Archosaur facts for kids

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Archosaurs
Temporal range: Lower Triassic – Recent
Crocodiles basking in the sun. Crocodiles can move quite fast on land by tucking their legs under their body: an Archosaur feature.
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Sauropsida
Subclass: Diapsida
(unranked): Archosauria
Cope, 1869

Archosaurs are a large group of reptiles, including all crocodiles, birds, dinosaurs, and pterosaurs (flying reptiles). There are also a number of smaller extinct groups, mostly from the Triassic period.

The Archosaurs are definitely a monophyletic clade, and do not include reptiles such as the Squamata (lizards and snakes) and the Sphenodontia (Sphenodon).

They have these diagnostic features, called synapomorphies in cladistics talk:

  • Teeth set in sockets, which makes them less likely to be torn loose during feeding. Some archosaurs, such as birds, are secondarily toothless.
  • Opening in the skull in front of the eyes but behind the nostrils, reduces the weight of the skull.
  • Small openings in the jaw bones, which reduces the weight of the jaw slightly.
  • Legs held under the body rather than sprawled. This improves both breathing and movement.
    • A special ridge for attaching muscles to the femur. This detail may have made it possible for dinosaurs to stand on two legs. All early dinosaurs and many later ones were bipedal.

The archosaurs or their immediate ancestors survived the catastrophic Permian–Triassic extinction event. Then, in the early and middle Triassic, there was rapid evolution into the types of land reptile which dominated the rest of the Mesozoic era.

Main forms

Pancrocodylia diversity
Examples of pseudosuchians. Clockwise from top-left: Longosuchus meadei (an aetosaur), Gavialis gangeticus, (a crocodilian), Saurosuchus galilei (a rauisuchian), Pedeticosaurus leviseuri (a sphenosuchian), Chenanisuchus lateroculi (a dyrosaurid), and Dakosaurus maximus (a thalattosuchian).
Panaves diversity
Examples of avemetatarsalians. Clockwise from top-left: Tupuxuara leonardi (a pterosaur), Alamosaurus sanjuanensis, (a sauropod), Tsintaosaurus spinorhinus (an ornithopod), Daspletosaurus torosus (a tyrannosaur), Pentaceratops sternbergii (a ceratopsian), and Grus grus (a neornithian).

Since the 1970s, scientists have classified archosaurs mainly on the basis of their ankles. The earliest archosaurs had "primitive mesotarsal" ankles: the astragalus and calcaneum were fixed to the tibia and fibula by sutures and the joint bent about the contact between these bones and the foot.

The Pseudosuchia appeared early in the Triassic. In their ankles, the astragalus was joined to the tibia by a suture and the joint rotated round a peg on the astragalus which fitted into a socket in the calcaneum. Early "crurotarsans" still walked with sprawling limbs, but some later crurotarsans developed fully erect limbs (most notably the Rauisuchia). Modern crocodilians are crurotarsans that can walk with their limbs sprawling or erect depending on speed of locomotion.

Euparkeria and the Ornithosuchidae had "reversed crurotarsal" ankles, with a peg on the calcaneum and socket on the astragalus.

The earliest fossils of Avemetatarsalia ("bird ankles") appear in the Carnian age of the late Triassic, but it is hard to see how they could have evolved from crurotarsans — possibly they actually evolved much earlier, or perhaps they evolved from the last of the "primitive mesotarsal" archosaurs. Ornithodires' "advanced mesotarsal" ankle had a very large astragalus and very small calcaneum, and could only move in one plane, like a simple hinge. This arrangement, which was only suitable for animals with erect limbs, provided more stability when the animals were running. The ornithodires differed from other archosaurs in other ways: they were lightly built and usually small, their necks were long and had an S-shaped curve, their skulls were much more lightly built, and many ornithodires were completely bipedal. The archosaurian fourth trochanter on the femur may have made it easier for ornithodires to become bipeds, because it provided more leverage for the thigh muscles. In the late Triassic, the ornithodires diversified to produce dinosaurs and pterosaurs.

