| Tatanectes | |
|---|---|
 | |
| Front paddle of theholotype | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Superorder: | †Sauropterygia |
| Order: | †Plesiosauria |
| Family: | †Cryptoclididae |
| Genus: | †Tatenectes O'Keefe and Wahl, 2003 |
| Species: | †T. laramiensis |
| Binomial name | |
| †Tatenectes laramiensis (Knight, 1900) [originallyCimoliosaurus] | |
| Synonyms | |
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Tatenectes is agenus ofcryptoclididplesiosaur known from theUpper Jurassic ofWyoming. Its remains were recovered from the Redwater Shale Member of theSundance Formation, and initially described as a new species ofCimoliosaurus byWilbur Clinton Knight in 1900. It was reassigned toTricleidus byMaurice G. Mehl in 1912 before being given its own genus by O'Keefe and Wahl in2003.Tatenectes laramiensis is thetype and only species ofTatenectes. While the original specimen was lost, subsequent discoveries have revealed thatTatenectes was a very unusual plesiosaur. Its torso had a flattened, boxy cross-section and itsgastralia (belly ribs) exhibitpachyostosis (thickening). The total length ofTatenectes has been estimated at 2–3 meters (6.6–9.8 ft).
Tatenectes is related toKimmerosaurus, although theirtaxonomic placement has varied. They were once considered to be close relatives ofAristonectes in the familyCimoliasauridae orAristonectidae, but later assigned to Cryptoclididae. The unusual body shape and pachyostotic gastralia ofTatenectes would have helped to make it more stable and resistant toturbulence. Based on stomach contents,Tatenectes fed oncephalopods andfish. It would have lived in the shallow waters of theSundance Sea, anepicontinental sea covering much of North America during part of theJurassic.Tatenectes shared its habitat withinvertebrates, fish,ichthyosaurs, and other plesiosaurs, including another cryptoclidid,Pantosaurus, and the largepliosauridMegalneusaurus.
All known specimens ofTatenectes come from the Redwater ShaleMember in the upper part of theSundance Formation. This formation is located inWyoming, in the EasternRocky Mountains. An incompleteplesiosaur skeleton preserving multitudinousvertebrae and a nearly completeforelimb from the Sundance Formation was described by Wilbur C. Knight in 1900. With this specimen as aholotype, which was never assigned aspecimen number, he named a new species ofCimoliosaurus,C. laramiensis.[1][2] In 1912,Maurice G. Mehl questioned the assignment ofC. laramiensis toCimoliosaurus, noting thatCimoliosaurus was acatch-all, and that since this genus was named based on vertebrae, the vertebrae ofC. laramiensis would have to be similar to those of the type species,C. magnus. He was unable to find such similarities, and provisionally reclassifiedC. laramiensis as a species ofTricleidus, due to the anatomy of its forelimbs.[3]
Although the holotype specimen ofT. laramiensis has since been lost, Knight's description of the specimen was sufficient to determine thatT. laramiensis is a valid species. In 2003, F. Robin O'Keefe and William Wahl, JR. referred another specimen housed at theTate Museum to this species. This specimen, listed under the specimen numbers UW 15943 and UW 24801, consists of vertebrae, ribs, a shoulder girdle, a partial forelimb, and a fragmentary skull and was assigned as aneotype. Additionally, O'Keefe and Wahl noted that theshoulder girdle of this species was quite different from that ofTricleidus, so they named a new genus,Tatenectes, to includeT. laramiensis. The nameTatenectes honours Marion and Inez Tate, founders of the Tate Museum inCasper, Wyoming in 1980 and combines this with theGreek wordnektes, meaning "diver."[4] O'Keefe and Hallie P. Street assigned more material toTatenectes laramiensis in 2009, including UW 24215, a partial skeleton including further cranial, vertebral, pectoral, and phalangeal elements.[2] Another skeleton, USNM 536976, was assigned to this species by O'Keefe and colleagues in 2011. This specimen preserved 22 vertebrae, ribs,gastralia, and completepelves.[5]
