Detailed explanation: Wyverns

Perhaps one of the biggest departures from the standard evolution in Az's history is the emergence of the wyvern order of the reptile classification, evolving many millions of years before present times. There exists no agreed upon origin point between scientists for this order of animals, though most likely they branched off from Az's equivalent to therapods or perhaps pterodactyls late in development. Unlike most other reptiles in the current setting, the majority of wyverns are warm-blooded, but still lay eggs and have scales lining their bodies.
  what truly sets this order of reptiles apart is the developed membranes that run along their sides, a majority of which over time have developed into wings that can bear them aloft in the sky, or enable advanced aquatic movement. For this reason there are three sub-orders of wyvern that currently exist; sky, Sea and Land wyverns.
 

Sky-wyverns

  Starting with sky wyverns, all possess membranous wings, long lizard-like tails and many bear accented plumage on their bodies akin to birds. With such fierce competition between birds and sky wyverns, these reptiles have needed to adapt quickly, a common niche being that they specify into hunting said birds, the scales on sky wyverns preventing them from receiving too much damage from the beaks and talons of their prey. Notable examples of sky wyvern species are the insectivorous Whip-tongue wyverns, small pixie wyverns and the larger vulture wyverns of Gurn.
  But the two most famous types are the Imperial wyvern of Ganneoux and the Frynnish mountains, and the Balloon wyvern found sailing the skies around the globe. These two are both exceptional specimens, both for their size and how they are able to fly despite their considerable weight. Closely related, the two species have developed an especially genius method for circumventing the pull of the planet. By producing a unique, lighter-than-air fluid in several glands, these reptiles then pump the fluid around vessels in the body, achieving an effect similar to that of a balloon, dramatically reducing the effective weight their wings must carry, hence the Balloon wyvern's name as it produces so much fluid that it swells up in certain areas.
 

Sea Wyverns

  Taking a different approach, sea wyverns are not quite as limited by the constraints of gravity as their airborne brethren, so often grow far larger in comparison. In place of wings, membranes become great fins and sails, allowing for seamless movement in the sea, while not completely rendering these animals helpless on the ground, as they still retain the developed limbs of their ancestors. Notable examples include river, lake, and the terrifying Gimeaen Lagoon wyvern, a large ambush predator with the ability to absorb water into its body before expelling it outwards violently, in order to jettison itself towards prey and deliver bites with dizzying amounts of force.
 

Land Wyverns

  This leaves the land wyverns, a rarer sub-order with only a few species composing its ranks. Incapable of flight, nor terrific at navigating waterways, this order specializes in either run-down or ambush tactics, using their considerable mass to over-power prey. Rock-creepers and Dune-stalkers are perhaps the most well-known, living around mountain ranges on Yddian, Quon'gir and the Jannid desert. Unlike most wyverns, these are cold-blooded reptiles as they can receive heat from the hot environments they live around with ease, hence individuals can grow to great sizes given enough time. A plague upon all sapient species.
  The final land wyvern specie is the ice wyvern, found solely in the northern fringes of the Tall Stretch. Very compact, quiet and deadly, these beasts are covered in a layer of downy feathers for heat retention and have repurposed the membranes on their flanks and arms to further retain heat. A potent example of polar gigantism, yet incredibly stealthy thanks to blending in to the white backdrop, ice wyverns are just as at ease ambushing unsuspecting mammoths as they are running down lone giants who stray to far out from their settlements.