Thalassomedon, meaning “lord of the sea”, was a 38 foot long elasmosaurid plesiosaur which lived within the Mowry Sea, the shallow sea that covered North America during the middle of the Cretaceous Period 97-95 million years ago.
In March 1939, employees for the Works Progress Administration (W.P.A.) were doing work on privately-owned land located thirteen and a half miles north of the small town of Pritchett in the northwestern part of Baca County, Colorado. While working, one of the W.P.A. employees named Fred Roth discovered some fossils weathering out of the side of a 20-foot shale cliff overlooking a creek bed; the fossils had been exposed as the result of a recent flood. Mr. Roth notified the property owner, Mr. Vernon Van Campen, about what had been found on his land. Mr. Van Campen in turn contacted Mr. Andrew A. Weresh, the principal of Pritchett High School who formerly worked as the school’s science teacher. Finally, Mr. Weresh contacted the Colorado Museum of Natural History (afterwards renamed to the Denver Museum of Natural History, and afterwards renamed again to the Denver Museum of Nature and Science) about the discovery (Annual Report of the Colorado Museum of Natural History for the Year 1939, pages 2, 34; Welles 1943, page 153).
In response to this notice, the museum dispatched Robert L. Landberg (assistant preparator, Department of Paleontology) and Harvey C. Markman (curator, Department of Geology) to investigate the site. When they arrived, they regrettably discovered that the locality had already been picked over by souvenir collectors and the remainder of the fossils were scattered everywhere. Even so, there was still a lot of material left, and it was carefully excavated after permission was granted by the property owner (Annual Report of the Colorado Museum of Natural History for the Year 1939, pages 2, 34; Welles 1943, page 153).
The fossils were found within the upper part of the Graneros Shale Formation, close to the formation’s uppermost level. The Graneros Shale Formation is part of the Benton Group (Welles 1943, page 153). Within Baca County, Colorado where the fossils were found, the Graneros Shale stands between the underlying Dakota Sandstone and the overlying Greenhorn Limestone (McLaughlin 1954, page 111). The specific rock layer which the fossils were recovered from was stated in the 1939 report to be dated to 100 MYA (Annual Report of the Colorado Museum of Natural History for the Year 1939, page 34). The stratum in which the skeleton was recovered from was described as bluish to brownish-grey mottled shale streaked with brown and yellow caused by iron oxides occasionally interbedded with thin layers of white clay or bentonite (Welles 1943, page 153). By examining these strata, and by comparing them with similar strata found nearby as well as the rock layers above and below them, it can be determined that the fossils were found within either the Mowry Shale Member or the Belle Fourche Shale Member, which are both sub-units of the Graneros Shale Formation. The Mowry Shale dates to the early part of the Cenomanian Stage of the middle Cretaceous Period, circa 100-98 MYA, while the Belle Fourche Shale dates to the middle Cenomanian circa 97-95 MYA (Collier 1922, pages 75-76, 79-84; Rubey 1931, pages 4-5; Bergendahl et al 1960, pages 650-652; Condon 2000, pages 6-7). Of these two options, it is more likely that the fossils were recovered from the Belle Fourche Member since the description of the strata given in Welles (1943) more closely matches the description of the Belle Fourche Member given in Berghendahl et al (1960) and Condon (2000). Indeed, Sachs et al (2021) affirmed that the fossils from Colorado came from middle Cenomanian strata (Sachs et al 2021: 104769).
Once the fossils were brought back to the museum’s lab, it was determined that they belonged to a nearly complete skeleton of large plesiosaur. The fossils consisted of a skull, a complete vertebral column, several ribs and gastralia, the clavicles, the sternal plates, part of the left scapula, the left humerus, ulna, and radius, one ilium, both left and right pubes and ischia, the pelvic girdle, both femora, and the left tibia and fibula. The specimen was provisionally identified in 1939 as Elasmosaurus, the quintessential large long-necked plesiosaur of the Cretaceous seas. However, further examination by Samuel P. Welles of the University of California revealed that, while this creature was definitely an elasmosaurid, there were several anatomical differences between Elasmosaurus and the bones belonging to the specimen which was recently recovered from Colorado. In 1943, Welles named this animal Thalassomedon haningtoni. The genus name Thalassomedon means “lord of the sea”, while the species name haningtoni is in honor of Charles H. Hanington, President of the Colorado Museum of Natural History. The fossils are currently housed within the collection of the Denver Museum of Nature and Science (collection ID code: DMNH 1588, formerly CMNH 1588) (Annual Report of the Colorado Museum of Natural History for the Year 1939, page 34; Welles 1943, pages 152-165; Annual Report of the Colorado Museum of Natural History for the Year 1943, pages 2, 32).
The holotype specimen of Thalassomedon haningtoni, as it was originally mounted within the Colorado Museum of Natural History in 1940. Annual Report of the Colorado Museum of Natural History for the Year 1940, page 38.
https://www.dropbox.com/s/bzmw67u7yar8qd2/AR_1940.pdf?dl=0.
