In total, two partial skeletons are known (a fractured skull and parts of two
postcranial skeletons, among which
paratype 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 long and around in mass, though such estimates apply to the
holotype, which is a
subadult. It had robust arms and powerful hind legs with four toes on each foot. Its
ischium is long. Its braincase resembles that of another megalosauroid, the megalosaurid
Piveteausaurus from France. A general resemblance to the theropod
Allosaurus was noted by Benton (1992); however there are key differences between the two. The scapular blade of
Piatnitzkysaurus is shorter and wider than more derived
tetanurans. The humerus represents 50 percent of the length of the femur, which is also a primitive condition present among basal
theropods. 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 of
Piatnitzkysaurus are proportionally longer than in
Allosaurus. Also primitive is the wide contact between the
pubis and
ischium, a condition that is much different than that of
Allosaurus and more derived tetanurans, in which contact is reduced. The pubis of
Piatnitzkysaurus also has a distal foot that is more modestly developed than in
Allosaurus. 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 cnemial crest and a first sacral vertebra with a shallower fossa in
Condorraptor.
Braincase The braincase of
Piatnitzkysaurus 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 of
Piatnitzkysauridae 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 the
tetanurans and is important for understanding not only Middle Jurassic
theropod evolution in the Southern Hemisphere, but also for knowledge of character evolution at the base of
tetanurae. The braincase of the holotype of
Piatnitzkysaurus 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 the
parasphenoid recess, which has only been described in two other non-avian theropods,
Sinovenator and
Sinornithosaurus. Given the distant phylogenetic position of the basal tetanuran
Piatnitzkysaurus and the advanced maniraptoran and deinonychosaurian taxa
Sinovenator and
Sinornithosaurus, the presence of this recess represents a convergence and can be considered an
autapomorphy of the former genus. Another noteworthy aspect of the braincase is the prominent hook-like
basisphenoidal wing, a feature that is also found in other theropods such as
Ceratosaurus,
Allosaurus, and
Tyrannosaurus. == Classification ==