Correct me if I’m wrong, but I believe there’s a general principle in biology that, given the opportunity, organisms will tend to evolve to larger body sizes.
A big benefit of being big is that it affords protection from predators. Of course, the predators are meanwhile evolving bigger, badder bodies too (T. rex, for instance), and that’s a perfect example of the well-known phenomenon of evolutionary arms race.
There are obvious limits to how big any species can grow: the strength of bones, the availability of food and habitat, and so on. And of course there are advantages to being small in some cases (for example, the pygmy elephants that evolved on small islands in the Mediterranean, where food and space were limited and major predators probably absent).
Speculation: wasn’t there more oxygen in the atmosphere during the dinosaur era? That, too, could have enabled them to grow larger and still keep all their tissues oxygenated.
My pet theory is that bipedal posture allowed the largest predator dinosaurs to grow much larger than any four-footed predator while remaining comparatively speedy. So the herbivorous dinosaurs increased in size to match.
I tend to favour the idea that they grew big simply because they could. In that era the climate was warmer and CO2 levels higher. This meant a superabundance of food, so much like Americans (and others) today, they just kept growing.
I don’t know, there were many small dinosaurs and mammals that survived by being small and fast and stealthy. So, why wouldn’t evolving smaller also be a natural trend. So, I believe the general trend/options would have been 1) get bigger 2) get smaller 3) develop some other defense. And basically human dominance has eliminated option #1.
This isn’t commenting on the size question but on the earth spinning faster by a fraction of a second each year. His idea is wrong then. The earth would have been rotating slower in the past. As far as the rotational speed needed to change the gravity around the equator by any noticable amount, pshaw. But Cecil could have commented on that.
My theory is that the ecological niches for smaller creatures tend to fill in faster. Over time, existing species will radiate into untapped niches, but the niches that can be filled by larger creatures will be filled slower. In addition, megafauna tend to be more vulnerable to ecological changes- like, you know, incoming comets and angry primates. The dinosaurs had a very, *very *long time to evolve to exploit those niches, and enjoyed a reasonably stable environment for quite a while, resulting in more megafauna than we see today.
In other words, the longer an environment is unchanged, the more species will evolve to fill the “large animal” ecological niche.
Yeah, I thought I had read that too but couldn’t be sure. I was only commenting on his statement: “I read somewhere that the Earth’s day is getting shorter by a fraction of a second every year.” So, if it was getting shorter as he thought, then it’s speeding up. Unless it’s a typo by Cecil? Nah. Couldn’t be.
It’s worth noting that while there have been giant mammal herbivores- mammoths, Paraceratherium, etc.- the upper limit on (land) mammal carnivore size has remained much smaller. We have no record of any mammal carnivore that ever exceeded one metric tonne in weight, and the vast majority were half that or less. The saber-toothed cats apparently adapted to hunting elephant-sized prey not by growing huge, but by evolving specialized fangs capable of fatally wounding large animals. Apparently a bigger four-footed predator would simply be too slow and lumbering even to ambush prey. The chief way dinosaurs were different was that the predator species were bipedal. If that made them faster, then the herbivores would have had a strong selective pressure to grow larger.
Crocodilians were a special case, being semi-aquatic ambush hunters. The test case would be non-dinosaur reptilian predators of the Permian era, but I don’t know much about them- anyone?
Bear-sized, with the only exception I could find being a semi-aquatic ambush hunter similar to later crocodiles.
Using the largest sizes:
[ul]
[li]Dimetrodon - Early Permian: 3.2 meters[/li][li]Titanopheneus - Middle Permian: 2.85 meters[/li][li]Titanosuchus - Late Permian: 2.5 meters[/li][li]Inostrancevia - Late Permian: 3.5 meters[/li][/ul]
All are Synapsids from lines that either produced mammals or with no extant descendants and all comparable to both the black bear (2.4 meters) and the polar bear (average 2.4 - 3, record 3.39 meters) and not much more than a quarter the size of later Tyrannosaursids.
The semi-aquatic ambush hunter was Anteosaurus of the Middle Permian coming in at 5 meters and believed to have hunted much like modern crocodiles. Not sure, but it may have been the largest non-marine Synapsid carnivore in history.
And nitpick, but pregnant polar bears can frequently exceed the 1 metric ton weight without difficulty hunting and obviously you’ll want to exclude pinnipeds and cetaceans from that mammalian carnivore weight limit.
Ok, I should have said “around” a metric tonne to not quibble a few kilos. But my point still stands: I’m unaware of any quadrupedal predators that significantly exceeded that weight, other than the semi-aquatic species mentioned.
I’m happy to be able to contribute to this thread. I have heard, and would be glad to have this verified,that as a general principal that as the size of an organism increases it’s energy demands (or required calorie intake) does not increase proportionately. That is to say that if you double in size, your energy demands will be less than double. This would give a clear evolutionary advantage to getting big, should time and circumstances be in your favour.
The question then would be why did this apply to dinosaurs more than any other group of animals? In particular, we can compare the dinosaurs to the reptiles that existed in the Permian period. The incredibly stocky Pareiasaurs look like they were maximizing their ratio of body mass to surface area but they were only about cow sized.
I wouldn’t suggest it applies to dinosaurs more than any other animal, except that perhaps surface area might play a part in facilitating the heating of cold blood. Most of the explanations in this thread make sense to me. Another way to put the question might be: Why weren’t dinosaurs bigger? Or any other animal for that matter.
I knew a dino expert who told me that it was not conclusive that T-rex fossils have shown actual full-grown size. Apparently, when crocodiles get old their bones fuse (as they do in humans, and other animals), but as yet no T-rex skeleton shows these tell-tale signs of old age. Also, the life spans of crocodiles show continual growth right up to the point of death.