So what exactly is an archosaur anyway?

This whole ‘Science Basics’ thing is not as easy as you might think. Even simple concepts take in several different branches of biology that some readers probably don’t know at all so while I can try and build up ‘A’ from scratch, it’s still quick tricky without a bit of background in ‘B’, which requires some knowledge of ‘C’, which is reliant on at least a partial understanding of ‘A’. And on and on and on. That is not to denigrate my audience, but I have to assume that at least some people reading this really don’t know much or any biology or palaeo at all, and since I’m not writing a straight through ‘what is biology’ series, but cherry-picking ideas I want to expand on, or I think are especially important or interesting, I can’t just do ‘A’ now, ‘B’ next week and ‘C’ the week after and hope the audience follows the thread. So, with that in mind, I’ll try to keep everyone on board and not bore the “experts” as I delve into the world of archosaurs.

Ideally it would help if I covered the basics of etymology, taxonomy, systematics, phylogeny, anatomical definitions and evolution before getting here, but mindful of the fact I’ve already been going for a few months now and the only place the word ‘archosaur’ appears is in the title, I really feel like I should tackle it as soon as possible. So I have cobbled this post together the best I can and with luck all will become clear over the next page or so. If you get lost, I’m sorry, with luck it will be enough for you to work out the details yourselves or be a decent primer for further reading, and my apologies if some of the references aimed at other readers don’t make sense.

We’ll start with the most obvious bit: what it means. ‘Archosaur’ is derived from ancient Greek and roughly translates as ‘ruling lizard’ or in this context more accurately ‘ruling reptile’. Of course really obsessive systematicists will immediately get annoyed at the use of ‘reptile’ being a paraphyletic group but this is etymology, not systematics and anyway, it is still a useful, practical term. You will also see me and others use Archosauria (not the capital ‘A’) which is basically the formal version of the word, with obviously ‘archosaur’ being the common anglicised version.

Next up, what are archosaurs, or rather what clades or groups fall under this term? The short (and techie) answer is “the most recent common ancestor of birds and crocodiles, and all their descendants”. Which in practical terms means the dinosaurs, crocodiles, birds, pterosaurs and a few other rather less well known groups like the phytosaurs and rauisuchids, plus a few odds and sods that don’t really belong to any group but none the less are archosaurs. They first appeared in late Permian period (around 250 around million years ago) and of course are still with us today. Archosaurs are a true, monophyletic clade, that is to say that all archosaurs are the descendents of a single common ancestor, and all of the descendants of that ancestor are considered archosaurs. That may sound unnecessarily circular, but when I get round to doing an essay on taxonomy and systematics this will hopefully become clear. It also goes a fair way to explaining the name of this blog as although I really don’t do much (well, nothing so far) apart from dinosaurs and pterosaurs ‘archosaurs’ has a rather nicer ring to it (and more people probably know the name) than ‘dinosauromorphs’, or ‘ornithodirans’ (the term for dinosaurs and pterosaurs) and of course long time readers will know I didn’t actually name the thing anyway. Depending on exactly who you ask, some palaeontologists prefer a slightly wider definition of archosaurs which includes a few more basal taxa beyond the crocs and birds and their descendents, but to be honest the point is pretty much moot as far as we are concerned.

One thing to note immediately is that there are of course lots of things that are *not* archosaurs that superficially look similar, or at least were big important animals running around at the same time that you might think were archosaurs. Obviously the lizards, snakes and turtles are excluded, but so two are pelycosaurs (like the famous Dimeterodon), ictyhosaurs, rhynchosaurs, prolacertiforms and plesiosaurs and various basal crocodile-like things. If you examine the tree I have kindly attached you can see how everything is related to everything else within the Archosauria, and I have marked up a few other relatives and clade names of things that are not archosaurs for reference too. Clades are inclusive so birds are of course dinosaurs, dinosaurs are ornithodirans, and ornithodirans are archosaurs and archosaurs are archosauromorphs.

The next obvious point is therefore how can you tell what is and what is not an archosaur? Systematicists have a nice big list of features which allow you to identify (in theory at least) any given clade or even any species. You ‘simply’ have to see what features the animal has and then tick it off on the list. If it has all (or actually typically any) of the features on the archosaur list, well then it’s an archosaur. I’ll not put them all down here as frankly they are technical, hard to explain, and boring, but it’s worth putting in a couple of the more obvious ones so that you can spot an archosaur yourself, and prove that I’m not just making this up.

1. Antorbital fenestra between naris and orbit. This is pretty easy to spot since for even a layman the orbit and naris (or eye socket and nostril) are easily identifiable, and then you just have to look for a big fat hole in the skull between them.

2. Laterally flattened serrated teeth. Not much to add to that really, except of course that this is the acquired evolutionary condition in archosaurs (i.e. they evolved these teeth and their ancestors did not have them) which means that it might be lost or change later on (like in birds most obviously).

3. A posterior mandibular fenestra. Again another nice easy one, simply find the lower jaw and look for a hole somewhere near the back.

And here they are as seen in the rauisuchiuan Prestosuchus. The teeth, antorbital fenestra, and the mandibula fenestra are all clearly visible (black arrows) and while we are here, you can see the upper and lower temporal fenestrae (white arrows) that also mark it out as a diapsid (a much larger and more important group of reptiles that includes snakes and lizards). That really is about it for now. You should have a decent idea of what anis and is not an archosaur, what they look like, and how they are related. I will expand on some of these ideas at some point in the future such as the details of taxonomy, spotting reversals (like tooth loss) that can confuse people, and of course providing more details about some of the groups that make up the Archosauria.

Now, (inevitably) back to the pterosaurs.

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