Megalneusaurus Rex

''Megalneusaurus'' is an extinct genus of large pliosaurs that lived during the Oxfordian and Kimmeridgian stages of the Late Jurassic in what is now North America. It was provisionally described as a species of ''Cimoliosaurus'' by the geologist Wilbur Clinton Knight in 1895, before being given its own genus by the same author in 1898. The only species identified to date is ''M. rex'', known from several specimens identified in the Redwater Shale Member, within the Sundance Formation, Wyoming, United States. A specimen discovered in the Naknek Formation in southern Alaska was referred to the genus in 1994, without a specific assignment. The binominal name literally means "king of large swimming lizards", due to the measurement of the fossils of the holotype specimen.

Estimated to be around long, ''Megalneusaurus'' is one of the largest known North American pliosaur. As its name suggests, the genus was considered the largest sauropterygian identified before the discovery of some ''Kronosaurus'' fossils in 1930. Like some other plesiosaurs, ''Megalneusaurus'' has four paddle-like limbs, a short tail, and most likely an elongated head and short neck, suggesting that it is a thalassophonean-like pliosaurid. Some researchers consider the fossil material to be non-diagnostic, making ''Megalneusaurus'' a possible ''nomen dubium'', although opposing views are nevertheless maintained.

The animal would have lived in the shallow waters of the Sundance Sea, an epicontinental sea covering much of North America during part of the Jurassic. ''Megalneusaurus'' shared its habitat with invertebrates, fish, ichthyosaurs, and other plesiosaurs, including the cryptoclidids '' Pantosaurus'' and ''Tatenectes''. Based on stomach contents, the animal fed on cephalopods and fish, although it is not excluded that it would have attacked and fed on contemporary plesiosaurs. The referred specimen known from Alaska also indicates that it would have also occupied colder waters located further north of the continent, although the fauna is less diverse.

Research history

In 1895, American geologist Wilbur Clinton Knight went to examine an oil field near the small town of Ervay, Wyoming, USA. The renowned paleontologist Edward Drinker Cope was originally supposed to accompany Knight on this examination, but instead, he sent an individual named Stewart, who worked for him at the American Museum of Natural History. Knight spent about 30 days in the field with Stewart, before being left with two other men to return home. On his way back, while re-examining the oil field, Knight discovered the fossils of a partially articulated but incomplete skeleton of a large pliosaur. Lacking the materials to exhume the specimen, he contacted a rancher living nearby who lent him tools a wagon. The lack of packing materials, however, led Knight to rebury much of the fossil specimen, intending to return to continue his work later. While the geologist was away from the discovery site, Stewart arrived nearby and was quickly informed about the finding by the rancher who had lent his colleague the tools. Stewart then recovered most of the reburied fossils and sent them to the American Museum of Natural History, urging his two colleagues he had previously met to help him with this task. Describing this decision as theft, Knight sent two letters in 1898, addressed respectively to Henry Fairfield Osborn and to a certain J. M. Garett, requesting the return of the pliosaur and shark fossils residing in Cope's collection at the museum, which were originally discovered by him.

The fossils first unearthed by Knight have since been sent to the University of Wyoming, where they have since been catalogued as UW 4602. These concerns ribs, vertebrae, two more or less partial flippers and part of the shoulder girdle, from a specimen that would have reached adulthood. Knight made his discovery official the same year via an announcement that was published by ''Science Magazine'', where he briefly described certain fossils. As the fossils were not exhumed from their geological matrix at the time of publication of his announcement, Knight was uncertain about the generic positioning of the specimen. He therefore erected a new species of ''Cimoliosaurus'', ''C. rex'', to which he provisionally classified it pending a more in-depth description. It was three years later, in 1898, that Knight described the specimen in more detail and erected the distinct genus ''Megalneusaurus'' to include it, the species thus becoming ''M. rex''. The generic name is formed from the words in Ancient Greek (, "great") and (, "swimmer") prefixed onto (, "lizard"), while the specific name ''rex'' means "king" in Latin. Combined, the binominal name literally means "king of large swimming lizards". Although no description of the meaning of this etymology was given in his 1898 description, Knight would likely have named the taxon thus due to the rather considerable measurements of this specimen for the time, which he judges according to his words to be "the largest known animal of the order Sauropterygia".

