Globicephalinae, Gray

Boessenecker, Robert W., Perry, Frank A. & Geisler, Jonathan H., 2015, Globicephaline whales from the Mio-Pliocene Purisima Formation of central California, USA, Acta Palaeontologica Polonica 60 (1), pp. 113-122 : 115-119

Globicephalinae Gray, 1850

Gen. et sp. indet. 1

Figs. 2–4 .

Material.— UCMP 219223 (the Seacliff Beach skull), a fragmentary skull collected from UCMP locality V99879, northern shore of Monterey Bay, USA, by Robin Eisenman. Mio-Pliocene Purisima Formation, early to middle Pliocene (5–2.47 Ma).

Description

Skull.—Although the “core” portion of the cranium is intact, most of the rostrum and the vertex is missing, as are most of the braincase elements, squamosals, lateral portions of the facial region, pterygoid hamuli, nasals, jugals, and lacrimals ( Figs. 2–4 ). The remaining portions of the skull are well preserved, and include much of the orbital region, medial facial region, anteroventral portion of the braincase, posterior palate, and the internal choanae. The skull is relatively large, with a slightly concave facial plane; if the vertex were complete, the face would likely have appeared more concave. Damage to the skull has exposed a partial endocranial cast. Skull measurements are presented in Table 1. A tentative reconstruction of the Seacliff Beach skull is given in Fig. 4.

Premaxilla.—The preserved rostral surface of the premaxilla is wide, flat, and smooth anteriorly, while tapering posteriorly to form a strongly convex and posterolaterally directed ridge ( Fig. 2A ). The lateral part of the premaxilla overlaps and is sutured to the maxilla anteriorly. The mesorostral canal is filled with matrix, and anteriorly measures 35 mm deep and 20 mm wide. The anteromedial sulcus lies just medial to the posterior end of the rostral surface of the premaxilla, delineating the elongate prenarial triangle. A large (15 mm wide) premaxillary foramen occurs at the posterior end of the anteromedial sulcus and medial to the rostral surface of the premaxilla. The premaxillary sac fossae are wide, shallow, exhibit a cancellous bone texture, and form a transversely concave basin.Although both nasal processes of the premaxillae are damaged, the right appears to have been wider than the left at the midpoint of the external nares, judging from the raised platforms of the maxillae that the premaxillae would have sutured to. The posterior terminations of the nasal processes are not preserved.

Maxilla.—Much of the lateral part of the rostral portion of the maxilla is missing, and only a very small portion of the ascending process remains on either side of the skull. The rostral portion is preserved as a thin sliver lateral to the rostral portion of the premaxilla ( Fig. 2A ). An accessory exposure of the maxilla lies medial to the right premaxilla and forms the anteromedial margin of the right naris. In dorsal view, the maxilla-premaxilla suture anterior to the anteriormost dorsal infraorbital foramen is slightly concave laterally.The anteriormost dorsal infraorbital foramen is positioned slightly more anteriorly on the right side than on the left one. The maxillae form a dorsoventrally deep palate ( Fig. 3A 1 ) with a trapezoidal cross section, and are posterolaterally underlapped by the palatine. Lateral to the palatine, the ventral surface of the maxilla is flat and wide. The ventral infraorbital foramen is positioned anteriorly within the antorbital fossa, which in turn is floored by the lacrimojugal crest of the maxilla.

Vomer and mesethmoid.—The vomer is not clearly visible on the anterior broken surface of the rostrum. Within the bony naris, the internasal septum is rotated clockwise about its vertical axis; in the absence of any diagenetic compaction, this feature is interpreted as natural.

Frontal.—The ventral orbital crest of the frontal is oriented anterolaterally, and forms a precipitous vertical crest that anteriorly demarcates the posterior boundary of the antorbital fossa ( Fig. 2B ). The optic foramen is laterally confluent with the frontal groove. Another groove is present in the frontal anteromedial to the frontal groove, forming an anterolateral continuation of the foramen rotundum; the anteromedial margin of this groove is adjacent to the ventral orbital crest.

Palatine and pterygoid.— The ventral extremities of these elements are missing. The laterally compressed and dorsoventrally narrow maxillary process of the palatine extends anteromedial to the antorbital fossa. A transversely thin lateral lamina of the palatine projects posteriorly ( Fig. 2B ), parallel to the posterior medial lamina of the pterygoid, demarcating the pterygoid sinus fossa.

Posterior braincase and basicranium.—Residual fragments of the parietals and alisphenoids adhere to recesses of the braincase endocast. A remnant of a well-developed internal sagittal crest is preserved between the cerebral hemispheres of the endocast. In Stenella attenuata , the internal sagittal crest actually represents the ossified falx cerebri of the dura mater ( Nojima 1988), and this appears to be the case in many other delphinids as well (Brian Beatty, personal communcation 2009), including the Seacliff Beach skull. Similarly, bony fragments preserved ventral to the region of the endocast corresponding to the right cerebral hemisphere are interpreted as ossified remnants of the tentorium cerebelli.

