Parareptilia
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Parareptilia ("at the side of reptiles") is a subclass or clade of
basal Basal or basilar is a term meaning ''base'', ''bottom'', or ''minimum''. Science * Basal (anatomy), an anatomical term of location for features associated with the base of an organism or structure * Basal (medicine), a minimal level that is nec ...
sauropsids ( reptiles), typically 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 and achieved their highest diversity during the Permian period. 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, roughly 299 to 270 million years ago. Mesosaurs were the first known aquatic reptiles, having apparently returned to an aquatic lif ...
s), the first bipedal reptiles (
bolosaurids Bolosauridae is an extinct family of ankyramorph parareptiles known from the latest Carboniferous (Gzhelian) or earliest Permian (Asselian) to the early Guadalupian epoch (latest Roadian stage) of North America, China, Germany, Russia and France. ...
such as ''
Eudibamus ''Eudibamus'' is an extinct genus of biped bolosaurid ankyramorph parareptile known from the Free State of Thuringia of central Germany. It had a very small size reaching only 25-26 cm in length. Discovery ''Eudibamus'' is known only fr ...
''), the first reptiles with advanced hearing systems ( nycteroleterids and others), and the first large herbivorous reptiles (the pareiasaurs). The only parareptiles to survive into the Triassic period 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 The supratemporal bone is a paired cranial bone present in many tetrapods and tetrapodomorph fish. It is part of the temporal region (the portion of the skull roof behind the eyes), usually lying medial (inwards) relative to the squamosal and latera ...
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 The maxilla (plural: ''maxillae'' ) in vertebrates 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 ...
, a broad prefrontal-
palatine A palatine or palatinus (in Latin; plural ''palatini''; cf. derivative spellings below) is a high-level official attached to imperial or royal courts in Europe since Roman times.
contact, and the absence of a supraglenoid foramen of the
scapula The scapula (plural 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 eithe ...
. 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 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 ...
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 sauropterygians or archosauromorphs, rather than parareptiles.


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, roughly 299 to 270 million years ago. Mesosaurs were the first known aquatic reptiles, having apparently returned to an aquatic lif ...
s with elongated snouts filled with hundreds of thin teeth, to the snub-nosed, knob-encrusted skulls of pareiasaurs. Parareptile teeth were quite variable in shape and function between different species. However, they were relatively homogenous on the same skull. While most
synapsid Synapsids + (, 'arch') > () "having a fused arch"; synonymous with ''theropsids'' (Greek, "beast-face") are one of the two major groups of animals that evolved from basal amniotes, the other being the sauropsids, the group that includes rep ...
s and many early eureptiles had a caniform 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 The maxilla (plural: ''maxillae'' ) in vertebrates 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 ...
. 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 (in Latin; plural ''palatini''; cf. derivative spellings below) is a high-level official attached to imperial or royal courts in Europe since 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 s ...
. 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 is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a ...
(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 and quadratojugal 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 now 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 postparietals, 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 (in the lower jaw) and the
quadrate Quadrate may refer to: * Quadrate bone * Quadrate (heraldry) * Quadrate lobe of liver * Quadrate tubercle The quadrate tubercle is a small tubercle found upon the upper part of the femur. It serves as a point of insertion of the quadratus femori ...
(in the upper jaw). In many parareptiles, the jaw joint is shifted forwards on the skull past the rear part of the braincase. Jaw muscles attach to the
coronoid process The Coronoid process (from Greek , "like a crown") can refer to: * The coronoid process of the mandible, part of the ramus mandibulae of the mandible * The coronoid process of the ulna The coronoid process of the ulna is a triangular process proj ...
, 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 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 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 al ...
-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, diadectomorphs, and seymouriamorphs. Another general ‘cotylosaurian’ feature in parareptiles is the ‘swollen’ appearance of their
vertebra The spinal column, a defining synapomorphy shared by nearly all vertebrates, Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characterist ...
e, which have wide and convex upper surfaces. Parareptiles lacked a supraglenoid foramen on the
scapula The scapula (plural 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 eithe ...
, a hole which is also absent in varanopids and neodiapsids. Most had a fairly short and thick
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 roun ...
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 (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...
generally has a poorly developed olecranon process, another trait in contrast with the earliest eureptiles. Most parareptiles had an
ilium Ilium or Ileum may refer to: Places and jurisdictions * Ilion (Asia Minor), former name of Troy * Ilium (Epirus), an ancient city in Epirus, Greece * Ilium, ancient name of Cestria (Epirus), an ancient city in Epirus, Greece * Ilium Building, a ...
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 ''Eudibamus'' is an extinct genus of biped bolosaurid ankyramorph parareptile known from the Free State of Thuringia of central Germany. It had a very small size reaching only 25-26 cm in length. Discovery ''Eudibamus'' is known only fr ...
'', an early Permian bolosaurid with very elongated hindlimbs.


