New mosasaur named Tylosaurus rex

Amelia Zietlow and colleagues have given a new name to a predator that swam the Cretaceous seaway covering what is now Texas: *Tylosaurus rex*. The species was described on May 21 in the *Bulletin of the American Museum of Natural History* after researchers reexamined fossils that had largely been assigned to another mosasaur, *Tylosaurus proriger*. The team said the Texas material represents a distinct giant species that lived about 80 million years ago and reached an estimated 13.2 meters, or about 43 feet, in length. The fossils were found over decades and are held across multiple museum collections. ### Why are paleontologists saying this was a different species, not just a big old specimen? The AMNH study said several fossils previously cataloged as *Tylosaurus proriger* share a set of traits that separate them from that species, even when specimens overlap in size. The authors wrote that *Tylosaurus rex* can be identified by a “unique suite of characters,” including features associated with increased jaw and neck musculature. They also said those differences could not be explained simply as growth-related changes. (digitallibrary.amnh.org) Amelia Zietlow, the study’s lead author, said the project began when she noticed a museum specimen that appeared to be misidentified. She and colleagues compared that fossil with the holotype of *T. proriger* at Harvard’s Museum of Comparative Zoology, then extended the comparison to more than a dozen similar fossils at other institutions. ### How big was this animal, and how unusual was it among mosasaurs? (digitallibrary.amnh.org) The paper estimated body lengths for referred specimens of *Tylosaurus rex* at 7.7 to 13.2 meters. That range is larger than the estimates the authors reported for the biggest known *T. proriger* specimens, at 3.9 to 9.5 meters. The researchers cautioned that some of that size gap could reflect preservation and sampling bias, but they still described the Texas animal as one of the largest mosasaurs known. (amnh.org) The American Museum of Natural History said the fossils range from about 25 feet to 43 feet long. National Geographic, citing the study and interviews with the researchers, described the animal as among the biggest known mosasaurs and said its size was comparable to a humpback whale. ### What made its bite stand out? The AMNH release said the fossils show finely serrated teeth, an uncommon feature in mosasaurs, along with adaptations for exceptionally strong jaw and neck muscles. (digitallibrary.amnh.org) Those features led the researchers to describe it as a powerful predator. Amelia Zietlow told National Geographic that “half of its characteristics are around it having a bigger jaw and bite.” That is the basis for the “skull-crushing” language used in some coverage: it reflects an inference from skull and muscle-related anatomy, not a direct measurement of bite force from a living animal. (amnh.org) ### Why does Texas matter so much in this find? (amnh.org) The AMNH release said most fossils assigned to the new species came from northern Texas and are about 4 million years younger than the better-known *T. proriger* material from Kansas. The study placed *Tylosaurus rex* in the Campanian stage of the Late Cretaceous, when a broad inland sea split North America. (nationalgeographic.com) That geographic split helped the researchers argue they were not looking at the same animal in different growth stages. The paper also said incomplete locality and stratigraphic data in older North American mosasaur collections may mean other distinct species are still hidden inside historical museum labels. ### Where can readers look next? (digitallibrary.amnh.org) The holotype skeleton, cataloged as PMNS 8029, is at the Perot Museum of Nature and Science in Dallas, according to the AMNH release. The formal species description is published as issue 482 of the *Bulletin of the American Museum of Natural History*, in a 77-page paper by Zietlow and co-authors that also updates the character list used in mosasaur phylogenetic analysis. (digitallibrary.amnh.org)