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| Arthropterygius | |
|---|---|
 | |
| Cranial elements ofA. chrisorum | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | †Ichthyosauria |
| Family: | †Ophthalmosauridae |
| Genus: | †Arthropterygius Maxwell,2010 |
| Type species | |
| †Arthropterygius chrisorum (Russel, 1993 [originallyOphthalmosaurus]) | |
| Other species | |
| Synonyms | |
| |
Arthropterygius is a widespreadgenus ofophthalmosauridichthyosaur which existed inCanada,Norway,Russia, andArgentina from the lateJurassic period and possibly to the earliestCretaceous.[2][3]
Arthropterygius appears to have been a relatively large ichthyosaur, with all species measuring between 3–5 m (9.8–16.4 ft) long.[2] The partially preserved specimen PMO 222.655 has been estimated at 3.8–4.3 metres (12–14 ft) based on comparisons to the contemporaryophthalmosauridUndorosaurus.[1][4] This specimen was probably mature or close to maturity at time of death, judging by the convex head of thehumerus and the smooth texture of the humeral shaft.[1][5]
The skull ofArthropterygius has become well known due to the discovery of a well preserved skull fromSvalbard.[6] It is unusual among ichthyosaurs in having a very short, yet also robust rostrum for its skull length. As a result, the orbit appears very large (about 0.34× the skull length).[4] The skull bears a very largepineal foramen, which is larger proportionately than that of any other ophthalmosaurid. This may potentially be an adaptation to the high latitude environment ofArthropterygius, but it is not yet well understood how the size of the pineal eye relates to latitude in ichthyosaurs. The presence of the foramen for theinternal carotid artery (a major artery that supplies blood to the brain) on the posterior surface of theparabasisphenoid is a distinguishing trait of this genus.[7] The teeth are relatively large, with robust, conical crowns, and are largely straight or slightly recurved.[2]
It is difficult to estimate the exact number of vertebral segments that were present in front of thesacrum; the preservedcentra suggest that there are at least 43. This is more than the 42 inAthabascasaurus,[8] the 41 inNannopterygius, and the 37 inPlatypterygius americanus,[9] but less than the 46 inPlatypterygius australis[10] and the 50 or more inUndorosaurus,[4]Aegirosaurus,[11] andPlatypterygius platydactylus. The posteriordorsal vertebrae are wider and taller than those near the front, and their front and rear faces are rounded. The first few caudal vertebrae are the proportionally tallest and widest of all the vertebrae,[12] but the vertebrae quickly become shorter vertically prior to the bend in the tail that supported the tail fluke.[1]
Eachneural spine is about the same length as the underlying centrum, which is similar toUndorosaurus[4] but notOphthalmosaurus (in which they are proportionally longer in the posterior cervical vertebrae). Similar toGengasaurus,[13] the first few neural spines are taller than the corresponding centra, but then decrease in height gradually. The tops of the neural spines, which are the thinnest parts of the bones, are straight, instead of being notched likeUndorosaurus[4] orPlatypterygius australis.[10] The ribs are shaped like afigure eight in cross section near their top ends, but this is less obvious closer to the bottom; such a morphology is typical among ophthalmosaurids, except forAcamptonectes[14] andMollesaurus.[15] The dorsal ribs are around 75–81 cm (30–32 in) long, while the first few caudal ribs are only 2–5 cm (0.79–1.97 in) long.[1]
InArthropterygius, thescapula has a relatively straight blade that was broadened at its front end into a fan-likeacromion process. This process is less prominent than that ofAcamptonectes[14] andSveltonectes,[16] instead being more similar to that ofUndorosaurus,[4]Platypterygius hercynicus, andSisteronia.[17] The scapula formed more of theglenoid than thecoracoid; likeSveltonectes[16] but unlikeUndorosaurus,[4] the glenoid does not extend onto the bottom face of the scapula. Thede facto blade of the scapula was widest near the middle, and angled slightly downwards; likeAcamptonectes[14] but unlike all other ophthalmosaurids, the blade has a relatively uniform thickness along its entire length.[1]
Theclavicle, which is not fused to other elements in thepectoral girdle, bears a thickened process on its frontal bottom edge which points towards the midline of the torso. This process is relatively short, square-shaped from the front, and bordered by a rim on its back edge. In most other ophthalmosaurids, this same process is more finger-like, although some specimens ofOphthalmosaurus have similar processes. The back part of the clavicle gradually narrows into a curved point, as inOphthalmosaurus andBaptanodon.[18][1]
