Posted: Jul 29, 2010 3:04 am
ginckgo wrote:num1cubfn wrote:I'm not expert, but I thought various species had traits of each. For example, sharks haven't changed much, nor had trilobites over 100+ million years, but animals like cows and chickens have undergone much larger changes in smaller times, because of increased selective pressure.
I could be completely wrong though.
You are completely wrong
Trilobites changed massively throughout their main regnum from Cambrian to Devonian. The fact that they all still retained their basic morphology (cephalon, thorax, pygidium, legs plus gills on each thoracic segment, etc) is no more 'unchanging' than all vertebrates retaining four limbs, a head, a spine, etc. We are just more biased to see minor variations as bigger than in other phyla. Having said that, sharks have also varied quite a bit.
/rant
Sorry if I implied that there were no changes to trilobites, but they WERE around for 300 million years. While there may have been changes, they stayed pretty much the same for a pretty damn long time. If you compare that to the changes in cows and chickens in a few thousand years of concentrated domestication, the difference is huge. That being said, I still don't know how to answer the OP's question.