So far all known piatnitzkysaurids have only been found in Jurassic deposits of the western hemisphere. Piatnitzkysaurus and Condorraptor hail from the Cañadón Asfalto Formation of Argentina, which has been dated to the Middle Jurassic. Marshosaurus was found in the Morrison Formation of the United States, which was dated to the Late Jurassic. Piatnitzkysaurids were among the first large theropods present in South America, and are evidence for a radiation of megalosauroids and other tetanurans in the middle Jurassic. Although there is strong support for their megalosauroid affinities, piatnitzkysaurids also share some traits with allosaurids, such as a 'sigmoidal' humerus. Piatnitzkysaurids can be distinguished from other megalosauroids due to the following synapomorphies :
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 and the spinosaurids. 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 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 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. 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, paraphyleticwith respect toAllosauroidea. 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:
