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| Aristonectes | |
|---|---|
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| Life restoration ofA. parvidens | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Superorder: | †Sauropterygia |
| Order: | †Plesiosauria |
| Superfamily: | †Plesiosauroidea |
| Family: | †Elasmosauridae |
| Subfamily: | †Aristonectinae |
| Genus: | †Aristonectes Cabrera, 1941 |
| Type species | |
| Aristonectes parvidens Cabrera, 1941 | |
| Other species | |
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| Synonyms | |
List
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Aristonectes (meaning "best swimmer") is anextinctgenus of largeelasmosauridplesiosaurs that lived during theMaastrichtianstage of theLate Cretaceous. Twospecies are known,A. parvidens andA. quiriquinensis, whosefossil remains were discovered in what are nowPatagonia andAntarctica. Throughout the 20th century,Aristonectes was a difficult animal for scientists to analyze due to poor fossil preparation, its relationships to other genera wereuncertain. After subsequent revisions and discoveries carried out from the beginning of the 21st century,Aristonectes is now recognised as thetype genus of thesubfamilyAristonectinae, a lineage of elasmosaurids characterized by an enlargedskull and a reduced length of theneck.
Measuring more than 10 m (33 ft) long,Aristonectes is a notably imposing plesiosaur. A referred specimen discovered in Antarctica has an estimated size of more than 11 m (36 ft) long, which would make this genus one of thelargest known plesiosaurs. Thepaddle-like limbs ofAristonectes are unusually large for an elasmosaurid, reaching almost 3 m (9.8 ft) in length, suggesting a totalwingspan of around 7 m (23 ft) for the animal. Theskull isogival-shaped and is flattened, possessing sharp forward-facingteeth. According to estimates, 13 teeth would have been present in eachpremaxilla, 50 teeth in themaxilla and 50 to 63 or more teeth in thelower jaw.
According to its morphology, mainly cranial,Aristonectes fed by mixingprey andsediment in thebenthic zones, like the moderngray whale. As in other plesiosaurs,gastroliths (stomach stones) would have been used byAristonectes either to helpdigest its food or to reduce excessbuoyancy, although there is little support for the latter hypothesis. According to itsgeographical distribution in the fossil record,Aristonectes would have regularlymigrated between Patagonia and Antarctica.
The firstAristonectes fossil was discovered long before thegenus was named byÁngel Cabrera in 1941.[1] In 1848, theFranco-ChileannaturalistClaude Gay described the first knownplesiosaur fromSouth America,Plesiosauruschilensis, on the basis of a singlecaudal vertebra discovered inQuiriquina Island, inConcepción Province,Chile.[2] In a book published in 1889,Richard Lydekker placed thisspecies in the genusCimoliosaurus.[3] In 1895, Wilhelm Deecke moved it into thepliosaurid genusPliosaurus asPliosaurus chilensis, and referred other fossils that have been discovered in the same locality to this species. Among these fossils are some vertebrae, afemur, fragments of anischium, andribs.[4] In 1918, the Russian paleontologistPavel A. Pravoslavlev [ru] moved it into the related genusElasmosaurus, asElasmosaurus chilensis.[5] In 1941, Cabrera described a different specimen fromChubut Province,Argentina as the new genus and speciesAristonectes parvidens. In the same publication, he listedElasmosaurus chilensis as "Plesiosaurus"chilensis, expressing uncertainty regarding its affinities.[6] In 1949,Edwin H. Colbert found that theholotype specimen of "P."chilensis (the caudal vertebra originally described by Gay) belongs to apliosauroid, but also was uncertain about its generic placement. Also in the article, he considers that the other fossil material previously attributed to "P."chilensis could come fromelasmosaurids.[7][8] In an analysis published in 2013 by José P. O'Gorman and colleagues, the holotype specimen of "P."chilensis was recognized as potentially belonging toA. parvidens. As this specimen merely consists of a single caudal vertebra, it was referred to asA.cf.parvidens to indicate uncertainty in its assignment.[1]
