Креталық кезеңнің қазба тісқабатты жәндіктері: Элазмозавр

Genus of reptiles (fossil)

Elasmosaurus (/ɪ/,/l//æ//z//m//ə/'/s//ɔːr//ə//s//, // //m//oʊ// /) is a genus of plesiosaur that lived in North America during the Campanian stage of the Late Cretaceous period, about 80.5million 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. Measuring in length, Elasmosaurus would have had a streamlined body with paddle like limbs, a short tail, a small head, and an extremely long neck. The neck alone was around long. Along with its relative Albertonectes, it was one of the longest necked animals to have lived, with the second largest number of neck vertebrae known, 72, 4 fewer than Albertonectes. The skull would have been slender and triangular, with large, fang like teeth at the front, and smaller teeth towards the back. It had six teeth in each premaxilla of the upper jaw, and may have had 14 teeth in the maxilla and 19 in the dentary of the lower jaw. Most of the neck vertebrae were compressed sideways, and bore a longitudinal crest or keel along the sides. The family Elasmosauridae was based on the genus Elasmosaurus, the first recognized member of this group of long necked plesiosaurs. Elasmosaurids were well adapted for aquatic life, and used their flippers for swimming. Contrary to earlier depictions, their necks were not very flexible, and could not be held high above the water surface. It is unknown what their long necks were used for, but they may have had a function in feeding. Elasmosaurids probably ate small fish and marine invertebrates, seizing them with their long teeth, and may have used gastroliths (stomach stones) to help digest their food. Elasmosaurus is known from the Pierre Shale formation, which represents marine deposits from the Western Interior Seaway.

In early 1867, the American army surgeon Theophilus H. Turner and the army scout William Comstock explored the rocks around Fort Wallace, Kansas, where they were stationed during the construction of the Union Pacific Railroad. Approximately northeast of Fort Wallace, near McAllaster, Turner discovered the bones of a large fossil reptile in a ravine in the Pierre Shale formation, and though he had no paleontological experience, he recognized the remains as belonging to an "extinct monster". In June, Turner gave three fossil vertebrae to the American scientist John LeConte, a member of the railway survey, to take back east to be identified. In December, LeConte delivered some of the vertebrae to the American paleontologist Edward Drinker Cope at the Academy of Natural Sciences of Philadelphia (ANSP, known since 2011 as the Academy of Natural Sciences of Drexel University). Recognizing them as the remains of a plesiosaur, larger than any he had seen in Europe, Cope wrote to Turner asking him to deliver the rest of the specimen, at the ANSP's expense. In December 1867 Turner and others from Fort Wallace returned to the site and recovered much of the vertebral column, as well as concretions that contained other bones; the material had a combined weight of The fossils were dug or pried out of the relatively soft shale with picks and shovels, loaded on a horse drawn wagon, and transported back to Fort Wallace. Cope sent instructions on how to pack the bones, which were thereafter sent in hay padded crates on a military wagon east to the railroad, which had not yet reached the fort. The specimen arrived in Philadelphia by rail in March 1868, whereafter Cope examined it hurriedly; he reported on it at the March ANSP meeting, during which he named it Elasmosaurus platyurus. The generic name Elasmosaurus means "thin plate reptile", in reference to the "plate" bones of the sternal and pelvic regions, and the specific name platyurus means "flat tailed", in reference to the compressed "tail" (actually the neck) and laminae of the vertebrae there. In 1986 a three dimensional reconstruction of the holotype skeleton was completed and is now displayed at the ANSP. This cast was later copied by the company Triebold Paleontology Incorporated, and replicas were provided to other museums. The replica at the Fort Wallace Museum measures about in length. In 2018, Davidson and Everhart documented the events leading up to the disappearance of these fossils, and suggested that a photo and drawing of Waterhouse's workshop from 1869 appear to show concretions on the floor that may have been the unprepared girdles of Elasmosaurus. They also noted that conceptual sketches of the Palaeozoic Museum show that the model Elasmosaurus was originally envisioned with a long "tail", though later updated with a long neck. Davidson and Everhart concluded that the girdle fossils were most likely destroyed in Hawkins' workshop. Fossils that may have belonged to the holotype were found by the American geologist Benjamin Franklin Mudge in 1871, but have probably been lost since. In 2007 the Colombian paleontologists Leslie Noè and Marcela Gómez Pérez expressed doubt that the additional elements belonged to the type specimen, or even to Elasmosaurus, due to lack of evidence. They explained that elements missing from the holotype may have been lost to weathering or simply not collected, and that parts may have been lost or damaged during transportation or preparation. Gastroliths may also not have been recognized as such during collection, since such stones were not reported from a plesiosaur until ten years after. In 2017 Sachs and Joachim Ladwig suggested that a fragmentary elasmosaurid skeleton from the upper Campanian of Kronsmoor in Schleswig Holstein, Germany, and housed in the Naturkunde Museum Bielefeld, may have belonged to Elasmosaurus. Additional parts of the same skeleton are housed at the Institute for Geology of the University of Hamburg, as well as in private collections. Combined, the specimen consists of neck, back and tail vertebrae, phalanges, a tooth, limb elements, 110 gastroliths, and unidentifiable fragments.

