The Pterosaurs — An Introduction
The name ‘Pterosaur’ literally translates to ‘winged reptile’, and that accurately describes what these animals were: reptiles with wings that lived during the same time as the dinosaurs (the Mesozoic era, including the Triassic, Jurassic and Cretaceous periods). But reducing them down to ‘flying reptiles’ really doesn’t do Pterosaurs justice. It’s like calling birds ‘feather things’. Pterosaurs were an incredibly diverse group of animals, showing as many different ecologies and morphologies as birds. In fact, if you can think of a bird lifestyle, Pterosaurs probably did it first (with a few notable, ostrich-shaped exceptions).
Pterosaurs have been studied extensively by scientists for centuries (e.g. Cuvier, 1801), meaning we have a good understanding of how diverse they were (e.g. Butler et al, 2013, Bestwick et al 2018), how they might have flown (e.g. Klein & Anderson), and even how their behaviour might have changed as they grew from hatchling to adult (Bestwick et al, 2020). However, a fairly crucial question has stood without a satisfactory answer for a long time: who were the closest relatives of pterosaurs?
Answering the question of pterosaur origins is important, as without a good response we can’t develop an accurate picture of how and why they evolved. This has proved a difficult question to answer though, as pterosaurs are one of those pesky groups of animals (into which generally many flying animals fall) which just seem to appear fully-formed in the fossil record, with some of the earliest species like Eudimorphodon already having fully formed wings. As a result, it has been hard to find enough similarities between pterosaurs and other animal groups to accurately place them within the tree of life.
Fortunately, new technology has allowed us to re-evaluate some fossils with a new, computer-augmented perception and has revealed some previously unknown similarities between pterosaurs and another unassuming reptile group, the Lagerpetids (Ezcurra et al, 2020). Before we discuss the question of who pterosaurs might have been, let’s take a step back and ask: what were pterosaurs, when were pterosaurs, and why were pterosaurs?
What were Pterosaurs?
Firstly, and importantly, let’s discuss what Pterosaurs aren’t. They were alive at the same time as the dinosaurs, had the same reptilian look as dinosaurs, but they were not dinosaurs. Dinosaurs are a very specific group of terrestrial animals, they lived on land andstayed mostly grounded on the land (with the exception of an extant group of dinosaurs called birds). Pterosaurs were probably very closely related to dinosaurs, but they were no more closely related to dinosaurs than crocodiles are. Pterosaurs are therefore not dinosaurs in the same way that crocodiles aren’t dinosaurs, despite having a prehistoric look.
As mentioned earlier, ‘Pterosaur’ literally means ‘winged reptile’, and that’s a good, if slightly reductive, description. Pterosaurs generally had large heads (relative to the rest of their body) attached to a long neck, which connected to a relatively short body (sometimes even shorter than the neck). Extending from this short body were a pair of relatively short legs and a comparatively enormous pair of arms with a massively extended fourth finger, the last finger on the hand. This enormous arm, hand, and finger combination was the frame for the membranous wing formed primarily of skin and possibly muscle-like structures called actinofibrils. The wing extended from the arm to attach to the body and even the legs, creating a surface like modern-day flying/gliding mammals, such as bats, flying squirrels or colugos.
Another interesting feature of pterosaurs is a downy covering of structures called pycnofibres (Unwin & Bakhurina,1994). These hair-like structures may have functioned similarly to hair on mammals, not being much help for flight, but being extremely helpful for retaining heat. This is interesting because if an animal is covered in insulation, it implies it has some heat it wants to insulate within it. Cold-blooded (or poikilothermic) animals don’t have insulation because they need to acquire heat from their environment, and insulation is just as effective as keeping heat in as it is keeping heat out. For this reason, only warm-blooded animals, which generate their own heat (endotherms), benefit from insulating feathers, fur, and pycnofibres. Like birds (including many non-bird dinosaurs) and mammals, then, pterosaurs were likely warm blooded beasts (Witton, 2013).
While pterosaurs all possessed roughly the same body shape, the head, tail and wing morphology (size and shape) differed quite significantly across the pterosaurs, though we can make some generalisations. The first pterosaurs, often called rhamphorhynchoids included species such as Dimorphodon and the titular Rhamphorhynchus, generally had toothed jaws and long tails connected to the legs by another wing membrane (called the uropatagium). This meant that all the limbs were connected to each other by a wing membrane, possibly giving these animals the same ungainly terrestrial motion as bats.
A group of pterosaurs which evolved later (sometimes called the pterodactyloids) generally had toothless beaks and a very small tail (if it was present at all). The absence of a tail also accompanied the loss the uropatagium which connected the hind limbs to the tail, meaning that the hind limbs were no longer connected to each other in some species. Though the arms are still connected to the legs by a part of the wing membrane (called the brachiopatagium), this greater independence between limbs would have allowed them to move with a bit more ease when on the ground.
