Saturday, 5 September 2015

Dawn of Dragons: The life and ecology of early Pterosaurs

Pterosaurs are a very well known group, both fossil-wise and through popculture. While Pteranodon is by far the most well known, Pterosaurs are actually quite diversely represented in popculture, as genera such as Dimorphodon, Rhamphorhynchus, and Quetzalcoatlus are also quite common sights. During most of the Mesozoic, these flying beasts were the rulers of the skies, with global distributions and an incredible diversity of species. But we are not going to talk about the later members of the group today, at least not most of them. This post is about the earliest Pterosaurs, how they lived, and where they came from, for while the media and documentaries tend to focus on the most impressive beasts, the early members of this group were just as interesting in their own right, and offer us an unique look into the evolution of flight.

Hypothetical proto-pterosaur, by Mark Witton

The first and most obvious thing that needs to be discussed when talking about Pterosaur evolution is what exactly they are, or rather, where they belong in relation to other animal groups. When first discovered, nobody was really sure what to make of them, and thus they were often depicted as Mammal-like, similar in appearance to Bats. This was in part to explain how these clearly "warm-blooded" animals could have existed, as at the time it was still believed that all Reptiles were sluggish and incapable of fast and efficient movement. Since then, as we learned more about these animals, it became apparent that they were indeed "Reptilian", more closely related to Squamates and Crocodiles than Bats or any other mammals. This view has only been strengthened since then, and for the last several decades there has been complete consensus that they are indeed Diapsids. The question often raised is where in the Diapsid tree they belong. In the past they have been variously proposed to be descended from basal-Archosauriforms, Protorosaurs, or Ornithodirans. Today the latter view is the most commonly held, and they are now regarded as a sister-group to Dinosauria, making them stem-Birds. 

It must also be mentioned that some, or rather, one person, has argued that Pterosaurs are not in fact Ornithodirans. Neither are they Archosaurs. In his opinion, Pterosaurs are a group of Lizards. In Squamata. This man's name is David Peters, and no qualified Pterosaur researchers agree with his views. He is not himself a Paleontologist, and as such has no expertise on the subject, yet has amassed an incredibly large web-presence via his two websites, ReptileEvolution and PterosaurHeresies. Neither of these sites are trustworthy, and should never be used as references for any extinct animals, Pterosaurs or not. 

Scleromochlus, also by Mark Witton

The earliest distinctive Pterosaur ancestors split off from the Dinosaur lineage around 230 million years ago, in the late Triassic. At this point, the animals had not yet gained their distinctive morphology, and the earliest Pterosaurs would have heavily resembled their Dinosauromorph-kin. We do not know of any confirmed non-flighted early Pterosaurs, so instead we tend to look to the basal Ornithodirans, who undoubtedly resembled early Pterosaurs in anatomy and lifestyle. The first of these animals we will be talking about is Scleromochlus. An early member of Ornithodira from the Carnian era of the Triassic, Scleromochlus was a small and agile animal. Similar in anatomy to the lizards of Squamata, one of its most distinctive features was the incredibly long legs it possessed. Due to this, it would probably have been capable of jumping quite effectively, as depicted in the above image. How exactly it lived is uncertain, but it would probably have subsisted mostly on Insects and other small Arthropods living in the many deserts of the Triassic.

Another typical early Ornithodiran was Lagosuchus. While not a Pterosaur ancestor, instead being a basal Dinosauromorph, Lagosuchus was, like Scleromochlus, probably representative of the basic anatomy and bauplan of the earliest Pterosaurs. As with Scleromochlus, Lagosuchus possessed long hind limbs, allowing it to move bipedally. It, along with its relatives such as Marasuchus, would have been effective predators of small animals, possibly including early Mammals, which were just beginning to emerge during their time. From these various genera, spread across multiple Ornithodiran lineages, we can conclude that the earliest Pterosaurs were probably small, bipedal creatures, dwelling in the Triassic deserts and forests. They may have been common parts of their faunas, though the lack of fossils makes this uncertain, and they were certainly diminutive animals, only attaining larger ecological roles when they finally took flight.

