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Niche separation in the fossil record

There is a slow but steady publication papers that describe new fossil taxa that state or imply that the presence of some new species is evidence for niche partitioning in the animal’s ecosystem. This is basically redundant and is akin to the classic ‘this new species adds to the known diversity’ as if it could do anything else. One of the fundamental ideas of niche theory is that two species will not occupy the exact same niche. If they do, one will go extinct (or be forced out) or will adapt. In other words, for two species to live alongside each other there will be, by definition, niche separation, so pointing it out as some revelation or important insight or gained knowledge from two species being present is really not the case.

Worse, it might be wrong. Most of the time with new finds we don’t have very much to go on, so we don’t actually know that it truly did occupy a unique niche. Perhaps it was a transient species and so could survive briefly in full competition with another species as it passed through or had only just invaded and in the fullness of time would outcompete (or be outcompeted) by another species. So a statement about two (or more) species with similar biology (perhaps they are close relatives) being evidence for niche partitioning is most likely either redundant or wrong. Either way, as currently reported, it’s really not needed as commentary on a lot of papers about palaeoecology.

As a related point, there is an idea floating around (online and in discussions rather than I think in the scientific literature) that large predators can’t coexist normally, especially if they are closely related. There’s an expectation that ecosystems with say three large theropods in (or even three large tyrannosaurs) would be really weird and somehow not normal or possible. I’m not sure why this idea is out there but I think it’s a misunderstanding of the above point about niche separation and is somehow a conflation of the idea that species somehow doing the same thing or eating the same prey means one will inevitably come out on top, but again this isn’t really correct.

First off, species can be catching prey in very different ways, but still competing with each other. You only need to see videos of a baitball and any combination of sharks, dolphins, whales, sealions, large fish, diving birds and others all going after the same small fish. Each has its own technique and feeds and processes food very differently, but they are in direct competition for that same resource. So just because birds are flying and have no teeth, doesn’t mean they don’t compete with the dolphins and there is at least some niche overlap between them. Even very similar and near identical species can still partition successfully depending on quite what they are eating and when. They may be separated by seasonal or daylight cycles, or be targeting different prey species even if they are hunting it in the same manner, they will be likely shifting their niches as they grow, and there could be all kinds of local differences in habitat that are hard enough to spot in living species let alone in the fossil record.

Back in my paper describing Zhuchengtyrannus I made this point and the idea that multiple similar species in ecosystems is not actually that strange. However, some ongoing work on a related issue with theropods has made me look again at this and pull out a couple of relevant examples from modern / recent ecosystems. A quick look at the distribution maps on Wikipedia (hardly the last word in science I know, but sufficient to make the point) shows that the estuarine crocodile C. prorosus overlaps with C. johnstoni in part of Australia, C. mindorensis in the Philippines and C. novaeguinea in New Guinea. C. siamensis and Tomistoma both overlap with it and each other in parts of Indonesia, and there is a similar 3-way overlap with both C. palustris and Gavialis in India. Similarly, in South America, Caiman crocodilus, Melanosuchus and two species of Paleosuchus all overlap with each other. Now, at a very local level in a given pond or a short stretch of river there might be only one or perhaps two species present, and in some cases there are some dramatic differences in skull shape and gross feeding ecology, but these overlaps and the inevitable occasional migrations or transport of individuals means they must be truly sympatric at times and probably under some competition.

For a more terrestrial example, the 2019 paper by Schnitzler and Hermann looking at fairly recent (historical) overlaps of large mammals in Asia (especially lions and tigers) has this wonderful quote [that I have modified a little for clarity] about carnivores in part of Western Asia. “The Western Asian area of the Palearctic Biogeographic Realm includes part of the continental interior of the Near East (the northeastern part of Anatolia in Turkey, and Transcaucasia – Georgia, Armenia, Azerbaijan), part of the Caspian lowlands and western part of Pakistan. [It] had an impressive assemblage of large mammalian carnivores (Asiatic lion, Caspian tiger, Asiatic cheetah, Anatolian leopard, lynx, brown bear, grey wolf, jackal, and striped hyena).”

That’s really quite a set of animals and while jackals were probably not much competing with lions for food, there’s a huge amount of overlap here. Even modern India has striped hyena, jackal, dhole, wolves, leopard, lion, tiger and sloth bears in various parts and until recently cheetah too (plus, of course, three large crocodylians) and while their ranges are now much restricted there would have been much greater overlap in the past. In short, while obviously dinosaurs are very different to mammals and crocs, the idea that a Mesozoic ecosystem couldn’t support two or three large theropod genera looks like a poor hypothesis against the kind of overlaps we see even in modern depauperate and stressed ecosystems. Multiple large carnivores, even including closely very related species from the game genus, that are known to hunt similar prey in similar ways, are commonly sympatric and there’s no clear reason to assume ancient systems were that different.

Schnitzler, A. and Hermann, L., 2019. Chronological distribution of the tiger Panthera tigris and the Asiatic lion Panthera leo persica in their common range in Asia. Mammal Review49(4), pp.340-353.

Dinosaur tails redux

Getting on for ten years ago, I published FANMATS 15470 New York Jets Golf Hitting Mat. The short version of that is that total length of tails in dinosaurs varies massively both between clades and even within groups (or within species!) which mean that a lot of the ‘total length X’ estimates for various dinosaurs are probably way out. Still, it wasn’t the biggest dataset and there’s not a lot of nuance to looking at total length vs body length, plus being restricted to only complete tails really cuts down on the number of specimens you can use.

Still, not too long after the paper was published, I set about trying to get a better dataset together as more dinosaur tails were coming out of the woodwork. That led to this appeal on here which helped reel in a few more specimens. Still, I wanted to do something more detailed and that led me to roping in my friend and colleague Steve LeComber.

At the Cheltenham Science Festival (L-R, Me, Steve Le Comber, Chris Faulkes, Jane Hallam)

Steve will be all but unknown to readers on the Musings as he never worked on dinosaurs before, though as a great science communicator he helped me out at a number of my events, especially when we went to the Cheltenham Science Festival together a few years ago.  Sadly, this will also be one of his last papers as he passed away at the end of 2019. Steve was one of my closest friends and colleagues, and was one of the most popular and friendly people I have ever met. He had an entire career as a journalist and writer before switching to science and was a superb statistician as well as a great biologist and a wonderful educator.It’s a testament to his work that papers are still coming out of his lab and his work on geographic profiling will have an important and lasting legacy in biology. He will be forever missed. (A scientific obituary was published for Steve here in the Journal of Zoology, where he was an editor for many years).

