Herpetocetus (Boessenecker, 2011)

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 : 864-871

Herpetocetus sp.

REFERRED MATERIAL. — UCMP 219479, a partial juvenile right squamosal, UCMP 219114, a right petrosal with attached compound posterior process, UCMP 219115, a left petrosal lacking posterior process, UCMP 219116, isolated posterior process of a left petrotympanic, UCMP 219117, a partial right tympanic bulla, and UCMP 219077, a partial right mandible, collected by R.W. Boessenecker from UCMP localities V99836 , V99840 , and V99859 .

STRATIGRAPHIC OCCURRENCE. — Middle and upper parts of the San Gregorio section of the Purisima Formation, Early to Late Pliocene (c. 5-2.5 Ma; Zanclean-Gelasian equivalent; Fig. 2 ).

DESCRIPTION

Ŋe squamosal (UCMP 219479) represents a juvenile or neonate based on its small size and lack of fused sutures with adjoining bones (e.g., alisphenoid, parietal, exoccipital). Dorsally, the squamosal is broadly convex where it forms the dorsolateral roof of the braincase and exhibits a longitudinal and elongate fossa along the lateral margin ( Fig. 27 ). Lateral to the fossa, the supramastoid crest is elongate, sharp, linear, and anteriorly oriented.Two ascending plates of the squamosal extend medial to the supramastoid crest; the squamosal plate extends dorsomedially from the posterior portion of the squamosal, while the falciform process runs anteromedially to form the anterior margin of the braincase ( Fig. 27B ). Ŋere is a large division between the squamosal plate and falciform process; in more complete crania of Herpetocetus , this wedge-shaped division accommodates the postparietal foramen ( Whitmore & Barnes 2008), the parietal (dorsally) and hypertrophied alisphenoid (anterodorsally). Ŋe zygomatic and postglenoid processes of UCMP 219479 are lost. Only a fragment of the medial edge of the zygomatic is present, but it shows that the process was anteriorly oriented. A knob-like process occurs along the supramastoid crest above the external acoustic meatus. Ŋe sternomastoid fossa is small, concave, triangular, and positioned posterior to the external acoustic meatus and immediately below the supramastoid crest ( Fig. 27D ). Ŋe lateral surface of the squamosal is otherwise flat. Ventrally, the external acoustic meatus is formed as a transversely oriented concavity posterior to the postglenoid process. Posteriorly, another semi-cylindrical concave fossa is present, oriented posterolaterally, and would have received the posterior process of the petrotympanic. Ŋe medial margin of the squamosal is concave where it forms the lateral wall of the petrosal fossa ( Fig. 27A ). A corrugated articular surface for the alisphenoid is present dorsomedially.

