Rhynchocephalia

Source: Wikipedia, the free encyclopedia.
Not to be confused with Rhynchosaur.

Rhynchocephalians
Temporal range:
Ma
The tuatara (Sphenodon punctatus), the only living rhynchocephalian
Fossil of Vadasaurus, a rhynchocephalian from the Late Jurassic of Germany
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Superorder: Lepidosauria
Order: Rhynchocephalia
Günther 1867
Type species
Sphenodon punctatus
, 1842
Subgroups

See text

Rhynchocephalia (/ˌrɪŋkoʊsɪˈfeɪliə/; lit. 'beak-heads') is an order of lizard-like reptiles that includes only one living species, the tuatara (Sphenodon punctatus) of New Zealand. Despite its current lack of diversity, during the Mesozoic rhynchocephalians were a speciose group with high morphological and ecological diversity. The oldest record of the group is dated to the Middle Triassic around 238 to 240 million years ago, and they had achieved a worldwide distribution by the Early Jurassic.[1] Most rhynchocephalians belong to the group Sphenodontia ('wedge-teeth'). Their closest living relatives are lizards and snakes in the order Squamata, with the two orders being grouped together in the superorder Lepidosauria.

Once representing the worlds dominant group of small reptiles, many of the niches occupied by lizards today were held by rhynchocephalians during the

sapheosaurs. There were even successful groups of aquatic sphenodontians, such as the pleurosaurs.[2]

History

cladistic analysis conducted in the 1980s providing a robust diagnosis for the definition of the group.[8]

Anatomy

Skeleton of the tuatara (Sphenodon punctatus)

Rhynchocephalia and their sister group Squamata (which includes lizards, snakes and amphisbaenians) belong to the superorder Lepidosauria, the only surviving taxon within Lepidosauromorpha.

Squamates and rhynchocephalians have a number of shared traits (

epiphyses, a knee joint where a lateral recess on the femur allows the articulation of the fibula, the development of a sexual segment of the kidney, and a number of traits of the feet bones, including a fused astralago-calcaneun and enlarged fourth distal tarsal, which creates a new joint, along with a hooked fifth metatarsal.[9]

Like some lizards, the tuatara possesses a

parietal bones, dubbed the "pineal foramen", which is also present in fossil rhynchocephalians. The parietal eye detects light monitoring the day-night and seasonal cycles, helping to regulate the circadian rhythm, among other functions.[10][11][12][13] While pineal eyes were widespread among early vertebrates, including early reptiles, they have been lost among most living groups.[12]

Rhynchocephalians are distinguished from squamates by a number of traits, including the retention of

hemipenes were probably present in the last common ancestor of rhynchocephalians and squamates.[15]

Skull of the basal rhynchocephalian Planocephalosaurus, which has an open lower temporal fenestra

The complete lower temporal bar (caused by the fusion of the

tympanic membrane in the ear and a corresponding quadrate conch, similar to those found in lizards, these have been lost in the tuatara and likely other derived rhynchocephalians. This loss may be connected to the development of back and forth motion of the lower jaw.[21]

Skull of the tuatara in oblique view

The dentition of most rhynchocephalians, including the tuatara, is described as

premaxillary teeth at the front of the upper jaw are merged into a large chisel-like structure.[23]

Rhynchocephalians possess palatal dentition (teeth present on the bones of the roof of the mouth). Palatal teeth are ancestrally present in tetrapods, but have been lost in many groups. The earliest rhynchocephalians had teeth present on the

vomer and pterygoid bones, though the vomer and/or the pterygoid teeth are lost in some groups, including the living tuatara, which only has palatine teeth.[24] A distinctive character found in all rhynchocephalians is the enlargement of the tooth row present on the palatine bones. While in other rhynchocephalians the palatine tooth row is oblique to the teeth of the maxilla, in members of Sphenodontinae (including the tuatara) and Eilenodontinae it is orientated parallel to the maxilla. In these groups, during biting, the teeth of the dentary in the lower jaw slot between the maxillary and palatine tooth rows. This arrangement, which is unique among amniotes, permits three point bending of food items,[25] and in combination with propalinal movement (back and forward motion of the lower jaw) allows for a shearing bite.[24][26]

Skeleton of the herbivorous elienodontine Priosphenodon avelasi
one of the largest known sphenodontians

The body size of rhynchocephalians is highly variable. The tuatara has an average total length of 34.8 and 42.7 centimetres (13.7 and 16.8 in) for females and males respectively.[27] Clevosaurus sectumsemper has an estimated total length of 12 centimetres (4.7 in),[28] while the largest known terrestrial sphenodontian, Priosphenodon avelasi has an estimated total length of just over 100 centimetres (39 in).[29] The aquatic pleurosaurs reached lengths of up to 150 centimetres (59 in).[30]

The tuatara has among the highest known ages of sexual maturity among reptiles,[31] at around 9 to 13 years of age,[32] and has a high longevity in comparison to lizards of similar size,[31] with wild individuals likely reaching 70 years, and possibly over 100 years in age.[33] Such a late onset of sexual maturity and longevity may have not have been typical of extinct rhynchocephalians.[30][34]

Classification

Homeosaurus maximiliani from the Late Jurassic of Germany

While the grouping of Rhynchocephalia is well supported, the relationships of many taxa to each other are uncertain, varying substantially between studies.

