Thread: Maximum size of top predators: dinosaurs vs mammals

I have a question regarding the maximum size of top predators. A NewScientist article explains the maximum size of presently living animals from their energy need. But I wonder how this worked with carnivorous dinosaurs and other extinct predators which obviously grew much larger and heavier than anything that still exists.

What it comes down to is: why did the bodies of dinosaurs function so much more efficiently than those of mammalian predators, allowing them to grow so much larger? Did their cold-bloodedness make such a huge difference, or could they simply get more food at the same effort?

From the article:

Extrapolating from this, Carbone's team predicts that carnivores should be unable to make a living at body sizes greater than about 1000 kg. Sure enough, the largest carnivore known from the fossil record, the short-faced bear, weighed in at 800 kg to 1000 kg, and the largest living species, the polar bear, averages just a little smaller.
Tight budget

Even large flesh-eating dinosaurs such as T. rex probably had energy needs roughly equal to that of a 1000 kg mammal, the researchers note.

Every organism can only be understood in the context of the larger ecosystem. Size, speed and so on evolves to meet immediate needs....natural selection.

I'm not a biologist but a paleontologist. A tyrannosaurid theropod had to eat. He ate meat. He had to eat enough to sustain his metabolism. I have a collection of theropod teeth and have ceratopsian and hadrosaur bones with distinct theropod teeth marks. So we can add that these top predators ate 'big herbivores'. I've collected material in the late Cretaceous in Alberta that spans about 20 million years from 85 to 65 million years ago. These animals and variations of them were quite stable and successful over a long span of time. Those are facts and the rest is all educated speculation.

Over that 20 million years there is, however, a slight increase in the size of predator and prey. The largest theropod (T Rex) is larger than his predecessor (Albertosaurus) and the largest herbivore (Triceratops) is larger than his predecessor (such as Centosaurus). It's quite striking, however, how little variation there is . Neither predator or prey went off on some tangent of evolution. This suggests that conditions were very stable in the Late Cretaceous. Not much pressure of natural selection to meet environmental pressures.

There was no tangent of large prey evolving to run much faster (develop longer legs, etc.). The large theropods, in turn, did not evolve any great speed. My speculation: This wasn't the Serengeti with lions chasing down impalas but rather a much more plodding relationship. I think the large theropods (the tyrannosaurids) spent a lot of time in low metabolism waiting for passing prey (like a croc does), more carrion eating, more taking killed prey from smaller theropods. The 'big guys' played the role we see today of crocs, kimono lizzards, grizzly bears and so on...and not the role of leopards and lions. These large predators didn't have a lot 'upstairs' as we see in more thinking, calculating predators. They muscled in on the food, filled their bellies and then took the next couple weeks off. A large animal with that might have got out of first gear for short spurts but then parked himself in the driveway.

I've posted some of my theropod fossils (Trex, velociraptor. etc.) on my paleontogical website (modestly called 'the World Paleontological Society'). Click back to the main page at the bottom if you'd like to view more dino fossils and other specimens)

http://www.geocities.com/joe2nora/fo...l?974833839820

I remember hearing that animals were able to get so much larger in the past because there was more oxygen in the atmosphere. As the oxygen levels dropped, so necessarily did the size limit. Not sure how accurate that is though.

Thanks for the info, and that's a nice collection you have jelly! :wink:

Yea I guess such 'laws' as "a preditor can't be bigger than 1000kg" really depends on the context. Alberta in the Cretaceous probably had an upper limit as well, but governed by different factors than in our present world (maybe water or temperature played a different role?). Or maybe the preditor to prey ratio was different, with relatively more prey in the Cretaceous making life for preditors easier.

By the way that oxygen theory of Neutrino, is there a logical relationship for that? Would be interesting.

average temperature during much of the Dino period 240m-65m years ago was much above what we see today. tropical conditions we see in some places today probably covered much of the planet. also there should have been a thick atmosphere or continuous high humidity. these tend to not burn and do create oxygen in higher degrees than perennial forest. i would think then oxygen levels increase during this period to near what we have today.

some know, i question the cause for the 3rd worst mass extinction 60-70 million years ago. i am trying to get a fix on barometric pressures and levels of atmosphere at or during this time. either has or could have resulted in this extinction, likewise be a cause for size.

the recent find in Japan of what was truly a left over from this period, normally lived at one mile depths in the ocean, found in shallow water and died shortly after in a shallow tank.

Just a reminder that '240 to 60 miilon years' and 'the world' are somewhat broad concepts. Today we ony have a few fossil 'snapshots' of ecosystems to make educated guesses from. 240 million years ago isn't 239 million years ago just as you couldn't judge the ecosystem of where I live from fossil from 'only' 50,000 years ago. Add that to the diversity of ecosystems across the globe and generalities can be misleading. An ecosystem like that of northern Scotland would yield different view of the world from that of a plateau in Chile or the island of Martinique. The large predator ecosystems of the Jurassic through to the end of the Cretaceous were niche systems but there was 'a lot more' going on that left little if no fossil record. Fossils are not rare but formations that yield fossils that can lend thenselves to comprehensive biostratigraphy (what I do) are few a nd far apart in the fossill record.

yes, i know and anything i could develop only a hypothesis. additionally there are continental shifts, maybe even an axis differential to consider. i just don't accept what was one of the longest known living species dieing out from something that was ongoing through its history. what ever the cause i also feel is the eventual cause for mans existence or re-existence.