The first specimen formally identified asA. parvidens was collected by Cristian S. Petersen in collaboration with local resident Victor Saldivia, in theCañadón de los Loros [ceb], near the municipality ofPaso del Sapo inChubut Province,Argentina.[6] The specimen was discovered in theLefipán Formation and was dated to theMaastrichtian.[9][10][11] In September 1940, Pablo Groeber sent this specimen to theMuseo de La Plata as a donation for theDirección de Minas y Geología del Ministerio de Agricultura (Directorate of Mines and Geology of the Ministry of Agriculture). Avertebra and incompletephalanges from the same region had previously been donated to theMuseo de La Plata by Mario Reguiló, and were subsequently referred to the holotype, catalogued as MLP 40-XI-14-6, as they most likely came from the same specimen.[6] The holotype consists of a partialskull attached to themandible, theatlas andaxis, 21cervical vertebrae (including 16 anterior cervicals which are articulated), 8 caudal vertebrae and an incomplete leftforelimb. The specimen was interpreted as anadult, or even an elderly individual.[9][12] Cabrera described the animal asAristonectes parvidens in his article published in 1941 by the museum that houses the fossil material.[6] Thegenus nameAristonectes comes from theAncient Greek words ἄριστος (áristos, "best", "superior") and νηκτός (nêktós, "swimmer"), and may be translated as "best swimmer", so named to indicate the best preserved specimen of plesiosaur from Argentina known at that time. The specific epithetparvidens means "small teeth", in reference to the rather small size of thetooth sockets.[13]
Since the unknown parts of the original specimen were heavily reconstructed, the precise anatomy ofAristonectes remained uncertain throughout the 20th century.[14][15][16] In 2003, Argentine paleontologistZulma Gasparini and colleagues re-prepared the holotype skull along with the atlas and axis, in an attempt to clarify its anatomy.[9] In a doctoral thesis published in 2013, O'Gorman informally assigns several caudal vertebrae toA. parvidens.[10] In 2016, the same author revised the holotype again.[11]
The first discovery of the second known species,A. quiriquinensis, dates back to the late 1950s, when Argentinian paleontologistRodolfo Casamiquela discovered a partial skeleton in Las Tablas Bay, located north of Quiriquina Island, Chile. The specimen, cataloged as SGO.PV.260, was found in Maastrichtianbeds of theQuiriquina Formation; initially, five vertebrae, ahumerus fragment, and the distal end of alimb were visible. The hardness of the rocks containing the fossils made subsequent preparation difficult. The preparation uncovered additional body parts, with the specimen including all four limbs, a posterior portion of theneck, most of thetrunk, and a completetail. It is interpreted as ajuvenile. This specimen was first analyzed in 2012 and identified as anindeterminatearistonectine,[17] before being assigned to thegenusAristonectes in 2013.[18]
In 2001, Chilean paleontologist Mario E. Suárez collected a partial skull, mandibular fragments and 12 anterior cervical vertebrae from a beach nearCocholgüe [es], located north ofTomé. This material was described in the following year and attributed toAristonectes, but identification as a separate species was then impossible due to lack of preparation.[19] In early 2009, a second excavation was carried out independently at the same site by a team from theUniversity of Concepción of Chile and theUniversity of Heidelberg ofGermany. This excavation recovered 119 blocks ofsandstone, most of them with bone material, but some were degraded by the periodic immersion ofsea water that caused brittle surfaces on the most delicate parts. Additionally, several anatomical contacts were lost as blocks of sandstone were cut from the beach using a rock saw at low tide. The precise location of the find and thetaphonomic distribution of the fossils show that they belong to the same specimen that was discovered in 2001. The bones were sent to theChilean National Museum of Natural History for the preparation and scientific analysis. The completely prepared fossil consists of the skull, the axis and the atlas, 12 anterior cervical vertebrae, 23 middle to posterior cervical vertebrae, most of the trunk, the two almost complete forelimbs and a significant part of the right hind limb. The specimen was likely a young adult, and is cataloged as SGO.PV.957. In 2014, Rodrigo Otero and colleagues made it the holotype of the new speciesA. quiriquinensis; these researchers also referred the earlier specimen, SGO.PV.260, to this species. The specific epithet is a reference to the formation of Quiriquina, which is thetype locality of this taxon.[12]
