Avemetatarsalia (meaning "bird
metatarsal
The metatarsal bones, or metatarsus, are a group of five long bones in the foot, located between the tarsal bones of the hind- and mid-foot and the phalanges of the toes. Lacking individual names, the metatarsal bones are numbered from the med ...
s") is a
clade
A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...
of
diapsid
Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The group first appeared about three hundred million years ago ...
reptiles
Reptiles, as most commonly defined are the animals in the Class (biology), class Reptilia ( ), a paraphyletic grouping comprising all sauropsid, sauropsids except birds. Living reptiles comprise turtles, crocodilians, Squamata, squamates (lizar ...
containing all
archosaur
Archosauria () is a clade of diapsids, with birds and crocodilians as the only living representatives. Archosaurs are broadly classified as reptiles, in the cladistic sense of the term which includes birds. Extinct archosaurs include non-avian d ...
s more closely related to
bird
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...
s than to
crocodilia
Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period ( Cenomanian stage) and are the closest living ...
ns.
The two most successful groups of avemetatarsalians were the dinosaurs and pterosaurs.
Dinosaurs
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
were the largest terrestrial animals for much of the
Mesozoic Era
The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising ...
, and one group of small feathered dinosaurs (Aves, i.e. birds) has survived up to the present day.
Pterosaurs
Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 to ...
were the first flying vertebrates and persisted through the Mesozoic before dying out at the
Cretaceous-Paleogene (K-Pg) extinction event. Both dinosaurs and pterosaurs appeared in the
Triassic Period
The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...
, shortly after avemetatarsalians as a whole.
The name Avemetatarsalia was first established by British palaeontologist
Michael Benton
Michael James Benton One or more of the preceding sentences incorporates text from the royalsociety.org website where: (born 8 April 1956) is a British palaeontologist, and professor of vertebrate palaeontology in the School of Earth Sciences ...
in 1999. 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 crocodilians.
Although dinosaurs and pterosaurs were the only avemetatarsalians to survive past the end of the Triassic, other groups flourished during the Triassic. The most
basal (earliest-branching) and
plesiomorphic
In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral character shared by all members of a clade, which does not distinguish the clade from other clades.
Plesiomorphy, symplesiomorphy, apomorphy, and ...
("primitive") known avemetatarsalians were the
aphanosaurs. Aphanosaurs were rare, four-legged carnivores which were only properly distinguished as a group in 2017.
The split between dinosaurs and pterosaurs occurred just after aphanosaurs branched off the archosaur family tree. This split corresponds to the subgroup Ornithodira, defined as the
last common ancestor
In biology and genetic genealogy, the most recent common ancestor (MRCA), also known as the last common ancestor (LCA) or concestor, of a set of organisms is the most recent individual from which all the organisms of the set are descended. The ...
of dinosaurs and pterosaurs, and all of its descendants. Until the discovery of aphanosaurs, Ornithodira and Avemetatarsalia were considered roughly equivalent concepts.
Pterosauromorpha includes all avemetatarsalians closer to pterosaurs than to dinosaurs. True non-pterosaur pterosauromorphs have been historically difficult to determine. Small, insectivorous archosaurs of the family
Lagerpetidae
Lagerpetidae (; originally Lagerpetonidae) is a family of basal avemetatarsalians. Though traditionally considered the earliest-diverging dinosauromorphs (reptiles closer to dinosaurs than to pterosaurs), fossils described in 2020 suggest that ...
may potentially be examples, alongside the similar genus ''
Scleromochlus
''Scleromochlus'' (from el, σκληρός , 'hard' and el, μοχλός , 'lever') is an extinct genus of small pterosauromorph archosaurs from the Late Triassic period. The genus contains the type and only species ''Scleromochlus taylori'', ...
''.
Dinosauromorpha
Dinosauromorpha is a clade of avemetatarsalian archosaurs (reptiles closer to birds than to crocodilians) that includes the Dinosauria (dinosaurs) and some of their close relatives. It was originally defined to include dinosauriforms and lager ...
, on the other hand, includes all avemetatarsalians closer to dinosaurs than to pterosaurs. Probable non-dinosaur dinosauromorphs include the diverse and widespread
silesaurids
Silesauridae is an extinct family of Triassic dinosauriforms. It is most commonly considered to be a clade of non-dinosaur dinosauriforms, and the sister group of dinosaurs. Some studies have instead suggested that most or all silesaurids compri ...
, as well as more controversial and fragmentary taxa such as ''
Marasuchus
''Marasuchus'' (meaning " Mara crocodile") is a genus of basal dinosauriform archosaur which is possibly synonymous with ''Lagosuchus''. Both genera lived during the Late Triassic in what is now La Rioja Province, Argentina. ''Marasuchus'' cont ...
