Sanjuansaurus

A diagnosis is a statement of the anatomical features of an organism (or group) that collectively distinguish it from all other organisms. Some, but not all, of the features in a diagnosis are also autapomorphies. An autapomorphy is a distinctive anatomical feature that is unique to a given organism or group.

According to Alcober and Martinez (2010), Sanjuansaurus can be distinguished based on the following characteristics:[1]

• the cervical vertebrae have shelf-like, posterolaterally directed transverse processes
• the neural spines of the sixth to eighth dorsal vertebrae, at least, bear acute anterior and posterior processes
• the glenoid has everted lateral margins
• the pubis is relatively short, measuring 63% of the length of the femur
• a pronounced, rugose scar is present on the medial surface of the femur at the level of the fourth trochanter

Sanjuansaurus holotype specimen PVSJ 605 was discovered in 1994, in gray-green sandstone of the Cancha de Bocas Member near the base of the Ischigualasto Formation, in Ischigualasto Provincial Park in San Juan, Argentina. This horizon dates to approximately 231.4 Ma, during the late Carnian Stage of the Late Triassic.[1]

Skeletal reconstruction of Sanjuansaurus gordilloi. Known elements in white and unknown in dark gray.

In the Ischigualasto Formation, dinosaurs constituted only about 6% of the total number of fossils,[2] but by the end of the Triassic Period, dinosaurs were becoming the dominant large land animals, and the other archosaurs and synapsids declined in variety and number.[3]

Studies suggest that the paleoenvironment of the Ischigualasto Formation was a volcanically active floodplain covered by forests and subject to strong seasonal rainfalls. The climate was moist and warm,[4] though subject to seasonal variations.[5] Vegetation consisted of ferns (Cladophlebis), horsetails, and giant conifers (Protojuniperoxylon). These plants formed highland forests along the banks of rivers.[6] Sanjuansaurus lived in the jungles of Late Triassic South America alongside early dinosaurs, Eoraptor, Herrerasaurus, Chromogisaurus, and Panphagia, as well as Saurosuchus,[7] a giant land-living rauisuchian (a quadrupedal meat eater with a theropod-like skull); the broadly similar but smaller Venaticosuchus, an ornithosuchid; and the predatory chiniquodontids. Herbivores were much more abundant than carnivores and were represented by rhynchosaurs such as Hyperodapedon (a beaked reptile); aetosaurs (spiny armored reptiles); kannemeyeriid dicynodonts (stocky, front-heavy beaked quadrupedal animals) such as Ischigualastia; and therapsid traversodontids (somewhat similar in overall form to dicynodonts, but lacking beaks) such as Exaeretodon. These non-dinosaurian herbivores were much more abundant than early ornithischian dinosaurs like Pisanosaurus.[8]