Elasmosauridae

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Elasmosauridae
Temporal range: Hauterivian-Maastrichtian, 130–66  Ma
Elasomosaurus Face Clean.png
Reconstructed skeleton of Elasmosaurus platyurus in the Rocky Mountain Dinosaur Resource Center in Woodland Park, Colorado.
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Superorder:† Sauropterygia
Order:† Plesiosauria
Clade:† Xenopsaria
Family:† Elasmosauridae
Cope, 1869
Genera
Synonyms
  • Cimoliasauridae
    Persson, 1960

Elasmosauridae is an extinct family of plesiosaurs, often called elasmosaurs. They had the longest necks of the plesiosaurs and existed from the Hauterivian to the Maastrichtian stages of the Cretaceous, and represented one of the two groups of plesiosaurs present at the end of the Cretaceous alongside Polycotylidae.

Description

Restoration of Thalassomedon haningtoni. Thalassomedon haningtoni.jpg
Restoration of Thalassomedon haningtoni .

The earliest elasmosaurids were mid-sized, about 6 m (20 ft). In the Late Cretaceous, elasmosaurids grew as large as 11.5–12 m (38–39 ft), such as Styxosaurus , Albertonectes , and Thalassomedon . Their necks were the longest of all the plesiosaurs, with anywhere between 32 and 76 (Albertonectes) cervical vertebrae. They weighed up to several tons.

Classification

Early three-family classification

Though Cope had originally recognized Elasmosaurus as a plesiosaur, in an 1869 paper he placed it, with Cimoliasaurus and Crymocetus , in a new order of sauropterygian reptiles. He named the group Streptosauria, or "reversed lizards", due to the orientation of their individual vertebrae supposedly being reversed compared to what is seen in other vertebrate animals. [13] [14] He subsequently abandoned this idea in his 1869 description of Elasmosaurus, where he stated he had based it on Leidy's erroneous interpretation of Cimoliasaurus. In this paper, he also named the new family Elasmosauridae, containing Elasmosaurus and Cimoliasaurus, without comment. Within this family, he considered the former to be distinguished by a longer neck with compressed vertebrae, and the latter by a shorter neck with square, depressed vertebrae. [15]

In subsequent years, Elasmosauridae came to be one of three groups in which plesiosaurs were classified, the others being the Pliosauridae and Plesiosauridae (sometimes merged into one group). [16] In 1874 Harry Seeley took issue with Cope's identification of clavicles in the shoulder girdle of Elasmosaurus, asserting that the supposed clavicles were actually scapulae. He found no evidence of a clavicle or an interclavicle in the shoulder girdle of Elasmosaurus; he noted that the absence of the latter bone was also seen in a number of other plesiosaur specimens, which he named as new elasmosaurid genera: Eretmosaurus , Colymbosaurus , and Muraenosaurus . [17] Richard Lydekker subsequently proposed that Elasmosaurus, Polycotylus, Colymbosaurus, and Muraenosaurus could not be distinguished from Cimoliasaurus based on their shoulder girdles, and advocated their synonymization at the genus level. [18] [19]

Seeley noted in 1892 that the clavicle was fused to the coracoid by a suture in elasmosaurians, and was apparently "an inseparable part" of the scapula. Meanwhile, all plesiosaurs with two-headed neck ribs (the Plesiosauridae and Pliosauridae) had a clavicle made only of cartilage, such that ossification of the clavicle would turn a "plesiosaurian" into an "elasmosaurian". [20] Williston doubted Seeley's usage of neck ribs to subdivide plesiosaurs in 1907, opining that double-headed neck ribs were instead a "primitive character confined to the early forms". [21] Charles Andrews elaborated on differences between elasmosaurids and pliosaurids in 1910 and 1913. He characterized elasmosaurids by their long necks and small heads, as well as by their rigid and well-developed scapulae (but atrophied or absent clavicles and interclavicles) for forelimb-driven locomotion. Meanwhile, pliosaurids had short necks but large heads, and used hindlimb-driven locomotion. [22] [23]

