A quick post hopefully as a prelude to a return to more regular offerings.
Last year a short symposium in honour of the Finnish palaeontologist Professor Mikael Fortelius‘ 60th birthday was held in Helsinki. As part of the celebrations, a large number of researchers were approached with the view to publishing a Festschrift volume of the Finnish journal Annales Zoologici Fennica. The expectation was a decent number would say yes and a volume could be produced. In the end, practically everyone asked produced a paper, so the volume now totals 25 papers (including the preface and a history of Finnish palaeontology) and a gargantuan 284 pages!
Because of Mikael’s great interest in teeth, alongside macroevolution and ecology (and a soft spot for rhinos), a large number of the papers feature research heavily focused on dentitions of various types. Indeed, the largest of the three subsections of the volume is titled ‘Teeth, Diet and Ecology’! I count 14 papers directly focused on some aspect of dental evolution, morphology, function and development; more than half of the research papers in the volume, with at least a few of the others more indirectly using tooth data. There should be something there for anyone with an interest in tetrapod teeth.
Anyway, the volume (Annales Zoologici Fennica 51: 1-2) is now out, and thanks to a generous donation from the Helsinki EvoDevo community, the entire thing is available as open access papers! That’s right, 25 top notch articles, at least 14 on teeth, available free for everyone. I was a coauthor on one of the papers, entitled ‘Gastrointestinal and dental morphology of herbivorous mammals: where does the Laotian rock rat fit?‘, and hopefully will post a little about that paper and some of the concepts involved in the near future. But please, head over to the journal now (clicking here takes you to the index, unfortunately there is no direct link to the volume which, as above, is Annales Zoologici Fennica 51: 1-2) and read as many free papers on teeth as you fancy!
OK, so that was more irregular than I would have liked (at the start of the last post, the ‘Hello World’ welcome to the blog one, I stated that Tetrapod Teeth & Tales would be updated on a semi-regular basis, but maybe I should aim for semi-irregular and see how it goes). Life, work, and a great conference on ‘Tooth Morphology and Differentiation’ in France followed by an excellent lab visit to Professor Marcelo Sanchez at the Paläontologisches Institut, Zurich University (go there for awesome images of developing living and fossil vertebrates) got in the way. But here’s the post I was drafting before all that…
(Oh and the weather in Finland has been almost the warmest anywhere in Europe recently, over 30 degrees C inside the Arctic Circle in Lapland, so the snow pics below are even more incongruous. We pretty much transitioned straight from winter to summer, with the happenings below, er, happening in the couple of springlike weeks. But other than the timing, the rest of the post still stands!)
A couple of weeks ago in Helsinki, right around the time I was writing the first post of this blog (on how ridiculously insanely huge vole incisors are), the snow in the garden was melting pretty quickly. This winter saw another huge snowfall in southern Finland, much above long term averages, so to get from the road to our front door we had to shovel snow. A lot of snow. Right next to the path the shovelled snow went on top of the already fallen snow, which was then covered in fresh snow, and more shovelled snow, and so on and on for about 5 months (it was still snowing at easter), making for some pretty tall snow piles. But come spring time it tends to melt fairly quickly, and so after a couple of weeks of melting all that was left in the front garden was the remains of a few of the piles of shovelled snow, close to the path.
Strangely, one of the piles of snow had a circle in the middle where it had melted through to the lawn first. I walked past that a couple of times a day for a few days before stopping to investigate. And this was what could be seen:
Some moss and grass, oak leaves, and the remains of acorns. The circular shape of the mossy structure had clearly been made, but by what? My mum was staying with us at the time, and while I was at work she did some quick research and reckoned it could be either voles or lemmings. We looked up the distribution of lemmings – basically in Finland the Arctic circle very roughly marks the southern limits of their southern distribution, and although Helsinki is a fair way north, that’s still 700km or so north of here. So lemmings were out, but a little more research suggested based on the acorn remains that a few of the Finnish rodents could have been the culprits in addition to voles.
As the snow continued to melt, more details of the nest became clear. With the surrounding snow gone, a number of slightly dug out trackways leading in various directions from the nest emerged:
A few of these led to holes in the ground, and a few to small piles of either whole acorns or the shelled remains of acorns. Additional piles of acorns or acorn shells were scattered about the rest of the garden, though the nest makers were very neat as the remaining meals and acorn rubbish were pretty much never both found in the same cache.