Endocast.—The right side of the brain endocast is nearly complete, with a slightly abraded and damaged surface ( Fig. 3A, B ). The cerebral hemispheres are large, subspherical, and separated medially by the internal sagittal crest/ossified falx cerebri ( Fig. 3B–D ). The cerebral hemisphere is separated from the cerebellum by a transverse fissure (sensu Colbert et al. 2005). On the lateral surface of the cerebral hemisphere, a faintly preserved ridge preserves the impression of the vertically oriented middle meningeal artery. The cerebellum appears to have been ventrally concave, and dorsoventrally thickened laterally near the cranial hiatus. The anteroventral apex of the cerebellar cast likely includes part of the cranial hiatus.

Gen. et sp. indet. 2

Fig. 2 .

Material.— UCMP 219487 and 219488, two left petrosals collected from the pebble marker bed ( UCMP locality V99869, northern shore of Monterey Bay, USA) by Stanley Jarocki (Watsonville, California, USA). Mio-Pliocene Purisima Formation, early to middle Pliocene (4.5–3.5 Ma).

Description.— The two isolated left petrosals resemble each other, and are inferred to represent the same taxon ( Fig. 5 ). Petrosal measurements are presented in Table 2 . In dorsal view, the outline of the petrosal is sigmoidal, with an anteromedially directed anterior process, a laterally thickened parabullary ridge, a medially convex pars cochlearis, and a posterolaterally projecting posterior process. In ventral view, the pars cochlearis is globular with a round medial margin. The ventral surface of the pars cochlearis is nearly flat.A subtle, anteroposteriorly aligned ridge extends from the margin of the fenestra rotunda, medial to which the pars cochlearis is slightly concave. The aforementioned ridge is much more apparent in UCMP 219488. The fenestra rotunda is D-shaped, with its flat edge positioned dorsally. The fenestra rotunda is confluent with a dorsally directed, V-shaped furrow.

The long axis of the fenestra ovalis is oriented anteromedially-posterolaterally. Lateral to the fenestra ovalis is the distal opening of the facial nerve canal. The stapedial muscle fossa and the facial sulcus are separated by a minute ridge in UCMP 219487, but not in 219488. The stapedial muscle fossa is narrow and elongate in both specimens, and not excavated beyond the level of the dorsal margin of the fenestra rotunda. The internal acoustic meatus is anteroposteriorly elongate ( Fig. 5A 2, B 2 ), owing to the inclusion of the facial nerve canal within the meatus. The meatus is teardrop-shaped, pointing anteriorly. The foramen singulare is medial to the spiral cribriform tract, with the partition separating the two being much lower than the transverse crest. The dorsal crest is not developed.

The anterior process is stocky and short, measuring approximately 81–90% of the length of the pars cochlearis Fig. 5A 1 , A 3 , B 1 , B 3 ; Table 2 ). The parabullary ridge is pronounced and overhangs more dorsal portions of the lateral side of the petrosal. In lateral view, the ventral margin of the anterior process is concave. The anterior margin of the anterior process is straight, giving the entire process a squared-off appearance. In anterior view, the anterior process appears triangular with ventral, dorsal, and medial apices.

The posterior process is relatively long (64 and 85% of the length of the promontorium in UCMP 219488 and 219487, respectively; Table 2 ) and projects posterolaterally ( Fig. 5A 1, B 1 ). Its ventral side is dominated by a large, transversely concave, pentagonal posterior bullar facet with longitudinal grooves. The apex of the pentagon points posterolaterally, and its base parallels the trough that includes the facial nerve sulcus and stapedial muscle fossa. The angle formed between the anterolateral edge of the posterior bullar facet and the lateral edge of the body of the petrosal in ventrolateral view is ca. 90º and 100º in UCMP 219487 and 219488, respectively. Note, however, that measurement of angles is highly sensitive to slight differences in viewing perspective. Anteriorly, the posterior process is separated from the parabullary ridge by a deep hiatus epitympanicus.