I had turned to Steve because he was tremendously creative with analyses and I had no idea how to approach the next problem I wanted to tackle – what was happening with individual vertebrae in the tails of dinosaurs? Very little has ever been written about this, and what there is implies or even states that as you go down the tail, each vertebra is shorter than the last. But you only have to look at a couple of specimens to see that this really isn’t the case. It’s true for big chunks of the tail, but the part closest to the hips often has short vertebrae but after that they tend to get longer, and in very long tails like those of sauropods you can find multiple sets of vertebrae that are lengthening. But how to capture this information? If groups of vertebrae are letting longer or shorter, and if this changes slowly or dramatically, or is just an oddity, can we capture it? Happily, he had some ideas.

As I was by now established at Queen Mary, I really lacked the time and opportunities to revisit collections and measure individual vertebrae so we then roped in Scott Persons, who has his own interest in dinosaur tails and had measurements we could use, or was able to get into some of the Canadian collections to procure more data. Various issues delayed the paper on numerous occasions but it is now out so here’s some quick take-home results (the full paper is in PeerJ and open access so you can see all the figs and data there).

First off, with more and better data, we did revisit the issue of overall dinosaur tail length and it is still very variable and unpredictable. Total length estimates without most of the tail present could easily be very wrong, and even some tails that you might think are pretty complete could easily truncate suddenly or go on much longer than you might think. There’s still a place for these of course (especially for engaging the public) but the standard ‘my theropod is longer than yours’ battles really need to stop. Total length isn’t a great indicator of size (mass is) and tail lengths, and by extension total lengths, are very hard to estimate without a near complete tail.

Obviously we do now have a much better Spinosaurus tail, but this old image by Scott Hartman demonstrates just how wildly different tail lengths (and so total lengths) of an animal could be for the same body size.

On to patterns within tails. First off we do find that individual vertebrae within tails simply don’t tend to get shorter as you go along them. There’s some interesting and cool patterns going on and I don’t want to cover all of them here (for example Coeplohysis is all over the place, and Juravenator seems really weird) but here’s a few of the more interesting ones. We use broken-stick regressions where we can have multiple different trajectories of sections of the tail lengthening, staying the same length or shortening. It’s a great tool to see what is happening and is visually nice and easy to follow, without getting mired down in the odd vertebra that’s rather out of place with the others.

First off, that means that it’s good for spotting changes in patterns of vertebrae lengths and also deal with (bits of) missing data quite well. This is also really useful for predicting the lengths of missing vertebrae and this is likely to be useful for things like working out total lengths of animals and the sizes of individual verts when reconstructing fossils. It’s also absolutely ideal for putting together skeletals and even mounted skeletons in the future.

Second, many dinosaurs have a pattern of a set of short vertebrae, then longer ones, and then the rest of the tail does indeed taper off. The second switch (were the long ones stop and it starts to taper) coincides with the ‘transition point’ in the tail, where the main leg muscles terminate, suggesting an important link between the two and from this we hypothesis that this short-longer- tapering pattern is a functional one linked to tail flexibility and muscle power. This clearly needs more work, but it’s a very interesting starting point.

Next, some exceptions. Plenty of dinosaurs don’t fit this pattern for various reasons (some it’s probably just missing data or it’s a subtlety like the tapering happens in two different phases), including some that do just generally taper. There’s some huge intraspecific variation in some but others are very consistent. All three specimens of Archaeopteryx we included show a weird humped distribution which is also very similar to Microraptor, and also different to other dromaeosaurs. That rather implies that this is flight related and that this is an important convergence, though again quite what and how is well beyond what we cover in the paper, it’s an area that hopefully others will pick up on. And aside from Microraptor, the dromaeosaurs appear to be highly variable which we attribute to their ‘sheath’ of elongated supporting rods for most of the tail length which would dominate any other functional issues and might leave the lengths of individual vertebrae to be fairly free of constraints.

I’ll leave it there since the paper is freely accessible and there’s lots that can be extracted from it, but I think this covers some of the more interesting points. The methods in particular should work well for any repeating units and while we have focused on dinosaur tails here, they should apply equally to any vertebral series or things like ribs, arthropod segments, and so on. I’d really hope that people will immediately see the use of this for describing things like sauropod or plesiosaur necks, pterosaur tails, or the lengths of neural spines or size teeth in a series. Of course I also need to say thanks to Scott and Steve and various referees and editors for helping get this published, and especially thanks to all those who contributed data to get this moving.

Hone, D.W.E., Persons, W.S.C. & LeComber, S.C. 2021. New data on tail lengths and variation along the caudal series in non-avialan dinosaurs. PeerJ. 9:e10721.

Protoceratops socio-sexual signalling again

It’s Protoceratops time again (hooray!). I have now published half a dozen papers about (or at least with a healthy dose of) Protoceratops andrewsi and a large part of that is because of the high number of really well-preserved specimens which make it one of the best dinosaurs to work on. There’s young juveniles through to large adults, large numbers of skulls and even skeletons, and all from a very narrow time and space that make it near enough (in palaeontological terms) a single population.

Among other things, I’ve used this to look at the growth and development of the frill and the implications for this major bit of dinosaurian anatomy being linked to sexual selection and dominance signals. (Here’s a recent round-up I did of all this work). The new paper out today is led by my former PhD student Andy Knapp and takes this previous simple work that only examined a few simple linear measurements and turns it up to 11.

Andy got to see almost every good Protoceratops skull going, including trips to New York, Warsaw and Mongolia to see material. Taking numerous photos of each allowed him to build 3D photogrammetry models of them and then get going with some quality morphometrics and analyses to look at how the skull changes in shape as the animals grow. It also shows that the skull can be divided up into distinct units and that these are somewhat independent of each other in their changes to size and shape and suggests evolution could act on each separately. The frill, rather inevitably, turned out to be a single unit and one that changed the most and one that is the most variable.

This fits very nicely with the prediction that the frill is used as a socio-sexual signal and if it’s true for Protoceratops then it’s probably true of other ceratopsians as well. I’d also suggest that this is good as we get very similar results from this very intensive and complicated methodology as we did from taking a few simple measurements of the skull and that gives some confidence that for future studies, the much quicker and easier methods may be more than sufficient to determine what kind of growth patterns are at play. There’s inevitably some more nuance and detail in the paper but happily this is another fully Open Access one, so click on this link here to be transported to a magical world of colourful ceratopsian skull bits in 3D.

I’ll finish here, but obviously huge congratulations to Andy on his great work on this long and complicated project and on getting an important paper out of it. I should also thank the Jurassic Foundation who gave Andy a grant to help with his travel and work on this project.

Knapp, A., Knell, & Hone, D.W.E.2021. Three-dimensional geometric morphometric analysis of the skull of Protoceratops andrewsi supports a socio-sexual signalling role for the ceratopsian frill. Proceedings of the Royal Society, Series B.

