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Description

A selection of small maniraptoran theropods known as alvarezsauroids, to scale.

At one stage alvarezsaurs were considered birds, admittedly very strange flightless ones. They certainly have some features in common with birds including an elongate keeled sternum found in volant avians. With the discovery of more basal forms, it appears that the similarities to birds is however convergent. Other groups such as dromaesaurids, therizinosaurs and oviraptorosaurs are more closely aligned to the base of true avians than the alvarezsauroids. The first discovered species was allied to ornithomimosaurs which are the sister group to all maniraptorans.

Much of the anatomy is very similar to other theropods, although the top half of alvarezsaurs were highly specialised. A muscular chest, very short arms and compact hands which ended with deep claws were very distinctive. Combined with a tube shaped snout and tiny teeth, these are features in common with ant and termite-eating mammals. Ants were not common during the Cretaceous and mound-building termites do not turn up in the fossil record until the Eocene. Wood-nesting termites were a common part of the insect fauna. While alvarezsaurs had the claws for insect foraging, the short arms would have necessitated resting very close to achieve it. Or it is possible they were hunting different prey. One Chinese species has been found surrounded by eggshell fragments which suggests that alvarezsaurs may have been nest raiders. Certainly the robust claws were capable of breaking into eggs. All species possessed long legs and a long tail giving alvarezsaurs a good turn of speed.

* One specimen of the Mongolian species Shuvuuia has been found surrounded by poorly preserved tube-like structures, that following analysis showed they contained the same keratins found in feathers.

* Discovery of Haplocheirus from the Late Jurassic showed that basal alvarezsauroids shared features in common with other maniraptorans, such as skull structure, sharp teeth and three clawed fingers on the hand. It has been proposed that Haplocheirus may in fact be a giant compsognathid, although the results are far from conclusive.

* At 39 cms long Parvicursor was one of the smallest non-avian dinosaurs known. In comparison the largest species was Bonapartenykus at a modest 2.6 metres long. Notably the only specimen was female with eggs either still in the oviduct or that she was incubating on a nest.

The group survived up to the K-Pg extinction event.

Description

A selection of small maniraptoran theropods known as alvarezsauroids, to scale.

At one stage alvarezsaurs were considered birds, admittedly very strange flightless ones. They certainly have some features in common with birds including an elongate keeled sternum found in volant avians. With the discovery of more basal forms, it appears that the similarities to birds is however convergent. Other groups such as dromaesaurids, therizinosaurs and oviraptorosaurs are more closely aligned to the base of true avians than the alvarezsauroids. The first discovered species was allied to ornithomimosaurs which are the sister group to all maniraptorans.

Much of the anatomy is very similar to other theropods, although the top half of alvarezsaurs were highly specialised. A muscular chest, very short arms and compact hands which ended with deep claws were very distinctive. Combined with a tube shaped snout and tiny teeth, these are features in common with ant and termite-eating mammals. Ants were not common during the Cretaceous and mound-building termites do not turn up in the fossil record until the Eocene. Wood-nesting termites were a common part of the insect fauna. While alvarezsaurs had the claws for insect foraging, the short arms would have necessitated resting very close to achieve it. Or it is possible they were hunting different prey. One Chinese species has been found surrounded by eggshell fragments which suggests that alvarezsaurs may have been nest raiders. Certainly the robust claws were capable of breaking into eggs. All species possessed long legs and a long tail giving alvarezsaurs a good turn of speed.

* One specimen of the Mongolian species Shuvuuia has been found surrounded by poorly preserved tube-like structures, that following analysis showed they contained the same keratins found in feathers.

* Discovery of Haplocheirus from the Late Jurassic showed that basal alvarezsauroids shared features in common with other maniraptorans, such as skull structure, sharp teeth and three clawed fingers on the hand. It has been proposed that Haplocheirus may in fact be a giant compsognathid, although the results are far from conclusive.

* At 39 cms long Parvicursor was one of the smallest non-avian dinosaurs known. In comparison the largest species was Bonapartenykus at a modest 2.6 metres long. Notably the only specimen was female with eggs either still in the oviduct or that she was incubating on a nest.

The group survived up to the K-Pg extinction event.