Duvenoy

Thanks, Mr. D.

Actually, I was thinking more along the lines of terrestial species and had forgotten all about icthyosaurs.

Thinking about it, I'm now a little amazed that penguins haven't evolved it. Consider the Emperor Penguin. This unfortunate bird hatches it's young in the middle of Antartica's dark, howling, misery of a winter. The birds huddle together in a flock, each with an egg on it's feet that's covered by a sort of up-side-down pouch of skin. The partner not incubating must walk a long way on short legs to find open water to feed.

Wow!

doov

MrDarwin

I don't think a species can adapt to unusual and unpredictable events, but a species can already have certain adaptations that allow it to survive, which I think might be what Neruda was getting at. For example, scavengers, omnivores, and generalist herbivores are probably more likely to survive a mass extinction than creatures with more specialized dietary habits. A plant species that is adapted to exploiting disturbed habitats, is pollinated and/or disseminated by wind (or can self-pollinate), and is able to survive as dormant seeds in the soil for years at a time (we usually call them "weeds") is more likely to survive than an epiphyte that grows at a certain level in trees, depends on certain pollinators, and has seeds that are spread by certain animals and remain viable for only a few days.

In other words, mass extinctions are survived by weeds and scavengers. If we and all the large mammals die off in another mass extinction, the sentient beings that look back on it 20 million years from now will probably be descended from rats, and their crops will be descended from ragweed and dandelions.

pz

That's almost certainly it. Calcification of the egg shell is thought to have evolved primarily as a readily available calcium reserve for the embryo, and not as a protective mechanism. Every stage of embryonic development may be adapted to having an easily mobilized supply of calcium available, and losing it would just mess up far too many processes to make it a likely proposition.

Kosh

The origins of this thread have all the markings of a drive by posting....

MrDarwin

I think it's too late for them (i.e., I don't think penguins or any of their descendants will ever evolve ovovivipary). I suspect that the marine reptiles--plesiosaurs, ichthyosaurs, mosasaurs, etc.--were descended from land-living reptiles that were already ovoviviparous. I'm willing to bet that modern sea snakes are descended from ovoviviparous land snakes (anybody have any data?). For whales and other marine mammals, it was never a problem (and perhaps things like otters and sea lions are on their way to a fully marine lifestyle, once they "figure out" how to give birth in the water). But for birds, they were already committed to laying hard-shelled eggs before ever becoming aquatic. Lacking a specific pre-adaptation, they're stuck.

Second thoughts regarding whales, otters, and sea lions:

Perhaps the ancestors of whales already had another pre-adaptation: babies that weren't helpless. I'm not sure how widespread it is in hoofed mammals (the group from which whales evolved) but their young can (usually?) stand and walk within minutes of birth, in other words are mobile and and at least somewhat self-sufficient--certainly a prerequisite for giving birth in the water. Carnivores, on the other hand, frequently (always?) have helpless young that are cared for in a nest by the parents for several days or weeks; given this, perhaps sea lions and otters are doomed to an amphibious lifestyle, like penguins. The requirement for parental care of the young, like the requirement to lay eggs, may be an insurmountable obstacle to a fully aquatic lifestyle.

[ September 25, 2002: Message edited by: MrDarwin ]</p>

MrDarwin

Ugh. I went searching for information on sea snake evolution and this is the first website that came up on a Google search:

<a href="http://www.users.bigpond.com/rdoolan/illusion.html" target="_blank">Evolution’s illusions</a>

Duvenoy

The site's a doozy!

Good luck finding much on sea snake evolution. I don't know of anything on the topic. Indeed, serpent evolution info in general is a little sparce.

Sea snakes are Elapidae, and as such are egg-layers. Here's a site for venom info. Dr. Brian Fry has been working with them and venoms in general, for a long time. It's a site well worth exploring.

<a href="http://www.kingsnake.com/toxinology/welcome.html" target="_blank">http://www.kingsnake.com/toxinology/welcome.html</a>

doov

Doubting Didymus

Yes, that is very likely indeed, if only there is a big enough advantage for them to do it. I think you underestimate evolution. Give it enough time and the right circumstances, and practically anything is possible. Remember that we got the original tetrapods from fish of all things.

That's not the point. Most things remain unchanged where there is no advantage to change, My point is that there is nothing stopping giant fearsome bipedal lizards from roaming the earth again, as per the original posters question (who unfortunately seems to have quickly lost interest). I think it is possible that lizards could become bipedal, winged, marine, or even intelligent. It won't happen anytime soon, and if there is no selection advantage for massive change, it will obviously never happen at all.

Exactly. There would need to be a fair shake up of the niches before change like that will occur. Someone here once used an analogy of a bucket of plastic shapes and blocks, each representing a species. The blocks are going to settle into place, and are not likely to shift around. However, if you give the bucket a great kick then all the blocks come out of place, and then settle again into new positions.

Whoever that was, I thought it was great. It is a perfect analogy for stasis in ecology, and how large scale change can restructure the niches.

Again, I think you underestimate evolution. A single cell can become an eagle. A jellyfish can become a man. A lizard on all fours can become a lizard on two legs.

True, the reptiles lucked out by getting into all the animal niches first, and mammals have got them all filled now. I also agree that mammals or birds would probably get there first.

That's awfully fatalist of you. What makes you think that the large mammals are doomed?

Doubting Didymus

Posted by morpho:

Table 1 tells me that oxygen levels during the jurassic and triassic were lower than even todays levels.

KeithHarwood

It's quite likely that mammals as a group will be overtaken by something else, but some will survive, just as amphibians and various reptiles do today. And the chances are that whatever overtakes mammals is already here and has been since shortly after the K-T exinction. It is probably known to science under the technical description of `A Funny-looking Mammal'. (Probably not marsupials, almost certainly not monotremes; any bets on the naked mole rats of Kenya?) But it won't get its chance until the next big extinction.

That's what happened last time. At the Permian extinction large numbers of ecological niches were declared vacant. Reptily things evolve to fill those niches and mammals evolve from reptily things. By the time the mammals have their act together most of the niches are full and they have to remain a niche player (sorry about that) until the K-T exinction. Now the mammals evolve to fill the vacant niches, though it looked touch and go for a while there when, according to `Walking with Beasts', the mammals were the prey and the predators were birds. And the mammals also evolve into the funny-looking-mammals, but, again, by the time they got their act together most of the more desirable niches are full. The next time the sky falls on our heads the flm's get their chance.