Phocoena, Cuvier, 1816

Boessenecker, Robert W., 2013, A new marine vertebrate assemblage from the Late Neogene Purisima Formation in Central California, part II: Pinnipeds and Cetaceans, Geodiversitas 35 (4), pp. 815-940 : 896-900

cf. Phocoena

REFERRED MATERIAL. — UCMP 219123, partial skull consisting part of right maxilla and premaxilla missing the anterior portion of the rostrum, antorbital process, and most of the ascending process of the maxilla, collected by R.W. Boessenecker from UCMP locality V99833 .

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

DESCRIPTION

Anteriorly, the rostrum is dorsoventrally compressed with a flat dorsal surface ( Fig. 46 ). Ŋe premaxilla

has parallel lateral and medial margins, and anteriorly, the premaxilla slopes slightly laterally. Medially, the premaxilla forms a sharp ridge along the dorsal margin of the mesorostral canal; posteriorly the medial surface is flat. Ŋe anteromedial sulcus is straight and runs posterolaterally from the margin of the mesorostral canal to the premaxillary foramen. Ŋe latter is positioned in the middle of the premaxilla anterior to the premaxillary eminence. Ŋe posteromedial sulcus is not apparent, and the posterolateral sulcus is developed lateral to the pre- maxillary foramen and extends posteriorly along the lateral margin of the premaxillary eminence; an additional posteromedial sulcus (sensu Murakami et al. 2012b) is not apparent. Ŋe prenarial triangle is bordered medially by the mesorostral canal and laterally by the anteromedial sulcus; it is composed of porous bone. Ŋe premaxillary eminence is elevated dorsally ( Fig. 46C ), dorsally convex in lateral aspect, and laterally convex in dorsal aspect ( Fig. 46A ). Ŋe eminence is anteroposteriorly short and laterally overhangs the maxilla; within this overhang is a small, laterally facing foramen in the premaxilla. Ŋere is a small indentation in the posteromedial corner of the premaxillary eminence, where the accessory exposure (= maxillary ossicle) of the maxilla was probably situated. Ŋe ascending process of the premaxilla is restricted to a small cone-shaped process extending posterolateral to the premaxillary eminence. Ŋis process dorsally overlaps the maxilla, and is short along the lateral side of the bony naris. Ŋe premaxilla/maxilla suture runs along the base of the premaxillary eminence. Ŋe dorsal surfaces of the maxilla of UCMP 219123 and ascending process of the maxilla are relatively flat ( Fig. 46C ). Although damaged, the rostrum clearly tapers anteriorly. Ŋe anteriormost dorsal infraorbital foramen occurs anterolateral to the premaxillary eminence and opens anteriorly. Ŋe palatal surface of the maxilla is transversely convex. A poorly preserved alveolar groove occurs adjacent to the lateral margin of the rostral portion of the maxilla.

