptus clavulus Biozone prevailed. Theinvestigated nodule most probably comes from that bio-zone. Based on the location, attributes of nodules from thelocality and their fossil content, and the shape, size andlithology of the studied nodule it seems that it comes fromlayers corresponding to the lowermost part of layer 9a(BouËek 1926) which was exposed in the former Vokovi-ce brickfield (Chlup·Ë pers. com.).

The nodule weathered out from shales of the ä·rkaFormation (Middle Ordovician, Darriwilian, Llanvirnian).This formation consists of dark shales with horizons ofsiliceous nodules of early diagenetic origin. In contrastto the fossils from the shales, those from the nodules are3-D preserved and their spatial orientation can be stud-ied well. However, the investigated fossils are preservedjust as external moulds, which is one of the typical modesof preservation in the ä·rka Formation nodules. Thusonly outer surfaces of fossils can be studied.

Description

The fossils are preserved in a broad spindle-like siliceousnodule which is 56 mm long and max. 26 mm in diame-ter. Its longitudinal shape is determined by the containedremain of the axis-fossil. This fragment is 48 mm long andis of circular outline, maximum 7.5 mm in diameter. Themain axis is two or possibly three times laterally branched.

The interval between two apparent secondary branches al-ternating to opposite sides is 27 mm. The surface of thisfossil, by contrast with the agnostids, is heavily li-monitized. The limonite forms a thin layer (ìcrustî), insome portions extended up to the infilling of the space ofthe external mould. The outline of this fossil is diffusedrather than sharp in some portions because of the limoniti-zation. Its visible surface is coarse and tuberous with someirregular longitudinal linear structures indicated. The axesbear on their surface spine-like processes. These hair-likeprocesses are some 2 mm long and not straight but slight-ly wavy. They are also limonitized and thus well markedas brown lines in the dark grey material of the split nod-ule. The spiny processes were observed only sporadical-ly, but in one portion four in a row indicate their quitedense distribution. These 4 processes occur within 2.5 mm.

Eight specimens of agnostid trilobites, all enrolled, aresituated close to the axis-fossil. All determinable speci-mens belong to the species Geragnostus (Geragnostella)tullbergi (Nov·k, 1883). One specimen is further away(5 mm) from the fragment of the axis fossil. The otherspecimens were found in close proximity to its mainbranch. Two of them occur individually, one situated justbelow the first preserved second order branch of the axis-fossil, the other isolated on the opposite side, some 5 mmabove the row of the four spine-like processes (see above).The remaining five agnostids are aligned in a row alongthe main axis in the upper portion of the axis-fossil frag-ment. This row is placed on the opposite side to the latterisolated trilobite. The upper limit of the row is on the same

Fig. 1 1a, 1b ñ Counterparts of the nodule with enrolled Geragnostus (Geragnostella) tullbergi in relationship with the axis-fossil, Praha-Vo-kovice ñ field near Bo¯islavka, L 36056, coated with ammonium chloride, scale bar = 10 mm; 2 ñ Detail of spine-like processes, portion withfour ones indicated by arrow, scale bar = 5 mm.

1a 1b 2

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level as the second apparent branchingof the axis-fossil. These specimens ina row are situated close to each other,at distances less than 1 mm. In somecases they are almost in contact.

The spatial orientation of each of thetrilobites is slightly different, but all areoriented with the free margins of thecephala and pygidia (tests) facing theaxis-fossil.

There are also other fossils close tothe axis-fossil. The latter isolated tri-lobite is placed near a flat fragment ofimpossible determination. Near thelower limit of the aligned row thereare visible two transverse cross sec-tions of hyolithids. The axes of shellsare oriented perpendicular to the planeof the split nodule and perpendicularto slightly oblique to the axis of theagnostid row. One hyolithid can be de-termined as Elegantilites elegans (Bar-rande, 1847).

Discussion

The described axis-fossil could repre-sent the robust part of an algal strand.However, its branching pattern doesnot resemble that of an alga (KvaËek pers. com.).

Some fragments of ichnofossils, which occur in nod-ules of the ä·rka Formation, resemble it in some respectsas well. However, in contrast to the ichnofossils, it main-tains no constant diameter and it shows no wall linings astypical for the ichnogenus Thalassinoides; the branchingis different from that of Planolites or Palaeophycus espe-cially because the second order branches are of smallerdiameter than the main branch (Mikul·ö pers. com.).