supratemporal fenestra (an opening of the skull), the palatine teeth are further reduced from the condition in eusphenodontians to a single lateral tooth row, the number of pterygoid tooth rows are reduced to one or none, and the posterior border of the ischium is characterised by a distinctive process.[23] In 2021 the clade Acrosphenodontia was defined, which is less inclusive than Sphenodontia and more inclusive than Eusphenodontia, and includes all sphenodontians with fully acrodont dentition, excluding basal partially acrodont sphenodontians.[37] In 2022 the extinct clade Leptorhynchia was defined, including a variety of neosphenodontians, at least some of which were aquatically adapted, characterised by the elongation of the fourth metacarpal, the presence of a posterior process on the ischium, and the antorbital region of the skulls is between a third and a quarter of the total skull length.[19] The clade Opisthodontia has been used for the grouping of all sphenodontians more closely related to Priosphenodon (a member of Eilenodontinae) than to Sphenodon.[38] Not all studies use this clade, as some studies have found the scope of the clade to be identical to Eilenodontinae.[19]

The family

Sphenodontinae, which are characterised by a completely closed temporal bar.[17]

The following is a

Bayesian analysis):[19]

Rhynchocephalia

Gephyrosaurus bridensis

Sphenodontia

Diphydontosaurus avonis

Acrosphenodontia

Planocephalosaurus robinsonae

Rebbanasaurus jaini

Eusphenodontia

Polysphenodon mulleri

Opisthiamimus gregori

Clevosauridae
Neosphenodontia

Cladogram after Simoes et al. 2022 (based on Bayesian inference analysis):[17]

Sphenodontia

Clades and genera

Gallery of rhynchocephalian skull diversity

Ecology

venomous, based on presence of grooves on two enlarged teeth at the front of the lower jaw[44] though this interpretation has been questioned by other authors.[44] The body of Pamizinsaurus from the Early Cretaceous of Mexico was covered in osteoscutes, similar to those of helodermatid lizards like the Gila monster, which is unique among known sphenodontians, which probably served to protect it against predators.[45]

Evolutionary history

Skulls of Clevosaurus hudsoni (left) and Clevosaurus cambrica (right)

The timing of when Rhynchocephalia is estimated to have diverged from Squamata is disputed. Older estimates place the divergence between the Middle Permian and earliest Triassic, around 270 to 252 million years ago,[36] while other authors posit a younger date of around 242 million years ago.[1] The oldest known remains of rhynchocephalians are those of Wirtembergia known from the Erfurt Formation near Vellberg in Southern Germany, dating to the Ladinian stage of the Middle Triassic, around 238-240 million years old, which is also the most primitive rhynchocephalian known.[18] Rhynchocephalians underwent considerable diversification during the Late Triassic,[3] and reached a worldwide distribution across Pangaea by the end of the Triassic, with the Late Triassic-Early Jurassic genus Clevosaurus having 10 species across Asia, Africa, Europe, North and South America.[46] The earliest rhynchocephalians were small animals, but by the Late Triassic the group had evolved a wide range of body sizes.[47] During the Jurassic, rhynchocephalians were the dominant group of small reptiles globally,[48] reaching their apex of morphological diversity during this period, including specialised herbivorous and aquatic forms.[3] The only record of Rhynchocephalians from Asia (excluding the Indian subcontinent, which was not part of Asia during the Mesozoic) are indeterminate remains of Clevosaurus from the Early Jurassic (Sinemurian) aged Lufeng Formation of Yunnan, China. Rhynchocephalians are noticeably absent from younger localities in the region, despite the presence of favourable preservation conditions.[49] Rhynchocephalians remained diverse into the Late Jurassic,[50] and were more abundant than lizards during the Late Jurassic in North America.[48]

Rhynchocephalian diversity declined the Early Cretaceous, disappearing from North America and Europe after the end of the epoch,[51] and were absent from North Africa[52] and northern South America[53] by the early Late Cretaceous. The cause of the decline of Rhynchocephalia remains unclear, but has often been suggested to be due to competition with advanced lizards and mammals.[54] They appear to have remained prevalent in southern South America during the Late Cretaceous, where lizards remained rare, with their remains outnumbering terrestrial lizards in this region by a factor of 200.[52] Late Cretaceous South American sphenodontians are represented by both Eilenodontinae and Sphenodontidae (including Sphenodontinae).[55] An indeterminate rhynchocephalian is known from a partial lower jaw of a hatchling from the latest Cretaceous or possibly earliest Paleocene Intertrappean Beds, in what was then the isolated landmass of Insular India, which appears to be an acrosphenodontian, possibly related to Godavarisaurus from the Jurassic of India.[50] The youngest undoubted remains of rhynchocephalians outside of New Zealand are those of the sphenodontid Kawasphenodon peligrensis from the early Paleocene (Danian) of Patagonia, shortly after the Cretaceous–Paleogene extinction event.[56] Indeterminate sphenodontine jaw fragments bearing teeth are known from the early Miocene (19–16 million years ago) St Bathans fauna, New Zealand, that are indistinguishable from those of the living tuatara. It is unlikely that the ancestors of the tuatara arrived in New Zealand via oceanic dispersal, and it is thought that they were already present in New Zealand when it separated from Antarctica between 80 and 66 million years ago.[54]

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Further reading

External links
Retrieved from "https://en.wikipedia.org/w/index.php?title=Rhynchocephalia&oldid=1217439764"