Drepanosaur

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Drepanosaurs
Temporal range: Late Triassic, 230–201.3  Ma
Drepanosaurus unguicaudatus.JPG
Fossil specimen of Drepanosaurus unguicaudatus
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Neodiapsida
Clade: Drepanosauromorpha
Renesto et al., 2010
Subgroups

Drepanosaurs (members of the clade Drepanosauromorpha) are a group of extinct reptiles that lived between the Carnian and Rhaetian stages of the late Triassic Period, approximately between 230 and 210 million years ago. [1] The various species of drepanosaurs were characterized by a bird-like skull, a barrell shaped body, and a horizontally narrow tail. A number of drepanosaurs had specialized grasping limbs and often prehensile tails similar to those of chameleons. Drepanosaurs are generally thought to have been arboreal (tree-dwelling), [2] and probably insectivores. [3] Some studies have alternately suggested fossorial (digging) and aquatic lifestyles for some members. [4] Fossils of drepanosaurs have been found in North America (Arizona, New Mexico, New Jersey, Utah) and Europe (England and northern Italy). The name is taken from the family's namesake genus Drepanosaurus , which means "sickle lizard," a reference to their strongly curved claws.

Contents

Some studies have included Drepanosaurs within the group Avicephala, which also includes the gliding Weigeltisauridae, but the close relationship between the two groups has been doubted by other authors. Their phylogenetic position has been disputed, with some studies considering them to be members of Archosauromorpha (and thus more closely related to modern birds and crocodilians than to lizards), while other studies have considered them to be basal neodiapsids that are not related to any modern reptiles. [2]

Description

Skeletal restoration of Megalancosaurus preonensis Megalancosaurus skeletal.png
Skeletal restoration of Megalancosaurus preonensis

Drepanosaurs are notable for their distinctive, triangular skulls, which resemble the skulls of birds. Some drepanosaurs, such as Avicranium, had pointed, toothless, bird-like beaks. This similarity to birds may have led to the misattribution of what may be a drepanosaur skull to the would-be "first bird," Protoavis . [5]

Megalancosaurus preonensis restoration Megalancosaurus BW.jpg
Megalancosaurus preonensis restoration

Drepanosaurs featured a suite of bizarre, almost chameleon-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 crocodiles, 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. [6] The tail of this genus was extremely deep and non-prehensile: much more fin-like than members of the more exclusive group Drepanosauridae. [7] 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. [8] While the suggestion has not been ruled out entirely, it has since been largely dismissed, due to Megalancosaurus' clunky, chameleon-like anatomy. [5] Hypuronector, however, is much more likely to be a glider or flyer due to the elongated forelimbs. [9] Fossorial or digging-related adaptations have been recognized in three drepanosaur genera: Skybalonyx , Ancistronychus , and Drepanosaurus. In particular, Drepanosaurus may have been adapted to hook-and-pull digging, similar to modern-day anteaters. Skybalonyx possessed claws similar in shape to modern-day moles and echidna, both of which are humeral-rotation diggers. [4]

Vallesaurus cenensis fossil Vallesaurus cenensis.JPG
Vallesaurus cenensis fossil

The phylogenetic position of drepanosaurs is highly disputed. Various studies have proposed that drepanosaurs are protorosaurian archosauromorphs, [10] lepidosauromorphs related to kuehneosaurids, [11] non-saurian diapsids related to weigeltisaurids, [6] or (most recently) basal neodiapsids. [12]

Early studies

When Drepanosaurus and Dolabrosaurus were first discovered (in 1980 and 1992, respectively), they were each considered early lepidosaurians, ancestral to modern lizards. Megalancosaurus was first believed to be a thecodont (i.e. an archosauriform) upon its discovery in 1980, but later studies placed it as a prolacertiform, and perhaps even an ancestor to birds, although this latter hypothesis has not been supported by subsequent studies. [12]

Drepanosaurs outside Neodiapsida: Avicephala and Simiosauria

A 2004 study by Senter placed drepanosaurs with the coelurosauravids (weigeltisaurids) and Longisquama in a clade 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." [6] [13]

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 . He also recovered a close relationship between the drepanosaurids Dolabrosaurus and Megalancosaurus . [6]

The following cladogram was proposed by Senter in his 2004 analysis: [6]