COMPARISONS

Ŋis specimen exhibits many features identifying it as a phocoenid or true porpoise, including a well-developed premaxillary eminence and an ascending process of the premaxilla that does not extend posteriorly beyond the bony naris ( Fig. 46A ; Barnes 1985a). Ŋis specimen differs from other modern and fossil phocoenids in several ways. Although the premaxillary eminence is elevated in UCMP 219123 as in Neophocaena phocoenoides Cuvier, 1824 , Phocoena phocoena Linnaeus, 1758 , Phocoena dioptrica , Phocoena sinus Norris & Mc- Farland, 1958, and Phocoenoides dalli , it is more elevated and more strongly dorsally convex than in Archaeophocaena Murakami, Shimada, Hikida & Hirano, 2012 , Australithax Muizon, 1988 , Haborophocoena , Lomacetus Muizon, 1986 , Miophocaena, Numatophocoena Ichishima & Kimura, 2000 , Piscolithax , Pterophocaena , Salumiphocaena Barnes, 1985 , and Septemtriocetus Lambert, 2008 . UCMP 219123 differs from Pterophocaena in possessing a narrower premaxillary eminence that does not widely overhang the ascending process of the maxilla. Ŋe laterally convex margin of the premaxillary eminence in this taxon is similar with P. phocoena , Phocoena spinipinnis Burmeister, 1865 , P. dalli and the Miocene porpoises Archaeophocaena and Salumiphocoena, while it is not as convex as in N. phocoenoides and P. sinus ; all other fossil taxa exhibit a straight lateral margin ( Haborophocoena toyoshimai , Piscolithax , Septemtriocetus ) or an “angled” margin with a slight corner ( Australithax , Haborophocoena minutis , Lomacetus , Miophocaena ). UCMP 219123 differs from an undescribed broadheaded phocoenid from the San Diego Formation (SDNHM 38340) in its smaller size, and slightly convex palate. UCMP 219123 differs from Archaeophocaena , Haborophocoena , and Miophocaena in possessing a left premaxilla that terminates further anteriorly and near the anterolateral margin of the bony nares. Phocoena dioptrica also exhibits a relatively straight lateral margin of the premaxillary eminence. Additionally, P. dalli exhibits an exposure of the premaxilla lateral to the base of the eminence, as opposed to the suture running along the base as in UCMP 219123. In UCMP 219123, all extant species of Phocoena , and Salumiphocaena , the premaxillary eminence is anteroposteriorly short and dorsally highly convex; it is slightly longer in Neophocaena Palmer, 1899 and Phocoenoides , and much longer in all other fossil phocoenids. When eminence height is expressed as a percentage of anteroposterior length of the eminence, UCMP 219123 exhibits the most convex eminence at 37%, compared to 35% in P. dioptrica , 27% in P. phocoena , 25% in P.spinipinnis and N. phocoenoides , and 23% in P. dalli . Ŋe rostrum of Neophocaena is also dorsoventrally deeper than in UCMP 219123, in addition to possessing a transversely convex prenarial triangle, which is flat in UCMP 219123. Although incomplete, the morphology of the premaxilla and

supramastoid crest

maxilla agree most closely with extant porpoises ( Phocoena , Phocoenoides , and Neophocaena ). In particular, the short, wide, and dorsally and laterally convex premaxillary eminence suggests that this specimen belongs to the genus Phocoena ; no other fossil or extant phocoenids exhibit such an anteroposteriorly short and dorsally convex premaxillary eminence. However, UCMP 219123 differs from extant phocoenids and Miophocaena in its apparent lack of additional posterolateral sulci (sensu Murakami et al. 2012b).

In UCMP 219123, as well as all other extant phocoenids and Piscolithax , the ascending process of the premaxilla is reduced to a small, cone shaped process that dorsally overlaps the maxilla. In Archaeophocaena , H. toyoshimai , Miophocaena, Numatophocoena , and crania of the Haborophocoena -like Phocoenidae unnamed genus 1 (UCMP 128285, 219504), the ascending process is formed instead as a transversely broad sheet that overlaps the maxilla (as in delphinids); in all other fossil phocoenids ( Australithax , H. minutis , Lomacetus , Salumiphocaena , and Phocoenidae unnamed genus 2), the ascending process forms the anterior part of a cylindrical ridge that laterally borders the bony nares, the posterior part of which is formed by the maxilla. In this specimen, the rostral portion of the maxilla is relatively narrow, similar to P.phocoena , Australithax , Septemtriocetus , H. minutis , and Lomacetus . However, this differs from all other species of Phocoena , Neophocaena , Phocoenoides , as well as Salumiphocaena and Piscolithax tedfordi Barnes, 1984 , all of which have a maxilla that is much wider than the premaxilla on the rostrum. At the other extreme, Haborophocoena toyoshimai , Piscolithax boreios Barnes, 1984 , and Piscolithax longirostris have a rostral portion of the premaxilla that is wide and nearly wider than the maxilla on the rostrum. Ŋe small facet in the premaxillary eminence indicates that the accessory exposure of the maxilla (= maxillary ossicle) was small (although missing in UCMP 219123), like in most phocoenids (with the exception of Lomacetus , which had large and anteriorly extending maxillary ossicle). Lastly, unlike all extant phocoenids, UCMP 219123 exhibits a slightly convex palate, which also occurs in most fossil taxa; a convex palate appears to be plesiomorphic for phocoenids and delphinoids in general. In extant phocoenids, the palate at mid-rostrum is transversely flat or even slightly concave.

