| Drepanosaurs Temporal range: Late Triassic, 220–216.5 Ma |
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| Fossil specimen of Drepanosaurus unguicaudatus | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| clade: | Sauria |
| Infraclass: | Archosauromorpha |
| Node: | †Drepanosauromorpha Renesto et al., 2010 |
| Subgroups | |
Drepanosaurs (members of the clade Drepanosauromorpha) are a group of strange reptiles that lived during the Carnian stage of the late Triassic Period, between 220 and 216 million years ago.[1] The various species of drepanosaurid were characterized by odd specialized grasping limbs and often prehensile tails, adaptions for arboreal (tree-dwelling), and/or possibly aquatic lifestyles. Fossils of drepanosaurs, also known as Simiosaurs ("monkey lizards"), have been found in Arizona, New Mexico, New Jersey, 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.
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Drepanosaurs are notable for their distinctive, triangular skulls, which resemble the skulls of birds. Some drepanosaurs, such as Hypuronector, had pointed, toothless, bird-like beaks. This similarity to birds may have led to the possible mis-attribution of a drepanosaur skull to the would-be "first bird", Protoavis.[2]
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.[3] The tail of this genus was extremely deep and non-prehensile – much more fin-like than members of the more exclusive group Drepanosauridae.[4]
Drepanosaurs have been difficult to pin down in terms of their phylogenetic position. They have been assigned by some resarches to the prolacertiformes, though more recent studies place them together with the coelurosauravids and Longisquama in a clade called the Avicephala.[3][5]
Within Avicephala, Senter created the group Simiosauria to refer to all the species traditionally grouped together within the Drepanosauridae, reserving that name for a more exclusive sub-group. 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.[3]
The following cladogram was found by Senter in his 2004 analysis.[3]
| Simiosauria |
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Renesto et al. (2010)[6] demonstrated that Senter (2004) cladogram was based on poorly defined characters and dataset. 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 is 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 pronenesis. Another more inclusive taxon, Elyurosauria ("lizard with coiled tails"), was erected, in order to include all the drepanosaurs with coiled tails, Vallesaurus is thus more derived than Hypuronector (as clearly shown by its morphology). Drepanosaurus and Megalancosaurus are also in a new taxon named Megalancosaurinae.
The alternative cladogram presented in Renesto et al. (2010).[6]
| Drepanosauromorpha |
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