Journal of Paleontology, 89(5), 2015, p. 882–893 Copyright © 2016, The Paleontological Society 0022-3360/16/0088-0906 doi: 10.1017/jpa.2015.57


First remingtonocetid archaeocete (Mammalia, Cetacea) from the middle Eocene of Egypt with implications for biogeography and locomotion in early cetacean evolution


Ryan M. Bebej,1 Iyad S. Zalmout,2 Ahmed A. Abed El-Aziz,3 Mohammed Sameh M. Antar,3 and Philip D. Gingerich4


1Department of Biology, Calvin College, 1726 Knollcrest Circle SE, Grand Rapids, Michigan 49546-4403, USA ⟨bebej@calvin.edu⟩ 2Mammals Research Chair, Department of Zoology, College of Science, King Saud University, PO Box 2455, Riyadh 11451, Saudi Arabia


⟨izalmout@ksu.edu.sa⟩ 3Egyptian Environmental Affairs Agency, Wadi Al-Hitan World Heritage Site, Fayum, Egypt ⟨ahmedreactors@gmail.com⟩,


⟨wrpashark@yahoo.com⟩ 4Department of Earth and Environmental Sciences, Museum of Paleontology, University of Michigan, 1109 Geddes Ave, Ann Arbor, Michigan 48109-1079, USA ⟨gingeric@umich.edu⟩


Abstract.—Remingtonocetidae are Eocene archaeocetes that represent a unique experiment in cetacean evolution. They possess long narrow skulls, long necks, fused sacra, and robust hind limbs. Previously described remingtonoce- tids are known from middle Eocene Lutetian strata in Pakistan and India. Here we describe a new remingtonocetid, Rayanistes afer, n. gen. n. sp., recovered from a middle to late Lutetian interval of the Midawara Formation in Egypt. The holotype preserves a sacrum with four vertebral centra; several lumbar and caudal vertebrae; an innominate with a complete ilium, ischium, and acetabulum; and a nearly complete femur. The ilium and ischium of Rayanistes are bladelike, rising sharply from the body of the innominate anterior and posterior to the acetabulum, and the acetabular notch is narrow. These features are diagnostic of Remingtonocetidae, but their development also shows that Rayanistes had a specialized mode of locomotion. The expanded ischium is larger than that of any other archaeocete, supporting musculature for powerful retraction of the hind limbs during swimming. Posteriorly angled neural spines on lumbar vertebrae and other features indicate increased passive flexibility of the lumbus. Rayanistes probably used its enhanced lumbar flexibility to increase the length of the power stroke during pelvic paddling. Recovery of a remingtonocetid in Egypt broadens the distribution of Remingtonocetidae and shows that protocetids were not the only semiaquatic archaeocetes capable of dispersal across the southern Tethys Sea.


Introduction


Archaeocetes are Eocene stem cetaceans that document early stages in the evolutionary transition of whales from land to sea. They are typically divided into five families that include about 40 genera (Uhen, 2010). The three most basal families (Pakicetidae, Ambulocetidae, and Remingtonocetidae) have been known exclusively fromearly to middle Eocene strata in Indo-Pakistan on the southern margin of the ancient Tethys Sea (Fig. 1), and many authors have suggested that this regionwas the center of origin for cetaceans (e.g., West, 1980; Gingerich et al., 1998). The more derived Protocetidae and Basilosauridae are known from Indo- Pakistan, but they were more cosmopolitan and have been recovered from middle to late Eocene deposits on several con- tinents around theworld (e.g.,Gingerich, 2010;Uhen et al., 2011; Uhen, 2013). The archaeocete family Remingtonocetidae is generally


regarded as a sister group of the clade that includes Protocetidae, Basilosauridae, and later Neoceti (crown group Cetacea). Remingtonocetids can be recognized by their unusual cranial


and mandibular morphology (Kumar and Sahni, 1986; Gingerich, Arif, and Clyde, 1995a; Thewissen et al., 2011). The best known remingtonocetid is Remingtonocetus, which is known from crania, teeth, much of the vertebral column, ribs, and hind limb elements (Fig. 2). Remingtonocetid crania are generally about six times longer than they are wide across the frontals, their rostra comprise more than 60% of condylobasal skull length, and their mandibular symphyses extend back at least to P3 (Gingerich et al., 1998). However, remingtonocetids also have very distinctive postcranial morphology, which allows them to be identified with confidence even in the absence of cranial material. Remingtonocetids have long cervical vertebrae that are unusual for cetaceans; sacra with as many as four fused vertebral centra; robust pelves with bladelike ilia and ischia rising sharply from the body of the innominate anterior and posterior to the acetabulum; acetabular notches that are narrow to closed; and robust hind limbs with evidence of some capacity for weight-bearing (Gingerich et al., 1995a; Gingerich et al., 2001a; Madar, 1998; Thewissen and Bajpai, 2009; Bebej et al., 2012).


882


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104  |  Page 105  |  Page 106  |  Page 107  |  Page 108  |  Page 109  |  Page 110  |  Page 111  |  Page 112  |  Page 113  |  Page 114  |  Page 115  |  Page 116  |  Page 117  |  Page 118  |  Page 119  |  Page 120  |  Page 121  |  Page 122  |  Page 123  |  Page 124  |  Page 125  |  Page 126  |  Page 127  |  Page 128  |  Page 129  |  Page 130  |  Page 131  |  Page 132  |  Page 133  |  Page 134  |  Page 135  |  Page 136  |  Page 137  |  Page 138  |  Page 139  |  Page 140  |  Page 141  |  Page 142  |  Page 143  |  Page 144  |  Page 145  |  Page 146  |  Page 147  |  Page 148  |  Page 149  |  Page 150  |  Page 151  |  Page 152  |  Page 153  |  Page 154  |  Page 155  |  Page 156  |  Page 157  |  Page 158  |  Page 159  |  Page 160  |  Page 161  |  Page 162  |  Page 163  |  Page 164  |  Page 165  |  Page 166  |  Page 167  |  Page 168  |  Page 169  |  Page 170  |  Page 171  |  Page 172  |  Page 173  |  Page 174  |  Page 175  |  Page 176  |  Page 177  |  Page 178  |  Page 179  |  Page 180  |  Page 181  |  Page 182  |  Page 183  |  Page 184  |  Page 185  |  Page 186  |  Page 187  |  Page 188  |  Page 189  |  Page 190  |  Page 191  |  Page 192  |  Page 193  |  Page 194  |  Page 195  |  Page 196  |  Page 197  |  Page 198  |  Page 199  |  Page 200  |  Page 201  |  Page 202  |  Page 203  |  Page 204  |  Page 205  |  Page 206  |  Page 207  |  Page 208  |  Page 209  |  Page 210  |  Page 211  |  Page 212