Due to the discovery of other large pliosaurs found elsewhere in the world, the attention of ''Megalneusaurus'' by paleontologists has generally been "forgotten", being only very briefly mentioned in 20th and early 21st century scientific literature. In 1995, the Tate Geological Museum of Casper planned to create an exhibit showcasing marine reptiles discovered in the Sundance Formation. During the collections, researchers came across a cast of the original forelimb originally discovered by Knight. Based on this same casting, American paleontologist William Wahl of the Wyoming Dinosaur Center, began to take a particular interest in this taxon, which led him to open an investigation to find the location of the original type locality of ''Megalneusaurus''. It was during the summer of 1996 that the type locality was finally found, thanks among other things to letters and especially to a map drawn by Knight in 1901. The place corresponds to the Redwater Shale Member of the upper part of the Sundance Formation, which dates from the Oxfordian stage of the Late Jurassic. After this rediscovery, new excavations were launched in the field and additional fossils were found. About from the area where the first known fossils of ''Megalneusaurus'' were discovered, a large bone fragment probably coming from the shoulder or pelvic girdle was exhumed and made official in an 2007 article. In 2008, it was a fully articulated and almost complete forelimb coming directly from the holotype specimen which was exhumed, and it is described in detail in 2010. From the orientation of this forelimb, it is likely that the humerus, or part of it, was on or near the ground surface and could have been collected in 1895 by either Knight or Stewart. Fossils coming possibly from two other specimens have also been reported. The first is an isolated neural arch, cataloged as UW 24238, while the second is a propodial (upper limb bone), cataloged as WDC SS019. Other fossil finds referred to ''Megalneusaurus'' were made in this area over an additional period from 2009 until 2011, but these have not yet been officially described. In 2003, Richard Ellis reported that Robert Bakker should re-examine the holotype specimen in more detail, something that has apparently not been done since.

''Megalneusaurus'' was then historically only known from Wyoming, but another specimen referred to the genus was discovered in southern Alaska. It was in June 1922 that W. R. Smith of the United States Geological Survey received two bone fragments from a certain Jack Mason. These two bone fragments were collected from the Kejulik River, located on the Alaska Peninsula, and consist of the proximal and distal ends of the same large humerus, which would subsequently be cataloged as USNM 418489. The stratigraphic unit to which this specimen was discovered corresponds to the Snug Harbor Siltstone Member of the Naknek Formation, dating from between the Oxfordian and Kimmeridgian stages of the Late Jurassic. It was only in 1994 that Robert E. Weems and Robert B. Blodgett described this fossil in detail to which they referred it to the genus on the basis of comparisons made with the holotype specimen. However, as their description and comparison is only based on an isolated humerus, they refer it under the name ''Megalneusaurus'' sp., the authors being uncertain as to whether the specimen would belong to another species or to the type species.

Description

The various genera of pliosaurids that are regularly compared with ''Megalneusaurus'' are today classified in the Thalassophonea clade, which are characterized by an elongated skull connected to a short neck, unlike many other plesiosaurs, which have a long neck and a small head. Like all plesiosaurs, however, short tail, a massive trunk and two pairs of large flippers.

