| Rhynchocephalians | |
|---|---|
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| Thetuatara (Sphenodon punctatus), the only living rhynchocephalian | |
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| Fossil ofVadasaurus, a rhynchocephalian from the Late Jurassic of Germany | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Superorder: | Lepidosauria |
| Order: | Rhynchocephalia Günther 1867 |
| Type species | |
| Sphenodon punctatus Gray, 1842 | |
| Subgroups | |
Rhynchocephalia (/ˌrɪŋkoʊsɪˈfeɪliə/;lit. 'beak-heads') is anorder of lizard-likereptiles that includes only one living species, thetuatara (Sphenodon punctatus) ofNew Zealand. Despite its current lack of diversity, during theMesozoic rhynchocephalians were a speciose group with high morphological and ecological diversity. The oldest record of the group is dated to theMiddle Triassic around 244 million years ago,[1] and they had achieved global distribution by theEarly Jurassic.[2] Most rhynchocephalians belong to thesuborderSphenodontia ('wedge-teeth'). Their closest living relatives arelizards andsnakes in the orderSquamata, with the two orders being grouped together in the superorderLepidosauria.
Rhynchocephalians are distinguished from squamates by a number of traits, including the retention of rib-likegastralia bones in the belly, as well as most rhynchocephalians havingacrodont teeth that are fused to the crests of the jaws (the latter also found among a small number of modern lizard groups likeagamids).
Once representing the world's dominant group of small reptiles, many of the niches occupied by lizards today were held by rhynchocephalians during theTriassic andJurassic. Rhynchocephalians underwent a great decline during theCretaceous, and they had disappeared almost entirely by the beginning of theCenozoic. While the modern tuatara is primarilyinsectivorous andcarnivorous, the diversity of the group also included theherbivorouseilenodontines, and there were other rhynchocephalians with specialised ecologies like thedurophagoussapheosaurs. There were even successful groups of aquatic sphenodontians, such as the elongate-bodiedpleurosaurs.[3]
Tuatara were originally classified asagamidlizards when they were first described byJohn Edward Gray in 1831. They remained misclassified until 1867, whenAlbert Günther of the British Museum noted features similar to birds, turtles, and crocodiles. He proposed the order Rhynchocephalia (fromAncient Greekῥύγχος (rhúnkhos) 'beak' andκεφαλή (kephalḗ) 'head', meaning "beak head") for the tuatara and its fossil relatives.[4] In 1925,Samuel Wendell Williston proposed the Sphenodontia to include only tuatara and their closest fossil relatives.[5]Sphenodon is derived from Ancient Greekσφήν (sphḗn) 'wedge' andὀδούς (odoús) 'tooth'.[6][7][8] 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 unrelatedrhynchosaurs, which lived in theTriassic.[5] Studies in the 1970s and 1980s demonstrated that rhynchosaurs were unrelated, with computer-basedcladistic analysis conducted in the 1980s providing a robust diagnosis for the definition of the group.[9]
Rhynchocephalia and their sister groupSquamata (which includes lizards,snakes andamphisbaenians) belong to the superorderLepidosauria, the only surviving taxon withinLepidosauromorpha.
Squamates and rhynchocephalians have a number of shared traits (synapomorphies), including fracture planes within the tail vertebrae allowingcaudal autotomy (loss of the tail when threatened), transversecloacal slits, an opening in the pelvis known as the thyroid fenestra, the presence of extraossification centres in the limb boneepiphyses, 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 fusedastralago-calcaneun and enlarged fourth distaltarsal, which creates a new joint, along with a hooked fifthmetatarsal.[10]
Like some lizards, the tuatara possesses aparietal eye (also called a pineal eye or a third eye) covered by scales at the top of the head formed by the parapineal organ, with an accompanying hole in the skull roof enclosed by theparietal bones, dubbed the "pineal foramen", which is also present in fossil rhynchocephalians. The parietal eye detects light (though it is probably not capable of detecting movement or forming images), monitoring the day-night and seasonal cycles, helping to regulate thecircadian rhythm, among other functions.[11][12][13][14][15] While parietal eyes were widespread among early vertebrates, including early reptiles, they have been lost among most living groups.[13]
Rhynchocephalians are distinguished from squamates by a number of traits, including the retention ofgastralia (rib-like bones present in the belly of the body, ancestrally present intetrapods and also present in livingcrocodilians).[16] Unlike squamates, but similar to the majority of birds, the tuatara lacks a penis. This is a secondary loss, as a penis or squamate-likehemipenes were probably present in the last common ancestor of rhynchocephalians and squamates.[17]
The complete lower temporal bar (caused by the fusion of thejugal andquadtrate/quadratojugal bones of the skull) of the tuatara, often historically asserted to be aprimitive feature retained from earlier reptiles, is actually aderived feature among sphenodontians, with primitive lepidosauromorphs and many rhynchocephalians including the most primitive ones having an open lowertemporal fenestra without a temporal bar.[18][19] While often lacking a complete temporal bar, the vast majority of rhynchocephalians have a posteriorly directedprocess (extension) of the jugal bone. All known rhynchocephalians lack thesplenial bone present in the lower jaw of more primitive reptiles,[20] with the skulls of all members of Sphenodontia lackinglacrimal bones.[21] The majority of rhynchocephalians also have fusedfrontal bones of the skull.[22][20] While early rhynchocephalians possessed atympanic 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.[23]