In 2015, two other specimens from Quiriquina Island were referred toA. quiriquinensis. These are composed of a complete left femur and aproximal part of a humerus, cataloged respectively as SGO.PV.135 and SGO.PV.169, which were previously referred to the nowdubious genusMauisaurus.[20] In 2018, the holotype specimen was redescribed after further preparation. In the same publication, another specimen, SGO.PV.94, consisting of a series of anterior caudal vertebrae, was also referred toA. quiriquinensis.[21]
Aristonectes is a plesiosaur that had been particularly difficult for scientists to analyze because of its incompleteness.[15][16] Its relationships remained unclear until more recent research carried out onA. parvidens and the discovery of the second speciesA. quiriquinensis have made it possible to redescribe its anatomy and classify it among the elasmosaurids.[9][11][12][21][14] However, anatomical comparisons betweenA. parvidens andA. quiriquinensis are still limited to elements that are known from both species.[12][11]Aristonectes is a large representative of theplesiosauroids, the holotype ofA. quiriquinensis having an estimated size of about or more than 10 metres (33 ft) long.[21] A referred specimen ofAristonectes discovered in theLópez de Bertodano Formation inAntarctica, cataloged as MLP 89-III-3-1, is considered to be one of thelargest and heaviest plesiosaurs identified to date, having an estimated size between 11–11.9 metres (36–39 ft) long and a body mass of 10.7–13.5 metric tons (11.8–14.9 short tons).[22][a]
In the two known species, the skull ofAristonectes is flattened andogive-shaped.[9][11][12][21] The size of the skull varies very slightly between the two species, that of the holotype ofA. quiriquinensis having a proposed size between 65–70 cm (26–28 in), while from the holotype ofA. parvidens measures 73.5 cm (28.9 in).[12] InA. quiriquinensis thesquamosal bones extend well posterior to theoccipital condyle. Thepterygoid bones are large and project posterior to theneurocranium. The posterior extensions of the pterygoid bones are surrounding the axis and the atlas, which reduces the space between the skull and the vertebrae, and would therefore limit lateral movement at their joints.[21] Around 13teeth were present in thepremaxilla and 50 in themaxilla.[12][11] Among the unique characters of the skull ofA. quiriquinensis is the presence of a mental boss (a ridge along themandibular symphysis), a feature not observed in the holotype ofA. parvidens.[12]
The mandibular symphysis ofAristonectes is short.[6][9][11][12][21] Thedentary bones, although being partially preserved in both species, would have an estimated total of 50 teeth inA. quiriquinensis and 63 or more inA. parvidens.[9][12][23][11][b] The mandibular symphysis of the dentary bones inA. quiriquinensis is comparatively thicker and lacks the deep groove seen in ventral view inA. parvidens. The most posterior teeth ofA. parvidens were about 60 mm (2.4 in) rostrally from thecoronoid process. InA. quiriquinensis, there are no tooth sockets present, although 130 mm (5.1 in) of the mandible is preserved rostral to the coronoid process. Therefore, it is likely thatA. quiriquinensis would have fewer teeth thanA. parvidens. InA. parvidens, the labial contact between theangular bone and thesurangular, anterior to the glenoid fossa, bears a deep cavity which is not present inA. quiriquinensis.[12]
