So my 6 year old explained this to me and it blew me mind I had to read it up to be sure.

"Chickens didn't exist hundreds of billions of years ago. T Rex did, but all died of the meteor. Some cousins and friends of T Rex, but not Trex, didn't die, and had x-men like babies [mutations lol] and were more like bird x-men,and the xmen survived but kept having new x-men babies, that also had x-men babies. None of these x-men are around today, so we get to play with Chickens and Dinosaurs and the X-men toys we have because we don't know what powers the x-men birds have."