Archosaur classification

Clades:

Phylogeny

In many phylogenetic analyses, archosaurs have been shown to be a monophyletic grouping, thus forming a true clade. One of the first studies of archosaur phylogeny was authored by French paleontologist Jacques Gauthier in 1986. Gauthier split Archosauria into Pseudosuchia, the crocodilian line, and Ornithosuchia, the dinosaur and pterosaur line. Pseudosuchia was defined as all archosaurs more closely related to crocodiles, while Ornithosuchia was defined as all archosaurs more closely related to birds. Proterochampsids, erythrosuchids, and proterosuchids fell successively outside Archosauria in the resulting tree. Below is the cladogram from Gauthier (1986):



ProterosuchidaeProterosuchusDB flipped.jpg




ErythrosuchidaeErythrosuchus afr12DB.jpg




ProterochampsidaeChanaresuchus.jpg


 Archosauria 
 Pseudosuchia 

ParasuchiaSmilosuchus adamanensis flipped.jpg




AetosauriaDesmatosuchus spurensis flipped.jpg




RauisuchiaPostosuchus kirkpatricki flipped.jpg



CrocodylomorphaDescription des reptiles nouveaux, ou, Imparfaitement connus de la collection du Muséum d'histoire naturelle et remarques sur la classification et les caractères des reptiles (1852) (Crocodylus moreletii).jpg





 Ornithosuchia 

EuparkeriaEuparkeria BW flipped.jpg




OrnithosuchidaeOrnithosuchus BW white background.jpg



OrnithodiraMeyers grosses Konversations-Lexikon - ein Nachschlagewerk des allgemeinen Wissens (1908) (Antwerpener Breiftaube).jpg









In 1988, paleontologists Michael Benton and J.M. Clark produced a new tree in a phylogenetic study of basal archosaurs. As in Gauthier's tree, Benton and Clark's revealed a basal split within Archosauria. They referred to the two groups as Crocodylotarsi and Ornithosuchia. Crocodylotarsi was defined as an apomorphy-based taxon based on the presence of a "crocodile-normal" ankle joint (considered to be the defining apomorphy of the clade). Gauthier's Pseudosuchia, by contrast, was a stem-based taxon. Unlike Gauthier's tree, Benton and Clark's places Euparkeria outside Ornithosuchia and outside the crown group Archosauria altogether.

The clades Crurotarsi and Ornithodira were first used together in 1990 by paleontologist Paul Sereno and A.B. Arcucci in their phylogenetic study of archosaurs. They were the first to erect the clade Crurotarsi, while Ornithodira was named by Gauthier in 1986. Crurotarsi and Ornithodira replaced Pseudosuchia and Ornithosuchia, respectively, as the monophyly of both of these clades were questioned. Sereno and Arcucci incorporated archosaur features other than ankle types in their analyses, which resulted in a different tree than previous analyses. Below is a cladogram based on Sereno (1991), which is similar to the one produced by Sereno and Arcucci:

Archosauriformes 

ProterosuchidaeProterosuchusDB flipped.jpg




ErythrosuchidaeErythrosuchus afr12DB.jpg




EuparkeriaEuparkeria BW flipped.jpg




ProterochampsidaeChanaresuchus.jpg


 Archosauria 
 Crurotarsi 

ParasuchiaSmilosuchus adamanensis flipped.jpg




OrnithosuchidaeOrnithosuchus BW white background.jpg



SuchiaDescription des reptiles nouveaux, ou, Imparfaitement connus de la collection du Muséum d'histoire naturelle et remarques sur la classification et les caractères des reptiles (1852) (Crocodylus moreletii).jpg




 Ornithodira 


?Scleromochlus



PterosauriaAerodactylus MCZ 1505.png




DinosauromorphaMeyers grosses Konversations-Lexikon - ein Nachschlagewerk des allgemeinen Wissens (1908) (Antwerpener Breiftaube).jpg









Ornithodira and Crurotarsi are both node-based clades, meaning that they are defined to include the last common ancestor of two or more taxa and all of its descendants. Ornithodira includes the last common ancestor of pterosaurs and dinosaurs (which include birds), while Crurotarsi includes the last common ancestor of living crocodilians and three groups of Triassic archosaurs: ornithosuchids, aetosaurs, and phytosaurs. These clades are not equivalent to "bird-line" and "crocodile-line" archosaurs, which would be branch-based clades defined as all taxa more closely related to one living group (either birds or crocodiles) than the another.

