Rhynchocephalians Temporal range: Middle Triassic-Holocene ~
The tuatara (Sphenodon punctatus), the only living rhynchocephalian
Fossil of Vadasaurus, a rhynchocephalian from the Late Jurassic of Germany
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Superorder:	Lepidosauria
Order:	Rhynchocephalia Günther 1867
Type species
Sphenodon punctatus Gray, 1842
Clades and genera
†Wirtembergia  ?†Gephyrosauridae Sphenodontia Williston 1925 †Diphydontosaurus †Micromenodon †Paleollanosaurus? †Pelecymala †Rebbanasaurus †Sphenocondor Acrosphenodontia Chambi-Trowell et al., 2021 †Godavarisaurus †Planocephalosaurus †Theretairus Eusphenodontia Herrera-Flores et al. 2018 †Opisthiamimus †Brachyrhinodon †Clevosaurus †Colobops? †Lanceirosphenodon †Polysphenodon †Sigmala Neosphenodontia Herrera-Flores et al. 2018 †Lamarquesaurus †Pamizinsaurus †Tingitana †Whitakersaurus †Ankylosphenodon †Opisthodontia †Eilenodontinae Sphenodontidae †Leptorhynchia DeMar, Jones & Carrano, 2022 †Leptosaurus †Kallimodon †Homoeosaurus †Vadasaurus †Sapheosauridae †Pleurosauridae

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.

Many of the niches occupied by lizards today were held by sphenodontians during the Triassic and Jurassic, although lizard diversity began to overtake sphenodontian diversity in the Cretaceous, and they had disappeared almost entirely by the beginning of the Cenozoic. While the modern tuatara is primarily carnivorous, there were also sphenodontians with omnivorous (Opisthias), herbivorous (Eilenodontinae), and durophagous (sapheosaur) lifestyles. There were even successful groups of aquatic sphenodontians, such as pleurosaurs.^[2]

History of discovery

Tuatara were originally classified as agamid lizards when they were first described by John Edward Gray in 1831. They remained misclassified until 1867, when Albert Günther of the British Museum noted features similar to birds, turtles, and crocodiles. He proposed the order Rhynchocephalia (meaning "beak head") for the tuatara and its fossil relatives.^[3] In 1925 Samuel Wendell Williston proposed the Sphenodontia to include only tuatara and their closest fossil relatives.^[4] Sphenodon is derived from Greek σφήν sphen 'wedge' and ὀδούς odous 'tooth'.^[5][6][7] Many disparately related species were subsequently added to the Rhynchocephalia, resulting in what taxonomists call a "wastebasket taxon". These include the superficially similar (both in shape and name) but unrelated rhynchosaurs, which lived in the Triassic.^[4] These were resolved after use of computer based cladistics, which showed the core sphenodontian grouping to be monophyletic.^[8]

Classification and anatomy

Skeleton of the tuatara (Sphenodon punctatus)

Sphenodonts, 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 (synapomorphies), including fracture planes within the tail vertebrae allowing caudal autotomy (loss of the tail-tip when threatened), transverse cloacal slits, an opening in the pelvis known as the thyroid fenestra, the presence of extra ossification centres in the limb bone 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 squamates, the tuatara retains a parietal eye, which has been lost in the other groups of extant reptiles, the turtles and archosaurs.^[10] Rhynchocephalians are distinguished from squamates by a number of traits, including the retention of gastralia (rib-like bones present in the belly of the body, ancestrally present in tetrapods and also present in living crocodilians).^[11] Unlike squamates, but similar to the majority of birds, the tuatara lacks a penis. This is a secondary loss, as a penis or squamate-like hemipenes were probably present in the last common ancestor of rhynchocephalians and squamates.^[12]

The complete lower temporal bar (caused by the fusion of the jugal and quadtrate/quadratojugal bones) of the tuatara, often considered a primitive feature, is actually a derived feature among sphenodontians, with primitive lepidosauromorphs and many rhynchocephalians having an open lower temporal fenestra.^[13][14]

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.^[15] Clevosaurus sectumsemper has an estimated total length of 12 centimetres (4.7 in),^[16] while the largest known terrestrial sphenodontian, Priosphenodon avelasi has an estimated total length of just over 100 centimetres (39 in).^[17] The aquatic pleurosaurs reached lengths of up to 150 centimetres (59 in).^[18]