Thalassomedon belonged to the family Elasmosauridae, and as such was a close relative of Elasmosaurus and Styxosaurus. In fact, Welles remarked that Thalassomedon’s skull was similar in appearance to that of Styxosaurus snowii (then known as Cimoliasaurus). Thalassomedon was one of the largest of the long-necked plesiosaurs to exist, measuring 38 feet (11.6 meters) long, and it was possibly the largest marine reptile to inhabit the Mowry Sea during the Cenomanian Stage of the middle Cretaceous. The skull was found lying on its left side, and it was badly crushed. The upper jaw, as fully restored, measured 23.62 inches (60 cm long) from the tip of the nose to the back of the quadrate, and the entire skull would have measured 26 inches (66 cm) from front to back. The animal had a total of 114 vertebrae: 62 cervical, 3 pectoral, 25 dorsal, 3 sacral, and 21 caudal. Its neck made up half of its total body length (Welles 1943, pages 153-158). An in-depth study of the Thalassomedon’s skull was made in 2021 which confirmed its position within Elasmosauridae (Sachs et al 2016; Sachs et al 2021).
The skull of Thalassomedon haningtoni. Welles, Samuel Paul (1943). “Elasmosaurid plesiosaurs with description of new material from California and Colorado”. Memoirs of the University of California, volume 13 (1943). Page 155.
https://babel.hathitrust.org/cgi/pt?id=uiug.30112027227153&seq=11.
In 1964, a second specimen of Thalassomedon was found in Nebraska within the Graneros Shale Formation. In 1970, Samuel P. Welles published a short note about this discovery. The fossils are currently housed within the University of Nebraska State Museum in Lincoln, Nebraska (collection ID code: UNSM 50132) (Oceans of Kansas. “The Longest Neck in the Ocean”). In 2020, it was recommended that this specimen should be reclassified as Styxosaurus snowii due to more similarities in skull structure to Styxosaurus than to Thalassomedon (Smith 2020, page 44).
Thalassomedon haningtoni. © Jason R. Abdale (May 28, 2024).
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Bibliography
Annual Report of the Colorado Museum of Natural History for the Year 1939. Denver: Colorado Museum of Natural History, 1939. Pages 1-41.
https://www.dropbox.com/s/5fuk3wwmwhj1q36/AR_1939.pdf?dl=0.
Annual Report of the Colorado Museum of Natural History for the Year 1943. Denver: Colorado Museum of Natural History, 1943. Pages 1-36.
https://www.dropbox.com/s/297m1kjijc46gqn/AR_1943.pdf?dl=0.
Bergendahl, M. H.; Davis, R. E.; Izett, G. A. (1960). “Geology and Mineral Deposits of the Carlisle Quadrangle, Crook County, Wyoming”. U. S. Department of the Interior Geological Survey Bulletin 1082-I: Geology and Mineral Deposits of the St. Regis-Superior Area, Mineral County, Montana. Washington, DC: United States Government Printing Office, 1960. Pages 613-706.
https://books.google.com/books?id=N6YPAAAAIAAJ&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false.
Collier, A. J. (1922). “The Osage oil field, Weston County, Wyoming”. Contributions to Economic Geology, Part 2: Mineral fuels. U.S. Geological Survey Bulletin, 736-D (1922). Pages 71-110.
https://pubs.usgs.gov/bul/0736d/report.pdf.
Condon, Steven M. (2000). “Stratigraphic Framework of Lower and Upper Cretaceous Rocks in Central and Eastern Montana”. U.S. Geological Survey Digital Data Series DDS-57 (2000). Washington, DC: United States Government Printing Office, 2000. Pages 1-12.
https://pubs.usgs.gov/dds/dds-057/DDS57.pdf.
McLaughlin, Thad G. (1954). “Geology and Ground-Water Resources of Baca County, Colorado”. Geological Survey Water Supply Paper 1256. Washington, DC: United States Government Printing Office, 1954. Pages 1-232.
https://pubs.usgs.gov/wsp/1256/report.pdf.
Rubey, William W. (1931). “Lithologic studies of fine-grained Upper Cretaceous sedimentary rocks of the Black Hills region”. Shorter Contributions to General Geology, 1930: U.S. Geological Survey Professional Paper, 165-A. Pages A1-A54.
https://pubs.usgs.gov/pp/0165/report.pdf.
Sachs, Sven; Lindgren, Johan; Kear, Benjamin P. (2016). “Re-description of Thalassomedon haningtoni – an elasmosaurid from the Cenomanian of North America”. 5th Triennial Mosasaur Meeting: A global perspective on Mesozoic marine amniotes. Abstracts and Program (May 16-20, 2016). Pages 38-40.
https://www.researchgate.net/publication/303446624_Re-description_of_Thalassomedon_haningtoni_-_an_elasmosaurid_from_the_Cenomanian_of_North_America.
Sachs, Sven; Lindgren, Johan; Madzia, Daniel; Kear, Benjamin P. (2021). “Cranial osteology of the mid-Cretaceous elasmosaurid Thalassomedon haningtoni from the Western Interior Seaway of North America”. Cretaceous Research, volume 123 (July 2021): 104769.
https://www.sciencedirect.com/science/article/abs/pii/S0195667121000161.
Smith, Elliot Armour (2020). “Revision of the Genus Styxosaurus and Relationships of the Late Cretaceous Elasmosaurids (Sauropterygia: Plesiosauria) of the Western Interior Seaway”. Master of Science thesis, Marshall University (May 2020).
https://mds.marshall.edu/cgi/viewcontent.cgi?article=2340&context=etd.
Welles, Samuel Paul (1943). “Elasmosaurid plesiosaurs with description of new material from California and Colorado”. Memoirs of the University of California, volume 13 (1943). Pages 125-254.
https://babel.hathitrust.org/cgi/pt?id=uiug.30112027227153&seq=11.
Oceans of Kansas. “The Longest Neck in the Ocean”, by Mike Everhart (February 14, 2009).
http://oceansofkansas.com/longneck.html.
Categories: Paleontology, Uncategorized
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