Evolution of Archosaurs

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Archosaurs

Overview

Archosauria is a family of amniote tetrapods with the crocodiles and birds as the only existing representatives today. These are tetrapods that developed a pair of holes in both sides of their skulls around 300 million years in the late Carboniferous period. The term is generally used in the description of reptiles, but in a way that includes the birds. Some of the archosaurs that have gone extinct include the pterosaurs, dinosaurs, and the extinct members of the crocodilian family.

Latest studies by scientists have put the definition of this group to include the newest common ancestor of the crocodilians, living birds, and descendants. Archosaurs are under the archosauriform group which is the real archosauromorphs. However, the oldest of them and the oldest archosauriform existed well into the late Permian. 

Evolution

The archosaurs have a record that spans almost 250 million beginning following the Permian-Triassic extinction event and it remains in place today via a pair of extant clades, which are the birds and the crocodilians. The pair of extant lineages represent extreme examples in the complicated and diverse history of evolution. However, it must be stated that not much is known about the common ancestor of all these lineages. 

Recent studies in the early archosaurs have led to the unveiling of a lot more information on the recognized taxa in the past two decades. One of the major studies involved comprehensive analysis at the phylogenetic level and it covered 80 taxa at the species level. This study covered the most recent Permian to the initial part of the Jurassic utilizing a dataset made up of 412 characters. 

Every individual terminal taxon was described and examined in detail. In addition to these, practically all the specimens used in the study were clearly outlined. Illustrations used by the researchers involved drawing on specimen photographs. An innovative blend of new characters and deep-rooted character sampling has shed more light on older studies that concentrated on archosauriform subclades. 

Recent discoveries also showed a comprehensive consensus tree reflecting Archosauria made of two main branches, which include the bird and crocodilian lines. These clades include the Phytosauria, Dinosauria, Ornithosuchidae, Aetosauria, and Crocodylomorpha. But it must be stated that phytosaurs are the most connected taxon to the Archosauria, instead of the fundamental crocodilian line, even if that was done for the first time. 

Then there are the taxa named rauisuchians, which is a clade discovered as the closest taxon to a set of rauisuchians and the crocodylomorphs. Therefore, the crocodylomorphs are encased in the Postosuchus clade and are not well linked to the aetosaurs more than the other taxa like the Postosuchus. Fundamental crocodylomorphs like the Hesperosuchus and other forms like Sphenosuchia as found in the grade that ended in the Crocodyliforms clade. There is greater support in this regard among scientists for Dinosauria. 

Another clade includes the Silesaurus and other related forms and this is in conjunction with the Dinosauria. As for Pterosaurs, there is a lot of support in the science community, especially concerning the birds. A time-based phylogeny of the Archosauriformes revealed the origin and early diversification of the Archosauria in the Early Triassic after the Permian-Triassic extinction. In addition to these, the principal archosaur lineages apart from Crocodylomorpha were formed towards the latter part of the Anisian epoch. The initial stage of the archosaur evolution features impressive levels of extensive ghost lines, homoplasy, and high turnover of character evolution. What scientists have noticed with the evolution of archosaurs is that the initial part of the history indicates that there is no full recovery via the record of fossils. The Middle Triassic led to a diversification of the archosaurs, but the biogeographic distribution was complete by the conclusion of the Anisian epoch. 

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