This is not a project, but an idea I had ~KaptainWombat
In the world we live in, the mammals are dominant, and have been since the end of the cretaceous, when non-avian dinosaurs went extinct. Reptiles are still very much presant, and still soldier on as the crocodilians and squamates, with the tuatara of New Zealand representing a third group. But suppose that one other group of reptiles evolved in the mesozoic, and persisted to the presant? These reptiles are new, different, and have one highly distinctive feature. Living on most continents, in the sea, and even in the air, they have made this world quite different from the familiar cenozoic.
Welcome to the Declinozoic
The Declinozoic is in most ways the same as the Cenozoic we're used to, with the megafauna being mammalian, and reptiles largely being secondary. The one major difference is the presence of a suborder of squamates known as the tazelvermians, evolving from a common ancestor with the monitor lizards in the Early Cretaceous. These reptiles have prospered throughout the Cenozoic.
The tazelvermians are a suborder of lizards sharing a common ancestor with the Varanoidea in the Early Cretaceous. While their relatives enjoyed success in the Late Cretaceous, particularily in the oceans, the tazelvermians stayed small and conservative, mostly as small predators of the undergrowth. This is also when they developed an unusual mutation, a lack of hind legs. In the small species of the Mesozoic, this was of little consequence, but to become anything more than small generalists, they would have to evolve. When the K-Pg extinction hit, the dinosaurs and many other animals went extinct.
The tazelvermians prospered during this time due to the abundance of small mammals, and their front legs, being their only legs, evolved stronger muscles and became longer in order to chase prey down, and the ancestral venom produces by most reptiles evolved into a more potent mixture, comparable to that of some mildly venomous snakes. Despite these early adaptations, the mammals soon overtook them, and until the ETM, they were prey to the large mammals that evolved. At this time, the reptiles took to the seas, and eventually evolved into sea serpent-like creatures, the Aquatazelvermids, one of three lineages of tazelvermians to survive to the presant. After the ETM, the Tazelvermians, which spread worldwide in the Palaeocene, heavily diversified, and evolved into at least 10 different families, all of which are extinct but two. Some of the groups evolved into arboreal predators, using their powerful limbs and prehensile tail to climb trees. Much of their prey probably consisted of forest-dwelling ungulates, but our early ancestors were likely on the menu too, and this may be one of the reasons we fear large snakes today (in some places the name 'snake' applies both to actual snakes and to tazelvermians).
At least one group in Australia achieved apex predator status, and this group may have outcompeted Megalania in localised areas of southern Australia. Unlike Megalania, this monster reptile has survived up to the presant in some isolated areas, and this presance of a large predator means some of the megafauna of pre-human Australia have survived, with examples including a dwarfed Stenthurine kangaroo in the Flinders Ranges, and an also dwarf Diprotodon in some areas of eastern Australia. A similar circumstance in South America resulted in the survival of a small species of Ground Sloth, and of the common Macrauchenia.
Out of some of the arboreal Tazelvermians evolved a group of fully flighted reptiles, the Vorpodontopterygidae. These reptiles are found in Eurasia and parts of Africa, and are believed to have inspired myths about dragons.
Aquatazelvermidae (Sea Serpents)
This group split off early, in the Eocene, when the earth had much higher sea levels. They are believed to have fled to the seas to avoid predation by the large predatory mammals of the time, and once in the water, they were driven to the seas by competition with the then successful Choristoderes.
In the seas, they avoided competition with whales by staying coastal, and stayed relatively small, up to 2 metres long up to the ETM. After this minor extinction, which wiped out the primative whales, and more importantly, the Palaeophid sea snakes, the aquatazelvermids took over the niches of the later and got larger, with some evolving into the largest marine reptiles since the Mesozoic.
Whales beat them to fully pelagic niches, but the reptiles still evolved adaptations to live far from land. This includes reduced, though still presant, limbs, and an elongation of the neck, giving them a serpentine, streamlined shape. The tail evolved a fluke to effectively swim long distances. By the Miocene, all the features characterising modern aquatazelvermids had evolved, and the reptiles evolved into the first of a long lineage of giants that has persisted to the presant.
