Occithrissops willsoni was an 8 inch (20 cm) long saltwater fish which lived within the Sundance Sea during the middle Jurassic Period 165 million years ago.
During the middle of the Jurassic Period, much of the center of North America was covered in a warm shallow sea called the Sundance Sea, named after the town of Sundance, Wyoming where the rock layers were first identified. Numerous fossils of bivalves and ammonites have been found here, along with the remains of several marine reptiles such as the ichthyosaur Baptanodon, the plesiosaurs Pantosaurus and Tatenectes, and the pliosaur Megalneusaurus. Surprisingly, fish seem to be rather scarce, with ammonites being the dominant aquatic life here.
Among the few species of fish which did live in the Sundance Sea was Occithrissops, which was named in 1984 by Bobb Schaeffer and Colin Patterson. The name means “western Thrissops” due to its similarity to the genus Thrissops from Jurassic Europe. The species name is in honor of Earl Willson of Hulett, Wyoming who owned the property that the fossils were found on. At least twenty specimens of Occithrissops willsoni have been found so far, all of which have been found in the vicinity of the town of Hulett, Wyoming within the Stockade Beaver Member of the Sundance Formation; the largest of these measures just 20 cm (7.8 inches) long (Cavin et al. 2013, page 26; Schaeffer and Patterson 1984, page 40). According to Herrick and Schram (1978), the Stockade Beaver Member dates to the lower part of the Callovian Stage of the middle Jurassic Period (Herrick and Schram 1978, page 9), while Schaeffer and Patterson (1984) says that it dates to the upper part of the underlying Bathonian Stage (Schaeffer and Patterson 1984, page 40). The boundary between the underlying Bathonian and the overlying Callovian Stages is about 165 MYA, so it can be said that Occithrissops willsoni lived approximately at that time.
Occithrissops willsoni. © Jason R. Abdale (August 24, 2025).
Occithrissops was a primitive member of the order Ichthyodectiformes (Schaeffer and Patterson 1984, page 40), which included formidable creatures like Ichthyodectes, Cladocyclus, Gillicus, and Xiphactinus. In fact, Occithrissops is the oldest-known ichthyodectiform (Cavin 2008, page 172). One report assigned Occithrissops to the ichthyodectiform family Allothrissopidae, but this family is believed to be paraphyletic (Arratia 2020, page 132).
In overall shape Occithrissops resembled a miniature tarpon. The teeth of Occithrissops are so small that they’re microscopic, so it probably fed on plankton. This fish had what’s called a “superior” mouth, meaning that the mouth is angled upwards. This indicates that it was a pelagic swimmer which fed in the water column near the surface rather than a bottom-dwelling fish. Due to the large number of specimens found in a small area, it’s likely that Occithrissops was a schooling fish like modern-day herrings, sardines, or anchovies. It was also likely a major source of food for the marine reptiles that dwelt in the Sundance Sea during the middle Jurassic.
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Bibliography
Arratia, Gloria (2020). “Osteichthyes or Bony Fishes”. In Alderton, David; Elias, Scott A., eds. Encyclopedia of Geology, 2nd Edition, Volume 3 – History of Life. Cambridge: Elsevier Academic Press, 2020. Pages 121-137.
https://www.google.com/books/edition/Encyclopedia_of_Geology/lcfrDwAAQBAJ?hl=en&gbpv=1&dq=Occithrissops+Allothrissopidae&pg=RA2-PA132&printsec=frontcover.
Cavin, Lionel (2008). “Paleobiogeographgy of Cretaceous bony fishes (Actinistia, Dipnoi, and Actinopterygii”. In Cavin, Lionel; Longbottom, A.; Richter, M., eds. Geological Society Publication 295: Fishes and the Breakup of Pangaea. London: The Geological Society, 2008. Pages 165-183.
https://www.google.com/books/edition/Fishes_and_the_Break_up_of_Pangaea/j–XRbVl2DoC?hl=en&gbpv=1&dq=Occithrissops&pg=PA172&printsec=frontcover.
Cavin, Lionel; Forey, Peter L.; Giersch, Samuel (2013). “Supplementary Online Material – Osteology of Eubiodectes libanicus (Pictet & Humbert, 1866) and some other ichthyodectiformes (Teleostei): phylogenetic implications”. Journal of Systematic Paleontology, volume 11, issue 2 (2013). Pages 1-29.
https://www.tandfonline.com/action/downloadSupplement?doi=10.1080%2F14772019.2012.691559&file=tjsp_a_691559_sm7083.pdf.
Herrick, Elizabeth M.; Schram, Frederick R. (1978). “Malacostracan Crustacean Fauna from the Sundance Formation (Jurassic) of Wyoming”. American Museum Novitates, no. 2652 (June 30, 1978). Pages 1-12.
http://digitallibrary.amnh.org/bitstream/handle/2246/2966/v2/dspace/ingest/pdfSource/nov/N2652.pdf?sequence=1&isAllowed=y.
Schaeffer, Bobb; Patterson, Colin (1984). “Jurassic Fishes from the Western United States, with Comments on Jurassic Fish Distribution”. American Museum Novitates, no. 2796 (November 13, 1984). Pages 1-86.
https://www.biodiversitylibrary.org/item/317403#page/1/mode/1up.
Categories: Paleontology, Uncategorized
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