Refinement of plesiosaur taxonomy

Although the placement of Elasmosaurus in the Elasmosauridae remained uncontroversial, opinions on the relationships of the family became variable over subsequent decades. Williston created a revised taxonomy of plesiosaurs in a monograph on the osteology of reptiles (published posthumously in 1925). He provided a revised diagnosis of the Elasmosauridae; aside from the small head and long neck, he characterized elasmosaurids by their single-headed ribs; scapulae that meet at the midline; clavicles that are not separated by a gap; coracoids that are "broadly separated" in their rear half; short ischia; and the presence of only two bones (the typical condition) in the epipodialia (the "forearms" and "shins" of the flippers). He also removed several plesiosaurs previously considered to be elasmosaurids from this family due to their shorter necks and continuously meeting coracoids; these included Polycotylus and Trinacromerum (the Polycotylidae), as well as Muraenosaurus, Cryptoclidus , Picrocleidus , Tricleidus , and others (the Cryptoclididae). [24]

In 1940 Theodore White published a hypothesis on the interrelationships between different plesiosaurian families. He considered Elasmosauridae to be closest to the Pliosauridae, noting their relatively narrow coracoids as well as their lack of interclavicles or clavicles. His diagnosis of the Elasmosauridae also noted the moderate length of the skull (i.e., a mesocephalic skull); the neck ribs having one or two heads; the scapula and coracoid contacting at the midline; the blunted rear outer angle of the coracoid; and the pair of openings (fenestrae) in the scapula–coracoid complex being separated by a narrower bar of bone compared to pliosaurids. The cited variability in the number of heads on the neck ribs arises from his inclusion of Simolestes to the Elasmosauridae, since the characteristics of "both the skull and shoulder girdle compare more favorably with Elasmosaurus than with Pliosaurus or Peloneustes." He considered Simolestes a possible ancestor of Elasmosaurus. [25] Oskar Kuhn adopted a similar classification in 1961. [26]

Welles took issue with White's classification in his 1943 revision of plesiosaurs, noting that White's characteristics are influenced by both preservation and ontogeny. He divided plesiosaurs into two superfamilies, the Plesiosauroidea and Pliosauroidea, based on neck length, head size, ischium length, and the slenderness of the humerus and femur (the propodialia). Each superfamily was further subdivided by the number of heads on the ribs, and the proportions of the epipodialia. Thus, elasmosaurids had long necks, small heads, short ischia, stocky propodialia, single-headed ribs, and short epipodialia. [27] Pierre de Saint-Seine in 1955 and Alfred Romer in 1956 both adopted Welles' classification. [26] In 1962 Welles further subdivided elasmosaurids based on whether they possessed pelvic bars formed from the fusion of the ischia, with Elasmosaurus and Brancasaurus being united in the subfamily Elasmosaurinae by their sharing of completely closed pelvic bars. [28]

Persson, however, considered Welles' classification too simplistic, noting in 1963 that it would, in his opinion, erroneously assign Cryptoclidus, Muraenosaurus, Picrocleidus, and Tricleidus to the Elasmosauridae. Persson refined the Elasmosauridae to include traits such as the crests on the sides of the neck vertebrae; the hatchet-shaped neck ribs at the front of the neck; the fused clavicles; the separation of the coracoids at the rear; and the rounded, plate-like pubis. He also retained the Cimoliasauridae as separate from the Elasmosauridae, and suggested, based on comparisons of vertebral lengths, that they diverged from the Plesiosauridae in the Late Jurassic or Early Cretaceous. [26] However, D. S. Brown noted in 1981 that the variability of neck length in plesiosaurs made Persson's argument unfeasible, and moved the aforementioned genera back into the Elasmosauridae; he similarly criticized Welles' subdivision of elasmosaurids based on the pelvic bar. Brown's diagnosis of elasmosaurids included the presence of five premaxillary teeth; the ornamentation of teeth by longitudinal ridges; the presence of grooves surrounding the occipital condyles; and the broad-bodied scapulae meeting at the midline. [29]

