I was elated and disappointed in the show. Elated, because some of the concepts were really, really cool. Disappointed because they did such a half-*** job with them.
The following is just my belly-aching. I really liked a ton of the concepts, from the giant tortoise to what I like to call "the blue bird of happiness" (anyone who saw it knows what I mean - it was gorgeous). Many of the concepts were very creative and believable. The following is just things I wish they'd fixed.
1. The silver spiders. At that size, should have been shaped like tarantulas (or even thicker!) unless Earth's gravity has drastically changed in 100 million years as well. Thickened legs and reduced abdomens would have fixed this easily - maybe model the spiders off the african dinner plate-sized spiders instead of a spider that's about the size of my pinky fingernail.
2. The flish. Why have their scales not altered to provide them better lift? Since they primarily hunt a small, rounded swimmer, why do they have parrot beaks instead of something better adapted like a scoop or spear? Why didn't the narrative mention, even once, that the hypercanes cycled (probably monthly), since that would have made the bumblebeetles' life cycle a lot more believable?
3. Food & energy. The silver spiders are unlikely to be able to live on the energy of grass seeds converted into meat. You lose 90% of the energy/mass when you convert it by eating, so right off the bat, they've got mass equal to only 1/10th what the seeds would have provided. If they were growing grass from the seeds, which the mammal ate, it could have been more believable.
4. Food & energy 2. The giant land squid is too big, slow and stupid to be a predator - it's not going to catch enough food to feed its own bulk. It should either hunt much smaller prey that it can suck up from the ground, or become an herbivore. As it is, it will die out as a species within days.
5. Why the heck did sharks evolve, again? They didn't seem to be any more effective than normal sharks would be. In evolution terms, that equals "keep the old design".
6. Ecologies. The presented ecologies were extremely simplistic. While I understand the needs of the show, it would have been nice to have some background stuff, even if it wasn't explained.
7. The rain forest was not built for rain. Why did it develop mile after mile of horizontal gymnastics bars, but not huge, rain-catching leaves and flowers? Were there no scientists available to "believe" something about the plants of the future?
