A new basal odontocete from the upper Rupelian of South Carolina, U.S.A., with contributions to the systematics of Xenorophus and Mirocetus (Mammalia, Cetacea)

Abstract

ABSTRACT—We describe the odontocete, Ashleycetus planicapitis, gen. et sp. nov., based on a partial skull that was collected from the upper Rupelian (lower Oligocene) Ashley Formation near Charleston, South Carolina, and place it in the Ashleycetidae, fam. nov. Overall, the cranial morphology of this new species is plesiomorphic; it has an elongate, tabular intertemporal region, external nares well anterior to the antorbital notches, and anteromedially oriented lateral margins of the supraorbital processes. Xenorophus sloanii and Xenorophidae are rediagnosed, and the latter is also redefined as an apomorphy-based clade. This new definition should lead to greater stability because it excludes Archaeodelphis patrius, whose phylogenetic relationships have proven to be problematic. The holotype skull of the archaic odontocete Mirocetus riabinini, from probable upper Rupelian sediments of the lower Maikop beds of Azerbaijan, is redescribed, newly figured, and placed in the Mirocetidae, fam. nov. Previously regarded as an archaeocete, Mirocetus is here shown to be an odontocete. We added Ashleycetus, Albertocetus, and Mirocetus to a recently published supermatrix of morphological and molecular data. Although Ashleycetus, Archaeodelphis, and Mirocetus consistently occupy basal branch(es) within Odontoceti, their exact positions are sensitive to the value of a constant used in implied weighting for cladistics analysis. Three of the characters we find to be odontocete synapomorphies are closely associated with soft tissue structures implicated in the production and transmission of high-frequency sounds during echolocation. Detailed dissections of extant odontocetes are required to determine if these features can be considered evidence of echolocation in extinct taxa.

http://zoobank.org/urn:lsid:zoobank.org:pub:21FEC587-D1C1-4968-814B-D66B00323FF7

SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP

ACKNOWLEDGMENTS

Our primary thanks must go to J. Malcom for collecting the holotype skull of Ashleycetus planicapitis for The Charleston Museum and adding this important new taxon to our knowledge of archaic odontocetes. We also thank L. Edwards (U.S. Geological Survey [USGS]) for examining samples of the Ashley Formation matrix of the Ashleycetus holotype and R. E. Weems (USGS, retired) for valuable information regarding the geological features of the area in which that specimen was collected. The first author is grateful to the director and staff of the Central Geological Research Museum in St. Petersburg, Russia, for permitting him to examine the holotype material of Mirocetus riabinini in 1976, and we express special thanks to A. Sokolov of that institution for his excellent digital photographs of the anterior limbs of Mirocetus, one of which is shown in Figure 9. We are also grateful to Dr. A. Alizadeh and others at the Geological Institute of the Azerbaijani National Academy of Sciences, Baku, Azerbaijan, for crucial information about the region in which the holotype of Mirocetus was collected. The photographs for Figures 3, 4, and 5 were made by Bryan Stone, whose fine work is greatly appreciated. We are especially grateful to R. Patterson for drawing the reconstructions of Mirocetus and Ashleycetus in Figures 6, 10, and 11 and thank S. Money (The Charleston Museum) for making the photographs in Figures 10 and 11. The late I. Dubrovo (Paleontological Institute, Russian Academy of Sciences, Moscow) graciously provided the photographs utilized in Figures 6, 7, and 8. We also thank L. G. Barnes and S. A. McLeod for providing a cast of Archaeodelphis, and we appreciate the efforts of the Willi Hennig Society in making the computer application TNT widely available. We thank J. McCormick (The Charleston Museum) for access to the holotype of Ashleycetus during revisions of the manuscript. For their helpful reviews of the manuscript, we thank O. Lambert, M. Uhen, and one anonymous reviewer. Comments by Associate Editor E. Fitzgerald also greatly improved this paper.

Handling editor: Erich Fitzgerald.