ALVAREZSAURIA
CLASSIFICATION
Animalia: Vertebrata: Tetrapoda: Sauropsida: Archosauromorpha:
Ornithodira: Dinosauria: Theropoda: Tetanurae: Coelurosauria
CLADOGRAM:
--Alvarezsauria "Alvarezsaurus's taxon"
| INCERTAE SEDIS:
| ?Rapator
`--Alvarezsauridae non Sereno, 1999
|--Alvarezsaurus
`--+--Patagonykus
`--Mononykinae == {Mononykus + Parvicursor + Shuvuuia} =Parvicursorinae
| INCERTAE SEDIS:
| unnamed form -- late Maastrichtian -- Montana
|--Mononykus
|--"Ornithomimus" minutus
|--Parvicursor
`--Shuvuuia
ESSAY
| WHERE DO THEY BELONG?:This recently discovered group has proven difficult to place. The original member, Alvarezsaurus, was originally given its own family in Ceratosauria. When Mononykus was first discovered, it proved a bafflement. It had bird-like features (the skull, a keeled breastbone), unbirdlike features (the tail), features like arctometatarsalian coelurosaurs (pinched middle metatarsal), and some features completely unique (the single, hooked claw on its stubby arms). (NOTE: Some of these fossils have been reassigned to the very closely related Shuvuuia.) The discovery of Patagonykus, a creature seemingly transitional between the more primitive Alvarezsaurus and the more derived mononykines, showed that all three probably belonged to the same group. They are now generally though to be primitive birds, although some think they may be related to ornithomimosaurs. (One piece of alvarezsaur ankle found by O. C. Marsh in the late 1800’s was actually assigned to the genus Ornithomimus.) T HEIR NICHE:Alvarezsaurs were small, terrestrial animals with long legs. Their niche is far from certain. Since their stubby forearms were built so powerfully, it has been suggested that they used them for digging. (In fact, some think that their sterna were keeled as a digging adaptation, similar to moles, and that they evolved keeled sterna separately from birds, which evolved it for flying.) The overall body plan of alvarezsaurs is not that of a burrower, but it has been suggested that they might have fed on colonial insects, ripping into nests with their single-clawed hands. |