Comparisons between specimens.— Despite the similarities between UCMP 219487 and 219488, some differences are evident dorsally. The edge of the internal acoustic meatus is more elevated in UCMP 219487 and the posterior margin is slightly pointed, whereas that edge is more rounded (but likely also abraded) in UCMP 219488. The aperture of the cochlear aqueduct in UCMP 219487 occurs on a low tuberosity (as in extant Globicephala ); this structure is absent in UCMP 219488, and also appears to be abraded. The expanse of the pars cochlearis medial to the internal acoustic meatus is wider in UCMP 219488. In both specimens, the region lateral to the internal acoustic meatus is low and faces dorsally; however, in UCMP 219487 this region is flat, whereas in UCMP 219488 it is gently convex. As the dorsal face of the petrosal varies appreciably among extant delphinids (and cetaceans in general), these differences between UCMP 219487 and 219488 are attributed to intraspecific variation, as well as abrasion of the dorsal pars cochlearis in UCMP 219488.

Our regression analysis showed that the maximum width of the bony nares in delphinidans significantly increases with the length of the promontorium (p <0.001), and resulted a line with a slope of 7.9852 and a y-intercept of -57.559 ( Fig. 6 ). This equation slightly changes if it is based only on delphinids (n = 33, slope = 7.2787, y-intercept = -39.685) or globicephalines (n = 12, slope = 7.3159, y-intercept = -41.864). The Seacliff Beach skull has a maximum narial width of approximately 110.5 mm, which, based on the equation above, should be associated with a petrosal that has a promontorium 21 mm in length. By contrast, the promontoria in UCMP 219487 and UCMP 219488 are much smaller (14.95 and 15.24 mm, respectively; Fig. 6 ). This difference in size is not easily explained as the result of ontogeny, because in extant Globicephala the petrosal, and particularly the pars cochlearis, initially ossifies at near adult size, and the length of the petrosal shows no correlation with total body length ( Kasuya 1973). Thus, our analysis indicates the presence of at least two globicephalines in the Purisima Formation.

The maximum nares width we measured is likely an overestimate, because the premaxillae are damaged. In most extant delphinids, a medial shelf of the ascending process of the premaxilla overhangs the lateral margin of the bony nares, somewhat narrowing their transverse width. It seems unlikely that, if preserved, the premaxillae would have reduced the width by 45 mm (i.e., 41%), which would be required to bring the petrosals and the Seacliff Beach skull in line with the proportions seen in other delphinidans. Nevertheless, given the appreciable scatter around the best fit line (r 2 = 0.51954), it is possible, although unlikely, that the fossils we describe represent a single taxon with a proportionally small petrosal and/or unusually large external bony nares. More complete specimens are needed to test our hypothesis that the skull and petrosals represent separate taxa.

Figures

Fig. 2. Cranium of Globicephalinae gen. et sp. indet. 1 (UCMP 219223, Seacliff Beach skull), Pliocene, UCMP locality V99879, in dorsal (A) and ventral (B) views. Cross-hatching denotes damaged or missing bone, gray indicates matrix (including the endocast). Photographs (A 1, B1), interpretative line drawings (A 2, B2).

Fig. 3. Cranium of Globicephalinae gen. et sp. indet. 1 (UCMP 219223, Seacliff Beach skull), Pliocene, UCMP locality V99879, in lateral view, showing endocast (A). Endocast in posterior (B), lateral (C), and dorsal (D) views. Cross-hatching denotes damaged or missing bone, gray indicates matrix (including the endocast). Photographs (A 1, B 1), interpretative line drawings (A 2, B 2,C, D).

Fig. 4. Reconstruction of Globicephalinae gen. et sp. indet. 1, in dorsal view. Gray denotes the preserved portion of the cranium.

Fig.1. Geographic and stratigraphic context of Purisima Formation fossil globicephalines. A. Geologic map of Purisima Formation exposures in Northern California (modified from Boessenecker 2011). B. Location of inset map in California and North America. C. Stratigraphic column of the Santa Cruz section of the Purisima Formation, showing stratigraphic control of globicephaline fossils (modified from Powell et al. 2007 and Boessenecker and Perry 2011). Skull and petrosal symbols indicate stratigraphic position of fossils reported herein. Abbreviations: ms, mudstone; sls, siltstone; ss, sandstone. Asterisks denote age determinations.

Fig.5. Petrosals of Globicephalinae gen. et sp. indet. 2, early Pliocene, UCMP locality V99869. A. UCMP 219487, in ventral (A 1), dorsal (A 2), and medial A 3) views. B. UCMP 219488, in ventral (B 1), dorsal (B 2), and medial (B 3) views.

Fig. 6. Plot of maximum width of external bony nares versus length of promontorium for 62 delphinidan specimens, representing 34 species. Solid line is a reduced major axis regression (for all Delphinida). Horizontal dotted line is the nares width for the Seacliff Beach skull (UCMP 219223) and the vertical dashed lines are the lengths of the promontoria in UCMP 219488 (black) and 219487 (gray). In order to maximize the measurement area occupied by the specimens we sampled, the labeled y axis does not pass through the origin, but instead x = 8.