Diapsids ("two arches") are a clade of
sauropsids
Sauropsida ("lizard faces") is a clade of amniotes, broadly equivalent to the class Reptilia. Sauropsida is the sister taxon to Synapsida, the other clade of amniotes which includes mammals as its only modern representatives. Although early syn ...
, distinguished from more primitive
eureptiles by the presence of two holes, known as
temporal fenestrae, in each side of their
skull
The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...
s. The group first appeared about three hundred
million years ago
The abbreviation Myr, "million years", is a unit of a quantity of (i.e. ) years, or 31.556926 teraseconds.
Usage
Myr (million years) is in common use in fields such as Earth science and cosmology. Myr is also used with Mya (million years ago ...
during the late
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 ...
period. All diapsids other than the most primitive ones in the clade
Araeoscelidia are sometimes placed into the clade
Neodiapsida. The diapsids are extremely diverse, and include birds and all modern reptile groups, including
turtles
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 tu ...
, which were historically thought to lie outside the group. Although some diapsids have lost either one hole (lizards), or both holes (snakes and turtles), or have a heavily restructured skull (modern birds), they are still
classified
Classified may refer to:
General
*Classified information, material that a government body deems to be sensitive
*Classified advertising or "classifieds"
Music
*Classified (rapper) (born 1977), Canadian rapper
* The Classified, a 1980s American ro ...
as diapsids based on their ancestry. At least 17,084
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
of diapsid animals are extant: 9,159 birds,
and 7,925 snakes, lizards,
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 ...
, turtles, and crocodiles.
Characteristics
The name Diapsida means "two arches", and diapsids are traditionally classified based on their two ancestral skull openings (
temporal fenestrae) posteriorly above and below the eye. This arrangement allows for the attachment of larger, stronger
jaw muscles, and enables the jaw to open more widely. A more obscure ancestral characteristic is a relatively long lower arm bone (the
radius
In classical geometry, a radius (plural, : radii) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', ...
) compared to the upper arm bone (
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 ...
).
Classification
Diapsids were originally classified as one of four subclasses of the class
Reptilia, all of which were based on the number and arrangement of openings in the skull. The other three subclasses were
Synapsida (one opening low on the skull, for the "mammal-like reptiles"),
Anapsida (no skull opening, including turtles and their relatives), and
Euryapsida (one opening high on the skull, including many prehistoric marine reptiles). With the advent of
phylogenetic nomenclature
Phylogenetic nomenclature is a method of nomenclature for taxa in biology that uses phylogenetic definitions for taxon names as explained below. This contrasts with the traditional approach, in which taxon names are defined by a '' type'', which ...
, this system of classification was heavily modified. Today, the synapsids are often not considered true reptiles, while Euryapsida were found to be an unnatural assemblage of diapsids that had lost one of their skull openings. Genetic studies and the discovery of the Triassic ''
Pappochelys'' have shown that this is also the case in turtles, which are actually heavily modified diapsids. In phylogenetic systems, birds (descendants of traditional diapsid reptiles) are also considered to be members of this group.
Some modern studies of reptile relationships have preferred to use the name "diapsid" to refer to the crown group of all modern diapsid reptiles but not their extinct relatives. However, many researchers have also favored a more traditional definition that includes the prehistoric
araeoscelidians. In 1991, Laurin defined Diapsida as 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 ...
, "the most recent common ancestor of
araeoscelidians,
lepidosaurs
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 specie ...
, 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, and all its descendants".
[Benton, M. J., Donoghue, P. C., Asher, R. J., Friedman, M., Near, T. J., & Vinther, J. (2015). "Constraints on the timescale of animal evolutionary history." ''Palaeontologia Electronica'', 18.1.1FC; 1-106; palaeo-electronica.org/content/fc-1]
A cladistic analysis by Laurin and Piñeiro (2017) recovers
Parareptilia as part of Diapsida, with pareiasaurs, turtles, millerettids, and procolophinoids recovered as more derived than the basal diapsid ''
Younginia''.
A 2020 study by David P. Ford and Roger B. J. Benson also recovered Parareptilia as deeply nested within Diapsida as the sister group to
Neodiapsida, But this excludes
mesosaurs, who were found to be basal among the sauropsids.
Relationships
Below is a
cladogram
A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...
showing the relations of the major groups of diapsids.
Cladogram
A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...
after Bickelmann ''et al.'', 2009
and Reisz ''et al.'', 2011:
See also
*
Vertebrate paleontology
*
Synapsida
*
Anapsid
An anapsid is an amniote whose skull lacks one or more skull openings (fenestra, or fossae) near the temples. Traditionally, the Anapsida are the most primitive subclass of amniotes, the ancestral stock from which Synapsida and Diapsida evolv ...
a
*
Euryapsida
References
External links
*
Diapsida Michel Laurin and Jacques A. Gauthier. ''Tree of Life Web Project''. June 22, 2000.
{{Taxonbar, from=Q134688
01
Reptile taxonomy
Reptiles of the United States
Extant Pennsylvanian first appearances
Taxa named by Henry Fairfield Osborn