Paleontologists have described a new genus and species of small polycotylid plesiosaur from two specimens found in Wyoming and South Dakota in the United States.
Life reconstruction of Unktaheela specta in its proposed habitat of sunlit waters just below the surface, with eyes shaded by supraorbital ledge. Image credit: Clark et al., doi: 10.1016/j.cretres.2023.105812.
Unktaheela specta inhabited the waters of the Western Interior Seaway during the Late Cretaceous epoch, some 80.5 million years ago.
This marine reptile was a type of polycotylid, a family of plesiosaurs that evolved during the Early Cretaceous and radiated into multiple genera during the Late Cretaceous.
“Polycotylids were Cretaceous marine reptiles of the superfamily Plesiosauroidea,” said lead author Dr. Robert Clark and his colleagues at Marshall University.
“The earliest polycotylids are known from the Aptian of Australia, but they reached North America by the Albian and attained a cosmopolitan distribution before their demise with the rest of Plesiosauria at the end of the Maastrichtian.”
“Polycotylids had the plesiosaurian bauplan of a broad, hydrodynamic body with a short tail, four limbs modified into large paddles, and expanded pectoral and pelvic girdles, but they also possessed the derived condition of a short neck and large head with an elongated rostrum.”
“Although convergent with pliosaurids in these traits, the smaller gape and teeth of most polycotylids indicate they occupied a different ecological niche.”
The holotype of Unktaheela specta. Image credit: Clark et al., doi: 10.1016/j.cretres.2023.105812.
The two fossilzied skulls (holotype and paratype) of Unktaheela specta were recovered from the Baculites obtusus zone of the Sharon Springs Formation in the United States.
“The holotype is a cranium and mandible with nearly complete cervical, sacral, and caudal series, partial dorsal series, ribs, portions of the pelvic and pectoral girdles, both ilia, both fore-paddles, and both hind-paddles,” the paleontologists said.
“The paratype is a complete cranium, mandible in three pieces, and five small, fragmentary, unidentified postcranial elements.”
Unktaheela specta was a small polycotylid — between 2.3 and 2.6 m (7.5-8.5 feet) in length — with a wide skull and unique paddles.
It had flat processes over its large eyes that likely shaded them from sunlight. It may have visually pursued prey just below the water’s surface.
“Comparisons to extant tetrapods suggest that several features of Unktaheela specta may be adaptations for visual pursuit predation in a sunlit environment, an interpretation with ecological implications for other aquatic reptiles,” the researchers said.
Their findings were published in the journal Cretaceous Research.
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Robert O. Clark et al. A new genus of small polycotylid plesiosaur from the Upper Cretaceous of the Western Interior Seaway and a clarification of the genus Dolichorhynchops. Cretaceous Research, published online December 24, 2023; doi: 10.1016/j.cretres.2023.105812
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