Ŋe isolated petrosals (UCMP 219114, 219115) are small (pars cochlearis length 29-33 mm; Table 7 ) and roughly lenticular in ventral view ( Fig. 28 ). Ŋe pars cochlearis is small and medially elevated ( Fig. 28A, E ); the tensor tympani muscle origin is marked by deep groove on the medial surface of the anterior process ( Fig. 28C, F ). Ŋe anterior process is transversely compressed and bladelike, and appears rectangular in medial aspect.Dorsally adjacent to the groove for the tensor tympani muscle, a longitudinal ridge is present on the medial surface of the anterior process ( Fig. 28C, G ). Ŋe posterior cochlear crest has a strongly convex posterior margin in medial aspect; in ventral aspect it is well developed, and strongly overhangs both the stapedial muscle fossa and the stylomastoid fossa ( Fig. 28C, G, H ). Ŋe dorsal side of UCMP 219115 is convex and lacks a suprameatal fossa, while in UCMP 219114, it is nearly planar with a shallow suprameatal fossa. Ŋe canal for the vestibulocochlear nerve is large and circular, and is separated from the facial canal by a thick crista transversa ( Fig. 28B, F ). In dorsal aspect, the slit-like hiatus fallopii is confluent with the anterior margin of the facial canal, giving the facial canal a teardrop shape. Relative to the endolymphatic foramen, the perilymphatic foramen is much smaller ( Fig. 28B ); both are oriented slightly anterodorsally. At the position of the anterior part of the pars cochlearis, the lateral tuberosity is present along the ventral margin of the anterior process; it is ventrolaterally projecting as a flattened triangular flange in UCMP 219114 ( Fig. 28A ) and a large knob in UCMP 219115 ( Fig. 28E ). In UCMP 219115, a short groove (that widens and deepens anteriorly) is present along the anterior half of the lateral tuberosity, separating the lateral tuberosity and anterior process. Medial to the lateral tuberosity the broken anterior pedicle occurs as a small, anteroposteriorly short and bladelike crest. Between the anterior pedicle and the pars cochlearis is a relatively large, shallow mallear fossa. Ŋe fenestra vestibuli is oval and positioned posteromedial to the mallear fossa; the facial canal opens lateral to the fenestra vestibuli, and opens into the facial sulcus. Ŋe facial sulcus and fenestra vestibuli are separated by a thin crest ( Fig. 28A ). Posterior to the lateral tuberosity, a small convex ridge-like squamosal flange (sensu Geisler & Luo 1996) is ventrally prominent. Ŋe lateral surface of the petrosal is broadly convex in both specimens ( Fig. 28A, B, E, F ). Medially adjacent to the squamosal flange, the posterior pedicle is preserved in both specimens as a transversely thin crest.

Ŋe posterior process (UCMP 219115, 219116) is short ( Table 8 ) and plug-shaped ( Figs 28 A-D; 29D- F); in UCMP 219115, there is a thin, constricted neck. Ŋe lateral surface of the posterior process is flat, triangular, and slightly rugose. Ŋe dorsolateral apex is anteroposteriorly tapered ( Figs 28D ; 29F ). Ŋe short, deep facial sulcus occurs on the ventral side, and is bounded anterolaterally by a large, high, bladelike ridge ( Figs 28D ; 29F ). Posteromedial to the facial sulcus is an anteroposteriorly thick, transversely oriented ridge, which is anteromedially convex and spherical. Medially adjacent to the neck of the posterior process is a small but well defined circular pit.

The partial right tympanic bulla (UCMP 219117) is abraded and missing most of the outer lip ( Fig. 29 A-C; Table 5 ). Ŋe involucrum is relatively thick with a well developed dorsal posterior prominence ( Fig. 29B, C ). In ventral aspect, the bulla is posteriorly bilobate: a fossa-like median furrow separates the medial and lateral lobes. Ŋe involucral and main ridges are posteriorly divergent and widely separated ( Fig. 29C ). In medial aspect, the anterior portion of the involucrum is constricted, and the medial surface of the involucrum bears fine transverse striations.

Ŋe anterior half of the mandible (UCMP 219077) is missing, but the posterior portion is well-preserved ( Fig. 30 ; Table 6 ) and similar to UCMP 219079 with a few exceptions ( Table 6 ). Laterally adjacent to the dorsal crest, three mental foramina are preserved and open into anteriorly extending sulci ( Fig. 30A ). Two small gingival foramina occur on the medial surface, medially adjacent to the dorsal crest anterior to the coronoid process, also anteriorly opening but with shorter sulci than the gingival foramina. A large, deep, and anteroposteriorly elongate fossa on the lateral surface of the coronoid process extends anteriorly as a narrow concavity for the temporalis insertion below the anterior crest of the coronoid process. Ŋe mandibular condyle is laterally bilobate, with a slight fossa occurring on the lateral side dividing the condyle into dorsal and ventral lobes. A deep fossa occurs on the ventral

A coronoid process elevation inward mandibular condyle

side of the condyle, forming a sharp ventromedially curving crest ( Fig. 30D ). Ŋe angular process extends far posterior to the condyle and dorsally exhibits a longitudinal groove.

