Parareptiles
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Parareptilia ("near-reptiles") is an extinct group of basal sauropsids ("
reptiles Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...
"), traditionally considered the
sister taxon In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...
to Eureptilia (the group that likely contains all living reptiles and birds). Parareptiles first arose near the end of the
Carboniferous period The Carboniferous ( ) is a geologic period and system of the Paleozoic era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the Permian Period, Ma. It is the fifth and penultimate perio ...
and achieved their highest diversity during the
Permian period The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...
. Several ecological innovations were first accomplished by parareptiles among reptiles. These include the first reptiles to return to marine ecosystems (
mesosaur Mesosaurs ("middle lizards") were a group of small aquatic reptiles that lived during the early Permian period ( Cisuralian), roughly 299 to 270 million years ago. Mesosaurs were the first known aquatic reptiles, having apparently returned to a ...
s), the first
bipedal Bipedalism is a form of terrestrial locomotion where an animal moves by means of its two rear (or lower) limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning 'two feet' (from Latin ''bis'' ...
reptiles ( bolosaurids such as '' Eudibamus''), the first reptiles with advanced hearing systems ( nycteroleterids and others), and the first large herbivorous reptiles (the
pareiasaur Pareiasaurs (meaning "cheek lizards") are an extinct clade of large, herbivorous parareptiles. Members of the group were armoured with osteoderms which covered large areas of the body. They first appeared in southern Pangea during the Middle Per ...
s). The only parareptiles to survive into the
Triassic period 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 the ...
were the procolophonoids, a group of small generalists, omnivores, and herbivores. The largest family of procolophonoids, the procolophonids, rediversified in the Triassic, but subsequently declined and became extinct by the end of the period. Compared to most eureptiles, parareptiles retained fairly "primitive" characteristics such as robust, low-slung bodies and large supratemporal bones at the back of the skull. While all but the earliest eureptiles were diapsids, with two openings at the back of the skull, parareptiles were generally more conservative in the extent of temporal fenestration. In its modern usage, Parareptilia was first utilized as a cladistically correct alternative to Anapsida, a term which historically referred to reptiles with solid skulls lacking holes behind the eyes. Nevertheless, not all parareptiles have "anapsid" skulls, and some do have large holes in the back of the skull. They also had several unique adaptations, such as a large pit on the
maxilla In vertebrates, the maxilla (: maxillae ) is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The two maxil ...
, a broad prefrontal-
palatine A palatine or palatinus (Latin; : ''palatini''; cf. derivative spellings below) is a high-level official attached to imperial or royal courts in Europe since Roman Empire, Roman times.
contact, and the absence of a supraglenoid foramen of the
scapula The scapula (: scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on either side ...
. Like many other so-called "anapsids", parareptiles were historically understudied. Interest in their relationships were reinvigorated in the 1990s, when several studies argued that Testudines (
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 their kin) were members of Parareptilia. Although this would suggest that Parareptilia was not extinct after all, the origin of turtles is still heavily debated. Many other morphological or genetic analyses find more support for turtles among diapsid eureptiles such as
sauropterygia Sauropterygia ("lizard flippers") is an extinct taxon of diverse, aquatic diapsid reptiles that developed from terrestrial ancestors soon after the end-Permian extinction and flourished during the Triassic before all except for the Plesiosau ...
ns or archosauromorphs, rather than parareptiles. Several studes from the early 2020s have suggested that "Parareptilia" is not a
monophyletic In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent co ...
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 ...
but a
paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...
grade of primitive sauropsids, with some "parareptiles" more closely related to modern reptiles than to other "parareptilians".


