Drepanosaurs (members of the

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

Drepanosauromorpha) are a group of

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...

s that lived between the Carnian and

Rhaetian The Rhaetian is the latest age of the Triassic Period (in geochronology) or the uppermost stage of the Triassic System (in chronostratigraphy). It was preceded by the Norian and succeeded by the Hettangian (the lowermost stage or earliest age ...

stages of the late

Triassic Period 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 ...

, approximately between 230 and 210 million years ago. The various species of drepanosaurid were characterized by specialized grasping limbs and often

prehensile Prehensility is the quality of an appendage or organ (anatomy), organ that has Adaptation (biology), adapted for grasping or holding. The word is derived from the Latin term ''prehendere'', meaning "to grasp". The ability to grasp is likely der ...

tails, adaptions for arboreal (tree-dwelling) and fossorial (digging) lifestyles, with some having also been suggested to be aquatic.

Fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

s of drepanosaurs have been found in

Arizona Arizona ( ; nv, Hoozdo Hahoodzo ; ood, Alĭ ṣonak ) is a state in the Southwestern United States. It is the 6th largest and the 14th most populous of the 50 states. Its capital and largest city is Phoenix. Arizona is part of the Fou ...

New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano) , seat = Santa Fe , LargestCity = Albuquerque , LargestMetro = Tiguex , OfficialLang = None , Languages = English, Spanish ( New Mexican), Navajo, Ker ...

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England England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe b ...

, and northern

Italy Italy ( it, Italia ), officially the Italian Republic, ) or the Republic of Italy, is a country in Southern Europe. It is located in the middle of the Mediterranean Sea, and its territory largely coincides with the homonymous geographical re ...

. The name is taken from the family's namesake

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

Drepanosaurus ''Drepanosaurus'' (; "sickle lizard") is a genus of arboreal (tree-dwelling) reptile that lived during the Triassic Period. It is a member of the Drepanosauridae, a group of diapsid reptiles known for their prehensile tails. ''Drepanosaurus'' wa ...

'', which means "sickle lizard," a reference to their strongly curved claws.

Description

Drepanosaurs are notable for their distinctive, triangular skulls, which resemble the skulls of

bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...

s. Some drepanosaurs, such as '' Avicranium,'' had pointed, toothless, bird-like

beak The beak, bill, or rostrum is an external anatomical structure found mostly in birds, but also in turtles, non-avian dinosaurs and a few mammals. A beak is used for eating, preening, manipulating objects, killing prey, fighting, probing for food ...

s. This similarity to birds may have led to the misattribution of what may be a drepanosaur skull to the would-be "first bird," ''

Protoavis ''Protoavis'' (meaning "first bird") is a problematic taxon known from fragmentary remains from Late Triassic Norian stage deposits near Post, Texas. Much controversy remains over the animal, and there are many different interpretations of what ...

''.

Drepanosaurs featured a suite of bizarre, almost

chameleon Chameleons or chamaeleons (family Chamaeleonidae) are a distinctive and highly specialized clade of Old World lizards with 202 species described as of June 2015. The members of this family are best known for their distinct range of colors, bein ...

-like skeletal features. Above the shoulders of most species was a specialized "hump" formed from fusion of the vertebrae, possibly used for advanced muscle attachments to the neck, and allowing for quick forward-striking movement of the head (perhaps to catch insects). Many had derived hands with two fingers opposed to the remaining three, an adaptation for grasping branches. Some individuals of '' Megalancosaurus'' (possibly exclusive to either males or females) had a primate-like opposable toe on each foot, perhaps used by one sex for extra grip during mating. Most species had broad, prehensile tails, sometimes tipped with a large "claw," again to aid in climbing. These tails, tall and flat like those of newts and

crocodile Crocodiles (family (biology), family Crocodylidae) or true crocodiles are large semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term crocodile is sometimes used even more loosely to inclu ...

