Does plate tectonics prevent dominance of multiple clades ("Lost World Scenario")?

[Bulldog Psion]

Okay, time for one of my combined curiosity/world-building type threads here. :smallsmile:

I was thinking about our planet, and how certain clades dominate the entire planetary land surface at any specific time. Despite various fictional "Lost World" scenarios, there has never been a time, so far as I can tell, when such a situation actually existed on Earth.

During the age of dinosaurs, dinosaurs were the dominant clade on all continents, everywhere. Now, during the age of mammals, mammals are the dominant clade on every continent. Even with the "terror birds" in South America, most of the large land animals were mammals. Even with megalania in Australia, again, most of the large land animals were mammals there, too.

My question is, do plate tectonics make this kind of situation inevitable? I figure any planet capable of supporting widespread, complex, large, fairly intelligent life will have plate tectonics as part of the necessary natural cycles.

So, does that mean it would be literally impossible for, say, one large continent to be dominated by mammals, a second continent by dinosaurs, and a third by giant amphibians, for example? Because the continents invariably link up before this situation can arise, and consequently, the better adapted clade crowds out the less competitive clade?

DISCLAIMER: I realize that this is actually impossible to give a definitive answer to with our currently available information. I'm just looking for a discussion that I hope the participants will enjoy, and a few ballpark estimates on whether you could have multiple continents/multiple clades or not. I also realize that you might not see the simultaneous evolution of say, dinosaurs and mammals; I'm just trying to use clearly defined examples of diverse clades that people will find interesting, so please bear with me. :smallbiggrin:

[Comissar]

Very quick, cliff-notes version (I may get a couple of factual things wrong here, forgive me if I do);

What you need to remember about both Dinosaurs and Mammals is that they emerged during a time when the continents were joined as Pangaea. The Triassic saw Dinosaurs divide into Saurischian and Ornithischian, saw the division between Sauropoda and Theropoda in Saurischia, and saw the division between Thyreophora and Cerapoda in Ornithischia. The vast majority of major groups were, therefore, already present before Pangaea had broken up.

The same is true of Mammalia. The split between Monotremes and Placentals/Marsupials happens in the Triassic if memory serves (sketchy on this one, when I have more time I'll do some digging), and one of the earliest placental mammal is present within (I think) the early Jurassic, which means the split between Marsupials and Placentals must have happened before that.

We therefore already have all major groups present and ready to go on Pangaea. There's all sorts of environmental factors at play which favour certain groups over others, which I'll get more into in a follow-up post, but the Synapsids (mostly mammals by this point) and the Diapsids (most notably Archosaurs, which includes Dinosaurs and Crocodiles), are by far and away the dominant terrestrial groups throughout the Mesozoic and Caenozoic. The fact that they were all present in their various major divisions while they could still move freely across the world just means that there was no continent devoid of either.

The closest analogue I can think of to what you're asking about would be discrete faunal provinces, but these aren't distinct regions which are dominated by a single clade. Instead they're regions that have been isolated (normally geographically, think Galapagos) and gone on to evolve their own unique organisms. In fact, the Galapagos islands (pre-people, anyway) is probably the closest you'll see to what you're suggesting. So far as I'm aware, there was no real mammalian presence there until humans arrived. Instead, it was dominated by Diapsids.

As I said, I'll do a more detailed post later tonight when I have the time to look up specific deep time examples for you.

[Grinner]

Here's a scenario:

Several landmasses are linked together, upon which dinosaurs, mammals, and even giant amphibians develop. (How would a giant amphibian exist, anyway? Something like abyssal gigantism?)

At least one of the landmasses detaches and goes sailing off into the horizon. Later, a pathogen develops which kills off the dinosaurs or at least impacts their survivability enough* that Mother Nature takes care of the rest, leaving the mammals unaffected. The pathogen, however, never reaches the distant continent, whose ecology remains largely static, and the pathogen dies out. Humans evolve, develop agriculture, and eventually discover the lost continent. Velociraptors are deadly, but the humans have enough technology to match them. (Flintlock pistols, maybe?)

*Pathogens which are too deadly tend to kill off their host before they have the opportunity to spread.

Edit: Ninja'd.

[Lord Torath]

Well, Galapagos seemed to be dominated by giant tortoises. Granted, it's an archipelago rather than a continent, but it does seem to be a counter-example.

New Zealand had giant Emu Moas that seemed to be fairly dominant as well. Certainly bigger than the Tasmanian Devils. So at this point I would argue, no, it doesn't prevent multiple clades being dominant at the same time on different land masses. Any time those land-masses meet, there is going to be competition and die-off, with one clade probably displacing another. But if there are 10s or 100s of millions of years between contact, I can see different clades arising in different locations. Especially on landmasses that get different but stable climates.

I'm not really that familiar with the mega-fauna in the late Plasticine, though. Someone with better knowledge could probably shed better light on who really dominated New Zealand before people killed the giant emus moas, and who was really on top in the Galapagos Archipelago.

Edit1: Double-ninja'd!
Edit2: Fixed name of Large bird on New Zealand. Thanks, Dire Moose!

[halfeye]

Australia is the place with most of the marsupial mammals. There, the biggest predator is probably the crocodile. They can be pretty ferocious (not the same species) in Africa too.

