Avicephala is an extinct
cladeA clade is a term used in modern alpha taxonomy, the scientific classification of living and fossil organisms, to describe a monophyletic group, defined as a group consisting of a single common ancestor and all its descendants.[The term "monophyletic group" is used in this article...]
of bizarre
diapsidDiapsids 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...
reptileReptiles, or members of the class Reptilia, are air-breathing, generally "cold-blooded" amniotes that generally have skin covered in scales or scutes. They are tetrapods and lay amniote eggs, whose embryos are surrounded by the amnion membrane...
s that lived during the Late
PermianThe Permian
[The 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...]
and
TriassicThe Triassic is a geologic period and system that extends from about 251 to 199 Ma . 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) lifestyles.
The name "avicephala" means "bird heads", in reference to the distinctive triangular skulls of these reptiles that mimic the shape of
birdBirds are winged, bipedal, endothermic , vertebrate animals that lay eggs. There are around 10,000 living species, making them the most numerous tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Birds range in size from the Bee Hummingbird to the ...
skulls. A few avicephalans, such as
HypuronectorHypuronector 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.
Avicephala is an extinct
cladeA clade is a term used in modern alpha taxonomy, the scientific classification of living and fossil organisms, to describe a monophyletic group, defined as a group consisting of a single common ancestor and all its descendants.[The term "monophyletic group" is used in this article...]
of bizarre
diapsidDiapsids 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...
reptileReptiles, or members of the class Reptilia, are air-breathing, generally "cold-blooded" amniotes that generally have skin covered in scales or scutes. They are tetrapods and lay amniote eggs, whose embryos are surrounded by the amnion membrane...
s that lived during the Late
PermianThe Permian
[The 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...]
and
TriassicThe Triassic is a geologic period and system that extends from about 251 to 199 Ma . 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) lifestyles.
Anatomy and Lifestyle
The name "avicephala" means "bird heads", in reference to the distinctive triangular skulls of these reptiles that mimic the shape of
birdBirds are winged, bipedal, endothermic , vertebrate animals that lay eggs. There are around 10,000 living species, making them the most numerous tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Birds range in size from the Bee Hummingbird to the ...
skulls. A few avicephalans, such as
HypuronectorHypuronector 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
convergenceConvergent 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, birds and bats do, and are capable of powered flight. The wings are similar in...
. This similarity may also have led to the possible misidentification of the would-be "first bird",
ProtoavisProtoavis is the name given to archosaurian fossil bones from the Late Triassic , found near Post, Texas. These fossils have been described as a primitive bird which, if the identification is valid, would push back avian origins some 60-75 million years.The original describer of Protoavis...
.
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
drepanosauridDrepanosauridae is a family 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 ,...
s, featured a suite of bizarre, almost
chameleonThe family Chamaeleonidae 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, and the...
-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
MegalancosaurusMegalancosaurus 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
newtA newt is an amphibian of the Salamandridae family, 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. Newts metamorphose through three distinct developmental...
s and
crocodileA crocodile is any species belonging to the family Crocodylidae . The term can also be used more loosely to include all 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.
Phylogeny
The various avicephalan groups have been difficult to pin down in terms of their phylogenetic position. Some of these enigmatic reptiles, specifically the
drepanosauridsDrepanosauridae is a family 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
LongisquamaLongisquama 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
ProlacertiformesProlacertiformes 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, for which he coined the name Avicephala, as a sister taxon to
NeodiapsidaNeodiapsida 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 created the group
Simiosauria ("monkey lizards") for the extremely derived tree-dwelling forms. Senter found that
HypuronectorHypuronector 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
VallesaurusVallesaurus is an extinct genus of reptile, belonging to the family Drepanosauridae. Found in Northern Italy in 1975, this Triassic drepanosaur could be considered the most primitive example of the drepanosaurid reptiles...
. He also recovered a close relationship between the drepanosaurs
DolabrosaurusDolabrosaurus is a genus of extinct reptile and a member of the family Drepanosauridae....
and
MegalancosaurusMegalancosaurus 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 was found by Senter in his 2004 analysis.
External links