The teeth ofA. quiriquinensis have an oval cross-section, being very similar to elasmosaurids of theNorthern Hemisphere and theWeddellian Province (a geographical area that appeared after the isolation of Antarctica), as inKaiwhekea. Theroot of complete teeth is slightly longer than thecrown. The dentition ofA. quiriquinensis, and most likelyA. parvidens,[11][c] ishomodont, which means that the teeth are all of similar shape. The largest known tooth is very thin and pointed, with ridges on the lingual side of the crown. The smaller tooth is a short-rootedreplacement tooth with a similar morphology as the larger one.[12] All teeth are inclined toward the front and would not fit together when the jaws are closed.[21]
Although postcranial remains are known in both species, they are more completely known, and thus better documented, inA. quiriquinensis.[12][21] In 2018, Otero and his colleagues estimated thatA. quiriquinensis would have had a total of 109 vertebrae, including 43 cervical, 3 pectoral, 24 dorsal, 3sacral, and 35 caudal vertebrae.[21] The anteriorcervical ribs are recurved inA. parvidens and fuseddistally,[6] while those ofA. quiriquinensis are short and lack contacts.[12] The cervical ribs and cervical neural spines ofAristonectes are curved towards the head, similar to other members of thesubfamilyAristonectinae.[12][21] This morphology would have allowed for maintaining thetrunk higher than thehead.[21] The neural spines present on the middle of the cervical part look likeblades and are enlarged distally.[12]
The dorsal vertebrae are larger and taller than the cervical vertebrae and are also taller than they are wide. Thecentra of the dorsal vertebrae have amphicelous articular surfaces.[12] Between theglenoid cavities, thepectoral girdle ofA. quiriquinensis is nearly one meter in width. Theribs appear thickened on their distal ends. Thegastralia arepachyostotic and moderate in length. The caudal vertebrae ofA. quiriquinensis are wider than tall or long. The neural spines of the caudals become progressively blunt and short and have a thick dorsal apex with a flat surface which suggests the probable existence of a strong ligament along the dorsal part of thetail.[21]
Thelimbs ofA. quiriquinensis are very long for an elasmosaurid, the best-preservedswim paddle being about 3 m (9.8 ft) long. Combined with the aforementioned width of the pectoral girdle, this indicates a totalwingspan of 7 m (23 ft). In aquatictetrapods, such limb proportions are only observable in therhomaleosauridMeyerasaurus from theEarly Jurassic and in the modernhumpback whale.[21]A. parvidens would also have had a similar fin size. In both species, thephalanges of their limbs have large articular facets, featuring elongated coil-shaped bones. InA. quiriquinensis theulna is slightly shorter than theradius and thetibia is slightly wider than thefibula.[12]
Aristonectes was reclassified multiple times throughout itstaxonomic history.[12][11] In 1941, Cabrera classified it within the Elasmosauridae based on the shape of the cranial elements,[6] which will prove to be correct decades later.[24][25][21] In 1962,Samuel P. Welles consideredAristonectes to be an indeterminate pliosaur based on its rather enigmatic anatomy.[14] In 1960, Per O. Persson classified it in the Cimoliasauridae, a proposal which he reaffirmed in 1963 on the basis of new observations in the skull.[15] In 1981, David S. Brown movedAristonectes into theCryptoclididae based on the presence of 6 to 15 teeth in the premaxilla.[16] From the first revision of the genus carried out in 2003, Gasparini and his colleagues reclassifiedAristonectes in the family Elasmosauridae,[9] but this classification was rejected the same year by F. Robin O'Keefe and William Wahl Jr., who placed the genus in the Cimoliasauridae.[26] In 2009, O'Keefe and Hallie P. Street reviewed the validity of Cimoliasauridae, and found that this taxon is ajunior synonym of the Elasmosauridae. Therefore, they moved the generaAristonectes,Kaiwhekea,Kimmerosaurus andTatenectes into the newly erected familyAristonectidae.[27] In 2011, two years later, O'Keefe and colleagues were skeptical about the classification ofAristonectes andKaiwhekea withTatenectes, because their morphologies did not seem to correspond.[28] Otero and colleagues, in 2012, erected theAristonectinae as asubfamily within the Elasmosauridae. This group is characterized by a very enlarged skull compared to the width of the body, a moderately shortneck, and more than 25 teeth in the maxilla,[17] a higher number than in other elasmosaurids.[23] Along with the Elasmosaurinae, the Aristonectinae represents one of two recognized subfamilies within the elasmosaurids.[29] In many phylogenetic analyses,Aristonectes is the mostderived genus within the Aristonectinae.[24][25][29][30]
