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.
Highlights
The modern walrus, Odobenus rosmarus (Linnaeus, 1758), is an iconic Arctic species and the last surviving member of a lineage that first appeared in the middle Miocene (∼16 Ma)
We describe a new specimen of tuskless odobenid (OCPC 11141) from the upper Miocene Oso Member of the Capistrano Formation from Lake Forest, Orange County, California (Fig. 2)
Titanotaria orangensis, Pelagiarctos sp., and I. downsi represent a grade of phenetically similar taxa at the base of the clade that includes P. magnus with Dusignathus, G. pugnax, and Odobeninae
Summary
The modern walrus, Odobenus rosmarus (Linnaeus, 1758), is an iconic Arctic species and the last surviving member of a lineage that first appeared in the middle Miocene (∼16 Ma). For much of their evolutionary history, odobenids lacked the characteristic tusks, molluscivory, and Arctic distribution of O. rosmarus, but instead were much more diverse, widespread, and ecologically varied (Fig. 1). Multiple species of odobenid can be found in the same formation (Barnes, 1988; Deméré, 1994a; Tanaka & Kohno, 2015; Velez-Juarbe, in press) including even the coexistence of tusked and tuskless forms (Boessenecker, Perry & Schmitt, 2014: table 1)
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