Avicephala is a possibly polyphyletic and therefore disused clade

Clade

A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 of diapsid

Diapsid

Diapsids are a group of reptiles that developed two holes in each side of their skulls, about 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, and include all crocodiles, lizards, snakes, and tuatara...

 reptile

Reptile

Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

s that lived during the Late Permian

Permian

The PermianThe term "Permian" was introduced into geology in 1841 by Sir Sir R. I. Murchison, president of the Geological Society of London, who identified typical strata in extensive Russian explorations undertaken with Edouard de Verneuil; Murchison asserted in 1841 that he named his "Permian...

 and Triassic

Triassic

The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

 periods. Many species had odd specialized grasping limbs and prehensile tails, adapted to arboreal (and possibly aquatic

Aquatic animal

An aquatic animal is an animal, either vertebrate or invertebrate, which lives in water for most or all of its life. It may breathe air or extract its oxygen from that dissolved in water through specialised organs called gills, or directly through its skin. Natural environments and the animals that...

) lifestyles.

Description

The name "Avicephala" means "bird heads", in reference to the distinctive triangular skulls of these reptiles that mimic the shape of bird

Bird

Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

 skulls. A few avicephalans, such as Hypuronector

Hypuronector

Hypuronector is a genus of extinct reptile from the Triassic Period that lived in what is now New Jersey. The etymology of the name translates as "deep-tailed swimmer from the lake." A member of the Simiosauria, Hypuronector is related to the arboreal Megalancosaurus. It was a small animal,...

, even appear to have had pointed, toothless, bird-like beaks. This cranial similarity to birds has led a few scientists to theorize that birds descended from avicephalans like Longisquama, though a majority see the similarity simply as convergence

Convergent evolution

Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...

. This similarity may also have led to the possible misidentification of the would-be "first bird", Protoavis

Protoavis

Protoavis is a problematic taxon of archosaurian 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 Protoavis actually is...

.

Avicephalans possessed a variety of odd and distinctive characteristics in addition to their bird-like skulls. Some displayed unique dermal appendages, such as the feather-like dorsal plumes of Longisquama, and the laterally-oriented rib-like rods of Coelurosauravus, which supported membranes and may have been used to glide from branch to branch in an arboreal habitat.

Another avicephalan group, the drepanosaurid

Drepanosauridae

Drepanosaurs are a group of strange reptiles that lived during the Carnian stage of the late Triassic Period, between 220 and 216 million years ago. The various species of drepanosaurid were characterized by odd specialized grasping limbs and often prehensile tails, adaptions for arboreal , and/or...

s, featured a suite of bizarre, almost chameleon

Chameleon

Chameleons are a distinctive and highly specialized clade of lizards. They are distinguished by their parrot-like zygodactylous feet, their separately mobile and stereoscopic eyes, their very long, highly modified, and rapidly extrudable tongues, their swaying gait, the possession by many of a...

-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

Megalancosaurus

Megalancosaurus is a genus of extinct reptile from the Late Triassic period 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."-Anatomy:Megalancosaurus was...

(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 newt

Newt

A newt is an aquatic amphibian of the family Salamandridae, although not all aquatic salamanders are considered newts. Newts are classified in the subfamily Pleurodelinae of the family Salamandridae, and are found in North America, Europe and Asia...

s and crocodile

Crocodile

A crocodile is any species belonging to the family Crocodylidae . The term can also be used more loosely to include all extant members of the order Crocodilia: i.e...

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 other drepanosaurs.

History of classification

The various avicephalan groups have been difficult to pin down in terms of their phylogenetic position. Some of these enigmatic reptiles, specifically the drepanosaurids

Drepanosauridae

Drepanosaurs are a group of strange reptiles that lived during the Carnian stage of the late Triassic Period, between 220 and 216 million years ago. The various species of drepanosaurid were characterized by odd specialized grasping limbs and often prehensile tails, adaptions for arboreal , and/or...

 and Longisquama

Longisquama

Longisquama insignis is an extinct lizard-like reptile known only from one poorly preserved and incomplete fossil. It lived during the middle or late Triassic Period, 230-225 million years ago, in what is now Kyrgyzstan...

, have been assigned by some researches to the Prolacertiformes

Prolacertiformes

Prolacertiformes were an order of archosauromorph reptiles that lived during the Permian and Triassic Periods...

. Senter, however, found them to form a group with the coelurosauravids

Coelurosauravidae

Coelurosauravidae was a family of avicephalans from the Permian and Triassic time periods. They lived in trees, and were typically two feet long...

