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Overview

Brief Summary

Introduction

Diapsids are by far the most speciose group of amniotes, with about 14 600 extant species (Goin et al., 1978). They have invaded all major habitats, from the polar circles (many migratory birds) to deserts (many lizards) and even the ocean (sea snakes, sauropterygians). Diapsids include most flying vertebrates (birds) and most poisonous chordates (snakes and the Gila Monster).

The early history of diapsids is poorly documented. Until the late seventies, the oldest known diapsids were the Upper Permian (250 Myr old) younginiforms from South Africa and Madagascar (Harris and Carroll, 1977; Currie, 1980, 1981, and 1982), and a few other contemporaneous diapsids of uncertain affinities (Carroll, 1976a and b). However, recent work (Reisz, 1977) has extended the fossil record of diapsids to the Pennsylvanian (about 300 Myr ago), and greatly increased our knowledge of the diversity of early diapsids (Brinkman et al., 1984; Reisz et al., 1984; Laurin, 1991; deBraga and Reisz, 1995). The oldest known crown-diapsids (saurians) date from the Late Upper Permian (Carroll, 1975; Evans, 1987).

Figure 1. The lepidosaur Crotaphytus, a terrestrial iguanid from western and central North America. Photograph copyright © 2000 John Merck.

Extant diapsids are classified into either lepidosaurs (lizards and Sphenodon) or archosaurs (birds and crocodiles). Both of these clades are very successful and speciose (Fig. 1), and archosaurs include some of the most fascinating vertebrates that ever lived, such as the pterosaurs (flying reptiles of the Mesozoic) and the many extinct groups of dinosaurs. Indeed, the present diversity of archosaurs, even though it compares favourably with many other clades, is a mere shadow of what it was in the mesozoic.

Diapsida was named after the two fenestrae (holes) found in the temporal region of the skull of most early and some extant diapsids. The lower temporal fenestra is between the jugal, postorbital, squamosal, and quadratojugal. The upper temporal fenestra is between the postorbital, parietal, and squamosal. Some diapsids have lost the lower fenestra (lizards) or even both fenestrae (snakes, amphisbaenids), but their early ancestors had both fenestrae.

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Comprehensive Description

Hearing in diapsids

The earliest diapsids (and indeed, all known stem diapsids) lacked a tympanum and could not hear high-frequency air-borne sounds, as shown by the lack of a quadrate emargination (the structure that supports the tympanum in most extant diapsids) and the robust shape of their stapes. This indicates that the tympanum present in most saurians developed independently from the tympanum of turtles and mammals (except if turtles are saurians, as recently suggested by Rieppel and deBraga (1996).

A few saurians have lost their tympanum and their ability to hear high-frequency air-borne sounds. These include snakes and many other lepidosaurs, such as Sphenodon, amphisbaenids, Tympanocryptis, Aphaniotes (Barry, 1963). These saurians can only hear low-frequency seismic vibrations (sounds transmitted through the ground).

Early phylogenies of diapsids suggested that the tympanum and the ability to hear high-frequency air-borne sounds had appeared separately in lepidosauromorphs and in archosauromorphs because younginiforms, formerly believed to be the oldest known lepidosauromorphs, apparently lacked a tympanum (Benton, 1985; Evans, 1988). However, the reinterpretation of younginiforms as stem diapsids implies that the tympanum appeared only once in diapsids, probably soon before the divergence between lepidosauromorphs and archosauromorphs (Laurin, 1991).

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Classification of diapsids

Despite numerous studies on diapsid phylogeny and classification, diapsids taxonomy still suffers from a lack of consensus. However, according to the principles of priority suggested by de Queiroz and Gauthier (1990, 1992, 1994), several diapsid taxa have been formally defined phylogenetically, and these definitions should be respected if diapsid taxonomy is ever to be standardized. These taxa include:

