A New Horned Crocodile from the Plio-Pleistocene Hominid Sites at Olduvai Gorge, Tanzania Christopher A. Brochu1*, Jackson Njau2,3, Robert J. Blumenschine4, Llewellyn D. Densmore5 1 Department of Geoscience, University of Iowa, Iowa City, Iowa, United States of America, 2 Human Evolution Research Center, Department of Integrative Biology, University of California, Berkeley, California, United States of America, 3 National Natural History Museum, Arusha, Tanzania, 4 Center for Human Evolutionary Studies, Department of Anthropology, Rutgers University, New Brunswick, New Jersey, United States of America, 5 Department of Biological Sciences, Texas Tech University, Lubbock, Texas, United States of America Abstract Background: The fossil record reveals surprising crocodile diversity in the Neogene of Africa, but relationships with their living relatives and the biogeographic origins of the modern African crocodylian fauna are poorly understood. A Plio- Pleistocene crocodile from Olduvai Gorge, Tanzania, represents a new extinct species and shows that high crocodylian diversity in Africa persisted after the Miocene. It had prominent triangular ‘‘horns’’ over the ears and a relatively deep snout, these resemble those of the recently extinct Malagasy crocodile Voay robustus, but the new species lacks features found among osteolaemines and shares derived similarities with living species of Crocodylus. Methodology/Principal Findings: The holotype consists of a partial skull and skeleton and was collected on the surface between two tuffs dated to approximately 1.84 million years (Ma), in the same interval near the type localities for the hominids Homo habilis and Australopithecus boisei. It was compared with previously-collected material from Olduvai Gorge referable to the same species. Phylogenetic analysis places the new form within or adjacent to crown Crocodylus. Conclusions/Significance: The new crocodile species was the largest predator encountered by our ancestors at Olduvai Gorge, as indicated by hominid specimens preserving crocodile bite marks from these sites. The new species also reinforces the emerging view of high crocodylian diversity throughout the Neogene, and it represents one of the few extinct species referable to crown genus Crocodylus. Citation: Brochu CA, Njau J, Blumenschine RJ, Densmore LD (2010) A New Horned Crocodile from the Plio-Pleistocene Hominid Sites at Olduvai Gorge, Tanzania. PLoS ONE 5(2): e9333. doi:10.1371/journal.pone.0009333 Editor: Carles Lalueza-Fox, Institute of Evolutionary Biology (CSIC-UPF), Spain Received December 9, 2009; Accepted January 27, 2010; Published February 24, 2010 Copyright: 2010 Brochu et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: National Science Foundation (www.nsf.gov): NSF DEB 0444133, NSF DEB 0228648. Wenner-Gren Foundation for Anthropological Research (www. wennergren.org). Rutgers University Center for Human Evolutionary Studies (evolution.rutgers.edu). National Geographic Committee for Research and Exploration (www.nationalgeographic.com/field/grants-programs/cre.html). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: chris-brochu@uiowa.edu Introduction Until recently, it was thought that the ancestors of modern African crocodiles would be found among Oligocene through Pliocene fossils found in Africa [1,2,3,4]. Many of these resembled the living Nile crocodile (Crocodylus niloticus), but recent phyloge- netic analyses argue instead that many belong to an endemic clade with only one unambiguous living representative – the African dwarf crocodile Osteolaemus. Gross similarity with C. niloticus, along with misconceptions of crocodiles as evolutionarily static ‘‘living fossils,’’ obscured the diversity of this group through the Neogene of Africa, Madagascar, and possibly Aldabra Atoll and the Arabian Peninsula [5,6,7,8,9,10,11,12,13,14,15,16,17]. Just as living African crocodile species may represent cryptic species complexes [18,19,20], their fossil relatives were more diverse than previously supposed, with outwardly similar (though not always related) species mistaken for geographically widespread species with long stratigraphic ranges. Several questions remain. Fossil and molecular data suggest a Neogene divergence among living species of Crocodylus, and they usually support a close relationship between the C. niloticus and a clade of Neotropical species [21,22,23,24,25], but relationships among other species of Crocodylus are largely unresolved, as is the placement of the African sharp-nosed crocodile (Mecistops cataphractus), which may be related to either Crocodylus or Osteolaemus [23,25,26,27,28,29]. Thus, whether C. niloticus represents an African lineage separate from the osteolaemine radiation or a more recent immigrant is unclear [30]. A better understanding of Neogene African crocodylids is needed to resolve these issues. One of these, Rimasuchus lloydi, was long thought to be close to the ancestry of C. niloticus before phylogenetic analyses suggested an osteolaemine affinity [17,23]. But codings in these analyses are based on material from the Middle Miocene type locality in Egypt, and fossils from all over Africa, ranging in age from the Early Miocene through Quaternary, have been referred to R. lloydi [2,13,15,16,31,32]. The phylogenetic relationships of these other fossils remain untested. Some of these are from the Plio-Pleistocene deposits exposed in Beds I through IV at Olduvai Gorge, northern Tanzania. Bed I is the oldest level at Olduvai and is best known for key discoveries of PLoS ONE | www.plosone.org 1 February 2010 | Volume 5 | Issue 2 | e9333