I don't think that 20 varieties of turtle, and 10 varieties of penguin really do a lot to flesh out the world. Unless there are dynamic differences with how an animal interacts with its environment, how that animal looks, or how it affects civilized beings that interact with it, it may be best to stick with abstracted "types" of animals.
Frankly, "Deer" is quite specific enough, as is "Horse"... However, Snapping Turtle, Sea Turtles, and Tortoises? Though they share some superficially common aspects, they're different enough to warrant subtypes, since they have quite different capabilities and environmental requirements.
When coming up with animals, put yourselves in the shoes of a Dwarven Settler. What animals are going to stand out to you as different in a given biome? Which ones are going to seem the same? Are you going to be able to tell and Emu from an Ostrich, and moreover would differentiating them really affect your life, or the success of your expedition? How about differentiating a Pika from a Degu? A Tiger from a Lynx? A Coyote from a Fox? An Electric Eel from a Lamprey Eel?
On the subject of including extinct fauna of different time-periods and realism: The creation of a fossil is a relatively rare event... many species never made a single fossil that we have discovered on Earth. As such, it's quite likely that many Megafauna lived for a much longer time period than we can historically tell, and probably persisted for significantly longer after said record was created. Moreover, it is a fantasy world only modeled after Earth, and fantastic creatures already exist. Why, then, cling to paleontological accuracy so fiercely? Especially when talking about including Dinosaurs and sentient Hominids in the same world? I don't see why we would need a series of different "time period" critter packs, rather than one big cohesive one.
That's just my 2 cents, take it as you will.