Globidens - A Rare, Shell Crushing Mosasaur from the Pierre Shale (Late Cretaceous) of Western Kansas Copyright © 2000-2008 by Mike Everhart Updated 01/31/2008   LEFT: "Globidens feeding on clams" -Copyright © Dan Varner; used with permission of Dan Varner

Latest information on a new and nearly complete Globidens specimen from South Dakota HERE

RECENT NEWS: Jim Martin, South Dakota School of Mines and Technology, has recently described a third species of Globidens from the DeGrey Member of the Pierre Shale in central South Dakota. The new species, Globidens schurmanni nov. sp. (Martin, 2007) is also the most complete (and youngest) known specimen, and includes the skull, most of the vertebral column and partial limbs.

OOO

In early 1995, Pete Bussen, a retired rancher and experienced paleontologist from Wallace, Kansas, was prospecting for fossils in the Sharon Springs member of the Pierre Shale near McAllaster Buttes, Logan County, Kansas. The dark gray shale exposure was about a mile north of the locality where, in 1867, Dr. Theophilus H. Turner found and collected what eventually became E. D. Cope's infamous and professionally embarrassing "head-on-the-wrong-end" Elasmosaurus platyurus (Cope, 1868) plesiosaur.

The land that Pete now walked on had already produced the remains of a large plesiosaur, several mosasaurs and a concretion filled with Baculites. As he moved along the side of a steep gully, Pete noticed a large chunk of bone protruding from the shale. By its general appearance, it appeared to be the lower jaw (dentary) of a mosasaur but something was wrong with the teeth. Instead of being sharp, conical spikes for seizing and tearing flesh, the teeth were round and bulbous. The only mosasaurs that he knew of that had bulbous or globular teeth were a rare and unusual genus called Globidens.   The genus had never been found before in Kansas.

	Plate 39 from Gilmore (1912): Side view of left maxilla of Globidens alabamaensis, 3/4 natural size. Type-specimen, Cat. No. 6527, U. S. N. M.
	Plate 40 from Gilmore (1913): Oblique view of dental border of left maxilla of Globidens alabamaensis, 3/4 natural size. Type specimen, Cat. No. 6527, U. S. N. M.

Although the isolated, round teeth are found in many marine deposits around the world, the first Globidens to be described (G. alabamaensis, Gilmore, 1912) was found in the Mooreville Chalk of Alabama. Another species, Globidens dakotensis (Russell, 1975), was discovered in the Pierre Shale of South Dakota. A third, and very complete Globidens skeleton was found in 1993 in South Dakota but it has not yet been prepared and studied. Pete suddenly realized that he had just found the first evidence that Globidens had lived in the Western Interior Sea that covered Kansas millions of years ago. Looking around, he scanned the area for other evidence but found only a few small bone scraps.

Thinking that the jaw represented a piece of a still buried and hopefully more complete fossil, Pete called me and offered the specimen to him to dig. When they returned to the site later in the Spring, however, the only piece to be found was the tip of the jaw, containing four teeth. After removing several cubic yards of shale to make sure that nothing else remained, I concluded that the rest of the fossil had either eroded out years ago or that the jaw was all that had been preserved at that location.

Drawing by Russell Hawley, © Copyright 2000 by Russell Hawley

When the jaw was cleaned and examined, there were a total of 10 teeth, and empty sockets for four others. The largest of the globular teeth was 3 cm (slightly over an inch) in diameter. The front four teeth differed from the globular back teeth and were shaped more like rounded pegs than marbles. The nearly 14 inch long dentary was the front half of the mosasaur’s lower jaw and represents a total jaw that would have been between 24 and 30 inches in length. Since the lower jaw in mosasaurs approximates the length of the skull, the skull of this Globidens would have been 2 to 2 1/2 feet in length. This, in turn, would have scaled up to an animal that would have been about 20 feet in length.

	A labial view of the right dentary of the Kansas Globidens (FHSM VP_13828). (Scale =10 cm)
	A lingual (inside) view of the right dentary of the Kansas Globidens. (Scale = 10 cm)
	A close-up of the anterior portion of the Kansas Globidens dentary showing the differences in shape between the anterior, peg-like teeth and the more posterior ball-like teeth.

A review of the literature on Globidens showed that the specimen from Alabama is considered to be the oldest. The first specimen from South Dakota (G. dakotensis) came from approximately the same stratigraphic level in the Pierre Shale as the Kansas specimen. Another, undescribed specimen from Texas probably falls between G. alabamaensis and G. dakotensis in age. The latest specimen from South Dakota is apparently the most recent. On the basis of a brief field examination of that material, Jim Martin at the South Dakota School of Mines (personal communication, 1995) has determined that the specimen is probably not G. dakotensis. Globidens skulls appear to be adapted to stand the stresses of a crushing-type bite and the latest Globidens from South Dakota is even more robust than earlier species.

