A new tuskless walrus from the Miocene of Orange County, California, with comments on the diversity and taxonomy of odobenids

Abstract

We describe Titanotaria orangensis (gen. et. sp. nov.), a new species of walrus (odobenid) from the upper Miocene Oso Member of the Capistrano Formation of Orange County, California. This species is important because: (1) It is one of the best-known and latest-surviving tuskless walruses; (2) It raises the number of reported odobenid taxa from the Oso Member to four species making it one of the richest walrus assemblages known (along with the basal Purisima of Northern California); (3) It is just the second record of a tuskless walrus from the same unit as a tusked taxon. Our phylogenetic analysis places T. orangensis as sister to a clade that includes Imagotaria downsi, Pontolis magnus, Dusignathus spp., Gomphotaria pugnax, and Odobeninae. We propose new branch-based phylogenetic definitions for Odobenidae, Odobeninae, and a new node-based name (Neodobenia) for the clade that includes Dusignathus spp., G. pugnax, and Odobeninae. A richness analysis at the 0.1 Ma level that incorporates stratigraphic uncertainty and ghost lineages demonstrates maximum peaks of richness (up to eight or nine coeval lineages) near the base of Odobenidae, Neodobenia, and Odobenini. A more conservative minimum curve demonstrates that standing richness may have been much lower than the maximum lineage richness estimates that are biased by stratigraphic uncertainty. Overall the odobenid fossil record is uneven, with large time slices of the record missing on either side of the Pacific Ocean at some times and biases from the preserved depositional environments at other times. We recognize a provisional timescale for the transition of East Pacific odobenid assemblages that include “basal odobenids” (stem neodobenians) from the Empire and older formations (>7 Ma), to a mixture of basal odobenids and neodobenians from the Capistrano and basal Purisima (7–5 Ma), and then just neodobenians from all younger units (<5 Ma). The large amount of undescribed material will add new taxa and range extensions for existing taxa, which will likely change some of the patterns we describe.

Feeding in marine mammals: An integration of evolution and ecology through time

“…Odobenidae (walrus) diversity was also found to be correlated with three variables: δ 18 O, δ 13 C, and sea level changes. This fits with a Messinian peak diversity of fossil walruses and the evolution of more specialized feeding systems (i.e., tusks and molluscivory) (Magallanes et al, 2018). The drop in walrus diversity beginning at the end of the Pliocene is associated with the onset of Northern Hemisphere glaciation, large-scale sea level oscillations, and other oceanographic alterations (e.g., changes in productivity and ocean circulation).…”

Oldest record of monk seals from the North Pacific and biogeographic implications

“…15: 20190108 basins by identifying the timing of diversification, diversity patterns, local extinctions, and faunal turnovers (e.g. [26][27][28]). Phocids nowadays found along the eastern coast of the North Pacific include the phocine Phoca vitulina, and the monachine Mirounga angustirostris, and in the central North Pacific the monachine Neomonachus schauinslandi (Hawaiian monk seals).…”

Insights on the Dental Evolution of Walruses Based on New Fossil Specimens from California

“…Walruses (Odobenidae Allen, 1880) are represented by a single extant species (Odobenus rosmarus [Linnaeus, 1758]) that is restricted to the Arctic and has large, iconic, upper canine tusks. In contrast, the Miocene fossil history of odobenids includes coeval subarctic, tuskless lineages (reviewed in Magallanes et al, 2018). Odobenids can be divided into two groups that overlapped in time during the late Tortonian and early Messinian (∼9-6 Ma), the paraphyletic 'basal odobenids' (∼17-6 Ma) leading to the clade Neodobenia Magallanes, Parham, Santos, and Velez-Juarbe, 2018 (∼9 Ma to Recent).…”

“…Neodobenians have a more specialized dentition with reduced, peg-like teeth and tusks. The phylogeny of odobenids is driven by characters that are related to dental specialization through time, resulting in a phylogenetic tree with a pectinate topology (Magallanes et al, 2018).…”

“…During the late Miocene, the coexistence of tuskless basal odobenids and tusked neodobenians resulted in rich odobenid assemblages, with some formations in California boasting as many as four or five species of odobenid (Boessenecker, 2017;Magallanes et al, 2018;Powell et al, 2019). Most of these records are not known from enough material to warrant the description of new species, and so just three species of basal odobenids are currently named from the late Miocene (Imagotaria downsi, Mitchell, 1968; Pontolis magnus [True, 1905a]; Titanotaria orangensis Magallanes, Parham, Santos, and Velez-Juarbe, 2018). The 12 specimens described here include specimens referred to I. downsi and cf.…”