Alvarezsauridae
Encyclopedia
Alvarezsauridae is an enigmatic family
of small, long-legged running dinosaur
s. Although originally thought to represent the earliest known flightless bird
s, a consensus of recent work suggests that they are primitive members of the Maniraptora
. Other work found them to be the sister group to the Ornithomimosauria
. Alvarezsaurs are highly specialized. They bear tiny but stoutly proportioned forelimbs with compact birdlike hands and their skeleton suggests they had massive breast and arm muscles, possibly adapted for digging or tearing. They have tubular snouts, elongate jaws, and minute teeth. They may have been adapted to prey on colonial insects such as termites.
Alvarezsaurus, and thus Alvarezsauroidea, Alvarezsauridae, and Alvarezsauria are named for the historian
Don Gregorio Alvarez, not the more familiar physicist
Luis Alvarez, who proposed that the Cretaceous-Tertiary extinction event
was caused by an impact event
.
that may have reached 2.5 m (8 ft). Fossils attributed to alvarezsaurs have been found in North
and South America
and Asia
, and range in age from the late Jurassic
to the late Cretaceous
periods, about 160 to 65 million years ago.
Shuvuuia deserti
, preserved down-like, feather
y, integumental structures covering the fossil. Schweitzer et al. (1999) subjected these filaments to microscopic, morphological, mass spectrometric, and immunohistochemical studies and found that they consisted of beta-keratin
, which is the primary protein in feather
s.
.
Perle et al. (1993) described the next alvarezsaur to be discovered, naming it Mononychus olecranus (meaning “one claw”). A month later they changed the genus name to Mononykus
, because the earlier spelling was already the genus name of an extant butterfly.
Perle et al. (1993) mistakenly described Mononykus
as a member of Avialae
, and one more advanced than Archaeopteryx
. They argued that the family Alvarezsauridae was actually a group of Mesozoic flightless birds on the basis of derived features that were unique to birds.
Novas (1996) described another member of the class called Patagonykus
puertai. Karhu and Rautian (1996) described a Mongolian member of the family; Parvicursor
remotus. Chiappe et al.(1998) described another Mongolian member, Shuvuuia
mongoliensis and mistakenly found it to be even more derived, concluding that the alvarezsaurs were actually crown-group (that is, modern) birds.
These mistaken assignments of alvarezsaurs to birds were caused primarily by features that are strikingly, or even uniquely, avian. The sternum, for example, is elongated and deeply keeled for an enlarged pectoralis muscle, as it is in neognathous birds and volant
ratite
s. One bone in the skull of Shuvuuia
appeared to be an ectethmoid fused to a prefrontal. The ectethmoid is an ossification known only in Neornithes. Other birdlike characters included the palatine, foramen magnum, cervical and caudal vertebrae, and many others.
Several researchers disagreed with Perle et al. (1993) and Chiappe et al. (1998). Feduccia (1994), Ostrom (1994), Wellnhofer (1994), Kurochkin (1995), Zhou (1995), and Sereno (1997) considered it unlikely that alvarezsaurids were members of Avialae
. Martin (1997) performed a cladistic analysis but Sereno criticized it strongly, finding it flawed by incorrect codings, use of only select data, and results that did not support his conclusions. Sereno (1999) performed a new analysis, revising the anatomical interpretations and clarifying the characters. He found that alvarezsaurids were more parsimoniously related to the Ornithomimosauria
.
As the more primitive members of the Alvarezsauridae were better characterized, the monophyly of the clade was strongly supported, but the more primitive members lacked the most birdlike traits. Some of these traits had been misinterpreted, also. The remaining similarities between birds and alvarezsaurs, like the keeled sterna, are another case of homoplasy; where the derived alvarezsaurids developed birdlike characters through convergent evolution, rather than inheriting them from a common ancestor with birds.
