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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 08/28/2012

Copyright 2008-2012 by Mike Everhart

LEFT: A recently born Tylosaurus is attacked and killed by a giant Ginsu shark (Cretoxyrhina mantelli)

From Williston, 1904: "If the mosasaurs were exclusively marine animals, it would seem almost certain that they were not viviparous, as were the ichthyosaurs, and probably also the plesiosaurs, and as are some modern lizards. As Fraas has remarked concerning the European ichthyosaurs, if one searches carefully, he will find in many Kansas specimens the remains of the skin and stomach contents, but never has there been found anything which has the faintest suggestion of mosasaur embryos. No aquatic reptiles of the present time lay their eggs in the water. The sea snakes are viviparous, or at least all available information concerning them gives viviparous habits. The sea turtles and the crocodiles lay their eggs upon the beaches, the latter guarding their nests and young. Doubtless the crocodiles of the past had the same beach-laying habits, suggesting that they were never inhabitants of the open oceans. The mosasaurs must have been practically helpless upon land; still it is not impossible that they may have frequented the beaches for the deposition of their eggs, though it is highly improbable that they gave any attention or care to either their eggs or their young. That the eggs of the mosasaurs were more numerous than are those of the terrestrial lizards of the present time is not to be supposed. The waters in which the mosasaurs flourished swarmed with highly predaceous fishes, sharks, and plesiosaurs, to say nothing of the hordes of their own kind; and, unless the eggs were very numerous, or unless the young were jealously guarded by the parent. the young reptiles must have stood very little chance in the fierce struggle for existence.  

It is, of course, possible that the shallow waters of the bays and estuaries may have afforded sufficient protection for the young mosasaurs, but this is doubtful, in the entire absence of all remains of such animals in marine deposits. It seems more probable that the mosasaurs were brought forth, perhaps alive,  in fresh water, that the females ascended the rivers to breed, and that the young remained in such protected places until fairly able to care for themselves. That the mosasaurs, as also the aigialosaurs, were in part denizens of fresh water may be, perhaps, one reason for the great relative abundance of their remains in the deposits of inland or protected seas. Their occurrence in Kansas associated with great numbers of small turtles. pterodactyls. and birds would seem to be fairly good evidence that they were more littoral than pelagic in their habits."


            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.

MosaSizesa.jpg (17670 bytes) 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.

Tylo_T-rexa.jpg (15065 bytes)
babymoa.jpg (15242 bytes) 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!

MosaBirth.jpg (45313 bytes) 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.  


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.

BabyMosasaursa.jpg (58685 bytes)

Most of the evidence we have regarding young mosasaurs comes from the remains of meals by other predators.  In Kansas, that meant large sharks, like Cretoxyrhina mantelli. During the Late Cretaceous  (Coniacian-early Campanian) a Great white sized shark (7-8 m), Cretoxyrhina mantelli, was a formidable predator on many kinds of marine creatures, including mosasaurs. It had razor-sharp teeth and strong jaws capable of literally biting through bone…Most of what we know about young mosasaurs in the Smoky Hill Chalk of Kansas is the results of predation / scavenging by sharks. Note that a modern 4.5 m (16 ft) Great white shark would weigh about 1500 kg (3200 lb) compared to about 185 kg (400 lb) for a mosasaur of the same length!
FHSM VP-14845a.jpg (15647 bytes) 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

FHSM VP-2220a.jpg (14258 bytes) LEFT: FHSM VP-2220 – Partially digested Clidastes cf. propython premaxilla from Logan County (lower Campanian). Click here for another view
KUVP 5012a.jpg (14278 bytes) 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.  
FHSM VP-14846-7A.jpg (16813 bytes) 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.
FHSM VP-14848a.jpg (16158 bytes) 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
FHSM VP-6392-1a.jpg (19349 bytes) 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...

FHSM VP-13748a.jpg (23100 bytes) 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-17206a.jpg (23163 bytes) FHSM VP-17206 - Partially digested juvenile Tylosaurus cf. kansasensis premaxilla; Collected between MU 1 and 2, (upper Coniacian), south-western Trego County, Kansas.
UWM1501_2a.jpg (23358 bytes) 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)
UWM1501-24a.jpg (14322 bytes) LEFT: UW-M 1501-24 – Remains of a juvenile Clidastes cf. propython, Logan County, Kansas
UWM1501-56a.jpg (17086 bytes) LEFT: UW-M 1501-56 - Skull remains of a juvenile Tylosaurus proriger  - Upper chalk, Logan County, Kansas
KUVP 85515a.jpg (29284 bytes) 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.

MosaVert1a.jpg (14899 bytes)
MosaTail1a.jpg (30081 bytes) 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).

MosaTail2a.jpg (24747 bytes)
KUVP 1157a.jpg (31398 bytes) LEFT: KUVP 1157 – Left dentary from a small Platecarpus.


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 Sheldon, M.A. 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.,  Sheldon, M.A., Lamb, J.P. and Martin, J.E. 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 Sheldon, M.A. 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.

Houssaye, A. and Tafforeau, P. 2012.What vertebral microanatomy reveals about the ecology of juvenile mosasaurs (Reptilia, Squamata). Journal of Vertebrate Paleontology 32(5):1042-1048.

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.

Williston, S.W. 1904. The relationships and habits of the mosasaurs. Journal of Geology 12(4):43-51.

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