Archive for October, 2018

Testing for sexual selection

I had a new paper out a few weeks ago but it was at the very height of my busy start to teaching and so barely even got a tweet out about it and completely failed to do anything on here. That’s a shame as this is a paper that has some serious and major implications for trying to detect sexually selected structures in extinct animals (and indeed looking at some odd structures in living ones too). I’ve written a huge amount about dinosaur dimorphism and sexual selection and with numerous papers covering different aspects of the evolution and behviour of dinosaurs (and pterosaus) when it comes to signals and sexually selected things like crests, spines and horns.

The short version is that these are of course hard to look at becuase we can’t directly observe behaviour in extinct animals and coupled with small sample sizes, taxonomic uncertainty of specimens and then issues like extended growth periods and cryptic dimorphism and this is a frustratingly tricky subject to tackle. One standard, if imperfect, measure has been to look at the growth trajectory of the anatomical feature in question and to see if it grows more rapidly than the rest of the naimal, especially iof this happens relatively late in ontogeny. In short, animals don’t need sexaul display structures when they are not sexually mature but when they are this is important so things like horns tend to be small for a long time and then grow very quickly.

This paper led by Devin O’Brien and featuring a host of sexaul selection theroists and biologists posits that things may be more complex still. Features that directly rate to body size will be postively allometric (this can include things like horns and crests in dinosaurs) but those that are not (like say a moths’ antenna), will not. The former are accurate representations of the animals they are attached to and so act as a proxy for their size and quality, but other traits that can still be variable and under sexual selection are not acting in this way and so wouldn’t follow this pattern. There may even be some allometry in these latter traits (non-reproducing animals will not likely invest in such features until the can mate) but the allometry will be much greater, and the correlation with body size present in visual signals.

To help resolve this, we also reccommend in the paper that allometry be tested not jsut again body size but also some other reference trait that is likely to (or been shown to) grow close to isometry. So for example, don’t just measure your dinosaur horn as it related to overall skull size, but also compre it to something like tooth size or humerus lenght. That will help keep things clear when there are other traits around that can grow rapidly or are large but that don’t function as signals. One wonderful example of this we inlcude is a comparisons of the horns on the head of a chameleon with the lenght of the tongue. We used foot size as a reference trait andf show that while both tongues and horns do show allometry, the tongue is little more than isometric but the horns (used in combat and an obvious visual siganl to reflect that) have a much greater allometric slope and show greater variability which is likely to reflect differing quality.

We include a whole raft of such measures of various animals from insects up to mammals and covering both signal and non-signal traits. Two extinct animals were included based on dataset I’ve been working on for a while and may be of interest. One was the frills of Protoceratops which I and colleagues did some time ago but now updated with some extra specimens that we did now have before. These produced a simialr result to our analysis which is no big surprise but nice to see the previous results verified. The second one though was to look at the growth of the tail vane in Rhamphorhynchus.

The standard interpretation of basal pterosaur tail vanes has been that these functioned in steering in flight and acted as something of rudder. That works out quite well since many of the shapes adopted are surprisingly close to the rudders actually made for various aircraft and putting a small vane at the end of the tail would make mechanical sense to increase the effects. However, it is notable that the vanes for Rhamphorhynchus (the only pterosaur where we have a decent sample size) seem to change quite dramatically in shape as they grow and this is rather at odds with the idea that this is purely mechanical. Similarly, there is some serious variation between various basal pterosaurs in vane shape which suggests that the tail is unlikely o be (purely) mechanical in function and the fact that the pterodactloids gfot rid of theirs implies it is hardly critical for flight. Some people have suggested that these vanes were therefor acting as some form of signal and our analysis bears this out. The height of the vane grown very considerably and shows strong positive allometry as the vane changes from a narrow leaf shape in juveniles to a triangle in adults. The vane could of course be multi-functional and it could well be that it has been co-opted from something initially mechanical to function in signaling.

The fundamentals of the methods and theory described here have been around for some time, but the nuance is important to try and distinctuish between traits that are sexually selected and those which are also likely used in some form of display and even combat. It should make for a more reliable way of assessing these kinds of traits and that should be of real benefit to palaeontologists who have an interest in these things. I hope it is not long until more animals are formally assessed for their growth trajectories and what that might mean for understanding their behaviour.

The paper is open access and is freely avaiable here:

O’Brien, D.M., Allen, C.E., Van Kleeck, M.J., Hone, D.W.E., Knell, R.J., Knapp, A., Christiansen, S., & Emlen, D.J. 2018. On the evolution of extreme structures: static scaling and the function of sexually selected signals. Animal Behaviour.

Yet more on bite marks

Yes, I have a new paper out and it is another paper describing bite marks on bones. I have done a number of these now and it can easily seem that they are incremental publications with limited application, but this is important stuff. As has been shown across various papers and descriptions, piecing together the taphomonic history of a specimen and the environmental conditions around it, as well as the nature of the bites, is crucial to showing if bites were likely inflicted by feeding predators or scavengers as well as what species/ clades may have left these traces. If palaeontologists are going to be able to amke effective statements about what bites can tell us then it will help enormously if we have numerous detailed datapoints where we are confident about what information they provide.

So, enter a small and beaten up piece of ceratopsian frill. I was shown this a few years ago by Darren Tanke and Caleb Brown after it was found during a dig in Dinosaur Provincial Park in Alberta, Canada. It was unusual in that it was from a fairly young animal and the bite marks were quite small. It is also unusual that these are bites on a frill, it’s not the kind of place an animal would usually feed on becuase there’s bascially no meat there, just a bit of skin and bone which rather points towards these being scavenging traces from an animal that got to a very decayed carcass rather late.

The bites are hard to interpret with lots of cracks and breaks not helping things. There are two clear bites and they fit the classic morphology of theropod traces and we can rule out things like crocodiles, champsosaurs or mammals having been responsible, despite the small size. One looks more like a tyrannosaur bite (though it would have to be from a very small one) and a second looking more like it was from kind of deinonychosaur. It is certainly possible that more than one animal bit this same bit of bone, but equally bite can be variable and identifying them accurately can be very difficult or even impossible to accurately work out who the biting animal was. So despite the apparent possible different candidates it’s hard to say quite what happened here. That’s obviously disappointing, but it’s important to try and evaluate each bite on it’s merits if possible and this does a least provide evidence that even smaller centrosaurs were being bitten by the local theropods and these were not beyond trying to make a snack of a damaged squamosal.

The whole paper is freely available and open access and is online here if you want to see more:

Hone, D.W.E., Tanke, D.H., & Brown, C.M. 2018. Bite marks on the frill of a juvenile Centrosaurus from the Late Cretaceous Dinosaur Provincial Park Formation, Alberta, Canada. Peer J.

 

Behold the SummonEngh!

Congratulations go out to Brian Engh as he has been awarded this year’s Lazendorf palaeoart prize for his stunning ‘Savage Ancient Seas’ piece. If you don’t know Brian and his work it’s high time to catch up, he’s been an increasing force in palaeoart for some time now and he even has a small and distant connection to the Musings after his first ever commissioned work of a Spinosaurus popped up on here many moon ago.

To celebrate his win, Brian has pointed out that there’s too few art prizes for palaeart (well, one to be exact) so he is starting his own. Anyone can enter and as fits Brian’s interests, he’s especially keen on speculative, but reasoned, reconstructions of anatomy, behaviour and ecology. Here is a video of brian explaining the whole thing (there’s cash to be won!!!) and here’s the Facebook group he has set up for it. now, go make some art!

 


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