Ectenosaurus clidastoides


A unique Kansas mosasaur with preserved skin


Copyright ©2011 by Mike Everhart


  Page created 11/14/2011- Last updated 11/21/2011



Ectenosaurus clidastoides is a relatively rare mosasaur, currently known only from the Smoky Hill Chalk of western Kansas, USA. The species was initially described as Platecarpus clidastoides by Merriam (1894) from a specimen collected by C.H. Sternberg or G. Bauer from Logan County, Kansas. The type specimen was in the Bayerische Staatssammlung fur Palaontologie, but the specimen was probably destroyed in World War II:

"P. clidastoides nov. spec. Die Aufstellung dieser Species stützt sich auf ein Parietale, das ganze Hinterhaupt, das Quadratojugale, den Atlas, einige Dorsal- und 3 Caudalwirbel. Sie characterisirt sich durch das eigenthfimliche Parietale, das varn ein niedriges dreieckiges Feld zeigt, in dessen Mitte das verhtiltnissmassig kleine, runde, von der Coronalsutur weit entfernte Scheitelloch liegt. Am Hinterende ist das Scbeitelbein vertical verflacht und zeigt grosse Aehnlichkeit mit dem von Olidastes. Das ganze Aussehen dieses Knochens ist durchaus verschieden vondem der anderen Platecarpus-Species. Die Zugehörigkeit zu dieser Gattung wird jedoch durch das durchbohrte Basioccipitale und das Platecarpus-ähnliche Seitenstück des Atlas bewiesen. Die Wirbel sind so zerquetscht und verwittert, dass sie weder als Platecarpus- noch Clidastes-ähnlich zu erkennen sind." (From Merriam, 1894, p. 30)

In 1953, George Sternberg discovered a much better specimen (GFS 109-53) about 9 miles north of WaKeeney, Kansas. The mosasaur was about 9 feet in length, mostly articulated, but had previously lost the rear limbs and tail to erosion. Sternberg initially identified it in his field notes as Clidastes velox. His notes on the specimen are shown below:

                           CLIDASTES VELOX             Museum No. 7937                          VP-401

DESCRIPTION: The complete skull and jaws with about 9 feet of the skeleton present.

            The tail and hind limbs had been eroded away before it was discovered.

            The skull is mounted upside down showing the exact position it was in when found. It is removable and can be turned over so as to study the top of the head. All teeth are shown and the small teeth attached to the pterygoids, ten in number. There [are] seventeen or eighteen tooth sockets in the upper jaws. The distal end of the right lower dentary has been restored.

            There are twenty-nine continuous vertebrae present. The front paddles are quite complete. A number of cartilaginous ribs are present also a large section of the cartilaginous breast bone.

            The ribs share were complete when found but the lower ends of a number of them were so badly eaten with a low grade of iron that they had to be restored.

            All of the original matrix has been removed and the specimen is mounted on a plaster base.

            The slab is 97 ½” long by 36” wide by 3 ½” deep.

The mounted specimen was on exhibit in Sternberg Memorial Museum on the campus of Fort Hays State University from the mid-1950s to 1999 when the museum was closed in preparation to move to the current location. Currently the specimen is in storage. It was examined in 1963 by Dale Russell, then of the American Museum of Natural History, and determined to be Platecarpus clidastoides instead of Clidastes velox. In a letter from Dale Russell to Myrl Walker, he indicated that it represented the only specimen of a rare mosasaur species, and that he would rename it as Ectenosaurus clidastoides in a future publication. Russell published his Systematics and Morphology of American Mosasaurs in 1967, and Ectenosaurus clidastoides (new genus) is officially re-described and named on page 156. The front limb and the skull in dorsal view were included as Text-figs. 54 and 86. Aside from Russell's (1967) description, nothing else has been published on this specimen other than Bell 1997, and mentions by Everhart 2000, 2005. A second specimen (FHSM VP-13746) was reported by Everhart (2004) from shark scavenged remains discovered in Gove County. Everhart (2005, fig. 9.5) published photos of the skull. The preservation of scales associated with this specimen was mentioned in a Masters thesis by Schumacher (1993) and further discussed in an unpublished thesis by Valerio (2008). 

