| Dinosauromorphs | |
|---|---|
 | |
| From top to bottom and left to right, different type of dinosauromorphs:Asilisaurus,Borealopelta,Triceratops andGiganotosaurus. | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Ornithodira |
| Clade: | Dinosauromorpha Benton, 1985[1] |
| Subgroups | |
| |
Dinosauromorpha is aclade ofavemetatarsalians (archosaurs closer to birds than to crocodilians) that includes theDinosauria (dinosaurs) and some of their close relatives. It was originally defined to includedinosauriforms andlagerpetids,[3] with later formulations specifically excludingpterosaurs from the group.[4]Birds are the only dinosauromorphs which survive to the present day.
The name "Dinosauromorpha" was briefly coined byMichael J. Benton in 1985.[1] It was considered an alternative name for the group "Ornithosuchia", which was named byJacques Gauthier to correspond to archosaurs closer to dinosaurs than to crocodilians.[5] Although "Ornithosuchia" was later recognized as amisnomer (sinceornithosuchids are now considered closer to crocodilians than to dinosaurs), it was still a more popular term than Dinosauromorpha in the 1980s.[3] The group encompassed by Gauthier's "Ornithosuchia" and Benton's "Dinosauromorpha" is now given the nameAvemetatarsalia.[4]
In 1991,Paul Sereno redefined Dinosauromorpha as a node-basedclade, defined by alast common ancestor and its descendants. In his definition, Dinosauromorpha included thelast common ancestor ofLagerpeton (alagerpetid),Marasuchus (a possiblejunior synonym ofLagosuchus),Pseudolagosuchus (now considered a synonym of thesilesauridLewisuchus), 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.[3][6]
In 2011, Dinosauromorpha was redefined bySterling 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 byPasser domesticus, the house sparrow), rather than pterosaurs (represented byPterodactylus), ornithosuchids (represented byOrnithosuchus), or otherpseudosuchians (represented byCrocodylus niloticus, the Nile crocodile). Nesbitt's study supported the hypothesis that Pterosauromorpha (pterosaurs and their potential relatives) was thesister group of Dinosauromorpha. Pterosauromorphs and dinosauromorphs together formed the groupOrnithodira, which encompasses almost all avemetatarsalians.[4]
Dinosauriformes was coined in 1992 by F.E. Novas, who used it to encompass dinosaurs,Lagosuchus, "Pseudolagosuchus" (=Lewisuchus), and theherrerasaurids, which he did not consider to be "eudinosaurs" (true dinosaurs likeornithischians andsaurischians).[7] 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 ofLagosuchus and Dinosauria, and all its descendants.[7] Nesbitt (2011) provided a roughly equivalent definition, usingMarasuchus andPasser domesticus (the house sparrow, a representative of dinosaurs). In his analysis, Dinosauriformes included dinosaurs,silesaurids, andMarasuchus, but notlagerpetids, which were considered to be an earlier-branching family of dinosauromorphs.[4]
A phylogenetic analysis by Andrea Cau in2018 resolved two different topologies for dinosaur origins, depending on whether it was run usingparsimony orbayesian inference. Cau coined the termDracohors for the clade uniting all taxa closer to the theropodMegalosaurus bucklandi than the basal formMarasuchus lilloensis. The name is derived from theLatin words for "dragon" and "cohort",draco andcohors. Under parsimony results, Dracohors included onlySilesauridae andDinosauria, the latter including the groupsHerrerasauria,Sauropodomorpha,Theropoda andOrnithischia, along with the basal formEodromaeus. However, under bayesian results, Herrerasauria placed outside Dinosauria within Dracohors, and Dinosauriformes, Dinosauromorpha, andPan-Aves were synonyms, withMarasuchus in a clade with lagerpetids.Pisanosaurus was resolved within Silesauridae. Cau identified the synapomorphies of Dracohors as:[8]
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 generaKongonaphon,Ixalerpeton andLagerpeton, it was found that lagerpetids shared many features with the basal taxa ofPterosauria. Features of themaxillary bone, teeth,braincase and forelimb meant that the2020 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 likeLagosuchus.[9]
