Archelosauria is a

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...

grouping

turtle Turtles are an order of reptiles known as Testudines, characterized by a special shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) and Cryptodira (hidden necked t ...

s and

archosaur Archosauria () is a clade of diapsids, with birds and crocodilians as the only living representatives. Archosaurs are broadly classified as reptiles, in the cladistic sense of the term which includes birds. Extinct archosaurs include non-avia ...

s (birds and crocodilians) and their fossil relatives, to the exclusion of

lepidosaur The Lepidosauria (, from Greek meaning ''scaled lizards'') is a subclass or superorder of reptiles, containing the orders Squamata and Rhynchocephalia. Squamata includes snakes, lizards, and amphisbaenians. Squamata contains over 9,000 species, m ...

s (the clade containing

lizard Lizards are a widespread group of squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbaenia altho ...

snake Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes . Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with several more ...

s and the

tuatara Tuatara (''Sphenodon punctatus'') are reptiles endemic to New Zealand. Despite their close resemblance to lizards, they are part of a distinct lineage, the order Rhynchocephalia. The name ''tuatara'' is derived from the Māori language and m ...

). The majority of

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups ...

analyses based on molecular data (e.g. DNA and

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

s) have supported a sister-group relationship between turtles and archosaurs. On the other hand, Archelosauria has not been supported by most morphological analyses, which have instead found turtles to either be descendants of

parareptile Parareptilia ("at the side of reptiles") is a subclass or clade of basal sauropsids (reptiles), typically considered the sister taxon to Eureptilia (the group that likely contains all living reptiles and birds). Parareptiles first arose near th ...

s, early-diverging

diapsid Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The group first appeared about three hundred million years a ...

s outside of

Sauria Sauria is the clade containing the most recent common ancestor of archosaurs (such as crocodilians, dinosaurs, etc.) and lepidosaurs ( lizards and kin), and all its descendants. Since most molecular phylogenies recover turtles as more closely ...

, or close relatives of lepidosaurs within the clade

Ankylopoda Ankylopoda was a proposed clade that hypothetically contains turtles and lepidosaur The Lepidosauria (, from Greek meaning ''scaled lizards'') is a subclass or superorder of reptiles, containing the orders Squamata and Rhynchocephalia. Squama ...

Classification

Multiple

sequence alignment In bioinformatics, a sequence alignment is a way of arranging the sequences of DNA, RNA, or protein to identify regions of similarity that may be a consequence of functional, structural, or evolutionary relationships between the sequences. Al ...

s of DNA and

sequences and

inferences have shown that turtles are the closest living relatives to birds and crocodilians. There are about 1000

ultra-conserved element An ultra-conserved element (UCE) was originally defined as a genome segment longer than 200 base pairs (bp) that is absolutely conserved, with no insertions or deletions and 100% identity, between orthologous regions of the human, rat, and mouse ge ...

s in the genome that are unique to turtles and

s, but which are not found in

s. Other genome-wide analyses also support this grouping. Archelosauria was named in a 2015 article by Crawford ''et al''. The name is meant to evoke the archosaurs and chelonians (turtles), the two living subgroups of the clade. Crawford ''et al.'' defined Archelosauria as the clade formed by the descendants of the

most recent common ancestor In biology and genetic genealogy, the most recent common ancestor (MRCA), also known as the last common ancestor (LCA) or concestor, of a set of organisms is the most recent individual from which all the organisms of the set are descended. The ...

of ''

Crocodylus niloticus ''Crocodylus'' is a genus of true crocodiles in the family Crocodylidae. Taxonomy The generic name, ''Crocodylus'', was proposed by Josephus Nicolaus Laurenti in 1768. ''Crocodylus'' contains 13–14 extant (living) species and 5 extinct species ...

'' (the Nile crocodile) and ''

Testudo graeca The Greek tortoise (''Testudo graeca''), also known commonly as the spur-thighed tortoise, is a species of tortoise in the family Testudinidae. ''Testudo graeca'' is one of five species of Mediterranean tortoises (genera '' Testudo'' and '' Ag ...

'' (the Greek tortoise). A 2021 article by Joyce ''et al.'' modified the definition to specifically exclude the

'' Lacerta agilis'' from the group. Below is the phylogeny from Crawford ''et al.'', showing interrelationships of Testudines at family level down to Durocryptodira. Archelosauria was grouped within

(the clade formed by archosaurs and lepidosaurs), as the sister branch to Lepidosauria, the clade containing lizards,

s and the

. Analyses based on morphological data have generally recovered turtles either as non-

reptiles nested within

Parareptilia Parareptilia ("at the side of reptiles") is a subclass or clade of basal sauropsids (reptiles), typically considered the sister taxon to Eureptilia (the group that likely contains all living reptiles and birds). Parareptiles first arose near th ...

(a group of basal reptiles that lived from the

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferou ...

to the

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest per ...

), as early-diverging diapsids outside of Sauria, or as close relatives of Lepidosauria. The hypothetical clade formed by turtles and lepidosaurs to the exclusion of archosaurs is known as

. A 2022 analysis by Simões ''et al.'' found a monophyletic Archelosauria using only morphological data for the first time, thus agreeing with most molecular analyses. Archelosauria was diagnosed by two unambiguous

synapomorphies In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to ha ...

(shared derived traits): a

sagittal crest A sagittal crest is a ridge of bone running lengthwise along the midline of the top of the skull (at the sagittal suture) of many mammalian and reptilian skulls, among others. The presence of this ridge of bone indicates that there are exception ...

on the

supraoccipital The occipital bone () is a cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone overlies the occipital lobes of the cereb ...

bone, and the lack of an entepicondylar foramen on the

humerus The humerus (; ) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of a r ...

. A cladogram adapted from their analysis is shown below:

References

{{Taxonbar, from=Q19595522 Reptile taxonomy Saurians