Tatenectes is a small plesiosaur, a group ofmarine reptiles well adapted to aquatic life.[6]Tatenectes is a "plesiosauromorph", meaning that it would have had an elongated neck, small head, and longerhumeri thanfemora, as opposed to the short necks and large skulls and femora of "pliosauromorphs".[7] In 1900, Knight estimated thatTatenectes was probably less than 3.7 meters (12 ft).[1] O'Keefe and Street later estimated a smaller total length of around 2 meters (6.6 ft) in 2009,[2] although O'Keefe and colleagues gave a maximum length of 3 meters (9.8 ft) two years later.[5] The overall shape ofTatenectes is very unusual among plesiosaurs, with the body being flattened and rather short back-to-front. Thesquamosals, bones located at the rear of the skull, are tall, three-pronged elements which form an arch over the back face of the skull. This structure, the squamosal arch, is anidentifying plesiosaurian trait.[4][2] There is an opening between thepterygoids (bones that form part of thepalate) known as the anterior interpterygoid vacuity. Posterior to this opening, the pterygoids thicken and project downwards. When the skull is viewed from below, theparasphenoid (a bone located between the palate andbraincase) is obscured by other bones. This palatal configuration is a distinctive characteristic ofTatenectes. Another such trait is the shape of itsteeth, which were small and thin, bearing elongatedroots and lowenamel ridges.[5]
The total number of cervical (neck) vertebrae inTatenectes is unknown.[2] The neural spines of these vertebrae are short and angled posteriorly.[4] Many features of the cervical vertebrae can be used to identify this genus: the cervical centra (vertebral bodies) are considerably shorter (in length) than wide, and are not constricted in the middle; the articulations for thecervical ribs are short but pronounced; the articular faces of the cervical vertebrae are round and weakly defined; the subcentralforamina (two small openings on the underside of the centrum) of the cervical vertebrae are positioned further apart than typical in related plesiosaurs.[2] The form of thetorso inTatenectes is very distinctive. Flattened and spheroid in shape, the torso has a somewhat boxycross-section.[5] There are sixteendorsal (back) vertebrae inTatenectes, which, when articulated, form a considerably flatter arch than seen in relatedtaxa. The dorsalneural spines are inclined anteriorly. The dorsal ribs articulate with the vertebrae horizontally and are deflected backwards, contributing to the flatness of the body. The gastralia (belly ribs)Tatenectes are unusually large for its size that exhibitpachyostosis (thickening).[8] The medial (midline) gastralia are relatively straight, while the lateral (outer) gastralia are strongly J-shaped.Tatenectes bears foursacral (hip) vertebrae, which are smaller than the dorsal vertebrae. The centra of thecaudal (tail) vertebrae are anteroposteriorly short.[5] All of the vertebrae have amphicoelous centra (bearing concave articular faces).[1]
The morphology of the medial process of thescapula ofTatenectes is diagnostic of this genus. These processes are anteroposteriorly short, with the posterior ends of each process contacting the other along the midline and a notch separating their anterior ends. The top of this notch is overlain by a distinctiveclavicle, which is flat and simple in morphology. The pectoral fenestra (an opening enclosed by the scapula andcoracoid) is rounded and enlarged. The posterior region of the coracoid is much thinner than the rest of the pectoral girdle. The humerus has a slender, elongated shaft and is another bone with a distinctive shape. The surfaces of the humerus that articulated with the lower arm bones (radius andulna) have roughly equal lengths. The humerus also bears articulations that indicate there were two additional bones in the lower arm.[2]Tatenectes had sixcarpals (wrist bones). Themetacarpals and proximalphalanges are mildly flattened, with a groove on each side of their top surfaces, while the distal phalanges are flattened more strongly.[1] The pelvic girdle ofTatenectes is wider than long and has a flatter bottom surface than typical among plesiosaurs, making it short top-to-bottom. Eachpubic bone is flat and plate-like, with a notch on its front rim forming a projection known as an anterolateral horn. Theischium is more robust where it forms theacetabulum, thinning posteriorly. The straight, gracileilium forms an acute angle with the ischium when viewed from the side, yet another identifying characteristic.[5]
Tatenectes has had a complicated taxonomic history. While it had previously been considered a species ofCimoliasaurus orTricleidus, O'Keefe and Wahl found it to be the sister taxon ofKimmerosaurus in 2001. These two genera, as well asAristonectes, were considered to be part ofCimoliasauridae, withKaiwhekea being either a member of this family or its closest known relative.[4] In 2009, O'Keefe and Street considered that utilizing the name "Cimoliasauridae" for this group was a poor choice since they foundCimoliasaurus to actually belong to a different group,Elasmosauridae. The group containingTatenectes,Kimmerosaurus,Aristonectes, andKaiwhekea was subsequently named Aristonectidae.[2] However, in 2011, O'Keefe and colleagues noted the problematic nature ofAristonectes andKaiwhekea. Due to their unusual morphology, these two genera are difficult to classify, making their relation toTatenectes ambiguous.[5] Further studies have recoveredTatenectes as a member ofCryptoclididae, where it is still found to be a close relative ofKimmerosaurus, in addition toCryptoclidus.[9][10][11]Aristonectes andKaiwhekea, however, are now thought to bederived elasmosaurids known asaristonectines.[12][13][14]
