The teeth of this genus are large, triangular and very similar in appearance to those of the modern mako shark (Isurus oxyrinchus). The crowns are broadly based and moderately high, with a strong distal inclination in all but the most anterior teeth. They have razor-like (non-serrated) mesial and distal cutting edges. A single pair of cusplets may occur in a few teeth, especially those from the lateral and posterior portions of the jaw. The cutting edge of the tooth extends from crown to root. There is a broad lingual dental band present. The roots are strongly bilobate and rounded, with a prominent lingual protuberance in the anterior teeth. A nutrient groove is never present, and the central lingual foramen is small when present.
Dentition Design
Shimada's (1997b) study of the Cretoxyrhina dentition design yielded an odd tooth-set with its multiple symphyseal and intermediate teeth. In May 2006. I had the opportunity to study two associated sets from the Hubbell collection. Not wanting to be influenced by Shimada's conclusions, I spent nine months (on and off) attempting to arrange them based only upon the propensities of modern lamniform sharks.
The final arrangements (Fig. & ) compared remarkably well with Shimada's observations, and he (pers com 2007) noted no apparent errors.
These two arrangements do raise a question, is one species represented or two? The differences between the first Hubbell specimen (broad-form, Fig. ) and Shimada's (1997b: fig. 4) are largely confined to the number of teeth in the "parasymphyseal" and "intermediate" positions; variations of this nature can be seen in the recent sand tiger Odontaspis noronhai. Less explainable are the differences between the broad-form and narrow-form (Fig. ); the latter is smaller, but not enough to suggest an ontogenetic variation. The original locales are relatively close, therefore regional dimorphism is not a good option. Remaining possibilities include individual variations (unlikely), sexual dimorphism (possible) and chrono-variations (Hubbell material not documented to Hattin-level). Based largely upon the root development of the (assumed to be) first lower lateral. but also the sizing of the upper third anterior (Shimada's first intermediate), it strikes me that these tooth-sets might represent two species.
The odd root-design of the fourth lower tooth in the broad-form dentition (also represented in Shimada's reconstruction) is so unusual when compared with the first lower lateral of extant taxa, that it begs three questions: 1) is it positioned properly and if so, 2) could it reflect a tooth originating from what might be a lower intermediate position or 3) could it be a fourth lower anterior that is mis-oriented? Given time, these questions should be answered; but they were neither explained in Shimada (2007b) nor can they be here.
Selected References
Cappetta, H. & Case, G., 1975. Contribution à l'étude des sélaciens du groupe Monmouth (Campanien - Maestrichtian) du New Jersey. Palaeontographica Abteilung A, 151:1-46.
Shimada, K., 1997a. Periodic Marker Bands in Vertebral Centra of the Lamniform Shark, Cretoxyrhina mantelli, Copea, pp 233-235.
Shimada, K., 1997b. Dentition of the Late Cretaceous Lamniform Shark Cretoxyrhina mantelli, from the Niobrara chalk of Kansas, Journal of Vertebrate Paleontology, 17(2):269-279.
Shimada, K., 1997c. Skeletal anatomy of the Late Cretaceous lamniform shark, Cretoxyrhina mantelli, from the Niobrara Chalk in Kansas. Journal of Vertebrate Paleontology 17:642–652.
Shimada, K., 1997d. Gigantic Lamnoid Shark Vertebra from the Lower Cretaceous Kiowa Shale of Kansas, J. Paleontology, 7(13), pp 522-524.
Shimada, K., 1997e. Paleoecological Relationships of the Late Cretaceous Lamniform Shark, Cretoxyrhina mantelli (Agassiz), J. Paleontology, 71(5), pp. 926-933
Shimada, K., 1997f. Stratigraphic Record of the Late Cretaceous Lamniform Shark, Cretoxyrhina mantelli (Agassiz), in Kansas, Trans. Kansas Academy of Science, 100(3-4), pp 139-149
Siverson, M., 1992. Biology, Dental Morphology and Taxonomy of Lamniform Sharks from the Campanian of the Kristianstad Basin, Sweden, Paleontology, Vol. 35, Part 3, pp 519-554.
Siverson, M., 1996. Lamniform Sharks of the Mid Cretaceous Alinga Formation and Beedagong Claystone, Western Australia, Paleontology, Vol. 39, Part 4, pp 813-849
Siverson, M. and Lindgren, J., 2005. Late Cretaceous sharks Cretoxyrhina and Cardabiodon
from Montana, USA, Acta Palaeontologica Polonica 50 (2): pp 301-314.
Welton, B. and Farish, R., 1993. The Collector's Guide to Fossil Sharks and Rays from the Cretaceous of Texas. Before Time, Texas. 204 pp.