Duvenoy

And now you know, and I did indeed misspell it. I haven't bothered to touch it up.

All snakes posses Duvernoy's organ, giving them the potential to become venomous. It is interesting to note that the organ is diminished in constrictors such as the big Boids, who have found an alternant method of subduing prey.

Actually, I was responding to DD. Sometimes I don't word things well. That and speling will someday be my downfall.

doov

Doubting Didymus

I think they sure as heck could evolve bipedalism, given just a bit of time. The bipedal dinosaurs (and every other animal as well) came from fish after all. There would not be any insurmountable barriers rearranging the pelvic structure, and like you said some lizards can already do it for short periods (and some can do it over water for short periods).

I know what you are talking about, but magazines original question was not 'why didn't T-Rex come back'. It was the more sensible question: 'If the reptiles evolved once to become the big beasts at the top of the food chain, why couldn't they do it again?'.

I think Magazine probably understands that T-Rex doesn't come back once he is extinct, but it is kind of obvious that any organism that evolves to be big and nasty will have a huge selection advantage, so why did this selection advantage not produce more big nasty reptiles? The answer is simply because the mammals took the niche before the reptiles could jump back in. Yes, its convergent 'nonhomologous' evolution, but its the answer to magazines question. The selection advantage for bignastiness was always there, but the niche post-dinosaur was filled by the mammals.

They wouldn't really be dinosaurs, but they would be giant terrible reptiles.

Duvenoy

Yes, the Basilisks are especally known for running over water. Jesus ain't a patch on these remarkable lizards.

But I must disagree on the likelihood of lizards becoming bi-pedal. If you you've seen ever seen a slow motion vidio of the gait of a fleeing lizard on it's hind legs, you will notice that while it is fast, it is also clumsey. Their pelvic structures just aren't designsd for it and it is exausting for them to do it for any distance.

This isn't to say that it can't happen; just that it's highly unlikely. Lizards are very successful just as they are, showing an excellent, evolutionary design.

As evolution can only work with what parts happen to be laying around the shop, so to speak, changing the pelvis to support the body directly over the hind legs, as in all bi-pedal animals, seems to me to be not a good modification for an already great design. Not to mention all of the other modifications necessary - spinal attachment to the skull, spinal modification to support an entirely different weight distribution, shoulder modifications, and so forth. Lizards might evolve into something else, but unlikely I think, bi-pedal. I do not think that the Earth will ever see a new Dinosaur, at least not one that came from Family Squamata. Or Crocodilia.

So, what might turn out to be a 'new', bipedal dino? How 'bout a Cassoary (had one of these at a place where I worked. It had a truly vile disposition and was considered the most dangerous animal there - a very high rating, considering what else was on the premisis)? Or any of the flightless birds, including some of the Grebes? They're already built for it. Perhaps a several ton penguin

It is impossible to predict evolution. The best we can do is guess based upon the evidence we have before us. A future, extended change in climate, hotter or cooler can easily throw all of our thoughts out with the dish water. But hey, ain't it fun to speculate?.



doov

Doubting Didymus

I agree with you on the cassowary! I think birds are an excellent candidate for a large beast niche takeover should the large mammals die out for some reason. There have been some very dinosaur-like birds in the past, though none of them flew. I want to see that penguin!

But still I must disagree that there is some kind of barrier 'preventing' lizards from becoming bipedal.

If we humans can evolve near perfect upright bipedalism from a lobe finned fish (which doesn't even have legs), then a lizard should have no problems becoming bipedal, as long as there is a selection advantage for it.

I should also add that there was nothing specifically bipedal about the dinosaurs. We could certainly get massive four legged nasty reptiles from a lizard, crocodile, or even a turtle.

While it is true that we can not predict evolution, I think that bignastiness would evolve again and again any time you level the playing feild (with a big rock, if neccesary).

Vorkosigan

I must respectfully disagree. I dimly recall reading that oxy levels were different in the dino days, so dinos could get more of what they needed to grow.

ALso, I am wondering, since islands have "island effects" that induce dwarfism among their large creature populations, why that wouldn't, over enough time, apply to the earth as a whole. Maybe that explains why todays mammals are on the whole smaller than their ancient counterparts, and a lot smaller than dinos.

Michael

Doubting Didymus

Indricotherium, the largest ever mammal, was twice the height of an elephant and eight times the size of a rhino. Only about half the size of a diplodocus, but still. That was 30 million years ago to the dinosaurs 65. However, I am not sure where the oxygen levels you speak of dropped off. Got any links?

Mammoths, though much smaller than Dinos, were 10 feet tall at the shoulder. That was just 10 000 years ago.

I am not sure what you are talking about. I can not think of any reason why there might be a global trend toward shortness. Could you give me more to go on about this?