Thecoracoid is a kidney-shaped bone that is about as long as it is wide, compared toUndorosaurus,[4]Nannopterygius, andSveltonectes,[16] where it is not as wide proportionally. LikeOphthalmosaurus, the coracoid bears a prominent notch on its front edge; it also bears a ridge on the frontal part of its midline, likeOphthalmosaurus andAcamptonectes,[14] but unlikeCaypullisaurus[19] andPlatypterygius australis.[10] Itsarticular facets with the scapula and glenoid are clearly separate, as inSveltonectes[16] but notAcamptonectes.[14] The former is only about 45% of the latter in length; this figure is 50% inUndorosaurus.[4]
There are two prominent processes on thehumerus. The longer, less prominent, and more ridge-like of these is the dorsal process. This process is remarkably short compared to other ophthalmosaurids, being less than half of the length of the humeral shaft, a condition also seen inUndorosaurus andAegirosaurus;[11] inOphthalmosaurus,Brachypterygius,Nannopterygius, andUndorosaurus,[4] the process reaches the midpoint of the humerus, while it is even longer inPlatypterygius americanus,[9]Platypterygius australis,[10] andPlatypterygius hercynicus.[20] The other process is the deltopectoral crest, which is about half of the shaft length inOphthalmosaurus, and is almost as long as the entire shaft inSisteronia,[17]Acamptonectes,[14] andPlatypterygius americanus.[9][1]
At the midpoint of the humeral shaft, the bone is mildly constricted, being approximately 20% narrower than the maximum width. The bottom end of the humerus is larger than the top end; it bears three articular facets, with one for the preaxial accessory element (the smallest of the three), one for theradius (the tallest of the three), and one for theulna (which forms an angle of 120° with the radial facet).Sveltonectes,[16]Nannopterygius,Platypterygius hauthali,[21] andPlatypterygius platydactylus all only have two;Maiaspondylus,[22]Aegirosaurus,[11]Brachypterygius, andPlatypterygius americanus[9] also have three, but the second facet articulates with a different bone;Undorosaurus has two on the left humerus and three on the right;[4] andPlatypterygius australis[10] andPlatypterygius hercynicus[20] have up to four facets.[1]
Theilium ofArthropterygius is rather short compared to that of other ophthalmosaurids;Aegirosaurus is the only other ophthalmosaurid that possibly approachesArthropterygius in this respect.[11] The side of the ilium directed towards the back of the body is concave, likeOphthalmosaurus andAthabascasaurus,[8] with the curve being more pronounced in the latter two. Theacetabulum, on the lower end of the ilium, is thickened relative to the rest of the bone, and does not bear any distinct articular facets.Autapomorphically, the top end of the ilium is 1.5 times the width of the acetabular end.[1]
Theischium and thepubis are fused together into a single, continuous, solid trapezoid-shaped element known as the ischiopubis, with the wider edge (1.4 times the width of the other end, which is shorter proportionally than that ofAegirosaurus,[11]Ophthalmosaurus, andAthabascasaurus[8]) being at the midline of the body. This complete fusion is also seen inSveltonectes,[16]Athabascasaurus,[8]Aegirosaurus,[11]Caypullisaurus,[19] and possiblyPlatypterygius australis;[10] meanwhile,Ophthalmosaurus,Undorosaurus,[4] andNannopterygius retain a small hole in the ischiopubis. The probable portion of the ischiopubis that represents the pubis is thicker than the rest of the bone. Also uniquely among ophthalmosaurids, this fused element is shorter than thefemur.[1]
Both ends of the femur ofArthropterygius are approximately the same width, with the middle of the femoral shaft being slightly narrower. The top end was slightly thicker than the rest of the bone along its front rim. UnlikeOphthalmosaurus, the femoral dorsal and ventral processes were rather reduced. At the bottom end, there are two facets, one for thetibia and one for thefibula; this is typical of ophthalmosaurids, butPlatypterygius hercynicus,[20]Platypterygius americanus,[9]Platypterygius australis,[10] andNannopterygius all have three. Both facets inArthropterygius were roughly the same length, and the fibular facet was directed slightly to the back, forming an angle of 120° with the tibial facet.[1]
The genus was first named in2010 by Erin E. Maxwell as the generic replacement name forOphthalmosaurus chrisorum, which was named in 1993 from fossils found onMelville Island in theNorthwest Territories. Its fossils are the most complete of any ichthyosaur in theCanadian Arctic.A. chrisorum has several features that separate it from the genusOphthalmosaurus, including a highly angled articulation between theradius andulna and thehumerus. Maxwell 2010 found it to be thesister taxon ofCaypullisaurus, an ophthalmosaurid fromArgentina.[7] However, many recentcladistic analyses found it to be the basalmost member of theOphthalmosauridae.[23][14]
