Bioacoustics of Delphinidae

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.

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. Measurement UCMP 219487 UCMP 219488 Anteroposterior length 3.79 3.33 Length of pars cochlearis 1.49 1.52 Transverse width of internal acoustic meatus 0.54 0.63 Anteroposterior length of internal acoustic meatus 1.40 1.13 Length of anterior process 1.87 1.41 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.