Though the only known specimen of Elasmosaurus (holotype specimen ANSP10081) is fragmentary and missing many elements, related elasmosaurids show it would have had a compact, streamlined body, long, paddle like limbs, a short tail, a proportionately small head, and an extremely long neck. The neck of Elasmosaurus is estimated at in length; thus, Elasmosaurus and its relative Albertonectes were some of the longest necked animals ever to have lived, with the largest number of neck vertebrae of any known vertebrate animals. Like other elasmosaurids, Elasmosaurus would have had a slender, triangular skull. The snout was rounded and almost formed a semi circle when viewed from above, and the premaxillae (which form the front of the upper jaw) bore a low keel at the midline. It is uncertain how many teeth Elasmosaurus had, due to the fragmentary state of the fossils. It probably had six teeth in each premaxilla, and the teeth preserved there were formed like large fangs. The number of premaxillary teeth distinguished Elasmosaurus from primitive plesiosauroids and most other elasmosaurids, which usually had fewer. The two teeth at the front were smaller than the succeeding ones, and were located between the first two teeth in the dentaries of the lower jaws. The known teeth of the front part of the lower jaw were large fangs, and the teeth at the back of the jaws appear to have been smaller. The dentition of elasmosaurids was generally heterodont (irregular throughout the jaws), with the teeth becoming progressively smaller from front to back. The maxillae (largest tooth bearing bone of the upper jaw) of elasmosaurids usually contained 14teeth, whereas the dentaries (the main part of the lower jaws) usually contained 17 to 19. The teeth interlocked, and their tooth crowns were slender and rounded in cross section. The mandibular symphysis (where the two halves of the lower jaw connected) was well ossified, with no visible suture. The pectoral and pelvic girdles of the holotype specimen were noted as missing by 1906, but observations about these elements were since made based on the original descriptions and figures from the late 19thcentury. The shoulder blades (scapulae) were fused and met at the midline, bearing no trace of a median bar. The upper processes of the shoulder blades were very broad, and the "necks" of the shoulder blades were long. The pectoral girdle had a long bar at the middle, a supposedly advanced feature thought to be absent from juvenile plesiosaurs. The ischia (a pair of bones that formed part of the pelvis) were joined at the middle, so that a medial bar was present along the length of the pelvis, a feature usually not found in plesiosaurs. Accompanying his 1869 description of E. platyurus, Cope named another species of Elasmosaurus, E. orientalis, based on two back vertebrae from New Jersey. He distinguished E. orientalis from E. platyurus by the more strongly developed processes known as parapophyses on the vertebrae, in which he considered it to approach closer to Cimoliasaurus; however, he still assigned it to Elasmosaurus on account of its large size and angled sides. The first of these vertebrae was used as a doorstop in a tailor's shop, whereas the other was found in a pit by Samuel Lockwood, a superintendent. Cope gave the name orientalis to the new species, on account of it possibly having a more easterly distribution than E. platyurus. In 1952 Welles considered the species a nomen dubium, given how fragmentary it was. In an 1870 reply to Leidy, Cope himself stated that the generic placement of E. orientalis was in doubt, and that he had illustrated it with a short neck due to believing this was the condition of Cimoliasaurus. If more remains showed E. orientalis to have had a long neck like Elasmosaurus, he stated the image may instead represent Cimoliasaurus better. In the same 1869 publication wherein he named E. platyurus and E. orientalis, Cope assigned an additional species, E. constrictus, Cope recognized this as a natural condition, and considered constrictus to be "a species of Elasmosaurus or an ally". Per Ove Persson retained it as valid in 1963, noting the longitudinal ridge on the sides of the centra as an elasmosaurid trait. In 1995 Nathalie Bardet and Pascal Godefroit also recognized it as an elasmosaurid, albeit indeterminate. Cope discovered another elasmosaurid