Skull of the tuatara in oblique view

The dentition of most rhynchocephalians, including the tuatara, is described as acrodont, which is associated with the condition of the teeth being attached to the crest of the jaw bone, lacking tooth replacement and having extensive bone growth fusing the teeth to the jaws resulting in the boundary between the teeth and bone being difficult to discern. This differs from the condition found in most lizards (except acrodontans), which have pleurodont teeth which are attached to the shelf on the inward-facing side of the jaw, and are replaced throughout life. The teeth of the tuatara have no roots, though the teeth of some other rhynchocephalians possess roots.^[19] The acrodont dentition appears to be a derived character of rhynchocephalians not found in more primitive lepidosauromorphs.^[20] The most primitive rhynchocephalians have either pleurodont teeth or a combination of both pleurodont front and acrodont posterior teeth.^[19][21] Some rhyncocephalians differ from these conditions, with Ankylosphenodon having superficially acrodont teeth that continue deeply into the jaw bone, and are fused to the bone at the base of the socket (ankylothecodont).^[19] In most sphenodontians, the premaxillary teeth at the front of the upper jaw are merged into a large chisel-like structure.^[22]

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.^[23] In most rhynchocephalians, the teeth present on the pterygoid bone are lost, but the lateral tooth row present on the palatine bones are enlarged, and orientated parallel to the teeth of the maxilla. 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,^[24] and in combination with propalinal movement (back and forward motion of the lower jaw) allows for a shearing bite.^[23][25]

Internal systematics

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

While the grouping of Rhynchocephalia is well supported, the relationships of many taxa to each other are uncertain, varying substantially between studies.^[26] In modern cladistics, the clade Sphenodontia includes all rhynchocephalians other than Wirtembergia, as well as Gephyrosaurus and other gephyrosauids. Gephyrosaurids have found to be more closely related to squamates in some analyses.^[8][21] In 2018, two major clades within Sphenodontia were defined, the infraorder Eusphenodontia which is defined by the least inclusive clade containing Polysphenodon, Clevosaurus hudsoni and Sphenodon, which is supported by the presence of three synapomorphies, including the presence of clearly visible wear facets on the teeth of the dentary or maxilla, the premaxillary teeth are merged into a chisel like structure, and the palatine teeth are reduced to a single tooth row, with the presence of an additional isolated tooth. The unranked clade Neosphenodontia is defined as the most inclusive clade containing Sphenodon but not Clevosaurus hudsoni, which is supported by the presence of six synapomorphies, including the increased relative length of the antorbital region of the skull (the part of the skull forward of the eye socket), reaching 1/4 to 1/3 of the total skull length, the posterior (hind) edge of the parietal bone is only slightly curved inward, the parietal foramen is found at the same level or forward of the anterior border of the 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.^[22] 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.^[27] In 2022 the extinct clade Leptorhynchia was defined, including a variety of aquatically adapted neosphenodontians, characteristed 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.^[28] The clade Opisthodontia has been used for the grouping of all sphenodontians more closely related to Priosphenodon (a member of Eilenodontinae) than to Sphenodon.^[29] Not all studies use this clade, as some studies have found the scope of the clade to be identical to Eilenodontinae.^[28]

The family Sphenodontidae has been used to include the tuatara and its closest relatives within Rhynchocephalia. However the grouping has lacked a formal definition, with the included taxa varying substantially between analyses.^[8] The closest relatives of the tuatara are placed in the clade Sphenodontinae, which are characterised by a completely closed temporal bar.^[14]

The following is a cladogram of Rhynchocephalia after DeMar et al. 2022.^[28]

Younginia capensis Prolacerta broomi Lepidosauria Pan-Squamata Sophineta cracoviensis Pristidactylus Eichstaettisaurus schroederi Megachirella wachtleri Marmoretta oxoniensis Rhynchocephalia Gephyrosaurus bridensis Sphenodontia Diphydontosaurus avonis Acrosphenodontia Planocephalosaurus robinsonae Rebbanasaurus jaini Godavarisaurus lateefi Theretairus antiquus Eusphenodontia Polysphenodon mulleri Opisthiamimus gregori Clevosauridae Clevosaurus convallis Clevosaurus brasiliensis Clevosaurus hadroprodon Clevosaurus bairdi Clevosaurus hudsoni Clevosaurus cambrica Neosphenodontia Brachyrhinodon taylori Colobops noviportensis Sphenodontidae Sphenodon punctatus (tuatara) Cynosphenodon huizachalensis Sphenovipera jimmysjoyi Kawasphenodon expectatus Kawasphenodon peligrensis Pelecymala robustus Eilenodontinae Fraserosphenodon latidens Opisthias rarus Eilenodon robustus Sphenotitan leyesi Toxolophosaurus cloudi Priosphenodon avelasi Leptorhynchia Homoeosaurus maximiliani Kallimodon pulchellus Sigmala sigmala Vadasaurus herzogi Pleurosauridae Palaeopleurosaurus posidonae Pleurosaurus goldfussi Pleurosaurus ginsburgi Kallimodon cerinensis Sapheosauridae Sapheosaurus thiollierei Ankylosphenodon pachyostosus Oenosaurus muehlheimensis