s, have led some researches to conclude that they were aquatic rather than arboreal. In 2004, Senter dismissed this idea, while Colbert and Olsen, in their description of ''Hypuronector'', state that while other drepanosaurs were probably arboreal, ''Hypuronector'' was uniquely adapted to aquatic life. The tail of this genus was extremely deep and non-prehensile: much more fin-like than members of the more exclusive group Drepanosauridae. Aerial locomotion has been attributed to at least two drepanosaur genera: '' Megalancosaurus'' and '' Hypuronector''. The first was originally suggested by Ruben et al. 1998 on the basis of bird-like characters and limb proportions. While the suggestion has not been ruled out entirely, it has since been largely dismissed, due to ''Megalancosaurus''' clunky, chameleon-like anatomy. ''Hypuronector'', however, is much more likely to be a glider or flyer due to the elongated forelimbs.

Fossorial A fossorial () animal is one adapted to digging which lives primarily, but not solely, underground. Some examples are badgers, naked mole-rats, clams, meerkats, and mole salamanders, as well as many beetles, wasps, and bees. Prehistoric eviden ...

or digging-related adaptations have been recognized in three drepanosaur genera: ''

Skybalonyx ''Skybalonyx'' is an extinct genus of drepanosaur from the Chinle Formation in the Late Triassic. The type and only known species is ''Skybalonyx skapter'', meaning "digging dung claw" from Ancient Greek. ''Skybalonyx'' is only known from numero ...

'', '' Ancistronychus'', and ''

.'' In particular, ''Drepanosaurus'' may have been adapted to hook-and-pull digging, similar to modern-day

anteaters Anteater is a common name for the four extant mammal species of the suborder Vermilingua (meaning "worm tongue") commonly known for eating ants and termites. The individual species have other names in English and other languages. Together with ...

. ''Skybalonyx'' possessed claws similar in shape to modern-day

moles Moles can refer to: *Moles de Xert, a mountain range in the Baix Maestrat comarca, Valencian Community, Spain *The Moles (Australian band) *The Moles, alter ego of Scottish band Simon Dupree and the Big Sound People *Abraham Moles, French engineer ...

and echidna, both of which are humeral-rotation diggers. Vallesaurus cenensis

The

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 ...

position of drepanosaurs is highly disputed. Various studies have proposed that drepanosaurs are protorosaurian

archosauromorphs Archosauromorpha ( Greek for "ruling lizard forms") is a clade of diapsid reptiles containing all reptiles more closely related to archosaurs (such as crocodilians and dinosaurs, including birds) rather than lepidosaurs (such as tuataras, lizard ...

lepidosauromorphs Lepidosauromorpha (in PhyloCode known as ''Pan-Lepidosauria'') is a group of reptiles comprising all diapsids closer to lizards than to archosaurs (which include crocodiles and birds). The only living sub-group is the Lepidosauria, which contains ...

related to kuehneosaurids, non-saurian

diapsids 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 ago ...

related to weigeltisaurids, or (most recently) basal neodiapsids.

Early studies

When ''Drepanosaurus'' and ''

Dolabrosaurus ''Dolabrosaurus'' is a genus of extinct reptile and a member of the family Drepanosauridae. Fossils of ''Dolabrosaurus'' have been found in the Chinle Formation of New Mexico.lepidosauria The Lepidosauria (, from Greek meaning ''scaled lizards'') is a subclass or superorder of reptiles, containing the orders Squamata and Rhynchocephalia. Squamata includes snakes, lizards, and amphisbaenians. Squamata contains over 9,000 species, m ...

ns, ancestral to modern lizards. ''Megalancosaurus'' was first believed to be a thecodont (i.e. an

archosauriform Archosauriformes (Greek for 'ruling lizards', and Latin for 'form') is a clade of diapsid reptiles that developed from archosauromorph ancestors some time in the Latest Permian (roughly 252 million years ago). It was defined by Jacques Gauthier ...

) upon its discovery in 1980, but later studies placed it as a

prolacertiform Protorosauria is an extinct polyphyletic group of archosauromorph reptiles from the latest Middle Permian (Capitanian stage) to the end of the Late Triassic (Rhaetian stage) of Asia, Europe and North America. It was named by the English anatomis ...

