Ceratosauria
Ceratosaurs are members of the clade Ceratosauria, a group of dinosaurs defined as all theropods sharing a more recent common ancestor with Ceratosaurus than with birds. The oldest known ceratosaur, Saltriovenator, dates to the earliest part of the Jurassic, around 199 million years ago. Ceratosauria includes three major clades: Ceratosauridae, Noasauridae, and Abelisauridae, found primarily in the Southern Hemisphere. Originally, Ceratosauria included the above dinosaurs plus the Late Triassic to Early Jurassic Coelophysoidea and Dilophosauridae, implying a much earlier divergence of ceratosaurs from other theropods. However, most recent studies have shown that coelophysoids and dilophosaurids do not form a natural group with other ceratosaurs, and are excluded from this group.
Ceratosauria derives its names from the type species, Ceratosaurus nasicornis, described by O.C. Marsh in 1884. A moderately large predator from the Late Jurassic, Ceratosaurus nasicornis, was the first ceratosaur to be discovered. Ceratosaurs are generally moderately large in size, with some exceptions like the larger Carnotaurus and the significantly smaller noasaurs. The major defining characteristics of Ceratosauria include a robust skull with increased ornamentation or height and a shortening of the arms. Both of these characteristics are generally accentuated in later members of the group, such as the abelisaurs, whereas more basal species such as C. nasicornis appear more similar to other basal theropods. The highly fragmented nature of the ceratosaur fossil record means that the characteristics, relationships, and early history of Ceratosauria remain mysterious and highly debated.
History of study
Ceratosauria was first described by O.C. Marsh in the American Journal of Science in 1884. Writing about the newly discovered C. nasicornis, he noted the similarities between the firmly united metatarsals of C. nasicornis and those of Archaeopteryx. Since C. nasicornis was the only other dinosaur discovered at the time to share this trait, Marsh concluded that Ceratosauria must be placed very near Archaeopteryx and its related groups. Marsh also named the family Ceratosauridae in 1884 to contain C. nasicornis. Since then, a number of other species have been referred to this family, mainly from the genus Ceratosaurus.The idea of the Ceratosauria would be contested by Marsh's rival, Edward Drinker Cope. Cope argued that the taxon was invalid. The idea of the Ceratosauria would regain some support more than thirty years later when Gilmore argued in its favor in 1920. Despite Gilmore's support, few species were added to the group following World War I, and little emphasis was placed on it. In fact, the scientific community's most common interaction with Ceratosauria throughout much of the 20th century was the disputation of its existence, performed by the likes of Romer, Lapparent, Lavocat, Colbert, and Charig amongst others.
Ceratosauria's fortune changed in 1986 when Jacques Gauthier, in an attempt to clarify the evolution of birds, grouped the majority of theropods into either Ceratosauria or Tetanurae. In Ceratosauria, he placed the ceratosaurs and coelophysoids. Gauthier's paper brought Ceratosauria's use back in vogue, and by the early 1990s, Abelisauridae had also been included under Ceratosauria. The triumvirate model of ceratosaurs, coelophysoids, and abelisaurids would go unchallenged until the early 2000s. Beginning at the turn of the millennium, a large number of paleontologists began excluding coelophysoids from Ceratosauria. This view is now widely held thanks to several similarities between Ceratosauria and Tetanurae not found in coelophysoids.
Phylogeny
Most paleontologists have postulated that Ceratosauria split off from other theropods in the Late Triassic or earliest Jurassic. Despite this, no ceratosaurs have been discovered prior to the Early Jurassic, and even in the Middle Jurassic, species are sparse. Many scientists, such as Carrano and Sampson, have postulated the lack of specimens is due to a poor fossil record, rather than an indictment on the abundance of ceratosaurs at the time. A similarly large gap of specimens exist in the lower Cretaceous, particularly for Abelisauridae. More recent discoveries have resulted in varying phylogenetic results concerning the relationships between Elaphrosaurus and the derived Cretaceous noasaurids. The precise relationship between Ceratosaurus and the abelisaurids is also not clearly resolved.Currently, most paleontologists agree that Ceratosauria contains a slightly more exclusive clade, Neoceratosauria, which contains the groups Ceratosauridae and Abelisauroidea, with some variance as to which taxa are placed into basal polytomy. Abelisauroidea is further divided into the Abelisauridae and Noasauridae, with Abelisauridae, including Carnotaurinae. Recently, Rauhut and Carrano have placed Elaphrosaurinae inside Noasauridae while simultaneously moving the previous noasaurs into Noasaurinae. Into their new Noasauridae, they have uniquely included Deltadromeus and Limusaurus.