Depictions of Spinosaurus

New ideas (or at least new papers reviving ideas) on the appearance, behaviour and ecology of dinosaurs are often accompanied by a wealth of new palaeoart as people get invigorated and inspired by potential ideas and want to create them. The flipside of this is that there is a tendency for the previous ideas to be immediately written off as being wrong or out of date. Given the huge volume of images of Spinosaurus that have been produced in the last few years off the back of the swimming hypothesis, I think this is therefore probably worth addressing.

Spinosaurus by Bob Nicholls of Palaeocreations

As we say clearly in the paper (and I said in an earlier post), there’s no reason at all to think Spinosaurus did not swim (pretty much all tetrapods can) and it was hanging around, and in, water really quite a lot. I don’t think it was swimming that much though, and diving would have been strenuous and difficult, but probably not impossible. Even animals we don’t think of as swimmers can at least get in and around water – I once saw a large grey heron in Japan dive into the water from a high river bank, get completely submerged and come up a few seconds later. That doesn’t mean that’s normal for that individual let alone the species, but depicting a heron underwater grabbing a fish would not be wrong.

In short, the vast majority of sketches, drawings, paintings and even videos of Spinosaurus underwater are not now suddenly wrong, inaccurate, or dated. Still, if you think that the wading model is something worth illustrating or if you have seen other ideas and hypotheses in the paper and want to use that on the next round of Spinosaurus (or indeed other spinosaur) illustrations, here’s a few key points and details and suggestions. As above, none of these are inherently correct at this point, but these all come up and would be relevant if you were trying to do an animal according to our ideas in the paper.

We suggest that the tail ‘fin’ may have been a display structure (in addition to the dorsal sail and the head crest) and as such could have been brightly coloured or patterned with strong contrasting colours. We also noted that crocodylians do a head-and-tail-up display posture in water which would be a potential option for a spinosaur.

Based on work done on Irritator, the head may have been typically held at 45 degrees (though there’s lots of variation seen in these value as per the excellent recent bovid study on head posture). This could well be close to a natural posture when foraging with the snout underwater (and we reference this specifically in one of our figures).

This is one that seems to get missed. We don’t have a single good full Spinosaurus skull but we do have a good idea of the nostril position and it’s a long way from the top of the snout. There is a question about how far forwards it is, but it’s very much not near the front or top of the snout. If foraging with the snout underwater, the nostril is usually going to be kept clear of the water.

And on that note, we propose that the snout would likely be below the surface normally, either when waiting for fish to come past that can be attacked, or possibly even shovelling around the bottom. (Come to think of it, we don’t really explicitly state this in quite these words, but we do talk about stork and heron-like behaviours, and show a stork foraging stirring up mud and reference searching for benthic things, so I think we cover both of these possibilities in what we have written).

If the animal is foraging while standing in water would not be too deep so that it wouldn’t float. This would probably be anything up to the belly (or of course much less).

We did also suggest that it may have made crocodile like thrusts into water to grab things, using the legs and tail and probably generating a lot of splash.

It doesn’t have to be in or even near water. There’s good evidence that some were spending considerable periods on land as ‘normal’ theropods.

A lot of things were probably on the menu apart from fish. We know from isotopic data that some individuals were eating herbivorous dinosaurs and the like with little or no aquatic prey in the diet and the tooth and skull structure point to Spinosaurus at least having the capacity to eat things like turtles and crustaceans, while we know other spinosaurs took dinosaurs and pterosaurs, and it probably scavenged too.

In short, there’s lots of opportunities to expand the range of things Spinosaurus is shown doing (OK, so those opportunities were already there, but here’s some specifics with reasons) and hopefully this will help give some hints and tips. I’ve already seen lots of new artwork popping up, in addition of course to the wonderful rendition that Bob Nicholls did for us (shown at the top) that has already been plastered round the web. Coupled with the (revised? alternate?) skeletals that are out there by YOTOO Heavy Duty Air Compressor Accessory Kit 20-Piece, 1/4" NPT and Get Away Trike (which we used in our paper), this should be a productive time for drawing this and other spinosaurs outside of them being deep underwater. Pencils and tablets at the ready!

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I tried to emphasise in the last post that there is still more to come here. I’m sure other people are writing more about Spinosaurus and its behaviour, ecology and functional biology right now and there will inevitably be further discoveries and data coming out in future. While I am confident in the wading / heron-like idea, further work could easily provide major modifications to that or overturn it. Such is the way of science: things are not settled. Below, I thought I would put together a few ideas of hypotheses or aspects of the model that could be tested or assessed further to show how much more there is to come and move away from the ‘this is settled’ idea that I’m sure at least a few people will have articulated. Some of these are much easier than others to tackle (measuring toes is way easier than working out drag) but all are, I think, something we as a scientific community can tackle in the coming years based on the techniques we have and the available fossils.

What does naris and orbit position look like in an even wider range of reptiles and in birds?

What is the overall density and distribution of mass? In particular, how buoyant would it be?

What was the exact arrangement of the neck muscles?

How flexible is the neck?

How stiff is the dorsal series?

What is the exact arrangement of the dorsal sail?

How much drag would the sail / legs produce when swimming?

How much wave drag would there be at different depths?

How spread were the toes? Is this more than other theropods?

What swimming form would it use? (Whole body or just the tail)?

Would leg thrusts help in propulsion or add more drag than thrust?

How flexible is the tail?

How strong are the caudal neural spines?

How much muscle did it have in the tail and where?

Would increased flexibility help it provide propulsion?

Would a leg and tail driven thrust in water be effective, even for a single thrust?

How efficient would it be walking?

What would the efficiency calculations for the tail look like with a more accurate model with variable flexibility and different degrees of submergence?

Are there more general common features of various aquatic and semi-aquatic reptile lineages and can we quantify things like leg reduction, tail musculature, drag reduction etc. to look at this as more of a continuum than a binary state?

Is ungual curvature driven in part by size / evolutionary relationships / habitats?

What are the isotopic signatures like for teeth that are in situ in jaws?

How many different habitats and environments did Spinosaurus occupy? And what were these like?


I think that all shows that we can push this forwards considerably. The new paper covers all kinds of different bits of anatomy, ecology, mechanics and possible behaviours and some considerations of the environments too, but this is still all just starting points. This of course is all based on existing fossils and any future discoveries (an arm would be nice, or a complete skull for that matter) is only going to provide most data (or fuel to the fire, take your pick). Still, this hopefully provides a few ideas for people to be getting on with.

If you want even more discussion on Spinosaurus (and why wouldn’t you) then the first episode of the new series of my Terrible Lizards podcast is now up and it’s a whole hour of wading hell-herons (copyright Andrea Cau).

The evidence for Spinosaurus being a specialist aquatic predator and good swimmer is weak

Spinosaurus shown as a wading hunter. Artwork by and copyright too Bob Nicholls, used with permission.

