Saturday, 7 March 2015

Early Mammals: more than just a pretty smile

Early mammals, it's all just teeth. This is certainly the discouraging message you hear as a student learning the ropes in palaeoscience.  Like my compatriots, I acquired a negative mammal attitude by osmosis. After all, who wants to pick through a pile of gnashers the size of cous cous when you could be stroking the sexy serrations on a carnivorous theropod blade, or weight-lifting hefty sauropod humerus?

Turns out, I do. Actually the teensy teeth are still a mystery to me, but I'm working on that. Meanwhile, the rest of skeleton of early mammals is satisfyingly odd while their ecology is proving more complex than previously thought.

Recently some surprisingly diverse Mesozoic critters have been described. It turns out there's more to early mammals than a frustratingly minuscule smile. I want to introduce you to two of my favourite mammaliaform superheroes: Jurassic Beaver and Mesozoic Mole.

Jurassic Beaver

No, this is not a prehistoric porn-flick. The animal with this moniker is Casterocauda lutrasimilis, a docodont from the middle Jurassic (around 165 million years ago).

The fossil of Casterocauda lutrasimilis,  alongside a beautiful reconstruction by Mark A. Klingler/CMNH.

I'm sad to say Wikipedia lets you down a bit with docodonts. Not many pictures, not many species, it's an altogether sparse description for these fascinating wee beasts. Caster here is the only one pictured, and rightly so as he's pretty famous (he even made the cover of Science magazine). Castor got his nickname after Ji et al. described his adaptations to an aquatic lifestyle. Caster's tail is more flattened than a terrestrial Mesozoic mammal like Jeholodens, in fact it resembles modern semi-aquatic mammals like beavers and otters.

It doesn't take a scientist to see Caster's tail is more like a beaver's than a terrestrial mammal, so it probably had a flattened tail used in swimming. Being a water-dweller, it also had a unique diet, giving it a unique place in the Jurassic ecosystem.

So rather than the cliché little insect eaters people previously thought all Mesozoic mammals were, it turns out this Caster was dipping and diving for food 165 million years before beavers even existed. What a little hero.

Mesozoic Mole

I'm not going to lie to you, my first thought when I read Luo et al.'s paper on Docofossor brachydactylus, was Nosferatu.

I know what I would have named it after...
Those claws are exactly what led the team to realise Docofossor's life was subterranean, similar to a mole. With shortened, broadened front feet and a robust little body, this mammaliaform was living beneath the ground - and probably dining down there too, once again proving that there were surprising things happening in the Mesozoic. 

I'm going to trial the improved aphorism: don't make a mountain out of a Docofossor-hill. Not sure it'll catch on (and that is probably why I don't have very many friends...)

Docofossor and a golden mole (Amblysomus hottentotus - amazing name!). The mole makes a slightly cuter analogue than a blood-sucking vampire, and certainly more functionally accurate. Docofosso's skeleton is below, see how short and stubby those legs are?

So why does it matter if these little fur-piles dug up their snacks or dove in to get them? It's all about fitting them into food webs. If every little mammal is doing the same thing then the food web isn't very complex, and so the world itself is a lot less interesting. We hear about biodiversity in the modern natural world: if docodonts like Castor and Docofossor were all doing different things, their Jurassic ecosystems must have been that bit more biodiverse and complex than we thought.

I can't wait to find out what other early mammal superheroes there are out there - maybe I'll get a chance to discover some for myself?

Casterocauda lutrasimilis (middle) and Docofossor brachydactylus (bottom right) hang out in their niches in this charming Jurassic scene by April I. Neander / University of Chicago. But who is that I see in the tree above? Is it a bird, is it a plane? No, it's Agilodocodon scansorius! I'll tell you more about that amazing Mesozoic sap-muncher another day...