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Friday, 25 March 2016

The lives and times of flying reptiles as told by the fossil record, part 1: azhdarchid pterosaurs

The undignified end to the life of a large (6m wingspan) azhdarchid: being squabbled over by Saurornitholestes and other local ruffians. We know a scene like this must have occurred at least once thanks to an azhdarchid fossil discussed below, and a growing number of specimens are providing similar insights into the place of azhdarchids in Mesozoic food webs.

Discussions of the lifestyles of fossil animals primarily centre around their functional morphology - the use of comparative anatomy and biomechanical calculations to assess their anatomical suitability to certain habits and behaviours. We use these approaches to deduce their likely diets, locomotory strategies, likely interactions with ancient environments and other organisms, and so on. But despite the widespread and proven utility of these techniques, it's widely regarded that these approaches can only provide probabilities and hypotheses about fossil habits. They allow us to predict, but not necessarily know how ancient animals functioned in their respective ecosystems. For the latter, we rely on fossils to provide us with direct records of animal habits - gut content, evidence of animals attacking and ingesting each other, trace evidence of foraging strategies and so on. For vertebrates, these fossils are very rare - hence our general reliance on functional anatomy - and single examples of palaeoecologically-significant fossils can provide crucial insights into ancient lifestyles and ecosystems.

As you might expect, such fossils are particularly rare for pterosaurs. Flying reptiles were not prone to fossilisation at the best of times and this - through simple probability - dramatically lowers their chance of forming fossils with palaeoecological significance. As recently as the 1990s some authors lamented the lack of these fossils and tried to ascribe some significance to their rarity. But renewed interest in pterosaur science, an increased rate of discovery of flying reptile remains and greater alertness for details on fossil specimens has seen our sample size of palaeoecologically-significant pterosaur specimens grow considerably in the last few years. These records have been augmented more in some taxa than others so that our record of pterosaur palaeoecology is looking increasingly patchy: some groups now now have a half-dozen or more of these fossils to their names, others have none at all. There's probably no real significance to this other than the habits and anatomy of some pterosaurs being more suited to recording these fossils than others.

I thought it might be fun to take a look at two pterosaur taxa with increasingly good palaeoecological records. In an upcoming post, we'll look at the really quite excellent palaeoecological data for the Jurassic pterosaur Rhamphorhynchus muensteri, but today I want to discuss the palaeoecology of azhdarchids. That's right: the lifestyles of these sometimes giant, sometimes long necked, always edentulous, and increasingly famous flying reptiles are not only predicted by functional morphology, but can be deduced somewhat from fossils too. Thanks to a number of specimens discovered in recent decades we have an idea what happened to azhdarchids when they died, what sort of animals they must have encountered in day to day life, and maybe even how they foraged.

Meeting the (teeth of the) neighbours

The majority of the azhdarchid palaeoecological record pertains to specimens showing which animals ate them, their teeth and puncture marks being left on azhdarchid bones, or else azhdarchid remnants being found in the torsos of other animals as gut content. One of the most famous of these fossils is an incomplete azhdarchid skeleton from the upper Cretaceous Dinosaur Park Formation of Canada. This fossil is noteworthy for not only being one of the most substantial pterosaur fossils from Canada, but also for the tooth gouges and embedded dinosaur tooth found at the distal end of the tibia (Currie and Jacobsen 1995). The tooth belongs to the 2 m long dromaeosaur Saurornitholestes langstoni, and the size and provenance of the gouges indicate they were probably made by the same species, perhaps the same individual. The size discrepancy between the pterosaur and dinosaur here is pretty dramatic. Check out the size of the tibia of the (c. 6 m wingspan) pterosaur compared to the embedded tooth:

Immature azhdarchid tibia from the Dinosaur Park Formation of Alberta, Canada, with an embedded dromaeosaur tooth (arrowed). Note the size of the pterosaur remains compared to those of the dinosaur. Image by Liz Martin-Silverstone, borrowed from her Twitter account.
This is not the only known occasion of a dromaeosaur interacting with an azhdarchid. Recently Dave Hone and colleagues (2012) described an articulated Velociraptor mongoliensis specimen from Tugrikin Shireh (Gobi Desert, Mongolia) with fragmentary remains of an azhdarchid pterosaur in its gut (below). That this reflects true gut content, and not just chance association of pterosaur bones with those of a dinosaur, is indicated by the articulation of the specimen and location of the pterosaur remains within an envelope of dorsal ribs. It's presumed that the dinosaur was eating bits of pterosaur bone with meat attached, not just swallowing chunks of bone.

