Piatnitzkysauridae

Piatnitzkysauridae is an extinct family of megalosauroid or basal Allosauroid dinosaurs. It only consists of three known dinosaur genera: '' Condorraptor'', ''Marshosaurus'', and ''Piatnitzkysaurus.'' The most complete and well known member of this family is ''Piatnitzkysaurus'', which also gives the family its name.

Description

So far all known piatnitzkysaurids have only been found in Jurassic deposits of the western hemisphere/New world. ''Piatnitzkysaurus'' and ''Condorraptor'' hail from the Cañadón Asfalto Formation of Argentina, which has been dated from the Toarcian epoch of the Early Jurassic to the Bathonian epoch of the Middle Jurassic (approximately 179 to 168 million years ago). ''Marshosaurus'' was found in the Morrison Formation of the United States, which was dated to the Kimmeridgian epoch of the Late Jurassic.

Piatnitzkysaurids were among the first large theropods present in South America, and are evidence for a radiation of basal tetanurans in the middle Jurassic. Although there is support for basal megalosauroid affinities, piatnitzkysaurids also share some traits with allosaurids, such as a 'sigmoidal' humerus. It has been proposed that they are basal Allosaurs or Non Megalosaurian Megalosauroids.

If Piatnitzkysaurids are Megalosauroids, they can be distinguished from other megalosauroids and placed into their own category due to the following synapomorphies (distinguishing features):
* Nutrient foramina on the surface of the maxilla which are arranged in two parallel rows (also found in ''Eocarcharia'' and ''Shaochilong'').
* Vertically striated or ridged paradental plates (also found in Abelisauridae, ''Megalosaurus'' and ''Proceratosaurus).''
* Reduced parapophyses on the axis (also found in Coelophysoidea, ''Eustreptospondylus'' and ''Afrovenator'').
* Anteriorly inclined posterior dorsal neural spines (in parallel with Allosauroidea).
* A humerus with a canted (bent forward, away from the body) distal condyle (also in ''Dilophosaurus'', ''Poekilopleuron'', Allosauridae and ''Fukuiraptor''). This form has also been described as 'sigmoidal' (S-shaped).
In addition, this family shows several reversals to basal Tetanuran traits. These include:
* A short or absent anterior process of the maxilla (also in Metriacanthosauridae and Carcharodontosauridae).
* Moderately developed (rather than reduced or absent) diapophyses on the axis.
* No pleurocoels on the axis.

Classification

Piatnitzkysauridae is defined as all members of Megalosauroidea more closely related to ''Piatnitzkysaurus'' than to either ''Spinosaurus'' or ''Megalosaurus''. ''Condorraptor, Marshosaurus, and Piatnitzkysaurus'' were all placed in a clade by Benson in 2010 during a redescription of ''Megalosaurus''. This clade was defined as a family by Carrano ''et al'' in 2012''.'' Piatnitzkysauridae has been recovered as the sister taxa to Megalosauria, a clade containing the megalosaurids (such as ''Megalosaurus'' and ''Torvosaurus'') and the spinosaurids (such as ''Spinosaurus'' and ''Baryonyx'').

''Condorraptor'' and ''Piatnitzkysaurus'' are generally grouped together within this family in a clade excluding ''Marshosaurus''. This is not only due to ''Condorraptor'' and ''Piatnitzkysaurus'' being from the same locality and time period, but also due to them having flat anterior faces of presacral vertebrae as well as a pronounced ventral keel on the anterior dorsal vertebrae. Due to the strong resemblance to ''Piatnitzkysaurus'', it has been suggested that the sister taxa '' Condorraptor'' 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 in ''Condorraptor''.

'' Xuanhanosaurus'', a problematic tetanuran with uncertain affinities, was placed as the sister taxon of these three genera when their relation was first observed in 2010. However, due to the incompleteness of ''Xuanhanosaurus's'' remains, this placement is considered uncertain. Other studies have considered ''Xuanhanosaurus'' a basal tetanuran. Xuanhanosaurus is still sometimes found as a Piatnitzkysauridae.

In 2019, Rauhut and Pol described ''Asfaltovenator vialidadi'', a basal allosauroid displaying a mosaic of primitive and derived features seen within Tetanurae. Their phylogenetic analysis found traditional Megalosauroidea to represent a basal grade of carnosaurs, paraphyletic with respect to Allosauroidea. Because the authors amended the definition of Allosauroidea to include all theropods that are closer to '' Allosaurus fragilis'' than to either '' Megalosaurus bucklandii'' or Neornithes, the Piatnitzkysauridae was found to fall within Allosauroidea. A cladogram displaying the relationships they recovered is shown below:

References

{{Taxonbar, from=Q5837348

Prehistoric tetanurans
Late Jurassic first appearances
Late Jurassic extinctions
Prehistoric dinosaur families