Peteinosaurus, by Nobu Tamura

The first known Pterosaurs were already flighted, and lived during the late Triassic, indicating that the group probably first emerged during the later half of the mid Triassic. These animals included Preondactylus, a small and long-tailed creature that lived in Italy, 228 million years ago. It would have hunted aerial insects and other such creatures, and was probably a poor flier, as were most early Pterosaurs. It is quite possibly that, while capable of powered flight, these early members of their lineage in fact mainly used their wings for gliding or short flights, instead preferring to hunt terrestrially. Another early Pterosaur was Peteinosaurus, a relatively poorly known animal whose fossils consist of several juveniles, none of which have heads, and some of which may not even belong to Peteinosaurus. From what we can tell, it too was a long-tailed, small-bodied animal, much like Preondactylus. Another Italian Pterosaur was Eudimorphodon, which despite its name was not related to the Jurassic Pterosaur. This animal lived at the very end of the Triassic, a few million years before the T-J extinction event. Unlike many other Triassic Pterosaurs, Eudimorphodon was not in fact a basal member of the group, and had several more derived traits. Its dental structure is indicative of a Piscivorous diet, what that has since been verified by the discovery of fish in an individual's stomach. This is interesting, as there is little evidence of later Pterosaurs living off fish. The classic "skim-feeding" hypothesis, which posits that some Pterosaurs fished by swooping across the surface and scooping up fish, has become increasingly more unlikely the more we learn about Pterosaur anatomy, and as such, it may be that the Piscivorous Pterosaurs, Eudimorphodon included, hunted by swimming down and catching their prey in a more conventional manner. 

Dimorphodon, once again by Mark Witton

We now come to the only "famous" Pterosaur I will be discussing today, Dimorphodon. A fairly small and oddly proportioned animal, Dimorphodon lived during the early Jurassic, around 190 million years ago. It is notable for being the oldest Pterosaur for which three dimensional remains have been preserved, though no complete fossils have ever been found. Despite this, an abundance of individuals have been known for over a century, giving us a complete view of the animal despite the fragmentary fossils. Dimorphodon may be a relative of earlier Pterosaurs such as the aforementioned Peteinosaurus, but given the shaky state of Pterosaur phylogeny, this is far from certain. By far the most obvious and peculiar thing about this animal is, as the picture above quite aptly demonstrates, the size of its skull. While not particularly impressive compared to later animals such as the Azhdarchids, it is still far larger than that of other early Pterosaurs, and represents the beginning of a trend that would eventually result in truly odd-proportioned beasts. The reason why Pterosaurs, Dimorphodon included, were capable of having such enormous heads is because of the hollow and airfilled nature of their bones, meaning that what may look particularly large and clumsy for a Mammal, is in fact relatively lightweight on a Pterosaur. Dimorphodon was a very strong-legged animal, with particularly beefy hind limbs. During the 80s and 90s, this fueled quite a lot of debate over whether it was in fact a bipedal Pterosaur, but this has since been proven to be unlikely, since many other features of its anatomy indicate it would be very poorly suited for such a method of locomotion.

More plausible is the idea that Dimorphodon was an adept hunter on the ground, capable of chasing downs its prey. This is backed up by the fact that the animal had proportionally quite small wings, which may have meant it was a so-called "reluctant flier", much like Grouse and Anseriforms today, which prefer to remain on the ground. In fact, research by some scientists has indicated that Dimorphodon may have been a very poor flier in general, possibly only using its wings to escape predators. If so, this would represent an interesting side branch of the Pterosaur tree, one in which an early member of the lineage actually partly returned to the ground, a clear example of how evolution is not in fact a linear process.

Dorygnathus with a hypothetical tail form.

The last two genera that I will be discussing here are Campylognathoides and Dorygnathus, both from the early Jurassic of Germany. Campylognathoides had a comparatively short snout, with large eye sockets placed low in the skull, which has lead some researchers to speculate that it may in fact have been Nocturnal. The postcranial of this animal is not very well known, and like with Dorygnathus, the tail is a complete enigma. If it was a nocturnal animal, this would have explained how it avoided competing with its similarly sized contemporaries, who would have otherwise hunted much of the same prey. It is the most basal known member of Novialoidae, a group that lasted all the way to the end of the Cretaceous. Dorygnathus was also a member of Novialoidae, more specifically, it is an early Rhamphorhynchine, and basal to the group. A fairly typical early Pterosaur, it would probably have hunted bugs and other small Arthropods, though it may, as was the case with Dimorphodon, have been a capable hunter on the ground. Interesting to note is that evidence of Pycnofibres is reported on a specimen of Dorygnathus, which is yet more confirmation that this form of integument was ancestral to the group. 

Preondactylus from AMNH

All in all, the early Pterosaurs were fascinating, if not particularly diverse animals. Unlike Bats, who suddenly appear in the fossil record during the Paleogene, Pterosaurs allow us to much more thoroughly track the evolution of flight, giving us an insight into how this group first emerged. For evolution by natural selection to take place, the adaptations gained must be advantageous, and it is evident in Pterosaurs that even during their earliest days, they were graceful and perfectly well adapted creatures, hallmarks of their habitats as definitive as birds are today. While it is easy to think of them as an ancient and archaic group, Pterosaurs emerged at the same time as the ancestors of birds, and the only real difference is that they have not had the misfortune of meeting us. Thus, while they may no longer be with us today, it is important to understand their evolutionary history, in order to fully comprehend what role these amazing creatures played back in those days long past.

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