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Monday, 14 March 2016

The magnificent Caviramus, an early example of an anatomically 'extreme' pterosaur

The Carnian/Norian Swiss pterosaur Caviramus schesaplanensis, one of the earliest species to take pterosaur anatomy to strange new places. Anyone else want to make puns about 'Cave-iramus' with this picture? No? Anyone...?
What happens when you take the innate weirdness of the Triassic Period - the evolutionary equivalent of the late 1960s in terms of experimentation, weirdness and tragic ends to interesting lineages - and multiply it by a pterosaur? One answer is the marvellously strange Late Triassic flying reptile Caviramus schesaplanensis. This animal is a relative newcomer to the pterosaur roster, first being described in 2006 based on an incomplete lower jaw from Switzerland (Fröbisch and Fröbisch 2006), before better material in the form of an incomplete skeleton turned up a few years later* (Stecher 2008). Several cranial and dental features indicate Caviramus had kinship with other European pterosaurs such as Eudimorphodon and Campylognathoides, but that it was a rather distinctive animal compared to even close relatives. This 1.35 m wingspan animal had a chunky skull, large crests on both upper and lower jaws, densely packed and gnarly teeth, long, slender wings and a robust, lengthy set of hindlimbs (Stecher 2008). Several of these features are 'extreme' variants of pterosaur anatomy common to other Triassic animals, and others represent the most pronounced development of anatomical traits of any pterosaur. They mean that, even a decade after the first remains of this animal were published, Caviramus still gives us a lot to think about as goes its life appearance, lifestyle and functional anatomy.

*This skeleton was described as the holotype of a new genus and species, Raeticodactylus filisurensis, but a number of recent workers have shown it to be very likely congeneric, if not entirely synonymous, with C. schesaplanensis. I'm treating the two as the same taxon here.


Fossil material referred to Caviramus. Above, the holotype jaw of Caviramus schesaplanensis, below, the incomplete holotype skeleton of "Raeticodactylus filisurensis", a taxon now considered by most to be Caviramus and perhaps even Caviramus schesaplanensis itself. From Fröbisch and Fröbisch (2006) and Stecher (2008).
One atypical aspect of Caviramus is the cranial crest. Sure, pterosaur headcrests are really not uncommon (it's perhaps fair to say seems crestless species are more unusual than crested ones) but they remain fairly rare in the Triassic and, even compared to much later pterosaurs, Caviramus is pretty well endowed in the crest department. Rather than the low midline ridge typical of many pterosaur bony crests, this structure projects rudely from the front of the snout to reach well above the rest of the skull. We don't see anything like this again in the pterosaur record until the lower Cretaceous, when the famously elaborate tapejarids adopted a similar configuration. As with these pterosaurs, it's likely a soft-tissue component extended the Caviramus crest tissues in some way. The posterior crest border of the only known Caviramus skull is badly preserved, but the lateral crest surfaces have the same fibrous textures as pterosaurs known to have large soft-tissue crest components, such as Pterodactylus and Tupandactylus. The most parsimonious interpretation of this is that Caviramus had a big soft-tissue crest too, and - if the bony crest portion is indicative of the soft-tissue extent, as seems apparent from some pterosaur fossils - it might have been quite a spectacularly adorned animal. Caviramus seems to represent one of the first experiments with this sort of outlandish headgear, there being only one other Triassic species which could rival it for crest development (Austriadactylus cristatus - see Dalla Vecchia et al. 2002) .

Multiple aspects of the Caviramus jaw are of interest. It was probably a powerful biter, and perhaps regularly consumed relatively tough prey such as invertebrates with thick exoskeletons or fish with hard scales. Such a diet is indicated by its blunted and worn tooth tips, and the enamel of the anterior teeth being strongly rugose - some readers may recall from a recent article that these features also occur in other specialists of hard prey, such as the giant, turtle-eating Cretaceous crocodylian Deinosuchus. Caviramus dentition is morphologically complicated and, again, indicates some specialisation. As with many Triassic pterosaurs, the teeth are differentiated into large, curving anterior fangs at the jaw tips and complicated, multicusped teeth behind these. These posterior teeth are so numerous and tightly packed that they actually sit obliquely in the jaw, overlapping one another to form a continuous, 'megaserrated' cutting surface. The depression of the jaw joint (another atypical feature for a pterosaur, and one that won't reappear until later in pterosaur evolution) permitted these teeth to occlude simultaneously rather than gradually, as occurs in animals with jaw joints level with the toothrow. Areas of Caviramus jaw muscle attachment are large, including a broadly expanded posterior lower jaw. The mandible and skull are not, as with some pterosaurs, delicately built from slender struts but comprised of deep bars and robust bone junctions. Cross sections of the Caviramus holotype jaw indicate that some cavities were present in the cranial skeleton, but that bone volumes were superior in at least some places. This was clearly an skull capable of delivering and withstanding forceful bites, and its configuration recalls some dinosaur species which are sometimes considered to be omnivorous (e.g. many small ornithischians). Maybe it was equipped with powerful jaws so that it could tackle a wide range of tough foods, including nutritious plant matter.