Benton proposed the name Avemetatarsalia in 1999 to include all bird-line archosaurs (under his definition, all archosaurs more closely related to dinosaurs than to crocodilians). His analysis of the small Triassic archosaur Scleromochlus placed it within bird-line archosaurs but outside Ornithodira, meaning that Ornithodira was no longer equivalent to bird-line archosaurs. Below is a cladogram modified from Benton (2004) showing this phylogeny:

Archosauria 

Hyperodapedon (Rhynchosauria) Hyperodapedon BW2 white background.jpg




Prolacerta (Prolacertiformes) Prolacerta broomi.jpg




Proterosuchus (Proterosuchidae)ProterosuchusDB flipped.jpg




Euparkeria (Euparkeriidae)Euparkeria BW flipped.jpg




ProterochampsidaeChanaresuchus.jpg


Avesuchia
 Crurotarsi 


PhytosauridaeSmilosuchus adamanensis flipped.jpg



GracilisuchusGracilisuchus BW white background.jpg




OrnithosuchidaeOrnithosuchus BW white background.jpg


 Suchia 

StagonolepididaeDesmatosuchus spurensis flipped.jpg




PostosuchusPostosuchus kirkpatricki flipped.jpg



CrocodylomorphaDescription des reptiles nouveaux, ou, Imparfaitement connus de la collection du Muséum d'histoire naturelle et remarques sur la classification et les caractères des reptiles (1852) (Crocodylus moreletii).jpg





Fasolasuchus


 Prestosuchidae

TicinosuchusTicinosuchus BW white background.jpg



Prestosuchus



SaurosuchusSaurosuchus BW white background.jpg






 Avemetatarsalia 

Scleromochlus


 Ornithodira 

PterosauriaAerodactylus MCZ 1505.png


 Dinosauromorpha

Lagerpeton


 Dinosauriformes 

MarasuchusMarasuchus flipped.jpg


Dinosauria

OrnithischiaStegosaurus stenops sophie wiki martyniuk flipped.png


 Saurischia 

SauropodomorphaBarapasaurus DB.jpg


 Theropoda 

HerrerasaurusHerrerasaurus BW flipped.jpg



NeotheropodaMeyers grosses Konversations-Lexikon - ein Nachschlagewerk des allgemeinen Wissens (1908) (Antwerpener Breiftaube).jpg
















In Sterling Nesbitt's 2011 monograph on early archosaurs, a phylogenetic analysis found strong support for phytosaurs falling outside Archosauria. Many subsequent studies supported this phylogeny. Because Crurotarsi is defined by the inclusion of phytosaurs, the placement of phytosaurs outside Archosauria means that Crurotarsi must include all of Archosauria. Nesbitt reinstated Pseudosuchia as a clade name for crocodile-line archosaurs, using it as a stem-based taxon. Below is a cladogram modified from Nesbitt (2011):



PhytosauriaSmilosuchus adamanensis flipped.jpg


Archosauria
Pseudosuchia

OrnithosuchidaeOrnithosuchus BW white background.jpg


Suchia

GracilisuchusGracilisuchus BW white background.jpg



Turfanosuchus




Revueltosaurus



AetosauriaDesmatosuchus spurensis flipped.jpg





TicinosuchusTicinosuchus BW white background.jpg


Paracrocodylomorpha

PoposauroideaPoposaurus gracilis (1) flipped.jpg


Loricata

PrestosuchusPrestosuchus11DB.jpg




SaurosuchusSaurosuchus BW white background.jpg




Batrachotomus




Fasolasuchus




RauisuchidaePostosuchus kirkpatricki flipped.jpg



CrocodylomorphaDescription des reptiles nouveaux, ou, Imparfaitement connus de la collection du Muséum d'histoire naturelle et remarques sur la classification et les caractères des reptiles (1852) (Crocodylus moreletii).jpg











Avemetatarsalia/

PterosauromorphaAerodactylus MCZ 1505.png


Dinosauromorpha

Lagerpetidae


Dinosauriformes

MarasuchusMarasuchus flipped.jpg




SilesauridaeSilesaurus opolensis flipped.jpg


Dinosauria

OrnithischiaStegosaurus stenops sophie wiki martyniuk flipped.png


Saurischia

SauropodomorphaBarapasaurus DB.jpg



TheropodaMeyers grosses Konversations-Lexikon - ein Nachschlagewerk des allgemeinen Wissens (1908) (Antwerpener Breiftaube).jpg











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Archosaur Facts for Kids. Homework Help - Kiddle Encyclopedia.