Before I get into the depths of this post since I’m sure many people won’t read it all (and it’s good to prep the reader), here’s the TLDR: I am not saying Spinosaurus didn’t or couldn’t swim, or that it could not swim better than most other large theropods, but the evidence presented to date that it was a semi-aquatic animal and a specialist hunter in water (and specifically a pursuit predator) is really not well supported from the currently available evidence. It may even ultimately turn out to be correct, but as things stand, the evidence is weak and there are a lot of gaps and contradictions to this model.

So, there’s the essence of it. The purported arguments that Spinosaurus was some croc-like or even stem-whale like animal spending the vast majority of its time in water, and the way it has been illustrated swimming in deep water, and even diving and pursuing fish does not hold up to scrutiny. Instead, a wading model of a more heron- or stork-like animal that spent a lot of time in and around water, but fundamentally fished while standing rather than swimming, is supported. I really don’t want to go into everything in this post since the paper I have just out on the subject is some 13000 words of text followed by 120 references, so is incredibly long and detailed. The paper is open access so freely accessible and I think it’s fairly easy to read and follow, so a huge post here is rather redundant.

The new paper is an extension of my collaboration with Tom Holtz on spinosaur biology and both follows up and expands on our ideas from our 2017 paper, and tackles a bunch of interrelated hypotheses about the biology of Spinosaurus. In essence, it’s supposed to be unique among theropods (even compared to other spinosaurs) in being highly aquatic and a pursuit-predator of fish in water, powered by a deep tail. This stands in contrast to our own favoured concept (which is not original, but really an extension / modification of the basic ideas put forwards by others) with them standing in water to take primarily piscivorous prey. There is more depth and nuance of course but that’s the basic split, actively swimming after things, or wading and grabbing. There’s no real argument that spinosaurs were generally tied in some way to water and aquatic prey, but how much and how they hunted is the core issue here.

For the paper, we went through the whole functional anatomy of Spinosaurus and the claims and arguments for both models. We collected some original data to analyse but also looked at some possible analogues and a bunch of literature and existing data that is out there too. What follows are a bunch of extremely simplified and reduced points and as such they might not be entirely clear to everyone but the central idea should be clear enough.

A principal components analysis of the shape of the skull show that Spinosaurus is like other spinosaurs (which are not supposed to be as specialised) and also closer to other theropods than crocodiles or other semi-aquatic and aquatic reptiles.

Similarly, the nares and orbits are not dorsally positioned and are not like semi-aquatic taxa.

The posteriorly retracted nares would allow the jaw tips to be in water while foraging as they are in modern storks and herons.

The supposed sensory system of the jaw is similar to that seen in other terrestrial theropods.

The form of the teeth are similar to those of large aquatic and semi-aquatic reptiles, but specifically they conform to a ‘generalist’ feeder type.

Functionally the skull would work very similarly to that of Baryonyx even though they are supposed to be hunting in different ways.

Enamel isotopes data shows that some individuals spent considerable times in terrestrail environments and some individuals ate a lot of terrestrial dinosaur prey which doesn’t fit with an animal that is an aquatic specialist to pursue fish.

The neck is specialised for a downwards action which would fit with an animal standing in water striking down better than an animal already swimming in water which could strike in any direction.

The neck is relatively long and with bracing cervical ribs. Such support would not be needed in water, but is useful if standing and striking.

The possibility of webbing on the toes could help this animal swim but would be equally useful for walking around in mud and other soft substrates.

The pedal unguals are rather flat, but other large theropods have similarly flattened unguals. Only very few modern birds have these are not usually good swimmers and also include a number of waders.

The large neural spines in the tail don’t match those of other swimming animals and do match those of species which use the tail for display.

The efficiency calculations presented show that Spinosaurus was a much less efficient swimmer than crocodiles, but these themselves are not fast and efficient swimmers and are not pursuit predators.

The tail has limited muscle attachments compared to tail-driven swimmers suggesting it was not a fast or powerful swimmer.

The dorsal neural spines would have created massive surface drag meaning that they could only reach efficient swimming if they were submerged in very deep water.

Despite the evidence of high bone density, they still have lots of pneumaticity which would make them unstable in water and make diving a strenuous exercise.

The legs (while apparently reduced) and arms are not anything like as reduced as seen in fast swimming animals.

As I say, there’s even more than these points covered in the paper and there’s additional details and nuances that I’m not going to cover here (and there’s some further ideas we left out of the paper because we didn’t have room). Still, as you can hopefully see there are some major issues with the advocated hypothesis that Spinosaurus was extremely adept in water. As noted at the start, this does remain a possibility, but the arguments put forwards to date are weak and even contradicted by other data. So the idea of a highly-aquatic and pursuit-predator Spinosaurus should be shelved for now, while the wading model is currently well supported by various lines of evidence.

Water was obviously important to the spinosaurs and Spinosaurus does show some traits suggesting greater affinities with water and perhaps reliance on swimming than other members of the group. But fundamentally it’s very similar to Baryonyx and other spinosaurs in lots of important functional ways suggesting they were basically doing the same thing the same way. Spinosaurus shows only the slightest adaptations towards aquatic ecology compared to lots of other semi-aquatic and aquatic animals and doesn’t have lots of ones that have appeared repeatedly in numerous lineages. The analyses to date of its swimming ability have been primitive but a big animal with a sail would have ton of drag and it appears to be a low muscled and inefficient swimmer – that’s not an animal that is going to be actively chasing fish.

We all know there is more material of this enigmatic dinosaur being uncovered and new work is also coming on various other spinosaurs which will help clarify things further. I think it’s reasonable to say this is by far the most in depth assessment of the ecology and behaviour of these animals to date and helps redress the balance of some hypotheses that have been advocated with very little support and gives a much firmer foundation for working out which spinosaurs may have been doing what and how. These are fascinating animals who undoubtedly had unusual ecologies but we can test ideas about their biology and this is, I hope, a major step forwards in that.

Hone, D.W.E., & Holtz, T.R. 2021. Evaluating the ecology of Spinosaurus: shoreline generalist or aquatic pursuit specialist? Palaeontologica Electronica.

The paper is fully open access to anyone can access the link above. I’d like to thank Tom for all his work on this, the four referees and editors who put a lot of effort into reviewing such a big paper and all the various people who contributed little bits of data, papers and images (especially GetAwayTrike on Twitter for the skeleton and Bob Nicholls for the awesome artwork) and those who acted as sounding boards for various discussions.
A quick update, here’s the podcast special episode I did on this paper.

Revising the frog-mouthed pterosaurs: the anurognathids

If you hunt around the right bits of various websites, you can still find adverts for a book called ‘The Pterosauria’ that doesn’t exist. Conceived as a pterosaurian equivalent of the famous ‘The Dinosauria’ text book it was to have chapters devoted to each major group and some other big aspects of pterosaur biology. Originally scheduled to appear in 2009 it got put back again and again and then slowly collapsed as content failed to be produced. The ghost of it is still remains in various places where there are previews available and for many authors (including me) it was a source of colossal frustration. Months had been devoted to writing chapters that could not easily be published elsewhere as they were in specific formats and these kinds of very long and detailed species by species reviews are not accepted even by many review journals.

