Posts Tagged 'teeth'

With nasty big pointy teeth….

Modern birds do not have teeth. So much so simple, but that doesn’t mean that their beaks need be simply shears or forceps. The keratinious beak can be quite complex in shape and a good number of birds have serrations along the margins that increase their cutting abilities or grip. I’ve been bitten by a penguin and they have brutally serrated beaks that I can assure you slice open human hands most effectively. Pictured (courtesy of the Optimistic Painter himself) is an emu and while it is small, the lower right part of the jaw is well framed against the light background and the small serrations are clearly visible. Another little reminder that bones (sadly) can’t tell us everything about important details of the shape of he living animal and that the real appearance could in cases be quite different to what we expect.

A little more on teeth

Since it generated so many comments and discussion about the origins and use of the phrases, I went googling phrases associated with Tyrannosaurs teeth.

Here’s the search terms and the number of hits (to the nearest thousand).

Tyrannosaurus teeth bananas – 792 000

Tyrannosaurus teeth steak knives – 13 000

Tyrannosaurs teeth railroad spikes – 98 000

Tyrannosaurus teeth bananas steak knives – 8000

Tyrannosaurus teeth bananas railroad spikes – 3.2 million (which must be wrong)

Tyrannosaurus teeth steak knives railroad spikes – 8000

Tyrannosaurus teeth bananas steak knives railroad spikes – 6000

Having explored a bit some of these are probably high – people can discuss Tyrannosaurus relative to other theropod teeth and call those steak-knife like especially, so that could be misleading. However, what is clear is that these are all being used with great regularity. My exploring also revealed a profound lack of qualifiers (e.g. ‘big’, ‘crown’, ‘shaped’, ‘similar to’ etc.) and they generally seemed to be flat comparisons of the “teeth like bananas” kind. This included all manner of less-than-stellar sources like AiG and Yahoo answers, but all the way through to Wikipedia, various museum and university websites and more. In short, these really seem to be pretty ubiquitous on the web.

Teeth like steak knives or bananas or well, tyrannosaur teeth

You can’t go too far talking about Tyrannosaurus without coming across one of two great statements, that they had teeth ‘like steak knives’ or teeth ‘the size of bananas’. These are both really, really annoying, in that they sort of convey semi-accurate information while at the same time being really, really misleading. I thought I’d take a little time today to bust these two with a swift one-two and leave at least one corner of the internet with a little less pseudo-information.

Let’s start with the size issue. I can see the concept behind this, a big banana is not so far removed from a big Tyrannosaurs rex tooth. The only trouble is that of course there are lots of smaller teeth in the jaws than just the biggest ones in the maxilla. And of course bananas vary enormously in size and length, so it’s not the greatest unit of size for scale. And of course if you *look* at a Tyrannosaurus tooth, (ooh, look what photos I happen to have, how handy) then things get a bit more complex. Even the biggest crown is not that long, so actually even a pretty small banana is probably bigger (well, longer, another issue of course) than the crown. And since that is really what we should be dealing with, it becomes rather pointless. Ironically the roots are so big that if you include them, then the tooth is huge and much bigger than even the biggest bananas that I have seen. So as a measure a banana is either too big OR too small at the *same* time. Impressive! And not really the same shape either. In short, please people, let’s stop using this as a description of tooth size for Tyrannosaurus.

And so to steak knives. First off, take a look at this:

While the two images above were in lingual and labial views, this is an anterior shot of the tooth. If you get something that fat and rounded handed to you by a waiter the next time you order a steak you but wonder if he wanted you to tenderise it rather than cut it up. The tooth is damned near circular in cross-section and about as far as you can get from a ‘knife’ if you tried (and indeed is less blade-like than any other theropod tooth). As we know, Tyrannosaurus had a bone-crunching bite, so what’s with the knife analogy?

I’m certain it dates to a paper by Abler where he examined the effects of the serrations on Tyrannosaurus teeth and concluded that the *way* in which they cut was most similar (at the microscopic level) to that of a steak knife. Note that this means that those tiny serrations on the teeth are cutting *like* the giant serrations of a steak knife. This doesn’t mean that they acted like one though. If I stuck that thing into your arm or chest with the power of a rex bite I rather suspect what would most impress on you (other than the need for extremely urgent medical attention) would be the massive and sub-circular puncture wound and not the nice little edge to it in a couple of places.

As such, again, please stop describing these as ‘like steak knives’, a small part of them does, superficially cut like a steak knife, but they are not shaped like them and don’t act like them and aren’t used like them.

Dental batteries

In my recent post on sauropod teeth I noted how the jaws of Nigersaurus looked a little like those of hadrosaur and iguanodontids. Here is a real dental battery, or rather a cast of one. This sits on display in Japan and where people are encouraged to touch and feel it, hence the wear and loss of colour in the middle. Even so, you can see the large numbers of teeth, lying in interlocking banks with each tooth being rather diamond shaped.