  • Diapsida: The most recent common ancestor of araeoscelidians, lepidosaurs, and archosaurs, and all its descendants (Laurin, 1991).
  • Araeoscelidia: The most recent common ancestor of Araeoscelis and Petrolacosaurus, and all its descendants (Laurin, 1991).
  • Neodiapsida: Sauria plus all other diapsids that are closer to saurians than they are to araeoscelidians (Gauthier et al., 1988b).
  • Eosuchia: The most recent common ancestor of Coelurosauravus, Apsisaurus, younginiforms, lepidosaurs, and archosaurs and all its descendants (Laurin, 1991).
  • Younginiformes: The most recent common ancestor of Youngina, Acerosodontosaurus, and Hovasaurus and all its descendants (Laurin, 1991).
  • Sauria: All the descendants of the most recent common ancestor of birds, crocodiles, squamates, and Sphenodon (Gauthier, 1984; Gauthier et al., 1988b).
  • Lepidosauromorpha: Extant lepidosaurs and all extinct saurians that are closer to them than they are to extant archosaurs (Gauthier, 1984; Gauthier et al., 1988a).
  • Archosauromorpha: Extant archosaurs and all extinct saurians that are closer to them than they are to extant lepidosaurs (Gauthier, 1984, 1994).

Some of these taxa have been given multiple definitions, and some of these definitions have been applied to more than one taxon, but the definitions of taxa given above appear to have priority (de Queiroz and Gauthier, 1990, 1992, and 1994).

This classification, as well as most recent classifications of diapsids, does not recognize Lacertilia as a formal taxon (Lacertilia included lizards but not snakes and amphisbaenids) because this group is not monophyletic (it does not include all the descendants of a common ancestor). Indeed, snakes (Estes et al., 1988; Rieppel, 1988) and amphisbaenids (Wu et al., 1993) are the direct descendants of some lizards, so in this page, the word "lizards" includes these taxa as well.

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Characteristics

Only skeletal autapomorphies (unique derived characters) can be confidently assigned to Diapsida because all the basal lineages of diapsids are extinct. In addition to the presence of an upper and a lower lateral temporal fenestra, these include:

  • A suborbital fenestra (Fig. 2). This is a relatively large hole in the palate that is located between the palatine, the ectopterygoid, and the maxilla. Other taxa may have a foramen (a small hole just large enough to allow passage for a nerve or a blood vessel) in this region, but there is usually no fenestra.
  • A long radius, measuring between 70% and 90% of the length of the humerus. This ratio is variable in extant diapsids, but early diapsids consistently had a longer radius than other taxa, in which it was typically less than 70% of the humeral length.

Figure 2. Diapsid skulls in palatal view. A, Petrolacosaurus, a Pennsylvanian araeoscelidian; B, Claudiosaurus, an Upper Permian neodiapsid; C, Youngina, an Upper Permian younginiform; D, Clevosaurus, a Late Triassic sphenodontid (a saurian).
Redrawn from A, Reisz, 1981; B and C, Carroll, 1981; D, Fraser, 1988. Scale bar equals 1 cm. Copyright © 2000 Michel Laurin.

Skeletal autapomorphies can unambiguously be attributed to Sauria, and other characters (soft anatomical, physiological, etc.) present in saurians but not in other extant amniotes are also usually attributed to Sauria, although they may have been present in other diapsids and even in their close relatives ("Protorothyridids" and captorhinids). Saurian autapomorphies include:

  • A low concentration of urea in the blood plasma resulting from a loss or suppression of the urea cycle. Lissamphibians, mammals, and turtles have a higher concentration of urea in their plasma.
  • The loss of the apposition between the kidney and the adrenal gland. These two organs are juxtaposed in lissamphibians, mammals, and turtles.
  • The presence of Huxley's foramen. This foramen is a hole in the distal end of the extracolumella (a cartilaginous extension of the stapes that contacts the tympanum). Huxley's foramen is absent in lissamphibians, mammals, and turtles.
  • Temporal muscles originating on the dorsolateral surface of the skull table. In araeoscelidians and younginiforms, the temporal muscles originate from a fascia attached to the lateral edge of the skull table and from the ventral surface of the skull table.
  • Prefrontal-nasal suture anterolaterally oriented (Fig. 3D). This suture is parasagittal in araeoscelidians, Claudiosaurus, and younginiforms (Fig. 3A-C).
  • Squamosal confined to the dorsal half of the skull, except for a narrow ventral process supporting the quadrate. The squamosal of younginiforms, Apsisaurus, and araeoscelidians is broad ventrally.
  • Strong, broad contact between the paroccipital process and the cheek. This contact is weak and often cartilaginous in younginiforms and araeoscelidians.
  • Quadrate deeply emarginated posteriorly. The quadrate of saurians supports a tympanum in its deep posterior emargination. The quadrate of younginiforms has a very shallow emargination that probably did not support a tympanum. The quadrate of Apsisaurus and araeoscelidians is not emarginated.
  • Slender stapes. The stapes of saurians is modified to function as a middle ear ossicle to transmit high-frequency air-borne sounds from the tympanum to the inner ear. The stapes of younginiforms and araeoscelidians is more massive and it is not specialized as a middle ear ossicle.
  • Dorsal process of stapes with ossified connection to paroccipital process of opisthotic. In other taxa, this connection is cartilaginous (or it may consist of a tendon), when it is present.
  • Large retroarticular process. This is the insertion point for the muscles that open the lower jaw. The retroarticular process of araeoscelidians, Coelurosauravus, and younginiforms is much smaller.
  • Cleithrum absent. The cleithrum is a dermal bone located on the anterior edge of the scapula, dorsal to the clavicle. It is present in araeoscelidians, Coelurosauravus, and younginiforms.
  • Lateral manual centrale (a bone in the wrist) small or absent. The lateral and medial centralia are approximately of equal size in araeoscelidians and in some younginiforms primitively (in Acerosodontosaurus).
  • Fifth distal tarsal absent (this is a small bone in the ankle, proximal to the fifth toe). Araeoscelidians, Coelurosauravus, and youngina have five distal tarsals.
  • Fifth metatarsal hooked (this bone supports the fifth toe). This bone is straight in other diapsids.

Figure 3. Diapsid skulls in dorsal view. A, Petrolacosaurus, a Pennsylvanian araeoscelidian; B, Claudiosaurus, an Upper Permian neodiapsid; C, Youngina, an Upper Permian younginiform; D, Clevosaurus, a Late Triassic sphenodontid (a saurian).
Redrawn from A, Reisz, 1981; B and C, Carroll, 1981; D, Fraser, 1988. Scale bar equals 1 cm. Copyright © 2000 Michel Laurin.

The linked page Autapomorphies of diapsid clades provides a list of autapomorphies of Neodiapsida and other clades smaller than Diapsida but more inclusive than Sauria.

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Ecology

Associations

Animal / dung saprobe
Basidiobolus ranarum is saprobic in/on dung or excretions of dung of Reptilia

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Known predators

Reptilia is prey of:
Thamnophis sirtalis
Diadophis punctatus
Lampropeltis triangulum
Agkistrodon piscivorus
Butorides virescens
Egretta thula
Eudocimus ruber
Buteo lineatus
Pandion haliaetus
Falco biarmicus
Herpetotheres cachinnans
Otus asio
Otus trichopsis
Micrathene whitneyi
Strix varia
Agelaius phoeniceus
Corvus corax
Corvus caurinus
Catharus guttatus
Spermophilus lateralis
Tamias dorsalis
Tamias merriami
Onychomys arenicola
Mustela vison
Bassariscus astutus
Nasua nasua
Panthera onca
Cerdocyon thous
Otocyon megalotis
Alligator mississippiensis
Paleosuchus trigonatus
Didelphis albiventris
Dasycercus cristicauda
Dasyurus maculatus
Planigale tenuirostris
Oncifelis geoffroyi
Prionailurus viverrinus
Ardea alba
Asturina nitida
Ictinia mississippiensis
Cacatua alba
Leontopithecus chrysopygus
Leontopithecus caissara
Papio hamadryas
Hylobates klossii
Heloderma horridum
Pseudalopex griseus
Pseudalopex gymnocercus
Pseudalopex vetulus
Vulpes chama
Oreailurus jacobita
Galidia elegans
Mungotictis decemlineata
Bdeogale nigripes
Crossarchus obscurus
Herpestes edwardsii
Herpestes ichneumon
Suricata suricatta
Lutrogale perspicillata
Melogale everetti
Melogale personata
Conepatus semistriatus
Galictis cuja
Ictonyx striatus
Mustela altaica
Mustela putorius
Bassaricyon gabbii
Chaetophractus villosus
Solenodon cubanus
Limnogale mergulus
Hemiechinus aethiopicus
Crocidura leucodon
Plecotus austriacus
Macroderma gigas
Megaderma lyra
Prionailurus iriomotensis
Canis lupus dingo
Canis lupus familiaris
Papio anubis
Papio ursinus