Thanks to Mark Witton who produced this beautiful restoration of Jeholopterus for this paper.

In my case a chapter on pterosaur origins and another on anurognathids were left languishing and a couple of attempts to resurrect them didn’t work when I ran short of time and of course they slowly became more and more out of date. However, a recent block of time appeared and I decided to dust this off and find a home for it. Much of it hadn’t dated since, well describing all the known specimens and giving a general overview of their history and anatomy was going to be the same, but there’s been an absolute flurry of anurognathid discoveries and new taxa and unnamed specimens in recent years as well as some conflicting discussions about their phylogenetic position. (You can see some of the progressions from this old blogpost I wrote in 2008 when there were only four genera known and the work that would become this paper was planned).

The anurognathids are a wonderful group of small non-pterodactyloid pterosaurs known from Europe and various parts of Asia that are perhaps the most distinctive of the early pterosaur groups and probably the latest survivors. They had bizarrely short and broad skulls made of tiny spars of bone and with few teeth and remarkably short tails for non-pterodactyloids. They were mostly small and are interpreted as having been hawking for insect prey on the wing. There are few specimens (even with the recent discoveries) that are hard to tell apart because they are all so similar and yet almost every different specimen has been named as a new species.

So they are both really unusual and not very well known and that means even if this has taken time to come to fruition, a review of them would be rather handy. And so as you might imagine, this post coincides with a new paper doing exactly that. Somewhat inevitably there’s not a huge amount to talk about here since as it’s a review, it doesn’t contain too much that’s new – the primary role is to bring things together and synthesise them so most of what is there is already known (at least to people who keep up with the pterosaur literature). Reading the review will bring you up speed if you want all the basics, but I do want to talk here about a couple of the more interesting things I have added.

The first one is the validity of the various taxa. It’s hardly unknown for pterosaur clades to be made up of lots of species each represented by only a single specimen but the anurognathids are pushing even that. While I can’t immediately think of any calls for synonymy of any taxa, the fact that so few specimens have been described in detail and the poor quality of the preservation of many means that the available lists of diagnoses have been pretty weak to date. They are not much better now, but I have at least revised and updated the diagnosis of every taxon. There are two consequences of this that are important. First off, all the current taxa seem valid, and moreover, some of the recently illustrated, but not yet named, specimens also look like they are distinct taxa and there’s probably several new names needed. Secondly, the second species of Dendrorhynchoides, D. mutodongensis is as distinct, if not more so, than many other anurognathid genera and as such needs to be elevated to the genus level.

I didn’t want to name the other putative taxa without the permission of the original describers but in this case, I named D. mutodongensis with Junchang Lu so it’s only fair game for me to sort out the naming. JC, as he was known, sadly passed away recently and he had published on multiple anuroganthid specimens so it is appropriate that in his memory I erected the new genus Luopterus to house the species. 

Next up, the variation in the different species is quite odd. Anurognathids are weirdly conservative, even compared to other pterosaur groups and while the poor preservation of the specimens hasn’t helped up find distinguishing traits between them, once you sit down and really look it’s hard to find the kinds of traits that you might normally use to separate out genera and species. That said, there are some bits of variation which while commented on before are quite notable in this context (and there is more coming on this in a future paper that I’m involved in). The length of the tail is really variable and while these are as a whole short-tailed (even the longest of them is much shorter than other non-pterodactyloids) there is really quite some difference between the longest and the shortest. I don’t know what this means but it’s an area worthy of greater attention. Similarly, the smaller anurognathids tend to have extraordinarily large heads and the larger ones rather small ones. There could be ontogentic effects here since many of the smaller specimens are juveniles but it stands in contrast with the more general isometry of other pterosaurs, and could be linked to prey sizes or even eye size. If they are, any many people suspect, nocturnal then juveniles need huge heads to house huge eyes.

The holotype of Anurognathus, the first anuroganthid

Finally, there is the issue of the ‘folded’ wings. While some disarticulation can occur in decaying pterosaurs unless the specimen has disintegrated the various bones of the wing finger stay together. Presumably they are held together by numerous strong ligaments or they would not be able to hold up the forces of flight. It’s a very derived condition since of course all other archosaurs (indeed tetrapods generally) can flex their fingers. Anurognathids however, despite having some exquisitely preserved specimens, and nearly all of them being basically articulated, show the joints of the wing finger being flexed. This suggests that they are doing something really rather different with their wings, when flying or even when on the ground. One thing to note is that this is also seen in one other set of pterosaur specimens – embryos. That implies that either anurognathids have inherited this trait from their ancestors (if they are, as some suggest, the first branching group of pterosaurs) or have secondarily acquired what is essentially a paedomorphic trait of wing flexion.

I’ll leave it there for now. There’s plenty more in the paper that you can read and there is obviously more research to come (indeed I’m working on another anurognathid paper that’s come about in part through this work) so don’t want to go over this in detail when it’s already a review. Hopefully this does sort out a few issues and pave the way for a better understanding of these most interesting of pterosaurs.

The paper is currently available online as a preprint but a final formatted version should be out soon: Hone, D.W.E. 2020. A review of the taxonomy and palaeoecology of the Anurognathidae (Reptilia, Pterosauria). Acta Geologica Sinica.

Terrible Lizards, series 2

A few months ago I put up a post to launch a dinosaur-centric podcast called Terrible Lizards. I and my co-presenter, Iszi lawrence, really didn’t know how popular it might be or how much momentum it would get. As such we recorded one series and then crossed our fingers.

Happily, it has been well-received and encouraged we have recorded and are already releasing episodes for the second series. The first couple of episodes are already up and we’ve kicked off with two taxa that feature regularly in the Musings in Velociraptor and Protoceratops.

All of the episodes of both series 1 and 2 are available here. It’s also available on iTunes, Spotify and all kinds of other platforms so it should be easy enough to get hold of it on your favourite website or set-up. New episodes will be coming every Wednesday for the next few weeks and there’s extra stuff available for some of our patreons too.

 

A second specimen of Luchibang?

I was going though a bunch of files this week hunting down some photos of Chinese pterosaurs and came across this one. I took it in a small private museum in Liaoning ten years ago and so didn’t record any details at the time since the material was never likely to be accessible for study and I was only there for an hour or so. There’s also no scale and of course the lighting is less than ideal. My memory of it is sketchy at best, but I remember it being quite a large specimen, though if it has stuck in my mind any further it would have been obvious then (and indeed more recently) what it looked like – it could be a second specimen of Luchibang.

A second specimen of Luchibang?

