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Bigger theropods rocked flashier head gear

Dinosaurs come in all shapes and sizes, and were the weird wonderful show-offs of the Mesozoic world. From massive, plated body armour to elaborate frilly head shields and rock solid bone-heads, they sported just about every sort of flashy fashion possible at the time.

There is a large ongoing discussion between researchers as to why these sorts of ‘ornaments’ evolved – was it to differentiate between species, for sexual selection, for communicating between individuals, for fighting or defence, or for something else entirely?

One of the problems here for extinct dinosaurs is that we can’t actually go back in time (yet) and see exactly how these animals used their ostentatious ornaments. This is a bit of an issue if we want to learn more about how dinosaurs functioned not just as animals, but in a social context.

Allosaurus flashed these cute crests above its eyes, but what were they for? (click for source)

New research by Terry Gates and colleagues has shown that in theropod dinosaurs, the evolution of the weirdness of their head gear, mostly bony horns and crests, is closely tied to the progressive development of gigantism – larger body size.

This evolutionary trend towards gigantism occurred an order of magnitude faster in more flamboyant theropods than those who remained unadorned. Not only this, but it seems that in many different lineages, if you wanted to grow to over 1,000kg then this was dependent on the previous evolutionary acquisition of ornamentation. No crown, no entry into the big leagues for theropods, it seems. This is actually the first time such an evolutionary threshold has been found in any amniote group (those which lay eggs), making theropods even weirder than they were before!

Interestingly though, evidence for this trend is at its strongest in what we consider to be ‘basal’ theropods, those which are more towards the base of the theropod family tree.

ncomms12931-f2
Gates et al. (2016)

Maniraptorans, the more ‘advanced’ group that includes all modern birds and some of their closer theropod cousins including Velociraptor, generally lack these bony structures. This is unusual, as they achieved a whole range of body sizes, from whopping beasts as tall as a double decker bus, right down to chicken-sized cuties.

However, some species did evolve weird ornaments too. Almost all of the largest of these belong to a group called oviraptorosaurs, once thought to be sneaky egg (ovi) thieves (raptor), but now considered more to be just a bizarre evolutionary offshoot closely related to birds. Why this is remains a mystery for now, but it certainly raises intriguing questions about the sociology of this group.

The reason for this apparent body size related distinction might be due to the evolution of pennaceous feathers in maniraptorans – those which are structurally designed for flight like we see in most modern birds. Now this is where it gets really cool. What this suggests is that these theropods had a sort of evolutionary trade-off, where they shifted visual communication structures away from simple bony elements to the more elaborate and colourful patterns that you get with feather displays. What’s more attractive – a rhino or a peacock? (OK, subjective I know, but you get the point..)

It also suggests that different theropod groups evolved in highly distinct ways. Numerous recent studies have shown that in the maniraptoran lineage, there was a sustained and accelerated evolutionary trend towards decreasing body size that was probably ecologically driven by the ability to fly. This is distinct from other larger-bodied theropod groups, where it seems social or sexual factors were more at play in driving evolutionary trends in body size.

This study is a really nice piece in helping to understand dinosaurs in a more social context, and helps to breathe a new life into how we see them as living, interacting organisms.

Reference

Gates, Terry A., Chris Organ, and Lindsay E. Zanno. “Bony cranial ornamentation linked to rapid evolution of gigantic theropod dinosaurs.” Nature Communications 7 (2016): 12931. doi: 10.1038/ncomms12931 (link)

Header image source, by H. Kyoht Luterman (CC BY 2.5).

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