The following cladogram follows that of Roberts and colleagues, 2020.[9]
Pachyostosis, as seen inTatenectes, is unusual among plesiosaurs, and only a handful of genera, such asPachycostasaurus, exhibit it. While many skeletal elements ofPachycostasaurus are pachyostoic,Tatenectes is even more unusual in that the gastralia are the only pachyostotic bones, the dorsal ribs being unmodified. Pachyostosis describes the thickening of a bone'speriosteal cortex, thereby making the bone larger, and often occurs withosteosclerosis, or increased bone density. Conversely, an animal's bone density can be decreased throughosteoporosis. Despite its gastralia being pachyostotic,Tatenectes does not exhibit osteosclerosis, but it does have a layer of osteoporotic bone between the layers of pachyostotic bone, an unprecedented condition.[8] The pachyostotic bone is concentrated around the midline of the ventral thorax. This, together with the unusually flat body shape, led O'Keefe and colleagues to hypothesize thatTatenectes would have been more resistant toturbulence, particularlyroll, than other plesiosaurs, increasing its stability when swimming close to the surface in shallow water.[5]
In addition to pachyostosis, lung deflation could also have been used byTatenectes (and other plesiosaurs) to gain negativebuoyancy, helping it dive.[7] Plesiosaurs swallowed stones known asgastroliths. While it has been proposed that these could have been used for ballast or buoyancy control, the effectiveness of this is controversial. The stones may have had a digestive function, or their ingestion may have been accidental. It is also possible, although more controversial, that gastroliths were used for other purposes such as alleviation of discomfort or mineral supplements. Additionally, gastroliths may have had a variety of functions instead of being limited to just one.[15] Stomach contents ofTatenectes include the hooklets ofcoleoidcephalopod and teeth anddenticles from a smallhybodontshark, indicating thatTatenectes fed on these animals.[16] These prey items show thatTatenectes was not abottom feeder, unlike some elasmosaurids as indicated by their stomach contents.[5]
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Tatenectes comes from theOxfordian-aged (Upper Jurassic) rocks of the Redwater Shale Member of the Sundance Formation.[5] This member is about 30–60 meters (98–197 ft) thick. While mainly composed of grayish greenshale, it also has layers of yellowlimestone andsandstone, the former layers containing plentifulfossils of marine life.[18] The Sundance Formation represents a shallowepicontinental sea known as theSundance Sea.[2] From theYukon andNorthwest Territories ofCanada, where it was connected to theopen ocean, this sea spanned inland southwards toNew Mexico and eastward to theDakotas.[18][19] WhenTatenectes was alive, most of the Sundance Sea was less than 40 meters (130 ft) deep.[5] Based onδ18O isotope ratios inbelemnite fossils, the temperature in the Sundance Sea would have been 13–17 °C (55–63 °F) below and 16–20 °C (61–68 °F) above thethermocline.[18]
The paleobiota of the Sundance Formation includesforaminiferans andalgae, in addition to a variety ofanimals. Manyinvertebrates are known from the Sundance Formation, represented bycrinoids,echinoids,serpulidworms,ostracods,malacostracans, andmollusks. The mollusks include cephalopods such asammonites and belemnites,bivalves such asoysters andscallops, andgastropods.Fish from the formation are represented by hybodont[5] andneoselachianchondrichthyans as well asteleosts (includingPholidophorus). Marine reptiles are uncommon, but are represented by four species.[19] Plesiosaurs, in addition toTatenectes, include another cryptoclidid,[9]Pantosaurus striatus, as well as the largepliosauridMegalneusaurus rex. Besides plesiosaurs, marine reptiles are also represented by theichthyosaurOphthalmosaurus (or, possibly,Baptanodon)[20]natans, the most abundant marine reptile of the Sundance Formation.[21][17]
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