This is of course half of a mandible, but a similar pattern is seen in the maxilla such that there are two massed ranks of teeth on each side of the jaws. As the animals bite down these ranks of teeth rub past each other meaning that there are effectively massive grinding surfaces and will do real damage to any plant material in there. It’s a wonderfully effective way of chewing and if you’ve seen isolated teeth from these animals you’ll know just how much damage they can suffer as probably hundreds of times a day they would be abraded against a bank of teeth on the other half of the jaw. That’s guaranteed to do some serious damage to the enamel and even dentine. This is course is where the replacement rate of teeth becomes important as the new teeth coming up into the jaw to replace the old means that the dental battery is generally complete and in good condition. Quite the system really.

Identifying Middle Jurassic teeth – early deinonychosaurs?

Some parts of skeletons are very easy to identify and can be diagnostic right down to individual genera in some cases. Others are, sadly, far less easy to sort out and even very different species can have very similar features, or the range of variation can be so great that all manner of things can overlap making identification difficult at best. Such is the lot of theropod teeth (as we have seen before) and while there are *some* good characters which seem pretty conservative within groups, in general it can be hard to tell with confidence exactly which groups a given tooth might (or might not) belong to.

However, this doesn’t mean we can’t try and work this out and there are cases where positive (or at least reasonable) identifications can be made. And this can shed real light on the diversity and distribution of certain clades.

Theropod teeth from the Shishugou. From Han et al., 2011.

And so we move to the Shishugou Formation of Middle Jurassic age in China. The name might not be familiar but the theropods will be – Limusaurus, Guanlong, Haplocheirus and Zuolong are all from here and a giant tooth suggests the presence of Sinraptor or something very similar too. That’s already quite a bit of variety (ceratosaurs, tyrannosaurs and alvarezsaurs) but there’s more to come. A number of isolated theropod teeth collected a few years ago were given to one of Xu Xing’s Masters students as part of his thesis work and a paper on this has just been published. I feature in the authorship list in the obviously most important place of ‘last’ though Tom Holtz suggests that I should in fact advertise this as “special guest star”.

Anyway, this is a real challenge. Not only are many theropod teeth not diagnostic or highly variable, but the Middle Jurassic represents perhaps the peak of theropod diversity at least in terms of the clades that were around at the time. The early coelophysoids have died off by then but ceratosaurs, allosaurs, tyrannosaurs, spinosaurs, and every maniraptoran was, or should be, around. Ah. Still, Han Fenglu stepped up the the challenge and the result of his work and the rest of the team has come to fruition.

Possible troodontid teeth. Modified from Han et al., 2011.

Perhaps unsurprisingly given the conditions not all of these teeth could be identified down to any especially narrow set of possibilities but a couple were very good candidates for deinonychosaurs teeth and not much else. For those that don’t know this is the name of the group that consists of dromaeosaurs and troodontids and thus are the closest dinosaurian relatives of birds. While the discovery of Anchiornis has finally pushed back the record of this group to before Archaeopteryx, these teeth predate even that. This marks them out then as (possibly, or dare I say probably) one of the oldest records of the group. Hardly groundbreaking stuff, but quite interesting none the less. Filling in all those gaps in the fossil record is generally a piecemeal and occasional occupation in any case, but any chip away at it should be welcomed.

What’s also interesting is that while many of the teeth could not be assigned to any particular group, many of them could be fairly confidently kept apart from taxa we know are present. You might not know quite what a tooth came from, but you can compare it to Guanlong and the rest and see if it’s a match. If not, you might well have something new there. In this case there were several possible newbies which means that not only might we have early troodontids and dromaeosaurs but there are probably other new theropod taxa waiting to be discovered in the Shishugou as well. Time will tell if all or any of these interpretations are correct, but they are certainly intriguing.

Han, F., Clark, J.M., Xu, X., Sullivan, C., Choiniere, J., & Hone, D.W.E. 2011. Theropod teeth from the Middle-Upper Jurassic Shishugou Formation of northwest Xinjiang, China. Journal of Vertebrate Palaeontology, 31: 111-126.

Details, details

In discussions of palaeontology with non-experts one theme that often seems to emerge is the lack of appreciation of details. This is especially common with the often tedious discussions with the ‘want-to-be’ experts or those at the less sane end of the spectrum of internet commentators. I don’t mean this in a disparaging manner – it’s an observation, not a criticism, and those who really do wish to learn or understand and generally quick to realise and adapt their approach accordingly, but that real appreciation for detail is often lacking.

Details are though, critical. As seen to a degree with my posts on ontogeny and convergence, it’s easy to take a feature at face value and not examine the details and thus misinterpret it. Without looking at the details of the proboscis itself or the rest of the animal, it’s easy to make the mistake of assuming that say anteaters and aardvarks are close relatives, or that all animals with large claws must be digging or burrowing etc.