Modern phylogenetic context

Carpenter's 1997 phylogenetic analysis of plesiosaurs challenged the traditional subdivision of plesiosaurs based on neck length. He found that Libonectes and Dolichorhynchops shared characteristics such as an opening on the palate for the vomeronasal organ, the plate-like expansions of the pterygoid bones, and the loss of the pineal foramen on the top of the skull, differing from the pliosaurs. While polycotylids had previously been part of the Pliosauroidea, Carpenter moved polycotylids to become the sister group of the elasmosaurids based on these similarities, thus implying that polycotylids and pliosauroids evolved their short necks independently. [30]

F. Robin O'Keefe likewise included polycotylids in the Plesiosauroidea in 2001 and 2004, but considered them more closely related to the Cimoliasauridae and Cryptoclididae in the Cryptocleidoidea. [16] [31] [32] Some analyses continued to recover the traditional groupings. In 2008 Patrick Druckenmiller and Anthony Russell moved the Polycotylidae back into the Pliosauroidea, and placed Leptocleidus as their sister group in the newly named Leptocleidoidea; [33] Adam Smith and Gareth Dyke independently found the same result in the same year. [34] However, in 2010 Hilary Ketchum and Roger Benson concluded that the results of these analyses were influenced by inadequate sampling of species. In the most comprehensive phylogeny of plesiosaurs yet, they moved the Leptocleidoidea (renamed the Leptocleidia) back into the Plesiosauroidea as the sister group of the Elasmosauridae; [35] subsequent analyses by Benson and Druckenmiller recovered similar results, and named the Leptocleidoidea–Elasmosauridae grouping as Xenopsaria. [36] [37]

The content of Elasmosauridae also received greater scrutiny. Since its initial assignment to the Elasmosauridae, the relationships of Brancasaurus had been considered well supported, and it was recovered by O'Keefe's 2004 analysis [31] and Franziska Großmann's 2007 analysis. [38] However, Ketchum and Benson's analysis instead included it in the Leptocleidia, [35] and its inclusion in that group has remained consistent in subsequent analyses. [36] [37] [39] Their analysis also moved Muraenosaurus to the Cryptoclididae, and Microcleidus and Occitanosaurus to the Plesiosauridae; [35] Benson and Druckenmiller isolated the latter two in the group Microcleididae in 2014, and considered Occitanosaurus a species of Microcleidus. [37] These genera had all previously been considered to be elasmosaurids by Carpenter, Großmann, and other researchers. [40] [38] [41] [42]

Within the Elasmosauridae, Elasmosaurus itself has been considered a "wildcard taxon" with highly variable relationships. [43] Carpenter's 1999 analysis suggested that Elasmosaurus was more basal (i.e. less specialized) than other elasmosaurids with the exception of Libonectes. [40] In 2005 Sachs suggested that Elasmosaurus was closely related to Styxosaurus, [44] and in 2008 Druckenmiller and Russell placed it as part of a polytomy with two groups, one containing Libonectes and Terminonatator , the other containing Callawayasaurus and Hydrotherosaurus . [33] Ketchum and Benson's 2010 analysis included Elasmosaurus in the former group. [35] Benson and Druckenmiller's 2013 analysis (below, left) further removed Terminonatator from this group and placed it as one step more derived (i.e., more specialized). [36] In Rodrigo Otero's 2016 analysis based on a modification of the same dataset (below, right), Elamosaurus was the closest relative of Albertonectes , forming the Styxosaurinae with Styxosaurus and Terminonatator. [39] Danielle Serratos, Druckenmiller, and Benson could not resolve the position of Elasmosaurus in 2017, but they noted that Styxosaurinae would be a synonym of Elasmosaurinae if Elasmosaurus did fall within the group. [43] In 2021 a new topology placed Cardiocorax as a sister taxon of Libonectes, [45] representing an older lineage of elasmosaurids in the Maastrichtian.

The family Elasmosauridae was erected by Cope in 1869, and anchored on the genus Elasmosaurus .