This list may not be complete but is based on published studies.
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Known prey organisms

Reptilia preys on:
Arthropoda
Gallicolumba luzonica
Meriones crassus
Sorex araneus

Based on studies in:
USA: California, Coachella Valley (Desert or dune)

This list may not be complete but is based on published studies.
  • Polis GA (1991) Complex desert food webs: an empirical critique of food web theory. Am Nat 138:123–155
  • Myers, P., R. Espinosa, C. S. Parr, T. Jones, G. S. Hammond, and T. A. Dewey. 2006. The Animal Diversity Web (online). Accessed February 16, 2011 at http://animaldiversity.org. http://www.animaldiversity.org
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Evolution and Systematics

Evolution

Discussion of Phylogenetic Relationships

View Diapsida Tree

Phylogeny based on Gauthier (1994).

Please note that some authors place the turtles within the Diapsida. Please refer to the accessory page on Diapsid Phylogeny and to the Discussion of Phylogenetic Relationships on the Amniota page for more information on this issue.

Diapsid phylogeny has been intensively studied in the last decade, but the affinities of many groups are still controversial. For instance, the affinities of the Upper Permian diapsids Galesphyrus (Carroll, 1976a), Heleosaurus (Carroll, 1976b), and Heleosuchus (Carroll, 1987) from South Africa are uncertain because these taxa are represented by fragmentary remains. However, the relationships of the other major groups of stem-diapsids are now reasonably well understood. The affinities of araeoscelidians were initially debated (Peabody, 1952; Vaughn, 1955; Reisz, 1977), but they are now universally believed to be the earliest known group of diapsids (Laurin, 1991; deBraga and Reisz, 1995). Younginiforms were previously believed to represent early lepidosauromorphs (Benton, 1985; Evans, 1988; Gauthier et al., 1988a), but they are now considered to be close relatives of Sauria (Laurin, 1991; Gauthier, 1994).

Some diapsid taxa like Ichthyosauria, Sauropterygia, Placodontia, and Choristodera remain problematic. Placodontia and Choristodera do not appear on the main tree because recent work suggests that they are either lepidosauromorphs or archosauromorphs (Rieppel, 1993, 1994; Rieppel and deBraga, 1996; deBraga and Rieppel, 1997; Merck, 1997). Only ichthyosaurs and sauropterygians appear because they may be either stem-diapsids, archosauromorphs, or lepidosauromorphs (Caldwell, 1996; Mercki, 1997; Motani et al., 1998). Turtles may also be saurians, either lepidosauromorphs or archosauromorphs, but for a discussion of this topic, please refer to the page on Amniota. For more information, see the linked page on Diapsid phylogeny.

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Functional Adaptations

Functional adaptation

Organs detect scent: reptiles
 

The tongues of many reptiles help detect odors by gathering scent particles and transferring them to a chemoreceptor organ.

     
  "Many snakes and reptiles combine the senses of smell and taste. When a snake flicks its forked tongue in and out of its mouth, it is sampling the air. The snake does not even need to open its mouth to do this. The tongue is flicked out through a small hole in the snake's lips, so its two slender forks can collect scent particles from the air or from an object such as a stone. Back inside the mouth, the tongue's forks are pressed into a pair of domed pits in the roof of the mouth, which have a moist lining that is sensitive to the chemicals it has picked up. The olfactory particles are transferred to the pits, which are well supplied with nerve endings, and are collectively known as Jacobson's organ. Although most often found in snakes, this organ is also common in other reptiles, especially terrestrial lizards." (Shuker 2001:31)
  Learn more about this functional adaptation.
  • Shuker, KPN. 2001. The Hidden Powers of Animals: Uncovering the Secrets of Nature. London: Marshall Editions Ltd. 240 p.
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Molecular Biology and Genetics

Molecular Biology

Statistics of barcoding coverage

Barcode of Life Data Systems (BOLD) Stats
                                        
Specimen Records:11,687Public Records:6,803
Specimens with Sequences:9,653Public Species:934
Specimens with Barcodes:8,953Public BINs:1,933
Species:2,216         
Species With Barcodes:1,847         
          
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Barcode data

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Locations of barcode samples

Collection Sites: world map showing specimen collection locations for Reptilia

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