Are there still dinosaur sized animals?

Chris Smith put this to palaeontologist Jason Head...

Jason - Well, we do. We have animals alive today that are larger in terms of their body mass than any of the dinosaurs but they live in completely different environments. Dinosaurs are exclusively terrestrial in their fossil record I should say. So the largest terrestrial dinosaurs are a group called Sauropods. These are again, to go back to your bag of plastic dinosaur toys, these are the ones with the long necks and these animals include some absolute giants.

There’s a group called Titanosaurs, and several members of that clade may have exceeded 60 to 70 tonnes possibly, but the largest Baleen Whales today still exceed them in overall body mass. They are marine animals where you effectively have no weight in water. Nothing on land today approaches dinosaurs in their size and, in part, that’s because the anatomical physiology of dinosaurs compared to mammals.

Birds are effectively hollow boned dinosaurs whose lungs invade their skeletal tissues. They’re extremely light and it’s very likely they inherited that from dinosaurs that had evolved those strategies initially to achieve large body size.

Chris - When you say their lungs invade their skeleton, what do you man?

Jason - Birds have actually very neumitised vertebral columns, and they’re sacks of the lungs that occupy the spaces within the vertebrae. And so it’s part of this very complex flow through lung that birds have where respiration in a bird doesn’t actually have a dead phase. So, every time you inhale, as a mammal you get this good oxygen and every time you exhale, all you’re doing is getting rid of carbon dioxide. With the very complex lung system that birds have, they’re constantly moving oxygenated blood across their tissues to exchange gas, so they never have one of these dead phases where there’s no oxygen going to the bloodstream.

Chris - And that, obviously, enables them to be very active and to fly very fast and they’re not compromised, and it means they’re lighter?

Jason - Exactly.

Chris - So what’s the relevance to a dinosaur then of that?

Jason - This may very well have initially evolved as an adaptation for being large, so to be a fairly large, initially bipedal animal. The earliest dinosaurs were fairly small but they were bipeds bigger than most birds are today. And so you had to deal with issues of body mass, and one of the ways to lighten the load is to have hollow bones and to have the very efficient respiratory system.

Chris - Interesting theory, but don’t you end up needing bones that are really really strong to hold up a really big mass. Because I remember, I went on a safari in Africa about 20 years ago. I was in Zimbabwe and I found an elephant's leg bone. When you looked at it in cross section, unlike a bone you’d get in a butcher from a cow or something, which would have a very clear marrow cavity where the bone is quite different, it’s spongy in appearance, and that's where all the stem cells that make all your new blood are, in the elephant bone it was almost like solid rock right the way from the outside, the cortex, right the way to the centre. And I asked the safari guy why is it like that? And he said well, elephants make most of their blood in their pelvises and they actually have legs that are solid like tree trunks because they’re supporting 6 to 8 tons, and if they didn’t have that their bones would bend. So what does a dinosaur do then?

Jason - The largest dinosaurs had very massive elephant-like columnar legs. The arms and the legs are the weight supporting bones are incredibly dense, but the vertebral column is actually incredibly delicate. Especially in these giant Sauropods, the vertebrae are almost just a series of struts. They’re very very pneumatic, they’re very light, and they’re very specialised for basically resisting torsion, resisting forces while not being massive and dense.