Damn you (no, actually LOVE you for bringing this up), that's just what I'm doing right now. An island continent split off for millions of years, stuck with therapsids and dinosaurs and early birds and all that stuff. A couple of them will be sentient (one from the bird and one from the therapsid side, according to plans). And of course there will be the late arrivals (humans first, then a few thousand years later elves... hehe, elves who have the best ships in the world) who can mess things up really thoroughly.
(Yeah, I'm marching towards a BSc in Biology or Evolutionary Biology right now.)
It's still in the early stages, though. I think the idea itself is only a few months, perhaps a year old. So no stories for you yet, unfortunately.
(by the way, you get billions, not 300 million, of years, though the first 3 of them aren't that interesting fantasy-wise (more potential for sci-fi, I think)... but look at the Burgess Shale. THERE are some weird critters.
Eh, that's pretty close to my idea (I've got a short story in progress), but not enough that we're unknowingly riffing off each other.
For starters, I like my way of cutting off the continents better (something like a magically-imposed barrier), and I'm not using dinosaurs in my story. Strictly Mid-Late Permian.
Marine reptiles stuck around in the land that wasn't cut off, though, while the new one doesn't have any marine life above fish and a few small mesosaurs (and China Mieville's already used the concept of surviving Dinicthys/Dunkleosteus, so I'm not inclined to recycle it and have any major marine predators past sharks.)
Although we're both using therapsids. That could be a problem. Hmm...what branches of the group are you working with?
I've also got an idea kicking around for a couple centuries down the line, when the colonists' homeland finds itself facing problems from a different barrier collapsing and putting the nation-state face-to-face with a species evolved from their world's variant of Phorusrhacos.
Since I don't think I'll ever get published, you needn't fear my therapsids :)
Anyway, I'm not yet sure about any of the details except for the one sentient bird race for whose sake I put the whole new continent there in the first place. It's all pretty vague as of now.
There's possibly going to be a serious mixing of real-world ages, since there's the early bird thing I made up, and I'm absolutely in love with gorgonopsids... but hey, I'm not obliged to copy real earth history as it is, so long as I make up a believable alternative XD
"I'm absolutely in love with gorgonopsids..."
Isn't everyone? Writing superb apex predators like those, even from a human point of view, is lots of fun. Especially when they're hungry. Same goes for their distant cousins, the anteosaurs. One of those (a very old, very large Titanophoneus potens) presents a serious initial problem in my story.
Also, out of curiosity, are you using Inostrancevia, Gorgonops, or (most likely) a completely made-up group of the genus?
P.S. Don't think "I'll never get published." Tons of people get published. Just takes time, effort (one of my creative writing teachers claimed to have wallpapered a room with the rejection slips he received), and an interesting premise (which you certainly have.)