Narrowed Hands and Huge Toes

Chirostenotes, meaning "narrowed hands," is based on one of the skeletons on the bottom. One of the first dinosaurs discovered from the Late Cretaceous of Alberta, Canada, this dinosaur has received some of the more interesting attention among researchers due to the problematic nature of its completeness.

When discovered, it was found near and in the same horizon as a few other theropod dinosaurs, one of which was regarded as a bird. The first, a pair of mandibles and later associated teeth, would be known as Richardoestesia ("for Richard Estes," named to honor a researcher in fossil lizards, including most of the Albertan fossil faunas). These jaws would seem to fit such a gracile, dromaeosaurid-like animal as Chirostenotes was first assumed to be.

Not much long after, description of some well-preserved feet, including a complete pes (ankle, metatarsals, toes) led to the creation of Macrophalangia ("big [or elongated] toes"), an animal that was quite different from the typical dromaeosaurid type that was known for Velociraptor, and very much unlike troodontid pes that were also known from the same formation. It was originally though to belong to a type of ornithomimosaur.

It would be years later when a block that was recovered close to the same time as the previously named dinosaurs was fully prepared that workers, looking back, reasoned that Chirostenotes and Macrophalangia, speculated at one time to be the same taxon, were shown to indeed be the same type of dinosaur, and an unusual one at that. This specimen, shown in the middle, also preserved details of its strange pelvis, and provides links to its affinities.

At the same time as the recovery of the fossil hands and feet, a lower jaw of what was considered to be a primitive bird was also found, and named Caenagnathus (or "delicate jaw"). Toothless, with a fused symphysis between the two dentaries, and with complete ends of the articular regions that attached to the upper skull, the jaw was refered to several ranks of taxa affirming its place in no other bird group, but definitively within Aves. This jaw is not shown.

Finally, the largest recovered specimen (at top) was referred to Chirostenotes, but for once not forming the basis of a new species or genus. This material not only including hands and feet, affirming the referral of Chirostenotes, but it preserved portions of the skull, including an edentulous (toothless) maxilla with a strong curve to it. The descriptors inferred that the curved maxilla would fit well with that of Caenagnathus' jaw, and so compiled all these taxa into one another:

Chirostenotes pergracilis, meaning "very slender and narrow hands." Because they were named after Chirostenotes, the other taxa I mention here are considered synonyms, and subsumed within the oldest diagnostic name. However, some complications arise:

Not all of these taxa are from the same formation, age, or region. The largest specimen is a few million years younger than the smaller ones, and some details of the ischium differ between it and the smaller specimen. In addition, several variant portions of ankles have been found that assume there were at least two different types of small caenagnathid theropod, but more over, two different types of jaw joints are known. The ankle preserved is very similar to Elmisaurus from Mongolia (named Elmisaurus elegans, but also called Chirostenotes elegans), while the jaw joint is only similar to the isolated mandible, as no other cranial material is described (and named Caenagnathus sternbergi, but also called Chirostenotes sternbergi); moreover, it has been speculated that these smaller species are each synonymous with the other, and also that they are all "morphs" of the same, single species.

Nonetheless, an extremely interesting taxon, and one that only further and more complete collections in Alberta's Dinosaur Provincial Park may help to explain.