| Overoraptor | |
|---|---|
 | |
| Life restoration | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Clade: | Maniraptora |
| Clade: | Pennaraptora |
| Clade: | Paraves |
| Genus: | †Overoraptor |
| Type species | |
| †Overoraptor chimentoi Mottaet al.,2020 | |
Overoraptor (/ˌʊuvərʊuˈræptər/, meaning "piebald thief") is anextinct genus ofparaviantheropod ofuncertain affinities from theLate CretaceousHuincul Formation of ArgentinianPatagonia. The genus contains asingle species,O. chimentoi, known from several bones of thehands,feet, andhips alongside somevertebrae.
The fossil specimens ofOveroraptor were unearthed by Dr. Roberto Nicolás Chimento in the Violante Farm locality, which is southeast of the Ezequiel Ramos-Mexía lake, in the northwestern portion ofRio Negro Province, Argentina. This outcrop is part of theHuincul Formation, which is dated to theCenomanian-Turonian boundary in theCretaceous period. Thisformation has yielded some of the largestSouth American dinosaurs including the massivesauropodArgentinosaurus and thetheropodMapusaurus.[1]
The precise date of the discovery of the type series ofOveroraptor is not publicly known, but the specimens were reposited in the collections of theCarlos Ameghino Provincial Museum inCipolletti, which is near the outcrop of the Huincul Formation where the specimens were discovered. The fossils weredescribed as a new genus and species of paravian theropod in May 2020 by Matías Motta, Federico Agnolín, Federico Brissón Egli, andFernando Novas in the journalThe Science of Nature (orNaturwissenschaften).[1]
Theholotype specimen, MACN-Pv 805, was disarticulated when it was found and it is relatively fragmentary. It consists of twophalanges and ametacarpal of the right hand, twohemal arches, the rightscapula, the rightulna, a partialilium, a partialpubis, twometatarsals from each foot, and several phalanges andunguals from the left foot.[1]
Theparatype specimen, MPCA-Pv 818, was discovered at the same locality in association with the holotype. It is about 20% smaller than the holotype, which led the researchers describing it to conclude that they were likely not from a fully-grown animal. The paratype consists of two phalanges from the right hand, a fragment of the ilium, the right pubis, one of the right metatarsals, and one pedal phalanx from each foot.[1]
The generic name,Overoraptor, comes from theSpanish word "overo", meaning "piebald"—referencing the coloration of the fossils—and theLatin word "raptor", meaning "thief". Thespecific epithet, "chimentoi", was given in honor of Roberto Nicolás Chimento, who initially discovered its remains.[1]
In their 2020 description of the genus, Motta and colleagues estimated thatOveroraptor would have been about 1.3 metres (4.3 ft) in length, although they do not provide an estimate of the animal's mass.[1]Gregory S. Paul estimated the size ofOveroraptor to be about 3 metres (9.8 ft) long and weighing about 30 kilograms (66 lb).[2]
Motta and colleagues diagnosedOveroraptor as being distinct from all otherparavians from the following unique autapomorphies: a medial deflection at the distal end of thescapula, a reduced and ridge-likeacromial process, an extensive crest onmetacarpal I, a crest ending in a distal posterior tubicle onmetatarsal II, and ametatarsal III that is taller than it is wide at the midpoint along its length. Motta and colleagues further diagnosed the genus by the co-occurrence of the following autapomorphies: lateral ridges on thecaudalcentra, a robustulna, a saddle-shapedradial cotyle on the ulna, a non-hinged joint at the distal end of metatarsal III, and dorsally displaced pits on thesecond toe. In the description of itsosteology, Motta and colleagues considered both the holotype and paratype together, rather than describing each specimen individually.[1]
Thescapular anatomy ofOveroraptor resembles that of stem-avialans in the presence of a cup-shapedglenoidfossa, but it differs from most otherparavians in that the acromial process projects dorsally rather than medially. The overall morphology of the scapula resembles the enigmatic paravian taxonRahonavis because the scapular blade projects medially outward at its distal-most end. The ratio of the lengths of theulna to themetacarpals is similar to the ratio found indromaeosaurs and less than that of more derived avialans. The ulna itself is also curved, which is theplesiomorphic condition in paravians, unlike the straight ulnae ofAnchiornis and related taxa. However, the ulna ofOveroraptor resembles the derived avialan condition in the presence of a saddle-shaped cotyle on the ulnar-radial articular surface.[1]
The metacarpal bones ofOveroraptor were noted by Motta and colleagues to be unusual in shape, namely that it is proportionally much wider and shorter than those known from other non-avialan paravians and there is a distinctly asymmetrical ginglymoidal surface which articulates with the lateralcondyle. The manualphalanges are slender and elongate, which is common in the paravian taxa that retain their fingers. Similarly, the manualunguals are mediolaterally compressed, sharp, and curved, similar to the condition seen in dromaeosaurs.[1]
The foot bones ofOveroraptor are sub-arctometatarsalian, meaning the middle metatarsal narrows at the medial end but it is not completely pinched together by the other two bones of the ankle. The foot also bears the archetypal raptorial second digit (the characteristic toe claw of dromaeosaurs), which is short in length and proportionally robust relative to other taxa with this claw.[1]