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The dentition of most rhynchocephalians, including the tuatara, is described asacrodont, 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 (exceptacrodontans), which havepleurodont 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 (in addition, the precise technical meaning of term "acrodont" is somewhat ambiguous and the term is used with inconsistent meanings by different researchers).[24] The acrodont dentition appears to be a derived character of rhynchocephalians not found in more primitive lepidosauromorphs.[22] The most primitive rhynchocephalians have either pleurodont teeth or a combination of both pleurodont front and acrodont posterior teeth.[24][20] Some rhynchocephalians differ from these conditions, withAnkylosphenodon having superficially acrodont teeth that continue deeply into the jaw bone, and are fused to the bone at the base of the socket (ankylothecodont).[24] In many derived sphenodontians, thepremaxillary teeth at the front of the upper jaw are merged into a large chisel-like structure.[25]
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 thepalatine,vomer andpterygoid bones, though the vomer and/or the pterygoid teeth are lost in some groups, including the living tuatara, which only has palatine teeth.[26] 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 themaxilla, in members of Sphenodontinae (including the tuatara) and Eilenodontinae it is orientated parallel to the maxilla. In these groups, during biting, the teeth of thedentary in the lower jaw slot between the maxillary and palatine tooth rows. This arrangement, which is unique among amniotes, permitsthree point bending of food items,[27] and in combination with propalinal movement (back and forward motion of the lower jaw) allows for a shearing bite.[26][28]
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.[29]Clevosaurus sectumsemper has an estimated total length of 12 centimetres (4.7 in),[30] while large individuals of the largest known terrestrial sphenodontian,Priosphenodon avelasi reached total lengths of just over 100 centimetres (39 in).[31] The aquaticpleurosaurs reached lengths of up to 150 centimetres (59 in).[32]
The tuatara has among the highest known ages of sexual maturity among reptiles,[33] at around 9 to 13 years of age,[34] and has a high longevity in comparison to lizards of similar size,[33] with wild individuals likely reaching 70 years, and possibly over 100 years in age.[35] Such a late onset of sexual maturity and longevity may have or not have been typical of extinct rhynchocephalians.[32][36]
While the grouping of Rhynchocephalia is well supported, the relationships of many taxa to each other are uncertain, varying substantially between studies.[37] In modern cladistics, the clade Sphenodontia includes all rhynchocephalians other thanWirtembergia, as well asGephyrosaurus and othergephyrosaurids. Gephyrosaurids have been found as more closely related to squamates in some analyses.[38][20] In 2018, two major clades within Sphenodontia were defined, theinfraorderEusphenodontia which is defined by the least inclusiveclade containingPolysphenodon,Clevosaurus hudsoni andSphenodon, which is supported by the presence of threesynapomorphies, including the presence of clearly visible wear facets on the teeth of the dentary or maxilla, thepremaxillary 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 cladeNeosphenodontia is defined as the most inclusive clade containingSphenodon but notClevosaurus 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, theposterior (hind) edge of theparietal bone is only slightly curved inward, theparietal foramen is found at the same level or forward of theanterior border of thesupratemporal 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 ofpterygoid tooth rows are reduced to one or none, and the posterior border of theischium is characterised by a distinctive process.[25] In 2021 the cladeAcrosphenodontia 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.[39] In 2022 the extinct cladeLeptorhynchia 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.[21] The cladeOpisthodontia has been used for the grouping of all sphenodontians more closely related toPriosphenodon (a member ofEilenodontinae) than toSphenodon.[40] Not all studies use this clade, as some studies have found the scope of the clade to be identical to Eilenodontinae.[21]
The familySphenodontidae 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.[38] The closest relatives of the tuatara are placed in the cladeSphenodontinae, which are characterised by a completely closed temporal bar.[19]
The following is acladogram of Rhynchocephalia after DeMar et al. 2022 (based onmaximum parsimony, note that cladogram collapses into apolytomy underBayesian analysis):[21]
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Cladogram after Simoes et al. 2022 (based onBayesian inference analysis), with better resolved relations of Sphenodontidae and particularly Sphenodontinae:[19]
| Sphenodontia |
