Alvarezsaurids Temporal range: Late Cretaceous, | |
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Mounted holotype of Mononykus | |
Skeletal mount of Alvarezsaurus | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Clade: | Dinosauria |
Clade: | Saurischia |
Clade: | Theropoda |
Superfamily: | † Alvarezsauroidea |
Family: | † Alvarezsauridae Bonaparte, 1991 |
Type species | |
† Alvarezsaurus calvoi Bonaparte, 1991 | |
Subgroups | |
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Synonyms | |
Alvarezsauridae is a family of small, long-legged dinosaurs. Although originally thought to represent the earliest known flightless birds, they are now thought to be an early diverging branch of maniraptoran theropods. Alvarezsaurids were highly specialized. They had tiny but stout forelimbs, with compact, bird-like hands. Their skeletons suggest that they had massive breast and arm muscles, possibly adapted for digging or tearing. They had long, tube-shaped snouts filled with tiny teeth. They have been interpreted as myrmecophagous, adapted to prey on colonial insects such as termites, with the short arms acting as effective digging instruments to break into nests.
Alvarezsaurus , the type genus of the family, was named for the historian Gregorio Álvarez. [1]
Bonaparte (1991) described the first alvarezsaurid, Alvarezsaurus calvoi , from an incomplete skeleton found in Patagonia, Argentina. Bonaparte also named a family, Alvarezsauridae, to contain it. He argued that Alvarezsaurus might be most closely related to the ornithomimosaurs. [2]
In 1993, Perle et al. 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 beetle. [3] Perle et al. mistakenly described Mononykus as a member of Avialae, one more advanced than Archaeopteryx . They argued that the family Alvarezsauridae was actually a group of Mesozoic flightless birds on the basis of several features that were supposedly unique to birds. [4]
In 1996, Novas described another member of the group called Patagonykus puertai . [5] Karhu and Rautian (1996) described a Mongolian member of the family; Parvicursor remotus. [6] Chiappe et al.(1998) described another Mongolian member, Shuvuuia deserti, and found it to be a bird as in Perle et al.'s analysis. [7]
These mistaken assignments of alvarezsaurids 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 ratites. 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. [8]
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. [8]
As the more primitive members of the Alvarezsauridae were better characterized, the monophyly of the clade was strongly supported, [5] 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. [8]
Alvarezsaurids ranged from 50 centimetres (20 in) to 2 metres (6.6 ft) in length, although some possible members may have been larger, including the European Heptasteornis that may have reached 2.5 metres (8.2 ft) long. Fossils attributed to alvarezsaurids have also been found in North and South America and Asia, and range in age from about 86 to 66 million years ago. [9]
At least one specimen of alvarezsaurid, from the species Shuvuuia deserti , preserved down-like, feathery, 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 feathers. [10]
The lifestyle of alvarezsaurids has been debated since the nature of these dinosaurs was established. It has been suggested by numerous palaeontologists that they used their claws to break into ant and termite colonies, though the arm anatomy of an alvarezsaurid would require the animal to lie on its chest against a termite nest. It is also possible that the alvarezsaurids filled some ecological niche that has not yet been considered. [11] Studies of the tails in various alvarezsaur genera also suggest they were possessed of an incredible ability to change their rotational inertia, and combined with their forelimbs, this suggests their ecological niches were similar to those of aardvarks, pangolins, and anteaters. [12]
Additionally, it is known that alvarezsaurids, with their long legs, appear to be built for speed. What implications this has on possible lifestyle is unknown. [11] The discovery of Qiupanykus in association with oviraptorid eggs, indicates that the advanced alvarezsaurids may also have been specialists in nest raiding, using their robust thumb claws to crack open eggshells. [13]
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. [14]
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 maniraptorans, particularly in relation to the skull structure and development of the hand, has provided further support for that relationship. [15]
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 . [16] 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. [16] Another subfamily, Patagonykinae, has been named to include the South American Patagonykus and Bonapartenykus, but a few recent studies have placed them just outside Alvarezsauridae, some of which do not even recover them in a single clade, making Patagonykinae turn out to be paraphyletic. [17] [18] [19] [20] [21] [22]
The cladogram presented here follows a 2012 phylogenetic analysis by Agnolin and colleagues. [17]
Alvarezsauridae |
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Cladogram after Xu et al., 2011: [23]
Alvarezsauroidea |
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Troodontidae is a clade of bird-like theropod dinosaurs from the Late Jurassic to Late Cretaceous. During most of the 20th century, troodontid fossils were few and incomplete and they have therefore been allied, at various times, with many dinosaurian lineages. More recent fossil discoveries of complete and articulated specimens, have helped to increase understanding about this group. Anatomical studies, particularly studies of the most primitive troodontids, like Sinovenator, demonstrate striking anatomical similarities with Archaeopteryx and primitive dromaeosaurids, and demonstrate that they are relatives comprising a clade called Paraves.
