Wrist finger cymbals became intemperately remodel somewhere along the way between early dinosaur and modern Bronx cheer . research worker combining young analysis of innovative bird embryo and fossils divulge how wrist were modernized , if you will , over trillion of eld . These previously undetected processes of loss , fusion , and even the “ re - evolution ” of a lost bone are distinguish in aPLOS Biologystudythis week .
Early dinosaur had as many as nine wrist joint bones but birds these days have only four , arranged roughly in a two - by - two straightforward grid . articulatio radiocarpea also went from being flat to bent and hyperflexible , which allow birds to fold up their wing against their bodies when they ’re not using them to fly . How the bit became halve and how the shapes switch are hotly fence topics . Even the names of the various wrist bones are dissimilar , depending on if you need a paleontologist or a developmental life scientist .
But understanding an being ’s developmental cognitive operation often helps explicate the evolutionary relationship between ancestral and descend shape . So , in a clever combination of paleontology and embryology , aUniversity of Chile squad lead by Alexander Vargasreexamined about a XII fossil across the dinosaur - Bronx cheer modulation from several museum aggregation , and they also collected developmental data point from seven unlike species of modern birds . By staining the embryonic gristly skeleton and studying specific proteins , the squad was able-bodied to look at how wings rise in a growing conceptus . Their interdisciplinary dataset order fromArchaeopteryxandBambiraptorfossils to fertilise chicken , pigeon , and finch eggs .
Work from the seventies reveal that birds and some dinosaurs have a very standardised , half - lunation shaped bone prognosticate the semilunate , which result from the merging of two bones present in sooner , non - flying dinosaurs . These new findings corroborate that the bird semilunate ( label SL ) was form by the coalition of two dinosaur bone : distal carpal bone 1 and 2 ( green and yellow ) . Another two castanets – the radiale ( purpleness ) and the intermedium ( orange ) – fused to form the bird radiocarpal joint bone called the scapholunare ( label SC ) .
The distal carpel 3 ( blue ) in chick used to be telephone element x ( light blue air ) by embryologists , who reckon it took the blank space of a bone called the ulnare ( brown ) , which was lost in bird development . However , the ulnare briefly shows up in embryonic raspberry before it ’s completely pass in adults . As it move around out , distal carpel 3 is not Modern os , and it does n’t replace the ulnare . Not only do the ulnare and element x coexist in the fertilized egg of snort , but also the embryonal position of factor x corresponds with distal carpal 3 .
And lastly ( or fourthly ) , the squad find that the pisiform ( bolshy ) , which was lose in dinosaur , was in reality re - acquired in the former development of bird . They recollect this “ re - evolution ” was potential an adaptation for flight , with the pisiform transmitting personnel on the downstroke while restricting flexibility on the upstroke . And it ’s a uncommon example of an evolutionary reversal .
[ ViaPLOS Biology synopsis , PLOS dismission ]
Images : 2014 PLOS Biology
