REMAINS OF YOUNG MOSASAURS FROM THE SMOKY HILL CHALK (UPPER CONIACIAN-LOWER CAMPANIAN) OF WESTERN KANSAS Update of a Presentation at the Second Mosasaur Meeting Sternberg Museum of Natural History, Hays, Kansas May 3-6, 2007 Page created 10/25/2008; Last updated 05/06/2010 Copyright © 2008-2010 by Mike Everhart LEFT: A recently born Tylosaurus is attacked and killed by a giant Ginsu shark (Cretoxyrhina mantelli)

ABSTRACT:  REMAINS OF YOUNG MOSASAURS FROM THE SMOKY HILL CHALK (UPPER CONIACIAN-LOWER CAMPANIAN) OF WESTERN KANSAS

            EVERHART, Michael J., Sternberg Museum of Natural History, Fort Hays State                    University, Hays, Kansas, 67601, USA.

The remains of young mosasaurs, especially those that could be considered to be newly born, are poorly represented in the fossil record. This lack of specimens led some earlier workers to suggest that mosasaur babies were born or hatched in other localities such as distant shores, or along the banks of rivers or estuaries. However, no such birthing area or concentration of the remains of young mosasaurs has ever been located. On the other hand, specimens of immature mosasaurs have been found within the Smoky Hill Chalk of western Kansas. The chalk was deposited on the eastern shelf of the Western Interior Sea during late Coniacian through Early Campanian time, hundreds of kilometers from the nearest land. Most of the specimens recovered to date are the isolated premaxillae of small tylosaurines, arguably the most resistant part of the skull of a mosasaur at any age. Most also appear to be the remains of a larger predator’s meal because of the digested appearance of the bone and the fact that the teeth have been dissolved into their alveoli. Measurement of the specimens and comparisons to complete skulls of tylosaurines for scaling purposes indicates that the total skull length of these individuals was 18-25 cm. Assuming that the skull to body length proportions (14%) remained approximately equal through life in mosasaurs, the remains represent young tylosaurines that were 1.3 to 1.8 m in length at the time of death. Some specimens (KUVP 5012 and FHSM VP-14845) include additional skull fragments and one specimen (VP-14846 and VP-14847) includes the partially digested and commingled skull fragments of two small plioplatecarpines. Another tylosaurine specimen (FHSM VP-14848) represents a 2 m long individual that apparently reached the sea bottom relatively intact.

LEFT: We know that mosasaurs became the apex predators of the Earth’s oceans during the Late Cretaceous. Tylosaurines were among the largest of all mosasaurs, with Hainosaurus reaching lengths of 15+ meters by the Maastrichtian.   RIGHT: A comparison of the skeletons of a Tyrannosaurus rex and a Tylosaurus proriger ... both are 15 m (46 ft long).... That was about the maximum size of a T. rex, but mosasaurs (e.g. Hainosaurus and Mosasaurus) grew even larger.... If you think that encountering a fully grown T. rex would have been scary, you certainly would not want to go swimming in the oceans of the Cretaceous.

Even though mosasaurs grew to great lengths, their bodies were long and slender, so they were not nearly as heavy as an elasmosaur (barrel-shaped body) of the same length, and even less so compared to a modern whale (lots of blubber to stay warm): A 14 m Gray Whale (Eshrichtius robustus) weighs about 32,000 kg (35 tons); A 14 m Elasmosaurus platyurus would have weighed about 10,000 kg (11 tons); A 14 m Tylosaurus proriger would have weighed about 4000 kg (4.5 tons) LEFT: Perhaps the real question should be not how large mosasaurs grew, but how small they were at birth and how this affected their survival far from shore near the middle of the Western Interior Sea where the Smoky Hill Chalk was deposited over Kansas!

LEFT: A Plioplatecarpus specimen discovered in South Dakota showed conclusively that mosasaurs gave live birth, but we do not know how many babies or how large the babies were when born.   REFERENCES: Bell, G. L. Jr. and A. M. Sheldon. 2004. A gravid mosasaur (Plioplatecarpus) from South Dakota. Abstract book and field guide of the First Mosasaur Meeting, Schulp, A. S. and John W. M. Jagt (eds.), Natuurhistorisch Museum Maastricht, the Netherlands pp. 16 (Abstract).   Bell, G. L. Jr., M. A. Sheldon, J. P. Lamb and J. E. Martin, 1996. The first direct evidence of live birth in Mosasauridae (Squamata): Exceptional preservation in Cretaceous Pierre Shale of South Dakota. Journal of Vertebrate Paleontology 16(suppl. to 3):21A.   In modern reptiles, the young are typically about 15% of the mother’s length.   Go here for a chart of mosasaur length to mass measurements   Go here for a graphic showing how much heavier mosasaurs became as they grew longer.