This neatness was continued in a second surprising find. As the snow melted and I wanted to be able to ride my bike in to work, I did a little tidying in the garage. Going through a pile of bike, ski and hiking shoes in a corner, one of them rattled. I upended it and about 12 intact acorns came spilling out. Cool I thought, the nest maker has been coming into the garage and using my shoes as a fallback stash of acorns. But even more surprising was when I started tipping out all my shoes to see if any others had acorns in them – none did, but a shoe from a different pair had a neat pile of acorn shells in! The neatness of separating eaten meals from eaten shells extended to my boots! (The acorns were in a right boot and the shells in a left, whether that has any biological significance I have no idea…).
As you can see, the nest is a reasonable height – taller than my shoe – and about as long as my shoe across. It looks pretty comfortable, with a nice mossy structure and dry oak leaves woven into roughly the centre. We’d still not been able to determine what exactly has built it, though the tracks or runs from the nest suggested more strongly a vole of some description. Luckily, I work with people whose research is on rodents, so I knew exactly the folks to ask.
Professor Heikki Henttonen of the Finnish Forest Research Institute is a world expert on small mammals, especially rodents, and in particular those living in sub-arctic to arctic regions. I’ve previously loaned some samples of shrews from him for a research project on their teeth, so he seemed like the person to ask. After a couple of emails, and a couple of photos showing the pellets (poop) found in and around the nest, he had an answer for me: The runways around the nest suggested the vole Microtus agrestis, the field or short-tailed vole, and the pellets were consistent with that.
It looks like all this winter we’ve had a number of these delightful little guys running around under the snow, making nests, storing acorns and recycling the shells in neat piles, and occasionally visiting the garage to do the same in my shoes. The tunnels and runways they make under the snow are partly excavated from the ground (my lawn!) and partly tunnelled in the bottom layer of snow, forming a complete tunnel from the two halves. Once the snow has melted, they abandon the nests and trackways and retreat underground, so they are possibly still in the garden but in the tunnels leading underground from the holes on the edge of the lawn.
To give you an idea of the type of teeth that the field vole has, here’s the same CT scan of the vole lower jaw we looked at in the first post, but from a different angle so that we can see the top of the molar teeth. This surface, where the food is chewed up, is known as the occlusal surface, since it is this part of the teeth that meet or occlude with the corresponding upper molars in the skull:
As well as having ridiculously insanely huge incisors, vole molars are actually relatively large compared to those of other rodents, especially the first molar of the three, found towards the front of the jaw, the m1. (That’s on the left of the jaw above, after the gap – diastema – following that ridiculously insanely huge incisor).
The evolutionary history of vole teeth has seen the front part of this first molar increase in size, adding cusps (or triangles of enamel and cementum) to enlarge the tooth. It’s been said that this has allowed them to more efficiently chew and process food, in this case all sorts of plant matter, and so take over from other rodents (mice etc.) right across temperate northern hemisphere regions (Renvoise et al. 2009).
So – voles, not living in holes, in my front garden. This post fits into the ‘tales’ category of the blogs title, being only indirectly connected with teeth but, to my mind, pretty cool nonetheless!
Renvoisé, E., Evans, A. R., Jebrane, A., Labruère, C., Laffont, R., & Montuire, S. (2009). Evolution of mammal tooth patterns: new insights from a developmental prediction model. Evolution, 63(5), 1327-1340.
The first post of this blog, on vole incisors, was a kind of response to Mike Taylor’s SV-POW! post on squirrel incisors, so I didn’t really introduce either myself or the blog. So here we go.
In this blog,
shamelessly copying ideas from, inspired by science, palaeontology and evolution blogs such as SV-POW!, What’s In John’s Freezer, The Bite Stuff, Tetrapod Zoology and others, I’m aiming to present scientifically interesting and aesthetically pleasing pictures of tetrapod teeth on a semi-regular basis.
Because I find them fascinating and intriguing, and much of my scientific research is based around them, but also because they can be visually arresting and, in the literal sense of the word, awesome.
Teeth are funny. In many living animals, they are hidden away in the mouth by parts of the skull and/or a covering of soft or hard tissue. They become visible only when the mouth opens during feeding, communication, or even yawning. However, once an animal has died and we can look at the skull without any soft tissue (skin, hair etc.) covering it, teeth often become startlingly obviously a major part of that animal, even if they are often fairly small relative to the total size of the animal.
