Avemetatarsalia
Encyclopedia
Avemetatarsalia is a clade
name established by British palaeontologist Michael Benton
in 1999 for all crown group archosaur
s that are closer to bird
s than to crocodile
s. It includes a similarly defined subgroup, Ornithodira. An alternate name is Panaves, or "all birds", in reference to its definition containing all animals, living or extinct, which are more closely related to birds than to crocodiles.
Members of this group include the Dinosauromorpha
, Pterosauromorpha, and the genus Scleromochlus
. Dinosauromorpha contains more basal forms like Lagerpeton
and Marasuchus
and more derived forms like dinosaur
s, which according to most modern scientists the birds belong to as members of the theropods
. Pterosauromorpha contains Pterosauria
, which are the famous flying reptiles, and as far as is known the first vertebrate
s capable of true flight
. Most researchers think pterosaurians had neither an S-curved neck, nor an upright gait
: the clade Ornithodira is defined on basis of ancestry, not characters – and it is always possible later descendants show a change in characters.
stage of the Middle Triassic
about 245 million years ago, represented by the dinosauriform Asilisaurus
. However, Early Triassic
fossil footprints
reported in 2010 from the Świętokrzyskie (Holy Cross) Mountains
of Poland
may belong to a more primitive dinosauromorph. If so, the origin of avemetatarsalians would be pushed back into the early Olenekian
age, around 249 Ma. The oldest Polish footprints are from a small quadrupedal animal named Prorotodactylus
, but footprints belonging to the ichnogenus Sphingopus
that have been found from Early Anisian strata show that moderately large bipedal dinosauromorphs had appeared by 246 Ma. The tracks show that the dinosaur lineage appeared soon after the Permian-Triassic extinction event
. Their age suggests that the rise of dinosaurs
was slow and drawn out across much of the Triassic.
coined the name Ornithodira for a node clade, containing the last common ancestor of the dinosaur
s and the pterosaurs and all of its descendants. Paul Sereno
had in 1991 given a formal (and different) definition of Ornithodira, one in which Scleromochlus was explicitly added. It was thus a potentially larger group than the Ornithodira of Gauthier. However, at that point, there was no named clade that could encompass species with a basal position on the archosaurian branch leading to dinosaurs (as opposed to that leading to crocodiles). In 1999 Michael Benton
concluded that Scleromochlus was indeed outside of Gauthier's original conception of Ornithodira, so he named a new branch-based clade for this purpose: Avemetatarsalia, named after the birds (Aves), the last surviving members of the clade, and the metatarsal ankle joint that was a typical character of the group. Avemetatarsalia was defined as: all Avesuchia closer to Dinosauria than to Crocodylia. In 2004 Benton gave a formal definition of Ornithodira sensu Gauthier. In 2005 Sereno stated the opinion that Ornithodira was not a useful concept, whereas Avemetatarsalia was.
In 2001, the same clade was given the name Panaves (meaning "all [pan in Greek
] birds" [aves in Latin
]), coined by Jacques Gauthier. He defined it as the largest and most inclusive clade of archosaurs containing Aves (birds, anchored on Vultur gryphus) but not Crocodylia (anchored on Crocodylus niloticus). Gauthier referred Aves, all other Dinosauria, all Pterosauria, and a variety of Triassic
archosaurs including Lagosuchus
and Scleromochlus to this group.
Cladogram
after Nesbitt (2011):
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...
name established by British palaeontologist Michael Benton
Michael J. Benton
Michael J. Benton is a British paleontologist, a Fellow of the Royal Society of Edinburgh, and professor of vertebrate palaeontology in the Department of Earth Sciences at the University of Bristol....
in 1999 for all crown group archosaur
Archosaur
Archosaurs are a group of diapsid amniotes whose living representatives consist of modern birds and crocodilians. This group also includes all extinct non-avian dinosaurs, many extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most...
s that are closer to 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...
s than to 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. It includes a similarly defined subgroup, Ornithodira. An alternate name is Panaves, or "all birds", in reference to its definition containing all animals, living or extinct, which are more closely related to birds than to crocodiles.
Members of this group include the Dinosauromorpha
Dinosauromorpha
Dinosauromorpha is the name of a clade of archosaurs that includes the closest relatives of dinosaurs, and the order Dinosauria itself. Basal forms include Saltopus, Marasuchus, the perhaps identical Lagosuchus, the lagerpetonids Lagerpeton from the Ladinian of Argentina and Dromomeron from the...