In George Sternberg's 1953 note on the FHSM VP-401 specimen, there is an interesting statement: "These small section[s] of chalk rock show the fine scale impressions from a Cretaceous mosasaur, specimen #7937.   Russell (1967, p. 70) also mentioned that the Fort Hays Ectenosaurus specimen had scales, as did a Tylosaurus specimen (KUVP 1075) reported by Snow (1878a, 1878b, 1879) and a Platecarpus (KUVP 1001) reported by Williston (1898a). Williston (1898b, p. 215, pl. 69-70)  also commented on Snow's discovery and republished photographs of the Tylosaurus scale impressions. 

All of these specimens were collected from the Smoky Hill Chalk.  Subsequently, Lindgren, et al. (2009) reported scale impressions from a specimen of Plotosaurus in California, and Lindgren, et al. (2010) described scale impressions preserved in association with a Platecarpus specimen (LACM 128319) from western Kansas. The LACM 128319 scale impressions were first reported by Geist et al. 2002. 

Also, in August-October, 1951, G.F. Sternberg collected a fairly complete, 20 foot long Tylosaurus specimen (GFS 1-51; FHSM VP-327), including a 35 inch skull, from a roadside exposure along Highway 24 west of Hill City (Graham County) that included scale impressions in the abdominal region. The specimen was subsequently sent to the Los Angeles County Museum in an exchange, and has not been relocated.  All that remains in the records are three color photographs, two of which show the collected specimen (vertebrae, ribs and scale impressions) and the other which shows the locality. Sternberg's field notes indicated that the scales were fragile and might not be able to be saved. 

The latest report on mosasaur integument comes back to the FHSM VP-401 specimen.  On November 16, 2011, we reported not only the scale impressions of Ectenosaurus clidastoides (FHSM VP-401), but the three-dimensional, layered structure of the skin:

Lindgren, J., Everhart, M.J. and Caldwell, M.W. 2011. Three-dimensionally preserved integument reveals hydrodynamic adaptations in the extinct marine lizard Ectenosaurus (Reptilia, Mosasauridae). PLoS One. 

Abstract: The physical properties of water and the environment it presents to its inhabitants provide stringent constraints and selection pressures affecting aquatic adaptation and evolution. Mosasaurs (a group of secondarily aquatic reptiles that occupied a broad array of predatory niches in the Cretaceous marine ecosystems about 98–65 million years ago) have traditionally been considered as anguilliform locomotors capable only of generating short bursts of speed during brief ambush pursuits. Here we report on an exceptionally preserved, long-snouted mosasaur (Ectenosaurus clidastoides) from the Santonian (Upper Cretaceous) part of the Smoky Hill Chalk Member of the Niobrara Formation in western Kansas, USA, that contains phosphatized remains of the integument displaying both depth and structure. The small, ovoid neck and/or anterior trunk scales exhibit a longitudinal central keel, and are obliquely arrayed into an alternating pattern where neighboring scales overlap one another. Supportive sculpturing in the form of two parallel, longitudinal ridges on the inner scale surface and a complex system of multiple, superimposed layers of straight, cross-woven helical fiber bundles in the underlying dermis, may have served to strengthen the skin, thereby minimizing surface deformation and frictional drag during locomotion. Additional parallel fiber bundles oriented at acute angles to the long axis of the animal presumably provided stiffness in the lateral plane. These features are consistent with advanced hydrodynamic adaptations of fast swimming fishes, such as tuna and sharks, and of ichthyosaur reptiles, and thus suggest that mosasaurs were efficient swimmers. These features suggest that the anterior torso of Ectenosaurus was held somewhat rigid, whereas motion was restricted to the posterior body and tail; i.e., a sub-carangiform mode of swimming.   FREE DOWNLOAD

The photographs below show various aspects of the FHSM VP-401 specimen not previously published.

LEFT: The skull of the Ectenosaurus clidastoides (FHSM VP-401) specimen in right dorso-lateral view. 

RIGHT: The skull of the Ectenosaurus clidastoides (FHSM VP-401) specimen in ventral view, including the left lower jaw in lateral view. 

LEFT: The frontal-parietal bones, including the parietal foramen, of the Ectenosaurus clidastoides (FHSM VP-401) skull in dorsal view. 

RIGHT: The orbit of the right eye of the Ectenosaurus clidastoides (FHSM VP-401) skull in lateral view, including the postorbital frontal, frontal, prefrontal and jugal. 