It looks most similar to the fossils described recentlyby Kenrick et al. (1999) from the Barrandian KlabavaFormation which were interpreted as octocorals. Bothspecimens from the Klabava Formation and our axis-fos-sil possess the spine-like processes. The main differenceconsists in the different mode of branching (in triadesversus lateral in the specimens from the Klabava and ä·r-ka formations, respectively). Their robustness (slender vs.robust) and intervals of branching (relatively long vs.short) are also not in correspondence.

The octocoral Nonnegorgonides ziegleri Lindstrˆm,1978 described from the basal Volkhovian (Arenigian) ofSweden (Lindstrˆm 1978) is much more slender andsmooth, i.e. bearing no processes but with a similarbranching pattern in comparison with the specimen fromthe Bohemian ä·rka Formation.

No other similar axis-fossil from the ä·rka Formationhas been recorded in any public collection in the CzechRepublic. This problem could arise because such objectshave not been collected in the past. It is only during the

last 25 years, that some paleontologists and collectorshave focused on complete collecting of all faunal ele-ments. Even still, such ìnot interestingî objects (frag-ments of ichnofossils etc.) have usually been avoidedeven by them. They were considered to provide almostno possibility to be determined or do not offer any rele-vant information.

As mentioned above the studied agnostid associationseems to be monospecific. All determinable specimensbelong to the species Geragnotus (Geragnostella) tull-bergi. It is the most common agnostid of the ä·rka For-mation.

The agnostid trilobites in close proximity to the axis-fossil are supposed to have originally been attached to it.Even though it was not directly observed, it seems thatthey were attached to the spine-like processes. As the formof these processes indicates that they were flexible. Theobserved orientation of trilobites could be slightly differ-ent from the natural state. However, all enrolled trilobitesremain facing the axis-fossil with their free margins ofcephala and pygidia. Their sagittal orientation to the axis-fossil has not been observed but the prevailing one is lat-eral to almost lateral. These circumstances suggest that thestudied agnostids were attached to the spine-like process-es by thoracic appendages rather than cephalic and/or py-gidial ones, which could then remain free for feeding orother activities.

Stemming from an octocoral affinity for the axis-fos-sil, not only an epiplanktic mode of life, but also an

Fig. 2 1 ñ Latex cast of the half of nodule figured on Fig. 1-1a; 2 ñ Latex cast of the coun-terpart figured on Fig. 1-1b, detail of aligned row of five agnostids in oblique view. Bothcasts coated with ammonium chloride, scale bars = 10 mm.

1a 1b

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epibenthic one comes into consideration for agnostids.They could be at least part of their life (e.g. resting, feed-ing) attached to this sessile benthic organism which pos-sessed hair-like processes suitable for attachment. The re-lationship of enrolled agnostids with the robust axis-fossilalso illustrates that these trilobites could search for suchobjects if they offered structures fine enough for attach-ment.

The agnostids attached to the axis-fossil most likelydied and were buried suddenly together and thus foundin a natural position.

Similar rows (without any axis-fossil) of agnostids havebeen described by several authors from the Bohemian Or-dovician. Agnostids in such accumulations were mentionedby Prantl (1948), P¯ibyl ñ VanÏk (1976), Pek (1977) anddescribed as tubes formed by the agglutinated shields, asa part of the benthic fauna on algal strands and as sug-gested epibionts on floating algae, respectively.

Based on our study, the accumulations of agnostids areusually small in size. They consist only of agnostids orof these trilobites associated with other fossils. All purelyagnostid rows investigated are monospecific, consistingof solely enrolled specimens which are small in size. Therows with Leiagnostus bohemicus (Nov·k in Perner,1918) or Geragnostus (Geragnostella) tullbergi (Nov·k,1883) prevail.

Only rows with some common character of arrange-ment (orientation, no fragments etc.) can be consideredas primary ones. However, their relationship to any or-ganism, such as described above, cannot be proved. Un-arranged rows might represent secondary accumulationsor primary ones disordered by taphonomic processes.

In the course of our investigation it was recognized thatalmost all studied rows with agnostid trilobites (approx.30) come from the eastern part of the Prague Basin, fromthe vicinity of Praha. The only exception is one row foundin the western part of the basin in the Rokycany area. Thismay be the result of stratigraphical differences. In theRokycany area, there occur only the nodules coming fromthe Corymbograptus retroflexus Biozone. On the contrary,the nodules from the Corymbograptus retroflexus Biozone

Fig. 3 Idealized reconstructionof the axis-fossils interpreted asoctocoral and attached agnostidtrilobites. (Drawn by M. Koöù·k)

as well as the overlying Didymograptus clavulus Biozonehave been collected in the Prague area. The rows seem tocome predominantly from the upper biozone but palaeo-ecologic differences during the Llanvirnian (or Darriwil-ian) age in the Prague Basin cannot be excluded.

