ABSTRACT
We describe a partial skeleton of the Late Cretaceous shark, Cretodus, collected from the Blue Hill Shale (middle Turonian) in north-central Kansas, U.S.A. It consists of 134 disarticulated teeth, 61 vertebrae, 23 placoid scales, and fragments of calcified cartilage. The scale morphology suggests that Cretodus was a rather sluggish shark, and the vertebral morphology affirms its placement into Lamniformes. With a strong tendency towards monognathic heterodonty, the dental morphology indicates that the specimen belongs to a new species, C. houghtonorum, sp. nov., increasing the total known species of Cretodus to five. The five species can be divided into three distinct groups: the longiplicatus/semiplicatus-grade, gigantea/houghtonorum-grade, and crassidens-grade. Cretodus, that successively evolved by broadening the tooth crown. The individual of C. houghtonorum, sp. nov., is estimated to be about 515 cm in total length (TL). Our vertebra-based growth analysis suggests that the shark was about 118 cm TL at birth and that the species had an estimated maximum growth length of 684 cm TL. The large size at birth indicates that the intrauterine cannibalism behavior of embryos seen in extant lamniforms had already evolved by the Late Cretaceous. Where C. houghtonorum, sp. nov., preferred nearshore environments, the specimen co-occurred with isolated teeth of Squalicorax and fragments of two dorsal fin spines of a hybodont shark, circumstantially indicating that the individual of Cretodus fed on the much smaller hybodont and was scavenged by Squalicorax.
ACKNOWLEDGMENTS
We are indebted to K. and D. Houghton for allowing us access to their land, providing logistical support during the digs, and the donation of the specimen to the Sternberg Museum of Natural History. A local resident, G. Pearson, initially introduced M.J.E. to the locality. The specimen was discovered by another local resident, F. Smith, in conjunction with exploratory work done by M.J.E. in 2010. The assistance of G. Pearson and F. Smith with the excavation is greatly appreciated. We thank especially C. K. Rigsby, A. C. Nicholas, K. Gray, B. Karl, and J. Hickey (Children’s Memorial Hospital, Chicago, Illinois) for assisting us with CT scanning of the vertebra-bearing concretions; E. Bernard and K. Webb (NHMUK) for taking and supplying us the photographs of the holotype of Cretodus crassidens; J. Pollerspöck (www.shark-references.com, Stephansposching, Germany) for providing images of, and additional information about, the holotype of C. semiplicatus; and L. E. Wilson and C. Byrd (FHSM) for curating FHSM VP-17575, VP-19272, VP-19273, and VP-19274 for the purpose of this study. We also thank G. Guinot and M. Newbrey for their comments on our manuscript that significantly improved the quality of this paper. Financial support was made to K.S. by the Department of Environmental Science and Studies and the Department of Biological Sciences of DePaul University.