Extinct Horned Crocodile Figure 10. Variation in the morphology of the maxillary ramus of the ectopterygoid. All images from right side of skull in ventral view. A, NHM R.5893, Crocodylus anthropophagus, posterior end of right maxilla and partial maxillary ramus of ectopterygoid. B, KNM FLKNI, C. anthropophagus, partial right maxilla; articulation surface for ectopterygoid is preserved. C, USNM 194831, C. niloticus. D, USNM 248848, C. niloticus. E, FMNH 17157, C. niloticus. Arrow indicates cleft in maxillary ramus of ectopterygoid; questionably present in A, on medial margin of suborbital fenestra in D. Scale = 1 cm. doi:10.1371/journal.pone.0009333.g010 bears on interpretations of the Plio-Pleistocene crocodylian record in Africa. Fossils as old as the Miocene have been referred to C. niloticus [2,17]; whether these are conspecific with C. niloticus (or even assignable to Crocodylus) is doubtful [62], but some geologically younger specimens (e.g. specimens forming the basis of C. niloticus kaisensis from the Pleistocene of Uganda [63]) are more consistent with the living species than with C. anthropophagus (pers. obs.). At least two similar species of Crocodylus may have been present in East Africa during the Late Pliocene and Pleistocene, and in the absence of diagnostic features permitting precise identification [64,65], referral of fragmentary remains to the species level may not be advisable. Preliminary analyses of the phylogeny of Neogene African crocodiles suggested that Crocodylus might be a comparatively recent immigrant into Africa and not a native lineage [30]. This was based on incomplete taxonomic sampling, and more recent work including a wider range of Mio-Pliocene forms suggests a more complicated phylogenetic and biogeographic history for the group in the region [62], but assuming Crocodylus was absent from Africa in the Early and Middle Miocene, the presence of two species in at least the early Pleistocene, if not the Pliocene, suggests either multiple dispersal events or dispersal early enough to have radiated by the Pleistocene. Further analysis of Late Miocene and Pliocene fossils from the region is needed to test these scenarios, but regardless, crocodiles appear to have remained cryptically speciose in Africa beyond their peak of diversity in the Miocene. That the features distinguishing C. anthropophagus from C. niloticus are dominated by gradational differences raises the general problem of how we recognize species in the fossil record. It is possible that Olduvai Gorge crocodile is an extinct regional variant of the Nile crocodile and not a discrete species. Molecular evidence reveals considerable genetic variation between popula- tions of C. niloticus [18,66]. However, biogeographic variation in C. niloticus morphology is expressed almost entirely in scalation [67]. Different living populations of C. niloticus may ultimately be distinguishable osteologically, but the differences will be subtle and most apparent from morphometric rather than qualitative approaches. Qualitatively, the fossil Olduvai crocodile lies outside the range of osteological variation for C. niloticus, both within and between populations. Cranially, the Olduvai form can be consistently distinguished from C. niloticus, and we cannot at present conclude that one is phylogenetically closely related to the other, even if biogeography strongly suggests such a relationship. Paleoecology Fossil bones of at least two hominid individuals from Olduvai Gorge bear tooth marks characteristic of crocodile feeding [37]. These marks are similar to those produced by mammalian carnivores, except that they are bisected by the carinae of newly erupted to moderately worn crocodile teeth [68]. Both tooth- marked specimens are from the same Tuff IB-IC interval as NNHM-OLD-1001, and were found by the Leakeys (L. Leakey, 1959; L. Leakey et al, 1964, M. Leakey, 1971) at two sites within 100 m of the collection site for NNHM-OLD-1001. Both hominid sites contain concentrations of vertebrate fossils and Oldowan stone artifacts. The FLK NN Level 3 site yielded the tooth-marked Olduvai Hominid (OH) 8 foot, a paratype of H. habilis found in the same assemblage as the species holotype. In situ elements of the C. anthropohagus holotype are essentially contemporaneous with OH 8. The FLK Level 22 site yielded the tooth-marked OH 35 tibia and fibula, probably of H. habilis [69], from the same assemblage as the holotype of A. boisei. Both OH 8 and OH 35 are from the left leg of a juvenile or adult [69], and have been argued to represent a single individual on the basis of their close articulation [70], despite deriving from different sites. Recent stratigraphic correlations of the sites show that these formed on two allochronous land surfaces [71]. Curiously, the tooth mark patterning on both specimens indicates that each hominid individual lost its left foot to crocodiles during or shortly after capture, or when being scavenged [37]. The FLK 22 and FLK NN 3 sites formed in close proximity (,50 m) to wetland settings from which crocodile body and trace fossils are documented [35,71]. FLK 22 formed on a topohigh adjacent to a freshwater marshland, and FLK NN 3 formed on the base of a shallow floodplain channel. NNHM-OLD-1001 likely derives from the floodplain deposits adjacent to this channel. The tooth-marked hominids died and were fed on by crocodiles at either the wetlands or the sites at which their remains were found. Predation risk from crocodiles likely impacted the foraging and land use behavior of hominids at Olduvai and at other tropical and PLoS ONE | www.plosone.org 11 February 2010 | Volume 5 | Issue 2 | e9333