Globidens dakotensis - A Rare, Shell Crushing Mosasaur from the Pierre Shale (Late Cretaceous) of Western Kansas

	Plate 39 from Gilmore (1912): Lateral view of the left maxilla of Globidens alabamaensis (Type specimen) in the United States National Museum (USNM 6527).
	Plate 40 from Gilmore (1912): Oblique, medial view of dental border of the left maxilla of Globidens alabamaensis (USNM  6527.

Although the isolated, round teeth had been 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 (including stomach contents) was found in 1993 in South Dakota but has only recently been prepared and described. LEFT: Map of North America during the Late Cretaceous (Campanian) showing approximate localities of Globidens specimens: 1) Holotype of Globidens alabamaensis, Mooreville Chalk, central Alabama; 2) Holotype of G. dakotensis, Sharon Springs Member, Pierre Shale, western South Dakota; 3) SDSMT specimen (Globidens schurmanni nov. sp.) from the DeGrey Member of the Pierre Shale, central South Dakota; 4) FHSM VP-13828, Sharon Springs Member, Pierre Shale, western Kansas; 5) Undescribed Globidens material from the Ozan Formation, east Texas. (Base map adapted from Carpenter, 1990).

Pete Bussen 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 me the specimen to dig. When we returned to the site later in the spring, however, the only piece to be found was the distal end of the lower jaw, containing four peg-like teeth. After I removed 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 three 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. It came from a 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.

LEFT: The right dentary of the Kansas specimen of Globidens dakotensis (FHSM VP-13828) in lateral, dorsal, occlusal and medial views.  Note the differences in shape between the anterior, peg-like teeth and the more posterior ball-like teeth. Numbers represent tooth positions. The large areas between teeth 4 and 6, and  9 and 11 indicate that the teeth in those alveoli had fallen out before preservation. In case of tooth #10, exostitial bone growth around the alveolus indicates some sort of a bone infection prior to the death of the mosasaur. (Scale =10 cm)

LEFT: Anterior teeth of FHSM VP-13828 in lingual view. White circle indicates location of the pit formed by a replacement tooth behind and below the older tooth. Scale bar = 2 cm. RIGHT: A shed (fallen out) tooth of Globidens sp.   from the phosphates of Morocco in Africa.  The oval scar on the middle tooth in the lower row was caused as the replacement tooth absorbed the root of this tooth. A functional mosasaur tooth, including those of Globidens, would normally have a long, columnar root that is solidly attached to the surrounding bone. As the replacement tooth begins to grow, the root of the old tooth is absorbed, allowing it to fall out when the new tooth is ready to move into the alveolus in the jaw.

A review of the literature on Globidens showed that the specimen from Alabama is considered to be the oldest in terms of geologic time (Early Campanian). The first specimen from South Dakota (G. dakotensis) came from approximately the same stratigraphic level in the Pierre Shale as the Kansas specimen (Middle Campanian). 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) determined that the Kansas specimen is not the same species as new new specimen.  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 in that regard than the 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. (Adapted from a pen and ink drawing by Russell Hawley)

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  E. D. Cope, O. C. Marsh, C.H. Sternberg, S.W. Williston, 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 hard shelled invertebrates captured 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. LEFT: A ventral view of of the skull of type specimen of Globidens dakotensis (FMNH PR 846) in the Field Museum, Chicago, IL.

LEFT: 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 premaxilla and right maxillary of the type specimen of Globidens dakotensis (FMNH PR 846). Martin (2007) indicated that he believes the Kansas specimen should be identified as Globidens dakotensis.

LEFT: Ventral and lateral views of the type specimen of Globidens dakotensis (FMNH PR 846); drawing adapted from Russell, 1975. Skull is approximately 26 inches in length.

LEFT: A comparison between the FHSM VP-13828 (Scale bar = 10 cm) and a composite drawing of holotype of "Clidastes intermedius Leidy 1870 (ANSP 9023, 9024) from Pickens County, Alabama (no scale); adapted from Leidy, 1873 (Plate 34, figs. 1 and 2). Note that edentulous tip of the FHSM VP-13828 dentary does not compare favorably with the terminally located tooth on the left dentary of the type specimen of ‘Clidastes’ intermedius." The anterior portion of the jaw (ANSP 9023) is from the left dentary and is reversed from Leidy's original figure in this composite drawing. Drawing of FHSM VP-13828 by Russell Hawley.

References:

Dollo, L. 1913. Globidens fraasi, mosasaurien mylodonte nouveau du Maestrichtien (Crétacé supérieur) du Limbourg, et l'éthologie de la nutrition chez les mosasauriens. Arch. Biol. XXVIII 1913, 609-626, pls. xxiv, xxv. (in French)

Dollo, L. 1924. Globidens alabamaensis mosasaurien Américain retrouvé dans la Craie d'Obourg (Sénonien supérieur) du Hainaut, et les mosasauriens de la Belgique en général. Arch. Biol. 34;167-213, pl. V.

Everhart, M.J. 2008. Rare occurrence of a Globidens sp. (Reptilia; Mosasauridae) dentary in the Sharon Springs Member of the Pierre Shale (Middle Campanian) of Western Kansas. p.  23-29 in Farley G. H. and Choate, J.R. (eds.), Unlocking the Unknown; Papers Honoring Dr. Richard Zakrzewski, , Fort Hays Studies, Special Issue No. 2, 153 p., Fort Hays State University, Hays, KS.

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.

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)

Huene, E. von. 1935. Mosasaurier-Zähne von Timor. Centralblatt fur Mineralogie. Geologic und Palaeontologie; in Verbindung mit dens Neuen Jahrbuch fur Mineralogie, Geologie und Palaeontologie. Stuttgart. Abt. B 10 412-416, 3 figs. (in German)