The followingcladogram is modified from Otero & Acuña, (2020):[30]
As with most other plesiosaurs,gastroliths have been found in some specimens ofAristonectes. Among these are 5 elements discovered during the second exhumation of the holotype ofA. quiriquinensis,[12] and over 793 in the largest known specimen of the genus, MLP 89-III-3-1, from Antarctica. However, because the latter specimen was found disarticulated (i.e., the bones had moved out of their original anatomical position), it is likely that not all of its gastrolithes have been found.[31][d] The function of gastroliths in plesiosaurs is still controversial.[33] The two most frequently cited hypotheses are their use inbuoyancy control and their use as aids for digestion; the latter being the most widely accepted in recent studies.[34][35][36] In 2014, O'Gorman and colleagues analysed the gastrolithes of the Antarctica specimen and concluded thatAristonectes did not select individual stones for ingestion, but swallowed batches of sediment at random that contained stones of various sizes. This is indicated by the lack of size selection: the size distribution of the gastrolites is similar to that expected from a random sample of sediment.[31]
The swimming paddles and tail ofA. quiriquinensis resemble those of various moderncetaceans, and lateral movements of the head would have been limited. In addition, the animal's skull has an enlargedmouth that would have allowed the engulfment of a large volume ofwater. This indicates thatA. quiriquinensis fed inbenthic zones, mixingprey and sediment at the same time. This type of feeding pattern is also documented in moderngray whales. The presence ofdecapodcrustaceans and fishes within the Quiriquina Formation further reinforces this assertion.[21]
After the discovery ofA. quiriquinensis in 2014, several specimens were subsequently referred to this species. In a study published in 2015 by Otero and his colleagues, it was noted that during theirgrowth, the humerus and femur change from a flatcapitulum in juveniles to hemispherical shaped heads at adulthood.[20]
Aristonectes is known from variousgeological formations ofPatagonia and Antarctica that date to the Maastrichtian. It is known from the Quiriquina[12][21] andDorotea formations in Chile,[37] from the Allen, Jagüel and Lefipán formations in Argentina[11] and from the López de Bertodano Formation in Antarctica.[38][22] The presence ofAristonectes within these formations shows that it would have beenendemic in the Weddellian Province, a geographical area that appeared after the isolation of Antarctica.[37] This also indicates that the genus could havemigrated regularly between Antarctica and Patagonia, like many current cetaceans.[21] Manybony fishes,cartilaginous fishes,crustaceans andmolluscs are known from most localities from whichAristonectes is listed.[39][40][41][37][21][42]
The diversification of plesiosaurs seems to vary from formation to formation. In the Lefipán Formation, onlyAristonectes is known,[9][11] while in the Quiriquina Formation it was contemporary with its close relativeWunyelfia.[30] In the Dorotea and López de Bertodano formations, in addition toAristonectes there are many indeterminate elasmosaurids.[37][38] Exclusively in the López de Bertodano Formation, numerous genera of contemporarymosasaurs have been identified. These includeKaikaifilu,Moanasaurus,Mosasaurus,Liodon andPlioplatecarpus, although the validity of some of these genera is disputed as they are primarily based on isolated teeth.[43][44] Some of these mosasaurs could have attacked the contemporary plesiosaurs of the formation, includingAristonectes.[45]
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