Holtz, 2011
Turner et al. (2007) place the alvarezsaurs as the most basal group in the maniraptora
, one step more derived than Ornitholestes
and two more derived than the Ornithomimosauria
. The alvarezsaurs are more primitive than the Oviraptorosauria
.
Novas' 1996 description of Patagonykus
, demonstrated that it was a link between the more primitive (basal) Alvarezsaurus and the more advanced (derived) Mononykus, and reinforced their monophyly. Parvicursor
was discovered shortly after, and placed in its own family Parvicursoridae, and then Shuvuuia in 1998. Everything has since been lumped into Alvarezsauridae, with Mononykinae surviving as a subfamily.
There may be a relationship between the alvarezsaurids and the Ornithomimosauria
as sister clades within either Thomas Holtz's Arctometatarsalia or Paul Sereno
's Ornithomimiformes. The discovery of Haplocheirus
, which exhibits transitional features between the more derived alvarezsaurs and other maniraptora
ns, particularly in relation to the skull structure and development of the hand, has provided further support for that relationship.
The taxonomy of the alvarezsaurs has been somewhat confused, due to different authors using different names for groups with the same definition. The family
Alvarezsauridae was first coined by Jose Bonaparte in 1991, but given no specific phylogenetic definition. Novas later defined the group as the most recent common ancestor of Alvarezsaurus
and Mononykus
plus all its descendants, though others, such as Paul Sereno
, used a more inclusive definition, such as all dinosaurs closer to Shuvuuia
than to modern birds. In 2009, Livezey and Zusi used the name Alvarezsauroidea for the total group of all alvarezsaurs, restricting the name Alvarezsauridae to the clade defined by Alvarezsaurus + Mononykus. This was followed by Choiniere and colleagues in 2010, who described the first non-alvarezsaurid alvarezsauroid, Haplocheirus
. Some authors have used the name Mononykinae for the sub-group of alvarezsaurs including the advanced Mongolian species. However, Choiniere and colleagues argued that Parvicursorinae has priority, since its coordinate name under the ICZN Code
, Parvicursoridae, was named earlier.
The cladogram
presented here follows a 2009 phylogenetic analysis by Nick Longrich and Phil Currie
.
Cladogram
after Xu et al., 2011:
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
of small, long-legged running 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. Although originally thought to represent the earliest known flightless 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, a consensus of recent work suggests that they are primitive members of the Maniraptora
Maniraptora
Maniraptora is a clade of coelurosaurian dinosaurs which includes the birds and the dinosaurs that were more closely related to them than to Ornithomimus velox. It contains the major subgroups Avialae, Deinonychosauria, Oviraptorosauria and Therizinosauria. Ornitholestes and the Alvarezsauroidea...
. Other work found them to be the sister group to the Ornithomimosauria
Ornithomimosauria
The Ornithomimosauria, ornithomimosaurs or ostrich dinosaurs were theropod dinosaurs which bore a superficial resemblance to modern ostriches. They were fast, omnivorous or herbivorous dinosaurs from the Cretaceous Period of Laurasia...
. Alvarezsaurs are highly specialized. They bear tiny but stoutly proportioned forelimbs with compact birdlike hands and their skeleton suggests they had massive breast and arm muscles, possibly adapted for digging or tearing. They have tubular snouts, elongate jaws, and minute teeth. They may have been adapted to prey on colonial insects such as termites.
Alvarezsaurus, and thus Alvarezsauroidea, Alvarezsauridae, and Alvarezsauria are named for the historian
Historian
A historian is a person who studies and writes about the past and is regarded as an authority on it. Historians are concerned with the continuous, methodical narrative and research of past events as relating to the human race; as well as the study of all history in time. If the individual is...
Don Gregorio Alvarez, not the more familiar physicist
Physicist
A physicist is a scientist who studies or practices physics. Physicists study a wide range of physical phenomena in many branches of physics spanning all length scales: from sub-atomic particles of which all ordinary matter is made to the behavior of the material Universe as a whole...