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 "desert bird". The name Shuvuuia is derived from the Mongolian word shuvuu (шувуу) meaning "bird".
Parvicursor is a genus of tiny maniraptoran dinosaur with long slender legs for fast running.
Maniraptora is a clade of coelurosaurian dinosaurs which includes the birds and the non-avian dinosaurs that were more closely related to them than to Ornithomimus velox. It contains the major subgroups Avialae, Dromaeosauridae, Troodontidae, Oviraptorosauria, and Therizinosauria. Ornitholestes and the Alvarezsauroidea are also often included. Together with the next closest sister group, the Ornithomimosauria, Maniraptora comprises the more inclusive clade Maniraptoriformes. Maniraptorans first appear in the fossil record during the Jurassic Period, and survive today as living birds.
Coelurosauria is the clade containing all theropod dinosaurs more closely related to birds than to carnosaurs.
Ornithomimosauria are theropod dinosaurs which bore a superficial resemblance to the modern-day ostrich. They were fast, omnivorous or herbivorous dinosaurs from the Cretaceous Period of Laurasia, as well as Africa and possibly Australia. The group first appeared in the Early Cretaceous and persisted until the Late Cretaceous. Primitive members of the group include Nqwebasaurus, Pelecanimimus, Shenzhousaurus, Hexing and Deinocheirus, the arms of which reached 2.4 m (8 feet) in length. More advanced species, members of the family Ornithomimidae, include Gallimimus, Struthiomimus, and Ornithomimus. Some paleontologists, like Paul Sereno, consider the enigmatic alvarezsaurids to be close relatives of the ornithomimosaurs and place them together in the superfamily Ornithomimoidea.
Alvarezsaurus is a genus of alvarezsaurid dinosaur from the Late Cretaceous, living in Argentina approximately 86 - 83 million years ago. It was a small dinosaur, measuring 1–1.4 m (3.3–4.6 ft) long and weighing approximately 3 kg (6.6 lb). It was found in the Bajo de la Carpa Formation and was named by paleontologist José Bonaparte in 1991 after the historian Don Gregorio Alvarez. The type species is A. calvoi.
Mononykus is a genus of alvarezsaurid dinosaur that lived during the Late Cretaceous in what is now Asia on the Nemegt Formation, about 70 million years ago.
Patagonykus is a genus of theropod dinosaur from the Upper Cretaceous of Argentina. This alvarezsauroid was discovered in exposures of the Portezuelo Formation (Turonian-Coniacian) of the Rio Neuquén Subgroup in the Neuquén Basin, Neuquen Province of Patagonia, Argentina. The holotype consists of an incomplete but well-preserved skeleton, lacking a skull, but including many vertebrae, the coracoids, a partial forelimb, pelvic girdle, and hindlimbs. Patagonykus has been classed with the Alvarezsauridae, a family which includes such taxa as the Mongolian Mononykus and the Argentinian Alvarezsaurus. In 2010 Gregory S. Paul estimated its length at 1 meter and its weight at 3.5 kg.