Cross section through the posterior (specifically, coronoid) section of the Caviramus holotype jaw. Grey shading represents bone, white indicates hollow regions. From Fröbisch and Fröbisch (2006).
This strong skull and dental apparatus seems odd compared to the Caviramus humerus. This bone is about as long as expected for a pterosaur of this kind, but is distinctive for being very, very slender. Usually, pterosaur humeri are the most robust elements in the limb skeleton, but that of Caviramus is no wider than the more distal wing elements. So proportionally different is this bone that the shoulder and upper arm were probably much more slender in life than those of other pterosaurs. Quite what this means for Caviramus locomotion has not been looked into yet, and any attempt to assess this will be frustrated by the only known Caviramus humerus being somewhat imperfectly preserved. Still, it's known in enough detail to at least permit some basic comments.

One obvious question concerns what this humerus means for quadrupedal launch potential in this animal. A core basis to this hypothesis is that pterosaur humeri are much stronger than their femora (Habib 2008) but - going on a basic assessment of bone shape here - this is not obviously the case for Caviramus. We should not automatically default to assuming Caviramus was a bipedal launcher however, as it is small enough to not need atypically strengthened limb elements for launch. The limb bones of volant animals are expected to start showing strong signals of a launch strategy once their body mass hits 2 kg (i.e. it's above this mass where the humerus or femur strength starts to become disproportionately strong compared to the other limb elements - see Habib 2008 for details) but, at only 1.35 metres across the wings, Caviramus probably only massed a little over one kilo. I'm sure Caviramus did have a preference for a particular launch strategy (I'm not aware of any animals which can readily flip between quadrupedal and bipedal launch, except under special circumstances), but its size means we might need dedicated investigation to know which was more likely. Given that all other pterosaurs seem to be quad-launchers, my suggestion is to assume this as the null hypothesis for now until we have reason to assume otherwise.

Caviramus schesaplanensis skeletal reconstruction, somewhat updated from the original version in Witton (2013). Unknown elements based on Campylognathoides liassicus (see Padian 2008).
The slenderness of the Caviramus humerus might have impacted flight once airborne, too. Caviramus joins pterosaurs like Eudimorphodon and Campylognathoides in having very long wings, but lacks the stocky, probably powerfully muscled humeri of these species. These might have enabled Eudimorphodon et al. to be forceful fliers capable of rapid flapping, elevated speeds and high agility. But the delicately constructed humerus supporting a long distal wing in Caviramus might have curbed any potential for being a powerful flapper or aerial acrobat - its humerus would have been far more vulnerable to bending than those of other pterosaurs. This is not to say that it was purely restricted to gliding, however. Studies of avian wing construction show that their humeri do not have to be enormously strong to permit flapping flight (indeed, their humeral strength seems to scale more or less isometrically with body size - Witton and Habib 2010) and the fact Caviramus has a large deltopectoral crest to anchor flight musculature is a good indication it was an active flier. We might conclude that its long, slender wing bones were suited to soaring flight with limited flapping - long winged seabirds like gulls, albatross and terns might be a good modern flight analogue.

Readers familiar with the Caviramus illustration in my 2013 book Pterosaurs: Natural History, Evolution, Anatomy might note that the 2016 Caviramus (above) is rather differently posed. There's good reason for this - read on...
It's not only flight that this unusual humerus might have impacted: it might have imposed some restrictions for life on the ground. I don't think we should assume it was so slender that it was incapable of supporting the animal - again, we're not dealing with an enormously heavy species here - but its slenderness might have impacted how the limb functioned when grounded. Specifically, the apparent absence of an expanded elbow region suggests it had sprawling forelimbs. As noted above, the Caviramus humerus is a little imperfectly preserved in places, the distal end being the poorest bit, but the proximal ulna confirms that the elbow joint was not broad. Slender elbows have been interpreted as a signature of sprawling forelimbs in some pterosaurs, for two reasons (Witton 2015, also see this blog post). Firstly, they suggest that musculature operating the wrist was fairly reduced (remember that wrist action is controlled by muscles anchored around the elbow - see Fujiwara and Hutchinson 2012). This reflects both the stresses encountered when standing in a sprawling pose and practicalities of terrestrial locomotion. Walking animals need to clear their feet or hands from the ground when moving, and animals with erect limbs have to do this by collapsing limb joints to reduce the effective length of the limb. Sprawling animals can use motion of the upper limb bone (humerus or femur) to elevate the entire limb, and can therefore take steps without needing to collapse the distal joints. Secondly, slender pterosaur elbows seem to correlate with shoulder joints that prevent depression of the humerus below the horizontal, a bony stop at the base of the shoulder precluding adoption of erect forelimb poses in these species. Given what we see in other pterosaurs, then, we might assume Caviramus elbow morphology indicates it had sprawling forelimbs, although we really need better fossil material to verify this. As in all other pterosaurs, details of the femoral morphology indicate that the hindlimbs were likely held erect. The legs are long enough that even with a highly crouched forelimb the animal still looks very 'leggy', and, in spite of its sprawled forelimbs, it might have been a fast, sprightly terrestrial animal. I imagine long-legged, skinny-limbed Caviramus scuttling about the place might give some people the creeps if it were alive today - if there was ever a pterosaur that might indirectly trigger arachnophobia, it's this one.

