Trinacromerum was a 12 foot long plesiosaur which lived within the Mowry Sea, the shallow sea that covered North America during the middle of the Cretaceous Period 95 million years ago.
The holotype specimen was found in 1888 within the Fencepost Member of the Benton Shale, Osborne County, Kansas, USA. It is currently housed within the collections of the Smithsonian Institution in Washington, DC (collection ID code: USNM V10945) (“Trinacromerum bentonianum Cragin, 1888”). Just a few months after its discovery, Dr. Francis W. Cragin named this animal Trinacromerum bentonianum (Cragin 1888 pages 404-407). A follow-up article was afterwards published three years later (Cragin 1891, pages 171-174).
Since then, other fossils belonging to Trinacromerum bentonianum have been discovered. In December 1936 during road construction south of Concordia, Cloud County, Kansas, a partial skeleton was uncovered consisting of an excellently-preserved skull measuring 745 mm (29.33 inches) long, fifty vertebrae, ribs, and most of the pelvic area (collection ID code: KUVP 5070). The fossil was found in the Hartland Shale Member of the Greenhorn Limestone, which dates to the late Cenomanian Stage of the middle Cretaceous (Schumacher and Everhart 2005, pages 33-34). In 1944, Elmer Riggs named this animal Trinacromerum willistoni (Riggs 1944, pages 77-87). However, in 1996, it was determined that this was the same animal as T. bentonianum, and so the name T. willistoni was declared invalid (Carpenter 1996, page 279).
In the Autumn of 1932, northwest of Treherne, Manitoba, Canada, a series of fossils were discovered within the Assiniboine Beds, which are dated to the same time as the Benton Shale. These bones were excavated that year and in the Spring of 1933 by Dr. Stewart Raeburn Kirk, an assistant professor of geology at the University of Manitoba. Unfortunately, while the prep work on the fossils was still underway, Dr. Kirk died in May 1934 at the shockingly young age of 35 after suffering through a long illness (“Obituary for Stewart Raeburn Kirk”, page 5; “Recent Deaths”, page 9) – an extremely promising career tragically cut short. Loris S. Russell volunteered to pick up where his predecessor had left off, and on November 10, 1934, the skeleton was put on public display in the Manitoba Museum in Winnepeg. In 1935, Loris S. Russell named this species Trinacromerum kirki in honor of Professor Stewart R. Kirk. (Russell 1935, pages 385-389).
Trinacromerum belonged to the plesiosaur family Polycotylidae (Albright et al 2007, page 52). These animals looked like a cross between a seal and a long-nosed dolphin, and they probably filled the same ecological niches as those animals following the demise of the ichthyosaurs. The polycotylids were very abundant during the middle and late Cretaceous Period, and their anatomy suggests that they were very agile and active swimmers. A well-preserved fossil from the related genus Mauriciosaurus from Mexico shows that the body had a thick layer of blubber and a thick tail. Viewed from overhead, the body and tail together would have resembled a teardrop shape (Frey et al 2017, pages 87-134).
Trinacromerum bentonianum. © Jason R. Abdale (April 1, 2024).
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Bibliography
Articles
Albright III, L. Barry; Gillette, David D.; Titus, Alan L. (2007). “Plesiosaurs from the Upper Cretaceous (Cenomanian-Turonian) Tropic Shale of southern Utah, part 2: Polycotylidae”. Journal of Vertebrate Paleontology, volume 27, issue 1 (March 2007). Pages 41-58.
https://www.researchgate.net/publication/40662737_Plesiosaurs_from_the_Upper_Cretaceous_Cenomanian-Turonian_Tropic_Shale_of_southern_Utah_part_2_Polycotylidae.
Carpenter, Kenneth (1996). “A review of short-necked plesiosaurs from the Cretaceous of the western interior, North America”. Neues Jahrbuch für Geologie und Paläeontologie Abhandlungen, volume 201, issue 2 (1996). Pages 259-287.
https://www.researchgate.net/profile/Kenneth-Carpenter-2/publication/40662804_A_review_of_short-necked_plesiosaurs_from_the_Cretaceous_of_the_Western_Interior_North_America/links/58c6e65e4585150ab4207a7c/A-review-of-short-necked-plesiosaurs-from-the-Cretaceous-of-the-Western-Interior-North-America.pdf.
Cragin, F. W. (1888). “Preliminary description of a new or little known saurian from the Benton of Kansas”. American Geologist, volume 2. Pages 404-407.
https://books.google.com/books?id=QtzN3KkQxv0C&pg=PA404&lpg=PA404&dq=New+or+Little+Known+Saurian+from+the+Benton+of+Kansas&source=bl&ots=7KrQ2UbHcR&sig=kmfFpLZ8Qd5WZWK8IMCc9Sf7D9I&hl=en&sa=X&ei=OWs7U_ekH-mpsATFroC4Dw&ved=0CCcQ6AEwAA#v=onepage&q&f=false.
Cragin, F. W. (1891). “New observations on the genus Trinacromerum“. American Geologist, volume 8. Pages 171-174.
https://www.google.com/books/edition/The_American_Geologist/QO5LAAAAMAAJ?hl=en&gbpv=1&dq=1891+New+observations+on+the+genus+Trinacromerum&pg=PA171&printsec=frontcover.
Frey, Eberhard; Mulder, Eric W. A.; Stinnesbeck, Wolfgang; Rivera-Sylva, Héctor E.; Padilla-Gutiérrez, José Manuel; González-González, Arturo Homero (2017). “A new polycotylid plesiosaur with extensive soft tissue preservation from the early Late Cretaceous of northeast Mexico”. Boletín de la Sociedad Geológica Mexicana (Bulletin of the Mexican Geological Society), volume 69, issue 1 (2017). Pages 87-134.
https://www.researchgate.net/publication/314235466_A_new_polycotylid_plesiosaur_with_extensive_soft_tissue_preservation_from_the_early_Late_Cretaceous_of_northeast_Mexico.
Riggs, Elmer S. (1944). “A new polycotylid plesiosaur”. University of Kansas Science Bulletin, volume 30. Pages 77-87.
https://www.biodiversitylibrary.org/page/2994555#page/85/mode/1up.
Russell, Loris S. (1935). “A Plesiosaur from the Upper Cretaceous of Manitoba”. Journal of Paleontology, volume 9, issue 5 (July 1935). Pages 385-389.
https://archive.org/details/sim_journal-of-paleontology_1935-07_9_5/page/384/mode/2up.
Schumacher, B. A.; Everhart, Michael J. (2005). “A stratigraphic and taxonomic review of plesiosaurs from the old ‘Fort Benton Group’ of central Kansas: A new assessment of old records”. Paludicola, volume 5, issue 2 (2005). Pages 33-54.
https://www.researchgate.net/publication/237584752_A_stratigraphic_and_taxonomic_review_of_plesiosaurs_from_the_old_Fort_Benton_Group_of_central_Kansas_A_new_assessment_of_old_records.
Websites
Newspapers.com. “Obituary for Stewart Raeburn Kirk”. Red Deer Advocate, May 23, 1934, page 5.
https://www.newspapers.com/article/red-deer-advocate-obituary-for-stewart-r/112594048/.
“Recent Deaths”. Science, volume 80, issue 2062 (July 6, 1934), page 9.
https://www.science.org/doi/10.1126/science.80.2062.9-a.
Smithsonian. “Trinacromerum bentonianum Cragin, 1888”. https://www.si.edu/es/object/nmnhpaleobiology_3448749.
Categories: Paleontology, Uncategorized
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