One thing that was very difficult with naming that taxon was establishing that it was genuine given its unusual mixture of features and proportions. Despite a very extensive section in the supplementary information of the paper on nature of the specimen and extra preparation work to establish that is is genuine, I’ve still seen comments online (including from people who should know better) claiming it might be a composite. I have though also heard of other specimens in China that are long-legged istiodactylids and apparently I’d already seen one but forgotten.

This is clearly an istiodactylid based on the skull, with the classic rounded jaw tip and teeth limited to only the front of the mouth. Like Luchibang and indeed a number of Liaoning istiodactylids, the mandible has rotated and is not in lateral view like the rest of the skull (though here the skull is rather crushed). The neck vertebrae are similarly ornithocheiroid-like and also preserved in dorsal view. The wings and legs though are not like ornithocheiroids, with a wing-finger with distinctly azhdarchoid-like proportions and long hindlimbs with large feet. This would generally be an odd combination, but taking some quick measurements on the photo shows that the broad proportions of the jaw, coracoid, humerus, ulna, wing metacarpal, wing phalanges, femur, tibia and metatarsal are all very similar to those of Luchibang. At the bare minimum that makes this extremely intriguing and without looking further into it, does make this a potential second specimen.

That said, there needs to be caution here. Looking as closely as possible at this less than perfect photo, throws up some oddities. The toes are a rather odd colour compared to the rest of the skeleton (though they look like they might simply not have had lacquer put on them), the humeri look weirdly wide (though could be crushed), and the wrist elements appear to be missing. There’s some kind of odd effect around many of the bones which could be clean up work and some filler, but could also be where bones have been moved around to make things look better, or of course rather worse, have been added in from another specimen. Even so, as with Luchibang, there is very considerable overlap across numerous elements. The mandible overlaps one of the wings, the cervicals overlap with the scapulocoracoid, the proximal wings and femora overlaps with the mass of bones of the torso and other wing and leg parts are in close association with each other.

So while I’d preach caution about this specimen without much better photos (and of course far better still, seeing it in person), it is a credible candidate for a second long-legged istiodactylid. Despite the fact that it looks like it has had work done on it, it would be rather odd indeed that someone had created a composite where they had managed to find an istiodactylid skull with a first wing phalanx of the correct length underneath it and of the right colour and preservation type to match with an unrelated azhdarchid body of the right size and proportions, that happens to have an ornithocheird-like posterior cervicals on it, and where all the different elements are a match in size for a second, unrelated specimen. In short, while some details are a little questionable, it looks like the majority of the elements as presented are all from a single specimen and that’s an azhdarchoid-like winged and legged istiodactylid, and right now that means Luchibang.

More and better presented specimens with proper descriptions are really needed here, but I think on balance this provides reasonable mutual support of both specimens being genuine. The faked Chinese fossils I’ve seen have numerous obvious anatomical issues or the composite parts are of very different preservational quality and type. Even poorly faked and restored specimens are often sold for very large sums and the goal is to produce something extremely aesthetically pleasing, not scientifically plausible, so there’s little motivation to make exceptional and high-quality fakes, especially from specimens like this one where the skull is mashed up. As such, then as reported, there do appear to be more of these istiodactylids out there with the potential to explain a lot more about their unique proportions and ecology and this is hopefully only an indication of more to come from Chinese collections.

 

How to grow your dragon – pterosaur ontogeny

Life reconstructions of Rhamphorhynchus on display in Munich.

The giant pelagic pterosaur Pteranodon is probably the most famous, and is certainly the most iconic, of pterosaurs and specimens and casts of this show up in museums around the world. There’s something like 1100 specimens in public collection and plenty more in private hands. Unfortunately though, almost all of them a squashed very flat and they are often rather distorted and worse, the overwhelming majority are very incomplete and often composed of only a few elements. They are also almost all of a good size (‘subadult’ and up) with only one specimen recognised as being something close to juvenile in age. That means that while this is an amazing number of specimens, it’s also really quite hard to work with as the data is limited in lots of ways.

However, if we turn to Rhamphorhynchus we have only a fraction of the number of specimens but pretty much all the other issues are absent. Most specimens are complete or at least have a very healthy amount of the specimen present, they are often flat but show nothing like the distortion of Pteranodon and there are even fully 3D specimens. They also cover a near order of magnitude in size with everything for animals of c 30 cm wingspan up to nearly 2 metres and include everything from putative hatchling-sized animals to a couple of genuine outliers that are much bigger than other known individuals. Thus despite the relatively low numbers they represent and absolutely fantastic resource for studying various aspects of pterosaur biology.

The numbers of course are not tiny, well over 100 good specimens, and that alone would make them an exceptional sample of most terrestrial Mesozoic archosaurs. The legendary Solnhofen researcher Peter Wellnhofer catalogued over 100 of these in his amazing 1975 monograph on them and this dataset has become an industry standard for pterosaur research ever since. However, we are still discovering more and there are plenty sitting in various collections around the world that nave never entered the literature because, well, there’s already 100 of them out there. But even big samples are improved with the addition of more material and so for the last decade I’ve been scouring collections and databases and hunting down every specimen I can to add it to Peter’s data. That takes us from his total of 108 to 129. The ‘real’ total is actually a little lower since several of his were in private hands and two of mine are casts, though of unique specimens, and not all of these are complete. Even so, it represents a hefty increase in the available data and marks the first major increase in the catalogue in 45 years.

Obviously I’m not going to make a dataset like that and sit on it, so this post inevitably marks the publication of an analysis of growth in Rhamphorhyunchus. In a lot of ways, this mirrors Chris Bennett’s fantastic 1995 paper on this genus where he convincingly demonstrated that all specimens belonged to a single species and not multiple ones as previously thought, and part of his arguments for doing this looked at the relationships between various elements based on Wellhofer’s dataset. Chris’ point was that while there were some discreet clusters of specimens (which he attributed to year classes) most of the alleged differences between the putative species vanished when you put them on a graph and the rest were classic ontogenetic traits like the fusion of the pelvis in large individuals of big eyes in small ones. So while he didn’t really deal with growth as such, he was already showing similar patterns to what I and my coauthors confirm now – Rhamphorhynchus was weirdly isometric in growth.

In other words, in the case of the vast majority of their anatomy, young animals are basically just scaled down adults. This is a weird proposition for a terrestrial vertebrate as most undergo some quite notable and even extreme allometry with some parts proportionally growing and others shrinking as they grow. Think of young animals with big eyes, in big heads and large hands and feet, or antelope with especially spindly legs and so on. But in the pterosaurs even the smallest animals are, aside from the eyes, basically carbon copies of the adults.