, and perhaps even an ancestor to birds, although this latter hypothesis has not been supported by subsequent studies.

Drepanosaurs outside Neodiapsida: Avicephala and Simiosauria

A 2004 study by Senter placed drepanosaurs with the coelurosauravids ( weigeltisaurids) and ''

Longisquama ''Longisquama'' is a genus of extinct reptile. There is only one species, ''Longisquama insignis'', known from a poorly preserved skeleton and several incomplete fossil impressions from the Middle to Late Triassic Madygen Formation in Kyrgyzst ...

'' in a

called which he called Avicephala. Senter's analysis placed Avicephala within Diapsida but outside Neodiapsida, defined by Senter as the clade containing "all taxa phylogenetically bracketed by Younginiformes and living diapsids." Within Avicephala, Senter named the group Simiosauria ("monkey lizards") for the extremely derived tree-dwelling forms. Simiosauria was defined as "all taxa more closely related to Drepanosauridae than to '' Coelurosauravus'' or Sauria." However, Renesto and colleagues (see below) found drepanosaurids to lie within Sauria, which would make the clade Simiosauria obsolete. Senter found that '' Hypuronector'', originally described as a drepanosaurid, actually lies just outside that clade, along with the primitive drepanosaur ''

Vallesaurus ''Vallesaurus'' is an extinct genus of Late Triassic elyurosaur drepanosauromorph. First found in Northern Italy in 1975, it is one of the most primitive drepanosaurs. ''V. cenenis'' is the type species, which was first mentioned in 1991 but on ...

''. He also recovered a close relationship between the drepanosaurids ''

Dolabrosaurus ''Dolabrosaurus'' is a genus of extinct reptile and a member of the family Drepanosauridae. Fossils of ''Dolabrosaurus'' have been found in the Chinle Formation of New Mexico.Megalancosaurus''. The following cladogram was proposed by Senter in his 2004 analysis: A clade containing drepanosaurids, ''Longisquama'', and ''Coelurosauravus'' (as well as ''

Wapitisaurus Weigeltisauridae is a family of gliding neodiapsid reptiles that lived during the Late Permian, between 258 and 252 million years ago. Fossils of weigeltisaurids have been found in Madagascar, Germany, Great Britain, and Russia. A possible weigelt ...

'') was also recovered in a 2003 analysis conducted by John Merck; however, in Merck's analysis this clade was nested within Neodiapsida as 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 t ...

of Sauria. Drepanosaurus BW

In a 2006 study, Renesto and Binelli found that when

pterosaur Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 to ...

'' Eudimorphodon'' was added to Senter's original matrix, it was found to be a member of Avicephala. The authors also conducted a second analysis, this time based on a character set and matrix updated by scoring additional characters previously reported as unknown and by adding a few relevant characters. This analysis recovered drepanosaurids as the sister taxon of ''Eudimorphodon''; the clade containing pterosaurs and drepanosaurids was recovered as the sister taxon of Archosauriformes. ''Longisquama'' and ''Coelurosauravus'' were not found to be closely related to drepanosaurids, but instead were recovered as non-neodiapsid diapsids as in Senter's analysis. However, it is feasible that this arrangement might be a result of poor knowledge of ''Longisquama'' rather than a reflection of its true phylogenetic position. The authors did note that there are similarities in the structure of the forelimb and shoulder regions of ''Longisquama'' and all or some drepanosaurids (e.g. the

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 ...

of both ''Longisquama'' and ''Vallesaurus'' is as long as the fourth digit of the manus). They stressed that they could not rule out the possibility that at least some of the similarities are convergent due to a similar behaviors, and that they did not examine ''Longisquama'' firsthand. Therefore, further studies of drepanosaurids should take the hypothesis that ''Longisquama'' might be a drepanosaurid into consideration.