Simultaneously, Rodrigo Müller and Maurício Garcia published novel results that reduced the family Silesauridae to a grade of basal dinosaurs inOrnithischia.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. The reclassification of silesaurids left Lagerpetidae andLagosuchus as the only non-Dinosaurians in the Dinosauromorpha .[10]Below are the results of:
Cau (2018, parsimony results):[8]
Cau (2018, bayesian results):[8]
| Ezcurraet al. (2020):[9]
Müller and Garcia (2020):[10]
|
A variety of individual species and taxa have at times been found to place within Dinosauromorpha and its subgroups, but outside Dinosauria. The taxonMarasuchus has been consistently recovered as a dinosauromorph between lagerpetids and silesaurids, but may also be a junior synonym of the coexisting formLagosuchus, another dinosauromorph.[11]Pisanosaurus, traditionally considered an ornithischian, was recovered in an unpublished analysis as a dinosauriform outside other clades,[12] but has since been recovered only as a member of Silesauridae or Ornithischia.[10][13][14][15]Saltopus, an enigmatic taxon from theLate Triassic ofScotland, has been placed closer to dinosaurs thanMarasuchus, in a polytomy with Silesauridae and Dinosauria,[13] as a sister taxon toMarasuchus,[14][15] or within Dinosauria as a basal saurischian.[10] The British taxonAgnosphitys was originally described as a dinosauriform closer to Dinosauria thanHerrerasaurus,[16] but has also been classified as a dinosauriform more derived than silesaurids but basal toHerrerasauridae and Dinosauria,[15] a silesaurid,[13] or a basal saurischian.[10][14] The genusNyasasaurus from the early Late Triassic ofTanzania is known only from vertebra and a single humerus, making it difficult to classify. It has been variously found as the direct sister taxon of Dinosauria, the basalmost ornithischian, a basal theropod,[17] or a deeply-nested sauropodomorph.[13][14][15]
Dinosauromorphs appeared putatively around 242 to 244 million years ago by theAnisian stage of theMiddle Triassic, splitting from other ornithodires.Early Triassic footprints reported in October 2010 from theŚwiętokrzyskie (Holy Cross) Mountains of Poland may belong to a dinosauromorph. If so, the origin of dinosauromorphs would be pushed back into the EarlyOlenekian, around 249 Ma. The oldest Polish footprints are from a small quadrupedal animal namedProrotodactylus, but footprints belonging to theichnogenusSphingopus 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 thePermian-Triassic extinction event. Their age suggests that the rise of dinosaurs was slow and drawn out across much of the Triassic.[18] However, other researchers have questioned the dinosauromorph affinities ofProrotodactylus and suggested that the tracemaker may have been a basal archosauriform such as theEuparkeriidae.[19][20] The oldest known definitive dinosauromorph isAsilisaurus, asilesaurid which may have lived as early as theAnisian age of the middleTriassic period, about 245 million years ago,[21] although it is possible thatNyasasaurus is a dinosaur of the same age, pushing the origins of the groups back further.[13]
Putative basal dinosauromorphs includeSaltopus,[22][23]Marasuchus, the perhaps identicalLagosuchus, thelagerpetidLagerpeton from theLadinian ofArgentina andDromomeron from theNorian ofArizona,New Mexico, andTexas (all in the United States),Ixalerpeton polesinensis and an unnamed form from theCarnian (Santa Maria Formation) ofBrazil,[24][25] and thesilesaurids, which includeSilesaurus from theCarnian ofPoland,Eucoelophysis from the Carnian-Norian of New Mexico,Lewisuchus and the perhaps identicalPseudolagosuchus from the Ladinian ofArgentina,[26][27]Sacisaurus from the Norian ofBrazil,[28]Technosaurus from the Carnian of Texas,[29]Asilisaurus from theAnisian ofTanzania,[30] andDiodorus from the Carnian(?) to Norian ofMorocco.[31]
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