Sixty-five million years ago, the world was struck by the worst catastrophe since the end of the Permian. Often referred to as the K-T event by scientists, the extinction was powerful enough to wipe out nearly two thirds of all life on Earth. Victims of this extinction ranged from the massive azdarchid pterosaurs and mosasaurs, to more unassuming creatures like enantiornith birds and polyglyphanodont lizards. The worst of these casualties were the non-avian dinosaurs, which up until that point had been the dominant land organisms on the planet. In our world it was the mammals, and to a lesser extent the birds, who inherited and conquered the world after the K-T. But in this world, something different happened. While crocodilians, turtles, and amphibians continued on as normal after the extinction, it was squamates, lizards, snakes, and their kin, who would come to dominate the post-Mesozoic biosphere.
At first, the early part of this world's Cenozoic would look a lot like our own. But to a discerning eye, one could already see the changes starting to take place that would lead to a world ruled by squamates. Big komodo dragon-like monitors stalked pantodonts and “condylarths”, large iguanids related to Pristiguana spread all over the New World, and gigantic Titanoboa-like snakes swam in the waters. By the time the Early Eocene rolled around, the squamates had gained near-total control of the Earth. The advances that mammals and birds had made up until that time had been merely fleeting. Throughout the rest of the Eocene and Oligocene, squamates were the dominant land animals on Earth. But there were not the active, warm-blooded, erect-gaited squamates of today. Rather these were huge sprawling or semi-erect animals, making this part of the Cenozoic look rather like a neo-Permian or Triassic. But all of this would change in the Miocene. The giant sprawlers and semi-erect squamates that dominated the Oligocene weren’t alone. At the same time, they shared their world with early forms of erect-gaited, warm-blooded squamates, many of these ancestral to modern day groups. In the Early-Middle Miocene, these groups took over, their advanced gaits and metabolisms competing most of the big sprawlers and other primitive squamate groups to extinction. Eventually further environmental changes such as the ice ages would further cement these advanced squamates hold on the planet, making them the dominant group of animals on Earth
The dominant herbivores of the Old World are erect-gaited agamids, who come in a wide variety of forms, ranging from small, arboreal drepanosaur-like creatures to gigantic elephant-sized animals reminiscent of the extinct pareiasaurs; the largest land animals on Earth. The main tall, giraffe-like browsers of this world are the giganatids, anseriform birds slightly resembling the dromornids of our world. Giant armour-plated herbivorous and omnivorous cordylid skinks related to girdled lizards and sungazers take the place of ankylosaurs and glyptodonts, inhabiting scrublands, deserts and uplands across the Old World. Preying on these animals are endothermic, erect-gaited monitors, who act in many cases like wolves, hyenas, and saber-toothed cats, and predatory geckos, which are pantherine-like ambush predators. Other predators include the more primitive predatory monitors, the therolacertids, who act like the weasels, badgers, and other small carnivores in this world. In the Old World, giant predatory short-skulled amphisbaenians prey on large surface-dwelling animals like giant subterranean worms or crocodiles. Across both the Old and New Worlds, lacertids and skinks evolve unparalleled diversity, with small insectivores and mid-sized generalists occurring across all environments, from forest floors to tree tops. And of course, completely bucking the curve, Madagascar becomes dominated by hooved “murder crocs”, huge numbers of arboreal chameleons, arctocyonid condylarths, and oplurid iguanas.
While many different lineages of squamates have evolved endothermy and erect gait, fewer have evolved integumentary structures, and thus are poorly-equipped to withstand the cold. Therefore, the differences between the Old and New World is more marked than in our world. However, several lineages of squamates on both sides of the planet have managed to make the crossing, and establish themselves on the other side. A group of agamids, mountain-dwelling species similar in appearance to Scutellosaurus, came to the New World in the Pliocene, and subsequently took the place of bighorn sheep and mountain goats. At the same time, gracilisaurs and sauroraptors have managed to make the crossing in the other direction, the former becoming established as deer-like herbivores in Eurasia while the latter have taken a wide variety of small predator niches. Boreosaurids, the ferocious opportunists of the North, are found all around the Holarctic region, whether it be Old or New World.
Australia has always sort of been dominated by squamates in our world, and the same is true in this one. The dominant herbivores of Australia are the agamids, like in the rest of the Old World. However, these are not the familiar Laurasiagamids of Africa and Eurasia, but rather their own unique group, the Gondawanagamids. Gondawanagamids tend to have semi-erect gaits like a crocodile or a therapsid, rather than the erect gaits that charictarize Laurasiagamids. But in an odd twist, many lineages of Gondawanagamids have circumvented this by several lineages becoming bipedal, ranging from omnivorous opportunists to full-blown herbivores. Joining them on the plains as large herbivores are the odd placental tingamarrs, herbivorous birds thought to be distantly related to emus and cassowaries, and meolanid turtles, who are more diverse here than anywhere on the mainland. As can be expected, monitors are the dominant predators of Australia. However, compared to the predatory monitors of the mainland, these monitors seem very different in their adaptations, being more prone to bipedalism and other similar niches due to their harsh environment. The two Australias also have another trait in common; they both seem to be places where evolution has gone mad. While across most of the world monitors are terrestrial predators, and crocodilians are mostly restricted to aquatic habitats, the situation is reversed in Australia, where predatory mekosuchines compete with geckos for the cat niche, while aquatic monitors take the place of crocodilians alongside native crocs in the water.