, Pterosauromorpha, and the genus Scleromochlus
Scleromochlus
Scleromochlus is an extinct genus of small avemetatarsalian from the Late Triassic period. A lightly built cursorial animal, its phylogenetic position has been debated; as different analyses have found it to be either the basal-most ornithodiran, the sister-taxon to Pterosauria, or a basal member...
. Dinosauromorpha contains more basal forms like Lagerpeton
Lagerpeton
Lagerpeton is a genus of basal dinosauromorph from the Ladinian . Lagerpeton is known from several specimens of hindlimbs, hips, vertebrae, and feet. It was about 0.7 metres long and was found in the Chañares Formation of Argentina. It has unique feet, with an unusually long fourth toe...
and Marasuchus
Marasuchus
Marasuchus is a genus of dinosaur-like ornithodiran from the middle Triassic Chañares Formation of Argentina. The species Marasuchus lilloensis was originally described as a second species of Lagosuchus, L. lilloensis...
and more derived forms like dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s, which according to most modern scientists the birds belong to as members of the theropods
Theropoda
Theropoda is both a suborder of bipedal saurischian dinosaurs, and a clade consisting of that suborder and its descendants . Dinosaurs belonging to the suborder theropoda were primarily carnivorous, although a number of theropod groups evolved herbivory, omnivory, and insectivory...
. Pterosauromorpha contains Pterosauria
Pterosaur
Pterosaurs were flying reptiles of the clade or order Pterosauria. They existed from the late Triassic to the end of the Cretaceous Period . Pterosaurs are the earliest vertebrates known to have evolved powered flight...
, which are the famous flying reptiles, and as far as is known the first vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s capable of true flight
Flight
Flight is the process by which an object moves either through an atmosphere or beyond it by generating lift or propulsive thrust, or aerostatically using buoyancy, or by simple ballistic movement....
. Most researchers think pterosaurians had neither an S-curved neck, nor an upright gait
Gait
Gait is the pattern of movement of the limbs of animals, including humans, during locomotion over a solid substrate. Most animals use a variety of gaits, selecting gait based on speed, terrain, the need to maneuver, and energetic efficiency...
: the clade Ornithodira is defined on basis of ancestry, not characters – and it is always possible later descendants show a change in characters.
Origin
Bird-line archosaurs appear in the fossil record by the AnisianAnisian
In the geologic timescale, the Anisian is the lower stage or earliest age of the Middle Triassic series or epoch and lasted from 245 million years ago until 237 million years ago, approximately...
stage of the Middle Triassic
Middle Triassic
In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided. It spans the time between 245 ± 1.5 Ma and 228 ± 2 Ma...
about 245 million years ago, represented by the dinosauriform Asilisaurus
Asilisaurus
Asilisaurus , and Greek, σαυρος is an extinct genus of silesaurid archosaur. It is the oldest avian-line archosaur, dating to about 245 million years ago....
. However, Early Triassic
Early Triassic
The Early Triassic is the first of three epochs of the Triassic period of the geologic timescale. It spans the time between 251 ± 0.4 Ma and 245 ± 1.5 Ma . Rocks from this epoch are collectively known as the Lower Triassic, which is a unit in chronostratigraphy...
fossil footprints
Ichnite
An ichnite is a fossilised footprint. This is a type of trace fossil. Over the years, many ichnites have been found, around the world, giving important clues about the behaviour of the animals that made them...
reported in 2010 from the Świętokrzyskie (Holy Cross) Mountains
Swietokrzyskie Mountains
Świętokrzyskie Mountains , are a mountain range in central Poland, in the vicinity of the city of Kielce. The mountain range consists of a number of separate ranges, the highest of which is Łysogóry . The two highest peaks are Łysica at 612 meters and Łysa Góra at 593 meters...
of Poland
Poland
Poland , officially the Republic of Poland , is a country in Central Europe bordered by Germany to the west; the Czech Republic and Slovakia to the south; Ukraine, Belarus and Lithuania to the east; and the Baltic Sea and Kaliningrad Oblast, a Russian exclave, to the north...
may belong to a more primitive dinosauromorph. If so, the origin of avemetatarsalians would be pushed back into the early Olenekian
Olenekian
In the geologic timescale, the Olenekian is an age in the Early Triassic epoch or a stage in the Lower Triassic series. It spans the time between 249.7 ± 0.7 Ma and 245 ± 0.7 Ma . The Olenekian follows the Induan and is followed by the Anisian.The Olenekian saw the deposition of a large part of the...
age, around 249 Ma. The oldest Polish footprints are from a small quadrupedal animal named Prorotodactylus
Prorotodactylus
Prorotodactylus is an dinosauromorph ichnogenus known from fossilized footprints found in Poland and France. It may have been made by a dinosauromorph that was a precursor to the dinosaurs, possibly closely related to Lagerpeton. Fossils of Prorotodactylus date back to the early Olenekian stage of...