Luis Alvarez, who proposed that the Cretaceous-Tertiary extinction event
Cretaceous-Tertiary extinction event
The Cretaceous–Paleogene extinction event, formerly named and still commonly referred to as the Cretaceous-Tertiary extinction event, occurred approximately 65.5 million years ago at the end of the Maastrichtian age of the Cretaceous period. It was a large-scale mass extinction of animal and plant...
was caused by an impact event
Impact event
An impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Throughout recorded history, hundreds of minor impact events have been reported, with some occurrences causing deaths, injuries, property damage or other significant...
.
Description
Alvarezsaurs range from 0.5–2 m (20–80 inches) in length, although some possible members may have been substantially larger, including the European HeptasteornisHeptasteornis
Heptasteornis is the name given to a dubious genus of small dinosaur from the Late Cretaceous. The type species is Heptasteornis andrewsi, described as a presumed gigantic prehistoric owl in 1975...
that may have reached 2.5 m (8 ft). Fossils attributed to alvarezsaurs have been found in North
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
and South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
and Asia
Asia
Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
, and range in age from the late Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
to the late Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
periods, about 160 to 65 million years ago.
Feathers
At least one specimen of alvarezsaur, from the speciesSpecies
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
Shuvuuia deserti
Shuvuuia
Shuvuuia is a genus of bird-like theropod dinosaur from the late Cretaceous period of Mongolia. It is a member of the family Alvarezsauridae, small coelurosaurian dinosaurs which are characterized by short but powerful forelimbs specialized for digging. The type species is Shuvuuia deserti, or...
, preserved down-like, feather
Feather
Feathers are one of the epidermal growths that form the distinctive outer covering, or plumage, on birds and some non-avian theropod dinosaurs. They are considered the most complex integumentary structures found in vertebrates, and indeed a premier example of a complex evolutionary novelty. They...
y, integumental structures covering the fossil. Schweitzer et al. (1999) subjected these filaments to microscopic, morphological, mass spectrometric, and immunohistochemical studies and found that they consisted of beta-keratin
Beta-keratin
β-keratin or beta-keratin is rich in stacked β pleated sheets, in contrast to alpha-keratin, a fibrous protein rich in alpha helices.β-keratin is found in reptiles...
, which is the primary protein in feather
Feather
Feathers are one of the epidermal growths that form the distinctive outer covering, or plumage, on birds and some non-avian theropod dinosaurs. They are considered the most complex integumentary structures found in vertebrates, and indeed a premier example of a complex evolutionary novelty. They...
s.
History, study and debate
Bonaparte (1991) described the first alvarezsaurid, Alvarezsaurus calvoi, from an incomplete skeleton found in Patagonia, Argentina. Bonaparte also erected a monotypic family to contain it. He argued that it might be most closely related to the OrnithomimosauriaOrnithomimosauria
The Ornithomimosauria, ornithomimosaurs or ostrich dinosaurs were theropod dinosaurs which bore a superficial resemblance to modern ostriches. They were fast, omnivorous or herbivorous dinosaurs from the Cretaceous Period of Laurasia...
.
Perle et al. (1993) described the next alvarezsaur to be discovered, naming it Mononychus olecranus (meaning “one claw”). A month later they changed the genus name to Mononykus
Mononykus
Mononykus was a theropod dinosaur from late Cretaceous Mongolia with long, skinny legs. It moved about on two legs, was very nimble, and could run at high speeds, something that would have been useful in the open desert plains where it lived...
, because the earlier spelling was already the genus name of an extant butterfly.
Perle et al. (1993) mistakenly described Mononykus
Mononykus
Mononykus was a theropod dinosaur from late Cretaceous Mongolia with long, skinny legs. It moved about on two legs, was very nimble, and could run at high speeds, something that would have been useful in the open desert plains where it lived...
as a member of Avialae
Avialae
Avialae is a clade of dinosaurs containing their only living representatives, birds , and the most immediate extinct relatives of birds.-Competing definitions:...