Ecology

Preserved stomach contents of Styxosaurus show that it fed on fish. [46]

Related Research Articles

<span class="mw-page-title-main">Plesiosauroidea</span> Extinct clade of reptiles

Plesiosauroidea is an extinct clade of carnivorous marine reptiles. They have the snake-like longest neck to body ratio of any reptile. Plesiosauroids are known from the Jurassic and Cretaceous periods. After their discovery, some plesiosauroids were said to have resembled "a snake threaded through the shell of a turtle", although they had no shell.

<i>Elasmosaurus</i> Genus of reptiles (fossil)

Elasmosaurus is a genus of plesiosaur that lived in North America during the Campanian stage of the Late Cretaceous period, about 80.5 million years ago. The first specimen was discovered in 1867 near Fort Wallace, Kansas, US, and was sent to the American paleontologist Edward Drinker Cope, who named it E. platyurus in 1868. The generic name means "thin-plate reptile", and the specific name means "flat-tailed". Cope originally reconstructed the skeleton of Elasmosaurus with the skull at the end of the tail, an error which was made light of by the paleontologist Othniel Charles Marsh, and became part of their "Bone Wars" rivalry. Only one incomplete Elasmosaurus skeleton is definitely known, consisting of a fragmentary skull, the spine, and the pectoral and pelvic girdles, and a single species is recognized today; other species are now considered invalid or have been moved to other genera.

<span class="mw-page-title-main">Plesiosaur</span> Order of reptiles (fossil)

The Plesiosauria or plesiosaurs are an order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia.

<i>Umoonasaurus</i> Extinct species of reptile

Umoonasaurus is an extinct genus of plesiosaur belonging to the family Leptocleididae. This genus lived approximately 115 million years ago during the Early Cretaceous period, in shallow seas covering parts of what is now Australia. It was a relatively small animal around 2.5 metres (8.2 ft) long. An identifying trait of Umoonasaurus is three crest-ridges on its skull.

<i>Plesiopleurodon</i> Extinct genus of reptiles

Plesiopleurodon is an extinct genus of Mesozoic marine reptiles, belonging to Sauropterygia, known from the Late Cretaceous of North America. It was named by Kenneth Carpenter based on a complete skull with a mandible, cervical vertebra, and a coracoid. In naming the specimen, Carpenter noted "Of all known pliosaurs, Plesiopleurodon wellesi most closely resembles Liopleurodon ferox from the Oxfordian of Europe, hence the generic reference." It was initially described as a pliosaur due to it short neck, a common trait of the family, although it is in the order Plesiosauria. However, later exploration into the relationships of both orders indicate that not all pliosaurs have short necks and not all plesiosaurs have long necks.

<span class="mw-page-title-main">Polycotylidae</span> Extinct family of reptiles

Polycotylidae is a family of plesiosaurs from the Cretaceous, a sister group to Leptocleididae. Polycotylids first appeared during the Albian stage of the Early Cretaceous, before becoming abundant and widespread during the early Late Cretaceous. Several species survived into the final stage of the Cretaceous, the Maastrichtian.

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Tuarangisaurus is an extinct genus of elasmosaurid known from New Zealand. The type and only known species is Tuarangisaurus keyesi, named by Wiffen and Moisley in 1986.

<i>Styxosaurus</i> Extinct genus of reptiles

Styxosaurus is a genus of plesiosaur of the family Elasmosauridae. Styxosaurus lived during the Campanian age of the Cretaceous period. Three species are known: S. snowii, S. browni, and S. rezaci.

<i>Aristonectes</i> Extinct genus of marines reptiles

Aristonectes is an extinct genus of large elasmosaurid plesiosaurs that lived during the Maastrichtian stage of the Late Cretaceous. Two species are known, A. parvidens and A. quiriquinensis, whose fossil remains were discovered in what are now Patagonia and Antarctica. Throughout the 20th century, Aristonectes was a difficult animal for scientists to analyze due to poor fossil preparation, its relationships to other genera were uncertain. After subsequent revisions and discoveries carried out from the beginning of the 21st century, Aristonectes is now recognised as the type genus of the subfamily Aristonectinae, a lineage of elasmosaurids characterized by an enlarged skull and a reduced length of the neck.

Kaiwhekea is an extinct genus of plesiosaur from the Late Cretaceous of what is now New Zealand.

