Scansoriopterygidae was a family of theropod dinosaurs that lived during the Jurassic, the most well-known member of which was Yi Qi. They are thought to have been arboreal insectivores, and convergently evolved bat-like wings.
Paleontologists currently think that it may have been capable of very weak flapping, but was likely an exclusive glider. The Scansoriopterygids went extinct in the late Jurassic, about 156 mya, but had they continued to live through the Cretaceous, I'd say that they'd be at least potential K-Pg survivors.
In my project (See also these questions on cockatrices, hydras, griffins, giants, sea serpents and merfolk), the Earth is inhabited by many creatures from human mythology that are supposed to be as scientifically plausible as possible (Note that in the project, the biology is the primary focus, and the story comes second). One such animal I'd really like to include are dragons - or, rather, wyverns.
The question is: is the premise of large, flying theropods descended from Scansoriopterygids which survived into the Cretaceous and Cenozoic plausible? Let me explain a bit more about my wyverns:
- They are capable of powered flight
- They can grow to large sizes, with 15-20 metre wingspans (Equivalent to the largest pterosaurs)
- They have not "taken over the world", in the way that birds or mammals did. They are far from the dominant form of life.
One of the problems that may exist for this concept is that Scansoriopterygids were gliders - and, according to many people I've heard - "gliding animals can't evolve into flying ones". Is this correct? Can I achieve gliders evolving powered flight, without having them lose the ability to glide and then evolve flight? If Yi Qi really was capable of weak flaps, this problem could be overcome by enlarging the pectoral muscles.
Then, there is size. A lot of the time, it's hard to just scale up a small animal and end up with a biomechanically functional organism. So, before asking this question, I thought about why bats can't grow as big as birds or pterosaurs. The main ideas I came up with were:
- Birds were already the dominant diurnal fliers when bats evolved,
- Membraned wings are more susceptible to damage
- Soaring is harder at night due to lack of rising air currents.
Could these problems be overcome?
Lastly, there is the fact that they "haven't taken over the world". The bullet point was a bit vague, but I pretty much mean that I don't want them to be nearly as ubiquitous as mammals or birds. Ideally, there should be about 500 species or less, almost all of which will be small-ish, hawk-sized ones, with a dozen or two species that are very large.
If I haven't mentioned it in the list of bullet points, you don't need to address it in your answer - so the likes of fire-breathing, scales vs. feathers etc. can be left to me.
If you require more information, please say so, and do the same if you suggest it to be broad, off-topic or opinion-based.