|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...?|
*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.
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).|
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).|
|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...|
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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- 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.