, but footprints belonging to the ichnogenus Sphingopus
Sphingopus
Sphingopus is an ichnogenus of dinosauriform footprints found in sediments dating to 250 and 228 Ma. The exact species which created Sphingopus tracks have not been identified.-Specimens:Sphingopus type footprints are known from two locations...
that have been found from Early Anisian strata show that moderately large bipedal dinosauromorphs had appeared by 246 Ma. The tracks show that the dinosaur lineage appeared soon after the Permian-Triassic extinction event
Permian-Triassic extinction event
The Permian–Triassic extinction event, informally known as the Great Dying, was an extinction event that occurred 252.28 Ma ago, forming the boundary between the Permian and Triassic geologic periods, as well as the Paleozoic and Mesozoic eras...
. Their age suggests that the rise of dinosaurs
Evolution of dinosaurs
Dinosaurs evolved from the archosaurs 232-234 Ma in the Ladinian age, the latter part of the middle Triassic. Dinosauria is a well-supported clade, present in 98% of bootstraps...
was slow and drawn out across much of the Triassic.
Classification
In 1986 Jacques GauthierJacques Gauthier
Jacques Armand Gauthier is a vertebrate paleontologist, comparative morphologist, and systematist, and one of the founders of the use of cladistics in biology....
coined the name Ornithodira for a node clade, containing the last common ancestor of the dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s and the pterosaurs and all of its descendants. Paul Sereno
Paul Sereno
Paul Callistus Sereno is an American paleontologist from the University of Chicago who discovered several new dinosaur species on several continents. He has conducted excavations at sites as varied as Inner Mongolia, Argentina, Morocco, and Niger...
had in 1991 given a formal (and different) definition of Ornithodira, one in which Scleromochlus was explicitly added. It was thus a potentially larger group than the Ornithodira of Gauthier. However, at that point, there was no named clade that could encompass species with a basal position on the archosaurian branch leading to dinosaurs (as opposed to that leading to crocodiles). In 1999 Michael Benton
Michael J. Benton
Michael J. Benton is a British paleontologist, a Fellow of the Royal Society of Edinburgh, and professor of vertebrate palaeontology in the Department of Earth Sciences at the University of Bristol....
concluded that Scleromochlus was indeed outside of Gauthier's original conception of Ornithodira, so he named a new branch-based clade for this purpose: Avemetatarsalia, named after the birds (Aves), the last surviving members of the clade, and the metatarsal ankle joint that was a typical character of the group. Avemetatarsalia was defined as: all Avesuchia closer to Dinosauria than to Crocodylia. In 2004 Benton gave a formal definition of Ornithodira sensu Gauthier. In 2005 Sereno stated the opinion that Ornithodira was not a useful concept, whereas Avemetatarsalia was.
In 2001, the same clade was given the name Panaves (meaning "all [pan in Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
] birds" [aves in Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
]), coined by Jacques Gauthier. He defined it as the largest and most inclusive clade of archosaurs containing Aves (birds, anchored on Vultur gryphus) but not Crocodylia (anchored on Crocodylus niloticus). Gauthier referred Aves, all other Dinosauria, all Pterosauria, and a variety of 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...
archosaurs including Lagosuchus
Lagosuchus
Lagosuchus is a genus of small archosaur from the middle Triassic period. It is generally thought to be closely related to dinosaurs, as a member of the Dinosauromorpha...
and Scleromochlus to this group.
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...
after Nesbitt (2011):
Further reading
- Michael J. BentonMichael J. BentonMichael J. Benton is a British paleontologist, a Fellow of the Royal Society of Edinburgh, and professor of vertebrate palaeontology in the Department of Earth Sciences at the University of Bristol....
: Origin and relationships of Dinosauria. In: David B. Weishampel, Peter Dodson, Halszka Osmólska (Hrsg.): The Dinosauria. Zweite Auflage. University of California Press, Berkeley 2004, S. 7-19, ISBN 0-520-24209-2.