<i>Brancasaurus</i> Extinct genus of reptiles

Brancasaurus is a genus of plesiosaur which lived in a freshwater lake in the Early Cretaceous of what is now North Rhine-Westphalia, Germany. With a long neck possessing vertebrae bearing distinctively-shaped "shark fin"-shaped neural spines, and a relatively small and pointed head, Brancasaurus is superficially similar to Elasmosaurus, albeit smaller in size at 3.26 metres (10.7 ft) in length as a subadult.

Eromangasaurus is an extinct genus of elasmosaurid known from northern Queensland of Australia.

<span class="mw-page-title-main">Rhomaleosauridae</span> Extinct family of reptiles

Rhomaleosauridae is a family of plesiosaurs from the Earliest Jurassic to the latest Middle Jurassic of Europe, North America, South America and possibly Asia. Most rhomaleosaurids are known from England, many specifically from lower Blue Lias deposits that date back to the earliest Jurassic, just at the boundary with the Triassic. In fact, to date only two undisputed rhomaleosaurids were named from outside Europe - the closely related Borealonectes russelli and Maresaurus coccai from Canada and Argentina, respectively. These two species are also the only Middle Jurassic representatives of the family. Rhomaleosauridae was formally named by Kuhn in 1961, originally proposed to include Rhomaleosaurus cramptoni and its relatives, which have short necks and large heads relatively to plesiosauroids like Elasmosaurus and Plesiosaurus, but longer necks and smaller heads relatively to advanced pliosaurids like Pliosaurus and Kronosaurus.

<span class="mw-page-title-main">Timeline of plesiosaur research</span>

This timeline of plesiosaur research is a chronologically ordered list of important fossil discoveries, controversies of interpretation, taxonomic revisions, and cultural portrayals of plesiosaurs, an order of marine reptiles that flourished during the Mesozoic Era. The first scientifically documented plesiosaur fossils were discovered during the early 19th century by Mary Anning. Plesiosaurs were actually discovered and described before dinosaurs. They were also among the first animals to be featured in artistic reconstructions of the ancient world, and therefore among the earliest prehistoric creatures to attract the attention of the lay public. Plesiosaurs were originally thought to be a kind of primitive transitional form between marine life and terrestrial reptiles. However, now plesiosaurs are recognized as highly derived marine reptiles descended from terrestrial ancestors.

Vegasaurus is an extinct genus of elasmosaurid plesiosaur known from the Late Cretaceous Snow Hill Island Formation of Vega Island, Antarctic Peninsula. It contains a single species, Vegasaurus molyi.

<i>Kawanectes</i> Extinct genus of reptiles

Kawanectes is a genus of elasmosaurid plesiosaur, a type of long-necked marine reptile, that lived in the marginal marine environment of Late Cretaceous Patagonia. It contains one species, K. lafquenianum, described in 2016 by O'Gorman.

<i>Nakonanectes</i> Extinct genus of reptiles

Nakonanectes bradti is an elasmosaurid plesiosaur of the late Cretaceous found in 2010 the state of Montana in the United States. It is one of the most recently known elasmosaurids to have lived in North America. Unlike other elasmosaurids, it has a relatively short neck.

References

  1. ↑ F. Robin O'Keefe; Hallie P. Street (2009). "Osteology Of The Cryptoclidoid Plesiosaur Tatenectes laramiensis, With Comments On The Taxonomic Status Of The Cimoliasauridae" (PDF). Journal of Vertebrate Paleontology. 29 (1): 48–57. Bibcode:2009JVPal..29...48O. doi:10.1671/039.029.0118. S2CID   31924376.
  2. ↑ Schumacher, Bruce A.; Everhart, Michael J. (2022). "Washed Ashore – New Elasmosaurid Specimens (Plesiosauria: Sauropterygia) from the Late Cretaceous of Colorado and Kansas and Their Bearing on Elasmosaurid Lineages of the Western Interior Seaway". Transactions of the Kansas Academy of Science. 125 (3–4): 237–263. doi:10.1660/062.125.0313. ISSN   0022-8443. S2CID   253364262.
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