History of classification

The name Parareptilia was coined by
Olson Olson may refer to: * Olson (surname), people with the name ''Olson'' * Sigurd Olson Environmental Institute * Olson (constructor), a former racing car constructor * Olson database, also known as ''zoneinfo database'' * "Olson", a song by Boards o ...
in 1947 to refer to an extinct group of
Paleozoic The Paleozoic (or Palaeozoic) Era is the earliest of three geologic eras of the Phanerozoic Eon. The name ''Paleozoic'' ( ;) was coined by the British geologist Adam Sedgwick in 1838 by combining the Greek words ''palaiós'' (, "old") and ' ...
reptiles, as opposed to the rest of the reptiles or Eureptilia ("true reptiles"). Olsen's term was generally ignored, and various taxa now known as parareptiles were generally not placed into exclusive groups with each other. Many were classified as 'cotylosaurs' (a wastebasket taxon 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 (; ) 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 typically shared derived ch ...
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 (; 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 o ...
definitions for the names of many amniote
taxa In biology, a taxon ( back-formation from '' taxonomy''; plural 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 n ...
and argued that captorhinids 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 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 251.9 Mya. It is the last period of the Pale ...
and
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 period ...
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 ''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 ...
is as follows:
Laurin Laurin is both a surname and a given name. Notable people with the name include: Surname * Anna-Lena Laurin (born 1962), Swedish composer * Camille Laurin (1922–1999), psychiatrist and politician in Quebec, Canada * Dan Laurin (born 1960), Swedi ...
& 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 Reptilia. Mesosaurs were still considered sauropsids, as they were closer to reptiles than to synapsids. The traditional group 'Anapsida' was rejected as a
paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...
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 group first appeared about three hundred million years ...
s related to the sauropterygians. 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 com ...
. With turtles positioned outside of parareptiles, Tsuji and Müller (2009) redefined Parareptilia as "the most inclusive clade containing '' Milleretta rubidgei'' and '' Procolophon trigoniceps'', but not '' Captorhinus aguti''." 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 deeply nested within Diapsida as the sister group to Neodiapsida, which suggests that parareptiles were ancestrally diapsid. But this excludes mesosaurs, who were again found to be basal among the sauropsids.


Evolutionary history

The oldest known parareptiles are the bolosaur '' Erpetonyx'' and the acleistorhinid '' Carbonodraco'' from the Late Carboniferous (
Moscovian Moscovian may refer to: *An inhabitant of Moscow, the capital of Russia *Something of, from, or related to Moscow *Moscovian (Carboniferous) The Moscovian is in the International Commission on Stratigraphy, ICS geologic timescale a stage (strati ...
- Gzhelian) of North America, which represents the only currently 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 (P–T, P–Tr) extinction event, also known as the Latest Permian extinction event, the End-Permian Extinction and colloquially as the Great Dying, formed the boundary between the Permian and Triassic geologic periods, a ...
. Procolophonid diversity sharply declined beginning in the Middle Triassic, with the group becoming extinct by the end of the Triassic.


References

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