Teeth are perhaps an especially good example, though already we are delving somewhat to a detailed level when we might have two whole animals to compare. Most theropod teeth look quite similar being generally curved at the front, quite straight at the back, laterally compressed and with some degree of serrations on the carinae (a primer on teeth is here for those who missed it). You can see that they look pretty similar and that it might be reasonable to assume that they are from closely related animals doing similar things in similar ways. To a degree that’s true of course (they are, after all, theropod teeth from carnivorous members of the group), but it belies the truth of the matter. A much look closer and a myriad of details become apparent that have taxonomic and systematic importance and morpho-functional implications too. There is variation, and thus important detail, to be had here. Similar as these might look, and simple though it would be to pass them off as not worthy of further attention or irrelevant, this would be quite wrong.

Briefly then:

Tooth denticles can vary in shape, distribution up and down the tooth and between the two carinae, and vary in size along the tooth
The grooves that lie between the denticles can also vary in terms of their depth and length too.
The carinae that support the denticles can vary in size and shape too and sometimes twist around the tooth (as seen in tyrannosaur premaxiallry teeth, but not the others).
Teeth can vary in their curvature, cross-sectional shape, and degree of lateral compression (or which side is compressed)
Some teeth have wrinkles on the surface of the enamel and others don’t, though the shape and size of these wrinkles varies too. You get ornamented teeth too with big vertical ridges that may or may not be present on one or both faces of the too
Finally the shape and size of the root can vary and there can be a restriction at the base of the crown too in some teeth.

All of this is true when comparing teeth directly to one another that are directly linked (like say the 1st premaxillary tooth, or the 3rd dentary tooth). There is still more variation there though.  The number and position of the teeth in the jaws varies, and the teeth are often different in the upper and lower jaws, and the teeth can vary along the tooth row itself (theropod premaxillary teeth are often more robust than maxillary teeth, and anteiro teeth bigger than posterior ones for example).

Details then, matter. It’s not enough to look quickly at gross shapes and patterns and pronounce judgement, there are details there, often of bewildering complexity and number and they are important. To gloss over them, or not realise they are there, is to potentially lead to pitfalls of misunderstanding and incorrect deductions. They might be obvious and they might not even be discussed (I would write a monstrous piece on why birds are not pterosaurs without even beginning to chip away at the anatomical differences of the wrists and ankles for example) but they are there and we need to be aware of them.

Theropod teeth again

As with the dromaeosaur tails, the teeth of Sinornithosaurus also allow us to revisit a recent post on anatomy – the anatomy of theropod teeth. As before the photo isn’t perfect, but should be well enough to illustrate a few points.

The point where the root ends and the crown starts is quite well demarked. Although the teeth are loose in the sockets and have extruded, the crown is curved where the root is straight and there’s a slight colour difference too. There is also a deep groove on the tooth root and in some cases this extended part-way onto the crown.

Astute readers will be thinking about a recent paper on this very taxon and its tooth a jaw morphology, but I’m not going to comment here on it (that’s what the literature is for, not blogs, or comments as far as I’m concerned). I do think it’s safe and uncontroversial to say that more work is needed in this particular area however.

Theropod tooth morphology – some definitions

This is a subject I have been somewhat deliberately avoiding since it seemed a bit too technical and would involve me having to create some figures or do something more complex than just typing. However with an upcoming paper on teeth that I want to talk about on the blog it seems like I’ll have to do it sooner or later, and before is definitely more relevant than after. So in the best traditions of compromise / laziness, I’ve kept it as brief as possible.

I’ll be clear from the off that I’m only trying to cover the general features that turn up in theropod teeth and not describe the range or variation or which features appear in which taxa.  Even so, it’s worth noting that theropod teeth are fairly conservative and the general statement that they are laterally compressed, recurved and have serrations on the front and back edges covers most of them quite well. There is inevitably a quite a bit of deviation from this and some teeth are really quite diagnostic (like those of derived troodontids) while others are pretty generalised and hard to pin down in isolation (like those of basal tetanurans). Here then are the terms (as best as possible, since sadly they are not all standardised) that you’ll most commonly come across with descriptions of theropod teeth.

So obviously the bit at the bottom that sits in the jaw is the root and bit exposed on top (with the enamel covering) is the crown. So much so boring. Although the root is mostly missing (and the crown is broken too) you can see the junction between the root and crown as indicated by the blue arrow. This is usually indicated by a change in colour in the fossil and a slight constriction.

Most theropod teeth have a degree of recurvature, that is they curve back over themselves so that the tip of the tooth is closer to the rear than the front. Here I have marked the anterior face of the teeth with an orange arrow so you can see how they curve.

Also usually present are a pair of carinae, the sharp edges of the teeth. Obviously these are usually on the anterior and posterior edges in line with each other, but they can be offset from the centreline of the tooth or split in some cases.

Along the each carina there’s usually a series of serrations, or more properly, denticles. These are often absent (typically because they are not preserved or wore away) or highly modified and there’s quite a range of morphology if you look in detail. One carina is nicely raised up and clear here and you can make out (just) the individual denticles marked with a yellow arrow.

Theropod teeth can also have various degrees of ornamentation, most notably the wrinkled enamel effect, which is hard to describe accurately, but really quite self-evidence when you see it. Less common, but occasion, the tooth can be ornamented or fluted with various sub-parallel, vertically orientated striations.

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