Simiosauria

A clade containing drepanosaurids, Longisquama, and Coelurosauravus (as well as Wapitisaurus ) 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 of Sauria. [14]

Drepanosaurus unguicaudatus restoration Drepanosaurus unguicaudatus on a branch.jpg
Drepanosaurus unguicaudatus restoration

In a 2006 study, Renesto and Binelli found that when pterosaur Eudimorphodon was added to Senter's original matrix, it was found to be a member of Avicephala. [15] 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 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. [15]

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. [16] In a 2009 study, Susan E. Evans 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 and the rest of Kuehneosauridae; however, unlike Müller's analysis, drepanosaurids and kuehneosaurids were recovered as non-lepidosaurian lepidosauromorphs. Evans did note that the two families share few synapomorphies, 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 "and is likely to be functional (and thus potentially convergent)." The other synapomorphy, the enclosed thyroid fenestra in the pelvis, "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. [11]

Drepanosaurs as archosauromorphs and the abandonment of Avicephala

In 1998, Dilkes argued that drepanosaurs were close relatives of tanystropheids, and his phylogenetic analysis has been used by many other authors. [10] 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 . [17]

In a later study, Renesto et al. [1] 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. Vallesaurus is thus more derived than Hypuronector (as shown by its morphology). Drepanosaurus and Megalancosaurus were also placed in a new taxon named Megalancosaurinae.

The alternative cladogram presented in Renesto et al. (2010). [1]

Drepanosauromorpha

Hypuronector

Elyurosauria

Vallesaurus

Drepanosauridae

Renesto et al. (2010) used modified versions of the matrices from the earlier analyses of Laurin (1991) [18] and Dilkes (1998) [10] 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 , (Archosauromorpha), or in unresolved polytomy with Archosauromorpha and Lepidosauromorpha. The analyses using Dilkes' matrix recovered drepanosaurs either as "protorosaur" archosauromorphs and the sister taxon to the clade containing Tanystropheus , Langobardisaurus and Macrocnemus , or in unresolved polytomy with Lepidosauromorpha, Choristodera 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. [1]

The phylogenetic study published by Buffa et al. (2024) did not recover "avicephalans" as closely related. The authors' phylogenetic analysis recovered Drepanosauromorpha as allokotosaurian archosauromorphs, specifically as the sister group of trilophosaurids. [2]

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, this new finding suggests that the first members of the drepanosauromorph lineage may have evolved much earlier, in the Permian (about 260 million years ago). [12]

Related Research Articles

<span class="mw-page-title-main">Sauria</span> Clade of reptiles

Sauria is the clade containing the most recent common ancestor of Archosauria and Lepidosauria, and all its descendants. Since most molecular phylogenies recover turtles as more closely related to archosaurs than to lepidosaurs as part of Archelosauria, Sauria can be considered the crown group of diapsids, or reptiles in general. Depending on the systematics, Sauria includes all modern reptiles or most of them as well as various extinct groups.

<i>Dinocephalosaurus</i> Extinct genus of reptiles

Dinocephalosaurus is a genus of long necked, aquatic protorosaur that inhabited the Triassic seas of China. The genus contains the type and only known species, D. orientalis, which was named by Chun Li in 2003. Unlike other long-necked protorosaurs, Dinocephalosaurus convergently evolved a long neck not through elongation of individual neck vertebrae, but through the addition of neck vertebrae that each had a moderate length. As indicated by phylogenetic analyses, it belonged in a separate lineage that also included at least its closest relative Pectodens, which was named the Dinocephalosauridae in 2021. Like tanystropheids, however, Dinocephalosaurus probably used its long neck to hunt, utilizing the fang-like teeth of its jaws to ensnare prey; proposals that it employed suction feeding have not been universally accepted. It was probably a marine animal by necessity, as suggested by the poorly-ossified and paddle-like limbs which would have prevented it from going ashore.

<span class="mw-page-title-main">Archosauromorpha</span> Infraclass of reptiles

Archosauromorpha is a clade of diapsid reptiles containing all reptiles more closely related to archosaurs rather than lepidosaurs. Archosauromorphs first appeared during the late Middle Permian or Late Permian, though they became much more common and diverse during the Triassic period.