In 2012, a skull and forefin discovered of an ophthalmosaurid ichthyosaur discovered in theNeuquén Basin of Argentina were described.[3] This specimen was notable for presenting unusual braincase morphology including the opening for the internal carotid artery on the posterior surface of the parabasisphenoid, which is a diagnostic trait ofArthropterygius, which was at the time only known from Canada. This specimen was initially referred toArthropterygius sp., massively expanding the known range of the genus. This presents strong evidence that some Jurassic marine reptiles had near cosmopolitan distributions, much like manymarine mammals today. A paper published by Camposet al. in 2019 further describes this specimen, and designates it as the holotype of a new species,A. thalassonotus.[24]
Eight seasons of excavations led by the Spitsbergen Mesozoic Research Group on the island ofSpitsbergen,Svalbard,Norway from 2004 to 2012 recovered 29 ichthyosaur specimens from outcrops of the Slottsmøya Memberlagerstätte, which belongs to the greaterAgardhfjellet Formation. These outcrops likely date to theVolgian (which roughly corresponds to theTithonian-earlyBerriasian) based onammonitebiostratigraphy. From these, numerous new genera and species of ichthyosaur were described includingCryopterygius kristiansenae (later recognized to be synonymous withUndorosaurus gorodischensis[25]),Palvennia hoybergeti (named for PalVenn, the Friends of the Palaeontological Museum in Oslo, whose expedition led to the discovery of the type specimen),[4] andJanusaurus lundi (named for the mountain Janusfjellet, where the specimen was found).[18]Janusaurus andPalvennia were both named off of less extensive material theCryopterygius, but both were recognizably distinct to that genus and were deemed to be distinct genera. An additional ichthyosaur specimen was prepared at theUniversity of Oslo and subsequently described in 2016: PMO 222.655, the holotype ofKeilhauia nui, discovered from the Berriasian portions of the Slottsmøya Member in 2010. This specimen comprises an articulated partial skeleton, which was preserved lying on its left side, including part of the snout, the dorsal and anterior caudal vertebrae, the right forelimb andpectoral girdle, the majority of thepubic girdle, and both of thefemora.[1] The genus nameKeilhauia honours the Norwegian geologistBaltazar Mathias Keilhau, who conducted an expedition to Spitsbergen in 1827. Meanwhile, the species namenui is derived from the acronym of the environmental organizationNatur og Ungdom, the fiftieth anniversary of which occurred in 2017.[1]
An extensive revision ofArthropterygius conducted by Zverkov and Prilepskaya in 2019 revealedJanusaurus andPalvennia to be junior synonyms of that genus. They were reassigned as the speciesA. lundi andA. hoybergeti respectively. Additionally, they determined that the holotype and only known specimen ofKeilhauia is referrable toArthropterygius as well. However, the poor preservation of the material made it difficult to identify it at the species level, and thus they referred the specimen simply toA. sp. cf. chrisorum and designated the nameKeilhauia nui as anomen dubium.[2] They also found that the specimen PMO 222.669, which was referred toPalvennia by Delsett et al. (2018), shares all identifying features withA. chrisorum and that there are no sufficient differences between them to warrant it belonging toA. hoybergeti. This means that there were at least three species ofArthropterygius present in the Slottsmøya Member, and significantly expands the geographic range of the speciesA. chrisorum specifically. The recognition that these taxa are referable toArthropterygius also makes this now one of the best understood ichthyosaurs of the Late Jurassic. They also recognize the previously dubious taxon"Ichthyosaurus" volgensis fromSyzran in Russia as belonging to a newly recognized species ofArthropterygius,A. volgensis. This provides further support for the previously established idea that there was significant faunal interchange between theVolga region and Svalbard during the Volgian.[25]
While Zverkov and some others treatJanusaurus,Keilhauia andPalvennia withinArthropterygius, a study on the Jurassic Ichthyosaurs performed by Delsett et al. suggests the synonymy is not supported by the presence evidence.[26]
The following cladogram shows a possible phylogenetic position ofArthropterygius in Ophthalmosauridae according to the analysis performed by Zverkov and Jacobs (2020).[27]
| Ophthalmosauria |
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Arthropterygius appears to have been a particularly wide ranging genus, with fossils known from several high latitude localities across the Northern Hemisphere, as well as an isolated specimen from Argentina.