, and one more advanced than Archaeopteryx
Archaeopteryx
Archaeopteryx , sometimes referred to by its German name Urvogel , is a genus of theropod dinosaur that is closely related to birds. The name derives from the Ancient Greek meaning "ancient", and , meaning "feather" or "wing"...
. They argued that the family Alvarezsauridae was actually a group of Mesozoic flightless birds on the basis of derived features that were unique to birds.
Novas (1996) described another member of the class called Patagonykus
Patagonykus
Patagonykus is a genus of theropod dinosaur from the Upper Cretaceous of Argentina. This alvarezsaur was discovered in exposures of the Portezuelo Member of the Rio Neuquen Formation in the Neuquen Province of Argentina...
puertai. Karhu and Rautian (1996) described a Mongolian member of the family; Parvicursor
Parvicursor
Parvicursor is a genus of tiny maniraptoran dinosaur with long slender legs for fast running. At only about 39 cm from snout to end of tail, and 162 grams in weight, it is one of the smallest non-avian dinosaurs known from an adult specimen.Like other members of the family Alvarezsauridae,...
remotus. Chiappe et al.(1998) described another Mongolian member, Shuvuuia
Shuvuuia
Shuvuuia is a genus of bird-like theropod dinosaur from the late Cretaceous period of Mongolia. It is a member of the family Alvarezsauridae, small coelurosaurian dinosaurs which are characterized by short but powerful forelimbs specialized for digging. The type species is Shuvuuia deserti, or...
mongoliensis and mistakenly found it to be even more derived, concluding that the alvarezsaurs were actually crown-group (that is, modern) birds.
These mistaken assignments of alvarezsaurs to birds were caused primarily by features that are strikingly, or even uniquely, avian. The sternum, for example, is elongated and deeply keeled for an enlarged pectoralis muscle, as it is in neognathous birds and volant
Flying and gliding animals
A number of animals have evolved aerial locomotion, either by powered flight or by gliding. Flying and gliding animals have evolved separately many times, without any single ancestor. Flight has evolved at least four times, in the insects, pterosaurs, birds, and bats. Gliding has evolved on many...
ratite
Ratite
A ratite is any of a diverse group of large, flightless birds of Gondwanan origin, most of them now extinct. Unlike other flightless birds, the ratites have no keel on their sternum—hence the name from the Latin ratis...
s. One bone in the skull of Shuvuuia
Shuvuuia
Shuvuuia is a genus of bird-like theropod dinosaur from the late Cretaceous period of Mongolia. It is a member of the family Alvarezsauridae, small coelurosaurian dinosaurs which are characterized by short but powerful forelimbs specialized for digging. The type species is Shuvuuia deserti, or...
appeared to be an ectethmoid fused to a prefrontal. The ectethmoid is an ossification known only in Neornithes. Other birdlike characters included the palatine, foramen magnum, cervical and caudal vertebrae, and many others.
Several researchers disagreed with Perle et al. (1993) and Chiappe et al. (1998). Feduccia (1994), Ostrom (1994), Wellnhofer (1994), Kurochkin (1995), Zhou (1995), and Sereno (1997) considered it unlikely that alvarezsaurids were members of Avialae
Avialae
Avialae is a clade of dinosaurs containing their only living representatives, birds , and the most immediate extinct relatives of birds.-Competing definitions:...
. Martin (1997) performed a cladistic analysis but Sereno criticized it strongly, finding it flawed by incorrect codings, use of only select data, and results that did not support his conclusions. Sereno (1999) performed a new analysis, revising the anatomical interpretations and clarifying the characters. He found that alvarezsaurids were more parsimoniously related to the Ornithomimosauria
Ornithomimosauria
The Ornithomimosauria, ornithomimosaurs or ostrich dinosaurs were theropod dinosaurs which bore a superficial resemblance to modern ostriches. They were fast, omnivorous or herbivorous dinosaurs from the Cretaceous Period of Laurasia...