One of the less well preserved Rhamphorhynchus out there, it nevertheless has most elements intact

To put this in context we looked at another group of quadrupedal, powered flying vertebrates with bony spars supporting membraneous wings, the bats. Yes, obviously they are not ideal in terms of their ancestry but functionally they are about the best analogue you could get for a pterosaur. Looking at their development we see that juveniles have proportionally very small wings and right around the time they start to fly and become independent, their wings grow rapidly. This is the pattern we would expect, young animals have only so much they can invest in their development and growing wings that are not being used is what we would expect, exactly as things like sheep (and indeed dinosaurs) don’t grow their horns until they reach sexual maturity, they are not being used before then. We do though, see the bats developing their legs early as they need to grip into cave roofs and their mothers so it’s not a case of overall reduced development of limbs, but clearly selective growth.

Birds are functionally poor analogues of pterosaurs but are much closer phylogenetically and are the only other powered flying tetrapod so we also looked at some existing datasets for them too. Most birds, unsurprisingly have allometric growth of various elements, but like bats the legs develop before the wings with one notable exception, those that are hyperprecocial. Some birds like mallee fowl are capable of flying within days, or even hours of having hatched from the egg. These birds have isometric growth and this immediately then suggests that Rhamphorhynchus at least (as has been suggested before) was precocial and flying while young.

This may sound correct since if you are flying when young and flying when adult you probably want to be the same but that’s not the case. As a flying animal in particular, relying on wings to hold you up you have a problem. If you grow isometrically you wings will get longer and wider but your weight will increase much faster since you as a whole will get longer and wider and deeper. So mass will increase much faster than wing area and that can only have a profound impact on how you fly. There are two things that might offset this, first of all different animals can use different flying gaits at different sizes which might mean that performance is not quite as different as might be predicted from this (though we’d still expect juveniles to be more agile) and secondly, changes in pneumaticity. Birds increase penumaticity as they grow and there’s evidence this is the case in other pneumatic clades too and if so for pterosaurs, then the mass increase in adults would also be offset somewhat by a proportionally lower mass in adults for a given volume than juveniles.

Precociousness has been suggested in pterosaurs before based on the evidence for them flying while young, but it has also been challenged. It suggested that to be flying at that size would require a huge amount of effort and this would leave little energy for growth. That’s largely true, but overlooks that there could be post hatching parental parental care. That is normal for archosaurs (including dinosaurs) and we would expect it for pterosaurs. Being precocial in terms of the ability to move does not mean they have to be independent, things like horses have babies that are capable of running within hours of birth but are still suckled for months, and various ducks take their ducklings out to sea soon after hatching. That’s obviously not the quite same thing as the energetics of flight, but it does show that being a good locomotor is not mutually exclusive with parents protecting and feeding their offspring.

So in short, Rhamphorhynchus is perhaps the best pterosaur for large studies about populations and growth and this genius at least grew isometrically, and this may or may not be the same for other pterosaurs. This then may or may not have some big implications for pterosaur taxonomy which is often based on the ratios of various wing elements. But it does imply that young pterosaur could fly, and fly well and that adults and juveniles were probably flying in different ways to each other and that could then have implications for where and how they foraged and what they ate. This is an incremental step in our understanding of this group (and again, much of what we say has been said before but this firms things up nicely) and hopefully opens up the options for further research on them as living animals.

 

The paper is open access and available here:

Hone, D.W.E., Ratcliffe, J.M., Riskin, D.K., Hermanson, J.W. & Reisz, R.R. 2020. Unique near isometric ontogeny in the pterosaur Rhamphorhynchus suggests hatchlings could fly. Lethaia.

Terrible Lizards – a new dinosaur podcast

With a near global lockdown and people stuck at home there’s been a rash of new podcasts forming (or at least a rash of jokes about everyone starting new podcasts while they are stuck at home) and here is the latest (and by extension, greatest) – Terrible Lizards. In my defence, I’m no stranger to podcasts and actually this one had been in the works since January and the lockdown has merely hastened its arrival rather than being its origin.

I’m no stranger to podcasts having been interviewed for loads of them at various times, but I’ve certainly never run one so this is a big step up. It is something I’d been considering for quite some time but there were various barriers to getting it going (not least time and some real expertise) when a chance meeting with an old friend suddenly made everything viable.

At a mutual friend’s Christmas party, I couldn’t help but spot the distinctive figure of Iszi Lawrence who I’d not seen in nearly 15 years so went over to say ‘hello’. Iszi was starting out as both a stand-up comedian and an undergraduate student in Bristol back while I was doing my PhD and we lived in the same block of flats. We got on well and hung out a bit and then I jetted off to Germany and we lost touch (this was before Facebook and other things like that) and as so often happens that was the end of a small friendship.

However, as also so often happens, meeting again it was like no time had passed and we were soon chatting nineteen to the dozen and catching up. She’d continued on the comedy circuit and also now runs and hosts several podcasts and radio shows (as well as writing childrens’ books and doing other stuff – find it all here) and we talked about me doing a guest spot on one of the history ones to talk about the early days of palaeontology and cover people like Mary Anning and Gideon Mantell. This though quickly morphed into doing an actual, proper, new and dedicated dinosaur podcast and so here we are.

There are of course, plenty of natural history podcasts, those on palaeontology generally, dinosaurs specifically and all kinds of others. I don’t think there’s real competition between them since it’s not like people can’t listen to them all, but it does immediately beg the question of what’s different or special about this one. I think the answer there is that we are trying to reach a truly lay audience – this isn’t a podcast that’s aimed at dinosaur geeks and nerds or students and academics, or even children – but one for people who like science but may know little more than the names Tyrannosaurs, Triceratops and Diplodocus.

We try and delve into a different subject in each episode and this is aided, in the best possible way, by Iszi’s ignorance. She can steer me to what needs to be said and explained and given context and of course her wit is there to stop me rambling on about gastralia excessively.  Her experience and expertise also means she generally knows how to host and edit one of these things so against all odds I even end up sounding vaguely professional, it’s quite a marvel. If all of the wasn’t incentive enough, we’ve managed to secure a special guest for each episode so alongside comedian Jo Caufield, Richard Herring and Alice Fraser we have historian Tom Holland, podcaster Dan Schreiber, dino-nerd and cake-maker Ralph Attanasia and legendary biologist Chris Packham to ask me some obscure, odd and downright naughty (Richard Herring, inevitably) questions about dinosaurs.

Obviously readers on here won’t normally fit our key target audience but I’d still hope it would be enjoyable to listen to and you’d learn something from it. There’s so much to talk about and explore and recover that it should be appealing no matter your existing levels of knowledge. Do though please share this to anyone who might want a listen and might enjoy it, reaching out well beyond the dino aficionados is a key part of this and you can make a huge difference with a like and share and tweet and whatever. The first two episodes are up right now here on iTunes and on here website here and we’ll be adding one a week for the next few weeks. This is something of an experiment so if we don’t get a good number of followers and subscribers this may be a short series (so consider that either a warning or a blessed relief).

Do give it a try and do give it a share. First episode? Well it could hardly be anything else, could it?

