Dinosaurs were pretty big – and yes, that’s how evolution had them in mind

All the leaves are brown and the sky is grey – which means it’s time for the Geological Society of America annual meeting.

A couple of days packed with discussions and research presentations about stuff you did not know in advance you would turn out to find interesting (like ‘Anthropocene geomorphology‘) – to get you to bridge the long-days-baking-hot-Sun fieldwork season and the season of long lonesome nights filled with proper indoors academic paperwork, to romanticise the geoscientist’s bearded existence – like the size of dinosaurs

dinosaurs evolution
Dinosaur scale comparison. That’s you in the left corner standing and waving. That red one planning to eat you is an ornithopod species. And that extremely big one is a member of the sauropod clade, not so friendly if you’d happen to be a plant. These Cretaceous folks are so big because they were very, very patient for many millions of years, researchers say. Image Wikimedia Commons.

For many species the distribution of relatively large body sizes tends to increase with lattitude, towards the cold-climate poles. Conversely, flora and fauna shrink under climate warming.

Another remarkable trend known to evolutionary biologists is that species tend to increase in size over time [no, not just lifespan]. Quite often ´survival of the fittest´ equals survival of the biggest. Especially for mammalian evolution* this clearly holds true – as you may recall we survived that end-Cretaceous asteroid by being burrowing and rodent-sized…

[*) Did you know the biggest animal species ever to have lived on this planet is still with us today?]

Ruling paleontologist allows dinosaurs’ thigh bones to grow larger

In Biology it’s called Cope’s Rule [named after paleontologist Edward Cope] – and it applies to dinosaurs too, researchers Gene Hunt and Matthew Carrano of the Smithsonian Institution and Richard FitzJohn of the University of British Columbia have discovered, analysing fossil thigh bones, or femurs, as size proxies.

They then used that femur data in their statistical model to look for two things: directional trends in size over time and whether there were any detectable upper limits for body size. On Sunday they’ll present their findings at the GSA conference.

“What we did […] was explore how constant a rule is this Cope’s Rule trend within dinosaurs,” said Hunt. They looked across the family tree of dinosaurs and found that some groups, or clades, of dinosaurs do indeed trend larger over time, following the rule. Ceratopsids and hadrosaurs, for instance, show more increases in size than decreases over time.

Although birds evolved from theropod dinosaurs, the team excluded them from the study because of the evolutionary pressure birds faced to lighten up and get smaller so they could fly better. [Bird evolution works a bit differently anyway, we just learned yesterday.]

Want to keep growing? First land back on all four your limbs!

As for the upper limits to size, the results were sometimes yes, sometimes no. The four-legged sauropods [sauropod bodies were so large, their methane farts may have almost wiped out the planet – for real(!)] and ornithopod clades showed no indication of upper limits to how large they could evolve. And indeed, these groups contain the largest land animals that ever lived.

Theropods, which include the famous Tyrannosaurus rex, on the other hand, did show what appears to be an upper limit on body size. This may not be particularly surprising, says Hunt, because theropods were bipedal, and there are physical limits to how massive you can get while still being able to move around on two legs [individual humans can testify to this].

The primary evolutionary driver to grow ever larger is according to Hunt – both for herbivores and carnivores – to be able to outcompete the other. As there are still of course also advantages to being small and downsides to being big (like high calorie requirements) there is no definite understanding of why Cope’s Rule seems to apply so well across taxonomic orders.

© Rolf Schuttenhelm | www.bitsofscience.org

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