This pioneer was the genus Serpentazelvermis, which was the most snake-like of the group, and could achieve 4 metres in length. Its direct descendant, Basilophis, achieved an even greater size, at 10 metres adult length. Basilophis went extinct in the middle Miocene, at roughy the same time as the contempory whale Livyatan melvilli, for unclear reasons, but possibly related to the cooling of the earth around that time. Taking its place was the most prolific genus of the family, Aquatazelvermis itself, a sister genus to Basilophis. This genus would survive until the presant and is capable of reaching an enormous 11 metres in length. This genus is likely the inspitation of most reported living 'sea serpents', including several historic sightings.
Tazelvermidae (Tazelwurms, Burrunjor, etc)
This family contains the majority of living Tazelvermids, and are found on all continents except Antarctica. Most members have a snake-like body and short front legs, but two independant subfamilies, one in Africa, and the other in Australia, developed long, powerful and semi-erect legs.
The first subfamily, and the one the group derives its name from, is the Tazelverminae, commonly called Tazelwurms. These reptiles populate Eurasia, Africa and parts of North America. The body is long, and in most species, can be raised above the ground much like a cobra. The reptiles also have a remarkable tolerance to cold, enabling them to live in the arctic circle, one of only a few reptiles to do so.
This subfamily is of Gondwanan origin, and is represented by the South American Duovaranus and Australotazelvermis, and the Australian Burrunjorosaurus. Members of this group have long legs, and walk in a fully bipedal stance. Living members of the australian genus Burronjorosaurus appear to have evolved recently from a smaller ancestor, possibly in the last few hundred years, when the introduction of many new prey animals resulted in a dramatic increase in overall size.
Duovaranines are keystone species, and the survival of Burronjorosaurus, a large predator, may be one of the reasons that a dwarf population of Procoptodon has survived in the Flinders Danges, and a small semi-aquatic Diprotodon species in the mountains of Eastern Australia, which inspired the myth of the Bunyip. The same thing happened with the similar Duovaranus of South America, where herds of Macrauchenia still live on the savannas of southern Brazil.
This African group are arboreal hunters of small monkeys, forest-dwelling ungulates, and other small game. Little is known about this group, though fossils suggest they were the ancestors of the Vorpodontopterygidae, sometime in the Miocene. All 8 living species are classified in the genus Simiophis, although there are likely at least two genera in this subfamily.
Vorpodontopterygidae (Dragons, Ropens)
This group of Tazelvermians have the greatest relationship with people, for this family inspired the legend of the dragon. Vorpodontopterygids are the most studied Tazelvermians, and over 10 species are known. Even more existed in the Medieval times, with another 3 having been hunted to extinction in the last 1000 years. This includes the largest of all dragons, the Welsh Red, or Vorpodontopteryx poenicans, along with the smaller Aerotaurus germanicus, and the fast flying Pyrosagittarius scotticus.
Though these dangerous synanthropic species have gone extinct, the species have survived relatively completely compared to most other megafauna. Today, dragons still exist in Europe in the Alps, where wild Vorpodontopteryx still exist, and in the Himalayas, where the most successful recent genus of dragon, Pyrolator, which has recently expanded its range to parts of eastern Europe and Africa.
Another, more cryptic, genus of Vorpodontopterygid, the Ropen, the last surviving species of the genus Ropen. This species lives in small isolated patches of the Papuan jungle. This species is also very rare, with only 10 sightings since the start of the 20th century, when a specimen was gunned down in a remote area of northern Papua New Guinea. This species is also unusual because it harbours a sort of glowing fungus, though this seams to be a rather more parasitic relationship, with the fungus living on the ropen and spreading the spores. Ropens with the fungus, however, seem to be more healthy than the individuals without it, though the reason why remains unclear.
Vorpodontopterygids are strange in another way, their ability to breath fire. They do this by using a pouch in their throat which they fill with water when they drink. This water is then mixed with a strange secretion which turns it into its compositional elements, Hydrogen and Oxygen. All Tazelvermians have this feature, and Aquatazelvermids use the oxygen during long dives, enabling them to stay under water for up to 18 hours strait, a record for an air breathing tetrapod. The hydrogen is usually a waste product, but this hydrogen is the reason that Vorpodontopterygids breath fire.
Vorpodontopterygids have specialised molar-like teeth at the back of their mouth. These are used to crack bones during hard times, but they also chew rocks containing a catalyst, usually platinum. When the hydrogen is released, it ignites and broduces a flame. The inside of the mouth is heavily armoured to reduce burns, as is the valve connecting the throat pouch to the mouth. Some species, like the small Pyrosaggitarius, can control this to produce signals.