, for which he coined the name Avicephala, as a sister taxon to Neodiapsida

Neodiapsida

Neodiapsida is a clade or major branch of the reptilian family tree, that includes all Diapsids apart from some early primitive types known as Araeoscelidia....

 (the group which includes all modern diapsids and their extinct relatives).

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

Drepanosauridae

Drepanosaurs are a group of strange reptiles that lived during the Carnian stage of the late Triassic Period, between 220 and 216 million years ago. The various species of drepanosaurid were characterized by odd specialized grasping limbs and often prehensile tails, adaptions for arboreal , and/or...

 than to Coelurosauravus

Coelurosauravus

Coelurosauravus is a genus of basal diapsid reptile, with specialized wing-like structures allowing it to glide. These were rod like structures with skin stretched over them; this feature is unique to the genus....

or Sauria

Sauria

Sauria is a clade of reptiles that includes all living diapsids, as well as their common ancestor and all its extinct descendants. The ancestral saurian was probably a small lizard-like creature living in the Permian Period...

." However, Renesto and colleagues (see below) found drepanosaurids to lie within Sauria, which would make the clade Simiosauria obsolete.

Senter found that Hypuronector

Hypuronector

Hypuronector is a genus of extinct reptile from the Triassic Period that lived in what is now New Jersey. The etymology of the name translates as "deep-tailed swimmer from the lake." A member of the Simiosauria, Hypuronector is related to the arboreal Megalancosaurus. It was a small animal,...

, originally described as a drepanosaurid, actually lies just outside that family, along with the primitive drepanosaur Vallesaurus

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, named in 2006. A second species, V...

. He also recovered a close relationship between the drepanosaurs Dolabrosaurus

Dolabrosaurus

Dolabrosaurus is a genus of extinct reptile and a member of the family Drepanosauridae....

and Megalancosaurus

Megalancosaurus

Megalancosaurus is a genus of extinct reptile from the Late Triassic period 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."-Anatomy:Megalancosaurus was...

.

The following cladogram

Cladogram

A cladogram is a diagram used in cladistics which shows ancestral relations between organisms, to represent the evolutionary tree of life. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational...

 was found by Senter in his 2004 analysis.
Renesto et al. (2010) 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, redefining the latter under the PhyloCode

PhyloCode

The International Code of Phylogenetic Nomenclature, known as the PhyloCode for short, is a developing draft for a formal set of rules governing phylogenetic nomenclature...

 as Drepanosauromorpha. Drepanosauromorphs are closely related to the Prolacertiformes

Prolacertiformes

Prolacertiformes were an order of archosauromorph reptiles that lived during the Permian and Triassic Periods...

, especially genera like Langobardisaurus

Langobardisaurus

Langobardisaurus is an extinct genus of prolacertiform reptile....

and Macrocnemus

Macrocnemus

Macrocnemus is an extinct genus of prolacertiform reptile from the Middle Triassic of Europe. Two species have been named, the type species M. bassanii and the species M. fuyuanensis....

, if not part of the Prolacertiformes.
Among the drepanosauromorphs it was defined a more inclusive taxon, Elyurosauria ("lizard with coiled tails"), in order to include all the drepanosaurs with coiled tails, Vallesaurus

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, named in 2006. A second species, V...

is thus more derived than Hypuronector

Hypuronector

Hypuronector is a genus of extinct reptile from the Triassic Period that lived in what is now New Jersey. The etymology of the name translates as "deep-tailed swimmer from the lake." A member of the Simiosauria, Hypuronector is related to the arboreal Megalancosaurus. It was a small animal,...

(as clearly shown by its morphology). Drepanosaurus

Drepanosaurus

Drepanosaurus is a genus of arboreal reptile that lived during the Triassic Period. Only one specimen of Drepanosaurus has thus far been found. Sadly this specimen was incomplete and lacked a head and neck...

and Megalancosaurus

Megalancosaurus

Megalancosaurus is a genus of extinct reptile from the Late Triassic period 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."-Anatomy:Megalancosaurus was...

are also in a new taxon named Megalancosaurinae.

The alternative cladogram as presented in Renesto et al. (2010).

External links

• Monkey Lizards of the Triassic - An illustrated article on drepanosaurs from HMNH.
• Prof. Silvio Renesto—Vertebrate Paleontology at Insubria University: Research - Images and discussion of Drepanosaurus.
• Prof. Silvio Renesto—Vertebrate Paleontology at Insubria University: Research - Images and discussion of Megalancosaurus.
• Gliding Mechanism in the Late Permian Reptile Coelurosauravus (Eberhard Frey, Hans-Dieter Sues, & Wolfgang Munk) - Abstract and available full text of the article in Science.