Size

''Megalneusaurus'' is the largest Jurassic pliosaurid identified in North America. Most studies and descriptions qualify ''Megalneusaurus'' as the largest pliosaur to have been identified in the North American continent of any period, but some related genera such as '' Megacephalosaurus'', dating from the Upper Cretaceous, are of comparable size. The taxon was even considered to be the largest known pliosaur in the world until some fossils of the Australian pliosaurid ''Kronosaurus'' were described in 1930. Several estimates of the size of ''Megalneusaurus'' have been given throughout research. In 2006, Wahl gave the animal a length of approximately , before the same author and his colleagues reduced its size to just the following year. In his 2009 thesis, Australian paleontologist Colin McHenry estimates the size of the animal at between long based on measurements of the femurs given by Knight in 1898. However, most recent estimates reduce the size of ''Megalneusaurus'' to between long. Based on a skeleton preserved in the Museum of Comparative Zoology of Harvard attributed to ''Kronosaurus'', Bakker suggests that the animal would have had a skull measuring long, although the relevant material is not known. In 2024, Chinese paleontologist Ruizhe Jackevan Zhao suggests that ''Megalneusaurus'' would have been similar in size to '' Pliosaurus funkei'', which according to his model was approximately long with a body mass of . The referred specimen from Alaska is smaller in measurement than the holotype specimen, although no estimate of its size has been given.

Postcranial skeleton

The majority of the anatomy of ''Megalneusaurus'' is only known from the holotype specimen and concerns only the postcranial parts of the animal, no cranial materials being known. In addition, additional parts including cervical, dorsal and caudal vertebrae, a large part of the shoulder girdle and ribs, have also been lost, the only remaining descriptions of them being made by Knight in 1898.

According to Knight's descriptions, the vertebrae of ''Megalneusaurus'' are twice as long as they are wide, although their morphology varies considerably. The anterior cervical vertebrae have a cupped anterior surface and a slightly concave posterior surface, having neural arches that are sutured together. The dorsal vertebrae are cylindrical in shape, overhanging forward from their upper part, their neural spines being low and crest-like. The caudal vertebrae have surfaces that are slightly concave. The coracoids are long and wide, being produced at the glenoid fossa of the scapula.

When Knight reported the discovery of the holotype specimen in 1895, he initially identified the flippers as hind limbs. However, in his more detailed description published three years later, he reidentified them as forelimbs. The discovery of the third swimming paddle in 2008, however, confirms that the first identification was correct, as it was 15% smaller than those first discovered. This also confirms that ''Megalneusaurus'' has longer hind limbs than forelimbs, like other pliosaurids. The humerus of ''Megalneusaurus'' is proportionally longer than that of ''Peloneustes'', indicating that it would have used a more powerful swimming method.

Classification and validity

When Knight made the discovery of the first known fossils of ''Megalneusaurus'' official in 1895, they were first classified in the genus ''Cimoliosaurus'', a plesiosauroid currently classified in the family Elasmosauridae. However, this classification was deliberately provisional, because the fossils were at that time unprepared to allow the specimen to have taxonomic clarification. It was in 1898 that Knight discovered that it was in fact a pliosaur, not hesitating to compare it to the pliosaurids ''Pliosaurus'' and ''Peloneustes''. Studies published from the second half of the 20th century agree that ''Megalneusaurus'' would most likely be a pliosaurid. In 2009, McHenry questioned the validity of ''Megalneusaurus'', judging the holotype specimen to be non diagnostic and possibly representing a ''nomen dubium'' or another taxon better known in other localities around the world. Two years earlier, in 2007, French paleontologist Marie-Céline Buchy was already perplexed by the attribution of the Alaskan material to this genus, judging it as non-diagnostic as well. Even before the writing of Buchy and McHenry's theses, Ellis already mentioned in 2003 that many researchers doubted the diagnostic nature of the holotype specimen due to the loss of the previously mentioned fossils, although he also points out that some are of the opposite opinion. This would explain why the name ''Megalneusaurus'' is still preserved in some later studies.

