New heterodont odontocetes from the Oligocene Pysht Formation in Washington State, U.S.A., and a reevaluation of Simocetidae (Cetacea, Odontoceti)

Abstract

Odontocetes first appeared in the fossil record by the early Oligocene, and their early evolutionary history can provide clues as to how some of their unique adaptations, such as echolocation, evolved. Here, three new specimens from the early to late Oligocene Pysht Formation are described further increasing our understanding of the richness and diversity of early odontocetes, particularly for the North Pacific. Phylogenetic analysis shows that the new specimens are part of a more inclusive, redefined Simocetidae, which now includes Simocetus rayi, Olympicetus sp. 1, Olympicetus avitus, O. thalassodon sp. nov., and a large unnamed taxon (Simocetidae gen. et sp. A), all part of a North Pacific clade that represents one of the earliest diverging groups of odontocetes. Amongst these, Olympicetus thalassodon sp. nov. represents one of the best known simocetids, offering new information on the cranial and dental morphology of early odontocetes. Furthermore, the inclusion of CCNHM 1000, here considered to represent a neonate of Olympicetus sp., as part of the Simocetidae, suggests that members of this group may not have had the capability of ultrasonic hearing, at least during their early ontogenetic stages. Based on the new specimens, the dentition of simocetids is interpreted as being plesiomorphic, with a tooth count more akin to that of basilosaurids and early toothed mysticetes, while other features of the skull and hyoid suggest various forms of prey acquisition, including raptorial or combined feeding in Olympicetus spp., and suction feeding in Simocetus. Finally, body size estimates show that small to moderately large taxa are present in Simocetidae, with the largest taxon represented by Simocetidae gen. et sp. A with an estimated body length of 3 m, which places it as the largest known simocetid, and amongst the largest Oligocene odontocetes. The new specimens described here add to a growing list of Oligocene marine tetrapods from the North Pacific, further promoting faunistic comparisons across other contemporaneous and younger assemblages, that will allow for an improved understanding of the evolution of marine faunas in the region.

New Skeletons of the Ancient Dolphin Xenorophus sloanii and Xenorophus simplicidens sp. nov. (Mammalia, Cetacea) from the Oligocene of South Carolina and the Ontogeny, Functional Anatomy, Asymmetry, Pathology, and Evolution of the Earliest Odontoceti

“…This optimization occurs because of Olympicetus avitus and CCNHM 1000, which are positioned between these clades and have lower nasals. If CCNHM 1000 and Olympicetus avitus form a clade, which is supported by a recent study of Simocetidae [176], then this character would likely once again be a synapomorphy of Odontoceti or the clade of all odontocetes, minus Mirocetus. Three putative odontocete synapomorphies suggested by Sanders and Geisler [5], are also not supported by the present study: maxilla of intermediate length, loss of cranial hiatus, and high tegmen tympani adjacent to IAM.…”

“…Specifically excluded from this clade are the Xenorophidae. The phylogenetic position of Ashleycetus and Mirocetus varies between analyses, and these taxa occur occasionally as the earliest diverging odontocete outside the Xenorophidae + Amblyoccipita clade, at the base of Amblyoccipita, or as sister to Xenorophidae ( [5,25,176]; this study). These taxa may belong to Amblyoccipita but require further study.…”

“…Given that it is more basal than Simocetus, its morphology could decisively resolve this uncertainty. Though not coded into our matrix, Olympicetus thalassodon was published while this study was under review [176]; crucially, this taxon presents a tooth count similar to Simocetus and Basilosauridae and lacks polydonty, further suggestive of multiple origins of polydonty within Odontoceti.…”

“…A similar condition, albeit with a smaller lacrimal, is evident in many other stem odontocetes, such as Olympicetus sp. (CCNHM 1000; [51]), Olympicetus thalassodon [176] Nihohae matakoi ( [204]: Figure 2A), Simocetus rayi ([18]: Figure 3), Waipatia maerewhenua ( [17]: Figure 2A), and undescribed specimens from Charleston included within our phylogenetic analysis (aff. Agorophius, ChM PV 5852; Ankylorhiza sp., CCNHM 1075 and ChM PV 2764; aff.…”

“…Composition: Based on our phylogenetic analysis, Amblyoccipita (Figure 89) includes Crown Odontoceti and most family-level stem odontocete taxa/grades including "squalodontids", "waipatiids", "agorophiids", Simocetidae (sensu [176]), and other odontocetes variably placed in the stem group or within the crown group (Inticetus, Prosqualodon, Squaloziphiidae, "Chilcacetus-clade", Eurhinodelphinidae). Specifically excluded from this clade are the Xenorophidae.…”