| Scalamagnus | |
|---|---|
 | |
| Life restoration | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Superorder: | †Sauropterygia |
| Order: | †Plesiosauria |
| Superfamily: | †Plesiosauroidea |
| Family: | †Polycotylidae |
| Subfamily: | †Polycotylinae |
| Genus: | †Scalamagnus Clark, O'Keefe & Slack,2023 |
| Type species | |
| †Scalamagnus tropicensis Schmeisser McKean,2012 | |
| Synonyms | |
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Scalamagnus is anextinct genus ofpolycotylid plesiosaur from theLate CretaceousTropic Shale Formation of the United States. The genus contains asingle speciesS. tropicensis, known from a skull and two partial skeletons.Scalamagnus was historically considered to represent a species of the genusDolichorhynchops before it was moved to its own genus.[1]
The taxon was first named as a species ofDolichorhynchops by Rebecca Schmeisser McKean in2012. Thespecific name is derived from the name of theTropic Shale, in which the two specimens were found. It is known from theholotypeMNA V10046, an almost complete, well-preserved 3.2 metres (10 ft) longskeleton including most of theskull and from the referred specimen MNA V9431, fragmentarypostcranial elements. It was collected between 2003 and 2005 by theMuseum of Northern Arizona from a single locality within the Tropic Shale ofUtah, dating to the earlyTuronian stage of the earlyLate Cretaceous, about 93.5-91million years ago.D. tropicensis extended the known stratigraphic range forDolichorhynchops back by approximately 7 million years.[2] Previously three additional polycotylid taxa,Eopolycotylus,Palmulasaurus andTrinacromerum, have been named from the same formation, two of which are currentlyendemic to theTropic Shale.[3] A 2023 study assignsD. tropicensis to a new genus,Scalamagnus. The generic name is a combination of theLatinscala, "staircase", andmagnus, "large", referring to theGrand Staircase-Escalante National Monument.[1]
Clark, O'Keefe & Slack (2023) recoveredScalamagnus as apolycotylid member of the plesiosaur cladeLeptocleidia, as thesister taxon to the clade formed byTrinacromerum and theDolichorhynchia within thePolycotylinae. The results of theirphylogenetic analyses are shown in thecladogram below:[1]
The holotype is associated with 289gastroliths, which is unusual in comparison to most polycotylid skeletons that generally lack gastroliths. Ranging from less than 0.1 grams to 18.5 grams, the total mass of the gastroliths was about 518 grams. About three-quarters of the stones weighed less than 2 grams, with the mean mass and median mass of the stones respectively estimated at 1.9 grams and 0.8 grams. The gastroliths had high mean value and variability in sphericity, suggesting that this individual was obtaining its stones from rivers located along the western side of the Western Interior Seaway.[5]
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