Acknowledgement. We are grateful to Prof. Ivo Chlup·Ëwho discovered the studied specimen and provided valu-able information on locality and stratigraphy. Kevin Brettand Prof. Euan Clarkson are thanked for constructive crit-icism of the manuscript and checking of the English. Wealso thank Dr. Radek Mikul·ö and Prof. Zlatko KvaËekfor discussion on comparison of axis-fossil with ichno-fossils and algae and Dr. Martin Koöù·k for his help withdrawing of reconstructions. Petr Kraftís part was pre-pared under the support of the Grant of Ministry of Ed-ucation MSM 113100006. This is a contribution to theIGCP 410.

Submitted February 15, 2001

References

Bergstrˆm, J. (1973): Organization, life, and systematics of trilobites. ñFoss. and Strata, 2: 1ñ69.

BouËek, B. (1926): P¯ÌspÏvek ku stratigrafii vrstev ö·reck˝ch dγ1 ËeskÈho

ordoviku. ñ Rozpr. »es. Akad. VÏd UmÏnÌ, T .̄ II, 35(43): 1ñ11.Clarkson, E. N. K. ñ Ahlberg, P. ñ Taylor, C. M. (1998): Faunal dynamics

and microevolutionary investigations in the Upper Cambrian OlenusZone at Andrarum, Skåne, Sweden. ñ GFF, 120: 257ñ267.

Fatka, O. ñ Pek, I. (1999): Ordovician agnostid trilobites of the PragueBasin (Barrandian area, Czech Republic. ñ In: Kraft, P. ñ Fatka, O.(Eds) Quo vadis Ordovician? ñ Acta Univ. Carol., Geol., 43(1ñ2):381ñ384.

Fortey, R. A. ñ Owens, R. M. (1999): Feeding habits in trilobites. ñPalaeontology, 42(3): 429ñ465.

Jaekel, O. (1909): ‹ber die Agnostiden. ñ Z. Dtsch. geol. Gesell., 53:380ñ401.

Jago, J. B. (1973): Cambrian agnostid communities in Tasmania. ñLethaia, 6: 405ñ421.

Kenrick, P. ñ KvaËek, Z. ñ Bengtson, S. (1999): Semblant land plantsfrom the Middle Ordovician of the Prague Basin reinterpreted asanimals. ñ Palaeontology, 42(6): 991ñ1002.

Lindstrˆm, M. (1978): An octocoral from the Lower Ordovician of Swe-den. ñ Geologica et Palaeont., 12: 41ñ52.

Lochman, C. (1940): Fauna of the basal Bonneterre Dolomite (upperCambrian) of southeastern Missouri. ñ J. Paleont., 14: 1ñ53.

Lochman-Balk, Ch. - Wilson, J. (1958): Cambrian biostratigraphy in NorthAmerica. ñ J. Paleont., 32: 312ñ350.

M¸ller, K. J. ñ Walossek, D. (1987): Morphology, ontogeny, and life habitof Agnostus pisiformis from the Upper Cambrian of Sweden. ñ Foss.and Strata, 19: 1ñ124.

Nielsen, A. T. (1997): A review of Ordovician agnostid genera (Trilobita). ñTrans. Roy. Soc. Edinburgh, Earth Sci., 87: 463ñ501.

Pek, I. (1977): Agnostid trilobites of the Central Bohemian Ordovician. ñSbor. geol. VÏd, Paleont., 19: 7ñ44.

Prantl, F. (1948): Some terebelloid remains from the Ordovician ofBohemia. ñ VÏst. Kr·l. »es. SpoleË. Nauk, T .̄ mat.-p¯ÌrodovÏd., 8:1ñ8.

P¯ibyl, A. ñ VanÏk, J. (1976): Palaeoecology of Berounian trilobites fromthe Barrandian area. ñ Rozpr. »es. Akad. VÏd, ÿ. mat. p¯Ìr. VÏd, 86(5):3ñ39.

Robison, R. A. (1972): Mode of life of agnostid trilobites. ñ 24th Interna-tional Geological Congress, Montreal, Session, 7: 33ñ40.

Schovsbo, N. H. (2000): Environmental fluctuations in the Olenus Zone(Upper Cambrian), southern Scandinavia: A geochemical approach. ñBull. Geol. Soc. Den., 47: 53ñ61.

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