.
As the more primitive members of the Alvarezsauridae were better characterized, the monophyly of the clade was strongly supported, but the more primitive members lacked the most birdlike traits. Some of these traits had been misinterpreted, also. The remaining similarities between birds and alvarezsaurs, like the keeled sterna, are another case of homoplasy; where the derived alvarezsaurids developed birdlike characters through convergent evolution, rather than inheriting them from a common ancestor with birds.
Classification
- Alvarezsauridae
- AchillesaurusAchillesaurusAchillesaurus is a genus of alvarezsaurid theropod dinosaur from the Santonian-age Upper Cretaceous Bajo de la Carpa Formation of Rio Negro, Argentina. It was a relatively large, basal alvarezsaurid, and a contemporary of Alvarezsaurus...
- AlvarezsaurusAlvarezsaurusAlvarezsaurus is a genus of small alvarezsaurid dinosaur from the Late Cretaceous period of Argentina, approximately 86 - 83 million years ago. Estimates suggest that it measured about 2 meters in length and weighed approximately 20 kg...
- BradycnemeBradycnemeBradycneme is a genus of theropod dinosaur from the early Maastrichtian-age Upper Cretaceous Sânpetru Formation of the Haţeg Basin, Transylvania, Romania. It is known only from a partial right lower leg , which its describers believed came from a giant owl.-History:Harrison and Walker described...
- CeratonykusCeratonykusCeratonykus is a genus of alvarezsaurid theropod dinosaur that existed in the late Cretaceous period. The discovery of a fragmented skeleton in Mongolia in early 2009 has led scientists to question where alvarezsaurids fit taxonomically into Theropoda.-Description:Ceratonykus was a small,...
- HeptasteornisHeptasteornisHeptasteornis is the name given to a dubious genus of small dinosaur from the Late Cretaceous. The type species is Heptasteornis andrewsi, described as a presumed gigantic prehistoric owl in 1975...
- PatagonykusPatagonykusPatagonykus is a genus of theropod dinosaur from the Upper Cretaceous of Argentina. This alvarezsaur was discovered in exposures of the Portezuelo Member of the Rio Neuquen Formation in the Neuquen Province of Argentina...
- Parvicursorinae
- AlbertonykusAlbertonykusAlbertonykus is a genus of alvarezsaurid dinosaur from lower Maastrichtian-age rocks of the Horseshoe Canyon Formation of Alberta, Canada. It is known from forelimb and hindlimb remains from multiple individuals. All but two of the specimens come from a bonebed dominated by Albertosaurus...
- AlbinykusAlbinykusAlbinykus is a genus of alvarezsaurid dinosaur. Fossils have been found from Late Cretaceous-age deposits in the Gobi Desert of Mongolia. The type species A. baatar was named by Sterling J. Nesbitt, Julia A. Clarke, Alan H. Turner and Mark A...
- KolKol (dinosaur)Kol is a genus of alvarezsaurid theropod dinosaur from the Late Cretaceous of Mongolia. The type specimen was excavated from the Ukhaa Tolgod locality of the Djadochta Formation, dating to about 75 million years ago. It is believed to have been about twice the size of the contemporary Shuvuuia...
- LinhenykusLinhenykusLinhenykus is an extinct genus of alvarezsaurid theropod dinosaur from the Late Cretaceous of Inner Mongolia, China. It is the most basal known member of the Parvicursorinae. The genus gets its name from Linhe, a city near the site where fossils were first found and Greek onyx, "claw"...
- MononykusMononykusMononykus was a theropod dinosaur from late Cretaceous Mongolia with long, skinny legs. It moved about on two legs, was very nimble, and could run at high speeds, something that would have been useful in the open desert plains where it lived...
