Avisauridae is a
family
Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...
of
extinct
Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
enantiornithine
The Enantiornithes, also known as enantiornithines or enantiornitheans in literature, are a group of extinct avialans ("birds" in the broad sense), the most abundant and diverse group known from the Mesozoic era. Almost all retained teeth and cla ...
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 ...
from the
Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...
period, distinguished by several features of their ankle bones. Depending on the definition used, Avisauridae is either a broad and widespread group of advanced enantiornithines (following Cau & Arduini, 2008),
or a small family within that group, restricted to species from the Late Cretaceous of North and South America (following Chiappe, 1992).
Description
Avisaurids were among the largest and last enantiornithines to have lived, although they are also among the most poorly preserved. The majority of them are known primarily from fossilized
tarsometatarsal bones, the part of a bird's leg formed by fused
metatarsals
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 medi ...
(the bones which comprise the foot in humans). As a result, members of this family are distinguished from other enantiornithines exclusively by features of the tarsometatarsal and pedal
phalanges
The phalanges (singular: ''phalanx'' ) are digital bones in the hands and feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are classed as long bones.
...
(toe bones).
Unlike in some prehistoric birds, avisaurid tarsometatarsals were not completely fused, with the distal (outer) parts of the metatarsals being separate from each other. The proximal (near) half of metatarsal III (the long bone in the middle of the tarsometatarsus) is convex from the front. The inside edge of this bone's trochlea (toe joint) has a bony tab which points downward, known as a plantar projection. The innermost bone of the tarsometatarsus, metatarsal I, is small, laterally compressed (flattened from side-to-side), and J-shaped from the side. It is connected to a reversed hallux sporting a very large and curved claw.
[Chiappe, Luis M. (1992)]
Enantiornithine (Aves) Tarsometatarsi from the Cretaceous Lecho Formation of Northwestern Argentina
''Journal of Vertebrate Paleontology.'' September, 1992. 12 (3): 344-350
Chiappe and Calvo (1994) found that the Avisauridae shared adaptations of the foot — including a fully reversed and distally placed
hallux
Toes are the digits (fingers) of the foot of a tetrapod. Animal species such as cats that walk on their toes are described as being '' digitigrade''. Humans, and other animals that walk on the soles of their feet, are described as being '' pl ...
with a large claw — that indicated the ability to perch in trees. They argued that an arboreal habit was most likely for all of the Avisauridae.
[Chiappe, Luis M., Calvo, Jorge O. (1994)]
''Neuquenornis volans'', a New Late Cretaceous Bird (Enantiornithes: Avisauridae) from Patagonia, Argentina
. ''Journal of Vertebrate Paleontology''. June 22, 1994 Volume 14 No. 2 pp.230-246.
History and classification
Avisauridae was erected as a family by Michael Brett-Surman and
Gregory S. Paul
Gregory Scott Paul (born December 24, 1954) is an American freelance researcher, author and illustrator who works in paleontology, and more recently has examined sociology and theology. He is best known for his work and research on theropod dino ...
in 1985. At that time the family consisted of a few fossils that they believed belonged to small non-avian dinosaurs. They doubted that these fossils belonged to birds due to the presence of several features of the tarsometatarsus. In ''
Avisaurus
''Avisaurus'' (meaning "bird lizard") is a genus of enantiornithine bird from the Late Cretaceous of North America.
Discovery
''Avisaurus archibaldi'' was discovered in the Late Cretaceous Hell Creek Formation of North America (Maastrichtian, fr ...
'' (the only avisaurid known at that time), only the proximal parts of the metatarsals were fused, the proximal part of metatarsal III was wide, and the hypotarsus (a wide ridge extending down the back of the tarsometatarsus) was poorly developed.
However, Chiappe later reassigned the Avisauridae to the
class
Class or The Class may refer to:
Common uses not otherwise categorized
* Class (biology), a taxonomic rank
* Class (knowledge representation), a collection of individuals or objects
* Class (philosophy), an analytical concept used differentl ...
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 ...
(which is equivalent to the clade
Avialae
Avialae ("bird wings") is a clade containing the only living dinosaurs, the birds. It is usually defined as all theropod dinosaurs more closely related to birds (Aves) than to deinonychosaurs, though alternative definitions are occasionally used ...
in modern usage) and the
subclass Enantiornithes
The Enantiornithes, also known as enantiornithines or enantiornitheans in literature, are a group of extinct avialans ("birds" in the broad sense), the most abundant and diverse group known from the Mesozoic era. Almost all retained teeth and cla ...
in 1992. He noted that the features used to exclude avisaurids from birds are in fact present in some early birds such as ''
Archaeopteryx
''Archaeopteryx'' (; ), sometimes referred to by its German name, "" ( ''Primeval Bird''), is a genus of bird-like dinosaurs. The name derives from the ancient Greek (''archaīos''), meaning "ancient", and (''ptéryx''), meaning "feather" ...
'', as well as various Cretaceous bird groups. Avisaurids also had a thin metatarsal IV (the outermost long bone of the tarsometatarsus) and a bony knob on the front of metatarsal II (the most innermost long bone of the tarsometatarsus) for the insertion of ''
M. tibialis cranialis'' (the muscle which flexes the ankle, also known as the ''M. tibialis anticus'' or ''M. tibialis anterior''), both believed to be enantiornithean features.
Chiappe in 1993 defined the family as the common ancestor of ''
Neuquenornis volans'' and ''
Avisaurus archibaldi'' plus all its descendants. In 2008, the family was given a broader definition courtesy of Cau and Arduini. They redefined the group as ''
Avisaurus archibaldi'' and all
genera
Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...
more closely related to it than to either ''
Longipteryx
''Longipteryx'' is a genus of prehistoric bird which lived during the Early Cretaceous (Aptian stage, 120.3 million years ago). It contains a single species, ''Longipteryx chaoyangensis''. Its remains have been recovered from the Jiufotang Forma ...
'', ''
'' or ''
Sinornis
''Sinornis'' is a genus of enantiornithean birds from the Lower Cretaceous Jiufotang Formation of the People's Republic of China.
When it was described in 1992, this 120 million-year-old sparrow-sized skeleton represented a new avian sharing "pr ...
''.
Matt Martyniuk gave the name Avisauroidea to this group, although the erection of that name has been criticized by Cau.
Under this broader definition, several other enantiornitheans, such as ''
Enantiophoenix
''Enantiophoenix'' is a genus of enantiornithine birds. Fossil remains were recovered from Lebanon
Lebanon ( , ar, لُبْنَان, translit=lubnān, ), officially the Republic of Lebanon () or the Lebanese Republic, is a country in Wes ...
'', would qualify as members of the family. Nevertheless, enantiornithean taxonomy is notably difficult to resolve, and some analyses on enantiornitheans have not resolved the family.
However, this may be due to such analyses focusing on early Cretaceous enantiornitheans (which are numerous and well-preserved) rather than fragmentary late Cretaceous taxa, such as most avisaurids.
The following is a
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 ...
based on Cau and Arduini (2008):
References
{{Taxonbar, from=Q4703066
Cretaceous birds
Prehistoric dinosaur families