Gliding (and flying?) mammals of the Mesozoic

looksee wrote: »

This is a somewhat zombie thread but it exactly relates to the question I cam in here to ask. I just read an article on rte https://www.rte.ie/brainstorm/2020/0309/1121174-dinosaur-myths/ which mentioned gliding dinosaurs and my first thought was, how did they get high enough to launch a glide.

Flying squirrels are primarily animals that climb and live in trees and have incidentally developed flaps of skin that allow them to glide, generally between trees. In the articles about dinosaurs though it is all about their gliding capabilities with no mention of adaptations to climb trees. A very quick google suggest that there were no tree inhabiting dinosaurs, so how did the gliding dinosaurs repeatedly get high enough to glide?

It's a very interesting question. The big problem is that even paleontologists aren´t sure about which feathered dinosaurs were gliders, and which were true, powered flyers. Theropods are all very similar anatomically, and very bird-like, so much that you find lots of flight-related characteristics in forms that obviously didn´t fly, like Velociraptor or Deinonychus, so you when you find a small, feathered theropod it isn´t always obvious whether you are looking at a flighted, flightless or gliding animal, or even something in between.

Let's take a look at the dinosaurs that have been proposed as gliders.


Archaeopteryx



The first skeleton of this animal was found in 1861, and around a dozen more have been discovered since, most of them with preserved feather impressions; these show Archaeopteryx had broad, bird-like wings with asymmetrical feathers, a characteristic of flying birds.

Despite this, paleontologists resisted for a long time the idea that Archaeopteryx could fly. Because no other animal but birds was known to be feathered at the time, it was classified as the earliest known bird, and it didn´t make sense for the earliest bird to be an advanced flyer. Instead, it was suggested that Archaeopteryx was a glider, using its wings and tail feathers to parachute from tree to tree, or from tree to the ground, and using its claws to climb back up.

Today we know that despite its age, Archaeopteryx was actually a rather advanced flyer. Everything in its anatomy points to powered flight, and a recent scan of its braincase found it comparable to modern birds in complexity. Also, as it turns out, Archaeopteryx was not a forest dweller; it lived in a semi-arid environment, a series of subtropical islands with very little annual rainfall and no large trees. Mostly, it would've been open landscape with shrubs here and there, enough to take refuge in but hardly comparable to the forested habitat of today's gliding mammals and reptiles. Everything seems to indicate Archaeopteryx was mostly a ground dweller that hunted small, terrestrial creatures and would've been capable of short but powerful bursts of flight- useful maybe to escape the unwanted attentions of the compsognathids it shared the islands with.

Personally, I imagine Archaeopteryx as somewhat akin to a roadrunner. This bird too lives in semi-arid shrubland, has a similarly long tail and broad wings, and preys on small terrestrial animals. It can both fly and glide, as seen in the videos below, and depending on the needs of the moment, but it mostly moves around by walking, running and hopping around.







Microraptor



One of several feathered dinosaurs found in China's Jehol province, it was more recent than Archaeopteryx, but unlike it, it was never classified as a true bird; instead, its anatomy shows it was a dromaeosaur- a member of the same family as Velociraptor and Deinonychus, among others.
Microraptor caused sensation in the early 2000s for several reasons; at the time, it was the smallest known non-avian dinosaur (although it has since been beaten for that title), and its existence strongly suggested dromaeosaurs were descended from volant ancestors. Most uniquely, it had all four limbs modified into wings with flight feathers, something never before seen.


Because of its strange configuration, Microraptor too has been the subject of debate among paleontologists regarding its flight capabilities. Even more than Archaeopteryx, it seemed well suited to glide from tree to tree; the creature was often restored with limbs outstretched, the hind wings providing additional lift, and there is indeed some evidence that suggests its legs and hip were articulated so that it could splay more than other dromaeosaurs.



And unlike Archaeopteryx, Microraptor did live in a forest environment.