Cladogram after Simoes et al: 2022:^[14]

Sphenodontia	†Diphydontosaurus Eusphenodontia †Planocephalosaurus †Clevosaurus Neosphenodontia †Homeosaurus †Pleurosauridae †Leptosaurus +†Kallimodon †Sapheosauridae Sphenodontidae †Eilenodontinae Sphenodontinae †Navajosphenodon †Cynosphenodon †Sphenofontis †Kawasphenodon Sphenodon (tuatara)

Gallery of rhynchocephalian skull diversity

• 
  Skull reconstruction of Gephyrosaurus a possible basal rhyncocephalian
• 
  Reconstruction of the skull of Diphydontosaurus a basal member of Sphenodontia
• 
  Reconstruction of the skulls of Clevosaurus hudsoni (A) and Clevosaurus cambrica (B)
• 
  Skull of the basal eusphenodontian Opisthiamimus
• 
  Skull of Sphenotitan, a neosphenodontian and possible member of Eilenodontinae
• 
  Reconstruction of the skull of Navajosphenodon, an early member of Sphenodontinae
• 
  Reconstruction of the skull of the eilenodontine Priosphenodon
• 
  Skull of the neosphenodontian Vadasaurus

Paleobiology

Skeleton of Pleurosaurus, an aquatically adapted sphenodontian from the Late Jurassic of Germany

The fossil record of rhynchocephalians demonstrates that they were a diverse group that exploited a wide array of ecological niches.^[3][24] Early rhynchocephalians possess small ovoid teeth designed for piercing, and were probably insectivores.^[30] Like modern tuatara, extinct members of Sphenodontinae were likely generalists with a carnivorous/insectivorous diet.^[31] Amongst the most distinct rhynchocephalians are the pleurosaurs, known from the Jurassic of Europe, which were adapted for marine life, with elongated snake-like bodies with reduced limbs, with the specialised Late Jurassic genus Pleurosaurus having an elongated triangular skull highly modified from those of other rhynchocephalians. Pleurosaurs are thought to have been piscivores (consuming fish).^[18] Several other lineages of rhyncocephalians have been suggested to have had semi-aquatic habits.^[32] Eilenodontines are thought to have been herbivorous, with batteries of wide teeth with thick enamel used to process plant material.^[33] The sapheosaurids, such as Oenosaurus and Sapheosaurus from the Late Jurassic of Europe possess broad tooth plates unique amongst tetrapods, and are thought to have been durophagous, with the tooth plates being used to crush hard shelled organisms.^[34][8]

Evolutionary history

Homeosaurus maximiliani from the Late Jurassic of Germany

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,^[8] 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.^[21] Rhynchocephalians underwent considerable diversification during the 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.^[35] The earliest rhynchocephalians were small animals, but by the Late Triassic the group had evolved a wide range of body sizes.^[36] During the Jurassic rhynchocephalians reached their apex of morphological diversity, including specialised herbivorous and aquatic forms.^[3] The only record of Rhynchocephalians from Asia 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.^[37]

Rhynchocephalians disappeared from North America and Europe after the Early Cretaceous,^[38] and were absent from North Africa^[39] and northern South America^[40] 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.^[41] 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.^[39] Late Cretaceous South American sphenodontians are represented by both Eliendontinae and Sphenodontidae (including Sphenodontinae), with the youngest records of Eliendontinae dating to the Turonian.^[42] An indeterminate rhynchocephalian is known from the latest Cretaceous or possibly earliest Paleocene of Insular India, which appears to be a basal acrosphendontian.^[43] 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.^[44] 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.^[41]

References

Further reading

External links

• Media related to Sphenodontia at Wikimedia Commons
• Data related to Rhynchocephalia at Wikispecies