LEFT: The paired pterygoid bones of the Ectenosaurus clidastoides (FHSM VP-401) skull, partially exposed under the left lower jaw.

RIGHT: The posterior corner of the Ectenosaurus clidastoides (FHSM VP-401) skull in right lateral view, including the squamosal, postorbital frontal and posterior portion of the jugal.  

LEFT: Dorsal and ventral views of the Ectenosaurus clidastoides (FHSM VP-401) skull. See labeled drawing from Russell (1967) here.

RIGHT: Lateral and medial views of the right quadrate of the Ectenosaurus clidastoides (FHSM VP-401) skull.

LEFT: A detail view of several teeth on the right maxilla of the Ectenosaurus clidastoides (FHSM VP-401) skull.

RIGHT: A detail medial view of a complete tooth in the right maxilla, showing bud teeth forming in the medial side of the maxilla. The teeth are described by Russell (1967, p. 156) as "bicarinate, vertically striated and medially recurved."

LEFT: The post-cranial skeleton of FHSM VP-401. The rear limbs and tail were lost to erosion prior to discovery by G. F. Sternberg

RIGHT: Preserved cartilage of the sternal ribs and sternum of FHSM VP-401. 

LEFT: The well preserved front limbs of the FHSM VP-401 specimen. The fifth digit is apparently greatly reduced in Ectenosaurus clidastoides.

RIGHT: A close-up photo of the left front paddle of FHSM VP-401. See labeled drawing from Russell (1967) here.

The following four close-up photos were taken in 2005 when I became interested in what I thought were "just skin impressions" (Everhart 2005). 

LEFT: A series of scale impressions in the matrix from the FHSM VP-401 Ectenosaurus clidastoides specimen. A note included with the specimen said that the impressions were discovered in the neck region of the specimen (between the lower jaws and the front limbs).

RIGHT: Two more small fragments of the matrix with scale impressions. The scales measure about 2.7 mm in length by 2 mm in width, and where preserved, they are about 0.1 mm thick.

LEFT: Another fragment of the matrix with scale impressions. Initially I had reported (Everhart 2005) that the scale impressions did not show a central keel, but upon closer examination it became evident that they do have a keel similar to those found in Tylosaurus (KUVP 1075). 

RIGHT: Another fragment of the matrix with scale impressions.  This piece, however, does show some of the three-dimensionality of the preserved skin layers, including some of the associated fibrous structures.

After the 2nd Mosasaur Meeting in May, 2007, Johan Lindgren and I started talking about the skin preservation in the Ectenosaurus specimen. That conversation continued during a September 2010 meeting at the University of Kansas when we examined the KUVP 1001 Platecarpus specimen for skin impressions. In the meantime, Lindgren and others (2009, 2010) had published papers regarding the discovery of scales in Plotosaurus and Platecarpus. 

In 2010, Johan and I began to look more critically at the FHSM VP-401 matrix fragments. As might be expected, the more we looked, the more structure we found.  The following six photos, taken through a binocular microscope and using a single light source, are examples of preserved scales, scale impressions, and underlying integument preserved in the matrix surrounding the bones of FHSM VP-401.  

LEFT: This fragment preserves what appears to be the underside of the scales.

RIGHT: Another fragment showing the underside of a group of scales, but also demonstrating layers of structure in the preservation. 

LEFT: This piece shows what appears to be the mid-line between two groups of scales, trending from lower left to upper right. as well as layers of structures.

RIGHT: Another fragment showing the underside of a group of scales, but also demonstrating the layering of structures in the preservation. 

LEFT: Another instance showing the underside of the scales, including layering, and in addition, a patch of phosphatized tissue (fiber bundles) on the left side of the photo.  

RIGHT: This specimen preserves a patch of scales, showing the central keels in some areas, plus layering and soft tissue remains.

Additional photos at higher resolution and using scanning electron microscope (SEM) technology are included in our publication published on November 16, 2011 in PLoS ONE.  

So, are keeled scales a holdover from the mosasaur's terrestrial ancestors, or an adaptation to more efficient swimming? At this point, the answer is simply that we don't know.

LEFT: This 30 cm Garter snake (Thamnophis sirtalis) has keeled scales that are roughly the same size and shape as those of the 5 m Ectenosaurus specimen shown above.


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