Ceratonykus is a monospecific genus of alvarezsaurid dinosaur from Mongolia that lived during the Late Cretaceous in what is now the Barun Goyot Formation. The type and only species, Ceratonykus oculatus, is known from a fragmentary skeleton, including an incomplete skull, of an adult individual. It was named and described in 2009 by Vladimir Alifanov and Rinchen Barsbold. Its describers questioned the traditional placement of alvarezsaurs in Theropoda, instead suggesting they were ornithischians, but this has not been accepted since. Ceratonykus has an estimated length of 75 centimetres and weight of 760 grams. It has been considered as a possible junior synonym of Parvicursor.
Kol is an extinct genus of coelurosaurian theropod dinosaur from the Late Cretaceous of Mongolia. The type and only species is Kol ghuva. 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 contemporaneous Shuvuuia. However, unlike Shuvuuia, which is known from many well preserved specimens, and although Ukhaa Tolgod has been thoroughly explored, Kol is known only from one complete foot, suggesting that it must have been relatively rare in that ecosystem.
Haplocheirus is an extinct genus of theropod dinosaur from the Middle Jurassic Shishugou Formation of Xinjiang in China. It is generally considered to be an alvarezsauroid, although some researchers have questioned this assignment. The genus contains a single species, H. sollers, which is known from a mostly complete skeleton including the skull.
Alvarezsauroidea is a group of small maniraptoran dinosaurs. Alvarezsauroidea, Alvarezsauridae, and Alvarezsauria are named for the historian Gregorio Álvarez, not the more familiar physicist Luis Alvarez, or his son geologist Walter Alvarez who jointly proposed that the Cretaceous–Paleogene extinction event was caused by an impact event. The group was first formally proposed by Choiniere and colleagues in 2010, to contain the family Alvarezsauridae and non-alvarezsaurid alvarezsauroids, such as Haplocheirus, which is the basalmost of the Alvarezsauroidea. The discovery of Haplocheirus extended the stratigraphic evidence for the group Alvarezsauroidea about 63 million years further in the past. The division of Alvarezsauroidea into the Alvarezsauridae and the non-alvarezsaurid alvarezsauroids is based on differences in their morphology, especially in their hand morphology.
Linhenykus 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 the fossil was first found and Greek nykus, "claw". The specific name is derived from Greek monos, "single", and daktylos, "finger", a reference to the fact that it is the only known non-avian dinosaur to have had but a single digit.
Bonapartenykus is a monospecific genus of alvarezsauroid dinosaur from Argentina that lived during the Late Cretaceous (Campanian-Maastrichtian) in what is now the upper Allen Formation of the Río Negro Province. The type and only species, Bonapartenykus ultimus, is known from a nearly articulated but partial skeleton that was found in close association to two incomplete eggs and several clusters of eggshells belonging to the oogenus Arriagadoolithus. Bonapartenykus was named in 2012 by Federico L. Agnolin, Jaime E. Powell, Fernando E. Novas and Martin Kundrát. Bonapartenykus has an estimated length of 2.5 m (8.2 ft) and weight of 72 kg (159 lb), making it the largest member of the clade Alvarezsauroidea.
Aorun is a genus of carnivorous theropod dinosaur first discovered in 2006, with its scientific description published in 2013. It is possibly one of the oldest known coelurosaurian dinosaurs and is estimated to have lived ~161.6 million years ago during the Late Jurassic Period, though some researchers consider it to be a carnosaurian instead.
Qiupanykus is a genus of alvarezsaurid coelurosaur theropod from the Late Cretaceous Qiupa Formation of southern China.
Khulsanurus is an extinct genus of alvarezsaurid theropod dinosaur from the Late Cretaceous Barungoyot Formation of the Khulsan Locality in the Gobi Desert region of Mongolia. The type and only species is Khulsanurus magnificus.