Cimoliasaurus, meaning “chalk lizard” due to its fossils being found within layers of chalk, was a genus of elasmosaurid plesiosaur which lived at the end of the Cretaceous Period 70-66 million years ago, making it one of the last plesiosaurs to have existed before the group went extinct. The name was originally ascribed by Prof. Joseph Leidy in 1851 to large plesiosaur remains found within New Jersey.
For a long time afterwards, the name Cimoliasaurus was treated as a “waste basket taxon” by paleontologists, and any fossils belonging to large plesiosaurs which dated to the late Cretaceous which could not be identified were automatically ascribed to this genus. In the past, as many as twenty-eight different species were ascribed to Cimoliasaurus, but now, only one is recognized – the type species, C. magnus. Even then, many paleontologists consider Cimoliasaurus to be a nomen dubium since no clear diagnostic features are present on the fossil bones which can immediately differentiate it from the bones of many other plesiosaurs (Welles 1943, page 209; Colbert 1949, page 17).
At the moment, the only remains which we have of Cimoliasaurus are vertebrae and a few isolated teeth which either may or may not belong to this animal. Examination of the vertebrae have shown a similarity to the plesiosaurs of the family Elasmosauridae, which include genera such as Elasmosaurus itself, Styxosaurus, and Thalassomedon (Cope 1871, page 235; Cope 1875, page 15; Seeley 1892, page 151; Hay 1902, pages 457-459). Notably, the vertebral centra are shorter in length compared to other elasmosaurids, meaning that Cimoliasaurus might have had a more moderately-sized neck, shorter than the elasmosaurids but longer than the polycotylids (Cope 1870, page 41; Seeley 1892, pages 135-136; Williston 1906, pages 222-223; O’Keefe and Street 2009, page 51). Several websites state that Cimoliasaurus is estimated to have measured 25 feet long, but I haven’t been able to find any justification for that exact number.
Because of Cimoliasaurus‘ compressed neck vertebrae, Samuel Paul Welles believed that Cimoliasaurus might actually be a short-necked pliosauroid, but he provided no evidence to support this hypothesis. In the past, the short-necked plesiosaurs of the family Polycotylidae which include species like Polycotylus, Dolichorhynchops, and Trinacromerum were placed within the super-family Pliosauroidea. In fact, Welles himself was the one who proposed this, and this belief persisted until the early 2000s until it was determined that the polycotylids actually belonged to Plesiosauroidea, not Pliosauroidea. So perhaps Welles imagined that Cimoliasaurus could be a polycotylid, not an elasmosaur. Even so, Welles stated in his 1943 report that the majority of the remains ascribed to Cimoliasaurus are elasmosaurid in form (Welles 1943, page 209; Mindat. “Polycotylidae”).
Below is a hypothetical reconstruction of Cimoliasaurus magnus, based upon moderately-necked plesiosaurs like Brancasaurus and Leptocleidus. Whether or not this creature actually looked like this is anyone’s guess, though. Here’s to hoping that we may find a complete specimen one day.
Cimoliasaurus magnus. © Jason R. Abdale (November 23, 2024).
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Bibliography
Colbert, Edwin H. (1949). “A new Cretaceous plesiosaur from Venezuela”. American Museum Novitiates, number 1,420 (July 10, 1949). Pages 1-22.
https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=e7af97756fd667a6764e5fef5573318c36bd1727.
Cope, Edward D. (1870). “Synopsis of the Extinct Batrachia, Reptilia and Aves of North America, Part I”. Transactions of the American Philosophical Society, volume 14 (1870). Pages 1-252.
https://archive.org/details/sim_transactions-of-the-american-philosophical-society_1870_14/mode/2up.
Cope, Edward D. (1871). “On the homologies of some of the cranial bones of the Reptilia, and on the systematic arrangement of the class”. Proceedings of the American Association for the Advancement of Science, volume 19. Pages 194-247.
https://iucn-tftsg.org/wp-content/uploads/file/Articles/Cope_1871.pdf.
Cope, Edward D. (1875). “Check-list of North American Batrachia and Reptilia; with a systematic list of the higher groups, and an essay on geographical distribution”. Bulletin of the United States National Museum, volume 1 (1875). Pages 1-109.
https://repository.si.edu/bitstream/handle/10088/26116/SmithMiscNorthAmerican.pdf.
Hay, Oliver P. (1902). “Bibliography and Catalogue of the Fossil Vertebrata of North America”. Bulletin of the United States Geological Survey, volume 179 (1902). Pages 1-868.
https://www.biodiversitylibrary.org/item/59973#page/463/mode/1up.
O’Keefe, F. Robin; Street, Hallie P. (2009). “Osteology of the cryptocleidoid plesiosaur Tatenectes laramiensis, with comments on the taxonomic status of the Cimoliasauridae”. Journal of Vertebrate Paleontology, volume 29, issue 1 (2009). Pages 48-57.
https://mds.marshall.edu/cgi/viewcontent.cgi?article=1067&context=bio_sciences_faculty.
Seeley, H. G. (1892). “The Nature of the Shoulder Girdle and Clavicular Arch in Sauropterygia”. Proceedings of the Royal Society of London, volume 51. Pages 119-151.
https://wellcomecollection.org/works/qab9wswa/items?canvas=19&query=Cimoliasaurus
Welles, Samuel Paul (1943). “Elasmosaurid plesiosaurs with description of new material from California and Colorado”. Memoirs of the University of California, volume 13, issue 3 (1943). Pages 125-254.
https://plesiosauria.com/pdf/welles_1943_elasmosaurid_plesiosaurs.pdf.
Williston, Samuel W. (1906) “North American Plesiosaurs: Elasmosaurus, Cimoliasaurus, and Polycotylus”. American Journal of Science, volume 21, issue 123 (March 1906). Pages 221-244.
https://commons.wikimedia.org/w/index.php?title=File%3ANorth_American_Plesiosaurs-_Elasmosaurus%2C_Cimoliasaurus%2C_and_Polycotylus.pdf&page=1.
Mindat. “Polycotylidae”. https://www.mindat.org/taxon-3239138.html.
Categories: Paleontology, Uncategorized
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