Why did dinosaurs achieve so little?

14 November 2017

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Question

Dinosaurs were around for over a hundred million years, why did they achieve so little?

Answer

Chris Smith asked palaeo-zoologist, Jason Head, from the University of Cambridge whether dinosaurs were under-achievers? 

Jason - I love this question so much because it allows us to probe a few things about how we think about dinosaurs. Dinosaurs get their start around the same time as mammal groups do. Where we are today we have about 5500 living species of mammals and we’re pretty impressive in terms of the diversity of sizes and ecologies that we do today.

Dinosaurs today are somewhere between 10,000 and 18,000 living species, depending on how you measure species in modern birds. They evolved intelligence; they do amazing things; they fly better than any mammal. Then when you think about the mesozoic record of dinosaurs, they achieved giant sizes on land, they had an incredible diversity of behaviours, body sized, and ecologies.

So it’s important to not think of dinosaurs as just being big lumbering things that lived in the past. They are one of the most successful and diverse groups of vertebrates in our history.

Chris - You’ve got a lump of one in front of you. That looks incredible… what’s that?

Jason - This is actually not a dinosaur. This is better than a dinosaur. This is the cast of the vertebra of the giant snake Titanoboa. Titanoboa is a fossil snake related to modern boas and anacondas and this animal occurred about 6 million years after the end of the Cretaceous, so after the end of the non-bird dinosaurs, and it was about 15 metres total length.

Chris - So how long ago is that dating from in millions of years?

Jason - We’re looking at about 58 to 60 million years ago.

Chris - Okay. Can you just describe, so people at home can actually picture this in their minds?

Jason - Yeah. So this is basically kind of a squarish vertebrae with a ball and socket joint on it. It is about 5 to 6cm wide and about 4cm long.

Chris - It completely covers my hand if you put it across the palm of my hand. This backbone bone is completely covering my hand. So that would have been along the spine of the snake and the ribs would have opened up off of that and come round towards the front of the snake. What would have been the diameter of the snake’s body then where that was?

Jason - This is one of about 300 vertebrae, probably, that would have composed the spine of the animal, and then the ribs coming off would have given the snake a diameter of about half to three quarters of a metre.

Chris - Uuph!

Jason - And the total weight of the animal would have been somewhere between three quarters of a ton and a ton and a quarter.

Chris - Goodness! What would it have eaten?

Jason - That is a really interesting question. It occupied an ecosystem with giant freshwater turtles, giant primitive crocodile relatives, giant lungfish. And the answer is... whatever it wanted!

Chris - And it’s a constrictor so it would presumably have crept up on these things, grabbed them, and then inspiraled them, would it?

Jason - With a constriction force that no biological tissue could withstand. I cannot remember the number, but it is exponentially larger than the constriction force of boas and pythons today, and those animals are extremely powerful.

Chris - Lee?

Lee - I just love being a scientist because only scientists would use the understatement of every time we find a big fossil call it Titanoboa. It’s just fantastic.

Jason - Well, the original name my co-author, John Block, University of Florida, wanted to call it Tiranoboa, and I told him no. I do also want to point out - this is important - Lee’s Homo specimens, you can actually see the scans of them at morphosorus.org. The Titanoboa’s there as well. So if you’d actually like to see a 3D reconstruction of these vertebrae or download the datasets to print your own, that’s where you should go.

Chris - Why did that organism become so big? Why is that not here today - where’s it gone?

Jason - The hypothesis that we’re working on is that the reason you can get snakes that large is that during this time period, after the demise of the non-bird dinosaurs, and what we call the paleogene period of the cenozoic, this is one of the warmest intervals in the history of Earth. You have no ice at the poles; you have incredibly warm oceans; you have tropical rainforests that are extending up toward high latitude; you have crocodiles and palm trees at the poles. And with what we used to call “cold-blooded” or a “poikilothermic” animal, we know that body size is going to be ultimately limited by the temperature within which is lives. So what we’re able to do is actually do the math to calculate how hot it would have to be to keep an animal this size alive, and so we’re able to estimate temperature for this interval in time. They’re consistent with a lot of other data and so for our main hypothesis it is that these animals got this big because the climate allowed them to do so.

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