<i>Tanystropheus</i> Extinct genus of reptiles

Tanystropheus is an extinct genus of archosauromorph reptile which lived during the Triassic Period in Europe, Asia, and North America. It is recognisable by its extremely elongated neck, longer than the torso and tail combined. The neck was composed of 13 vertebrae strengthened by extensive cervical ribs. Tanystropheus is one of the most well-described non-archosauriform archosauromorphs, known from numerous fossils, including nearly complete skeletons. Some species within the genus may have reached a total length of 6 meters (20 ft), making Tanystropheus the longest non-archosauriform archosauromorph as well. Tanystropheus is the namesake of the family Tanystropheidae, a clade collecting many long-necked Triassic archosauromorphs previously described as "protorosaurs" or "prolacertiforms".

<span class="mw-page-title-main">Neodiapsida</span> Clade of reptiles

Neodiapsida is a clade, or major branch, of the reptilian family tree, typically defined as including all diapsids apart from some early primitive types known as the araeoscelidians. Modern reptiles and birds belong to the neodiapsid subclade Sauria.

<i>Protorosaurus</i> Extinct genus of reptiles

Protorosaurus is an extinct genus of reptile. Members of the genus lived during the late Permian period in what is now Germany and Great Britain. Once believed to have been an ancestor to lizards, Protorosaurus is now known to be one of the oldest and most primitive members of Archosauromorpha, the group that would eventually lead to archosaurs such as crocodilians and dinosaurs.

<span class="mw-page-title-main">Avicephala</span> Extinct clade of neodiapsid reptiles

Avicephala is a potentially polyphyletic grouping of extinct diapsid reptiles that lived during the Late Permian and Triassic periods characterised by superficially bird-like skulls and arboreal lifestyles. As a clade, Avicephala is defined as including the gliding weigeltisaurids and the arboreal drepanosaurs to the exclusion of other major diapsid groups. This relationship is not recovered in the majority of phylogenetic analyses of early diapsids and so Avicephala is typically regarded as an artificial or unnatural grouping. However, the clade was recovered again in 2021 following a redescription of Weigeltisaurus, raising the possibility that the clade may be valid after all, although subsequent analyses have not supported this result.

<i>Hovasaurus</i> Extinct genus of reptiles

Hovasaurus is an extinct genus of basal diapsid reptile. It lived in what is now Madagascar during the Late Permian and Early Triassic, being a survivor of the Permian–Triassic extinction event and the paleontologically youngest member of the Tangasauridae. Fossils have been found in the Permian Lower and Triassic Middle Sakamena Formations of the Sakamena Group, where it is amongst the commonest fossils. Its morphology suggests an aquatic ecology.

<i>Megalancosaurus</i> Extinct genus of reptiles

Megalancosaurus is a genus of extinct reptile from the Late Triassic Dolomia di Forni Formation and Zorzino Limestone of northern Italy, and one of the best known drepanosaurids. The type species is M. preonensis; a translation of the animal's scientific name would be "long armed reptile from the Preone Valley."

<i>Hypuronector</i> Extinct genus of reptiles

Hypuronector is a genus of extinct drepanosaur reptile from the Late Triassic Lockatong Formation of New Jersey. The etymology of the name translates as "deep-tailed swimmer from the lake", in reference to its assumed aquatic habits hypothesized by its discoverers. Hypuronector was related to the arboreal Megalancosaurus. It was a small animal, estimated to be only 12 cm (4.7 in) long in life. So far dozens of specimens of Hypuronector are known, though scientists have not found any complete skeletons. This makes attempts to reconstruct Hypuronector's body or lifestyle highly speculative and controversial.

<i>Drepanosaurus</i> Extinct genus of reptiles

Drepanosaurus 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 was probably an insectivore, and lived in a coastal environment in what is now modern day Italy, as well as in a streamside environment in the midwestern United States.

<i>Vallesaurus</i> Extinct genus of reptiles

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 only formally described in 2006. A second species, V. zorzinensis, was named in 2010.

Langobardisaurus is an extinct genus of tanystropheid archosauromorph reptile, with one valid species, L. pandolfii. Its fossils have been found in Italy and Austria, and it lived during the Late Triassic period, roughly 228 to 201 million years ago. Langobardisaurus was initially described in 1994, based on fossils from the Calcare di Zorzino Formation in Northern Italy. Fossils of the genus are also known from the Forni Dolostone of Northern Italy and the Seefeld Formation of Austria.

