Gli squali fossili
Cretoxyrhina is a Cenomanian to lower Campanian genus present in the fossil faunas of Africa, Europe and the Americas. In North America, it is represented by Cretoxyrhina mantelli (AGASSIZ 1843), common in Upper Cretaceous sediments of the Western Interior Seaway.
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.
In Kansas, Cretoxyrhina mantelli
is found in the Carlile Shale (middle Turonian), Fort Hays Limestone (lower
through middle Coniacian) and Smoky Hill Chalk (upper Coniacian through
lower Campanian). C. mantelli is the most common shark from the family
Cretoxyrhinidae in the Smoky Hill chalk. It is found from the base of the
chalk upwards, but becomes extinct during early Campanian time. Cretoxyrhina
teeth have not been found in the last few meters of this formation. Studies
have shown a general increase in size in Cretoxyrhina teeth through time,
but fieldwork by the author indicates that the species reaches its largest
size by the end of the Coniacian. In the Smoky Hill chalk, the largest
teeth have crown heights in the 52-54 mm range. Welton and Farish (1993)
cite a 63 mm Cretoxyrhina tooth from gravels derived from the Coniacian
/ Santonian Austin group in Texas. The Smoky Hill chalk of Kansas has been
the source of several well preserved and nearly intact shark "mummies",
with complete dentitions, dermal scales, vertebral columns, stomach contents,
and fossilized cartilage. This shark probably reached lengths of 6 meters
or more in the Western Interior Seaway during the late Cretaceous.
GENUS SPECIFIC REFERENCES
Shimada, Kenshu, 1997. Stratigraphic Record of the Late Cretaceous Lamniform Shark, Cretoxyrhina mantelli (Agassiz), in Kansas, Trans. Kansas Academy of Science, 100(3-4), pp 139-149
Shimada, Kenshu, 1997. Paleoecological Relationships of the Late Cretaceous Lamniform Shark, Cretoxyrhina mantelli (Agassiz), J. Paleontology, 71(5), pp. 926-933
Shimada, Kenshu, 1997. Dentition of the Late Cretaceous Lamniform Shark Cretoxyrhina mantelli, from the Niobrara chalk of Kansas, Journal of Vertebrate Paleontology, 17(2):269-279.
Shimada, Kenshu, 1997. Periodic Marker Bands in Vertebral Centra of the Lamniform Shark, Cretoxyrhina mantelli, Copea, pp 233-235
Shimada, Kenshu, 1997. Gigantic Lamnoid Shark Vertebra from the Lower Cretaceous Kiowa Shale of Kansas, J. Paleontology, 7(13), pp 522-524
Siverson, Mikael, 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, Mikael, 1996. Lamniform Sharks of the Mid Cretaceous Alinga Formation and Beedagong Claystone, Western Australia, Paleontology, Vol. 39, Part 4, pp 813-849
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