Chapter 14 Giraffidae 2020-03-08آ  T. Harrison (ed.), Paleontology and Geology of Laetoli: Human Evolution

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    Abstract 792 specimens attributed to the Giraffidae were recovered by the Eyasi Plateau Paleontological Expedition (EPPE) from the three Pliocene stratigraphic units at Laetoli, with Giraffa stillei the most common taxon in all three levels. Giraffids are notably well represented in the Upper Laetolil Beds, with further evidence gathered by EPPE for the three previously recognized species from this unit. In the Lower Laetolil Beds Giraffa stillei is provisionally identified, as is Sivatherium. A third, large giraffid species may also be present. Based on a specimen recovered by Kohl-Larsen’s team during the first extensive exploration of Laetoli, we now provisionally recognize Giraffa pygmaea from the Upper Ndolanya Beds, along with Giraffa stillei and Sivatherium maurusium. Evidence for Giraffa jumae in the Upper Ndolanya Beds is not as convincing, as it is based on a small number of postcranial bones. In the time between the formation of the Upper Laetolil Beds and the Upper Ndolanya Beds, it appears that Giraffa stillei increased in size, which has been documented at other contemporary East African localities. This may relate to competition from the smaller Giraffa pygmaea.

    Keywords Giraffidae • Giraffa • Sivatherium • Artiodactyla • Pliocene • Africa • Tanzania

    Introduction

    The diversity of giraffids in the Pliocene of Africa was con- siderably greater than it is today, and Laetoli is similar to most other African Pliocene sites in having multiple giraffid taxa represented in all of the stratigraphic units. In the Upper Ndolanya Beds, for example, four giraffid species are now identified with the recognition of Giraffa pygmaea, Harris

    1991 based on a mandibular specimen described below. These same four taxa have been identified at the Pliocene sites of Hadar, Koobi Fora and the Omo Valley (Boaz et al. 1982; Harris 1991; Reed 2008). While the diversity of spe- cies is similar to other sites, previous studies of fossil giraf- fids have noted the unusually high number of giraffe specimens at Laetoli compared to other East African Plio- Pleistocene sites (Harris 1976a, 1987, 1991), which could be related to favorable ecological conditions and/or a reduced number of competitors compared to other early Pliocene sites. The Laetolil Beds are unusual with respect to the num- ber of giraffids recovered, but this abundance diminishes in the Upper Ndolanya Beds, possibly due to competition from the increased number of bovids.

    Three species of Giraffa have been at least provisionally identified from the various stratigraphic units at Laetoli, Giraffa jumae, Leakey 1965, Giraffa pygmaea and Giraffa stillei, Dietrich 1942. The earliest appearance of Giraffa in East Africa is currently in the late Miocene at Aramis and Lothagam (WoldeGabriel et al. 1994; Harris 2003). At that time it appears that the genus had already split into at least two species, probably G. jumae and G. stillei. The G. stillei specimens from the Lower Laetolil Beds described below represent some of the earliest fossils in East Africa that can be provisionally attributed to Giraffa stillei. Based on the current evidence, G. pygmaea does not appear until the late Pliocene, where competition with G. stillei seems to have resulted in increased size in the latter taxon (see below).

    Also present at Laetoli, as it is in many East African faunal assemblages, is Sivatherium maurusium, Pomel 1892. The S. maurusium specimens from the Upper Laetolil Beds rep- resent the earliest known representatives of that species, although there are a number of earlier sivathere specimens that cannot be attributed to species. A single sivathere pre- molar was recovered from the Lower Laetoli Beds, which are similar in age to sites in North, East and South Africa from which Sivatherium hendeyi, Harris 1976 has been identified (Harris 1976a, 1999; Likius 2002; Vignaud et al. 2002). The tooth could represent this taxon or its possible descendant, S. maurusium.

    C.A. Robinson (*) Department of Biology, Bronx Community College, 2155 University Avenue., Bronx, NY 10453, USA e-mail: chris.robinson@bcc.cuny.edu

    Chapter 14 Giraffidae

    Chris A. Robinson

    T. Harrison (ed.), Paleontology and Geology of Laetoli: Human Evolution in Context. Volume 2: Fossil Hominins and the Associated Fauna, Vertebrate Paleobiology and Paleoanthropology, DOI 10.1007/978-90-481-9962-4_14, © Springer Science+Business Media B.V. 2011

