Tetanurae

Tetanurans have two basic skull morphologies.[6] The first skull type, typical in large theropods such as Allosaurus, is common within ceratosaurs and may be primitive for tetanurans. In this type, the skull is about three times longer than tall, with a blunter snout and frequent elaborations such as horns or spikes along the lacrimals, nasals, and frontals.[6] In the second skull type, the skull is lower and longer, with a less elaborated skull roof and a more elongated snout.[6] Shared tetanuran features include the maxillary fenestra (an opening in the antorbital fossa), a pneumatic excavation in the jugal, and the position of the maxillary teeth anterior to the orbit.[7] The posterior skull is little modified in tetanurans, except within Spinosauridae.

In the postcranial skeleton, tetanurans transition between the most primitive theropod morphologies in basal tetanurans towards more derived, bird-like states in coelurosaurs.[6] Most tetanurans possess specialized wrist bones, the absence or reduction of the fourth digit of the hand, a strap-like scapula, stiffened tails, and a laminar astragalar ascending process.[7][9][13] Advanced tetanurans would have possessed a sophisticated air-sac-ventilated lung system similar to birds, and an advanced circulatory system.[9][13] In megalosaurids and allosaurids, the orientation of the femoral head is anteromedial such as in ceratosaurs, but in avetheropods this orientation is fully medial.[6] Tetanuran locomotor morphology is relatively generalized, with few variations between taxa.[6]

Basal tetanurans were the first theropod clade to achieve truly giant body sizes, with both megalosauroid and allosauroid taxa weighing over 1 ton.[6] Sequential temporal appearances of large body size in subsequent clades suggest a pattern of size-cycles, with the extinction of incumbent giant forms allowing for replacement with a new, more bird-like theropod group that then also evolved giant body size.[6] It is, however, possible that more than one giant tetanuran existed at a time in the same paleoenvironment, perhaps with feeding habit variations. Within most dinosaur clades, body size tended to increase over time along a lineage according to Cope's rule.[17] Coelurosaurian theropods are the notable exception to the pattern of body size increases.[6]

Tetanurae was recognized and named by Gauthier in 1986.[18] The earliest-discovered tetanuran is the earliest-named dinosaur, Megalosaurus.[6] For a century after the description of Megalosaurus, most large carnivorous dinosaurs were serially arrayed into the family Megalosauridae within the order Theropoda.[6] In 1914, Friedrich von Huene separated small, lightly built forms into the infraorder Coelurosauria and larger taxa into the infraorder Pachypodosauria.[6] Later, he transferred large, carnivorous taxa to the new infraorder Carnosauria, which came to include all known large-bodied carnivores other than Ceratosaurus.[6] The size-based arrangement persisted until Gauthier, who redefined Carnosauria and Coelurosauria based on new cladistic analyses but retained the terms.[18] Gauthier defined Coelurosauria as a taxon comprising birds and theropods closer to birds than to Carnosauria, and listed within Carnosauria several large-bodied theropod taxa but did not formally define the group.[18] Many of these original carnosaurs have since been reclassified as coelurosaurs or primitive tetanurans, and Carnosauria has now been defined as Allosaurus and all Avetheropods closer to Allosaurus than to birds.[19]

Initial cladistics studies supported the arrangement of primitive megalosaurs as serial outgroups to a clade of allosaurids, followed by the Coelurosauria. Subsequent studies have discovered that many of these basal tetanurans formed a true clade, termed Megalosauroidea or alternatively Spinosauroidea.[6]

Current phylogeny agrees on a monophyletic Tetanurae that includes a series of generally large-bodied basal taxa outside a monophyletic Coelurosauria.[6] Coelophysoids are basal to Tetanurae, with Ceratosauria forming a sister taxa that diverged during the late Triassic.[7]

After their initial appearance, Tetanurae radiated into two main clades, Spinosauroidea or Megalosauroidea and Avetheropoda or Neotetanurae.[19] Spinosauroidea are believed to represent basal Tetanurans. At the Neotetanurae/Avetheropoda node, allosaurids split from the Coelurosauria. Tyrannosauridae has been placed within Coelurosauria. The allosaurids and their closest relatives form a reconstituted Carnosauria.[6] Debate persists about whether the allosaurids form a clade with spinosauroids/megalosauroids, and whether Allosauroidea belongs in Avetheropoda with Coelurosauria or forms a sister taxa to Megalosauroidea, and whether Megalosauroidea forms a valid clade.[6]

The cladogram presented below follows a phylogenetic analysis published by Zanno and Makovicky in 2013.[14]

The biogeographical history of non-avian Tetanurae spans over 110 million years and all continents.[6] The presence of major lineages prior to the breakup of Pangaea implies wide dispersal of these clades, with later absences indicating regional extinctions or dispersal failure.[6] The density of sampling is currently insufficient to provide a detailed analysis of biogeographical evolution for the Tetanurae.[6]

Tetanurae and Ceratosauria likely diverged during the late Triassic, more than 200 mya. By the Early Jurassic, Tetanurae fossils appear in the fossil record and reached global distribution by the Middle Jurassic.[6] In the Late Jurassic, the fossil record demonstrates widespread presence of multiple clades within both megalosauroids and avetheropods.[6] The Megalosauroidea contained high diversity with two Jurassic clades, Piatnitzkysauridae and Megalosauridae, as well as the Cretaceous Spinosauridae.[6] Tetanuran evolution appears to exhibit waves of diversification, although this may be due to uneven sampling.[6] During the Late Jurassic and Early Cretaceous, large spinosaurids and allosaurids flourished, but the latter possibly died out before the end of the Cretaceous due to the Cenomanian-Turonian boundary event, while spinosaurids are known from the Santonian. Soon afterwards the niche of terrestrial apex predator ceratosaurs and tyrannosaurid coelurosaurs, which dominated terminal Cretaceous terrestrial ecosystems.[14] Coelurosaurs persisted through the end of the Mesozoic Era.[14] Modern birds are the only living representatives of the Tetanurae.[14]