# Noasaurus

Noasaurus ("Northwestern Argentina lizard") is a genus of ceratosaurian theropod dinosaur from the Maastrichtian (Late Cretaceous) of Argentina. The type and only species is N. leali. The fragmentary holotype specimen of Noasaurus, PVL 4061, consisting of a few cranial and postcranial bones, was discovered from strata from the Lecho Formation of Southern Salta in 1975 by a team lead by José Fernando Bonaparte. When described by Bonaparte and in PhD student Jaime Powell in 1980, it was believed to be a coelurosaur theropod and assigned to a family of its own; this family, Noasauridae, still exists, though has been reassigned to Ceratosauria.
Noasaurus was a fairly small theropod, with PVL 4061 measuring somewhere between 1.6–2 m (5.2–6.6 ft) in length. Initially, it was believed that two strongly curved unguals (claws) found alongside the holotype were evidence of raptorial foot claws, like those of dromaeosaurids. However, it is now known that they belonged to Noasaurus' forelimbs, and were thus functionaly similar to those of spinosaurids. Therefore, rather than converging on dromaeosaurids, it may have been an opportunistic mesopredator, feeding on small vertebrates, including fish.

## Discovery and naming

In the mid-seventies, the fragmentary skeleton of a small theropod was discovered by Jaime Eduardo Powell and José Fernando Bonaparte at the El Brete fossil site in Salta Province, Argentina. El Brete, 2.7 km (1.7 mi) from the town of El Brete Estancia, is a fossiliferous (fossil-bearing) site that is part of the Lecho Formation. The tetrapod fossils of El Brete were first recorded by Boneparte et al. in 1977, including the theropod recovered. The type species, Noasaurus leali, was named and described by Bonaparte and Powell in 1980. The generic name begins with a usual abbreviation of noroeste Argentina, "northwest Argentina". The specific name honours the discoverer of the site, Juan Carlos Leal.
The holotype of Noasaurus, PVL 4061, was found in a layer of the Lecho Formation dating from the late Cretaceous period, more precisely the early Maastrichtian stage, about seventy million years ago. It consists of a partial skeleton with skull. It contains the maxilla, the quadrate bone, two neck vertebrae, two neck ribs, the centrum of a back vertebra, two hand claws, a finger phalanx and the second right metatarsal bone. One of the hand claws was initially identified as a second toe claw. In 2004, it was recognised as a hand claw, at which occasion the second hand claw was referred.
In 1999, a neck vertebra found at the site, specimen MACN 622, was identified as oviraptorosaurian, a rare proof that the Oviraptorosauria had invaded the Gondwanan continents. In 2007 however, it was reidentified as a noasaurid vertebra, probably belonging to the Noasaurus holotype.

## Description

Noasaurus was a small theropod. Gregory S. Paul estimated its length at 1.5 m (4.9 ft) and its weight at 15 kg (33 lb). In 2024, Hendrickx et al. used the reconstructed size of the skull and the dimensions of the second metatarsal to provide length estimates of 1.6 m (5.2 ft) and 2 m (6.6 ft), respectively. They abstained from providing a mass estimate due to the lack of the necessary limb elements, and the possibility that the Noasaurus holotype was a juvenile. A histological analysis could not be performed on the holotype, as it was prohibited, thus its age is currently unknown.

### Skull

The preserved skull elements of Noasaurus are a left maxilla and a right quadrate. The alveolar margin of the maxilla was concave, and the dorsal (top) part of the maxillary fossa bore a diagonal ridge. At least eleven teeth are present on the maxilla, though only five are preserved. They are recurved, and bear serrations on the front and rear edges. The body of the quadrate was strongly arched, and the pterygoid flange had a low anterior margin.

### Postcranial elements

Based on Noasaurus' elongated cervical vertebrae and comparisons with other noasaurids, its neck was long and sigmoidal, bearing an S-shaped curve. The vertebrae are vertically compressed, with low neural spines and long epipophyses, traits characteristic of abelisauroids. The only preserved element of the hind limb is the second right metatarsal, which is long and gracile.
Two claws were found in association with Noasaurus' holotype. Initially, they were believed to represent raptorial foot claws, similar to those of the unrelated dromaeosaurids. Subsequent studies, however, have demonstrated that the claws actually came from the manus. The claws are strongly curved, essentially blades, with parallel base sides in top view, and a deep triangular cavity at the base underside.

## Classification

Noasaurus is today considered to be a member of the Ceratosauria. Originally, it was seen as a member of the Coelurosauria. Bonaparte and Powell assigned it to a family of its own, the Noasauridae. In 1988, Gregory S. Paul saw them as members of the Abelisauridae and coined a Noasaurinae within that group. He also incorrectly thought they were Megalosauria. Later, the noasaurids were recognised as close relatives of the larger abelisaurids; they are both derived from the same basal abelisauroid ancestor.
The following cladogram is based on the phylogenetic analysis conducted by Rauhut and Carrano in 2016, showing the relationships of Elaphrosaurus among the noasaurids:
In 2024, Hendrickx et al. recovered Noasaurus in a polytomy with Laevisuchus, Masiakasaurus, Velocisaurus,and Vespersaurus, likely representing a radiation of small-bodied noasaurids that occurred during the Late Cretaceous.

## Paleobiology

In 1980, it was thought that the presumed foot claw functioned as a sickle claw. Paul in 1988 saw the noasaurines as the South-American counterparts of the Asian and North-American dromaeosaurids, in a process of convergent evolution. Noting that abelisaurids tend to have very short arms, he wondered whether the forelimbs of Noasaurus were of limited length also, forcing the animal to employ a kicking technique instead of grasping the back of a victim in order to disembowel it with the foot claws, a method he assumed the dromaeosaurids used. This hypothesis was undermined when it was determined that the foot claw was in fact a hand claw. Instead, as proposed by Hendrickx et al. in 2024, it may have been utilised to snag fish from the water, making it instead convergent with the enlarged thumb claw of spinosaurids like Baryonyx. They concluded that it was likely an opportunistic mesopredator that fed on small vertebrates, such as fishes. This lifestyle is similar to that proposed for Masiakasaurus.