Aratasaurus

Aratasaurus is an extinct genus of basal coelurosaurian theropod dinosaur from the Early Cretaceous (Aptian) Romualdo Formation of Brazil. The genus contains a single species, A. museunacionali, known from a partial right leg. Aratasaurus represents the only tetrapod fossil known from the lower levels of the Romualdo Formation.

Discovery and naming
The Aratasaurus holotype, MPSC R 2089, was discovered in 2008 in Mina Pedra Branca, a plaster mine representing outcrops of the lower Romualdo Formation near Santana do Cariri in Ceará state of northeastern Brazil. The specimen consists of a partial articulated right leg, including the distal end of the femur, the proximal end of the tibia, part of metatarsals I–IV, several phalanges, and three pedal unguals. The specimen represents only part of the preserved individual, but the remainder was broken and lost during mining activity.

After its discovery, the fossil was taken to the Plácido Cidade Nuvens Museum of Paleontology to be prepared and described. Between 2008 and 2016, histological sample slices were made of the bones to observe the microscopic tissues. In 2016, the specimen was deposited at the National Museum of Brazil. On September 2, 2018, the museum was heavily damaged in a fire, but the area where the holotype was stored remained intact.

In 2020, Sayão and colleagues described Aratasaurus museunacionali as a new genus and species of early coelurosaurian theropod based on these fossil remains. The generic name, Aratasaurus, combines the Tupi words "ara", meaning "born", and "atá", meaning "fire", with the Greek "σαῦρος" ("sauros"), meaning "lizard". The specific name, museunacionali, honours the fire-devastated National Museum of Brazil, which is the country's oldest science institution. The full binomial name is intended to mean "the dinosaur born from the National Museum fire".

Description
As a coelurosaur, Aratasaurus likely had some form of feather covering, although no integument was preserved in the holotype. Some of the Aratasaurus bones—especially the tibia—compare favorably to Zuolong, a theropod from the Late Jurassic Shishugou Formation of China. The pes is more similar to the Jurassic theropods Aorun and Tanycolagreus. Based on osteohistological studies, Sayão et al. (2020) proposed that the holotype of Aratasaurus represents a juvenile or young adult that was about four years old at the time of its death. It has an estimated body length of 3.12 m and a body weight of around 34.25 kg. However, since the individual was not mature, it would have been able to grow larger.

Classification
In their phylogenetic analyses, Sayão et al. (2020) recovered Aratasaurus as the sister taxon to Zuolong, in the basalmost clade of coelurosaurs. Their results are displayed in the cladogram below:

Paleoecology


Aratasaurus is known from the Romualdo Formation of Brazil, which dates to the Aptian–Albian ages of the early Cretaceous period. It is the only theropod—and the only fossil tetrapod—currently known from the base of the formation, which has an estimated date of 115 Ma. However, at least three non-avian theropods have been named from the upper layers, including the spinosaurine Irritator challengeri and the coelurosaurs Santanaraptor placidus and Mirischia asymmetrica. This formation is very well known for its diverse pterosaur fauna, including abundant well-preserved anhanguerians and tapejarids. Several fossil crocodylomorphs, fish, and turtles are also known from the formation.