| Archaeornithoides | |
|---|---|
 | |
| Skull reconstruction, known material in brown | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Family: | †Troodontidae |
| Genus: | †Archaeornithoides Elzanowksi & Wellnhofer, 1992 |
| Type species | |
| †Archaeornithoides deinosauriscus Elzanowski & Wellnhofer, 1992 | |
Archaeornithoides is agenus ofmaniraptorantheropoddinosaur of theLate Cretaceous ofMongolia.
In 1965, a Polish-Mongolian paleontological expedition found a fossil of a small dinosaur atBayn Dzak,Mongolia. In 1983, the find was reported byAndrzej Elzanowski.[1] The remains were named as thetype speciesArchaeornithoides deinosauriscus by Elzanowski andPeter Wellnhofer in 1992.[2] In 1993, they were described in more detail by the same authors.[3]
The generic name (Archaeornithoides) means “shaped like an ancient bird” inAncient Greek, from ἀρχαῖος,archaios, "ancient"; ὄρνις,ornis, "bird"; and εἶδος,eidos, "form". Thespecific descriptordeinosauriscus, "little dinosaur", alludes to the animal's small size for a dinosaur.
Theholotype,ZPAL MgD-II/29, was discovered in Late Cretaceous river sandstones of theDjadokhta Formation beds, dating from the lateCampanian. It consists of an articulated but fragmentary skull and lower jaws comprehending pairedmaxillae, a partialjugal, palate bones and dentaries. The specimen represents a juvenile individual.[3]
The holotype ofArchaeornithoides was a very small individual. The head fragment as preserved measures just twenty-seven millimetres in length, indicating an original skull length of about five centimetres. The body length was estimated at fifty to sixty centimetres,[3] making theArchaiornithoides type one of the smallest known non-avian dinosaurs. Adult length is uncertain.
The snout ofArchaeornithoides features a longantorbital fenestra, stretching over three quarters of the length of the maxilla. The maxilla bears at least eight teeth. These are small, conical and smooth, lacking wrinkles, serrations or carinae. Thepalatine bone seems to show the presence of a secondary fenestra.[3]
Elżanowski & Wellnhofer (1993) claimed thatArchaeornithoides was the closest known relative to birds orAvialae. This conclusion rested on key bird-like features; an interdigitated suture between the premaxilla and maxilla, broad palatal shelves, pneumatic sinuses, lack of interdental plates, and unserrated teeth. Since publication, though, all of these features have been discovered in new fossils of adult and/or juveniletroodonts anddromaeosaurs.[4] The describing authors in 1993 also concluded thatArchaeornithoides was closely related to troodontids,Spinosauridae andLisboasaurus, all these taxa together with birds forming aclade, showing that birds originated from a more basal position than normally assumed.[3] However, subsequent research has not supported a close relationship between spinosaurids and troodontids andLisboasaurus was later shown to be a lizard or acrocodylomorph.
Some scientists had suggested that the juvenile specimen ofArchaeornithoides may belong to a previously known species of Mongolian troodontid, eitherSaurornithoides orByronosaurus. However, studies of a juvenileByronosaurus skull showed that theropod dinosaurs possess many distinctive adult characteristics even as hatchlings or embryos, and that the lack of characters solidly linkingArchaeornithoides to known adult specimens shows that it is probably a distinct species. Bever and Norell (in 2009) found no evidence to support the placement ofArchaeornithoides close to Avialae, and only weak support for the idea that it is a juvenile troodontid.[5]
Elzanowski and Wellnhofer noted that the specimen has distinct bite marks while the back of the head fragment was ragged, and suggested that the jaws were bitten off from its braincase by aDeltatheridium mammal the size of a weasel (adding that these are common in the Bayn Dzak assemblage).[3] Clark and colleagues (2002) noted that it may have also passed through the digestive tract of the predator before fossilization. If true, this may be the first known evidence ofMesozoic mammals feeding on dinosaurs (seeRepenomamus).
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