- "Ornithomimus" minutus
- ParvicursorParvicursorParvicursor is a genus of tiny maniraptoran dinosaur with long slender legs for fast running. At only about 39 cm from snout to end of tail, and 162 grams in weight, it is one of the smallest non-avian dinosaurs known from an adult specimen.Like other members of the family Alvarezsauridae,...
- ShuvuuiaShuvuuiaShuvuuia is a genus of bird-like theropod dinosaur from the late Cretaceous period of Mongolia. It is a member of the family Alvarezsauridae, small coelurosaurian dinosaurs which are characterized by short but powerful forelimbs specialized for digging. The type species is Shuvuuia deserti, or...
- XixianykusXixianykusXixianykus is a genus of alvarezsauroid theropod dinosaur from the late Cretaceous period of China. It was discovered in Xixia County in Henan, China...
- Albertonykus
- Achillesaurus
Turner et al. (2007) place the alvarezsaurs as the most basal group in the maniraptora
Maniraptora
Maniraptora is a clade of coelurosaurian dinosaurs which includes the birds and the dinosaurs that were more closely related to them than to Ornithomimus velox. It contains the major subgroups Avialae, Deinonychosauria, Oviraptorosauria and Therizinosauria. Ornitholestes and the Alvarezsauroidea...
, one step more derived than Ornitholestes
Ornitholestes
Ornitholestes was a small theropod dinosaur of the late Jurassic of Western Laurasia . To date, it is known only from a single partial skeleton, and badly crushed skull found at the Bone Cabin Quarry near Medicine Bow, Wyoming, in 1900...
and two more derived than the Ornithomimosauria
Ornithomimosauria
The Ornithomimosauria, ornithomimosaurs or ostrich dinosaurs were theropod dinosaurs which bore a superficial resemblance to modern ostriches. They were fast, omnivorous or herbivorous dinosaurs from the Cretaceous Period of Laurasia...
. The alvarezsaurs are more primitive than the Oviraptorosauria
Oviraptorosauria
Oviraptorosaurs are a group of feathered maniraptoran dinosaurs from the Cretaceous Period of what are now Asia and North America. They are distinct for their characteristically short, beaked, parrot - like skulls, with or without bony crests atop the head...
.
Novas' 1996 description of Patagonykus
Patagonykus
Patagonykus is a genus of theropod dinosaur from the Upper Cretaceous of Argentina. This alvarezsaur was discovered in exposures of the Portezuelo Member of the Rio Neuquen Formation in the Neuquen Province of Argentina...
, demonstrated that it was a link between the more primitive (basal) Alvarezsaurus and the more advanced (derived) Mononykus, and reinforced their monophyly. Parvicursor
Parvicursor
Parvicursor is a genus of tiny maniraptoran dinosaur with long slender legs for fast running. At only about 39 cm from snout to end of tail, and 162 grams in weight, it is one of the smallest non-avian dinosaurs known from an adult specimen.Like other members of the family Alvarezsauridae,...
was discovered shortly after, and placed in its own family Parvicursoridae, and then Shuvuuia in 1998. Everything has since been lumped into Alvarezsauridae, with Mononykinae surviving as a subfamily.
There may be a relationship between the alvarezsaurids and the Ornithomimosauria
Ornithomimosauria
The Ornithomimosauria, ornithomimosaurs or ostrich dinosaurs were theropod dinosaurs which bore a superficial resemblance to modern ostriches. They were fast, omnivorous or herbivorous dinosaurs from the Cretaceous Period of Laurasia...
as sister clades within either Thomas Holtz's Arctometatarsalia or 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...
's Ornithomimiformes. The discovery of Haplocheirus
Haplocheirus
Haplocheirus is a genus of alvarezsauroid theropod dinosaur. It is the most basal member of its clade. It is the oldest known alvarezsauroid, predating all other members by about 63 million years. This also makes it about 15 million years older than the oldest known bird Archaeopteryx...