A Velociraptor mongoliensis chest cavity with pterosaur remains, probably belonging to an azhdarchid, as gut content. Black arrows show pterosaur bone, white arrow indicates a pathological rib. Borrowed from Dave Hone's Archosaur Musings.
Crocodyliforms are also thought to have consumed azhdarchids. Possible puncture wounds from these reptiles are found on the holotype specimen of the Eurazhdarcho langendorfensis, from the upper Cretaceous Sebeş Formation of Transylvania (Vremir et al. 2013). Round bite marks and crushing are found on the cervical vertebrae (below) and distal metacarpal IV of this specimen. In this instance the azhdarchid was unlikely to have been receiving selective treatment by the local reptile fauna, as crocodyliform biting traces are common to many bones from this formation (Vremir et al. 2013).

Holotype cervical vertebrae III and IV of Eurazhdarcho langendorfensis, with possible crocodyliform tooth marks outlined and arrowed. From Vremir et al. (2013).

Azhdarchid bones as an insect hotel?

A further possible record of azhdarchid consumption pertains to strange oval punctures at the back of a Quetzalcoatlus sp. skull (Kellner and Langston 1996). These may record tooth marks, although there is no obvious indication as to what animal might have made them. A rather different take on them was suggested by Kellner et al. (2010), who noted that insect borings had been found the same Quetzalcoatlus material. Kellner et al. suggest that these borings were described by Kellner and Langston (1996) but, to my knowledge, this suggestion is an error as I can find no mention of insect borings in the 1996 paper. I assume this comment provides a different interpretation of the skull punctures, but it might not. I'd really like to know more about this, as there are as yet no confirmed cases of pterosaur bones being utilised by insects. I wonder if this is a simple mistake, or might pertain to details of as-yet undescribed portions of Quetzalcoatlus anatomy?

An azhdarchid foraging trace?

Pterosaur foraging traces are known from a number of Jurassic and Cretaceous tracksites (Lockley and Wright 2003). Most look very similar to those known for birds - paired impressions made in the substrate surfaces by beaks pecking at the ground. Interested parties need only check out the mudflats frequented by wading birds to see analogous traces left by modern fliers. Sometimes we find longer scrapes or gouges made by pterosaur beaks sweeping low across sediments, too. These are sometimes interpreted as tail drags, but it's difficult to envisage sensible scenarios where short-tailed pterodactyloid pterosaurs - those thought to have been creating the pterosaur tracks we know of - randomly sweep their tails across the ground.

Such a beak scrape might be known preserved alongside a possible azhdarchid track from the Late Cretaceous (Campanian) Cerro del Pueblo Formation of North Mexico (Rodriguez-de la Rosa 2003). This track comprises one well-preserved footprint and a series of handprints, alongside several sharp, linear gouges. Other tracks from this locality - known as the El Pelillal tracksite - include turtles, crocodylomorphs, theropods and mammal-like creatures, and the sedimentary setting is considered a shallow, freshwater environment. When described, these pterosaur prints were considered to represent the generic pterodactyloid trace Pteraichnus (Rodriguez-de la Rosa 2003), but the footprint bears little similarity to the broad, triangular-shaped impressions of this ichnotaxon (below). In my 2013 book I argued that the narrow form, pronounced heel impression and short, blunt toes of this print much more reminiscent of Haenamichnus, a trace thought with good reason to represent the footfalls of azhdarchids (below, Hwang et al. 2002; Witton 2013). The age of the specimen is further indication of an azhdarchid identity, the Campanian being a stage of the Mesozoic where azhdarchids seem to largely dominate pterosaur evolution.
The El Pelillal pterosaur trace described by Rodriguez-de la Rosa (2003), argued here and elsewhere to represent an azhdarchid trace (Haenamichnus) rather than a generic pterosaur track (Pteraichnus) - see for yourself in the inset. Illustrations after Rodriguez-de la Rosa 2003 and Hwang et al. 2002.
If this is an azhdarchid track, could the sediment gouges represent marks made by the same animal? The original 2003 description provides little commentary little on these marks, but, to be fair, we only really started discussing pterosaur foraging traces in the early 2000s and prospects of terrestrial foraging were considered poor to non-existent by most workers at that time. Perhaps the concept of pterosaur foraging traces were just not on the radar for many researchers in 2003. In any case, these gauges do look similar to marks thought to record sweeping pterosaur beaks at other track sites (Lockley and Wright 2003), so the El Palillal gouges might indeed represent similar phenomena. If this is the case, this specimen might have some important implications for how we interpret azhdarchid pterosaur habits. Speaking of which...