It is difficult to discern possible synapomorphies of Ceratosauridae from autapomorphies of Ceratosaurus because the remains of the related Genyodectes are so fragmentary; e.g. Ceratosaurus is different from other ceratosaurians by the very prominent horn on its snout; Genyodectes, however, was not found with a complete skull; whether it had a horn is unknown, so it cannot establish that the horn was a shared derived feature of the group. However, due to the shared similarities between the teeth of the two genera, synapomorphies have been recognized in the teeth. The synapomorphies that do exist include: overlap of the second and third premaxillary alveoli in palatal view, largest crown in subadults/adults higher than six centimeters, subquadrangular mesial denticles at two-thirds of the crown in lateral teeth. Currently the only generally-recognized ceratosaurid species outside the genus Ceratosaurus is Genyodectes from the Cretaceous or Patagonia. The taxa Eoabelisaurus and Ostafrikasaurus are also probable ceratosaurs, but it is unknown if they belong to Ceratosauridae. Delcourt defined Ceratosauridae as "the most inclusive clade containing Ceratosaurus nasicornis but not Carnotaurus sastrei".
Abelisauroidea is a diverse superfamily of ceratosaurians and the sister taxon of Ceratosauridae. It is typically regarded as a Cretaceous group, though the earliest abelisaurid remains are known from the Middle Jurassic of Argentina and possibly Madagascar. Possible abelisaurid remains were also discovered in Late Jurassic Tendaguru Beds in Tanzania.
Abelisauroids flourished in the Southern Hemisphere during the Cretaceous period, but their origins can be traced back to at least the Middle Jurassic, when they had a more global distribution. By the Cretaceous period, abelisauroids had apparently become extinct in Asia and North America, possibly due to competition from tyrannosauroids. However, advanced abelisauroids of the family Abelisauridae persisted in the southern continents until the Cretaceous–Paleogene extinction event million years ago.
In an assessment of the phylogenetic position of Eoabelisaurus, the analysis found it as the most basal member of the Abelisauridae. Abelisaurid synapomorphies include the laterally covered lacrimal antorbital fossa, broad cervical prespinal fossae, anteroposteriorly short anterior caudal neural spines, absence of a ventral groove in the anterior caudals, presence of rudimentary centrodiapophyseal laminae in the anterior mid-caudals, reduced distal ginglymus in the manual phalanges, and the presence of a flexor depression in the pedal unguals. Alternative phylogenetic placements of Eoabelisaurus are significantly suboptimal, except for a slightly more basal position. Noasaurids had longer arms than their relatives the abelisaurids, whose arms were tiny and diminished. Although by no means as large or specialized as the arms of advanced bird-like theropods, noasaurid arms were nevertheless capable of movement and use, possibly even for hunting in large-clawed genera such as Noasaurus. Some genera such as Limusaurus did have somewhat reduced arms and hands, but far from the extent that abelisaurids acquired. Noasaurids were also nimble and lightly built, with feet showing adaptations for running such as a long central foot bone. Noasaurids varied in size, from the small Velocisaurus under long, to much larger genera such as Elaphrosaurus and Deltadromeus, which were more than in length.
The oldest known ceratosaur currently described is Saltriovenator zanellai which is dated to the Early Sinemurian, 199-197 Ma. The origin of Ceratosauria could have been in Northern Pangea where Saltriovenator, its close relative Berberosaurus, and Carmelopodus footprints have been found.
The following family tree illustrates a synthesis of the relationships of the major theropod groups based on various studies conducted in the 2010s and demonstrates the position of Ceratosauria within theropods.
The following cladogram shows the internal relationships within Ceratosauria following an analysis by Diego Pol and Oliver W. M. Rauhut, 2012.
A different conclusion was reached in a 2017 paper on Limusaurus ontogeny. Unlike other analyses, Noasauridae was placed more basal than Ceratosaurus, with the latter being within Abelisauridae by definition. This was later expanded on in a 2018 paper on ceratosaur paleobiology, which named a new clade Etrigansauria, which contained the families Abelisauridae and Ceratosauridae. The following cladogram is a consensus tree of the latest phylogenies shown in the paper.