'', ''
Lagosuchus
''Lagosuchus'' is an extinct genus of avemetatarsalian archosaur from the Late Triassic of Argentina. The type species of ''Lagosuchus'', ''Lagosuchus talampayensis,'' is based on a small partial skeleton recovered from the early Carnian-age Cha ...
'', ''
Nyasasaurus
''Nyasasaurus'' (meaning " Lake Nyasa lizard") is an extinct genus of avemetatarsalian archosaur from the putatively Middle Triassic Manda Formation of Tanzania that may be the earliest known dinosaur. The type species ''Nyasasaurus parringtoni' ...
'', and ''
Saltopus
''Saltopus'' ("hopping foot") is a genus of very small bipedal dinosauriform containing the single species ''Saltopus elginensis'' from the late Triassic period of Scotland. It is one of the most famous Elgin Reptiles.
Description
''Saltopus ...
''. Lagerpetids were also traditionally considered dinosauromorphs,
though this has been more recently debated.
Description
![AdvMesotarsal](https://upload.wikimedia.org/wikipedia/commons/4/4b/AdvMesotarsal.png)
The foundational characteristic is the "advanced mesotarsal" ankles, which are characterized by a large
astragalus
''Astragalus'' is a large genus of over 3,000 species of herbs and small shrubs, belonging to the legume family Fabaceae and the subfamily Faboideae. It is the largest genus of plants in terms of described species. The genus is native to tempe ...
and a small
calcaneum
In humans and many other primates, the calcaneus (; from the Latin ''calcaneus'' or ''calcaneum'', meaning heel) or heel bone is a bone of the tarsus of the foot which constitutes the heel. In some other animals, it is the point of the hock.
St ...
. This ankle orientation operated on a single hinge, allowing for better mobility. Probably as a result of this change, the common ancestor of the avemetatarsalians had an upright, bipedal posture, with their legs extending vertically, similar to that of mammals.
Feather
Feathers are epidermal growths that form a distinctive outer covering, or plumage, on both avian (bird) and some non-avian dinosaurs and other archosaurs. They are the most complex integumentary structures found in vertebrates and a premier ...
s and other filamentary structures are known across the avemetatarsalians, from the downy pycnofibers of pterosaurs, to quill-like structures present in
ornithischia
Ornithischia () is an extinct order of mainly herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds. The name ''Ornithischia'', or "bird-hipped", reflects this similarity and is derived from the Greek s ...
n dinosaurs, such as ''
Psittacosaurus
''Psittacosaurus'' ( ; "parrot lizard") is a genus of extinct ceratopsian dinosaur from the Early Cretaceous of what is now Asia, existing between 126 and 101 million years ago. It is notable for being the most species-rich non-avian dinosaur gen ...
'' and ''
Tianyulong
''Tianyulong'' (Chinese: 天宇龍; Pinyin: ''tiānyǔlóng''; named for the Shandong Tianyu Museum of Nature where the holotype fossil is housed) is an extinct genus of heterodontosaurid ornithischian dinosaur. The only species is ''T. confuc ...
'', to feathers in theropod dinosaurs and their descendants, birds.
Two clades of avemetatarsalians, pterosaurs and birds, independently evolved flight. Pterosaurs are the earliest
vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...
s known to have evolved powered flight. Their wings are formed by a membrane of skin, muscle, and other
tissues stretching from the ankles to a dramatically lengthened fourth finger.
Birds evolved flight much later. Their wings formed from elongated fingers and their arms, all covered with
flight feathers
Flight feathers (''Pennae volatus'') are the long, stiff, asymmetrically shaped, but symmetrically paired pennaceous feathers on the wings or tail of a bird; those on the wings are called remiges (), singular remex (), while those on the tail ...
.
Avemetatarsalians were generally more lightly built than
crocodile-line archosaurs. They had smaller heads and usually a complete lack of
osteoderm
Osteoderms are bony deposits forming scales, plates, or other structures based in the dermis. Osteoderms are found in many groups of extant and extinct reptiles and amphibians, including lizards, crocodilians, frogs, temnospondyls (extinct amp ...
s.
Origin
Bird-line archosaurs appear in the fossil record by the
Anisian
In the geologic timescale, the Anisian is the lower stage or earliest age of the Middle Triassic series or epoch and lasted from million years ago until million years ago. The Anisian Age succeeds the Olenekian Age (part of the Lower Triassic Ep ...
stage of the
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 in chronostratigraphy. The Middle Triassic spans the time between Ma and ...
about 245 million years ago, represented by the
dinosauriform
Dinosauromorpha is a clade of avemetatarsalian archosaurs (reptiles closer to birds than to crocodilians) that includes the Dinosauria (dinosaurs) and some of their close relatives. It was originally defined to include dinosauriforms and lager ...
