One of the strangest fossils discovered at the Flaming Cliffs by the famous Roy Chapman Andrews Mongolian expedition in the 1920s was the skeleton of a small “birdlike dinosaur”. This specimen was filed away in the collection of the American Museum of Natural History, where it sat unprepared and unexamined for nearly 70 years. As this specimen gathered the last of its dust, paleontologists of the joint Mongolian/American field project were collecting several new specimens of this same “birdlike” taxon, which was finally given a name and a description in 1993, as Mononykus olecranus, the “one-clawed elbow”, in reference to its huge hand claw and robust forearm. Although very little fossil material was known from the head of Mononykus (and little is still known about its cranium), the postcranial anatomy of this new species strongly hinted that it belonged somewhere within birds. Like birds, Mononykus has a wrist that is fused to the metacarpals, a keeled sternum, a thin, posteriorly-projecting pubis, an extra ridge medial to the cnemial crest, and unserrated teeth with constricted roots. However, the very short, powerful arm of Mononykus could not have been used in flight. Scientists at the time hypothesized that Mononykus was a member of a flightless lineage of early birds, and that it might have used the robust forelimb to dig out insects.
Meanwhile, in South America, a bizarre, fragmentary fossil theropod named Alvarezsaurus calvoi was described in 1991. Alvarezsaurus has interesting features of the skeleton including a powerful first claw of the hand, but at the time of its description, little was known about this feature in other theropods. A second South American theropod with a powerful first claw was described in 1996 and named Patagonykus puertai. This animal preserved a little more of the skeleton, including a short, robust forearm, and Dr. Fernando Novas recognized that Patagonykus, Alvarezsaurus and Mononykus formed a group of theropods that were closely related to each other. He named this group the Alvarezsauridae.
When the skull of a species closely related to Mononykus, named Shuvuuia deserti, popped up a few years later during the Mongolian expeditions, it seemed to provide more evidence of a position within birds for the Alvarezsauridae. The skull of Shuvuuia is lightly built, and probably had some degree of cranial kinesis (flexibility of the snout). As in birds, skull of Shuvuuia is lightly built and loosely sutured around the eye. The foramen magnum is very large and the occipital condyle is very small, the jugal doesn’t contact the postorbital, the quadratojugal is fused to the jugal and doesn’t contact the quadrate, and the head of the quadrate has two articular surfaces – one with the squamosal (typical for theropods) and one with the braincase. Perhaps most interestingly, Shuvuuia has dozens of small, unserrated teeth in the jaws, shaped like little grains of rice. Other specimens of Shuvuuia showed that next to the enormous first digit, there are two short, scrawny fingers that probably had little functional importance.
Other bits of alvarezsaurids were discovered in North America (an isolated pelvis from Montana and recently a partial forelimb from Canada), and in Romania (distal femur, distal tibia), so that by 2002, scientists had good evidence that the Alvarezsauridae were a diverse family of theropod dinosaurs in the Late Cretaceous and were living on at least four continents at that time (there is still no evidence for alvarezsaurids in Africa, Australia or Antarctica).
Despite the similarities between alvarezsaurids and birds, careful reanalysis of the data began to show a different picture of the phylogenetic position of alvarezsaurids. Using a more comprehensive dataset, Novas and Pol showed that alvarezsaurids were not birds, as originally hypothesized, but instead were basal members of Maniraptora, a group of theropods closely related to birds that includes things like ovirpatorosaurs, troodontids, theirizinosaurs and others. This new hypothesis was important – it meant that the characteristics shared by birds and alvarezsaurids were convergently evolved. Later phylogenetic analyses supported this more basal position for the Alvarezsauridae. Sereno also pointed out some similarities between alvarezsaurs and the Ornithomimosauria (the “ostrich-dinosaurs”), a group not particularly close to bird origins.
In 2000, James M. Clark (George Washington University, USA) and Xu Xing (Institute of Vertebrate Paleontology and Paleoanthropology, China) started to collect fossils in the province of Xinjiang, northwestern China. They were looking in beds known to be from the Middle-Late Jurassic. This time period is important because Archaeopteryx is from the Late Jurassic, and scientists hypothesize that the early relatives of birds should therefore have been diverging during this time period. In 2004, their research group collected the 3m long skeleton and skull of a lightly-built theropod dinosaur, and in 2005 the fossil was prepared well enough to begin describing it. The task fell to me.
That’s it for today what with the press embargo and all on the new paper which will appear in Science tomorrow. I’ll be putting up the next post as soon as I’m allowed.
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