890


Journal of Paleontology 89(5):882–893


the distinct lateral flange present on the femoral shaft of R. domandaensis (GSP-UM 3054).


Remarks.—Rayanistes afer is important in extending the geographic range of Remingtonocetidae to the African continent, and it is also important in adding to what we know of remingtonocetid pelvic and hind limb morphology. Complete sacra were known previously for Remingtonocetus, Dalanistes, and Kutchicetus, and reasonably complete femora were known for Remingtonocetus, Kutchicetus, and Andrewsiphius. However, Remingtonocetus was the only remingtonocetid represented by innominates with a well preserved ilium and ischium. Rayanistes is the first to yield all three elements (sacrum, innominate, and femur) well-preserved in association in one individual specimen.


Biogeographical implications


The broadening geographic range of archaeocetes seems to correlate closely with their relative degree of adaptation to an aquatic lifestyle. Fully aquatic Basilosauridae achieved a worldwide distribution (Uhen, 2010). Semiaquatic Protocetidae are not known from as many localities, but they too are known from several continents. The oldest and phylogenetically most basal protocetids, such as Artiocetus (Gingerich et al., 2001b), Maiacetus (Gingerich et al., 2009), and Rodhocetus (Gingerich et al., 1994; Gingerich et al., 2001b), are all restricted to Pakistan. Younger and more derived protocetids, such as Qaisracetus (Gingerich et al., 2001a) and Babiacetus (Trivedy and Satsangi, 1984; Gingerich, Arif, Bhatti, Raza, and Raza, 1995b; Bajpai and Thewissen, 1998) from Indo-Pakistan, Protocetus and Eocetus (Fraas, 1904) from Egypt, and Georgiacetus (Hulbert, Petkewich, Bishop, Bukry, and Aleshire, 1998), Carolinacetus (Geisler et al., 2005), and Crenatocetus (McLeod and Barnes, 2008) from the eastern United States, achieved a much broader geographic distribution. Prior to the recovery of Rayanistes afer, the three


geologically-oldest and phylogenetically most basal families of archaeocetes (Pakicetidae, Ambulocetidae, and Remingtonoce- tidae) were known exclusively from Indo-Pakistan, the putative site of origin for Cetacea. Rayanistes afer extends the range of Remingtonocetidae to North Africa (Fig. 1), adding an additional family of archaeocetes to those known from outside of Indo-Pakistan. Remingtonocetidae and Protocetidae were broadly


contemporaneous in the middle to upper Lutetian strata of Indo-Pakistan, but they have been found to predominate in different environments. In the Domanda Formation of Pakistan, remingtonocetids tend to be more common in shallow marine environments, while protocetids tend to be more common in deeper marine environments (Gingerich et al., 1998). This environmental distribution, along with differences in cranial and postcranial anatomy, contributed to widespread interpretation of remingtonocetids as shallow water ambush predators and protocetids as pursuit-oriented predators able to swim with greater efficiency offshore (Gingerich et al., 1995a, 1998; Gingerich, 1998; Bajpai et al., 2011). Given their presumably greater swimming capabilities, protocetids were thought to be


the most basal archaeocetes to disperse away from Indo- Pakistan (Geisler, Sanders, and Luo, 2005). Recovery of R. afer in Egypt demonstrates that


remingtonocetids had the locomotor capability to disperse across the southern Tethys Sea between Indo-Pakistan and northern Africa. This does not negate any ecological differences in where remingtonocetids and protocetids may have lived or how they hunted, but it does suggest that the distinctively specialized remingtonocetids were more skilled as swimmers than is commonly acknowledged. Discovery of a more basal clade of archaeocetes outside


Indo-Pakistan also raises the question of whether Indo-Pakistan was the geographic center of cetacean origins. Given the current state of the evidence, this is still the most likely scenario because the oldest and most primitive archaeocetes are concentrated in that region. However, it should be noted that Ypresian and early Lutetian localities elsewhere have yet to be explored so intensely. Fieldwork in Wadi Al-Hitan and the surrounding areas in northern Egypt has yielded an excellent record of archaeocetes, but most have come from upper Lutetian, Bartonian, and Priabonian strata (Gingerich, 2008, 2010; Gingerich et al., 2013). The oldest known Egyptian archaeocete is the middle Lutetian Protocetus atavus (Fraas, 1904) from the Lower Building Stone Member of the Gebel Mokattam Formation (Strougo et al., 1982; Gingerich, 1992; Zalmout and Gingerich, 2012). Recovery of the remingtonocetid R. afer described here and recovery of additional undescribed protocetid taxa from Lutetian strata in Egypt (Gingerich, 2010; Gingerich et al., 2013) raises the possibility that further exploration of Lutetian formations in Egypt may uncover more primitive taxa like Pakicetidae andAmbulocetidae. If pakicetids and ambulocetids are discovered in northern Africa, then our understanding of where cetaceans originated and how and when they dispersed to other regions will need to be reevaluated.


Functional implications


Relatively little of the postcranial skeleton was recovered for Rayanistes afer; however, important functional insights can be gained from the features preserved. Bebej and colleagues (2012) interpreted Remingtonocetus domandaensis as a highly specialized foot-powered swimmer, and this interpretation appears apt for R. afer as well. In both taxa, concave surfaces on the dorsal aspects of the sacra, broad ilia of the innominates, and high greater trochanters on the femora supported


well-developed gluteal musculature (e.g., m. gluteus super- ficialis and m. gluteus medius) for powerful retraction of the hind limb (Getty, 1975; Schilling et al., 2009; Fisher et al., 2010; Bebej et al., 2012). However, anatomical differences in the hind limb and vertebrae indicate that Remingtonocetus and Raya- nistes did not swim in an identical fashion. These similarities and differences are discussed in turn.


Vertebral column.—The lumbar vertebra known for Rayanistes afer is similar to those of Remingtonocetus domandaensis in many ways. The centrum is of similar proportions, the trans- verse processes are roughly the same size and angled to the same degree, the neural canal is about the same size and shape, and the zygapophyses are gently curved and face mediolaterally.


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