''
Asilisaurus
''Asilisaurus'' ( ); from Swahili, ''asili'' ("ancestor" or "foundation"), and Greek, (, "lizard") is an extinct genus of silesaurid archosaur. The type species is ''Asilisaurus kongwe.'' ''Asilisaurus'' fossils were uncovered in the Manda Beds ...
''. However,
Early Triassic
The Early Triassic is the first of three epochs of the Triassic Period of the geologic timescale. It spans the time between Ma and Ma (million years ago). Rocks from this epoch are collectively known as the Lower Triassic Series, which is a un ...
fossil footprints
A fossil track or ichnite (Ancient Greek, Greek "''ιχνιον''" (''ichnion'') – a track, trace or footstep) is a fossilized footprint. This is a type of trace fossil. A fossil trackway is a sequence of fossil tracks left by a single organism ...
reported in 2010 from the
Świętokrzyskie (Holy Cross) Mountains of
Poland
Poland, officially the Republic of Poland, is a country in Central Europe. It is divided into 16 administrative provinces called voivodeships, covering an area of . Poland has a population of over 38 million and is the fifth-most populous ...
may belong to a more primitive
dinosauromorph
Dinosauromorpha is a clade of avemetatarsalian archosaurs (reptiles closer to birds than to crocodilians) that includes the Dinosauria (dinosaurs) and some of their close relatives. It was originally defined to include dinosauriforms and lag ...
.
If so, the origin of avemetatarsalians would be pushed back into the early
Olenekian
In the geologic timescale, the Olenekian is an age in the Early Triassic epoch; in chronostratigraphy, it is a stage in the Lower Triassic series. It spans the time between Ma and Ma (million years ago). The Olenekian is sometimes divided into ...
age, around 249 Ma. The oldest Polish footprints are classified in the
ichnogenus
An ichnotaxon (plural ichnotaxa) is "a taxon based on the fossilized work of an organism", i.e. the non-human equivalent of an artifact. ''Ichnotaxa'' comes from the Greek ίχνος, ''ichnos'' meaning ''track'' and ταξις, ''taxis'' meaning ...
''
Prorotodactylus
''Prorotodactylus'' is a dinosauromorph or pterosauromorph ichnogenus known from fossilized footprints found in Poland and France. The prints may have been made by a dinosauromorph that was a precursor to the dinosaurs, possibly closely related ...
'' and were made by an unknown small quadrupedal animal, but footprints called ''
Sphingopus'', 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
The Permian–Triassic (P–T, P–Tr) extinction event, also known as the Latest Permian extinction event, the End-Permian Extinction and colloquially as the Great Dying, formed the boundary between the Permian and Triassic geologic periods, as ...
. Their age suggests that the
rise of dinosaurs was slow and drawn out across much of the Triassic.
The primitive traits found in the
quadrupedal
Quadrupedalism is a form of locomotion where four limbs are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four limbs is said to be a quadruped (from Latin ''quattuor' ...
aphanosaur ''
Teleocrater
''Teleocrater'' (meaning "completed basin", in reference to its closed acetabulum) is a genus of avemetatarsalian archosaur from the Middle Triassic Manda Formation of Tanzania. The name was coined by English paleontologist Alan Charig in his ...
'' shows that the earliest avemetatarsalians had many pseudosuchian-like features, and that the traits typical for the group evolved later.
Classification
In 1986,
Jacques Gauthier
Jacques Armand Gauthier (born June 7, 1948 in New York City) is an American vertebrate paleontologist, comparative morphologist, and systematist, and one of the founders of the use of cladistics in biology.
Life and career
Gauthier is the so ...
defined the name Ornithosuchia (previously coined by
Huene) for a branch-based clade including all archosaurs more closely related to birds than to crocodiles.
In the same year, Gauthier also coined and defined a slightly more restrictive node-based clade, Ornithodira, containing the last common ancestor of the
dinosaur
Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
s and the
pterosaurs
Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 to ...
and all of its descendants.
Paul Sereno
Paul Callistus Sereno (born October 11, 1957) is a professor of paleontology at the University of Chicago and a National Geographic "explorer-in-residence" who has discovered several new dinosaur species on several continents, including at sites ...
in 1991 gave a different definition of Ornithodira, one in which ''
Scleromochlus
''Scleromochlus'' (from el, σκληρός , 'hard' and el, μοχλός , 'lever') is an extinct genus of small pterosauromorph archosaurs from the Late Triassic period. The genus contains the type and only species ''Scleromochlus taylori'', ...