So Microraptor could, possibly, be a gliding dinosaur. However, there's also some good evidence suggesting it may have been a powered flyer; for example, the arms are articulated in such a way that it allowed for a vertical flapping motion, and several details of its anatomy, including the presence of an alula and the wrist articulation are similar to what you see in flying birds. So it may be that Microraptor, like Archaeopteryx, was able to both glide and fly to an extent.



As for whether it was arboreal or not, nobody knows for certain. Some have suggested its claws were well suited for climbing, whereas others think its anatomy most resembles ground birds. It may be that Microraptor was a rather versatile animal, exploiting different levels of its environment and equally capable on trees and on the ground; evidence for this are the animal's own preserved gut contents which have been found to include both terrestrial and even aquatic prey, such as mammals and fish, as well as tree-dwelling birds. We also known, thanks to the gut contents of a large compsognathid, that Microraptor types sometimes got caught and eaten by non-volant predators.

Sinornithosaurus



Also found in China's Jehol province, it was one of several feathered dinos (along with Microraptor and the non-volant Sinosauropteryx) that caused sensation during the 90s and early 2000s. Sinosauropteryx too was a dromaeosaur ("raptor" dinosaur).
Like Microraptor, it seemingly had long, splaying limbs and was feathered, so it may have been able to glide or fly to some extent. The most usual argument against this has been its size; at 1.2 m and around 3 kg it was considerably larger than Microraptor or Archaeopteryx. However, many forest dwelling birds today are as big as this, or bigger, and still fly and glide; peafowl, for example, can weigh up to 6 kg.
Sinornithosaurus was imagined at one point as an arboreal predator that would swoop down on ground dwelling prey, killing with a venomous bite. This was based on its seemingly having long, grooved, fang-like teeth. However, the teeth were later found to have partially slipped out of their sockets giving the false impression of being fangs; as for the grooves, they are not necessarily indicative of venom. One way or the other, it is obvious that Sinornithosaurus was a predator, but exactly what it was preying on, and whether it hunted from the trees, on the trees, or on the forest floor is unknown.

Gliding dinosaur? Maybe, but not certain.


Scansoriopterygids



Here's where things become interesting. The Scansoriopterygids are the only family of dinosaurs that has been interpreted as arboreal from the very beginning, ever since the discovery of the first specimen (a juvenile) in 2002.



The Scansoriopterygids's forelimbs were longer than their hindlimbs, and they had claws, toes and joints indicative of an arboreal lifestyle. Their very name is meant to reflect this (scansorial means tree-climbing). Originally, the describers of Scansoriopteryx also suggested that the stiff tail of the animal may have served as a prop much like the stiff tail feathers of a woodpecker, to aid in climbing.
The most unique characteristic of the Scansoriopterygids were the very elongated fingers, which were at first interpreted as a tool to probe for wood-boring insect larvae, not unlike what we see in the aye-aye today.



However, a recent and surprising discovery proved that the elongated fingers were actually the support for a bat-like wing, something never before seen in dinosaurs. They would've looked like miniature dragons! They even had a special, elongated bone (the styliform bone) on their wrists, which offered further support to the wing; this is an adaptation we see, with some variation, on multiple flying and gliding animals, from bats and pterosaurs to flying squirrels, colugos and anomalures.



Up to this moment, there is no consensus as to whether Scansoriopterygids could only glide or truly fly, but they certainly were doing at least one of them. Some early studies have suggested that they may have been akin to the modern kakapo, a parrot that mostly glides but can also control its descent a bit by flapping its wings. It is unlikely that they were as acomplished flyers as modern day bats, considering their proportions.

What does seem certain is that Scansoriopterygids were living in forested habitats and most likely were tree-dwellers.

So, to answer your question, how did the gliding dinosaurs repeatedly get high enough to glide? It depended on the dinosaur. Some already lived in trees (Scansoriopterygids), some apparently divided their time between trees and the forest floor (Microraptorans), and some (Archaeopteryx, possibly Microraptorans) had enough powered flight capability to take off from the ground, like the roadrunner, and reach high perches from which they may descend later with minimal energy expenditure by means of gliding.