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Quilmesaurus is a genus of carnivorous theropod dinosaur from the Upper Cretaceous (Campanian to Maastrichtian faunal stage) Allen Formation of Argentina. It was a member of Abelisauridae, closely related to genera such as Carnotaurus. The only known remains of this genus are leg bones which share certain similarities to a variety of abelisaurids. However, these bones lack unique features, which may render Quilmesaurus a nomen vanum (more commonly known as a nomen dubium, or "dubious name").
The holotype and currently only specimen was designated the collection number MPCA-PV-100, in the Museo Provincial "Carlos Ameghino". It consists of the distal (lower or outermost) half of the right femur (thighbone), and a complete right tibia (inner shinbone), collected from the Allen Formation of the Malargüe Group in the Neuquén Basin. These deposits date from the Campanian to Maastrichtian. The specimen came from the fluvial at the bottom of the Allen Formation. The taxon is notable as it represents one of the youngest records of a non-Bird theropod from Patagonia.
Unlike members of Megalosauroidea, the tibia of Quilmesaurus does not possess a noticeable anteromedial buttress, and instead it includes a large cnemial crest. Quilmesaurus is also not a Coelurosauria due to the distal part of the tibia being asymmetrical in shape as well as having a socket for the Talus bone which is lower than that of coelurosaurs. Finally, the shallow and wide (rather than deep and thin) extensor groove excludes Quilmesaurus from Carnosauria, as does the possession of parallel upper and lower edges of the cnemial crest.
However, some features do support its placement within Ceratosauria. These include a pronounced cnemial crest of the tibia and large mesiodistal crest of the femur. The asymmetrical distal part of the tibia and small socket for the astragalus specifically place it within the family Abelisauridae. The preserved bones share features with various abelisaurid taxa throughout the family, although such similarities are widespread and seemingly pop up at random among the taxa, thus making more specific placement difficult. The hook-like shape of the cnemial crest suggests that Quilmesaurus was a member of the subfamily Carnotaurinae, which Paul Sereno (1998) defined to include all abelisaurids closer to Carnotaurus than to Abelisaurus., the validity of Carnotaurinae has been debated. Although Valieri et al. (2007) considered the subfamily to include taxa such as Majungasaurus, Carnotaurus, Aucasaurus, and Rajasaurus, other studies have found different results. Tortosa et al. (2014) found that Carnotaurinae was an invalid group, as very few abelisaurids could actually apply to the definition set forth by Sereno. According to their analysis, Aucasaurus and Carnotaurus were actually closer to Abelisaurus than they were to Majungasaurus and Rajasaurus, thus forcing the latter two taxa to be excluded from the subfamily. Quilmesaurus was retained as close to Aucasaurus and Carnotaurus, although Sereno's name and definition of Carnotaurinae was completely demolished. In its place the tribe Carnotaurini was used, which includes all abelisaurids descended from the last common ancestor of Aucasaurus and Carnotaurus. Tortosa et al. (2014)'s result has largely been supported over that of Valieri et al. (2007). Filippi et al. (2016) created a new clade, Furileusauria, to include abelisaurids more closely related to Carnotaurus than to Ilokelesia, Skorpiovenator, or Majungasaurus. They included Quilmesaurus among the furileusaurians.
Valieri et al. (2007) were unable to establish a single autapomorphy (distinctive or unique trait) of the taxon, concluding that Quilmesaurus were a nomen vanum.
Remains of land animals were also common in this formation. An indeterminate is known, as well as numerous snake taxa including the Madtsoiidae Patagoniophis and Alamitophis. Other non-dinosaur animals in the area include the pterosaur Aerotitan and a variety of mammals.
Dinosaur remains recovered from the Allen Formation include a diverse and abundant assortment of ( Saltasaurus, Aeolosaurus, Laplatasaurus, Rocasaurus, etc.) and a hadrosaurid of dubious validity ( Willinakaqe). Theropods other than Quilmesaurus were also present; they include the large Unenlagiinae Dromaeosauridae Austroraptor, a basal Ornithurae bird ( Limenavis), and a Cimolopterygidae bird ( Lamarqueavis). A tooth has been referred to the family Carcharodontosauridae; this tooth is one of the most recent carcharodontosaurid fossils found as more well known members of this family ( Giganotosaurus, Mapusaurus) lived millions of years earlier in the Cretaceous. Indeterminate Nodosauridae remains have also been found at this formation, consisting of , , a femur, and a tooth.
The Allen Formation is also notable for the high amount of sauropod eggs discovered there. Nesting grounds have been discovered in the bajo de Santa Rosa area of the upper Allen Formation. Some (but not all) of these eggs were designated as the oogenus Sphaerovum. The structure of their eggshells indicate that they were laid in a very damp environment.
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