Missing Bits

The fossils of vertebrates (with the possible exception of fishes and some marine reptiles) are only rarely complete. That is, when you find a fossil you don’t get a whole skeleton, but only bits of one. Fossils can of course range from a complete and articulated skeleton (i.e. not only are the bits there, but are have not come apart and moved, so the legs attach the hip and the head is on the neck etc.) to a fragment of bone or part of a tooth. Palaeontologists are obviously rarely interested in such fragments since there is very little you can do with them normally (though I have a paper in review on just such a set of fragments which *are* interesting) but it is well worth considering how and why you get fossils that consist of only a few bits, or why some pieces are often missing.

I have already posted about scavenging and degradation effects on skeletons and indeed about the related issues of bias in the fossil record (and these should probably be read before you start here). However, this post will deal with parts of the actual skeleton itself – what can cause toes and tails to vanish for example?

While it is perfectly possible to find a combination of almost any two (or a few) bones you care to mention (a claw and a rib, a humerus and a palatine, an ilium and a jugal) these are likely to be more or less random associations from a scattered skeleton taken apart completely by scavengers or transported in water. What is of interest here is finding a partial skeleton with only some parts missing. These parts are often the same ones, or part of an obvious subset so let’s take the skeleton apart and look at each section in turn.

Remember that to even become a fossil the carcass has to be buried by sediment, typically underwater, but also occasionally on land (e.g. in volcanic ash). This means we already have a combination of factors to contend with: did the animal die on land or in the water, was it killed and dismembered by a predator or just keeled over from age or disease, did scavengers get to it, was it transported in water or sank immediately, was it buried quickly or over days or even years, or exposed and recovered multiple times, and in what kind of sediments, was it an adult or juvenile, how big was it? Thus unless I want to sink into the quagmire of every single possible factor and how they affect every kind of vertebrate or archosaur, I’ll soon be buried in details and this will turn into a book. This is just a quick look at the primary factors based on the bones themselves rather than all of the possible interactions and combinations of circumstances that will affect them based around their likelihood of separating from a body, being preserved, transported and / or destroyed.

Skull: this (as a whole unit) can hang around since there is little meat on it and so scavengers can only do so much. That said the individual pieces of the skull can easily split apart and be lost, and of course the head can easily come off the rest of the skeleton, and the jaw can easily come off the skull, so both parts can often be found in isolation (either from a skeleton or from each other). Weak bits like the sclerotic ring, nasal turbinates, ear bones and so on are easily broken or simply have too little bone to be well preserved if the survive at all and thus are rare. Skulls in general aren’t too common as compared to say a nice robust femur, the bones are pretty weak and fragile.

Vertebral column (back and neck): individual bones can be well bound together with ligaments and so one and will be found as whole units while others can easily separate out and vanish (notably the axis and atlas that support the skull). While they are commonly found (they are very numerous and typically robust) often fragile bits like the neural spine get broken off or simply do not preserve.

Pelvis: the pelvis and sacral vertebrae is obviously a robust and chunky set of bones that are unlikely to be destroyed during feeding, or dragged away by predators or washed away with flooding. Other bones of the pelvis (like the pubic bones) can be separated and lost however.

Tail: the distal tail is almost always missing from a combination of factors – scavenging, small size or simply being washed away or missed during excavation. The bones tend to separate out (except linked / fused things like dromaeosaur tails) and thus can be easily lost, as can the chevrons for the same reason.

Ribs and gastralia: a quick burial of a dead body will likely ensure that everything is intact and articulated and that is by far your best chance typically of finding ribs or gastralia at all. They are obviously often fragile and while ribs can be big, they are also often thin and any predator wanting a decent meal is going to go through the ribcage to get to it, so it’s not surprising when ribs are often disarticulated, broken or lost.

Main limb bones: can be found intact and articulated, or broken and scattered, I’ve not noticed any obvious pattern here. One thing of not is how often one finds bird and pterosaurian wings articulated by isolated from the rest of the skeleton (and occasionally wingless bodies) since they clearly detach easily after death.

Fingers and toes: these again are often missing (or found in isolation) since being small they tend to detach, and are easily moved.

So there you have it, in general fingers and toes, tail bits, skulls and ribs tend to go missing or be found separated from the ‘main’ body. That means you should often find something consisting of part of a vertebral column, with a pelvis and a few major limb bones. That at least is my impression and is at least partly back by my experience in the field and just looking over collections and fossils, though it would be interesting if someone did (or has done) a specific study of this. As ever there are strong variables based on all kinds of things, (sauropods only very rarely preserve skulls at all but these are common in isolation for ceratopsians) so don’t take it as gospel by any stretch of the imagination, but it certainly illustrates a few problems (like getting accurate length measurements when you never have a full tail – how much is missing?).

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