Osteological correlates

I’ve now passed the 500 odd post mark (if you include all the old stuff on DinoBase) and frankly it’s getting hard to remember what I have and have not said before. While I don’t think this post is a real repeat of anything I’ve said before I’ll be surprised if I’ve not mentioned it at least in passing and may well have gone into some detail. Since I try to avoid covering ‘new’ things to do with archosaurs, and focus on the basics palaeontology and the mechanics of research on archosaurs I don’t really have the luxury of always ‘moving forwards’. As such it’s really only a matter of time before I end up completely repeating myself in a post or two (or three).

However, I’m working under the assumption that this is unlikely to be a problem. New readers will not necessarily have read my whole back catalogue (though I would ask, if not, why not? It’s all great, honest.) and regular readers won’t necessarily remember what I said two years ago (since I can’t, I doubt they can). Should this end up repetitive then consider it like revision – something rarely stays in the mind permanently after one round, so a little brush up, or re-examination of an old topic is hardly a disaster. With that in mind, onto the largely (I think) uncovered world of osteological correlates and their place in palaeontology.

These have been mentioned in passing before but not as explicitly as they could, or perhaps should, have been. Obviously the ‘osteo’ part of the term relates to bones so what we are talking about here are features of the bony skeleton that correlate with other things such as soft tissues, physiology or behaviours. This is mostly used for soft tissues understandably where bony knobs, extensions, crests and rough patches can be used to work out were various muscles attached and especially in conjunction with the EPB when it comes to archosaurs.

It has its uses further afield though. For example it has been shown that having an elongate penultimate phalanx (the bone that comes before the ungual in the hand or foot) closely correlates with an ability to grip in living taxa. Thus, when you find a fossil with elongate penultimate phalanges, it’s a fair assumption that it could grip well with the hand or foot. The lack of direct information (and let’s face it, you are not likely to find a fossil animal conveniently gripping something in its hand) is not too much of a problem because the correlation is pretty strong though of course there are exceptions (this is biology, not physics). Similarly, the lack of a correlate can be used to challenge a hypothesis. To stick with gripping, it used to be thought that pterosaurs might have hung from branches or ledges much in the same way that bats do. However, animals that hang in this manner have feet (and / or hands) where the ends of the digits all stop at the same point (i.e. each digit, regardless of the number of bones is the same overall length) which helps distribute their weight evenly across the foot to provide a stronger grip. You see this in things like sloths and bats, but not in pterosaurs. No correlate, probably not hanging.

The strength of the inference is of course linked to the strength of the correlation or their accurate determination and of course consideration of other evidence. For example, nasal turbinates (very thin sheets of bone in the nasal cavity of the skull) are present in mammals and birds and clearly have a role in regulation body temperature and water loss in breathing. Since both groups are endothermic and dinosaurs lack them, this would imply that the non-avian dinosaurs were ectothermic. However, given how fragile these bony sheets are (and that they can be cartilaginious or bony) it possible that many dinosaurs had them and they were simply not preserved and in a few rare cases they are absent in specialised extant taxa as well. Even so, they appear to be genuinely absent in dinosaurs which would place them alongside all manner of ectotherms which also lack these features. However in this case there is of course much more evidence to consider and despite this relatively good osteological correlation between thermal biology and turbinates, it does not account for the information we have on their growth rates, ability to survive in very cold climates, aerobic capacity and others. I really don’t want to get dragged into the question of dinosaur physiology, merely to use this as an example, but the short version here is that the turbinates may say ‘no’ quite strongly, but to the evidence still suggests ‘yes’ to the question of dinosaurian endothermy.

Overall though such correlates are exceptionally useful and an important part of palaeontology. We can make a lot of good and solid inferences about biology and behaviour based on the bones which of course is generally all we have. What is generally lacking is the knowledge about the extant taxa that allow us to make stronger inferences about extinct organisms, though this is a well known, if little tackled, issue.

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