  • 340 C.A. Robinson

    Cranial Specimens and Taxonomic Attribution of Giraffids

    Species of Giraffidae are generally differentiated from one another on the basis of size and cranial morphology, especially on the shape of their ossicones (Harris 1976a, b, 1987, 1991; Churcher 1978; Hamilton 1978; Geraads 1986, 1994; Likius 2002; Solounias 2007). No giraffid cranial specimens were recovered by EPPE. Harris (1987) described two Giraffa stillei ossicones from the Upper Laetolil Beds, and another (MB Ma 42325 in Berlin) was published by Dietrich (1942) that is likely to be derived from the Upper Laetolil Beds (T. Harrison, personal communication), confirming the presence of the smaller Giraffa in this unit. In addition, following publication of Harris (1987), right and left frontal ossicones (LAET 76-4193) from this unit were described and assigned to Sivatherium maurusium (Harris, 1991). This makes it possible to confidently recognize the presence of Sivatherium mauru- sium in the Upper Laetolil Beds. It should be noted that, while most authors identify the cranial appendages of sivatheres as ossicones (Harris 1987, 1991; Harris 2003; Harris et al. 1988; Solounias 1988, 2007; Churcher 1990), Geraads (1986, 1991) argues that this term should be reserved for the cranial append- ages of Giraffa, Okapia and, possibly, Palaeotragus. Geraads (1991) describes the ossicone as “a bone originally indepen- dent from those of the cranial roof, ossifying from a cartilagi- nous matrix”. He also notes that the “true” ossicones of extant giraffids are “hyper-ossified” and shifted posteromedially, fur- ther from the supraorbital region. The ramified cranial append- ages of Sivatherium, which show evidence of having had blood vessels on them, appear to grow in a manner more similar to cervids or bovids than Giraffa and do not exhibit the extremely dense bone of that genus (Geraads 1986).

    Other than in its cranial morphology, especially the form of its ossicones, Giraffa jumae closely resembles Giraffa camelopardalis, Linnaeus 1758 (Harris 1987, 1991; Geraads et al. 2004). There are no G. jumae cranial specimens known from Laetoli. Consequently, it is not possible to confirm the specific attribution of giraffid fossils that are similar in size to extant giraffes. However, the Upper Laetolil Beds, where a number of specimens similar in size and morphology to G. jumae from other sites are well documented, are dated to between approximately 3.5 and 3.8 Ma (Drake and Curtis 1987; Deino 2011), making it likely that these specimens represent G. jumae rather than G. camelopardalis (Harris, 1987). There are no specimens of G. camelopardalis from the Pliocene, whereas G. jumae is known from a number of Pliocene sites in East and South Africa (Harris 1976a–c, 1991; Churcher 1978; Harris et al. 2003; Kullmer et al. 2008). The earliest diagnostic evidence for G. cameloparda- lis is from the Nariokotome Member at West Turkana, dated to between 1.2 and 1.3 Ma (Harris et al. 1988; Harris 1991; McDougall and Brown 2006).

    Because Dietrich (1942) did not designate a holotype for Okapia stillei, later referred to the genus Giraffa (Harris, 1976b), Harris (1987) selected two specimens from Laetoli to be lectotypes of Giraffa stillei. One of the lectotypes, based on Dietrich (1942) Figure 170, was identified as a par- tial mandible with P

    3 -P

    4 and M

    2 -M

    3 , although the teeth in the

    figure are actually P 4 -M

    3 (Dietrich 1942; T. Harrison, per-

    sonal communication). Moreover, as Dietrich often did, the lectotype is comprised of a composite of specimens from dif- ferent individuals and localities (Geraads et al. 2004; T. Harrison, personal communication). It is proposed here that the mandibular fragment with M

    2 and M

    3 (MB Ma

    39078) be retained as a lectotype, as it clearly belongs to the smaller Giraffa species from the Upper Laetolil Beds, and conforms to the concept of Okapia stillei as proposed by Dietrich (1942). The other unassociated specimens should be recognized as paralectotypes.

    Lower Laetolil Beds

    Eleven giraffid specimens were recovered from the Lower Laetolil Beds during the 1998–2005 field seasons (Table 14.1). All were identified to taxon on a provisional basis because of the lack of diagnostic specimens. Nine of these specimens are attributed to Giraffa aff. stillei.

    In the following descriptions the terminology used for the postcranial elements follows that of Harris (1987, 1991) and colleagues (Harris et al. 2003) other than for the external cuneiform. The term external cuneiform is used rather than lateral cuneiform to avoid confusion, since this bone is formed by the fusion of the middle and lateral cuneiforms in fossil giraffids from Laetoli.

    Giraffa aff. stillei is best represented by a left mandibular fragment from Kakesio retaining M

    1 -M

    3 (EP 854/04)

    (Fig. 14.1). The preserved morphology and dimensions of its well worn molars are similar to those of smaller G. stillei specimens from the Upper Laetolil Beds (see Fig. 14.4). Small ectostylids are present on M

    1 and M

    2 , and an ento-

    stylid on M 3 , as they are on a nu