One area I have steered clear of on the Musings (among quite a few others to be honest) is dinosaur physiology. It’s just something I’m not that into and have not read too much about and it’s an extremely complex and tortuous subject to write on. However, while writing another piece I referred to LAGs and realised that I’d not mentioned them before, so it seemed like time to dust them off and get them out of the way.
We have before dealt with determining the age of a vertebrate animal, at least in general terms, by the patterns in which bones fuse up and change during ontogeny. LINK However, in addition to just being able to say if an animal is young, maturing, and adult or perhaps an old individual, it is possible to (tentatively) get an actual date on the specimen – in other words, tell you how old, in years, it was when it died.
First off of course, I should as ever note that this method certainly has a few caveats and while I’d hardly say the results are ever wrong as such, some caution needs to be exercised with this and not everything can be taken at face value. The short version is that as bones grow they expand outwards fairly steadily, but any sudden change in the metabolism of the animal can mean the lay down a layer of rather denser bone that leaves a characteristic trace behind, a ‘line of arrested growth’ (or LAG). Since the only thing that is likely to cause a fairly regular change in metabolism is a change of season, then it follows that these are likely to be annual and so each LAG represents the passing of one year. Each time winter comes around, there weather gets colder, food is more scarce, growth slows up and a LAG is formed. In other words, cut a long bone in half (like a femur or radius as opposed to say a rib) and count the rings to get the age of the dinosaur. Simple, right?
Too simple of course, hence the caveats. First off, as animals get bigger and older their bones remodel themselves so that LAGs, even if laid down, might later be lost. Some younger animals don’t seem to have them at all, and in some cases the number of LAGs can vary between different bones of the same animal (so LAGs counted in the femur might be a different number to those in the tibia). LAGs also vary in living organisms too and of course, while even in the modern tropics there is still seasonality, an animal going though an especially good patch might not leave any LAGs at all, or one with stunted growth might be getting older while not laying down any new ones if the metabolism is low. LAGs are also largely un-recordable in some animals – the hollow bones of pterosaurs (and couple with their generally flattened preservation) make them pretty unsuitable subjects.
Even allowing for all of this, increasing work both on the patterns of LAGs in dinosaurs and how they are laid down in living taxa make this an increasingly accurate science. The lack of LAGs in some young animals and the loss of some in older ones means that a LAG count should often be taken as a minimum, but as a more detailed measure of age than just juvenile-subadult-adult they are useful. There is of course more to this that I am covering here, but these are the bare essentials and should give you a good start the next time you are left with a broken theropod bone and asked how old the animal was.
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