[ September 24, 2002: Message edited by: Doubting Didymus ]

[ September 24, 2002: Message edited by: Doubting Didymus ]</p>

Quetzal

Since we're just speculating - Morpho's Fun Facts:

1. DD and Duv: Cassawry shmassawry. Birds already had their shot. See Diatryma and the rest of the Paleocene Gastornithidae. Egg sneaking mammals already put paid to the birds' bid for mastery.

2. Local name for basilisk in Nicaragua is "jesus lizard". Funniest damn thing I ever saw was a basilisk skittering across a creek near Matagalpa. Why couldn't they eventually develop true bipedalism? Looks like they're already on the way...

3. A good article on oxygen content and atmospheric density leading to both giantism and flight: <a href="http://www.eeb.uconn.edu/Courses/Eeb477/Dudley_98.pdf" target="_blank">Atmospheric oxygen, giant paleozoic insects and the evolution of aerial locomotor performance</a>. See table 1.

Duvenoy

It's a matter of specialization. Consider, is it likely that the whales could return ashore?

Lizards are not nearly as specalized as the Whales, nor even seals. They have retained a design far more ancient. It's a design that shows up in amphibians such as modern salamanders and such ancient creatures as Ichthyostega and others. In short, it works, has worked for a long time - since the Devonian, and that's all evolution asks.

I wouldn't call it a 'barrier'. After all, even whales ultiminatly evolved from this basic tetrapod form. I'm merely saying that lizards are successful enough, fill enough ecological niches, that they are unlikely to become bi-pedal.

That is not to say that, given the right conditions over an extended period, they couldn't get large. Witness the afore mentioned Komodo Monitor. This species evolved from a mainland species that was probably no more than 6 or 7 feet in length, max (about average for most of the larger Monitors). No competing preditor / scavengers around allowed it to become what it is today. A fearsome and fascinating beast indeed, but still a mere Monitor lizard.

To become bi-pedal, or even to become dino-like, the animal must stand up; get it's belly off the ground. It's legs must support the body directly, not have it hanging between them. This would require such a great number of changes to the basic lizard that I find it unlikely.

Also, I might mention that at one time or another in their lives, all reptiles are prey species, even the Crocs, the Cobras, and the Komodos. Many, if not most, are prey for all of their lives.

But somewhere along the line of time, the dinos evolved from that same, basic, tetrapod form. However, I think that there were a lot more niches open then than there would be if only the large mammals dissappeared today. And too many other creatures better designed to exploit them. Take that same Cassowary, modify it's diet, and put an eagle's beak on it! Yikes!! That sucka don't gots to get big to be bad!

There are also the small mammals to consider. In the absence of the larger ones, these would prosper and soon begin to fill abandoned niches (assuming that whatever killed off the larger ones didn't affect them). I'd think that they would have a head start on reptiles (a 1000 pound Skunk? Yikes!!).

If all the larger mammals were to vanish, and someday we certainly will, I do not think that they will be replaced by reptiles, giant or otherwise. There would be too much competition and reptiles lack both speed and stamina (hard to believe for anyone who has ever tried to catch a Race runner or sneak up on an Alligator, but true). More likely, I believe, they would become one of the main, dietaty staples of whatever filled the vacated, predatory niches. As indeed, they pretty much are today.

Interesting speculations.

doov

Duvenoy

Amazing, aren't they?

However, this bi-pedalism is strictly a defensive posture, not unlike a Cobra's hooding. It is to avoid danger. The Basilisk still does it's traveling and tends to it's business with it's belly on the ground. Until it can get it's legs under it instead of splaying out to the side, it will never be bi-pedal. This would require a lot of changes to the basic lizard. As is, even though it is fast, the basilisk's (and others) bi-pedalism is very inefficent. The lizard can't maintain it for any length of time.

I'd never say it can't happen, but I will say that it's highly unlikely.

Birds are actually quite successful in defending their eggs. Otherwise, there'd be no birds. Perhaps the birds turn has yet to come.

Evolution is a wonderful thing. Many serpents have become ovaviviperous. I am currently raising 3 batchs of neonates from snakes who didn't bother to go through the egg-laying phase. Why could not some terrestrial birds evolve to hatch the chick(s) in it's body? Why, I wonder, have some of them not already? I further wonder if some dinos were ovaviviperous. Why not? They certainly had time to evolve it and the advantages are obvious.

Open a nich in the ecology and something's going to fill it. I think that birds and small mammals would be more competitive than reptiles.

doov

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

MrDarwin

Some dinosaurs may or may not have been ovoviviparous. But certain other extinct reptiles, like ichthyosaurs, are known to have been. In fact, this was critical to their evolving a fully marine lifestyle.

Similarly, ovovivipary would certainly be helpful to certain birds, like penguins. Think about it--they could become as big as whales if they didn't have to come ashore to lay eggs! But whereas several different groups of reptiles have independently evolved ovovivipary, not a single bird ever has, suggesting that egg laying in this group has gone down some genetic path that is difficult to modify, even given the right selective pressures (possibly related to the calcification of the shells?).