Avicephala | |
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Skeleton of the drepanosaur Drepanosaurus unguicaudatus | |
Skeleton of the weigeltisaurid Weigeltisaurus jaekeli | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Reptilia |
Clade: | Neodiapsida |
Clade: | † Avicephala Senter, 2004 |
Subgroups | |
Avicephala ("bird heads") 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 unnatural grouping. However, the clade was recovered again in 2021 in a redescription of Weigeltisaurus , raising the possibility that the clade may be valid after all.
Avicephalans were named in reference to their pointed, lightly constructed, superficially bird-like skulls. The resemblance is especially striking in some drepanosaurs such as Avicranium which possess toothless beaks, forward-facing eyes and large, rounded craniums. These superficial similarities to birds led some scientists to hypothesise that birds were derived from various avicephalians, although the well established relationship of birds to theropod dinosaurs indicates that these similarities arose only through convergent evolution. [1]
Although bird-like in some respects, avicephalans also possess a variety of archaic and unique characteristics as well, including some associated with an arboreal lifestyle. Drepanosaurs possess a suite of chameleon-like skeletal features, such as opposable digits on the hands and feet and prehensile tails—tipped with a bony hook in some species like Drepanosaurus . [2] Weigeltisaurids on the other hand possessed wing-like gliding membranes (patagia) supported by elongated bony rods along their bodies, novel structures analogous to ribs of the modern gliding Draco lizard. Weigeltisaurids also had toes that were similarly proportioned to living arboreal lizards. Although both groups are highly derived, they do share some similarities, namely they have relatively unossified skeletons lacking intercentra between their vertebrae, as well as proportionately tall, thin shoulder blades. [3]
The enigmatic Triassic reptile Longisquama was initially included as an Avicephalian when the clade was first defined as a relative of Coelurosauravus . [1] However, due to the limited knowledge of its anatomy, Longisquama has been excluded from many subsequent phylogenetic analyses as its true relationships are difficult to discern. [4]
The phylogenetic relationships of both drepanosaurs and weigeltisaurs, as well as Longisquama, have historically been difficult to pin down. Drepanosaurs and weigeltisaurids were first suggested to form a clade together by John Merck in an abstract presented to the 2003 annual meeting of the Society of Vertebrate Paleontology. [5] The same conclusion would be reached by Phil Senter in a paper in 2004, who named this clade Avicephala and found it to be the sister taxon to Neodiapsida (defined wherein as the clade containing "Younginiforms" and all living diapsids).
Within Avicephala, Senter found two clades, one that he named Simiosauria (equivalent to Drepanosauromorpha), and another containing Coelurosauravus and Longisquama. The cladogram below depicts the result of his analysis: [1]
Avicephala | |
Later research, including Renesto and Binelli (2006) and Renesto et al. (2010), argued that the phylogenetic analysis of Senter (2004) was flawed due to the absence of available data for drepanosaur skulls at the time and the exclusion of particular diapsid groups, such as pterosaurs. Renesto and colleagues (2010) recommended abandoning Avicephala for these reasons (as well as defining a new clade, Drepanosauromorpha, to replace Simiosauria following the guidelines of the PhyloCode). They instead found drepanosauromorphs to be archosauromorphs, possibly related to Prolacertiformes, and unrelated to weigeltisaurids or other early diapsids. [4] [6]
Drepanosauromorphs would later be demonstrated to be early-diverging diapsids after all following the description of the three-dimensionally preserved skull of Avicranium in 2017, revealing archaic features of their skull anatomy. Still, drepanosaurs and weigeltisaurids remained as separate lineages in the study's phylogenetic analysis, with each being successively closer to living diapsids. [7]
Avicephalia would not be recovered again until 2021 following the redescription of Weigeltisaurus by Adam Pritchard and colleagues, wherein the phylogenetic analysis found a clade of drepanosauromorphs and weigeltisaurids. They identified four unambiguous synapomorphies (shared unique traits) for Avicephalia: the absence of both cervical and dorsal intercentra, a length/height ratio for the scapula between 0.4 and 0.25, and no outer process on the fifth metatarsal. They also updated Senter's definition of Avicephala, as the modern definition of Neodiapsida now includes both drepanosaurs and weigeltisaurids. Avicephalia is now defined as "including all taxa more closely related to Weigeltisaurus jaekeli Weigelt 1930 and Drepanosaurus unguicaudatus Pinna 1979 than to Petrolacosaurus kansensis Lane 1945, Orovenator mayorum Reisz, Modesto & Scott, 2011, Claudiosaurus germaini Carroll, 1978, Youngina capensis Broom 1914, or Sauria Macartney 1802". [3]
Despite this result, they noted that should Avicephala turn out to not be monophyletic in future analyses they recommend that the taxon be abandoned once again. The cladogram of their results is shown below: [3]
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.
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.
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.
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 Kyrgyzstan. It is known from the type fossil specimen, slab and counterslab and five referred specimens of possible integumentary appendages. All specimens are in the collection of the Paleontological Institute of the Russian Academy of Sciences in Moscow.
Coelurosauravus is an extinct genus of gliding reptile, known from the Late Permian of Madagascar. Like other members of the family Weigeltisauridae, members of this genus possessed long, rod-like ossifications projecting outwards from the body. These bony rods were not extensions of the ribs but were instead a feature unique to weigeltisaurids. It is believed that during life, these structures formed folding wings used for gliding flight, similar to living gliding Draco lizards.
Claudiosaurus is an extinct genus of diapsid reptiles from the Late Permian Sakamena Formation of the Morondava Basin, Madagascar. It has been suggested to be semi-aquatic.
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.
Weigeltisaurus is an extinct genus of weigeltisaurid reptile from the Late Permian Kupferschiefer of Germany and Marl Slate of England. It has a single species, originally named as Palaechamaeleo jaekeli in 1930 and later assigned the name Weigeltisaurus jaekeli in 1939, when it was revealed that Palaeochamaeleo was a preoccupied name. A 1987 review by Evans and Haubold later lumped Weigeltisaurus jaekeli under Coelurosauravus as a second species of that genus. A 2015 reassessment of skull morphology study substantiated the validity of Weigeltisaurus and subsequent authors have used this genus. Like other Weigeltisaurids, they possessed long rod-like bones that radiated from the trunk that were likely used to support membranes used for gliding, similar to extant Draco lizards.
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."
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.
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.
Drepanosaurs 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. The various species of drepanosaurid were characterized by specialized grasping limbs and often prehensile tails, adaptions for arboreal (tree-dwelling) and fossorial (digging) lifestyles, with some having also been suggested to be aquatic. Fossils of drepanosaurs have been found in Arizona, New Mexico, New Jersey, Utah, 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.
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.
Kuehneosauridae is an extinct family of small, lizard-like gliding diapsids known from the Triassic period of Europe and North America.
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.
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.
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.
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.
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.
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.