| Piatnitzkysaurus | |
|---|---|
 | |
| Skeleton cast | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Family: | †Piatnitzkysauridae |
| Genus: | †Piatnitzkysaurus Bonaparte,1979 |
| Species: | †P. floresi |
| Binomial name | |
| †Piatnitzkysaurus floresi Bonaparte, 1979 | |
| Synonyms | |
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Piatnitzkysaurus (/ˌpiːətnɪtskiːˈsɔːrəs/pee-ət-NITS-kee-SOR-əs; meaning "Piatnitzky's lizard") is agenus oftetanurantheropoddinosaur that lived approximately 179 to 177million years ago during the lower part of theJurassicPeriod in what is nowArgentina.Piatnitzkysaurus was a moderately large, lightly built,bipedal, ground-dwellingcarnivore that could grow up to 6.6 m (21.7 ft) long.
Theholotype specimen ofPiatnitzkysaurus, PVL 4073, was collected during expeditions in 1977, 1982, and 1983 at theCañadón Asfalto Formation in sediments that were deposited during the Middle-LateToarcianstage of theJurassic period, approximately 179 to 177 million years ago.[1][2] The specimen was very complete and is one of the best known from a megalosaur, including a partial skull and partial anterior postcranial skeleton of a subadult individual preserved in semi-articulation.[3] Later in 1986, several bones of an adult individual were referred to the taxon and are the only described material besides the type.[3]
The type species,Piatnitzkysaurus floresi, was described byJose Bonaparte in 1979.[2] It was named to honorAlejandro Matveievich Piatnitzky (1879–1959), a Russian-born Argentine geologist.[2] In Bonaparte’s 1979 paper on the taxon, he briefly described it along with the sauropodsPatagosaurus fariasi (acetiosaurid)andVolkheimeria chubutensis (an earlyeusauropod) from the same deposits.[2] It wasn’t until 1986 that Bonaparte fully described the theropod, classifying it in Allosauridae along withAllosaurus,Acrocanthosaurus,Eustreptospondylus, andDilophosaurus.[3]Piatnitzkysaurus was later reclassified as an abelisaurid,[4] basal carnosaur, basal tetanuran,[5] and non-megalosaurid megalosauroid[6] in phylogenetic analyses, but was classified in its own family in 2012, the Piatnitzkysauridae.[6]
In total, two partial skeletons are known (a fractured skull and parts of twopostcranial skeletons, among whichparatype MACN CH 895) and is the most completely known theropod from the Middle-Late Jurassic Period of the Southern Hemisphere.Piatnitzkysaurus was a relatively lightly built medium-sized bipedal carnivore that was around 4.3 metres (14 ft) long and around 450 kilograms (990 lb) in mass, though such estimates apply to theholotype, which is asubadult.[7] It had robust arms and powerful hind legs with four toes on each foot. Itsischium is 423 millimetres (16.7 in) long.[8] Its braincase resembles that of another megalosauroid, the megalosauridPiveteausaurus from France.[9]
A general resemblance to the theropodAllosaurus was noted by Benton (1992); however there are key differences between the two.[10] The scapular blade ofPiatnitzkysaurus is shorter and wider than more derivedtetanurans. The humerus represents 50 percent of the length of the femur, which is also a primitive condition present among basaltheropods. The relative lengths of the ulna in respect to the humerus and femur are also similar to those of basal theropods, which means the forelimbs ofPiatnitzkysaurus are proportionally longer than inAllosaurus. Also primitive is the wide contact between thepubis andischium, a condition that is much different than that ofAllosaurus and more derived tetanurans, in which contact is reduced. The pubis ofPiatnitzkysaurus also has a distal foot that is more modestly developed than inAllosaurus.[11]
Due to the strong resemblance toPiatnitzkysaurus, it has been suggested that the sister taxaCondorraptor could be better interpreted as the result of individual variation within the species, and not as separate taxa. The main noted differences between the two dinosaurs include both a less well-developed enemial crest and a first sacral vertebra with a shallower fossa inCondorraptor.[11] However, detailed redescription of the appendicular anatomy ofPiatnitzkysaurus revealed many differences between the two taxa.[12]
The braincase ofPiatnitzkysaurus has been reviewed in detail by Oliver Rauhut; the review constitutes one of the few detailed accounts of braincase morphology in basal theropods.Piatnitzkysaurus is the only member ofPiatnitzkysauridae with cranial material preserved, for which two maxillae, a frontal, a braincase, and a partial dentary are known.Piatnitzkysaurus is among the most basal members of thetetanurans and is important for understanding not only Middle Jurassictheropod evolution in the Southern Hemisphere, but also for knowledge of character evolution at the base oftetanurae.[9]
The braincase of the holotype ofPiatnitzkysaurus floresi (PVL 4073) is rather well preserved and shows no signs of deformation. The sutures between the individual bones of the skull are only partially visible, which indicates that the holotype represents a subadult individual. This is also consistent with the state of ossification in the post-cranial skeleton. An unusual feature of the braincase is theparasphenoid recess, which has only been described in two other non-avian theropods,Sinovenator andSinornithosaurus. Given the distant phylogenetic position of the basal tetanuranPiatnitzkysaurus and the advanced maniraptoran and deinonychosaurian taxaSinovenator andSinornithosaurus, the presence of this recess represents a convergence and can be considered anautapomorphy of the former genus. Another noteworthy aspect of the braincase is the prominent hook-likebasisphenoidal wing, a feature that is also found in other theropods such asCeratosaurus,Allosaurus, andTyrannosaurus.[11]
The most basal clade withinMegalosauroidea containsCondorraptor,Marshosaurus,Piatnitzkysaurus andXuanhanosaurus. The next most basal clade comprisesChuandongocoelurus andMonolophosaurus. However, the affiliation of these clades with Megalosauroidea is poorly supported by tree support metrics, and it is possible that they will be classified outside of Megalosauroidea by future analyses.[13]
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