 

Gharials, dinosaurs, sexual selection, dimorphism, communication and conservation

Male (above) and female (below) gharial skulls. Photo courtesy of Larry Witmer.

So, yes, new paper time and which the concept behind this one was quite simple the outcome (as is so often the case) rather spiralled out into a bunch of other, very interesting aspects. As I noted in the run up to this post, I’ve been working a lot on sexual selection and what it means for dinosaurs in particular and wanted to use gharials as the perfect model for dinosaurs but lacked a dataset on these rare animals. A chance post by Larry Witmer led me to contact him about his dataset but it turned out to be only three animals, not the dozens I’d hoped for.

It was though, enough stimulus to get me hunting and with Jordan Mallon roped in with his interest in testing these ideas we just needed to get enough data. Happily, my former undergrad student Patrick Hennessey wanted to get engaged in some research and had time on his hands, so while I e-mailed every museum curator and croc research I could asking for photos of skulls, he set off to visit every collection in the south of the UK that was accessible. Some months later and we had an incredible set of over 100 specimens. We know of more too from photos that lacked scalebars (we were unuseable) or were in museums where we couldn’t get a response from the curator, or had various bits of skin preserved which concealed key bits of data. (We also found a good few mislabelled specimens of Tomistoma while we were at it). Still, 100 is a massive dataset for this kind of work and especially for such a rare animal and this gave us an excellent platform for our analyses.

Digging into the gharial literature though we soon found other issues. Despite the fame of these animals, their rarity means the literature on them is very small and very little is known in detail or was last written about in detail decades ago. To complicate things further, the two distinctive male traits (a fossa on the snout that correlates with the ghara, and a pair of palatal bullae) have never been truly convincingly shown to be definitively male accoutrements. Happily, an analysis of the data did suggest that the fossa was clearly a male feature and the bullae most likely were too.

Moving onto the central point of the project, analysis of the dataset showed that without pre-existing evidence for a given specimen being male or female, discovering any evidence of dimorphism was very hard, even for a dataset of over 100 animals. Gharials are strongly dimorphic in body size but the overlap between larger females and smaller males across much of the data, and the unknown sex of juveniles (which shown neither fossae nor bullae) makes finding this signal impossible. This matches what Jordan and I have said in a previous paper, and suggests that short of very large datasets and / or very strong dimorphsm (even more than seen here) or very good evidence for the sex of most specimens, it will be hard to find. That means that for the average data set we have for even well-represented species of dinosaurs (well under 100 incomplete specimens, no idea of levels of dimorphism but unlikely to be well above what we see in modern species, and no data on sex) we are not going to get a signal on dimorphism even if it’s there. I’m sure dimorphism is common in dinosaurs but I’m also sure we’re not finding it.

Female (left) and male (right) gharial snouts, the latter showing the expansion of the snout and the narial fossa anterior to the opening that makes the nares. Image courtesy of Larry Witmer.

That is, of course, based on things like body size or where a feature is expressed in both sexes (as, for example, ceratopsian fills appear to be). Presence-absence dimorphism (where one sex has a feature the other does not) should still show up relatively clearly with much smaller sets of data, but we’re not aware of any species that would obviously fit this criterion. The fossil record isn’t giving up numerous horn-less Allosaurus or dome-less Pachycephalosaurus specimens and while there are things like the two Khaan specimens with different tail anatomy, it’s just those two for now rather than a nice dataset of a dozen or so. Well-known taxa like Centrosaurus and Coelophysis are distinctly lacking in obvious dimorphism.

All of this is hopefully interesting and important for understanding sexual selection in the fossil record and as a guide for future research, but this work also threw up some interesting information for the gharials themselves which is worthy of comment. First of all, we were able to show that the fossa on the snout which is the correlate for the ghara is strongly positively allometric. This is no big surprise but it’s good confirmation that this feature is under sexual selection, and conforms with the (limited) evidence that the ghara starts growing around the time that these animals become sexually mature. We also note that it likely serves as an honest signal, since it would generate tremendous drag on the tip of the snout and that’s pretty critical for an animal with a super thin and presumably hydrodynamic set of jaws used to catch fish.

Surprisingly though, the bullae don’t show this pattern. They first appear on skulls around the same time as the fossa suggesting they are also linked to reproduction, but they first appear just before the fossa. We suggest that this is because the ghara while still small, may not need a fossa to hold it onto the skull and so the ghara and bullae may start growing at the same time, but the bullae would appear on the skeleton first. The bullae are also not allometric, so while they are larger in larger males, they are not disproportionately larger. This suggest that while they are an important part of the reproductive biology (and presumably as part of the palatal sinuses, potentially in making noise) it might be there merely to indicate sexual maturity rather than be an actual attractor. Either way, these give us some hints about the reproductive biology of these animals which gives us some hypotheses to test.

One last thing we spotted is that the very largest males are quite disproportionately robust. They have unusually wide skulls (including the normally slender snout) and also have very thick teeth, with animals only 20% smaller having teeth about half as thick. To our knowledge this has not been observed before and quite what this means isn’t certain. We hypothesise that these very large individuals might either have especially strong heads and teeth for fighting each other, or perhaps because they are entering a different niche and are able to exploit much larger prey than others. Either way, this points to an important issue given how endangered gharial populations are.

Very young gharials, yet to display any external features that might indicate their sex.

With animals under strong sexual selection, a few individual males will have a disproportionate amount of the mating opportunities in a population. But those males are also likely very well adapted to the prevailing conditions. They have, essentially, a good combination of genes allowing them to grow so big and maintain such a large ghara. If they are operating in a different niche and that isn’t taken into account (they may be eating much larger fish species compared to other gharial for example) when trying to protect them and conserve their habitats, then they might be especially vulnerable. If your genetically best adapted and fittest individuals are at most risk, that’s potentially very bad news and is unlikely to be good for the long term survival and genetic health of the population. This is of course, potentially rather speculative, but it’s supported by what we understand of strong sexual selection and the observations about the largest male skulls. It’s certainly something that is worth checking out in more detail and at the bare minimum it’s an interesting observation about their ontogeny and what that might mean for our taxonomy in the fossil record.

So here ends a very long process to analyse and assess dimorphism in gharials as a model for dinosaurs. It has thrown up far more complexity and nuance, especially in the living species themselves, than I ever thought but that has been in itself most interesting. It only remains for me to thank my coauthors for their contributions on this paper, and the huge number of curators and researchers who generously checked catalogues and sent in photos for us, the paper really would not exist with them all.

Hone, D.W.E., Mallon, J.C., Hennessey, P., & Witmer, L.M. 2020. Ontogeny of a sexually selected structure in an extant archosaur Gavialis gangeticus (Pseudosuchia: Crocodylia) with implications for sexual dimorphism in dinosaurs. Peer J.

 


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