As an example, above is a picture of a living Eurasian lynx Lynx lynx (by David Castor), and below a skull and jaw of the same species that I am studying at the moment from the Finnish Museum of Natural History. The teeth of the lynx are pretty arresting. As a carnivore and member of the Felidae (cats and their relatives), its teeth are used for both capturing prey and slicing meat. The enlarged canines help with prey capture, and the relatively few but enormous (compared to skull size) premolars and molars have long, sharp cutting edges that work like scissors as the upper and lower teeth move during chewing to cut up meat.
The genus Lynx (including the North American Bobcat and Canadian lynx) is unusual even among felids in having an especially reduced tooth count. It has only two premolar teeth and one molar tooth in the lower jaw, and just one premolar and one molar in the upper jaw (plus occasionally a tiny, vestigial second molar). This allows each of these post-canine teeth to be especially large relative to the skull, and for a reasonable gap or diastema between the canine and post-canine teeth.
These two modifications are both presumably for better capturing, killing, and eating of live prey. Three of the four lynx species, including the bobcat, mostly eat small and medium mammals like rabbits, hares, and rodents. The slightly larger Eurasian lynx tends to eat a larger proportion of large mammals, like reindeer, moose, red deer or roe deer, than smaller mammals. (And topically – see the recent report here on Tetrapod Zoology of a wild living lynx shot and killed in England in 1903, unusual because lynx and other big cats have been extinct in the UK for centuries…).
But back to where we were – teeth as a major and important part of an animal. As the above picture and text should make clear, teeth that are hidden from view during much of an animals day to day life are vitally important in obtaining and processing food. Because not all food items are like the soft but slightly fibrous meat eaten by the lynx, different types of teeth are needed by animals that eat different foods. This was noted as long ago as in Greek times, with Aristotle in ‘On the Parts of Animals’ noting that the major use of teeth was in reducing food, but that different shaped teeth served different dietary needs. Much of the variation in tooth shape, the ‘endless forms most beautiful and most wonderful’ (Darwin 1859 in ‘On the Origin of Species…’ – he wasn’t talking specifically about teeth, but I think it’s appropriate), are to do with the different diets and requirements for breaking down food that different animals have. The the link between tooth shape and diet will probably be a theme running through the blog. Some other functions do modify tooth shape, though, including sensing, grooming, use as an offensive or defensive weapon etc, and tooth shape or composition can respond to changes in environment too. I plan to cover some or all of these subjects in further blog posts.
So, though not always visible, teeth can be an important part of an animals body. The huge diversity and variation of their shapes can be linked to various functions, though principally diet. That’s enough reason for me to be interested in studying and researching them on their own. However, a second reason for my interest in teeth is that in the fossil record, and particularly the mammalian fossil record, it’s frequently the hard, mineralised teeth that are the only parts of animals to survive the process of fossilisation. This means that it is often teeth that provide much of the data available to evolutionary biologists and palaeontologists. A high proportion of the information allowing identification of species in the fossil record and the tracing of their evolutionary histories is obtained from differences in tooth shape, especially for mammals.
This was recognised by the great French 19th century scientist Georges Cuvier, who, in comparing the teeth and bones of living animals to those of fossils, helped establish the subjects of comparative anatomy and palaeontology. Cuvier said that simply by looking at the teeth, the whole system of digestive organs appropriate for the type of food consumed could be figured out. He also believed that, through the use of comparative anatomy, a single tooth fossil or bone fragment could be used to recreate extinct animals. His ability to identify species from a single bone or especially tooth was legendary!
A third important reason for my interest in teeth is that the genetic and molecular contributions to tooth shape are increasingly well understood. Teeth have been used as a model system for studying how an organ grows or develops, and what the short term genetic and long term evolutionary forces controlling this development are and have been.
Figuring out the links between all of these different aspects (tooth shape, diet, evolution, genetics, growth and development, and more) interests a number of scientists around the world. They can be from many different specialities – geology, palaeontology, evolutionary biology, developmental biology, environmental sciences etc., but all concentrating on teeth. I hope to explore many of these areas of research in this blog.
In summary – welcome to Tetrapod Teeth and Tales, a blog showing awesome pictures of teeth and talking about some of the science being carried out on them, on a semi-irregular basis.
Who am I?
I’m Ian Corfe, a British palaeontologist and evolutionary biologist with a Ph.D, currently working as a postdoctoral researcher in an Evolutionary and Developmental Biology (EvoDevo) lab in Helsinki, Finland. See the ‘about’ button for more.
