Wednesday, September 23, 2009

In Praise of Notoungulates

I've spent the last couple of weeks having my first experience of teaching at Yale. Naively, having tutored a small army of Oxford undergraduates in during my time there, I thought that Yale students would present few surprises. However, from my limited experience to date, I'd have to say that they are scary-smart and very motivated. Either Yale knows how to pick 'em, or else things have got a lot tougher since my day.

The course in question is E&EB 171a, "The Collections of the Peabody Museum" taught by YPM's Curator of Invertebrates, Leo Buss. I think it's a great course (and no, I'm not just trying to kiss up to Leo, who is a very nice man, but also a very smart one, and would see through my ruse in an instant). What it does is take freshman or sophmore biology students and teaches them, through a short research project, how we use museum collections to underpin research.

Personally I think this is a great idea; there are significant numbers of people (including a few curators) who actually work in museums who have no idea what collections are for, let alone the wider community of researchers. So when I was asked to come up with a couple of projects for undergraduates, I didn't hesitate for an instant when it came to saying yes. All the hesitation came later on, when I actually had to devise the projects. Which is where notoungulates come in.

One of the biggest challenges for paleontologists is that they have to learn about the biology of their organisms indirectly - it may seem obvious to say, but most of the things that they are studying are extinct. Sometimes they have living relatives that you can examine for clues as to how your fossil beasties lived. But often you're totally out of luck. And problems don't come more acute than the South American ungulates.

For most of the last 80 million years, South America was an island continent. After the breakup of the supercontinent of Pangaea and its southern portion, Gondwana, tectonic plate movements kept the South America’s mammals isolated from those of other continents. So while mammals in the northern hemisphere were busy evolving into the familiar forms that we see today, South America developed a unique mammal fauna, most of which went extinct after the formation of the Isthmus of Panama led to an influx of mammals from North America.

With a few exceptions (opossums, sloths, anteaters, and armadillos), almost all of the modern mammals that we think of as typically South American are either relatively recent arrivals from North America (lamas, jaguars, tapirs) or more ancient immigrants from Africa (primates and rodents). So what was living in South America pre-Panama?

The answer is some very weird things. First, there were a whole gang of marsupials. A lot of these were much like that familiar opossum that you see pancaked on the highway in the morning, but there were also the Sparassondonta, a huge radiation of carnivorous marsupials that included just about every body plan you would expect for a carnivore, up to and including a jaguar-sized, saber-toothed animal called Thylacosmilus (the name means "pouch-knife," which says it all really).

As a former student of marsupials, I used to think that these big metatherian carnivores were the coolest thing about pre-Isthmian South America. And indeed, they are very, very cool. But that was before I gained a true appreciation of the ungulates. And to my mind, they are even cooler.

Consider these basic facts. South America split off and went its own separate way back when dinosaurs (of the non-flying, non-feathery variety) still roamed the earth. So the ancestors of those ungulates were extremely primitive. They parted company with the rest of the mammalian family tree so long ago that no-one really knows where exactly they fit - to try and place them in the tree requires poking around some very deep nodes, of the sort that can prove quite intractable to phylogenetic analyses. This (plus the total absence of any preserved DNA) may explain why most "super-trees" of placentals ignore them altogether. They then survived in splendid isolation for 80 million years, diversified spectacularly into 13 families and around 150 genera, then went extinct leaving nothing that is even remotely related to them. How cool is that?

Anyway, all of the above makes notoungulates a great teaching tool for the budding vertebrate paleontologist. To my way of thinking, there are two skills that are critical if you want to study fossils. First, you need to be able to observe - to look at something and truly see it. This may sound a little bit Zen, but it's a genuine ability and not everyone has it. The second is that you have to be able to compare - in the absence of relatives, you may need to cast your net very widely to find analogues. And these are two activities that our collection is well-placed to support; we have an exceptional collection of fossil mammals from South America and a large teaching collection of comparative osteology to refer to.

S0 I turned my student loose in the collection and she came back with a specimen of the Miocene notoungulate Hegetotherium which she is now in the process of describing. Without access to any publications on notoungulates. Yes, I am that much of a sadist. My rationale is that she needs to learn to observe without preconceptions. I am like a paleontological Mr. Miyagi. Anyway, we'll see what she makes of it. Meanwhile I have just a few weeks to flesh out project #2. On paper this looks like an exciting chance to delve into the evolution of whales. Sadly, I fear the reality is several weeks of poking around in mesonychid ankle bones for the unlucky candidate. Whoever said working with fossils was glamorous?

[By the way, I'm just a tourist where South American ungulates are concerned. If you want to really experience how cool these things are, go take a look at Darrin Croft's webpage]

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