It’s important to note that the rhamphorhynchoid-pterodactyloid dichotomy is not a hard rule, and most scientists don’t recognise them as distinct groups (e.g. Lü et al, 2008 & Witton 2013). Nevertheless, it’s sometimes a useful descriptor to highlight differences between different types of pterosaurs.
Across all pterosaur species, there was a remarkable amount of diversity, with these animals occupying a range of different ecological niches. Some were coastal or sea-faring piscivores (fish-eaters), such as Rhamphorhynchus with its toothy grin and nimble wings (Hone et al, 2015), Pteranodon with a similar soaring-suited morphology (Witton & Habib, 2010 & Goto et al, 2020), or the shorter wings of Alcione suggesting it could have behaved like modern gannets (Longrich et al, 2018).
In shallower waters and wetlands, Pterodaustro and Ctenochasma had filter-feeding beaks like a flamingo (Sanderson & Wassersug, 1993; Henderson, 2017), while Dsungaripterus had a strange, slender yet robust skull thought to be used to grab shellfish or detach molluscs from rocks before crushing them with its flat teeth (Chen et al, 2020). Some of the largest pterosaurs, the azhdarchids (including the giraffe-sized Quetzalcoatlus), may have behaved like herons and cranes, sharing the same slender, long necks, legs and beaks (trackways of these animals have also been found in fossil formations from wetlands or shallow waters; Unwin, 2007). Alternatively, these enormous animals may have flown short distances across land to pick off small dinosaurs: their long legs and relatively small wings compared to the rest of the body, at least) suggest the largest azhdarchids spent more time on the ground and less time in the air than other pterosaurs, meaning terrestrial foraging is a more likely lifestyle for these winged behemoths (Witton & Naish, 2008; 2013).
When were the pterosaurs alive?
With the myriad niches exhibited by pterosaurs, it’s safe to say they were a very diverse group, just as diverse as birds are today. The pterodactyloid-type pterosaurs don’t appear in the fossil record until the Mid-Jurassic (Andres et al, 2014), and until that point the rhamphorhynchoid-type pterosaurs were a diverse bunch. Once the pterodactyloid-type pterosaurs appear though, they overtake their smaller, longer-tailed cousins in diversity, with the former having mostly disappeared by the end of the Jurassic (Witton 2013). The exact level of diversity is quite hard to determine, however, so these conclusions should come with a heavy pinch of salt. This is because our fossil record is imperfect, and many smaller pterosaurs (like some of the rhamphorhynchoid-type pterosaurs) lived in forests which are famously poor for fossil preservation. This might have biased our conclusions about diversity towards the larger, generally pterodactyloid-type pterosaurs which occupied coastal habitats where fossils are far more likely to form.
We also have to contend with the Signor-Lipps effect, a principle proposed by Philip W. Signor and Jere H. Lipps (Signor & Lipps, 1982). This principle states that, due to the incompleteness of the fossil record, we will almost certainly never find the first or last individual of a species. As a result, species appear to have gone extinct before they actually went extinct, and so our understanding of pterosaur diversity towards the End Cretaceous extinction is particularly unreliable.
A popular, though now controversial, theory to explain why pterosaurs appear to become less diverse (assuming our fossil record is actually representative of what was really happening) is that birds were outcompeting them (e.g. Benson, et al, 2014). Birds start to become diverse during the Cretaceous, during which time average pterosaur body size becomes larger and smaller species seem to go extinct. However, this theory has been losing favour, in part because of the sampling biases introduced by the fossil record (Butler et al, 2009), as well as evidence that some pterosaur species may have outcompeted birds to reclaim niches in some cases (Longrich, N.R et al 2018). Competition with birds may have been a factor in explaining changes in pterosaur diversity like the loss of rhamphorynhcoid-types and the increase in average body size, but this more recent research makes it clear that it likely wasn’t the only factor.
Where did the pterosaurs come from?
Pterosaur diversity didn’t appear from nowhere, however, every mega-diverse group had to start from some humble beginnings, but what did this start look like? As mentioned earlier, we don’t really know. The earliest pterosaurs like Dimorphodon already look like pterosaurs, we don’t have any Archaeopteryx-style transitional fossils which tell us how pterosaurs evolved from their nearest non-pterosaur ancestors.
Plenty of research has been devoted towards trying to find the nearest ancestors to pterosaurs. This research has often used phylogenetic techniques to position pterosaurs in the tree of life based on characteristics of the skeleton they might share with other groups. Some characteristics of the ankle, for example, place a genus of archosaurs, Scleromochlus, as a sister group to pterosaurs and very close relative to early dinosaurs. This would mean that a reptile that looked like Scleromochlus (but not Scleromochlus itself) gave rise to Pterosaurs and Scleromochlus. However, a more recent analysis (Bennett, 2020) suggests that Scleromochlus is part of a completely separate group of archosaurs (the group that includes crocodiles, pterosaurs, dinosaurs and birds). This type of repositioning tends to happen when the fossils you have access to aren’t complete, and when species share characteristics with several different groups. Similarly, a species at the base of the dinosaur-like archosaur family tree, Euparkeria has also been thought to be a close relative to pterosaurs based on its skeletal anatomy. The fact the pterosaurs appear to be related to these very early dinosaur-like archosaurs suggests that pterosaurs diverged quite early in the Triassic away from dinosaurs and other archosaurs.
