Old Earth Ministries Online Dinosaur Curriculum
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Lesson 31 - Alvarezsauridae
Alvarezsauridae is an enigmatic family (not genus) of small, long-legged running dinosaurs. Although originally thought to represent the earliest known flightless birds, 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.
Lentgh: 20 - 80 inches
Date Range: 160 - 65 Ma
|Artist impression of Ceratonykus
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.
Alvarezsaurs range from 0.5–2 m (20–80 inches) in length, although some possible members may have been substantially larger, including the European Heptasteornis 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.
At least one specimen of alvarezsaur, 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.
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 Ornithomimosauria.
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, or 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 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.
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.
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