A new clade of putative plankton-feeding sharks from the Upper Cretaceous of Russia and the United States

ABSTRACT

Eorhincodon casei from Russia and Megachasma comanchensis from the United States are two Cretaceous taxa initially described as putative planktivorous elasmobranchs, but the type specimens of these two taxa were subsequently reinterpreted to represent taphonomically abraded teeth of an odontaspidid, Johnlongia Siverson (Lamniformes: Odontaspididae). Here, we redescribe the type materials of ‘E. casei’ and ‘M. comanchensis’ and describe additional specimens of these species from other Late Cretaceous localities in Russia and the United States. These specimens demonstrate that (1) the two fossil taxa are valid species; (2) they warrant the establishment of a new genus of presumed planktivorous sharks, Pseudomegachasma, gen. nov., to accommodate the two species; and (3) the new genus is sister to Johnlongia and together constitute a new subfamily Johnlonginae, subfam. nov., tentatively placed in the family Odontaspididae sensu stricto. This taxonomic placement indicates that the putative planktivorous clade was derived from a presumed piscivorous form (Johnlongia), with an implication that Pseudomegachasma, gen. nov., evolved a plankton-eating habit independent of the four known planktivorous elasmobranch clades (Rhincodontidae, Megachasmidae, Cetorhinidae, and Mobulidae). It also indicates that planktivorous diets evolved independently at least three times in the order Lamniformes (i.e., Megachasmidae, Cetorhinidae, and Odontaspididae), and more significantly, Pseudomegachasma, gen. nov., would represent the oldest known plankton-feeding elasmobranch in the fossil record. The present fossil record suggests that Pseudomegachasma, gen. nov., evolved in a relatively shallow-water environment in Russia in the early Cenomanian or earlier and subsequently migrated to the North American Western Interior Seaway by the mid-Cenomanian.

http://zoobank.org/urn:lsid:zoobank.org:pub:D5D0400FD438-4A95-8301-DD47991572F6

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

ACKNOWLEDGMENTS

We thank the late J. H. McLellan who collected NMMNH P-67453 in 1985, S. G. Lucas and J. Spielman (NMMNH) who curated the specimen for the purpose of this study, and R. W. Boessenecker for providing us with useful information. We also thank A. V. Lapkin (Saratov-Moscow) for kindly donating specimens SSU 155/86 and 155/88 and other relevant samples to the SSU collection, E. M. Pervushov (SSU) for stratigraphic discussion of Russian localities, A. V. Birukov, F. K. Timirchev (SSU), and SSU geology students for field assistance at the Melovatka-5 locality during 2011–2012, and A. O. Averianov (ZIN) for providing access to Albian–Cenomanian shark materials from the Belgorod Province (Lebedinskij and Stoilenskij quarries), including the collection of the late L. Nessov. The second author's (E.V.P.) portion of this study was supported by the Russian Fund for Basic Researches (RFBR grant 14-05-00828) and by the SSU (internal fund granted by vice-rector A. V. Stalmakhov). We thank G. Guinot, an anonymous reviewer, and the editors for their constructive comments on the drafts of the manuscript, which significantly improved this paper.

Handling editor: Charlie Underwood.

Citation for this article: Shimada, K., E. V. Popov, M. Siversson, B. J. Welton, and D. J. Long. 2015. A new clade of putative plankton-feeding sharks from the Upper Cretaceous of Russia and the United States. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2015.981335.