Avisauridae is a

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce 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 c ...

dinosaurs Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...

from the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

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 (: metatarsi) are a group of five long bones in the midfoot, located between the tarsal bones (which form the heel and the ankle) and the phalanges (toes). Lacking individual names, the metatarsal bones are nu ...

(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 (: phalanx ) are digit (anatomy), digital bones in the hands and foot, feet of most vertebrates. In primates, the Thumb, thumbs and Hallux, big toes have two phalanges while the other Digit (anatomy), digits have three phalanges. ...

(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 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 ''plantigrade''; ...

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. Their large size and tarso-metatarsus anatomy also indicate that they occupied a niche similar to modern

birds of prey Birds of prey or predatory birds, also known as (although not the same as) raptors, are hypercarnivorous bird species that actively predation, hunt and feed on other vertebrates (mainly mammals, reptiles and smaller birds). In addition to speed ...

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. He is best known for his work and research on theropoda, theropod dinosaurs and his detailed illustrations, both l ...

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'' (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, Classes, 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 d ...

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 lightweight ...

(which is equivalent to the clade

Avialae Avialae ("bird wings") is a clade containing the only living dinosaurs, the birds, and their closest relatives. It is usually defined as all theropod dinosaurs more closely related to birds (Aves) than to Deinonychosauria, deinonychosaurs, though ...

in modern usage) and the subclass

Enantiornithes The Enantiornithes, also known as enantiornithines or enantiornitheans in literature, are a group of extinct Avialae, avialans ("birds" in the broad sense), the most abundant and diverse group known from the Mesozoic era. Almost all retained teet ...

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 (''archaîos''), 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 (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...

more closely related to it than to either '' Longipteryx'', '' Gobipteryx'' or '' Sinornis''. 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'', 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. Another member of Avisauridae is, Elsornis, due to its pectoral skeleton, it's likely that it could not fly, however, it may have glided like the

. The following is a

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

based on Cau and Arduini (2008): A 2024 study reduces the group to just six taxa: MOR 3070, '' Soroavisaurus'', two '' Avisaurus'' species, '' Mirarce'' and '' Gettyia''.Clark AD, Atterholt J, Scannella JB, Carroll N, O’Connor JK (2024) New enantiornithine diversity in the Hell Creek Formation and the functional morphology of the avisaurid tarsometatarsus. PLoS ONE 19(10): e0310686. https://doi.org/10.1371/journal.pone.0310686

References

{{Taxonbar, from=Q4703066 Dinosaur families Cretaceous dinosaurs