Pterosaurs are practically unknown from some stages, for example, the Bajocian in the Middle Jurassic, but abundant (relatively speaking) in others, such as the Barremian in the Lower Cretaceous. This unevenness is also encountered in their geographic distribution. As the maps in Figure 2.2 show, pterosaur body fossils have now been found on every continent, even Antarctica, but the vast majority have emerged from sites in Europe and North America, mainly because that is where most of the effort to collect them has been made so far.
As we saw earlier, most of the pterosaur fossil record consists of petrified bones and teeth. Virtually all of the components of the skeleton are fully known, many of them in fine detail, thanks to several beautifully preserved complete, uncrushed fossil pterosaurs from the Santana Formation, in which even the subtlest features, such as muscle scars and openings for the passage of nerves and blood vessels, can be identified. How skeletal anatomy varies among pterosaurs is also quite well-understood, because skulls and a substantial part of the rest of the skeleton are known for approximately half of the more than 100 species found so far.
The fossil record of pterosaur soft parts is largely confined to relatively tough materials, such as the skin and its various derivatives. Thus, apart from the skin itself, we also have evidence of several structures that were constructed from modified skin— the horny covering of the beak, throat sacs, skull crests, claw sheathes, tail flaps, webs of skin between the toes and wing membranes. Other soft parts, such as stiffening rings in the wind pipe and what might be sections of the gut and patches of muscle, are exceptionally rare, and their exact identity is often disputed. An easily overlooked but important aspect of fossilized soft parts is that they have been found in several different pterosaurs that, collectively, represent most of the main evolutionary lines. This means that not only have we gained some information about pterosaurs' soft parts but, occasionally, it is even possible to see how these structures, such as the wing membranes, varied.
The pterosaur fossil record is not extensive, especially compared with that of most other groups of backboned animals, but with the steady improvement in our knowledge of where to find the fossils and how to prepare them more accurately, it is slowly but surely getting bigger and better. Although scattered in collections all over the world, the fossils that we already have are sufficient for us to be able to begin to establish some fundamental aspects of pterosaur biology. The trick to this has been not to rely on single "Rosetta Stone" specimens, but to combine knowledge from as many specimens and as many different types of fossils— skeletons, soft tissues and tracks— as possible. Thanks to a few pterosaurs who gazed upon Medusa, we now have fossils that reveal how these animals were constructed, how they flew, how they moved on the ground, even how they managed to evolve into flying giants. But, before we begin that part of the story, we first need to meet some of the principal actors.
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