What’s a tetrapod?
The short answer is a vertebrate with four limbs and digits at the end of these (so arms and legs, fingers and toes in humans). The slightly longer answer is in the linked ‘about’ section of the blog, or over there on the right somewhere, or possibly up the top (I need to figure out how to control the position of these links!). The even longer version is in a link from the about page.
The actual answer is that the precise definition of the group “Tetrapoda’ is still up for grabs, or ‘under intense discussion and debate’ in scientific speak. I don’t have a horse in this particular race, though the ‘winner’ if one emerges will affect the names I can can call some of my favourite extinct animals, the ‘nearly mammals’ and ‘earliest mammals’ from around 200 million years ago. (Hopefully more on these in the blog soon).
What’s a tooth?
The short answer is a mineralised part of a vertebrate’s body found in the mouth, and usually but not exclusively used for eating. (A vertebrate is an animal with a backbone). The longer answer, including the components of teeth, the locations of teeth, the growth of teeth, the shape of teeth, the use of teeth, and the origin of teeth, I hope to answer as the blog grows.
Mike Taylor of the always excellent Sauropod Vertebra Picture of the Week blog recently acquired the skeleton of a squirrel, and commented on the ridiculousness of the size of the single incisor in the lower jaw. In squirrels these run for about three quarters of the length of the entire jaw in addition to the crown of the incisor visible at the front end, and are heavily curved and so at some points lie just inside the bone margins of the jaw. Mike also commented that ‘the tooth literally could not be any bigger’. Here’s the blog post, and the killer image from it is reproduced below:
It just so happens that my colleagues at the University of Helsinki, Aki Kallonen of the Laboratory of Microtomography, Department of Physics, and Elodie Renvoise, my fellow postdoctoral researcher in the Jernvall EvoDevo lab, Institute of Biotechnology, have been X-Ray Computer Tomography (CT) scanning vole jaws in 3D. Mike Taylor was able to remove his squirrel incisor without damage to the jaw, but CT scanning allows a look inside the jaw with the teeth still in place. Aki and Elodie kindly allowed me to use their data to make a picture that shows the single incisor inside the vole jaw:
The image shows differing density values, in the same way a traditional X-ray image shows the denser bone as white with the less dense soft tissue, muscles, skin, fat etc, as transparent. In this case, it’s coloured from transparent = lowest density (in the vole jaw this is thin bone, as there was no soft tissue on the specimen scanned), through orange representing intermediate density, with yellow the highest density.
As can be seen, the incisor curves below the molar teeth and reaches all the way to the rear of the jaw, excluding the sticky out bits at the top and bottom (the condyle and angular process respectively). In comparison, in the squirrel it only reaches about three quarters of the way back. The last section of the incisor in the vole appears as a dark oval in the CT scan, indicating that although there is space at the back of the jaw for the end of the incisor, it isn’t mineralised and so shows as a region of very low density (and that the bone surrounding it here must also be thin). You can then see the developing tooth increasing in density as you go forwards, becoming orange where the tooth dentine has mineralised.
The lower part of the front half of the incisor ,which is the only portion covered in enamel, the hardest and densest tooth part, shows up as an orangey yellow grading into a bright yellow, indicating a very high density in the CT scan. Similarly, the outlines of the vertical ridges of the three molar teeth, also enamel covered, are a bright orange or yellow, and so high density. Their unusual shape is because, like a horse molar tooth, the vole molar teeth grow continuously for all, or nearly all, of the animals lifespan. The most rearward part of the incisor space is probably full of the stem cells that allow the incisor to continuously grow.
In addition to extending further rearwards than in the squirrel, I’d say that the vole incisor sticks out a little further than the squirrel incisor at the front of the jaw, and occupies more of the jaw in the diastema (the gap in front of the molar teeth) and below the molars. The incisor is actually longer than the entire lower jaw. As other parts of the vole jaw are also smaller than in the squirrel, I’d say that the vole incisor is larger than the squirrel incisor relative to the jaws containing them. This means that for the squirrel incisor,
the tooth literally could not be any bigger the tooth could be considerably bigger! It also means that the vole jaw is reaching the absolute limits of how big the incisor can be. The entire jaw consists pretty much of just the incisor, with the molars and the three processes for muscle attachment (coronoid and angular) and articulation with the skull (condyle) basically hanging off the incisor’s edges. In fact, if you look closely, you can see that the molar tooth roots have to bend around the incisor to fit in the dentary bone…