Megacephalosaurus eulerti was a one of the last of the pliosaurs – a group of short-necked big-headed plesiosaurs – living during the Late Cretaceous (~93 million years ago) in what is now the Midwestern United States, a region that at that time was covered by the Western Interior Seaway.
Although known only from fossil skulls and a few neck bones, based on the proportions of related pliosaurs it probably reached around 9m long (~30') with its 1.75m (~5'9") head alone making up 20-25% of that measurement.
Its elongated jaws were lined with pointed conical teeth, and pits in the bones of its snout may have housed a complex sensory system, possibly giving it the ability to detect the movements or even bioelectric fields of nearby prey.
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References:
Foffa, Davide, et al. "Complex rostral neurovascular system in a giant pliosaur." Naturwissenschaften 101 (2014): 453-456. https://doi.org/10.1007/s00114-014-1173-3
Madzia, Daniel, Sven Sachs, and Johan Lindgren. "Morphological and phylogenetic aspects of the dentition of Megacephalosaurus eulerti, a pliosaurid from the Turonian of Kansas, USA, with remarks on the cranial anatomy of the taxon." Geological Magazine 156.7 (2019): 1201-1216. https://doi.org/10.1017/S0016756818000523
Schumacher, Bruce A., Kenneth Carpenter, and Michael J. Everhart. "A new cretaceous pliosaurid (reptilia, plesiosauria) from the carlile shale (middle turonian) of russell county, kansas." Journal of Vertebrate Paleontology 33.3 (2013): 613-628. https://doi.org/10.1080/02724634.2013.722576
Wikipedia contributors. “Megacephalosaurus” Wikipedia, 30 Mar. 2025, https://en.wikipedia.org/wiki/Megacephalosaurus