Paleobiology

Feeding

The stomach contents of the holotype specimen of ''M. rex'' contain numerous hooklets of coleoid cephalopods as well as a few rare fragments of fish bones. Similar stomach contents have also been documented in other contemporary plesiosaurs, such as ''Tatenectes'' and '' Pantosaurus'', proving that these were common food sources for marine reptiles of the Sundance Formation. Being a large pliosaurid, ''Megalneusaurus'' would likely have fed on other contemporary plesiosaurs, particularly juveniles or subadults. A juvenile plesiosaur fossil identified in this formation also shows bite marks on the bones of one of its flippers, although their origins have not been formally determined. Nevertheless, despite the lack of direct evidence of interactions, ''Megalneusaurus'' is interpreted as an opportunistic predator that likely occupied the top of the food web in the Sundance Formation.

Paleopathology

The hind limbs of the ''M. rex'' holotype specimen are affected by avascular necrosis. A large number of other more or less related plesiosaur fossils also show this disease in both the humerus and the femur. This is the result of ascent too quickly after a deep diving. It is nevertheless uncertain to deduce the depth to which the animal would have descended because it is also possible that the avascular necrosis would have been caused by a few very deep dives, or by a large number of relatively shallow descents. The vertebrae, however, do not show such damage, being, during the lifetime of plesiosaurs, probably protected by a superior blood supply, made possible by the arteries penetrating the bone through the two ''foramina subcentralia'', large openings in their lower face.

Paleoecology

Wyoming

''Megalneusaurus'' comes from the Oxfordian-aged (Upper Jurassic) rocks of the Redwater Shale Member of the Sundance Formation. This member is about thick. While mainly composed of grayish green shale, it also has layers of yellow limestone and sandstone, the former layers containing plentiful fossils of marine life. The Sundance Formation represents a shallow epicontinental sea known as the Sundance Sea. From the Yukon and Northwest Territories of Canada, where it was connected to the open ocean, this sea spanned inland southwards to New Mexico and eastward to the Dakotas. When ''Megalneusaurus'' was alive, most of the Sundance Sea was less than deep. Based on δ¹⁸O isotope ratios in belemnite fossils, the temperature in the Sundance Sea would have been below and above the thermocline.

The paleobiota of the Sundance Formation includes foraminiferans and algae, in addition to a variety of animals. Many invertebrates are known from the Sundance Formation, represented by crinoids, echinoids, serpulid worms, ostracods, malacostracans, and mollusks. The mollusks include cephalopods such as ammonites and belemnites, bivalves such as oysters and scallops, and gastropods. Fish from the formation are represented by hybodont and neoselachian chondrichthyans as well as teleosts (including ''Pholidophorus''). Marine reptiles are uncommon, but are represented by four species. Of all the plesiosaurs, ''Megalneusaurus'' is the only pliosaurid identified in the Sundance Formation. The other plesiosaurs known from this formation are the cryptoclidids ''Tatenectes'' and '' Pantosaurus''. Besides plesiosaurs, marine reptiles are also represented by the ichthyosaur ''Ophthalmosaurus'' (or, possibly, '' Baptanodon'') ''natans'', the most abundant marine reptile of the Sundance Formation.

Alaska

USNM 418489 likely originates from Oxfordian to Kimmeridgian rocks of the Snug Harbor Siltstone Member of the Naknek Formation. When the specimen was alive, this territory was located at a paleolatitude of more than 60°N, making the region a boreal environment. The sparse limestone sediments and the few mollusk fossilS in the Naknek Formation indicate that the peninsular terrane from which the specimen originates was located in cooler or even colder waters than in the Sundance Formation. The Naknek Formation also has a less diverse invertebrate fauna than the Sundance Formation, being mainly represented by mollusks. These include bivalves and cephalopods of the ammonite and belemnite groups. Paleobiota also include foraminifera, radiolarians, and fish fossil debris.

See also

* List of plesiosaur genera
* Timeline of plesiosaur research

Notes

References

External links

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{{Taxonbar, from=Q1911639

Late Jurassic plesiosaurs
Late Jurassic reptiles of North America
Jurassic plesiosaurs of North America
Sauropterygian genera
Fossil taxa described in 1898