Dinosauromorphs

Dinosauromorpha is a clade of avemetatarsalian archosaurs (reptiles closer to birds than to crocodilians) that includes the Dinosauria (dinosaurs) and some of their close relatives. It was originally defined to include dinosauriforms and lagerpetids, with later formulations specifically excluding pterosaurs from the group. Birds are the only dinosauromorphs which survive to the present day.

Classification

The name "Dinosauromorpha" was briefly coined by Michael J. Benton in 1985. It was considered an alternative name for the group "Ornithosuchia", which was named by Jacques Gauthier to correspond to archosaurs closer to dinosaurs than to crocodilians. Although "Ornithosuchia" was later recognized as a misnomer (since ornithosuchids are now considered closer to crocodilians than to dinosaurs), it was still a more popular term than Dinosauromorpha in the 1980s. The group encompassed by Gauthier's "Ornithosuchia" and Benton's "Dinosauromorpha" is now given the name Avemetatarsalia.

In 1991, Paul Sereno redefined Dinosauromorpha as a node-based clade, defined by a last common ancestor and its descendants. In his definition, Dinosauromorpha included the last common ancestor of ''Lagerpeton'' (a lagerpetid), ''Marasuchus'' (a possible junior synonym of ''Lagosuchus),'' ''Pseudolagosuchus'' (now considered a synonym of the silesaurid ''Lewisuchus''), Dinosauria (including Aves), and all its descendants. This definition was intended to correspond to a clade including lagerpetids and crownward bird-line archosaurs, but not pterosaurs or other archosaurs.

In 2011, Dinosauromorpha was redefined by Sterling Nesbitt to be a branch-based clade, defined by including reptiles closer to one group than to another. Under this definition, Dinosauromorpha included all reptiles closer to dinosaurs (represented by ''Passer domesticus'', the house sparrow), rather than pterosaurs (represented by ''Pterodactylus''), ornithosuchids (represented by ''Ornithosuchus''), or other pseudosuchians (represented by ''Crocodylus niloticus'', the Nile crocodile). Nesbitt's study supported the hypothesis that Pterosauromorpha (pterosaurs and their potential relatives) was the sister group of Dinosauromorpha. Pterosauromorphs and dinosauromorphs together formed the group Ornithodira, which encompasses almost all avemetatarsalians.

Dinosauriformes was coined in 1992 by F.E. Novas, who used it to encompass dinosaurs, ''Lagosuchus'', "''Pseudolagosuchus''" (=''Lewisuchus''), and the herrerasaurids, which he did not consider to be "eudinosaurs" (true dinosaurs like ornithischians and saurischians). Contrary to Novas, most paleontologists since 1992 have considered herrerasaurids to be true dinosaurs, though many other dinosaur-like reptiles still fall within his definition of Dinosauriformes. Novas (1992) defined Dinosauriformes as a node-based clade containing the most recent common ancestor of ''Lagosuchus'' and Dinosauria, and all its descendants. Nesbitt (2011) provided a roughly equivalent definition, using ''Marasuchus'' and ''Passer domesticus'' (the house sparrow, a representative of dinosaurs). In his analysis, Dinosauriformes included dinosaurs, silesaurids, and ''Marasuchus'', but not lagerpetids, which were considered to be an earlier-branching family of dinosauromorphs.

Phylogeny

A phylogenetic analysis by Andrea Cau in 2018 resolved two different topologies for dinosaur origins, depending on whether it was run using parsimony or bayesian inference. Cau coined the term Dracohors for the clade uniting all taxa closer to the theropod ''Megalosaurus bucklandi'' than the basal form '' Marasuchus lilloensis''. The name is derived from the Latin words for "dragon" and "cohort", ''dracos'' and ''cohors''. Under parsimony results, Dracohors included only Silesauridae and Dinosauria, the latter including the groups Herrerasauria, Sauropodomorpha, Theropoda and Ornithischia, along with the basal form ''Eodromaeus''. However, under bayesian results, Herrerasauria placed outside Dinosauria within Dracohors, and Dinosauriformes, Dinosauromorpha, and Pan-Aves were synonyms, with ''Marasuchus'' in a clade with lagerpetids. ''Pisanosaurus'' was resolved within Silesauridae. Cau identified the synapomorphies of Dracohors as:

The anterior tympanic recess, the axial epipophyses, the centrodiapophyseal laminae in the presacral vertebrae, the relative size enlargement of the postacetabular process of ilium, the elongation of the pubis, the proximal sulcus and the reduction of the ligament tuber in the femoral head, and the further reduction in length of the fourth metatarsal and toe compared to the third.

