Herpetocetus morrowi, El Adli & Deméré & Boessenecker, 2014

El Adli, Joseph J., Deméré, Thomas A. & Boessenecker, Robert W., 2014, Herpetocetus morrowi (Cetacea: Mysticeti), a new species of diminutive baleen whale from the Upper Pliocene (Piacenzian) of California, USA, with observations on the evolution and relationships of the Cetotheriidae, Zoological Journal of the Linnean Society 170 (2), pp. 400-466 : 457-458

HERPETOCETUS MORROWI

One of the inherent problems faced by systematic palaeontologists is how to deal with morphological variation in the recognition and diagnosis of fossil species. As in extant organisms, morphological variation within a given population is expressed as intraspecific variation amongst individuals of the same ontogenetic age, amongst individuals of different ontogenetic ages, and between individuals of different sexes. As vertebrate palaeontologists often have only a few specimens available for study for any given taxon, it is not unusual for specimens representing each of these different expressions of intraspecific variation to be assigned to different species. This problem of the biological meaning of morphological variation lies at the core of the age-old conundrum of taxonomic lumping vs. taxonomic splitting. It needs to be borne in mind that just as failure to recognize intraspecific variation can lead to overestimates of palaeobiodiversity and related phylogenetic errors, failure to recognize interspecific variation can lead to underestimates of palaeobiodiversity and errors in phylogeny. Thus, when the opportunity arises to study a relatively large suite of vertebrate fossil specimens of a single species, it is cause for celebration.

A good example of ontogenetic variation within H. morrowi is seen in the morphological series of squamosals starting with the smallest and most juvenile specimen, SDNHM 125833, and followed by the progressively more mature specimens, SDNHM 34155, UCMP 124950, and SDNHM 65781, respectively. When viewed as an ontogenetic series, these specimens reveal variation in the degree of anterolateral rotation of the postglenoid process, wherein the almost transversely orientated process in SDNHM 125833 becomes progressively more anterolaterally rotated in successively more mature specimens. Anterolateral rotation of the postglenoid process has also resulted in a broad lateral exposure of the external surface of the postglenoid process in adult herpetocetines, which has given the posterior portion of the squamosal its broadly triangular appearance ( Fig. 24 ). In juvenile individuals (SDNHM 125833) the posterior surface of the postglenoid process is orientated roughly perpendicular to the lateral surface of the zygomatic process of the squamosal and forms the anterior margin of the external auditory meatus. In successively more mature individuals (SDNHM 34155, UCMP 124950, and SDNHM 65781) the anterolateral rotation of the postglenoid process results in the posterior surface of the process assuming a more lateral orientation wherein this surface no longer forms the anterior margin of the external auditory meatus, but instead is positioned roughly in the same plane as the lateral surface of the zygomatic process. This same squamosal morphological series also reveals that the relative form of the articular surface of the glenoid fossa, which is transversely and longitudinally convex in less mature specimens (SDNHM 125833 and SDNHM 34155), becomes transversely and longitudinally planar to slightly concave in more mature specimens (SDNHM 65781). Further, the apex of the postglenoid process in juvenile individuals tapers distally to a relatively sharp edge, whereas in more mature individuals (SDNHM 65781), the apex of the postglenoid process is expanded and flattened to form an approximately lenticular (in ventral view), flattened heel that extends continuously from the posteromedial corner of the postglenoid process anteriorly around and onto the anterolateral margin of the process ( Figs 18 , 19 ).

The exoccipital also preserves an instructive morphological series that probably translates into an ontogenetic series. The occipital condyles in SDNHM 90484, SDNHM 31455, and UCMP 124950 are relatively broad and closely appressed to the adjacent planar surfaces of the exoccipitals, whereas the occipital condyles in SDNHM 65781 (a developmentally more mature individual) are more well defined and set off from the exoccipitals by a distinct neck. This same degree of morphological variation was used by Whitmore & Barnes (2008) to propose that the holotype skulls of H. transatlanticus and N. eremus represented immature individuals, whereas the holotype skull of H. bramblei (with its well-defined occipital condyles) was suggested to represent a mature individual.

The variable degree of development of the anterior and posterior processes of the petrosal in H. morrowi appears to represent yet another ontogenetic series, with juvenile specimens (SDNHM 38689) possessing an anteroposteriorly short anterior process and distally short and narrow posterior process, whereas more mature specimens (UCMP 124950 and SDNHM 63690) display a relatively longer anterior process and more distally elongated and wider posterior process. Bisconti (2001) noted a similar ontogenetic series in the petrosals of juvenile and adult fin whales ( Balaenoptera physalus Linnaeus, 1758 ).

Figures

Figure 18. Herpetocetus morrowi sp. nov., left squamosal in ventral view. A, SDNHM 125833; B, SDNHM 34155; C, SDNHM 65781. Scale bars = 5 cm.

Figure 19. Herpetocetus morrowi sp. nov., left squamosal in lateral view. A, SDNHM 125833; B, SDNHM 34155; C, UCMP 124950; D, SDNHM 65781. Scale bars = 5 cm.

Figure 24. Herpetocetus morrowi sp. nov., UCMP 124950, holotype skull in posterolateral view. Scale bar = 5 cm.