The Slottsmøya Member of theAgardhfjellet Formation, from which fossils ofA. chrisorum,A. lundi, andA. hoybergeti have been recovered, provides perhaps the best glimpse into the ecosystems and habitats this genus was a part of. It consists of a mix ofshales andsiltstones and was deposited in a shallow watermethane seep environment, near a patch of deepermarine sediment.[28] The seafloor, which was located about 150 metres (490 ft) below the surface, seems to have been relatively dysoxic, or oxygen-poor, although it was periodically oxygenated byclastic sediments.[29] Despite this, near the top of the member, various diverse assemblages of invertebrates associated with cold seeps have been discovered; these includeammonites,lingulate brachiopods,bivalves,rhynchonellate brachiopods,tubeworms,belemnoids,tusk shells,sponges,crinoids,sea urchins,brittle stars,starfish,crustaceans, andgastropods, numbering 54 taxa in total.[30] Outside of the cold seeps, many of these invertebrates were also present in abundance.[29] Though direct evidence from Slottsmøya is currently lacking, the high latitude of this site and relatively cool global climate of the Tithonian mean that sea ice was likely present at least in the winter.[31][32]
In addition toArthropterygius, the Slottsmøya Member presents a diverse assemblage of other marine reptiles, including the ichthyosaursUndorosaurus gorodischensis,Nannopterygius borealis, and a partial skull attributed toBrachypterygius sp..[25][27][33] Additionally, 21plesiosaurian specimens are also known from the site, including two belonging to the largepliosauridPliosaurus funkei, which would have been the apex predator of the ecosystem, three toColymbosaurus svalbardensis, one toDjupedalia engeri, one toOphthalmothule cryostea, and one each toSpitrasaurus wensaasi andS. larseni. Many of these specimens are preserved in three dimensions and partially in articulation; this is correlated with high abundance of organic elements in the sediments they were buried in, as well as a lack of invertebrates locally.[29]
Material belonging to four species ofArthropterygius,A. chrisorum,A. volgensis,A. lundi, andA. hoybergeti, has been reported from the Volga region of Russia, which gives the Volgian stage its name.[2] Though very little is known or published about the fossils of these localities, fossils of a number of marine animals have been recovered, including species of the ichthyosaursNannopterygius andUndorosaurus. The presence of these taxa indicates that there was significant faunal exchange across the seas of Northern Europe during this time period.[25] In addition to these three genera, fossils of the ichthyosaurGrendelius, the pliosaurPliosaurus rossicus, and indeterminate remains belonging to ametriorhynchid, as well as a high diversity of ammonites including the large-bodied taxonTitanites, are also known from the Volgian-aged sediments of this region.[34]
A. thalassonotus hails from theVaca Muerta Formation of the Neuquén Basin inPatagonia, Argentina.[24] This is arguably one of the best marine reptile-bearing fossil sites of the Southern Hemisphere, and has revealed a great number of specimens of Late Jurassic marine reptiles. In addition toArthropterygius, the ichthyosaursCaypullisaurus bonapartei andOphthalmosaurus sp. as well as two species of the genusPliosaurus,P. patagonicus andP. almanzaensis, and a variety of marineturtles are known from the formation.[35] It also features a uniquely diverse array of metriorhynchids, with species belonging to the generaDakosaurus,Cricosaurus,Purranisaurus, andGeosaurus all having been reported.[36][37] The extent to which the fauna reported from the Vaca Muerta Formation resembles that known from sites in the Northern Hemisphere, such as theKimmeridge Clay, suggests that many marine reptile genera may have had near cosmopolitan distributions in the Late Jurassic.
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