Developing a scientifically accurate species of dragon-like humanoids

Question

I'm trying to figure out a way to make this species as I envision it while still maintaining scientific realism. I want to get it as close to "yes, this could exist in the real universe" as possible. If you want a quick visual primer: something like this or this - but with wings capable of flight and tail. I'm not an expert in biology or astronomy, so everything below is up for criticism. Thanks in advance!

My rough draft of their origin is: they are descendant from an ancient dragon-like being that was killed long ago, its remains preserved and left adrift in the vacuum of space. Eventually it's captured in the gravity of a young, habitable planet and plummets to the surface, where a panspermia-type event helps to create new life. Evolution does its thing over the course of many, many years, and sooner or later come this race, evolved in the image of the original creature. How far along the planet was in its development I'm not sure, but they are meant to be the first sapient species on the planet. Again, all of this is subject to change based on my lack of expert knowledge.

Here are some basic traits that I'd like - with diet and genetics playing a factor when appropriate.

• Height: An average somewhere around 6'8" (2.03m) - 7'2" (2.18m)
• Weight: Anywhere between 200-350 pounds (90.7-158.8kg)?
• Basic anatomy: Bipedal, humanoid anatomy - does it make sense that they would be upright, humanoid creatures without the presence of humans? With three fingers and a thumb for each hand. Digitigrade or plantigrade feet with three or four toes.
• Lifespan: ??? - I could use some help with this one - not sure how much of a factor all of these different traits would play into the average lifespan.

And for some more complex features...

• Wings: Made of a durable and light material, but not sure what exactly. Capable of powered flight, ideally - would this be possible without violating square-cube law? Ideally capable of providing enough lift to fly anywhere, but gliding once jumping from a high enough area could also be an option. Maybe a second pair of lungs that stores lifting gases like hydrogen.
• A tail: For stabilization during flight. Length and thickness will vary with genetics and diet, but typically as long as or slightly longer than the length from waist to feet. Serrated ridges along the top for protection and to use as a weapon. Would the tail be an extension of the backbone, or would it have its own vertebrae and ligaments?
• Fire-breathing: Could maybe have glands somewhere that store flammable gases like octane or methane, plus some sort of oxidant and ignition mechanism. I think this is the one that I need most help with, not too sure about the anatomy of storing all the required parts.
• Horns: Just because I want them! Their shape and size would be determined mostly by genetics. Protruding out from either side of the forehead and can curve while growing. Useful for protecting the head.

Answer 1

There's a few interesting things possible here, but let's get something out of the way. Powered flight is impossible. I mean, okay, if you play around with atmosphere and gravity it isn't, but as it stands now, it's completely impossible. Like you said, it violates the square cube law. An upright reptile averaging 6'8" to 7'2", realistically, is going to average around 300 pounds, maybe more. That means massive, gargantuan wings, which take massive gargantuan muscles, and you'll end up less with dragon-like humanoid and basically a pair of wings which happens to have a face on it.

Gliding is more realistic, and if you feel like it, settle them on some kind of volcanic world with plenty of updrafts from lava pits that they'd be able to ride. The wings in question would be giant flaps of skin, normally locked to the dragonoids back, which they'd be able to snap into a giant framework, giving the dragon a pair of massive sails. (I'd assume cartilage would be involved in this framework - more flexible than bone, more rigid than just flaps of skin.)

A tail - reptilians tails are an extension of the spine, in general, so I'd assume this would be no different. They also tend to be strong.

Fire-breathing - there's already a few answer to this, apparently. If you're looking for anatomy to copy, I'd recommend taking a look at spitting cobras.

Horns - this is really the easiest. Horns are made out of keratin, the same stuff human hair is made out of. Except that keratin usually crops up in creatures that have hair, aka mammals. So, if you want to keep to the reptile theme, these would actually be bone horns. (Well, keratin horns are also made from bones, but there's only a small nub, than keratin grows around it. These would be entirely bone. Probably hurt a lot if it breaks.)

Lifespan - tricky one. Reptiles are basic lifeforms, but these are advanced versions, lots of complex parts. Throw it a fully developed autoimmune system, and add natural armor. Not as long as a human, but not significantly shorter. Probably around 60 years.