What these fossils might indicate about azhdarchid pterosaur palaeobiology

What we have here is the start of a fossil dataset on azhdarchid palaeoecology, comprising several indications of which animals ate them, possible indications of animals using their remains for shelter, and possible trace evidence of a foraging azhdarchid. The data is currently of a small sample size - only five specimens in total - but already offers several points of interest those wanting to understand azhdarchid habits. 

Firstly, these occurrences show azhdarchids interacting with animals known to have existed in inland habitats. Several of the involved animals are entirely terrestrial, and the most aquatically adapted species yet known to have accosted an azhdarchid bone is a crocodyliform. Given that the latter group is a primarily freshwater lineage (and, indeed, the crocodyliforms in question lived in a freshwater deposit), this data is not inconsistent with this model. As regular readers may appreciate, an 'inland' palaeoecological signature is consistent with the 'terrestrial stalker' mode of azhdarchid life proposed in by myself and Darren Naish in 2008 (see below, also Witton and Naish 2008, 2015), where we argued that azhdarchids were not aquatic or marine adapted species, but much more at home in woodlands and plains, picking up small game with their oversize beaks. I see these palaeoecologically-relevant fossils as a test of this idea, and am happy to see that - so far - they are consistent with the model we proposed. 

Azhdarchid terrestrial stalking, the infographic. From Witton and Naish 2015.
Secondly, if we do indeed have an azhdarchid foraging trace, proposals that azhdarchids can reach the ground with their jaw tips to feed are vindicated. Darren and I initially encountered some resistance to our 2008 'terrestrial stalker' proposal because some peers thought the azhdarchid neck would not permit the jaws to reach the ground. We argued that the jaw is so long that only minimal neck motion, or even just a bit of forelimb flexion, would see the jaws reaching the ground without problem. Azhdarchid beak scrapes would indicate that this was indeed the case and put this minor debate to rest. 

Thirdly, the terrestrial stalker model might predict the presence of beak scrapes alongside azhdarchid footprints. After all, azhdarchids looking for or striking at prey may sometimes have held their jaw tips close to the ground - modern animals track prey in this fashion, after all. The potential existence of beak marks with the El Pallilal track may be 'smoking gun' evidence of azhdarchids foraging on the ground in the manner proposed in 2008. I stress that the El Pallilal tracks need re-examination to confirm these ideas, but, if I'm correct, these traces augment the 'terrestrial stalker' argument considerably.

One question I expect will be asked about these fossils will be whether they tell us much about azhdarchid susceptibility to predation. This is something which has been discussed at a technical level (see this blog post for details), but I'm not sure these specimens help us understand it further. My issue is that, although various ideas have been published on the likelihood of certain azhdarchid fossils representing scavenging or predation (mainly involving relative masses of the animals involved), I'm not sure we can account for the many factors behind the circumstances recorded in these remains. Those azhdarchids outlined above were certainly dead, or near death, when being chewed on as their bones show no signs of healing from the inflicted damage, but a number of scenarios could account for this. We know that modern predatory events are influenced by the health of the animals involved, the skills and behaviour of different predatory species and individuals, and circumstances of time and place. These are difficult to account for with those bitten and chewed azhdarchid fossils we currently have, so I prefer to have no opinion on these matters and focus on the significance of things we can say more positively.

OK, that's all for now. In the next, and concluding part of this series we'll see what the fossil record tells us about the rhamphorhynchiest pterosaur of all, Rhamphorhynchus muensteri

