CAENAGNATHUS

With the help of a long-forgotten dinosaur, packed up and in storage since 1923, Currie and Russell realised that the remains of Caenagnathus (Recent jaw), Chirostenotes (Narrow hands) and Macrophalangia (Large toes) all belonged to the same dinosaur in 1979. Of the three, Chirostenotes was named first, so that's what this Frankenstein-o-saur became known as, and strictly speaking it should've had dibs on a replacement name for Caenagnathidae — the family that was chalked off along with Caenagnathus — right? Wrong. Chirostenotes, its valid derivatives, and closest relatives were all moved to Elmisauridae; a group of swift running oviraptorosaurs that were anchored by Elmisaurus. You may think that Elmisaurus is nothing but an opportunistic queue jumper at this point because it was the last named member of this "new" group. But with Caenagnathidae gone, and Chirotenotes having never been a group name-bearer, ICZN laws state that the next available family name should take precedence, and that was Elmisauridae. Nevertheless, things were not as clear cut as initially hoped.

Despite its meagre remains, Caenagnathus turned out to be valid, which saw its family -- Caenagnathidae -- reinstated and Elmisauridae sunk as its junior synonym. All but one of the once called "Elmisaurids" — with their short arms and long legs with pinched metatarsus for swift running — are not dissimilar to oviraptorids and, in fact, that's exactly what they may turn out to be. The odd one out, funnily enough, is Caenagnathus, who's sole remains (a lower jaw) seems to be of a more primitive design than the corresponding part of its purported closest relatives. If further research bears this out then the merry-go-round will begin again; Caenagnathus would belong further down the oviraptorosaur family tree so the more advanced "elmisaurids" would belong to Elmisauridae again. And Caenagnathidae would be sunk once more... unless Caenagnathus drummed up some new relatives in its more basal position.

Upon discovery in 1936, Sternberg thought Caenagnathus was a primitive relative of the ostrich based solely on features of its lonely lower jaw, and four years later he used it to anchor Caenagnathidae; a family of weird flightless birds. This opinion was followed into the 1970s when more finds began to shed some light on its actual affinities, and we now know that Caenagnathus—despite its beak, body plan, and a probable feathery coating—were not birds, but had evolved several bird-like features via convergent evolution.