Collectively, these points suggest Caviramus represents one of the oldest deviations from what might be considered a 'standard' pterosaur bauplan and perhaps one of the first developments of anatomical 'extremes' in the group, at least as goes skull and wing anatomy. What makes this remarkable is that Caviramus lived so soon after the pterosaurs evolved in the first place - it seems to have wasted no time in pushing the pterosaur skeleton to weird new places. Unfortunately, it's currently difficult to say how successful these experiments were. The Triassic pterosaur record is extremely poor, particularly outside of Europe, and it is difficult to provide any meaningful evaluation of the abundance or longevity of lineages from this period. In a broad sense, however, it might be significant that we don't find Caviramus-like humeri or jaws in the better understood pterosaur faunas of the Jurassic or Cretaceous. Maybe Caviramus represents a configuration that was unsuited to life beyond conditions of the Triassic or, alternatively, perhaps the more 'typical' anatomies of other pterosaurs were just more adaptable in the long run. Whatever the reality here, Caviramus is a good example of how diverse and adaptable pterosaur anatomy can be and how much we have to learn about the early history of this group.

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References

  • Dalla Vecchia, F. M., Wild, R., Hopf, H., & Reitner, J. (2002). A crested rhamphorhynchoid pterosaur from the Late Triassic of Austria. Journal of Vertebrate Paleontology, 22(1), 196-199 .
  • Fröbisch, N. B., & Fröbisch, J. (2006). A new basal pterosaur genus from the Upper Triassic of the Northern Calcareous Alps of Switzerland. Palaeontology, 49(5), 1081-1090.
  • Fujiwara, S. I., & Hutchinson, J. R. (2012). Elbow joint adductor moment arm as an indicator of forelimb posture in extinct quadrupedal tetrapods. Proceedings of the Royal Society of London B: Biological Sciences, 279(1738), 2561-2570.
  • Padian, K. (2008). The Early Jurassic pterosaur Campylognathoides Strand, 1928. Special papers in Palaeontology, 80, 65-107.
  • Stecher, R. (2008). A new Triassic pterosaur from Switzerland (Central Austroalpine, Grisons), Raeticodactylus filisurensis gen. et sp. nov. Swiss Journal of Geosciences, 101(1), 185-201.
  • Witton, M. P. (2013). Pterosaurs: natural history, evolution, anatomy. Princeton University Press.
  • Witton, M. P. (2015). Were early pterosaurs inept terrestrial locomotors?. PeerJ, 3, e1018.
  • Witton, M. P., & Habib, M. B. (2010). On the size and flight diversity of giant pterosaurs, the use of birds as pterosaur analogues and comments on pterosaur flightlessness. PloS one, 5(11), e13982.

6 comments:

  1. Given the find site's position on the coast of the late-Triassic Tethys Ocean, it seems reasonable to assume that the 'hard target' jaws could be aimed at crustaceans and bivalves. Are there suitably sized terrestrial hard-shell prey items that we know of in the region?

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  2. Omnivorous, huge crested and possibly fairly competent in the ground. If not for the soaring flight speciation it'd be a good tapejarid-mimic (or ar tapejarids Caviramus mimics? Food for thought)

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    1. The one problem I have with it being competent on the ground is that it still had to be a very light animal or that humerus would be extremely limiting. That being said, eagles that size regularly take on heavier animals on the ground, and that jaw looks fairly powerful. I'm going with either a diet of seafood or terrestrial animals that are large enough to put up a fight.

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    3. Looking back at the jaw, I'm imagining this thing in a predatory role. Maybe similar to a bobcat that flies or something.

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  3. I'd also point out that having very long slender wings like that would make this an excellent glider. Ultra-high aspect wings that don't flap very hard or pull Gees are very good for conserving energy in the air, they're almost reminiscent of a sailplane glider.

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