, which exhibits transitional features between the more derived alvarezsaurs and other maniraptora
Maniraptora
Maniraptora is a clade of coelurosaurian dinosaurs which includes the birds and the dinosaurs that were more closely related to them than to Ornithomimus velox. It contains the major subgroups Avialae, Deinonychosauria, Oviraptorosauria and Therizinosauria. Ornitholestes and the Alvarezsauroidea...
ns, particularly in relation to the skull structure and development of the hand, has provided further support for that relationship.
The taxonomy of the alvarezsaurs has been somewhat confused, due to different authors using different names for groups with the same definition. The family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
Alvarezsauridae was first coined by Jose Bonaparte in 1991, but given no specific phylogenetic definition. Novas later defined the group as the most recent common ancestor of Alvarezsaurus
Alvarezsaurus
Alvarezsaurus is a genus of small alvarezsaurid dinosaur from the Late Cretaceous period of Argentina, approximately 86 - 83 million years ago. Estimates suggest that it measured about 2 meters in length and weighed approximately 20 kg...
and Mononykus
Mononykus
Mononykus was a theropod dinosaur from late Cretaceous Mongolia with long, skinny legs. It moved about on two legs, was very nimble, and could run at high speeds, something that would have been useful in the open desert plains where it lived...
plus all its descendants, though others, such as 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...
, used a more inclusive definition, such as all dinosaurs closer to Shuvuuia
Shuvuuia
Shuvuuia is a genus of bird-like theropod dinosaur from the late Cretaceous period of Mongolia. It is a member of the family Alvarezsauridae, small coelurosaurian dinosaurs which are characterized by short but powerful forelimbs specialized for digging. The type species is Shuvuuia deserti, or...
than to modern birds. In 2009, Livezey and Zusi used the name Alvarezsauroidea for the total group of all alvarezsaurs, restricting the name Alvarezsauridae to the clade defined by Alvarezsaurus + Mononykus. This was followed by Choiniere and colleagues in 2010, who described the first non-alvarezsaurid alvarezsauroid, Haplocheirus
Haplocheirus
Haplocheirus is a genus of alvarezsauroid theropod dinosaur. It is the most basal member of its clade. It is the oldest known alvarezsauroid, predating all other members by about 63 million years. This also makes it about 15 million years older than the oldest known bird Archaeopteryx...
. Some authors have used the name Mononykinae for the sub-group of alvarezsaurs including the advanced Mongolian species. However, Choiniere and colleagues argued that Parvicursorinae has priority, since its coordinate name under the ICZN Code
International Code of Zoological Nomenclature
The International Code of Zoological Nomenclature is a widely accepted convention in zoology that rules the formal scientific naming of organisms treated as animals...
, Parvicursoridae, was named earlier.
The 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...
presented here follows a 2009 phylogenetic analysis by Nick Longrich and Phil Currie
Phil Currie
Philip John Currie, AOE is a Canadian palaeontologist and museum curator who helped found the Royal Tyrrell Museum of Palaeontology in Drumheller, Alberta and is now a professor at the University of Alberta in Edmonton...
.
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 Xu et al., 2011:
External links
- RE: Alvarezsauridae splitting, by Thomas R. Holtz, Jr.Thomas R. Holtz Jr.Thomas Richard Holtz, Jr., Ph.D. is a vertebrate palaeontologist and senior lecturer at the University of Maryland's Department of Geology. He has published extensively on the phylogeny, morphology, ecomorphology, and locomotion of terrestrial predators, especially on tyrannosaurids and other...
, from the Dinosaur Mailing List. - Vertebrates 350.200 Aves: Metornithes, from Palæos.
- The Holy of Holies... Dinosauria II, by Thomas R. Holtz, Jr., from the Dinosaur Mailing List (mentions in passing the Rapator = ?alvarezsaurid hypothesis seeing print for the first time; it had been rumored on the list for several years prior)