Physeteroidea Gray 1868

cf. Physeteroidea gen. et sp. indet.

REFERRED MATERIAL. — UCMP 219108, an isolated and abraded squamosal collected by R.W. Boessenecker from UCMP locality V99836.

STRATIGRAPHIC OCCURRENCE. — Middle part of the San Gregorio section of the Purisima Formation, Early Pliocene (c. 5-3.35 Ma; Zanclean-Piacenzian equivalent; Fig. 2).

DESCRIPTION

Ŋis abraded, partial small squamosal (Fig. 47) is robust and blocky, with a blunt zygomatic process. Much of the postglenoid process is missing, although part of the external acoustic meatus is preserved at about the level of the ventral margin of the zygomatic process. An arcuate and shallow dorsomedial fossa is present on the zygomatic process. Ŋe supramastoid crest is robust and posteriorly elevated (Fig. 47A). A sharp dorsomedial ridge is present on the zygomatic process, forming the medial margin of the mandibular fossa; in dorsal aspect, it is medially convex. Ŋe mandibular fossa is broad, rectangular, and shallowly concave. A large fossa occurs on the posterolateral surface of the squamosal, and appears to have been taphonomically enlarged. A number of deep grooves on the posterior surface mark the articular surface with the exoccipital.

REMARKS AND COMPARISONS

Although clearly a cetacean squamosal, UCMP 219108 compares poorly with most groups of Neogene cetaceans; comparisons were made between this specimen and large cetaceans including mysticetes, ziphiids, physeteroids, and globicephaline delphinids. UCMP 219108 lacks features common to Neogene mysticetes including a large and paddle-shaped postglenoid process, and is transversely more robust and absolutely larger than the squamosal of extant ziphiids and globicephaline delphinids. UCMP 219108 exhibits a posterodorsally elevated supramastoid crest, which Bianucci & Landini (2006) identified as a synapomorphy of the Physeteroidea, and it is here tentatively identified to this clade. In terms of the extreme bluntness of the zygomatic, it compares relatively well with the Miocene Belgian stem physeteroid Eudelphis mortezelensis Du Bus, 1872; however, UCMP 219108 is significantly younger (c. 3.35-5 Ma, Early Pliocene; Zanclean- Piacenzian equivalent) than Eudelphis Du Bus, 1872 (Middle Miocene; Lambert 2008b: fig. 19). In lateral aspect, the zygomatic process exhibits a similar morphology to hvalassocetus Abel, 1905, Brygmophyseter Kimura, Hasegawa & Barnes, 2006, Orycterocetus Leidy, 1853, and Aulophyseter Kellogg, 1927 (all Early to Middle Miocene physeteroids). Ŋe zygomatic process of UCMP 219108 is shorter and more blunt than those of Zygophyseter Bianucci & Landini, 2006, and Acrophyseter Lambert, Bianucci & Muizon, 2008, and transversely thicker than all known physeteroids. Although incomplete, the squamosal of Livyatan Lambert, Bianucci, Post, Muizon, Salas-Gismondi, Urbina & Reumer, 2010 appears to have been more gracile and anteriorly tapering than UCMP 219108. Because Ontocetus oxymycterus Kellogg, 1925 lacks squamosals, it cannot be compared with UCMP 219108. Ŋis specimen is substantially larger and more robust than the squamosals of all known kogiids (e.g., Aprixokogia Whitmore & Kaltenbach, 2008; Kogia Gray, 1846; Praekogia Barnes, 1973; Scaphokogia Muizon, 1988) and can be differentiated from these taxa. Because the squamosal morphology of UCMP 219108 does not match any previously described physeteroid taxon, it cannot be confidently identified to the generic level.