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The fossil record of rhynchocephalians demonstrates that they were a diverse group that exploited a wide array of ecological niches.[4][27] Early rhynchocephalians possess small ovoid teeth designed for piercing, and were probablyinsectivores.[41] Like modern tuatara, extinct members of Sphenodontinae were likely generalists with a carnivorous/insectivorous diet.[42] Amongst the most distinct rhynchocephalians are thepleurosaurs, 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 genusPleurosaurus having an elongated triangular skull highly modified from those of other rhynchocephalians. Pleurosaurs are thought to have beenpiscivores (consuming fish).[32] Several other lineages of rhynchocephalians, such asKallimodon andLeptosaurus have been suggested to have had semi-aquatic habits,[43] with fish found as gut contents in oneKallimodon specimen.[44]
Eilenodontines are thought to have been herbivorous, with batteries of wide teeth with thickenamel used to process plant material.[45] Thesapheosaurids, such asOenosaurus andSapheosaurus from the Late Jurassic of Europe possess broad tooth plates unique amongst tetrapods, and are thought to have beendurophagous, with the tooth plates being used to crush hard shelled organisms.[46][38]Sphenovipera from the Jurassic of Mexico has been suggested to have beenvenomous, based on presence of grooves on two enlarged teeth at the front of the lower jaw[47] though this interpretation has been questioned by other authors.[47] The body ofPamizinsaurus from the Early Cretaceous of Mexico was covered inosteoscutes, similar to those ofhelodermatid lizards like theGila monster, which is unique among known sphenodontians, which probably served to protect it against predators.[48] The limb bone proportions and shape of the hand and foot bones ofSphenodraco from the Late Jurassic of Germany indicate that it was a primarilyarboreal tree climbing animal, unlike the largely terrestrial tuatara. Other extinct rhynchocephalians with relatively long limbs such asNavajosphenodon andHomoeosaurus may also have exhibited climbing capabilities.[44]
The timing of when Rhynchocephalia is estimated to havediverged from Squamata is disputed. Older estimates place the divergence between theMiddle Permian and earliest Triassic, around 270 to 252 million years ago,[38] while other authors posit a younger date of around 242 million years ago.[2] The oldest known definitive rhynchocephalian isAgriodontosaurus from theHelsby Sandstone Formation ofDevon, UK dating to the upperAnisian stage of theMiddle Triassic, approximately244 to 241.5 million years ago.[1] The next earliest rhynchocephalian isWirtembergia which is known from theErfurt Formation nearVellberg in Southern Germany, dating to theLadinian stage of theMiddle Triassic, around 238-240 million years old.[20] Rhynchocephalians underwent considerable diversification during the Late Triassic,[4] and reached a worldwide distribution acrossPangaea by the end of the Triassic, with theLate Triassic-Early Jurassic genusClevosaurus having 10 species across Asia, Africa, Europe, North and South America.[49] The earliest rhynchocephalians were small animals, but by the Late Triassic the group had evolved a wide range of body sizes.[50] During the Jurassic, rhynchocephalians were the dominant group of small reptiles globally,[51] reaching their apex of morphological diversity during this period, including specialised herbivorous and aquatic forms.[4] The only record of Rhynchocephalians from Asia (excluding theIndian subcontinent, which was not part of Asia during the Mesozoic) are indeterminate remains ofClevosaurus from the Early Jurassic (Sinemurian) agedLufeng Formation ofYunnan, China. Rhynchocephalians are noticeably absent from younger localities in the region, despite the presence of favourable preservation conditions.[52] Rhynchocephalians remained diverse into the Late Jurassic,[53] and were more abundant than lizards during the Late Jurassic in North America.[51]
Rhynchocephalian diversity declined during theEarly Cretaceous, disappearing from North America and Europe after the end of the epoch,[54] and were absent from North Africa[55] and northern South America[56] by the earlyLate 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.[57] 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.[55] Late Cretaceous South American sphenodontians are represented by both Eilenodontinae and Sphenodontidae (including Sphenodontinae).[58] An indeterminate rhynchocephalian is known from a partial lower jaw of a hatchling from the latest Cretaceous or possibly earliestPaleoceneIntertrappean Beds, in what was then the isolated landmass ofInsular India, which appears to be an acrosphenodontian, possibly related toGodavarisaurus from the Jurassic of India.[53] The youngest undoubted remains of rhynchocephalians outside of New Zealand are those of the sphenodontidKawasphenodon peligrensis from the early Paleocene (Danian) ofPatagonia approximately 64-63 million years ago, shortly after theCretaceous–Paleogene extinction event.[59] Indeterminate sphenodontine jaw fragments bearing teeth are known from the earlyMiocene (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 viaoceanic dispersal, and it is thought that they were already present in New Zealand when it separated fromAntarctica between 80 and 66 million years ago.[57]
Media related toSphenodontia at Wikimedia Commons
Data related toRhynchocephalia at Wikispecies
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