Thursday, 15 January 2015

I’m a Therodontid Eutherian – and so are you



In most disciplines, getting to grips with the terminology is one of the challenges of being a student. This is even more of a problem for those in biology, wrapping their heads around evolutionary taxonomy. You know you are Homo sapiens (you are actually Homo sapiens sapiens, denoting that you are an anatomically modern human rather than one of those old-fashioned ones from >200,000 years back), and doubtless you know you are a primate, and a mammal and an animal. But you are so much more.

simpsons geeksOne of the surprises of studying evolutionary biology and palaeontology is discovering the tongue-twisting, mind-boggling array of names for both extant and extinct creatures. Palaeontology get togethers remind me of similar geek-fests like Comic-Con, where conversations (and often squabbles) are carried out in bewildering detail regarding the backstory of obscure characters with bizarre monikers. (“I think, heh-hem, you’ll find that Morganucodon was a tricodont mammaliophorm that was functionally dyphodont, you fool!”)

I marvel at people’s capacity for memorising film/game/book/comic trivia, but those same skills are often employed in science.

As well as learning terms, a student (and aren’t we all perpetual students?) must learn how to place the terms in context. As for humans, any animal has a plethora of labels that tell you the various taxonomic groupings it belongs to. These labels get more and more inclusive – or exclusive, depending on what direction you approach them from.

Skull holesOne of the major barriers to understanding all of this is that – on top of remembering the terms, what they mean, and what animals they include – you have to overcome the problem that many sources use terms in misleading ways. As you can imagine, mass market documentaries, much as we love them, are especially guilty of this. But even experts can be quite vague about terminology, switching between terms that refer to a clade and a group without explaining the difference, and using outdated classifications. For learners this is really difficult to unpick.
Let me give you an example.

The term synapsid refers to how many holes (fenestra) are in the skull, other than the eye socket and nostrils. There are 3 main clades based on these holes: anapsids (no holes), synapsids (one hole), and diapsids (two holes).

Now take a look at this cladogram below:
Synapsids cladogram
This is a simplified cladogram of synapsid origins and relationships, with an indication of time down the left hand side. You can see how the therapsids – the mammal-like reptiles – arise in the middle of the Permian, and the synapsids then disappeared. Looking at this cladogram – which is based on others in text books and papers – you’d be inclined to think that the synapsids ‘became or were replaced by’ the therapsids, then the therapsids ‘became or were replaced by’ theriodontids. The word replaced is commonly used in the literature, and it seems to support the assumption that one group took over from the other.

But you’d be wrong.

The truth is that everything on that cladogram is a synapsid. You are a synapsid. You are also a therapsid, and a theriondontid, and a cynodont. A way to clarify this is below:

Synapsids cladogram 2

To further confuse the issue, mammal-like reptiles are not related to reptiles and they also aren’t mammals!

If you want to speak with complete technical correctness you’d have to adopt terms like non-therapsid Pelycosauria and non-mammalian therapsid. Despite making things clearer for students, it is also quite a clunky way to talk, so most palaeontologists shy away from it; adopting terms that they admit aren’t ideal. For example Kemp (2006) said when discussing early synapsids “the situation is sufficiently well known and straightforward that no misunderstanding is likely to arise from using the terms Pelycosauria** and Therapsida, rather than the synonymous but awkward expressions ‘non-therapsid Pelycosauria*’ and ‘non-mammalian Therapsida’ ”.

It may be straightforward to an expert, but it’s pretty confusing to everyone else.

The next setback arises when terms that are not at the same taxonomic level are used together. I recently had a conversation with a fellow student about early mammals. We gave up after becoming hopelessly entangled between terms at order level, terms at family level, and terms that referred to the shape of the mammal’s teeth, all of which are thrown together in various textbooks and papers discussing the evolution of early mammals.

This confusion partly springs from the fact that taxonomy isn’t straightforward. Like with the synapsid example, explanation is often required and it can be as hard to convey as it can be to understand. Diagrams help, but with so many terms it will always take time and effort (and lots of repetition and double checking) before you really know your stuff. Perhaps clearer, if a little clunky, terminology is necessary during teaching so that students can get a firmer grasp on how the terms piece together? It means lecturers have to be more exacting. There are some lecturers who already hold themselves to this high standard of specificity, and it shows.

For the public, is it possible to convey these taxonomic complexities to people with no scientific training? As someone who has tackled this issue with the public in previous work roles, I believe it can be done. It takes care with language to prevent yourself from being misleading, but it is worth the effort. The best solution though, would be to introduce taxonomy in schools. It’s not just naturalist pedantism, knowing nomenclature helps us understand  biological relationships. Even a basic introduction would lay the groundwork for appreciating later forays into science, even if it’s just watching the latest BBC natural history documentary.

And when kids get to the point where they are correcting adults… well as long as they are getting it right, I say good for them.


* Pelycosaurs are early synapsids, most famously including sail backed animals like Dimetrodon. The term is not accurate however, and is now used only informally – which in practice means it is still used quite often.

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