skeleton in 1876. He named it as a new species, E. serpentinus, in 1877, and differentiated it by the lack of compression in the rear neck vertebrae, the presence of few sessile ribs among the first few dorsals, and the presence of "weak angles" below the front tail vertebrae. Cope had also discovered another large skeleton that bore great resemblance to the known remains of E. orientalis from the black shale of the "Cretaceous bed No.4"; he excavated it with the help of George B. Cledenning and Capt. Nicholas Buesen. In 1943 Welles removed E. serpentinus from Elasmosaurus, and placed it in a new genus, Hydralmosaurus. Subsequently, all Hydralmosaurus specimens were moved to Styxosaurus in 2016, rendering the former a nomen dubium. Williston published a figure of another E. serpentinus specimen in 1914; Elmer Riggs formally described it in 1939. Welles moved this specimen to the new genus and species Alzadasaurus riggsi in 1943. Subsequently, a series of 19 neck and back vertebrae from the Big Bend region of the Missouripart of the Pierre Shale formationwere found by John H. Charles. Cope, upon receiving the bones at the Academy of Natural Sciences, considered them yet another species of Elasmosaurus. The vertebrae were, according to Cope, the shortest among members of the genus (approaching Cimoliasaurus in this condition), but he still considered them as belonging to Elasmosaurus due to their compressed form. He named it E. intermedius in 1894. However, in his 1906 revision of North American plesiosaurs, Williston regarded the vertebrae as "all more or less mutilated", and found no distinct differences between the remains of E. intermedius and E. platyurus. however, he proceeded to label it a nomen dubium in 1962. In 1874 he and Mudge discovered a specimen in Plum Creek, Kansas. he subsequently recognized the elasmosaurid nature of its humerus and coracoids. Thus, he renamed the species E. snowii. A second specimen, discovered by Elias West in 1890, was also assigned by him to E. snowii. in the same year, Carpenter assigned both to Styxosaurus snowii. Several Russian species, based on poorly preserved vertebral remains, were assigned to Elasmosaurus by N. N. Bogolubov in 1911. One was E. helmerseni, which was first described by W. Kiprijanoff in 1882 from Maloje Serdoba, Saratov, as Plesiosaurus helmerseni. Some material from Scania, Sweden, was assigned to P. helmerseni in 1885 by H. Schröder. Vertebral and limb remains from Kursk initially assigned by Kiprijanoff to P. helmerseni were also moved by Bogolubov to the new species E. kurskensis, which he considered to be "identical with Elasmosaurus or related to it". He also named E. orskensis, based on "very large" neck and tail vertebra remains from Konopljanka, Orenburg; and E. serdobensis, based on a single neck vertebra from Maloje Serdoba. However, the validity of all these species has been questioned. Welles considered E. kurskensis as an indeterminate plesiosaur in 1962. In 2000 Storrs, Archangelsky, and Vladimir Efimov concurred with Welles on E. kurskensis, and labelled E. orskensis and E. serdobensis as indeterminate elasmosaurids. this classification was followed by Alexander Averianov and V. K. Popov in 2005. Then, in 1916, Православлев, Павел Александрович named E. amalitskii from the Don River region, based on a specimen containing vertebrae, limb girdles, and limb bones. Persson considered it a valid species, and a relatively large member of the elasmosaurids; In a work published in 1889, Richard Lydekker assigned this species to Cimoliasaurus. Wilhelm Deecke moved chilensis to Pliosaurus in 1895, a classification which was acknowledged by Pravoslavlev. Edwin Colbert later assigned the type vertebra in 1949 to a pliosauroid, and also assigned other assigned remains to indeterminate elasmosauroids; the type vertebra was recognized as potentially belonging to Aristonectes parvidens by José O'Gorman and colleagues in 2013. Another was E. haasti, originally Mauisaurus haasti, named by James Hector in 1874 based on remains found in New Zealand. Although its validity was supported for a considerable time, M. haasti is regarded as a nomen dubium as of 2017. Pravoslavlev recognized another species from New Zealand, E. hoodii, named by Owen in 1870 as Plesiosaurus hoodii based on a neck vertebra. Welles recognized it as a nomen dubium in 1962;