Drepanosaurs as relatives of kuehneosaurids

Drepanosaurids were also found to be non-saurian neodiapsids in a 2004 analysis conducted by Johannes Müller; however, in this analysis Drepanosauridae were not found to be closely related to '' Coelurosauravus'', but rather were recovered as the sister taxon of Kuehneosauridae. In a 2009 study,

Susan E. Evans Susan E. Evans is British palaeontologist and herpetologist. She is the author or co-author of over 100 peer-reviewed papers and book chapters. She received a BSc in Zoology at Bedford College in 1974, and in 1977 a PhD in vertebrate palaeon ...

conducted a phylogenetic analysis using a modified version of Müller's matrix. Evans also recovered Drepanosauridae as the sister taxon of the clade containing basal kuehneosaurid ''

Pamelina ''Pamelina'' is an extinct genus of basal kuehneosaurid known from Early Triassic (Olenekian age) rocks of Czatkowice 1, Poland.. It was first named by Susan E. Evans in 2009 and the type species is ''Pamelina polonica''. It is the oldest known ...

'' and the rest of Kuehneosauridae; however, unlike Müller's analysis, drepanosaurids and kuehneosaurids were recovered as non-

lepidosauria The Lepidosauria (, from Greek meaning ''scaled lizards'') is a subclass or superorder of reptiles, containing the orders Squamata and Rhynchocephalia. Squamata includes snakes, lizards, and amphisbaenians. Squamata contains over 9,000 species, m ...

n lepidosauromorphs. Evans did note that the two families share few

synapomorphies In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have ...

, with Müller citing only two. One of them is the increased angulation of the zygapophyses in the posterior dorsal vertebrae; Evans noted that this character is also present in the skeletons of lizards belonging to the modern genus ''

Draco Draco is the Latin word for serpent or dragon. Draco or Drako may also refer to: People * Draco (lawgiver) (from Greek: Δράκων; 7th century BC), the first lawgiver of ancient Athens, Greece, from whom the term ''draconian'' is derived * D ...

'' "and is likely to be functional (and thus potentially convergent)." The other synapomorphy, the enclosed thyroid fenestra in the

pelvis The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis, or pelvic skeleton). The ...

, "may be variable in the British kuehneosaurs and remains unknown in ''Pamelina,''" according to Evans. The author also noted that there are many differences between the skulls of drepanosaurids and kuehneosaurids, and that the only skull characters shared by members of both families are primitive neodiapsid characters and thus cannot be used to support a close relationship between the two clades.

Drepanosaurs as archosauromorphs and the abandonment of Avicephala

In 1998, Dilkes argued that drepanosaurs were close relatives of

tanystropheids Tanystropheidae is an extinct family of mostly marine archosauromorph reptiles that lived throughout the Triassic Period. They are characterized by their long, stiff necks formed from elongated cervical vertebrae with very long cervical ribs. So ...

, and his phylogenetic analysis has been used by many other authors. Gottmann-Quesada and Sander (2009) included one member of Drepanosauridae, ''Megalancosaurus'', in their analysis of archosauromorph relationships; it was found to be one of the most basal known members of Archosauromorpha and the sister taxon of '' Protorosaurus''. In a later study, Renesto et al. demonstrated that Senter's 2004 cladogram was based on poorly defined characters and data. The resulting phylogeny was therefore very unusual compared to any other previous study on drepanosaurs or related taxa. The new cladogram proposed in this last study abandoned both Avicephala (because it was polyphyletic) and Simiosauria. Senter's definition of Simiosauria included Sauria as an external specifier, causing the clade to become obsolete in Renesto et al.'s study (where drepanosaurs nested within Sauria). Renesto and colleagues instead defined a new clade, Drepanosauromorpha, as the least inclusive clade containing ''Hypuronector limnaios'' and ''Megalancosaurus preonensis''. A more inclusive taxon, Elyurosauria ("lizard with coiled tail"), was erected in order to include all the drepanosaurs with coiled tails. ''

'' is thus more derived than '' Hypuronector'' (as shown by its morphology). ''