'' was explicitly added.
[Sereno, P. C. 1991. Basal archosaurs: phylogenetic relationships and functional implications. ''Journal of Vertebrate Paleontology'' ''Memoir 2'', 11(4, Supplement):1–53.] It was thus a potentially larger group than the Ornithodira of Gauthier. In 1999
Michael Benton
Michael James Benton One or more of the preceding sentences incorporates text from the royalsociety.org website where: (born 8 April 1956) is a British palaeontologist, and professor of vertebrate palaeontology in the School of Earth Sciences ...
concluded that ''Scleromochlus'' was indeed outside Gauthier's original conception of Ornithodira, so he named a new
branch-based clade for this purpose: Avemetatarsalia, named after the birds (
Aves
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...
), the last surviving members of the clade, and the metatarsal
ankle joint
The ankle, or the talocrural region, or the jumping bone (informal) is the area where the foot and the leg meet. The ankle includes three joints: the ankle joint proper or talocrural joint, the subtalar joint, and the inferior tibiofibular join ...
that was a typical character of the group. Avemetatarsalia was defined as all Avesuchia closer to Dinosauria than to
Crocodylia
Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period (Cenomanian stage) and are the closest living ...
. 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" (, from Greek + 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
The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...
archosaurs, including ''
Lagosuchus
''Lagosuchus'' is an extinct genus of avemetatarsalian archosaur from the Late Triassic of Argentina. The type species of ''Lagosuchus'', ''Lagosuchus talampayensis,'' is based on a small partial skeleton recovered from the early Carnian-age Cha ...
'' and ''Scleromochlus'', to this group.
[Gauthier, J. and de Queiroz, K. (2001). "Feathered dinosaurs, flying dinosaurs, crown dinosaurs, and the name "Aves"". pp. 7–41 in Gauthier, J. and L. F. Gall (eds.), ''New Perspectives on the Origin and Early Evolution of Birds: Proceedings of the International Symposium in Honor of John H. Ostrom''. New Haven: Peabody Museum of Natural History, Yale University. .]
In a 2005 review of archosaur classification, Phil Senter attempted to resolve this conflicting set of terminology by applying strict priority to names based on when and how they were first defined. Senter noted that Ornithosuchia, the earliest name used for the total group of archosaurs closer to birds than to crocodiles, should be the valid name for that group, and have precedence over later names with identical definitions, such as Avemetatarsalia and Pan-Aves. While this has been followed by some researchers, others have either continued to use Avemetatarsalia or Ornithodira, or have followed Senter only reluctantly. Mike Taylor (2007) for example noted that, while Senter is correct in stating that Ornithosuchia has priority, this is "undesirable" because it probably excludes the eponymous family
Ornithosuchidae
Ornithosuchidae is an extinct family of pseudosuchian archosaurs (distant relatives of modern crocodilians) from the Triassic period. Ornithosuchids were quadrupedal and facultatively bipedal (e.g. like chimpanzees), meaning that they had the abi ...
, and questioned the utility of using priority before the
PhyloCode
The ''International Code of Phylogenetic Nomenclature'', known as the ''PhyloCode'' for short, is a formal set of rules governing phylogenetic nomenclature. Its current version is specifically designed to regulate the naming of clades, leaving the ...
is implemented to govern it.
In fact, the name Ornithosuchia may be "illegal" under the PhyloCode because it does not include its eponymous taxon as part of its definition.
[
]Cladogram
A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to d ...
after Nesbitt ''et al.'' (2017), with clade names from Cau (2018).
Kammerer ''et al''. (2020) and Ezcurra ''et al''. (2020) supported an alternative hypothesis regarding the relationships of lagerpetids. They were interpreted as non-pterosaur pterosauromorphs. This phylogeny would shorten the morphological and chronological gap perceived between pterosaurs and other stem-birds, and explain the origin of this group. Bennett (2020) argued that ''Scleromochlus'', a genus historically considered a relative of ornithodirans or even a basal pterosauromorph, was instead a non-archosaur archosauriform (possibly a doswelliid
Doswelliidae is an extinct family of carnivorous archosauriform reptiles that lived in North America and Europe during the Middle to Late Triassic period. Long represented solely by the heavily-armored reptile ''Doswellia'', the family's composit ...
).
References
Sources
*
{{Taxonbar, from=Q133187
Anisian first appearances
Extant Middle Triassic first appearances
Taxa named by Michael Benton