Description


Skull

Parareptilian skulls were diverse, from
mesosaur Mesosaurs ("middle lizards") were a group of small aquatic reptiles that lived during the early Permian period ( Cisuralian), roughly 299 to 270 million years ago. Mesosaurs were the first known aquatic reptiles, having apparently returned to a ...
s with elongated snouts filled with hundreds of thin teeth, to the snub-nosed, knob-encrusted skulls of
pareiasaur Pareiasaurs (meaning "cheek lizards") are an extinct clade of large, herbivorous parareptiles. Members of the group were armoured with osteoderms which covered large areas of the body. They first appeared in southern Pangea during the Middle Per ...
s. Parareptile teeth were quite variable in shape and function between different species. However, they were relatively homogenous on the same skull. While most
synapsid Synapsida is a diverse group of tetrapod vertebrates that includes all mammals and their extinct relatives. It is one of the two major clades of the group Amniota, the other being the more diverse group Sauropsida (which includes all extant rept ...
s and many early eureptiles had a
caniform Caniformia is a suborder within the order Carnivora consisting of "dog-like" carnivorans. They include dogs (wolves, foxes, etc.), bears, raccoons, and mustelids. The Pinnipedia ( seals, walruses and sea lions) are also assigned to this gr ...
region of enlarged fang-like teeth in the front half of the skull, very few parareptiles possessed caniform teeth. Many amniotes have a row of small pits running along bones at the edge of the mouth, but parareptiles have only a few pits, with one especially large pit near the front of the
maxilla In vertebrates, the maxilla (: maxillae ) is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The two maxil ...
. The rest of the skull was often strongly-textured by pits, ridges, and rugosities in most parareptile groups, occasionally culminating in complex bosses or spines. The maxilla is usually low, while the prefrontal and lacrimal bones in front of the eye are both fairly large. In all parareptiles except mesosaurs, the prefrontal has a plate-like inner branch which forms a broad contact with the
palatine A palatine or palatinus (Latin; : ''palatini''; cf. derivative spellings below) is a high-level official attached to imperial or royal courts in Europe since Roman Empire, Roman times.
bone of the
palate The palate () is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but in most other tetrapods, the oral and nasal cavities are not truly sep ...
. A prominent hole, the foramen orbitonasale, is present at the intersection of the prefrontal, palatine, and lacrimal. Parareptilian palates also have toothless and reduced ectopterygoid bones, a condition taken to extremes in mesosaurs, which have lost the ectopterygoid entirely. Most parareptiles had large
orbits In celestial mechanics, an orbit (also known as orbital revolution) is the curved trajectory of an physical body, object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an satellite, artificia ...
(eye sockets), significantly longer (from front-to-back) than the region of the skull behind the eyes. The jugal bone, which forms the lower and rear edge of the orbit, has a very thin suborbital process (front branch), usually no subtemporal process (lower rear branch), and a thick dorsal process (upper rear branch). The
squamosal The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestra ...
and
quadratojugal The quadratojugal is a skull bone present in many vertebrates, including some living reptiles and amphibians. Anatomy and function In animals with a quadratojugal bone, it is typically found connected to the jugal (cheek) bone from the front and ...
bones, which lie behind the jugal, are quite large and are embayed from behind to accommodate the internal ears. Parareptiles were traditionally considered to have an " anapsid"-type skull, with the jugal, squamosal, and quadratojugal firmly sutured together without any gaps or slits between them. This principle still holds true for some subgroups, such as pareiasaurs. However, a growing number of parareptile taxa are known to have had an infratemporal fenestra, a large hole or emargination lying among the bones behind the eye. In some taxa, the margins of such openings may include additional bones such as the maxilla or postorbital. When seen from above, the rear edge of the skull is straight or has a broad median embayment. From inside to outside, the rear edge of the skull is formed by three pairs of bones: the
postparietal Postparietals are cranial bones present in fish and many Tetrapod, tetrapods. Although initially a pair of bones, many lineages possess postparietals which were fused into a single bone. The postparietals were Dermal bone, dermal bones situated alo ...
s, tabulars, and supratemporals. Parareptiles have particularly large supratemporals, which often extend further backwards than the tabulars. Apart from the long, slender jaws of mesosaurs, most parareptile jaws were short and thick. The jaw joint is formed by the
articular The articular bone is part of the lower jaw of most vertebrates, including most jawed fish, amphibians, birds and various kinds of reptiles, as well as ancestral mammals. Anatomy In most vertebrates, the articular bone is connected to two o ...
(in the lower jaw) and the quadrate (in the upper jaw). In many parareptiles, the jaw joint is shifted forwards on the skull past the rear part of the
braincase In human anatomy, the neurocranium, also known as the braincase, brainpan, brain-pan, or brainbox, is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the calv ...
. Jaw muscles attach to the coronoid process, a triangular spur in the rear half of the jaw. Both the tooth-bearing dentary bone and the posterior foramen intermandibularis (a hole on the inner surface of the jaw) reach as far back as the coronoid process. The
surangular The surangular or suprangular is a jaw bone found in most land vertebrates, except mammals. Usually in the back of the jaw, on the upper edge, it is connected to all other jaw bones: dentary, angular bone, angular, splenial and articular. It is o ...
bone, which forms the upper rear part of the jaw, is narrow and plate-like.