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References

  • Currie, P. J., & Jacobsen, A. R. (1995). An azhdarchid pterosaur eaten by a velociraptorine theropod. Canadian Journal of Earth Sciences, 32(7), 922-925.
  • Hone, D., Tsuihiji, T., Watabe, M., & Tsogtbaatr, K. (2012). Pterosaurs as a food source for small dromaeosaurs. Palaeogeography, Palaeoclimatology, Palaeoecology, 331, 27-30.
  • Hwang, K. G., Huh, M., Lockley, M. G., Unwin, D. M., & Wright, J. L. (2002). New pterosaur tracks (Pteraichnidae) from the Late Cretaceous Uhangri Formation, southwestern Korea. Geological Magazine, 139(04), 421-435.
  • Kellner, A. W., & Langston Jr, W. (1996). Cranial remains of Quetzalcoatlus (Pterosauria, Azhdarchidae) from Late Cretaceous sediments of Big Bend National Park, Texas. Journal of Vertebrate Paleontology, 16(2), 222-231.
  • Kellner, A. W., Rich, T. H., Costa, F. R., Vickers-Rich, P., Kear, B. P., Walters, M., & Kool, L. (2010). New isolated pterodactyloid bones from the Albian Toolebuc Formation (western Queensland, Australia) with comments on the Australian pterosaur fauna. Alcheringa, 34(3), 219-230.
  • Lockley, M. G., & Wright, J. L. (2003). Pterosaur swim tracks and other ichnological evidence of behaviour and ecology. Geological Society, London, Special Publications, 217(1), 297-313.
  • Rodriguez-de la Rosa, R. A. (2003). Pterosaur tracks from the latest Campanian Cerro del Pueblo Formation of southeastern Coahuila, Mexico. Geological Society, London, Special Publications, 217(1), 275-282.
  • Vremir, M., Kellner, A. W., Naish, D., & Dyke, G. J. (2013). A new azhdarchid pterosaur from the Late Cretaceous of the Transylvanian Basin, Romania: implications for azhdarchid diversity and distribution. PLoS One, 8(1), e54268.
  • Witton, M. P. (2013). Pterosaurs: natural history, evolution, anatomy. Princeton University Press.
  • Witton, M. P., & Naish, D. (2008). A reappraisal of azhdarchid pterosaur functional morphology and paleoecology. PLoS One, 3(5), e2271.
  • Witton, M. P., & Naish, D. (2015). Azhdarchid pterosaurs: water-trawling pelican mimics or “terrestrial stalkers”?. Acta Palaeontologica Polonica, 60(3), 651-660.

5 comments:

  1. I suspect that pterosaurs (or at the very least azhdarchids) were far less predator-prone as is often suggested. A contemporaneous tyrannosaurid might be far heavier than a Quetzalcoatlus sized azhdarchid, but the pterosaur has a distinct height advantage and presumably be quite an intimidating animal! I'm sure the beak could deliver some incredibly nasty jabs and the forelimbs could pack a similarly nasty punch in a pinch. There's a video on YouTube of a crane fending off a hawk with nothing more than some aggressive posturing and feinting pecks. Interactions between a theropod predator and an azhdarchid probably weren't much different.

    On that note, I wonder why the public had run away with the idea that dromaeosaurids must've hunted and killed big azhdarchids. I know that humerus is the catalyst, but I feel like the entire concept of it being the result of predation is downright goofy.

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    1. I dare say what you would see with pterosaurs that were not newly-hatched juveniles would be behaviour similar to modern corvids: before landing at any food source, the animals would have a look around the area for any possible danger, and would take reasonable care not to go into areas where they couldn't take off fairly quickly.

      This also ties into how most predators operate. Predators can only feed if they themselves are in tip-top condition. They cannot afford to be carrying injuries, as quite minor injuries can be eventually fatal. For this reason, predators never look for fair prey but tend to stick to preying on animals that are young, injured or rather old, or which are much smaller than themselves.

      A large azhdarchid therefore wouldn't land next to anything big enough to cause it trouble, wouldn't stand around on the ground if anything anywhere near its own size hove into view, and would likely give a very good account for its self if confronted by any mid-range predator. In such a situation, the pterosaur doesn't actually have to win the confrontation at all, all it has to manage is to make the other animal back off for long enough for the pterosaur to take flight. Once flying, that's the last any predator is going to see of that pterosaur.

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    2. I definitely agree that corvids are a good analogue, excluding the rare circumstances that the pterosaur can't actually get off the ground for some reason or another. That's what I had in mind with the above comment. Any pterosaur that picks fights with dinosaurs its own size is likely a very stupid one!

      One topic that stands out in my mind is where azhardchids (and other pterosaurs capable of dismembering or swallowing reasonably sized animals) fall in terms of scavenging hiearchy. A big azharchid could presumably bully small-mid sized dromaeosaurs away from their kills so long as it was something they could swallow. Of course, I doubt any pterosaurs were regular kleptoparasites.

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  2. Great article Mark! As a quick note, there is a typo in one of the captions - the Dinosaur Park element pictured is the tibia.

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