Bebej et al.—First remingtonocetid archaeocete from Egypt from Egypt
But there are some notable differences. The less prominent attachment sites for ligamenta flava in the anterior laminae of L6 in R. afer indicate reduction of these ligaments, while the less robust metapophyses suggest reduction of the m. multifidus lumborum. The ligamenta flava are anteroposteriorly oriented ligaments that connect the neural arches of adjacent vertebrae and have been shown to be highly resistant to ventral flexion (Dumas et al., 1987; Gál, 1993; Ponseti, 1995). Branches of the m. multifidus lumborum run between the metapophyses and zygapophyses of one vertebra and the neural spines of a more anterior vertebra. These muscles, like ligamenta flava, serve with other muscles of the transversospinalis system primarily to stabilize the lumbar region (English, 1980; Evans, 1993; Pabst, 1993; Schilling and Carrier, 2010). Taken together, the inferred reductions of the ligamenta flava and m. multifidus lumborum in R. afer relative to those of R. domandaensis suggest that the lumbar region of R. afer was comparatively less resistant to ventral bending (i.e., more passively flexible). Another key difference is the orientation of the neural
spines in the lumbar region. In R. afer, the base of the neural spine of L6 and a neural arch disarticulated from an unidentified posterior thoracic or lumbar vertebra demonstrate that the neural spines in this region were oriented posteriorly. Slijper (1946) described how the orientation of the neural spines in a vertebral region is indicative of the relative dominance of various groups of epaxial muscles. Lumbar neural spines with a posterior inclination signal a reduction in the importance of the m. longissimus lumborum. This muscle is the primary lumbar extensor in mammals (Carlson, 1978; Alexander et al., 1985; Pabst, 2000), although it may also serve more of a rheostatic function in some cases (Zhou et al., 1992). In R. domandaensis, the lumbar vertebrae lack anapophyses and possess transverse processes that are short and exhibit relatively little anterior or ventral inclination, indicating diminished leverage for the m. longissimus to extend the spine and suggesting that this muscle served more of a stabilizing role (Bebej et al., 2012). Although we are unable to assess the presence or absence of anapophyses or the angulation of the transverse processes in the anterior lumbar region of R. afer, L6 is like the lumbar vertebrae of R. domandaensis with regard to these traits, so it is reasonable to hypothesize that the m. longissimus did not serve to actively extend the lumbar region of R. afer. Therefore, if the m. longissimus played more of a stabilizing role in R. afer and the posteriorly inclined neural spines of the lumbar vertebrae signal a decrease in the relative development of this muscle, then these traits also suggest that the lumbar spine of R. afer was more passively flexible than that that of R. domandaensis. The unique features of the sacrum in R. afer (such as the
large sacral foramina and the lack of fusion between various sacral elements) may have little functional significance. Fusion patterns within the sacrum are highly variable within mammals, with complete fusion sometimes not occurring until well into adulthood (e.g., Passalacqua, 2009; Robertson and Shadle, 1954; Sánchez-Villagra, 2002), so these features may not have persisted throughout the animal’s life. But regardless, the partial to complete fusion exhibited by S1–S3 would have precluded any intervertebral movement, and the overall length of the sacrum would have disrupted any functional continuity between
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the lumbus and the anterior part of the tail. Thus, while the lumbar region appears to have been more passively flexible, the morphology of the sacrum is consistent with the interpretation that R. afer did not utilize dorsoventral undulation of its vertebral column to generate propulsion during swimming.
Innominate.—Rayanistes afer exhibits a very broad ischiatic table. The archaeocetes Pakicetus attocki (Fig. 4.10) and Ambulocetus natans (Fig. 4.4) had ischia that were expanded to various degrees, but no archaeocetes had an ischium as broad as that of R. afer. Among modern mammals, ruminant artiodactyls have complex, multifaceted ischia with multiple tuberosities (Getty, 1975), while the modern hippo Hippopotamus amphibius (Fig. 4.5) also has an expanded ischium (Pickford, 2008). This part of the innominate serves as the origination surface for muscles like the m. gluteobiceps (or m. biceps femoris), m. semitendinosus, and m. semimembranosus, which collectively serve to retract the femur and flex the knee (or stifle) joint (Getty, 1975; Evans, 1993; Schilling et al., 2009; Fisher et al., 2010; Deban et al., 2012). Fisher and colleagues (2010) noted that these muscles in hippos are robust, possess extensive fusions, and exhibit more distal insertions, characteristics that signal a significant increase in power to aid in propelling the hippo’s large body through the water. Given the expansive ischiatic table of R. afer, its femoral retractor muscles must have been similarly robust and able to provide an effective power stroke during pelvic paddling.
Femur.—The narrow acetabular notch and lack of a distinct fovea capitis femoris on the femur indicate reduction of the round (or teres) ligament anchoring the femoral head in the acetabulum. This ligament is one of the primary stabilizers of the hip joint, and its reduction compromises the ability of a mammal to support its weight on land. Rayanistes afer is similar to Remingtonocetus domandaensis in possessing this characteristic (Gingerich et al., 2001a; Bebej et al., 2012). The lateral keel on the femur of R. domandaensis indicates a well-developed m. adductor magnus, which extends and adducts the femur (Gingerich et al., 1995a; Bebej et al., 2012). Absence of a lateral keel on the femur of R. afer indicates that the m. adductor magnus was less substantial, suggesting that the femur of R. afer may have been more habitually abducted relative to the femur of R. domandaensis. The more vertical orientation of the femoral head and neck in R. afer is consistent with this inference. Modern bovids that live in closed, forested habitats that require higher degrees of hind limb maneuverability tend to exhibit more vertically oriented femoral heads compared to bovids that live in more open environments (Kappelmann, 1988). This connection between the orientation of the femoral head and maneuverability of the hip joint is also evident in other mammals (e.g., Fleagle and Meldrum, 1988; White, 1993), suggesting an increase in multi-directional maneuverability of the femur in R. afer relative to the condition in R. domandaensis.
Summary.—Features of the vertebral column and hind limb in Rayanistes afer suggest that it swamin a way both similar to and different from its relative Remingtonocetus domandaensis. The robust hind limb and expansive innominate of R. afer
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