REMARKS

Ŋese comparisons suggest that UCMP 219123 is most phenetically similar to the extant harbor porpoise P. phocoena , and is identified as cf. Phocoena . Ŋis is the earliest fossil record of an extant phocoenid genus, and the first fossil record of an extant phocoenid from the eastern Pacific.Previously, fossils of extant phocoenid taxa have been reported only from the Pleistocene: N. phocaenoides from Japan ( Kimura & Hasegawa 2005) and P. phocoena from Champlain Sea deposits ( Harington 1977). Two additional Phocoena -like taxa are present in Upper Pliocene strata of California: an undescribed broad-headed phocoenid with spatulate teeth from the San Diego Formation (SDNHM 38340), and a small phocoenid with a convex palate and sulci on the premaxillary eminences (USNM 23885) from the Purisima Formation near Pillar Point (San Mateo County). Ŋis proposed Pliocene record of cf. Phocoena has implications for the timing of divergence of crown phocoenids.

Ŋe evolutionary biogeography of phocoenids was hypothesized by Fajardo-Mellor et al. (2006) as follows: 1) phocoenids originated in the eastern North Pacific; 2) Neophocaena and the common ancestor of the Phocoena - Phocoenoides clade dispersed into the southern hemisphere during the Pliocene (3-2 Ma; Piacenzian-Gelasian) following cooling caused by the closure of the Panama seaway; and 3) Pleistocene cooling allowed the ancestors of Phocoena sinus and the common ancestor of the P. phocoena - P. dalli clade to each disperse northwards into the North Pacific. Ŋe presence of an Early Pliocene (4.5-3.35 Ma; Zanclean-Piacenzian equivalent) record of cf. Phocoena suggests that the clade may have diverged by 4 Ma. Ŋe phylogenetic relationship of the southern hemisphere P. spinipinnis as sister to a Phocoenoides + P. phocoena clade (Fajardo-Mellor et al. 2006) still supports a southern hemisphere origin for the common ancestor of these taxa, although it is possible that basal members of the entire Phocoena - Phocoenoides clade remained in the North Pacific during the Late Pliocene. More fossils are needed for further evaluation.

Figures

FIG. 2. — Stratigraphy of the Purisima Formation: A, Stratigraphic column of the San Gregorio section of the Purisima Formation (in meters), showing stratigraphic position of UCMP vertebrate localities, dated ash beds, diatom floras, and species of Carcharodon; thickness measurements start over above missing section in uppermost part due lack of overlapping strata; epoch and stage boundaries marked as "?" are estimated; modified from Boessenecker (2011a); Pliocene-Pleistocene boundary after Hilgen et al. (2012); B, photograph of 3-4 m thick ash within Purisima Formation at mouth of San Gregorio Creek correlated with 2.5 Ma Ishi Tuff (Powell et al.2007); C, trace fossils in pervasively bioturbated sandstone,Purisima Formation,mouth of San Gregorio Creek; D, swaley cross-stratified sandstone, mouth of San Gregorio Creek. Abbreviations: Qt, Pleistocene terrace deposits; Tp, Purisima Formation; C, claystone; St, siltstone; Ss, sandstone. Scale bars: A, 2 m; B, C, 10 cm.

FIG. 46.— Partial skull of cf.Phocoena (UCMP 219123):A, dorsal view; B, ventral view; C, right lateral view;D, interpretive line drawing in right lateral view; E, interpretive line drawing in dorsal view,with fossil mirrored. Gray shading on line drawing represents preserved portions, and dashed line represents unknown features. Scale bars: 3 cm.