As opposed to the Old World, which is dominated by predatory monitors and geckos, the New World is dominated by all manner of iguanines. Perhaps the epitome of this are the crotaphytids. While in our world crotaphytids are restricted to leopard and collared lizards, in this world crotaphytid iguanians are some of the most successful squamates of all time, evolving erect gaits and endothermy in the Americas and become long-legged giant cursorial predators. These animals, the sauroraptors, resembling the long-extinct non-avian theropods, and take up a variety of niches including those of small animal catchers and wolf analogues.. Some of the latter even spread across to the Old World, and are prevalent there. Anoles take the place of cats in the New World, evolving into big-game hunters, arboreal predators, and terrestrial stalkers. The New World is also home to parabirds, strange squamates descended from anoles that seem to converge a bit on the basal birds of the Mesozoic, but rather than becoming more advanced, they have taken their Archaeopteryx-like body plan and run with it. Strange basal iguanines have evolved into their own unique predator group, the paramonitors; sphenacodontian-like animals which hunt the deserts of the southwest. Most of the large herbivores of the New World are members of a special group of iguanid-derived iguanines, the ungulosaurs, which range from primitive trilophosaur-like species to huge prosauropod, and ground sloth-like animals. The phrynosomatids take part in this diversification too, becoming analogues of peccaries, bison, and the extinct ceratopsians. The anguids also become diverse, evolving into a variety of forms including prairie dog-like burrowers, raccoon analogues, ankylosaur analogues, nectar-eaters and ferocious northern predators. Helodermatids are also more diverse in this world, taking the place of saber-toothed cats and bear-dogs.
For the majority of the Cenozoic South America was an island continent, isolated from the rest of the world. Because of this it developed its own, unique fauna. Giant, sprawling megalania-like teiids, predatory bear-like iguanids, and sebecosuchian crocodiles stalked meiolanid turtles, sloth lizards, native ungulosaurs, and grazing corytophanids across the pampas and plains. But that all changed when North America came and linked to the continent by way of the Panamian land bridge. No longer isolated, South America was buffeted by an invasion of northerners. Gilas, sauroraptors, advanced ungulosaurs, terror owls, and many other adaptable northern groups spread south, muscling out many of the natives. Today, South America’s fauna is mostly made up of these northern invaders. However, some species proved to be adaptable and survive. One of these groups were the corytophanids, which took the place of the sauroraptors, herons, and even some grazers in the South. Their diversity has been severely cut, but they have survived. Teiids have also been heavily hit, formerly huge carnivores as big as Megalania, now they are reduced to smaller monitor lizard analogues. Meiolanid turtles have survived, though not as diverse as before, and have even managed to spread north into southernmost North America. Sebecosuchians too have mostly been able to hold their own, but are now found as smaller predators.
While squamates are the dominant land animals in this world, they are by no means its only inhabitants. Champsosaurs survived the K-T event, and subsequently take the place of gharial-like fish eaters all over the world, except Australia. Crocodiles are less diverse in this timeline, but more varied. Familiar crocs and alligators prowl the waterways of the world, except in Africa, where their niche is taken up by dyrosaurs. There are even several varieties of terrestrial crocodilians, including pristichampsid "murder crocs" and sebecosuchians. Aquatic turtles are more diverse than in our time, but terrestrial turtles are conspicuously absent. The exception to this are the meiolanid turtles, ankylosaur-like chelonians found across Australia with a few genera in the New World. Birds, the sole surviving group of dinosaurs, have been quite successful in this world, though not as much as in our own. There are many groups of birds unique to this world or who have had greater success here than in our world. Pseudodontornithes soar over the seas as albatross or sea bird analogues. Giant anseriformes browse from the trees, as presybyornids take the place of ducks and other aquatic waterfowl. Terror owls, small animal hunters the size of a troodont and related to modern day owls, hunt for prey in the undergrowth across every continent except Australia. Mammals, while far from dominant, have also managed to become somewhat diverse. Multituberculates take the place of rodents and in some cases larger herbivores, alongside condylarths and pantodonts. Bats have evolved in this timeline, swooping overhead as insectivores. Some mammals have even become predators; leptictids and miacids hunt small mammals, birds, and squamates in the undergrowth.
Because mammals never really diversify in this world, numerous lineages of squamates take to the sea and become aquatic. Arguably, the most successful of these are the snakes, who fill the sea with hundreds of species, ranging from small ones to giant species that put our modern sea snakes to shame. Monitors too have tried to become aquatic once again, and patrol the seas and bays like reptilian sharks or the odontocetes of our world in search of fish and marine squamates. Some are semi-terrestrial and come on land to rest like seals, while others are entirely marine. Carnivorous squamates are not the only squamates to try and go marine. The ungulosaurs too developed their own aquatic lineages, the clownguanas and the sirenosaurs. Pseudodontornithes soar over the ocean like albatrosses, while true gulls live much like gulls always have. Like in our world, the seas are filled with huge amounts of plankton. And also like in our world, there are filter feeders that have evolved to take advantage of this marine bounty. Overall, the filter feeders can be divided into three general size classes. The first of these are the presybyornids, who range in size from a small duck to a swan or a small seal. The next are the marine arctocyonids, which range from seal to dolphin-sized. The largest, and most abundant, filter feeders of this world are the sharks, which vaguely resemble the basking and whale sharks of our world.