Here I'm revisiting an idea I wrote about several issues ago: 

"What if California, including Baja California, had broken completely off of North America back in the Eocene or Oligocene. It heads south and west a little bit, carrying a full range of North American mammals of the era with it. That would include Tarsier and Lemur-type primitive primates, primitive carnivores like Creodonts, the extinct North American branch of the marsupials, lots of unique herbivores, and maybe even some real exotics like land-going crocodiles, the primitive rodent-like (and possibly egg-laying) Multituberculates, and a side-branch of the primates which developed  rodent-type teeth."

"Island California provides a Madagascar-style refuge for those primates, along with other North American mammals of the era. The primates develop into a full ecological range, sort of like the lemurs of Madagascar. You could have all kinds of unique animals, given millions of years and a fairly large island like that."

Now for the animals. Island California would not be an unchanging world with relicts of Eocene North American life doing the same thing that they did during the Eocene. Some of the initial load of animals would die out.  If Island California went the way of most islands that size, very large mammal carnivores would not survive very long there. At best, an island the size of New Guinea would be able to sustain a species or two of carnivores the size of a mid-sized dog. Larger mammal predators just couldn't have large enough populations to stay viable long-term. 

We'll have the small and medium-sized carnivore slots occupied by descendents of the Hyaenodont branch of Creodonts, with a huge range of species dominating niches ranging from weasel-sized "rodent-eaters" to cat-and-dog-like forms. There are even otter-like hyaenodonts along California's rivers. The largest hyaenodont isn't a carnivore, though it does sometimes catch other animals. It plays a bear-like role, though at 150 pounds it is very small for a bear.

The larger predator niches would be taken by lizards or other cold-blooded creatures. Cold-blooded animals can get by on a lot less food than warm-blooded ones—as little as one-tenth. That's why a little island like Komodo can have a viable population of Komodo dragons. The big lizards in theory can have the bulk of a large mountain lion while eating as much as a good-sized house cat. In spite of that advantage, carnivore lizards usually play a minor role in continent-sized ecologies. Warm-bloodedness usually pays for itself if there is enough food to sustain a viable population, and large carnivore lizards are usually restricted to niches where large predator mammals can't develop.

In the Old World, monitor lizards take on most of the cold-blooded predator roles, especially in Australia. To some extent they converge on mammals, with relatively large brains for reptiles and often a surprising turn of speed. There are currently no native monitor lizards in the New World. I'm not sure if there ever were, but it might be more fun to come up with something different anyway. Crocodiles have from time to time developed primarily land-dwelling forms. They've never been able to compete long-term on the continents, but in Island California we'll have them take over a wide range of large and medium-sized carnivore niches.

For the primate niches, I figure lemurs already have Madagascar to show what they can do, so we'll have the old North American Tarsiers take over the bulk of the monkey-type slots, with some neat forms that sort of resemble baboons, giant koalas, and even apes to some extent. In Eocene North America, some primates took on a rodent-like form, with the big rodent-like front teeth. We'll have those animals taking up most of the rodent niches, with a few surviving Multituberculates (distant, rodent-like relatives of the Platypus) holding on to a few niches. The most prominent surviving Multituberculate is a large (40 pounds or so) species with habits vaguely resembling a beaver's.

The plains and deserts of Island California are a fairly recent development, and one group of Tarsiers have taken advantage of that development to develop forms that resemble kangaroos. They develop from tree-hopping leaf-eaters to savannah-dwelling grass-eaters, then even push back into the rain-forests to challenge more traditional grazing and browsing animals on their home turf. Large ground-living birds that resemble ostriches also push into the grazing and browsing niches.

I haven't quite worked out what the more traditional grazers and browsers would be. A distant cousin of the horse, maybe with three toes, would be interesting. There would almost certainly be a dwarf species of elephant or mastodon, a comparative recent over-water arrival. It could be one of the more primitive varieties—maybe a shovel-tusker. I'll do some more digging to fill in the rest of the grazing and browsing niches.

Marsupials play a relatively minor role in this equation. They do dominate some of the insect-eating niches and compete with the night-living primates to some extent. The largest marsupial is not on island California itself but on one of the off-shore islands. There, a cat-sized, highly predatory opossum is the largest mammal predator in a miniature ecology.

So, what do you think? Should I do a more detailed ecology, and then go back and make the history more solid? I like this scenario, but I have a lot of irons in the fire, as anyone who read last issue knows. There were a lot of part ones in there that really deserve a part two, and not all of them are going to get them this issue.

Note: Starting next issue I'm planning to start an 'e-mail to the editor' section.  If you do e-mail me, please indicate whether or not I can use your e-mail in that section.