In order to evaluate thephylogenetic position ofOveroraptor withinparaves, Motta and colleagues employed the data sets used by Agnolin and Novas (2013), Brusatte and colleagues (2014), Gianechini and colleagues (2017), and Hu and colleagues (2018) and added the scored characters of the new taxon. They also modified these data sets to include data found in more recently published research for some taxa.[1]
Their analyses recoveredOveroraptor in a variety of positions including as a basalmaniraptoran, a basalpennaraptoran, and a basal paravian, but they found the tree with the highest support values in their analysis recoveredOveroraptor as the sister taxon ofRahonavis in a clade that is the sister-taxon ofavialae and to whichunenlagiidae is basal. TheOveroraptor-Rahonavis clade was recovered with the followingsynapomorphies: a complex set of ridges on the lateral surfaces of the caudalcentra and a medial deflection of the distal end of thescapular blade. Even considering the similarities between the two taxa, Motta and colleagues also remark that the recovery of this clade may be the result of the unique mix of plesiomorphic and derived characters present inOveroraptor. The recovery of this clade is also suggested to be an early indicator of a much more diverse community of paravians acrossGondwana that have yet to be fully uncovered.[1]
A number of unorthodox results were obtained from the analysis of Motta and colleagues. They recoveredmicroraptorians andunenlagiines as being outside ofdromaeosauridae as successive stem clades to avialae. They similarly foundtroodontids as being the sister group of all other paravians rather than being either the sister group of dromaeosaurs or avialans,[1] which have been the results obtained by other recent analyses.[3][4][5][6][7] An abbreviated version of the strict consensus tree from their analysis using the data set from Agnolin and Novas (2013) is shown below.[1]
In 2021, Andrea Cau and Daniel Madzia published their redescription of thetroodontidBorogovia, which contained a phylogenetic analysis using a wide variety ofcoelurosaur taxa. They used several different interpretive frameworks including the coding ofRahonavis as anunenlagiine,Balaur as avelociraptorine, andhalszkaraptorines as being basal to thetroodontid-dromaeosaurid split as well as an unweighted analysis without these constraints. The poorly-understood taxonFukuivenator was also coded as a paravian,[8] although recent studies have resolved that it is probably a basaltherizinosaur[9] or some other non-paravianmaniraptoran.[4] The most strongly-supported result of their analysis was one in which troodontidae was the sister taxon of avialae, which includesOveroraptor,Rahonavis,Balaur, and several other taxa. An abbreviated version of the tree recovered by their parsimony analysis can be seen below.[8]
| Paraves |
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Huinculsaurus was discovered in the Argentine Province ofNeuquén. It was originally reported from the Huincul Group of the Río Limay Formation,[10] which have since become known as theHuincul Formation and theRío Limay Subgroup, the latter of which is a subdivision of theNeuquén Group. Thisunit is located in theNeuquén Basin inPatagonia. The Huincul Formation is composed of yellowish and greenishsandstones of fine-to-medium grain, some of which aretuffaceous.[11] These deposits were laid down during the Upper Cretaceous, either in the middleCenomanian to earlyTuronian stages[12] or the early Turonian to lateSantonian.[13] The deposits represent thedrainage system of abraided river.[14]
The Huincul Formation is thought to represent an arid environment with ephemeral or seasonalstreams.[15] In some areas, it is up to 250 metres (820 ft) thick. It is mainly composed of green and yellowsandstones and can easily be differentiated from the overlying Lisandro Formation, which is red in color. TheCandeleros Formation, underlying the Huincul, is composed of darker sediments, making all three formations easily distinguishable.[16][17]
Fossilised pollen indicates a wide variety of plants were present in the Huincul Formation. A study of the El Zampal section of the formation foundhornworts,liverworts,ferns,conifers, and someangiosperms (flowering plants).[18]
TheHuincul Formation is among the richest Patagonian vertebrate associations, preserving fish includinglungfish andgar,chelid turtles,squamates,sphenodonts,neosuchiancrocodilians, and a wide variety of dinosaurs.[12][19] Vertebrates are most commonly found in the lower, and therefore older, part of the formation.[20]
The Violante Farm locality is one of the most productive in the Huincul Formation, having yielded specimens of a wide variety of non-aviandinosaur species. Among these were theabelisaurTralkasaurus, themegaraptoranAoniraptor, thecarcharodontosaurTaurovenator, and the enigmatic taxonGualicho in addition to a variety of remains from indeterminate abelisaurs andcoelurosaurs. Specimens of thetitanosaurChoconsaurus were also discovered at this locality and represent the largest animal confidently known to have coexisted at this locality.[21]
Many other non-avian dinosaurs are known from the Huincul Formation, although it is not known with confidence if all of them coexisted withOveroraptor.[22]Sauropods are represented by the giant titanosaursBustingorrytitan,Chucarosaurus, andArgentinosaurus as well as therebbachisauridsCathartesaura,Limaysaurus, andSidersaura.[23][24] Theropods are also diverse and include the abelisaursIlokelesia andSkorpiovenator, thenoasauridHuinculsaurus, the carcharodontosauridsMapusaurus andMeraxes.[25]
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