	LEFT: FHSM VP-14845: Partially digested remains (skull only) of a very young (neonate?) mosasaur from Gove County (early Santonian);   Note the incomplete development of the premaxilla (upper left, ventral view). Based on the nearly terminal tooth positions on the premaxilla and the tip of the left dentary (far left, third row down; medial view), it is likely that these remains are those of a baby Platecarpus. It is also possible that these remains are the result of scavenging on the unborn fetus of a pregnant mosasaur... Click here for a closer view of the ventral side of the premaxilla
	LEFT: FHSM VP-2220 – Partially digested Clidastes cf. propython premaxilla from Logan County (lower Campanian). Click here for another view
	LEFT: KUVP 5012 - Partially digested premaxilla from the skull of a small tylosaurine. The rest of the specimen is HERE. Based on the differences in the appearance of some of the bones, these remains may be part of a “mixed” specimen, two or more young mosasaurs eaten by the same predator.
	LEFT: FHSM VP-14846 and VP-14847. This specimen consists of a mixture of cranial material and represents the partially digested remains of two co-mingled cf. Platecarpus juveniles.
	LEFT: FHSM VP-14848: Some of the vertebrae from the remains of a small (< 2m) Tylosaurus kansasensis (upper Coniacian). Although the mosasaur was possibly scavenged (no limb material presents), the bones that were found had not been consumed (partially digested). Here are photos of the very small quadrate.;   A cervical vertebra, and; Two dorsal vertebrae
	LEFT: FHSM VP-6392 - Nine small caudal vertebrae measuring just over 15 cm... or less than 2 cm (3/4 inch) each.... Lower Smoky Hill Chalk, Trego County, KS.  Note the fused chevron bones on the ventral side of vertebrae. This is probably a portion of the tail of a small Clidastes, cf. C. liodontus. Click here for closer views of the last vertebra in this series...
	LEFT: FHSM VP-13748 - Severed, partially digested anterior premaxilla and maxillae in dorsal (left) and ventral view from a juvenile cf. Platecarpus (lower Santonian) – Gove County, Kansas.
	FHSM VP-17206 - Partially digested juvenile Tylosaurus cf. kansasensis premaxilla; Collected between MU 1 and 2, (upper Coniacian), south-western Trego County, Kansas.
	LEFT: UW-M 1501-2 - Nearly complete, articulated skull of a juvenile Platecarpus ictericus in left lateral view… upper chalk, Logan County, KS. Lower jaw length 28 cm. (Estimated body length = < 3 m)
	LEFT: UW-M 1501-24 – Remains of a juvenile Clidastes cf. propython, Logan County, Kansas
	LEFT: UW-M 1501-56 - Skull remains of a juvenile Tylosaurus proriger  - Upper chalk, Logan County, Kansas

LEFT: KUVP 88515: Partially digested vertebrae from a small mosasaur RIGHT: A dorsal vertebra from a small mosasaur in the upper Smoky Hill Chalk, Logan County (Early Campanian age). This vertebra is about 2 cm (about 3/4 inch) in length.

LEFT: The terminal end of the tail of a juvenile mosasaur (cf. Platecarpus). This 32 cm section (26 vertebrae) represents about 7% of the total length of the mosasaur (TL = 4.2 m).  Note the fusion of three of the caudal vertebrae resulting from a bone infection, probably caused by a bite. RIGHT: Close up of the last dozen vertebrae (about 4 inches).

LEFT: KUVP 1157 – Left dentary from a small Platecarpus.

REFERENCES:

Bell G.L., Jr. and Barnes, K.R. 2007. First record of stomach contents in Tylosaurus nepaeolicus and comments on predation among Mosasauridae. Second Mosasaur Meeting Abstract Booklet and Field Guide, Sternberg Museum of Natural History, Hays Kansas, pp. 9-10.

Bell, G. L. Jr., and M. A. Sheldon. 1986. Description of a very young mosasaur from Greene County, Alabama, Journal of the Alabama Academy of Science, Vol. 57, No. 2, pages 76-82.

Bell, G. L. Jr., M. A. Sheldon, J. P. Lamb and J. E. Martin, 1996. The first direct evidence of live birth in Mosasauridae (Squamata): Exceptional preservation in Cretaceous Pierre Shale of South Dakota. Journal of Vertebrate Paleontology 16(suppl. to 3):21A.

Bell, G. L. Jr. and A. M. Sheldon. 2004. A gravid mosasaur (Plioplatecarpus) from South Dakota. Abstract book and field guide of the First Mosasaur Meeting, Schulp, A. S. and John W. M. Jagt (eds.), Natuurhistorisch Museum Maastricht, the Netherlands, pp. 16 (Abstract).

Everhart, M. J. 1999. Evidence of feeding on mosasaurs by the late Cretaceous lamniform shark, Cretoxyrhina mantelli. Journal of Vertebrate Paleontology 17(suppl. to 3):43A-44A.

Everhart, M. J. 2002. Remains of immature mosasaurs (Squamata; Mosasauridae) from the Niobrara Chalk (Late Cretaceous) argue against nearshore nurseries. Journal of Vertebrate Paleontology 22(Supplement to 3): 52A.

Russell, D. A. 1967. Systematics and morphology of American mosasaurs. Peabody Museum of Natural History, Yale University, Bulletin 23.

Sheldon, M. A. 1990. Immature mosasaurs from the Niobrara: a sampling problem? Journal of Vertebrate Paleontology Abstract 10(Supplement to 3): 42A.