Archelosauria is a

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

grouping

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

s and

archosaur Archosauria () or archosaurs () is a clade of diapsid sauropsid tetrapods, with birds and crocodilians being the only extant taxon, extant representatives. Although broadly classified as reptiles, which traditionally exclude birds, the cladistics ...

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

lizard Lizard is the common name used for all Squamata, squamate reptiles other than snakes (and to a lesser extent amphisbaenians), encompassing over 7,000 species, ranging across all continents except Antarctica, as well as most Island#Oceanic isla ...

snake Snakes are elongated limbless reptiles of the suborder Serpentes (). Cladistically squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales much like other members of the group. Many species of snakes have s ...

s and the

tuatara The tuatara (''Sphenodon punctatus'') is a species of reptile endemic to New Zealand. Despite its close resemblance to lizards, it is actually the only extant member of a distinct lineage, the previously highly diverse order Rhynchocephal ...

). The majority of

phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

analyses based on molecular data (e.g.

DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

and

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

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

parareptile Parareptilia ("near-reptiles") is an extinct group of Basal (phylogenetics), basal Sauropsida, sauropsids ("Reptile, reptiles"), traditionally considered the sister taxon to Eureptilia (the group that likely contains all living reptiles and birds ...

s, early-diverging diapsids outside of Sauria, or close relatives of lepidosaurs within the clade

Ankylopoda Ankylopoda was a proposed clade that hypothetically contains turtles and lepidosaurs (tuatara, lizards and snakes) and their fossil relatives. This clade was historically supported based on microRNA analysis as well as some cladistic analyses. ...

. Some recent morphological analyses have also found support for Archelosauria.

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 biology, structural, or evolutionary relationships between ...

s of

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 ultraconserved element (UCE) is a region of the genome that is shared between evolutionarily distant taxon, taxa and shows little or no variation between those taxa. These regions and regions adjacent to them (flanking DNA) are useful for tracing ...

s in the genome that are unique to turtles and

s, but which are not found in lepidosaurs. 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 A most recent common ancestor (MRCA), also known as a last common ancestor (LCA), is the most recent individual from which all organisms of a set are inferred to have descended. The most recent common ancestor of a higher taxon is generally assu ...

of '' Crocodylus niloticus'' (the Nile crocodile) and '' Testudo graeca'' (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 Sauria (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- diapsid reptiles nested within

Parareptilia Parareptilia ("near-reptiles") is an extinct group of basal sauropsids (" reptiles"), traditionally considered the sister taxon to Eureptilia (the group that likely contains all living reptiles and birds). Parareptiles first arose near the en ...

(a group of basal reptiles that lived from the

Carboniferous The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...

to the

Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...

), 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 ...

(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 excepti ...

on the supraoccipital bone, and the lack of an entepicondylar foramen on the

humerus The humerus (; : humeri) 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 (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...

. A cladogram adapted from their analysis is shown below: Wolniewicz ''et al'' (2023) also found evidence for an expanded Archelosauria containing the three Mesozoic marine reptile clades of uncertain placement:

References

{{Taxonbar, from=Q19595522 Reptile taxonomy Sauria