Posted: Jun 02, 2010 3:03 am
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