[Dire Moose]

The marsupial-placental split happened sometime in the Middle Jurassic or earlier, as the earliest known member of either clade is the ~160 million-year-old placental Juramaia. At what point these diverged from monotremes is still unknown due to lack of evidence, but probably happened in the Early Jurassic after the earliest primitive mammals had appeared then.

As for the topic of the thread, while distinct faunas do develop when continents are in isolation (South America was home to several unique clades of mammals, as was Africa, and Australia still is), it's still the same general groups dominating. That said, it's still possible for some isolated landmasses to have other clades dominating. The best example I can think of is New Zealand, which as mentioned has no native land mammals; it has been dominated by birds instead. Also, minor nitpick: The giant flightless birds there were a separate group from emus known as moas.

[Khedrac]

Short answer is no it doesn't.

Look at the history of New Zealand. Before man arrive those islands were remote enough to be dominated by birds - and this is during the "age of mammals" in other places.
The biggest animal was probably the moa - which were wiped out by the Maori when humans reached there.
The top predator was the Haast's Eagle which is generally believed to be behind a Maori's mythological boogy-monster (it was a bird that had evolved to eat man-sized bipeds and then man arrived there...).

So yes, given sufficient isolation any of the existing clades can evolve to be dominant in a given area. What does require a lot more time is for new clades to develop.

OK, New Zealand is hardly a large continent, but given slow enough movement the same should apply to continents. As people have observed just look at the differences between the life on Australia and everywhere else (except for Papua New Guinea which seems to be a hybrid). If man was removed from the equation it might be interesting to see what will come to dominance on Antarctica after it moves away from the pole and looses its ice - my guess would be descendants of the penguins. Anything similar can clear a continent allowing something different to develop without affecting the other continents.

[Bulldog Psion]

So in other words, you could have radically different clades on different continents, assuming that plate tectonics functioned in such a way as to prevent those continents from ever linking up, even temporarily.

I'd imagine this works better with smallish continents like Australia, and it could work out even better if they were even more isolated in some fashion. Less island "stepping stones" and all that.

[cobaltstarfire]

They also have to be far enough apart, even without islands to "hop" across birds still manage to make their way between continents.

Like, the Cattle Egret has been spreading everywhere. It simply flew across the Atlantic from Africa and slowly crept up through the Americas. (and upon looking them up they don't even originate from Africa).

Other birds also get blown across the Atlantic regularly, I don't know if any of those species establish, if they're trapped forever alone, or if they can make it back to their home range though.

[noparlpf]

Depends on what you mean by "dominance," too. Dinosaurs still make up a pretty sizeable percentage of living animals. They've just adapted to a different environment. I don't know whether mammals would actually get as big as elephants in a Triassic or Jurassic climate regardless of large dinosaur predators.

[Manga Shoggoth]

Depends on what you mean by "dominance," too. Dinosaurs still make up a pretty sizeable percentage of living animals. They've just adapted to a different environment. I don't know whether mammals would actually get as big as elephants in a Triassic or Jurassic climate regardless of large dinosaur predators.

Also the insects and bacteria are far more successful than humans. They just don't get the good press...

[sktarq]

Actually just look at all the southern continents. After the KT boundary event (DireMoose I know that the formal terminology is now something else but hush) basically all the southern continents where dominated by non-placenal non-multiturbuculate mammals and went their own way and even when they did later get large placental mammal infusions (in S America and Africa) they then reisolated and developed very differently-hell look at Africa 35 mya. So yes it would be possible-but would require a lack of pangea events for a long time (in our case since the evolution of amniota)

[Bulldog Psion]

Depends on what you mean by "dominance," too. Dinosaurs still make up a pretty sizeable percentage of living animals. They've just adapted to a different environment. I don't know whether mammals would actually get as big as elephants in a Triassic or Jurassic climate regardless of large dinosaur predators.

Large animals who occupy most of the major habitats and large-scale niches. I'm not counting numbers because bacteria and insects are always numerous.

In my case, I'm talking about basically what the Stereotypical Big Game Hunters of the "Start of Darkness" book would notice when they arrived on a continent. They wouldn't say, "By Jove, look at the number of soil bacteria in this sample, old chap, never seen anything like it!" :smallbiggrin: It would be "What a remarkable assemblage of mammals, old bean!" or "I say, rather a lot of dinosaurs here, what?"

During what we might call the "Age of Dinosaurs," mammals were small and rather furtive, occupying the ecological niches the dinosaurs "left" unoccupied. In the Age of Mammals, birds are mostly small and occupy the ecological niches the mammals don't fill. I realize there are some exceptions like the ostrich, but in the main, as in the case of the "terror birds," when they came into direct competition with the mammals, the birds lost out. And mammals didn't occupy a broad range of niches until the bolide impact finished off the dinosaurs.

[LibraryOgre]

And a few changes to our plates would make them less connecty. For example, remove Asia, or make it essentially an "ocean plate" (i.e. we've got a lot more water, and the Asia plate is much lower). If you do that, North and South America are still quite close, as are Europe and Africa, but otherwise, there's not much closeness. There's less traffic between Africa/Europe and North America (which went over Asia and the land bridge about 12kybp), and no route from Africa to Australia via that land bridge/island hopping.

Do that, and you're looking at 140 million years of separation between Africa/Europe and the Americas, and about 100 million years separation between Australia and Antarctica.