In 1949 Welles named a new species of Elasmosaurus, E. morgani. It was named from a well preserved skeleton found in Dallas County, Texas. However, part of the specimen was accidentally thrown out during the relocation of the Southern Methodist University's paleontological collections. Despite its reassignment and the loss of its material, L. morgani is often considered an archetypal elasmosaurid. Data based on these lost elements were unquestionably accepted in subsequent phylogenetic analyses, until a redescription of the surviving elements was published by Sachs and Benjamin Kear in 2015. Persson assigned another species to Elasmosaurus alongside his 1959 description of "E."helmerseni remains from Sweden, namely E.(?)gigas. It was based on Schröder's Pliosaurus(?) gigas, named in 1885 from two dorsals; one was found in Prussia, the other in Scania. While they were incomplete, Persson recognized that their proportions and the shape of their articular ends differed greatly from pliosauroids, and instead agreed well with elasmosaurids. Given that, at the time of Persson's writing, "there [was] nothing to contradict that they are nearest akin to Elasmosaurus", he assigned them to Elasmosaurus "with hesitation". Theodor Wagner had previously assigned gigas to Plesiosaurus in 1914. Another species from Russia, E. antiquus, was named by Dubeikovskii and Ochev in 1967 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. 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. 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 1925. 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. Oskar Kuhn adopted a similar classification in 1961. and Franziska Großmann's 2007 analysis. However, Ketchum and Benson's analysis instead included it in the Leptocleidia, and its inclusion in that group has remained consistent in subsequent analyses. Within the Elasmosauridae, Elasmosaurus itself has been considered a "wildcard taxon" with highly variable relationships. Ketchum and Benson's 2010 analysis included Elasmosaurus in the former group. In 2020, O'Gorman formally synonymized Styxosaurinae with Elasmosaurinae based on the inclusion of Elasmosaurus within the group, and also provided a list of diagnostic characteristics for the clade. Topology A: Benson et al. (2013)

