Godfrey et al.—New specimen of Agorophius pygmaeus (Odontoceti, Cetacea)
The left periotic is complete; however, some of the posterior process is missing from the right periotic. For that reason, the descriptionwill be based on the details evidenced primarily on the ventral face of the left periotic. The periotic in SC 2015.51.1 compares most closely to that of Simocetus (Fordyce, 2002). In ventral view, the long axis of the periotic (43.5mm)
forms an angle of 62 degrees to the long axis of the skull. The anterior process is convex laterally, slightly concave medially and conspicuously thickened (deep) dorsoventrally (Figs. 6, 7). The depth of the anterior process (17.5mm) is greater than its length (15.5mm) and it reaches its maximum thickness at its anteriormost end (i.e. the apex and ventral-most end of the laterally bowed anterior keel). The bullar facet, a shallow 10- mm-long groove, occupies the lengthwise ventral face of the anterior process. The fovea epitubaria separates the posterior end of the bullar facet from the body of the periotic. The fovea is bounded laterally by an indentation that marks the anterolateral sulcus (Fig. 7), and medially by a ridge and groove complex that parallels the long axis of the bone, which in turn separates the fovea from the pars cochlearis. The aforementioned ridge and groove complex occupies the area identified by Fordyce (2002) in Simocetus as having held the origin of the tensor tympani. Furthermore, as in Simocetus Fordyce (2002), the anterolateral sulcus marks the path of the middle meningeal artery. Posterior to the fovea is the mallear fossa, a shallow 4mm-
wide hemispherical concavity that faces mostly postero- ventrally, but also slightly laterally. Lateral to the mallear fossa, the lateral tuberosity is tightly pressed up against the squamosal just anterior to the spiny process and the anterior meatal crest. Two tiny notches mark the bone between the fossa and the pars cochlearis. Behind the mallear fossa lays the fossa incudis, which is bound laterally by the anterior meatal crest and medially by the facial sulcus. Immediately adjacent to the facial sulcus is the fenestra ovalis (neither stapes, nor other auditory ossicles were preserved), both of which lay medial to the spiny process and the external auditory meatus. As in Simocetus, the pars cochlearis is long and narrow
(approximately 16mm×8mm respectively) with a tabular ventral surface. A small ridge marks the antero-internal angle of the pars cochlearis. The fenestra rotunda is centered within the slightly concave posterior face of the pars cochlearis. Posterolaterally, the pars cochlearis is drawn out into a cochlear crest (Fordyce, 2002).Adistinct ridge emerges from the fenestra rotunda passing posteromedially before turning dorsally around the back of the pars cochlearis. The distal opening of the facial canal opens between the
fenestra ovalis and the fossa incudis. Posteriorly, the fossa that held the stapedial muscle is visible in ventral view between the posteromedial margin of the pars cochlearis (the cochlear crest) and the posterior bullar facet. The long axis of the fossa points toward the paroccipital process. The long axis of the posterior bullar facet of the periotic lies
at about a 45 degree angle to the length of the skull. Most of this facet lies directly behind the posterior meatal crest. Distally, the posteriormost portion of the periotic re-curves posteriorly. The cranial surface of the cochlear portion of the periotic
features the raised rim of the internal acoustic meatus. Within the anterior third of the meatus, the transverse crest separates the proximal opening of the facial canal from the dorsal vestibular
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area (Fig. 7.3). Posterior to this ovoid opening is the aperture for the cochlear aqueduct. The circular aperture for the endo- lymphatic duct (vestibular aqueduct) is positioned immediately posterolateral to the acoustic meatus. Passing forward from the endolymphatic duct is an elongated and inverted V-shaped trough that parallels the lateral edge of the internal acoustic meatus, occupying the space between the meatus and the dorsal crest of the periotic. Within the larger context of cetacean ability to perceive
water-borne sound, the dorsolateral and lateral sides of the periotic in SC 2015.51.1 abut the squamosal approximately to the same extent as that illustrated for Squalodon by Luo and Gingerich (1999, fig. 27c “reduced sq. contact”). In SC 2015.51.1, there does not appear to be any contact between the periotic and the exoccipital; decidedly derived over the condition in Dorudon (Luo and Gingerich, 1999, fig. 27a), but not nearly as derived as in extant odontocetes (where the petrotympanic complex is enclosed in a peribullar cavity,
isolated from the rest of the cranium) (Luo and Gingerich, 1999, fig. 27d).
Cladistic analysis Methods.—To determine the phylogenetic relationships of SC 2015.51.1, we coded this specimen according to the morphological characters (to the extent to which they were preserved) listed in Geisler et al. (2012, 2014) and Sanders and Geisler (2015).Wealso reappraised the codings for the holotype specimen based on the tooth MCZ 8761 and existing litho- graphs, as well as ChM PV4256 and treated all three specimens as separate operational taxonomic units (OTUs) to test the hypothesis that all three specimens represent the same taxon. The matrix consisting of three outgroup taxa and 83 ingroup taxa coded for 313 characters is included here as Appendix 2 in the Supplemental Material. The matrix was analyzed using PAUP 4b10 using the heuristic search option with 10,000 replicates and a random addition sequence of taxa. Characters were weighted following Sanders and Geisler (2015) such that ordered multistate characters were down-weighted so that their minimum length was equal to the minimum length of binary characters (i.e., one step).
Results.—Six equally most parsimonious trees of length 184,859 were generated by the analysis. The strict consensus of the six trees is shown in Figure 8. The three specimens sus- pected to represent Agorophius pygmaeus all cluster together in a polytomy, and this polytomy is unambiguously placed near the base of the odontocete clade.
Remarks.—Because all of the three specimens suspected of representing Agorophius pygmaeus cluster in a polytomy, the hypothesis that they all represent the same taxon is supported. That said, the three specimens that we assigned to Agorophius pygmaeus (MCZ 8761, ChM PV4256, and SC 2015.51.1) are not identical. In ChM PV4256 and SC 2015.51.1 for example, a V-shape best describes the outline of the antero- dorsal margin of the supraoccipital, whereas in MCZ 8761, that margin is parabolic. Conversely, MCZ 8761 and ChM PV4256 possess four or more dorsal infraorbital foramina in each
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