Following the discovery and description of more cranial and postcranial material of the genera ''Kongonaphon'', ''Ixalerpeton'' and ''Lagerpeton'', it was found that lagerpetids shared many features with the basal taxa of Pterosauria. Features of the maxillary bone, teeth, braincase and forelimb meant that the 2020 phylogenetic analysis of Ezcurra and colleagues placed Lagerpetidae next to pterosaurs within Pterosauromorpha, removing the family from Dinosauromorpha. The contents of Dinosauromorpha was thus restricted to only Silesauridae, Dinosauria, and individual genera like ''Lagosuchus''.

Simultaneously, Rodrigo Müller and Maurício Garcia published novel results that reduced the family Silesauridae to a grade of basal dinosaurs in Ornithischia. ''Pisanosaurus'', considered by various authors to be either a silesaurid or basal ornithischian, was found to be intermediate between the grade of silesaurids and true ornithischians, explaining its peculiar combination of silesaurid and ornithischian features that has resulted in its phylogenetic inconsistency. ''Lewisuchus'', a carnivorous form, was found to be the most primitive form of ornithischian, which was almost universally considered to be an only-herbivorous clade before. Dinosauromorpha was reduced to only including Lagerpetidae and ''Lagosuchus'' as a result of the reclassification of silesaurids.

Below are the results of:

Cau (2018, parsimony results):

Cau (2018, bayesian results):

Ezcurra ''et al.'' (2020):

Müller and Garcia (2020):

A variety of individual species and taxa have at times been found to place within Dinosauromorpha and its subgroups, but outside Dinosauria. The taxon ''Marasuchus'' has been consistently recovered as a dinosauromorph between lagerpetids and silesaurids, but may also be a junior synonym of the coexisting form ''Lagosuchus'', another dinosauromorph. ''Pisanosaurus'', traditionally considered an ornithischian, was recovered in an unpublished analysis as a dinosauriform outside other clades, but has since been recovered only as a member of Silesauridae or Ornithischia. ''Saltopus'', an enigmatic taxon from the Late Triassic of Scotland, has been placed closer to dinosaurs than ''Marasuchus'', in a polytomy with Silesauridae and Dinosauria, as a sister taxon to ''Marasuchus'', or within Dinosauria as a basal saurischian. The British taxon ''Agnosphitys'' was originally described as a dinosauriform closer to Dinosauria than ''Herrerasaurus'',Nicholas C. Fraser, Kevin Padian, Gordon M. Walkden and A. L. M. Davis, 2002. Basal dinosauriform remains from Britain and the diagnosis of the Dinosauria. Palaeontology. 45(1), 79-95. but has also been classified as a dinosauriform more derived than silesaurids but basal to Herrerasauridae and Dinosauria, a silesaurid, or a basal saurischian. The genus ''Nyasasaurus'' from the early Late Triassic of Tanzania is known from multiple incomplete specimens, making it difficult to classify. It has been found as the direct sister taxon of Dinosauria, the basalmost ornithischian, a basal theropod, or a deeply-nested sauropodomorph.

Origins

Dinosauromorphs appeared putatively around 242 to 244 million years ago by the Anisian stage of the Middle Triassic, splitting from other ornithodires. Early Triassic footprints reported in October 2010 from the Świętokrzyskie (Holy Cross) Mountains of Poland may belong to a dinosauromorph. If so, the origin of dinosauromorphs would be pushed back into the Early Olenekian, around 249 Ma. The oldest Polish footprints are from a small quadrupedal animal named ''Prorotodactylus'', but footprints belonging to the ichnogenus '' Sphingopus'' that have been found from Early Anisian strata show that moderately large bipedal dinosauromorphs had appeared by 246 Ma. The tracks show that the dinosaur lineage appeared soon after the Permian-Triassic extinction event. Their age suggests that the rise of dinosaurs was slow and drawn out across much of the Triassic. The oldest known dinosauromorph is ''Asilisaurus'', a silesaurid which may have lived as early as the Anisian age of the middle Triassic period, about 245 million years ago, although it is possible that ''Nyasasaurus'' is a dinosaur of the same age, pushing the origins of the groups back further.

Putative basal dinosauromorphs include ''Saltopus'', ''Marasuchus'', the perhaps identical ''Lagosuchus'', the lagerpetid ''Lagerpeton'' from the Ladinian of Argentina and ''Dromomeron'' from the Norian of Arizona, New Mexico, and Texas (all in the United States), '' Ixalerpeton polesinensis'' and an unnamed form from the Carnian (Santa Maria Formation) of Brazil, and the silesaurids, which include ''Silesaurus'' from the Carnian of Poland, '' Eucoelophysis'' from the Carnian-Norian of New Mexico, ''Lewisuchus'' and the perhaps identical ''Pseudolagosuchus'' from the Ladinian of Argentina, '' Sacisaurus'' from the Norian of Brazil, ''Technosaurus'' from the Carnian of Texas, ''Asilisaurus'' from the Anisian of Tanzania, and ''Diodorus'' from the Carnian(?) to Norian of Morocco.

References

External links

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Anisian first appearances
Extant Middle Triassic first appearances
Taxa named by Michael Benton