A 2015 study concluded that locomotion was mostly done by the fore flippers while the hind flippers functioned in maneuverability and stability; a 2017 study concluded that the hind flippers of plesiosaurs produced 60% more thrust and had 40% more efficiency when moving in harmony with the fore flippers. The paddles of plesiosaurs were so rigid and specialized for swimming that they could not have come on land to lay eggs like sea turtles. Therefore, they probably gave live birth (viviparity) to their young like some species of sea snakes. Evidence for live birth in plesiosaurs is provided by the fossil of an adult Polycotylus with a single fetus inside. Elasmosaurid remains provide some evidence they were preyed upon. A humerus of an unidentified subadult elasmosaurid was found with bite marks matching the teeth of the shark Cretoxyrhina, while a crushed Woolungasaurus skull has tooth marks matched to the pliosaur Kronosaurus.

Cope, in 1869, compared the build and habits of Elasmosaurus with those of a snake. Although he suggested that the vertebral column of the trunk did not allow for much vertical movement due to the elongated neural spines which nearly form a continuous line with little space between adjacent vertebrae, he envisaged the neck and tail to have been much more flexible: "The snake like head was raised high in the air, or depressed at the will of the animal, now arched swan like preparatory to a plunge after a fish, now stretched in repose on the water or deflexed in exploring the depths below". Simulation of water flow on 3D models showed that more elongated necks, such as those of elasmosaurids, did not increase drag force while swimming compared to shorter necked plesiosaurs. On the other hand, bending the neck sideways did increase drag force, more so in forms with very long necks. Another study found the long necks of elasmosaurs would normally increase drag during forward swimming but this was cancelled out by their large torsos, and hence large body sizes may have facilitated the evolution of longer necks.

In 1869 Cope noted that scales and teeth of six species of fish had been discovered directly beneath the vertebrae of the Elasmosaurus holotype, and theorized that these fish would have had formed the diet of the animal. From these remains, Cope named a new species of barracuda, Sphyraena carinata. It has also been suggested that the predatory abilities of elasmosaurids have been underestimated; their large skulls, big jaw muscles, strong jaws, and long teeth indicate they could prey on animals between and long, as indicacted by stomach contents including those of sharks, fish, mosasaurs, and cephalopods. It is possible that Elasmosaurus and its kin stalked schools of fish, concealing themselves below and moving the head slowly up as they approached. The eyes of the animal were at the top of the head and allowed them to see directly upward. This stereoscopic vision would have helped it to find small prey. Hunting from below would also have been possible, with prey silhouetted in the sunlight while concealed in the dark waters below. Elasmosaurids probably ate small bony fish and marine invertebrates, as their small, non kinetic skulls would have limited the size of the prey they could eat. Also, with their long, slender teeth adapted for seizing prey and not tearing, elasmosaurids most certainly swallowed their prey whole. Several different functions have been proposed for gastroliths, including aiding in digestion, mixing food content, mineral supplementation, and storage and buoyancy control.

Elasmosaurus is known from the Sharon Springs Member of the Campanian age Upper Cretaceous Pierre Shale formation of western Kansas, which dates to about 80.5million years ago. The Pierre Shale represents a period of marine deposition from the Western Interior Seaway, a shallow continental sea that submerged much of central North America during the Cretaceous. At its largest, the Western Interior Seaway stretched from the Rockies east to the Appalachians, some wide. At its deepest, it may have been only deep. Two great continental watersheds drained into it from east and west, diluting its waters and bringing resources in eroded silt that formed shifting river delta systems along its low lying coasts. There was little sedimentation on the eastern margin of the Seaway; the western margin accumulated a thick pile of sediments eroded from the western land mass. The western shore was thus highly variable, depending on variations in sea level and sediment supply. The soft, muddy sea floor probably received very little sunlight, but it teemed with life due to steady rains of organic debris from plankton and other organisms farther up the water column. The bottom was dominated by large Inoceramus clams, which were covered with oysters; there was little biodiversity. Clam shells would have accumulated over the centuries in layers under the sea floor's surface, and would have provided shelter for small fish. Other invertebrates known to have lived in this sea include various species of rudists, crinoids and cephalopods (including squids and ammonites). Large fish known to have inhabited the sea include the bony fishes Pachyrhizodus, Enchodus, Cimolichthys, Saurocephalus, Saurodon, Gillicus, Ichthyodectes, Xiphactinus, Protosphyraena and Martinichthys; and the sharks Cretoxyrhina, Cretolamna, Scapanorhynchus, Pseudocorax and Squalicorax. In addition to Elasmosaurus, other marine reptiles present include fellow plesiosaurs Libonectes, Styxosaurus, Thalassomedon, Terminonatator, Polycotylus, Brachauchenius, Dolichorhynchops and Trinacromerum; the mosasaurs Mosasaurus, Halisaurus, Prognathodon, Tylosaurus, Ectenosaurus, Globidens, Clidastes, Platecarpus and Plioplatecarpus; and the sea turtles Archelon, Protostega, Porthochelys and Toxochelys. The flightless aquatic bird Hesperornis also made its home there. The pterosaurs Pteranodon and Nyctosaurus, and the bird Ichthyornis, are also known far from land.