<i>Helveticosaurus</i> Extinct genus of reptiles

Helveticosaurus is an extinct genus of diapsid marine reptile known from the Middle Triassic of southern Switzerland. It contains a single species, Helveticosaurus zollingeri, known from the nearly complete holotype T 4352 collected at Cava Tre Fontane of Monte San Giorgio, an area well known for its rich record of marine life during the Middle Triassic.

<span class="mw-page-title-main">Weigeltisauridae</span> Extinct family of reptiles

Weigeltisauridae is a family of gliding neodiapsid reptiles that lived during the Late Permian, between 259.51 and 251.9 million years ago. Fossils of weigeltisaurids have been found in Madagascar, Germany, Great Britain, and Russia. They are characterized by long, hollow rod-shaped bones extending from the torso that probably supported wing-like membranes. Similar membranes are also found in several other extinct reptiles such as kuehneosaurids and Mecistotrachelos, as well as living gliding lizards, although each group evolved these structures independently.

<span class="mw-page-title-main">Protorosauria</span> Extinct order of reptiles

Protorosauria is an extinct, likely paraphyletic group of basal archosauromorph reptiles from the latest Middle Permian to the end of the Late Triassic of Asia, Europe and North America. It was named by the English anatomist and paleontologist Thomas Henry Huxley in 1871 as an order, originally to solely contain Protorosaurus. Other names which were once considered equivalent to Protorosauria include Prolacertiformes and Prolacertilia.

<i>Pamelaria</i> Extinct genus of reptiles

Pamelaria is an extinct genus of allokotosaurian archosauromorph reptile known from a single species, Pamelaria dolichotrachela, from the Middle Triassic of India. Pamelaria has sprawling legs, a long neck, and a pointed skull with nostrils positioned at the very tip of the snout. Among early archosauromorphs, Pamelaria is most similar to Prolacerta from the Early Triassic of South Africa and Antarctica. Both have been placed in the family Prolacertidae. Pamelaria, Prolacerta, and various other Permo-Triassic reptiles such as Protorosaurus and Tanystropheus have often been placed in a group of archosauromorphs called Protorosauria, which was regarded as one of the most basal group of archosauromorphs. However, more recent phylogenetic analyses indicate that Pamelaria and Prolacerta are more closely related to Archosauriformes than are Protorosaurus, Tanystropheus, and other protorosaurs, making Protorosauria a polyphyletic grouping.

<span class="mw-page-title-main">Allokotosauria</span> Extinct clade of reptiles

Allokotosauria is a clade of early archosauromorph reptiles from the Middle to Late Triassic known from Asia, Africa, North America and Europe. Allokotosauria was first described and named when a new monophyletic grouping of specialized herbivorous archosauromorphs was recovered by Sterling J. Nesbitt, John J. Flynn, Adam C. Pritchard, J. Michael Parrish, Lovasoa Ranivoharimanana and André R. Wyss in 2015. The name Allokotosauria is derived from Greek meaning "strange reptiles" in reference to unexpected grouping of early archosauromorph with a high disparity of features typically associated with herbivory.

<span class="mw-page-title-main">Azendohsauridae</span> Extinct family of reptiles

Azendohsauridae is a family of allokotosaurian archosauromorphs that lived during the Middle to Late Triassic period, around 242-216 million years ago. The family was originally named solely for the eponymous Azendohsaurus, marking out its distinctiveness from other allokotosaurs, but as of 2022 the family now includes four other genera: the basal genus Pamelaria, the large horned herbivore Shringasaurus, and two carnivorous genera grouped into the subfamily-level subclade Malerisaurinae, Malerisaurus and Puercosuchus, and potentially also the dubious genus Otischalkia. Most fossils of azendohsaurids have a Gondwanan distribution, with multiple species known across Morocco and Madagascar in Africa as well as India, although fossils of malerisaurine azendohsaurids have also been found in the southwestern United States of North America.

<i>Avicranium</i> Extinct genus of reptiles

Avicranium is a genus of extinct drepanosaur reptile known from the Chinle Formation of the late Triassic. The type species of Avicranium is Avicranium renestoi. "Avicranium" is Latin for "bird cranium", in reference to its unusual bird-like skull, while "renestoi" references Silvio Renesto, a paleontologist known for studies of Italian drepanosaurs.

References

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