Wednesday, November 27, 2019

[PaleoIchthyology • 2019] Cretodus houghtonorum • A New Large Late Cretaceous Lamniform Shark from North America, with Comments on the Taxonomy, Paleoecology, and Evolution of the Genus Cretodus

  Cretodus houghtonorum
Shimada & Everhart, 2019

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, Cretodus 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.


Class CHONDRICHTHYES Huxley, 1880
Subclass ELASMOBRANCHII Bonaparte, 1838

Cohort EUSELACHII Hay, 1902
Subcohort NEOSELACHII Compagno, 1977

Order LAMNIFORMES Berg, 1958


Included Genera— Cretodus Sokolov, 1965; Eoptolamna, Kriwet, Klug, Canudo, and Cuenca-Bescos, 2008a; Leptostyrax Williston, 1900; Protolamna Cappetta, 1980; Pseudoscapanorhynchus Herman, 1977.

Genus CRETODUS Sokolov, 1965

Included Species— Cretodus crassidens (Dixon, 1850); C. gigantea (Case, 2001); C. houghtonorum, sp. nov. (this study); C. longiplicatus Werner, 1989; C. semiplicatus (Agassiz, 1843).


    Lamna semiplicata Agassiz, 1843: Cappetta, 1973:506, figs. 3.7, 3.7’.
    Odontaspis macrota Agassiz, 1843: Edward, 1976:67, fig. 1d–h.
    Cretodus semiplicatus (Agassiz, 1843): Wolberg, 1985a:10, figs. 3.10–3.21; Wolberg, 1985b:4, fig. 3f, g, j.
    Cretodus crassidens (Dixon, 1850): Welton and Farish, 1993:98, 1a, b, 2, two leftmost teeth on row ‘a’ and third tooth from the left on row ‘b’ on p. 99 (others on pp. 98–99 questionable).
    Cretodus semiplicatus (Agassiz, 1843): Williamson et al., 1993:figs. 5.6–5.8.
    Cretodus crassidens (Dixon, 1850): Cappetta and Case, 1999:21, fig. 2a–h; Cicimurri, 2004a:9–10, fig. 5e; Shimada, 2006b:166, figs. 3b, 4d; Becker et al., 2010:255, 257, figs. 5.6–5.9, 6.1?
    Cretodus sp.: Shimada et al., 2010a:fig. 4c.
    Cretodus crassidens: Bice and Shimada, 2016:177, fig. 3h (not 3g); Ouroumova et al., 2016: fig. 2l.

Diagnosis— Cretodus differing from all other known species of the genus by having teeth with moderately wide principal cusp, small lateral cusplets relative to principal cusp in tall, erect teeth, ‘V’-shaped crown base, relatively narrow tooth neck on lingual face, ‘V’-shaped basal root concavity, and short root lobes.

Etymology— The species name, houghtonorum, is in honor of Keith and Deborah Houghton, the owners of the land where the Cretodus specimen was found and who kindly donated the specimen to FHSM for the purpose of this present study.

Kenshu Shimada and Michael J. Everhart. 2019. A New Large Late Cretaceous Lamniform Shark from North America, with Comments on the Taxonomy, Paleoecology, and Evolution of the Genus CretodusJournal of Vertebrate Paleontology. e1673399. DOI: 10.1080/02724634.2019.1673399

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