There is a brand new book on pterosaurs out: Witton’s Pterosaurs: Natural History, Evolution, Anatomy, published by Princeton University Press. Vertebrate palaeontologist friends are telling me it’s a stunning and masterful book, suitable both for the lay public and as a reference work, so if you have any interest at all in pterosaurs, get it. And if you don’t know what pterosaurs are or why you should be interested in them, read on.
Cosimo Alessandro Collini was the first to describe a pterosaur back in 1784, based on specimens from the Late Jurassic of Germany. He described it as some sort of sea creature. In 1801, legendary palaeontologist Georges Cuvier took a look at it and reinterpreted it as a flying reptile, later naming Pterodactylus and setting the stage for the modern study of pterosaurs.
Only three vertebrate lineages have managed to develop the ability to fly: birds, bats, and pterosaurs. While in the early nineteenth century, some biologists tried to shove pterosaurs as strange bats or birds, it soon became clear that the three groups had evolved flight convergently and use different mechanisms and structures to achieve flight. Birds have feathers stuck in the skin of the arm and hand, and they act as the flight surface. The bat forelimbs have four elongated digits acting as supports for a membrane that acts as an aerofoil.
Pterosaurs achieved flight with a flexible and elastic membrane, the cheiropatagium (ch), stretching between the arm and legs, supported only by a single elongated fourth digit, the so-called wing finger (wf). The membrane was also reinforced by fibers inside it (Kellner et al., 2010). The three other digits served as grasping claws. They also had a cheripatagium (cr), a small mebrane in the elg, and a propatagium (pro) in front of the arm, supported by the pteroid bone (pt), the functional morphology of which has been the subject of debate for decades. Similar to birds, pterosaurs had a highly-efficient air sac system for respiration, saving a lot of weight by hollowing out the bones (Claessens et al., 2009). They also generally had very high growth rates (Padian et al., 2004), allowing a juvenile to reach the adult, flight-able morphology quickly – there are even indications that newly-hatched pterosaurs were able to fly (Wang & Zhou, 2004)!
Pterosaurs evolved way back in the Late Triassic, 225 million years ago, tens of millions of years before birds and bats were even close to originating, so they are the earliest flying vertebrates, althought here were some earlier gliding vertebrates (Evans & Haubold, 1987). Of course, the first flying animals, insects, came about a couple hundred million years earlier than that. With a wingspan of 12m, the pterosaur Quetzalcoatlus was the largest flying animal ever, although not all pterosaurs are large: Nemicolopterus was a small bird-sized 25 cm creature (Wang, 2008). Pterosaurs survived for over 155 million years before going extinct at the Cretaceous-Tertiary extinction event.
When they were alive, pterosaurs reached a high diversity both in terms of species and ecologically, being able to live and forage in many different habitats and ecosystems, as indicated by the head diversity illustrated above. As a stark example, Pterodaustro guinazu from the Lower Cretaceous of Argentina is the pterosaurian equivalent of a flamingo, with a specialised mouth for filter-feeding through mud (Codorniú & Chiappe, 2004). Some pterosaurs ate fish, others were eaten by fish (Frey & Tischlinger, 2012). Pterosaurs were diverse in the Early Cretaceous, 145-99 Ma (Prentice et al., 2011), and it was at this time that many of the weird pterosaurs started appearing, e.g. the Tapejaridae, the pterosaurs with a crest.
The study of pterosaurs is one fraught with difficulties due to a significant bias in the fossil record (Butler et al., 2009). The air sac system results in fragile skeletons and hollow bones that really do not fossilise well. Hence, pterosaur fossils come from Konservat-Lagerstätten, those fossil localities preserving exceptional fossils. These places will always be rare treasures, resulting in a very gappy fossil record. A relatively recent listing of pterosaur localities can be found in Barrett et al. (2008).
What this means is that we have very little reliable information on the origin of pterosaurs and their evolution from their archosaurian ancestors – the earliest fossils are, simply, pterosaurs. No “transitional forms” are known, to my knowledge. This means that placing them on a phylogenetic tree is rather difficult, although most authors will agree with the placement above, with the pterosaurs and dinosauromorphs sharing a last common ancestor (Hone & Benton, 2008).
Pterosaurs are roughly divided into two suborders. The monophyletic Pterodactyloidea are the famous large ones from the mid-Jurassic to the end-Cretaceous. They are derived from the Rhamphorynchoidea, a wastebasket taxon where the earlier non-pterodactyloids are dumped pending further research and sorting. They’re smaller, possess a long tail (it got reduced in the pterodactyloids), and lived from the Late Triassic to the early Cretaceous. A fossil that provides some insights into the early evolution of the pterodactyloids from their ancestors was described by Lü et al. (2010). The cool thing about it is that it exhibits mosaic evolution, with a mixture of pterodactyloid and non-pterodactyloid characters, hinting at which features evolved first. Apparently, it was the skull, since it has a pterodactyloid skull but a non-pterodactyloid postcranial skeleton.
Their biology is similarly full of interesting unanswered questions, although any material allows us to investigate it further. The lack of good modern analogues, especially for the giant pterosaurs, makes actualistic testing of hypotheses difficult – it’s identical to the study of dinosaurs. Pterosaurs were the very first fossils to be examined histologically, as I explained here, and that provides a wealth of information. In addition to standard skeletal modeling and reconstruction, trackways and footprints also provide many tidbits.
For example, how pterosaurs moved about on land is a classic controversy, but it’s now mostly resolved. They certainly did move on land, and some could even climb and live on trees (Witton & Habib, 2010). They were most likely quadrupedal, using both the front and hind limbs to walk. This is the most consistent model, as it’s supported both by skeletal analysis and trackway evidence (Wilkinson, 2008). The older counter-hypothesis, that they were bipedal, can be seen in Padian (1983).
Regardless of how they walked around, flight was integral to the pterosaur’s life, as is obvious from its anatomy. So central was it that pterosaurs, unlike birds and bats, invest very little energy in childcare. Female birds are well-known to go on calcium binges in order to make strong eggs. By contrast, the egg of the pterosaur is soft and leathery, as the spectacular fossil above shows (ie is the egg). What this hints at is that the pterosaur was so at the limit of being able to fly that the extra weight from having to lug a growing egg around proves to be too much of a compromise to the pterosaur’s lifestyle. By evolving this parchment-like egg, the baby is underprotected, but at least the mother can still fly around.
Flight in pterosaurs is best simulated nowadays by paragliders: they did not flap around, but they soared on thermals for slow flight. This is what modern aerodynamic studies tell us (Palmer, 2011), and as the authors point out, this is also in line with the anatomy of pterosaurs, with their thin bones not being able to sustain the high impact required of landing from high-speed flight.
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