COMPARISONS

Ŋese specimens can be referred to Herpetocetus owing to several characteristics.Ŋe squamosal (UCMP 219479) matches those of Herpetocetus as described by Whitmore & Barnes (2008) in possessing a large fossa for the posterior process of the petrotympanic, an elongate anteroposteriorly oriented trough-like fossa on the squamosal medial to the supramastoid crest, and a large wedge-shaped dorsomedial margin to accommodate the alisphenoid and postparietal foramen. Ŋe squamosal differs from Herpetocetus bramblei in exhibiting a small sternomastoid fossa that does not extend anterior to the level of the external acoustic meatus. Ŋe petrosals bear a laterally compressed, bladelike anterior process (which characterizes Herpetocetus and Nannocetus Kellogg, 1929 ; Geisler & Luo 1996; Whitmore & Barnes 2008), a large lateral projection (sensu Geisler& Luo 1996; = lateral tuberosity, Mead & Fordyce 2009) of the anterior process (unique to Herpetocetus ; Geisler & Luo 1996; Whitmore & Barnes 2008), and a short plug-shaped posterior process (a potential synapomorphy of herpetocetines; Bouetel & Muizon 2006; Steeman 2007; Whitmore & Barnes 2008). Ŋe squamosal (UCMP 219479) was placed in articulation with UCMP 219114 ( Fig. 27C ), a petrosal with attached posterior process from a separate individual (see description above); the tight articulation of these two elements strongly supports the identification of these as the same taxon.

Ŋe tympanic bulla (UCMP 219117) is referred to Herpetocetus sp. because of its small size, distinct dorsal posterior prominence, median groove, distinct transverse creases on the involucrum, and close phenetic similarity to previously published tympanic bullae of Herpetocetus ( Whitmore & Barnes 2008) . A newly referred (and nearly complete) skull of H. bramblei (UCMP 219111) introduced by Boessenecker & Geisler (2008) includes both petrosals and a tympanic bulla, the latter of which is not preserved in the holotype. Ŋe isolated tympanic bulla (UCMP 219117) differs from the well-preserved tympanic bulla of UCMP 219111 in its smaller size, less distinct transverse creases, much lower dorsal posterior prominence, a shallower furrow on the medial surface between the dorsal prominence and the posterior lobe, and lacking a sharp spur on the ventral tip of the dorsal posterior prominence. UCMP 219117 differs from Herpetocetus transatlanticus Whitmore & Barnes, 2008 , the referred specimen of H. “ sendaicus ”, and the holotype bulla of H. sendaicus , in having a transversely narrower involucrum with a smaller and more defined dorsal posterior prominence. It also differs from Piscobalaena and Nannocetus in exhibiting a more strongly developed dorsal posterior prominence and having a more constricted anterior portion of the involucrum.

Ŋe mandible (UCMP 219077) exhibits characteristic features of Herpetocetus , including an elongate and laterally hooked coronoid process, a posteromedial sloping mandibular condyle, and a posteriorly projecting and shelf-like angular process (see above remarks under H. bramblei ; Boessenecker 2011b; Whitmore & Barnes 2008). Ŋis specimen is unique among herpetocetines (and possibly among mysticetes) in having a fossa on the lateral surface of the condyle, giving it a bilobate morphology. It is further distinct from Piscobalaena , H. “ sendaicus ”, H. bramblei , and indeterminate herpetocetine mandibles ( Boessenecker 2011b) in having a fossa for the temporalis insertion that extends anteriorly beyond the coronoid process; this feature is slightly developed in H. scaldiensis . UCMP 219117 is roughly the same size as the mandible of H. “ sendaicus ”, and is much smaller than UCMP 219076 described above. As mentioned above, the mandibular morphology of herpetocetines may not be diagnostic at the genus level ( Boessenecker 2011b), but some subtle mandibular features may be taxonomically useful among species of Herpetocetus .

