Soft tissues and pterosaur taphonomy, but not as you might expect

In what now seems like a distant and past life, I briefly had a job in University College Dublin teaching in the biology department. Happily, this was on the floor above the earth sciences dept which had a healthy population of palaeontologists including some friends from my previous jobs in both Bristol and Germany. It meant that I had a good time chatting to colleagues on both sides of the ‘divide’ about various research aspects.

One day I was talking to Sue Beardmore (then doing her PhD) and her supervisor Paddy Orr about taphonomy. Through discussion with Paddy, Sue had developed a method of assessing the taphonomy of a vertebrate skeleton in aquatic settings, which could be used to compare environmental conditions among several localities, and infer differences and even changes through time. In theory, if we have the same or very similar species (that will essentially decay in the same way because of their similarities) preserved at two localities, it is possible that their final preserved state will still be different because they were subjected to different external processes. For example, they might have disarticulated to different degrees, suggesting differences in the relative time over which they had decayed before burial by sediments. If their completeness was different, it would suggest a greater number of, or more intense, (biostratinomic) processes. Perhaps one was exposed to stronger currents and less settled waters, which would move away any bits of the body that had separated during decay. In quiet water with few such processes, decay still occurs, resulting in the disarticulation of the skeleton without separated bits moving far from the main part of the carcass. Sue and Paddy have gone on to publish a series of papers exploring this idea, but I realized that it could also be turned around and used from an alternate perspective.

Differences in taphonomy between two related animals in the same environment should reflect differences in anatomy and in particular how well various body parts are secured to each other. In other words, the way in which various bits of the animals have decayed, disarticulated and / or lost allow us to infer something about the soft tissues, even though they are not preserved. This idea inevitably led me to pterosaurs and the huge numbers of Rhamphorhynchus and Pterodactylus specimens that have been recovered from the Solnhofen. They are pretty close relatives and certainly overlap strongly in time and space in these ancient lagoons but we also know that a profound shift in bony anatomy was going on between the two – is this also reflected in their soft tissue? Roping in Emma Lawlor who was then looking for a research project for her undergraduate dissertation, we then had a project to put together.

First off of course we needed to survey pretty much every specimen that we could (and as far as possible in person) leading to examining a whole lot of fossils and supported by photos where necessary. Essentially the animal is divided up into a bunch of segments (head, limbs, tail, body etc.) and are scored for articulation (attached to the right other bit of the body e.g. the wing to the shoulder, fingers to the wrist) and also completeness (so whether or not they are present on the specimen). A fossil could potentially be 100% complete but with 0% articulation, though the two factors are at least partly correlated since anything lost is also by definition disarticulated.

Going through the data there are some simple but fairly stark patterns that emerge. First off, a lot of the specimens are more or less complete and more or less articulated. That’s perhaps no big surprise – the Solnhofen waters are famously fairly anoxic and still, which is why we so often get lots of very well preserved specimens, even including fragile things like pterosaurs as well as soft tissues being retained. Still, it does highlight the general situation at play and that’s also importantly because pterosaurs were generally pretty pneumatic and less dense than many other vertebrates. That would imply that they could potentially float for a long time before sinking which would allow for lots of bits to come off and go missing. That this is generally fairly rare suggests that these effects were pretty limited. When we do see loss of articulation we also see loss of the elements, so decay when it did occur was likely mostly in the floating phase, and that things did not tend to fall apart much once the specimen had settled or we would see lower articulation with higher completeness. In short, there wasn’t much going on at the bottom, likely due to both low currents and limited bioturbation.

Generally, Pterodactylus specimens are less complete than Rhamphorhynchus which may point to them floating for longer (since they are more pneumatic) allowing things to be lost, but could also point to greater transport to sites before sinking and burial. There are also far fewer specimens of Pterodactylus available so this may be a result of the limited data exaggerating the differences a little.

Despite the long and presumably heavy tail of Rhamphorhynchus, this was preserved far more often than the much smaller one of Pterodactylus. This implies that in the former the tail was very strongly attached to the body and was held on with a strong set of muscles and / or ligaments and points to its greater use than in later shorter tailed pterosaurs. Where we see limb loss in Rhamphorhynchus this seems to coincide with the loss of the other limb from the same side – in short if you lose a left arm you also tend to lose a left leg. That points to the idea that the two are attached to each other quite firmly and tallies with the ankle attachment for the main wing membrane.

There’s some other issues at play in these patterns of course (and various other similarities and differences) which I won’t dwell on as that is what the paper is for, but this should give an idea of what we have done and what we can potentially infer with these methods. Sure, the information available is rather limited but it gives a framework for looking at certain anatomical areas in more detail, and it’s likely possible to combine this with other information to delve more deeply into our understanding of pterosaur soft tissues.

Beardmore, S.R., Lawlor, E., & Hone, D.W.E. 2017. The taphonomy of Solnhofen pterosaurs reveals soft-tissue anatomical differences between basal and derived forms. Naturwissenschaften.

 


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