Jormungandr walhallaensis

Jormungandr is an extinct genus of mosasaurid squamates from the early Campanian Pierre Shale of North Dakota, United States. The genus contains a single species, J. walhallaensis, known from a nearly-complete skull and partial skeleton. Jormungandr was a medium-sized mosasaur, at around 6-8 m long, and its skeletal anatomy exhibits a mix of features seen in both basal and derived mosasaurines.

Discovery and naming
The holotype specimen was first discovered as a single piece of bone in 2015 by Deborah Shepherd while revisiting a public fossil dig site near Walhalla, North Dakota. The fossil came from a bentonite layer of the Pembina Member of the Pierre Shale, radiometrically dated to about 80.04 ± 0.11 mya. Shepherd informed a park ranger about the bone, which led to the notification of North Dakota Geological Survey senior paleontologist Clint Boyd. Boyd subsequently led a volunteer excavation of the site, which over a multi-year period until 2018 recovered the remainder of the specimen: a near-complete disarticulated skull missing only the braincase, parietal bone, eye bones, and parts of the nose (nasals and septomaxilla); all seven cervical vertebrae, five dorsal vertebrae, eleven ribs, and additional postcranial bones that remain unprepared. The specimen, which Boyd nicknamed "Jorgie," was curated into the North Dakota Heritage Center as NDGS 10838.

A subsequent study was led by Amelia Zietlow, a PhD student at the American Museum of Natural History, with Boyd and West Virginia paleontologist Nathan Van Vranken as co-authors. The team initially suspected the specimen to be the oldest known Mosasaurus, but further preparation uncovered features indicating a new intermediate genus and species between Mosasaurus and the more primitive Clidastes. The study, published in 2023, named the species Jormungandr walhallaensis. The specific name "walhallaensis" references the North Dakota city, which was named after the great hall Valhalla from Norse mythology. This inspired the generic name Jormungandr, which is a Latinization of the Norse serpent Jǫrmungandr that was said to encircle the world's oceans.

Description
Jormungandr is a large mosasaur. The holotype skull measures 72 cm in total length and the lower jaw is 80.8 cm long. Based on these measurements, Zietlow and colleagues estimated a total body length of 5.4–7.3 m.

The fossil material of Jormungandr demonstrates features seen in both basal and derived mosasaurines; it shares a high dental count with the more basal Clidastes, as well as the subrectangular quadrate seen in the derived Mosasaurus.

The fourth dorsal vertebra has traces that probably represent bite marks that were formed during a single biting event right before death or some time after death based on the fact that they show no signs of healing. The describers of Jormungandr argued that these supposed bite marks are consistent with those made by mosasaurs but not sharks, and that the tracemaker may have consumed the posterior (back) portion of the skeleton, resulting in the remaining body parts being separated from it.

Classification
In their phylogenetic analyses, Zietlow, Boyd & van Vranken (2023) recovered Jormungandr as a mosasaurine member of the squamate clade Mosasauridae, with Clidastes consistently as its closest relative. The following cladograms represent the phylogenetic results from the strict consensus of parsimony analyses using several extant anguimorphs as outgroups, depicting only the Mosasaurinae clade. Topology A is recovered from the 86 most parsimonious unweighted trees, while Topology B is recovered from 5 trees with implied weighting (k=12). Definition of Mosasaurinae follows Madzia and Cau (2017).

Palaeoenvironment
Jormungandr was discovered in layers of the Pierre Shale (Pembina Member) in North Dakota, which dates to the early Campanian age of the late Cretaceous period, around 80.04 million years old. Many other fossil animals have been found here which would have likely been contemporaries of Jormungandr. These include many other mosasaur genera such as Clidastes, Latoplatecarpus, Platecarpus, Plioplatecarpus, and Tylosaurus, as well as birds (Brodavis, Hesperornis, and Ichthyornis), plesiosaurs (Dolichorhynchops, Elasmosaurus, Styxosaurus, and Trinacromerum), turtles (Protostega and Toxochelys), and several bony and cartilaginous fish.