REMARKS

Ŋese specimens appear to represent an undescribed species of Herpetocetus morphologically distinct from and geochronologically younger than H. bramblei ; additional material of this taxon is recorded from the Santa Cruz section of the Purisima Formation (see below). Ŋe squamosal, petrosals, and mandible exhibit several differences with H. bramblei (see above). Ŋese specimens are from higher up in the San Gregorio section than the mandible of H. bramblei (UCMP 219079), and occur in younger strata with occasional Patinopecten healelyi shells, the latter indicating a Pliocene age (Powell et al. 2007). Ŋe mandible (UCMP 219077) was collected from between two tephra layers dated at 2.5 and 3.35 Ma, indicating an Early to Late Pliocene age (Piacenzian- Gelasian equivalent) for this specimen. All other specimens were collected below the 3.35Ma tephra, and above the aforementioned 6.4-5.6 Ma (Late Miocene) diatom flora; they are likely Early-Late Pliocene (Zanclean-Piacenzian equivalent) in age due to their association with Pliocene mollusks and Carcharodon carcharias teeth ( Boessenecker 2011a). Ŋese specimens are all younger than the 6.79- 5.33 Ma age for the type locality of H. bramblei . Likewise, these specimens differ from H. bramblei . Ŋe squamosal (UCMP 219479) exhibits a small sternomastoid fossa that does not extend anteriorly past the external acoustic meatus ( Fig. 27D ); in H.bramblei , the sternomastoid fossa is much larger. A similarly small sternomastoid fossa occurs in a Herpetocetus sp. braincase (UCMP 219121) from the Lower Pliocene Purisima Formation near Santa Cruz (5.33-4.5 Ma; Zanclean equivalent). Ŋe petrosals (UCMP 219114 and 219115) are relatively similar to the holotype of H. bramblei , but both differ from it in having a posteriorly elongate and transversely thin posterior cochlear crest, and a thick crista transversa; the crista transversa in the H. bramblei holotype is thin, and the posterior cochlear crest is short. Ŋe H. bramblei holotype petrosal further differs in having elongate prongs of bone on the dorsal surface, and having a circular facial canal which lacks an anterior fissure (as opposed to the teardrop shaped facial canal described above in UCMP 219114 and 219115). UCMP 219115 also differs from H.bramblei (and all other herpetocetines) in exhibiting a bulbous lateral projection of the anterior process ( Fig. 28E ).

Ŋese specimens are distinct from H. bramblei , and indicate the presence of a second, younger undescribed species within the Purisima Formation. Additional material from the lower Pliocene section of the Purisima Formation (Zanclean-Piacenzian equivalent) near Santa Cruz includes a skull with earbones (UCMP 219121) and several isolated petrosals (UCMP 219118, 219119) that share this same morphology. As this more diagnostic material is from a locality separate from material described herein, it is beyond the scope of this study to describe this new taxon, and will be considered in a separate study.

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. 27. — UCMP partial right squamosal (UCMP 219479) of Herpetocetus sp.; A, ventral view; B, dorsal view; C, ventral view with right petrosal and posterior process (UCMP 219114) in articulation; D, lateral view (without petrosal). Scale bar: 3 cm.

FIG. 28. — Petrosals of Herpetocetus sp.; right petrosal with attached compound posterior process, UCMP 219114 in A, ventral view; B, dorsal view,and C, medial view;D, compound posterior process of UCMP 219114 in posterolateral view;left petrosal UCMP 219115 in E, ventral view, F, dorsal view, G, medial view, and H, posterior view. Scale bar: 3 cm.

FIG. 29. — Petrotympanic elements of Herpetocetus sp.; A, UCMP 219117, partial right tympanic bulla in medial view; B, UCMP 219117 in dorsal view; C, UCMP 219117 in ventral view; D, UCMP 219116, compound posterior process of left petrotympanic in anterior view; E, UCMP 219116, posterior process of left petrosal in posterior view; F, UCMP 219116, posterior process of left petrosal in posterolateral view. Scale bar: 3 cm.

FIG. 30. — Partial right mandible of Herpetocetus sp. (UCMP 219077); A, medial view; B, lateral view; C, dorsal view; D, interpretive line drawing in dorsal view with cross section outlines (lateral direction in cross-sections is to left side). Numbers adjacent to cross sections indicate distance from broken anterior surface (measured in centimeters). Scale bar: 10 cm.