Postcranial skeleton

There was some variation in the body shape of parareptiles, with early members of the group having an overall
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 ...
-like appearance, with thin limbs and long tails. The most successful and diverse groups of parareptiles, the pareiasaurs and procolophonids, had massively-built bodies with reduced tails and stout limbs with short digits. This general body shape is shared with other "cotylosaurs" such as
captorhinids Captorhinidae is an extinct family of tetrapods, traditionally considered primitive reptiles, known from the late Carboniferous to the Late Permian. They had a cosmopolitan distribution across Pangea. Description Captorhinids are a clade of sm ...
, diadectomorphs, and seymouriamorphs. Another general "cotylosaurian" feature in parareptiles is the "swollen" appearance of their
vertebra Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spina ...
e, which have wide and convex upper surfaces. Parareptiles lacked a supraglenoid foramen on the
scapula The scapula (: scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on either side ...
, a hole which is also absent in varanopids and neodiapsids. Most had a fairly short and thick
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 ...
which was expanded near the elbow. Unlike early eureptiles, the outer part of the lower humerus possessed both a small supinator process and an ectepicondylar foramen and groove. The
ulna The ulna or ulnar bone (: ulnae or ulnas) is a long bone in the forearm stretching from the elbow to the wrist. It is on the same side of the forearm as the little finger, running parallel to the Radius (bone), radius, the forearm's other long ...
generally has a poorly developed
olecranon process The olecranon (, ), is a large, thick, curved bony process on the proximal, posterior end of the ulna. It forms the protruding part of the elbow and is opposite to the cubital fossa or elbow pit (trochlear notch). The olecranon serves as a lever ...
, another trait in contrast with the earliest eureptiles. Most parareptiles had an ilium which was fan-shaped and vertically (rather than horizontally) oriented, an unusual trait among early amniotes. The sacral ribs, which connect the spine to the ilium, were usually slender or fan-shaped, with large gaps between them. The hindlimbs were typically not much longer than the forelimbs, and had thick reptilian ankle bones and short toes. There are some exceptions, such as '' Eudibamus'', an early Permian bolosaurid with very elongated hindlimbs.