Another interesting group that pterosaurs have been lumped with are the Protorosaurs. This is a poorly defined group but includes some rather charismatic species like Sharovipteryx. Superficially, Sharovipteryx looks like a good candidate for a pterosaur relative or ancestor, until you realise this animal’s wings are made from its legs, rather than its arms. Regardless of where Sharovipteryx chose to put its wings, Protorosaurs appear to share some skeletal similarities with pterosaurs, but not enough to confirm a consistently close relationship between them (Hone & Benton, 2010).
A recent study by Martin Ezcurra and colleagues, however, uses the anatomy of the inner ear (and some skeletal anatomy like a hook-shaped head of the femur) to group the lagerpetids as a sister group to the pterosaurs (Ezcurra et al, 2020). The lagerpetids have been thought to be a very close relative to dinosaurs, but using some high-resolution CT scans, Ezcurra et al found some interesting similarities in the shape of the inner ear and the floccular lobe of the brain. The inner ear is partly responsible for your sense of orientation, your sense of what is up or down, and the sensation of rotation, while the floccular lobe is involved in motor control of muscles. In pterosaurs and other flying animals, these structures are complex and well developed to afford the fine muscle control required for flight (which is bloody difficult, as any aerospace engineer will tell you).
The fact that lagerpetids have floccular lobes and inner ears which share a similar morphology to basal (early) pterosaurs, doesn’t necessarily imply that lagerpetids could fly, but it suggests they might have had an arboreal lifestyle in which balance and orientation is important, jumping between trees or spending time in tree canopies. Importantly, it also suggests a close relationship between lagerpetids and pterosaurs, potentially giving us an insight into the kind of animals from which pterosaurs might have evolved.
This is some strong evidence, but until we find a transitional fossil, pterosaurs will probably end up being grouped with yet more animals as more evidence is discovered and analysed. A clear trend though, is that pterosaurs are archosaurs, very closely related to dinosaurs, and diverged from them with a common ancestor early in the Triassic or during the Permian, and that their closest relatives were likely dinosaur-like archosaurs.
How did the pterosaurs evolve?
We now might know to whom pterosaurs were most closely related, but unfortunately, we still haven’t found a transitional fossil between these groups and pterosaurs, so we don’t have a strong idea of how they became animals that resembled pterosaurs.
The driving forces that might explain why a group like the lagerpetids would begin to evolve flight are easy to understand: being able to fly is extremely beneficial. It’s no coincidence that flying animals are also some of the most diverse. If you can fly, you can travel farther and more quickly to find food or mates, escape predators and environmental hazards more easily, and reach areas that other ground-bound animals simply cannot. This is partly why birds, bats, and insects occupy a huge variety of different niches, are globally distributed, and are the most speciose groups of animals on the planet.
These advantages are very apparent once you can fly, but an animal can’t just sprout wings. Their evolution is a gradual process, so there needs to be a benefit for the animal throughout the different stages of wing evolution. All the way from skin membranes barely recognisable as wings, through to majestic and effective flight surfaces, there must be an advantage for an animal which has the character, over an animal which doesn’t.
The evolution of wings and flight in vertebrates has two main schools of thought. Tree-down and ground-up (see Ostrom, 1979). The tree-down hypothesis states that the evolution of flight began as animals attempted to jump between trees or reach the ground safely, and having any sort of wing membrane, no matter how meagre, would allow you to glide greater distances between trees, or reach the ground safely from a greater height. These benefits are enhanced as membranes become larger, and as animals begin to flap these wings to generate more lift or thrust to control their descent (Norberg, 1985; Rayner, 2008). This process sees animals develop pre-wing membranes initially, and these membranes eventually become wings that can be flapped.
The ground-up hypothesis, on the other hand, is supported by evidence from ground-living birds like chickens or quails. As these birds run, particularly when they run uphill, they flap their wings as a method of gaining speed through generating thrust (Dial, 2003; Dial et al, 2003). Again, we can see a situation where even a simple, small skin membrane might be helpful in this activity, and the larger a membrane becomes, the more useful it becomes. This theory also explains why these animals might have evolved flapping flight, because they were flapping their arms anyway.
The poor fossil record of flying species makes it hard to know which, if either, of these theories are correct. In reality, a combination both ground-up and tree-down activities might explain the evolution of flight — there’s no reason that an animal that flaps its wings to help it climb up trees couldn’t then use those wings to glide between those same trees. But given that lagerpetids and their close relatives were arboreal, tree-dwelling animals, this lends support to the idea that perhaps a tree-down method of evolution might have occurred during the evolution of the pterosaurs. Hopefully, in the future, more fossil evidence will come to light which will give us insight into how this group of animals evolved and began to dominate the Mesozoic skies.
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