'' and '' Megalancosaurus'' were also placed in a new taxon named Megalancosaurinae. The alternative cladogram presented in Renesto et al. (2010). Renesto ''et al.'' (2010) used modified versions of the matrices from the earlier analyses of Laurin (1991) and Dilkes (1998) in order to determine the phylogenetic position of Drepanosauromorpha within Diapsida. The analyses using Laurin's matrix recovered drepanosaurs either as the sister group of the clade containing '' Prolacerta'', '' Trilophosaurus'' and ''

Hyperodapedon ''Hyperodapedon'' is a genus of rhynchosaurs (beaked, archosaur-like reptiles) from the Triassic, Late Triassic period (Carnian stage). Fossils of the genus have been found in Africa, Asia, Europe and North and South America. Its first discovery ...

'', (

Archosauromorpha Archosauromorpha (Greek for "ruling lizard forms") is a clade of diapsid reptiles containing all reptiles more closely related to archosaurs (such as crocodilians and dinosaurs, including birds) rather than lepidosaurs (such as tuataras, liza ...

), or in unresolved polytomy with Archosauromorpha and

Lepidosauromorpha Lepidosauromorpha (in PhyloCode known as ''Pan-Lepidosauria'') is a group of reptiles comprising all diapsids closer to lizards than to archosaurs (which include crocodiles and birds). The only living sub-group is the Lepidosauria, which contains ...

. The analyses using Dilkes' matrix recovered drepanosaurs either as "

protorosaur Protorosauria is an extinct polyphyletic group of archosauromorph reptiles from the latest Middle Permian (Capitanian stage) to the end of the Late Triassic ( Rhaetian stage) of Asia, Europe and North America. It was named by the English anato ...

" archosauromorphs and the sister taxon to the clade containing '' Tanystropheus'', ''

Langobardisaurus ''Langobardisaurus'' (, meaning Reptile of Langobardi, in reference to the Long Bearded People, an ancient Italian civilization) is an extinct genus of tanystropheid archosauromorph reptile, with one known species, ''L. pandolfii''. Its fossils ...

'' and ''

Macrocnemus ''Macrocnemus'' is an extinct genus of archosauromorph reptile known from the Middle Triassic (Late Anisian to Ladinian) of Europe and China. ''Macrocnemus'' is a member of the Tanystropheidae family and includes three species''. Macrocnemus bass ...

'', or in unresolved polytomy with Lepidosauromorpha,

Choristodera Choristodera (from the Greek χωριστός ''chōristos'' + δέρη ''dérē'', 'separated neck') is an extinct order of semiaquatic diapsid reptiles that ranged from the Middle Jurassic, or possibly Triassic, to the late Miocene (168 to 1 ...

and several archosauromorph clades. Renesto ''et al.'' (2010) concluded that avicephalan synapomorphies proposed by Senter (2004) are merely evolutionary convergences caused by common lifestyle shared by drepanosaurids, coelurosauravids and ''Longisquama''. The authors did not rule out the possibility that drepanosaurs and ''Longisquama'' might really be close relatives.

Drepanosaurs as basal diapsids

In 2017, Pritchard and Nesbitt employed a phylogenetic analysis in their description of ''Avicranium'', a new genus of drepanosaur. This study found that Drepanosauromorpha was one of the earliest diverging groups of diapsids in the analysis, even more basal than weigeltisaurids (such as '' Coelurosauravus''), tangasaurids (such as '' Hovasaurus''), and younginids (such as '' Youngina''). However, they were not found to be as basal as '' Petrolacosaurus'', one of the earliest and most primitive diapsids known. Although drepanosaurs are only known from the

late Triassic The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch ...

, this new finding suggests that the first members of the drepanosauromorph lineage may have evolved much earlier, in the

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 Paleoz ...

(about 260 million years ago).

References

External links

Monkey Lizards of the Triassic
- An illustrated article on drepanosaurs from HMNH.

- Images and discussion of ''Drepanosaurus''.

- Images and discussion of ''Megalancosaurus''. {{Taxonbar, from=Q3494615 Avicephalans Drepanosaurs Norian first appearances Late Triassic extinctions