The Kansas Globidens was found in the Sharon Springs member of the Pierre Shale (Middle Campanian), just below the contact with the Weskan member. From the stratigraphic evidence, it now appears that this specimen is approximately the same age as Globidens dakotensis. Unfortunately, because the type specimen of G. dakotensis does not have a lower jaw, the lower jaw from Kansas cannot be positively assigned to that or any other species.

Since Kansas was located under the Western Interior Sea between Alabama and Texas on the south and South Dakota to the North, it seems likely that Globidens would have been found here also. However, even with more than 100 years of fossil collecting in Western Kansas by such famous paleontologists as Dr. G. M. Sternberg, E. D. Cope, O. C. Marsh, Charles Sternberg and the many others that followed, no Globidens remains had been reported prior to 1996.

The globular teeth of Globidens are so radically different from the normal teeth associated with mosasaurs that it seems evident that this genus had a completely different life style and food source. A twenty foot long mosasaur weighing a thousand pounds or more could certainly eat whatever it wanted, and even round teeth are capable of killing smaller prey. It is most likely, however, given the adaptations that are apparent in the teeth and skull, that Globidens fed on shellfish on the sea bottom. It is likely that mosasaurs such as Prognathodon with their large, conical teeth had pursued shelled prey, such as ammonites or even turtles, for millions of years. Globidens may have been an extreme adaptation to that end.

In October, 1997, I had the opportunity to visit The Field Museum in Chicago during the Society of Vertebrate Paleontology meeting. While there, I was allowed the opportunity (Thanks, Bill!) to examine and photograph the type specimen of Globidens dakotensis that had been described by Dale Russell. The two pictures below show the underside of the skull and a comparison of the right dentary of the Kansas specimen with the maxillary teeth of G. dakotensis. While not scientifically significant at this point, I did make the observation that although the jaw length of both specimens is about the same, the teeth of G. dakotensis are about 50% larger in size. I would guess (with no way of telling for sure) that the Kansas specimen is from a slightly smaller individual compared to the type specimen.

	A ventral view of of the skull of type specimen of Globidens dakotensis in the Field Museum, Chicago, IL.
	A close-up showing the teeth of the right dentary of the Kansas Globidens sp. ((FHSM VP-13828), above) compared with the teeth on the premaxillary and right maxillary of the type specimen of Globidens dakotensis. Martin (2007) indicated that he believes the Kansas specimen should be identified as Globidens dakotensis.

References:

Everhart, M. J. and P. A. Everhart. 1996. First report of the shell crushing mosasaur, Globidens sp., from the Sharon Springs Member of the Pierre Shale (upper Cretaceous) of Western Kansas. Kansas Academy of Science, Transactions 15(Abstracts): 17. (Only Kansas specimen of Globidens)

Gilmore, Charles W. 1912. A new mosasauroid reptile from the Cretaceous of Alabama, Proceedings U.S. National Museum, 40(1870): 489-484, 3 fig., pl. 39-40  (Description of Globidens alabamaensis type)

Lingham-Soliar, T. 1999. The durophagous mosasaurs (Lepidosauromorpha, Squamata) Globidens and Carinodens from the Upper Cretaceous of Belgium and the Netherlands. Paleontological Journal. 33(6):638-647, translated from the Russian, Paleontologicheski Zhurnal 6:34-43.

Martin, J. E. and J. E. Fox. 2004. Molluscs in the stomach contents of Globidens, a shell crushing mosasaur, from the Late Cretaceous Pierre Shale, Big Bend area of the Missouri River, central South Dakota. Geological Society of America, 2004 Rocky Mountain and Cordilleran Regions Joint Meeting, Abstracts with Programs, 36(4):80.

Martin, J. E. 2007. A new species of the durophagous mosasaur, Globidens (Squamata: Mosasauridae) from the Late Cretaceous Pierre Shale Group of central South Dakota, USA. Pages 167-176 in Martin, J. E. and Parris D. C. (eds.), The Geology and Paleontology of the Late Cretaceous Marine Deposits of the Dakotas. Geological Society of America, Special Paper 427.

Martin, J. E. and Fox, J. E. 2007. Stomach contents of Globidens, a shell-crushing mosasaur (Squamata), from the Late Cretaceous Pierre Shale Group, Big Bend area of the Missouri River, central South Dakota. Pages 167-176 in Martin, J. E. and Parris D. C. (eds.), The Geology and Paleontology of the Late Cretaceous Marine Deposits of the Dakotas. Geological Society of America, Special Paper 427.

Russell, Dale A. 1975. A new species of Globidens from South Dakota. Fieldiana Geology, 33(13): 235-256. (Field Museum of Natural History)

Schulp, A.S. 2005. Feeding the mechanical mosasaur: what did Carinodens eat? Netherlands Journal of Geosciences / Geologie en Mijnbouw, 84(3), p. 345-358.