History of classification

The name Parareptilia was coined by Olson in 1947 to refer to an extinct group of
Paleozoic The Paleozoic ( , , ; or Palaeozoic) Era is the first of three Era (geology), geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic (the last era of the Proterozoic Eon) and ends 251.9 Ma a ...
reptiles, as opposed to the rest of the reptiles or Eureptilia ("true reptiles"). Olsen's term was generally ignored, and various taxa later known as parareptiles were generally not placed into exclusive groups with each other. Many were classified as "cotylosaurs" (a
wastebasket taxon Wastebasket taxon (also called a wastebin taxon, dustbin taxon or catch-all taxon) is a term used by some taxonomists to refer to a taxon that has the purpose of classifying organisms that do not fit anywhere else. They are typically defined by e ...
of stout-bodied "primitive" reptiles or reptile-like tetrapods) or " anapsids" (reptiles without temporal fenestrae, such as modern turtles). Parareptilia's usage was revived by
cladistic Cladistics ( ; from Ancient Greek 'branch') is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is ...
studies, to refer to those traditional "anapsids" that were thought to be unrelated to turtles. Gauthier ''et al.'' (1988) provided the first
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 ...
definitions for the names of many
amniote Amniotes are tetrapod vertebrate animals belonging to the clade Amniota, a large group that comprises the vast majority of living terrestrial animal, terrestrial and semiaquatic vertebrates. Amniotes evolution, evolved from amphibious Stem tet ...
taxa In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...
and argued that
captorhinids Captorhinidae is an extinct family of tetrapods, traditionally considered primitive reptiles, known from the late Carboniferous to the Late Permian. They had a cosmopolitan distribution across Pangea. Description Captorhinids are a clade of sm ...
and turtles were sister groups, constituting the clade Anapsida (in a much more limited context than typically applied). A name had to be found for a clade of various early-diversing
Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...
and
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 ...
reptiles no longer included in the anapsids. Olsen's term "parareptiles" was chosen to refer to this clade, although its instability within their analysis meant that Gauthier ''et al.'' (1988) were not confident enough to erect Parareptilia as a formal taxon. Their
cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...
is as follows: Laurin & Reisz (1995) found a slightly different topology, in which Reptilia is divided into Parareptilia and Eureptilia. They argued that Testudines (turtles) were members of Parareptilia; in fact, they explicitly defined Parareptilia as "Testudines and all amniotes more closely related to them than to diapsids". Captorhinidae was transferred to Eureptilia, while Parareptilia included turtles alongside many of the taxa named as such by Gauthier et al. (1988). There was one major exception: mesosaurs were placed outside both groups, as the sister taxon to the
crown group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor ...
Reptilia. Mesosaurs were still considered sauropsids, as they were closer to reptiles than to synapsids. The traditional group "Anapsida" was rejected as a
paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...
assemblage. The cladogram of Laurin & Reisz (1995) is provided below: In contrast, several studies in the mid-to-late 1990s by Olivier Rieppel and Michael deBraga argued that turtles were actually lepidosauromorph
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 earliest traditionally identified diapsids, the araeosc ...
s related to the
sauropterygia Sauropterygia ("lizard flippers") is an extinct taxon of diverse, aquatic diapsid reptiles that developed from terrestrial ancestors soon after the end-Permian extinction and flourished during the Triassic before all except for the Plesiosau ...
ns. The diapsid affinities of turtles have been supported by molecular phylogenies. The first genome-wide phylogenetic analysis was completed by Wang et al. (2013). Using the draft genomes of ''Chelonia mydas'' and ''Pelodiscus sinensis,'' the team used the largest turtle data set to date in their analysis and concluded that turtles are likely a sister group of crocodilians and birds (Archosauria). This placement within the diapsids suggests that the turtle lineage lost diapsid skull characteristics, since turtles possess an anapsid skull. This would make Parareptilia a totally extinct group with skull features that resemble those of turtles through
convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...
. With turtles positioned outside of parareptiles, Tsuji and MĂŒller (2009) redefined Parareptilia as "the most inclusive clade containing ''
Milleretta rubidgei ''Milleretta'' is an extinct genus of millerettid parareptile from the Late Permian of South Africa. Fossils have been found in the Balfour Formation.Ruta, M., Cisneros, JC., Liebrecht, T., Tsuji, L. A. and MĂŒller, J. 2011Amniotes through major b ...
'' and ''
Procolophon trigoniceps ''Procolophon'' (from , 'before' and , 'summit') is a genus of lizard-like procolophonidae, procolophonid Parareptilia, parareptiles that first appeared in the Early Triassic (Induan) of South Africa, Brazil, and Antarctica. It persisted throu ...
'', but not ''
Captorhinus aguti ''Captorhinus'' (from , 'to gulp down' and , 'nose') is an extinct genus of Captorhinidae, captorhinid reptiles that lived during the Permian period. Its remains are known from North America (Oklahoma, Texas) and possibly South America. Descr ...
''." The cladogram below follows an analysis by M.S. Lee, in 2013. The cladogram below follows the analysis of Li ''et al''. (2018). A 2020 study by David P. Ford and Roger B. J. Benson found that Parareptilia was nested within Diapsida as the sister group to Neodiapsida, with the clade containing Neodiapsida and Parareptilia dubbed Neoreptilia, which suggests that parareptiles were ancestrally diapsid. This excluded mesosaurs, which were again found to be basal among the sauropsids. Some studies have found Parareptilia to be paraphyletic, with some parareptiles more closely related to diapsids than to other parareptiles, with SimÔes et al. (2022) using Neoreptilia for the clade containing Procolophonomorpha+Neodiapsida. Cladogram after SimÔes et al. (2022):


Evolutionary history

The oldest known parareptiles are the bolosaur '' Erpetonyx'' and the acleistorhinid '' Carbonodraco'' from the Late Carboniferous ( Moscovian- Gzhelian) of North America, which represents the only known Carboniferous parareptiles, indicating that the initial diversification of the group took place in the Late Carboniferous. Numerous parareptile lineages appeared during the early Permian and the group reached a cosmopolitan distribution. Parareptile diversity declined towards the end of the Permian and procolophonoids, which first appeared during the Late Permian, were the only group of parareptiles to survive the
Permian–Triassic extinction event The Permian–Triassic extinction event (also known as the P–T extinction event, the Late Permian extinction event, the Latest Permian extinction event, the End-Permian extinction event, and colloquially as the Great Dying,) was an extinction ...
. Procolophonid diversity sharply declined beginning in the Middle Triassic, with the group becoming extinct by the end of the Triassic.


References

{{Taxonbar, from=Q1719140 Permian reptiles Triassic reptiles Pennsylvanian first appearances Rhaetian extinctions Taxa named by Everett C. Olson Polyphyletic groups