I recently covered the issues of ontogeny for taxonomy but deliberately did not cover how one can spot the features of a skeleton which might indicate that it is not an adult saving it for here. As ever this discussion is based primarily around archosaurs, though astute observers will note that many of these are applicable to other clades as well, and that there are other features not covered here that can be of use (such as finding a new set of teeth in a mammal’s jaw strongly suggests that it is not yet an adult).
The following features vary in their useful ness and applicability and are best used in comparison to other animals in the same clade that are known to be juveniles or adults. Of course these are not always 100% accurate (or even close) as indicators of an animal being a juvenile or not, but nevertheless, taken in concert these can provide solid evidence that an animal was young, or an adolescent, or an adult.
Overall size. Probably the least useful of the lot, but it still can be helpful. If you find a 3 m long psittacosaur it is very unlikely to be a juvenile, if you find a 3 m long tyrannosaur it is very unlikely to be an adult.
Gross proportions. As noted in the ontogeny and taxonomy essay baby tetrapods tend to have proportionally big heads and feet that then ‘shrink’ during ontogeny. These and other proportions can give you a good idea of how grown the animal is, especially if it is very young.
Bone surface texture. Very young animals often have a very grainy and wood-like texture to the surface of the bone and thus can be used to age even very partial bone fragments.
Epiphyses. Longbones (like the radius or fibula) often grow specifically in patches close to the ends of the bone so that the bone itself is effectively in three pieces – the main shaft and then the two ends being separate with the growth occurring in the gaps. These bits are called epiphyses and these typically only fuse with the main shaft of the bone when the animal is getting close to full grown and growth slows down. As such the fusion or otherwise of the epiphyses give a strong indication of ontogenetic stage.
Fusion. Likewise plenty of other bony bits fuse together only near the end of growth such as the main bones of the skull, the pelvic bones, the sacrum, the astragalus to the tibia and various others. Thus even very partial skeletons can belong to animals that are obviously juvenile or adult, though the exact timing of when this occurs can vary.
Neural arch fusion. The final fusion character and probably the most important. The vertebrae are made up of two parts, the lower centrum and upper neural arch which being separate but fuse together during ontogeny. In most archosaurs this occurs in late development (i.e. fairly close to adulthood) and also moves from back to front so that the neck is the last thing to fuse up and the sacrum one of the first. This is great for comparing multiple specimens with each other and since it occurs quite late in ontogeny and you often get specimens that consist largely of vertebrae it’s a useful indicator.
Secondary sexual characteristics. Obviously this rather relies on being able to identify secondary sexual characteristics but in general ornaments like horns and crests only appear late in ontogeny when the animal is close to adult size and are reduced or even non-existent in juveniles.
None of these characteristics can be taken as an absolute – the vagaries of intraspecific variation and the fact that this review is covering a huge variety of taxa means that you can get near adults with juvenile bone textures and young juveniles with a well fused sacrum. To make things a little more complex most archosaurs also undergo indeterminate growth (that is they have no fixed adult size like mammals do, but will continue to grow throughout their lives. As a result you can get some very big adult animals that can be twice the size of juveniles in addition to what you might expect from natural variation and gender differences. However taken in concert it is usually not too hard to positively identify an individual as a juvenile, adolescent or adult. Some of these factors generally occur in certain orders (e.g. the skull fusing up before all the neural arches are fused) to give a greater level of refinement to the age of the animal. Such patterns have been little studied (mostly do to the lack of numerous specimens at multiple ages which would be required) so relying on them as absolutes is a bad idea, though that doesn’t mean they are not very helpful or that generally recognition of ontogentic status based on them is not accurate.
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