-Darren Naish's staement-
Firstly, we have Squamate World. Mammals never really get going inthe latter part of the Cretaceous, and while amphibians, turtles,birds and crocodilians continue as normal, it is squamates that come to dominate the post-Mesozoic biosphere. Herds of millions of herbivorous agamids dominate Eurasia and Africa, and giant armour-plated (but still sprawling) omnivorous skinks and mastigures inhabit scrublands, deserts and uplands across the Old World. Lacertids and skinks evolve unparalleled diversity, with small insectivores and mid-sized generalists occurring across all environments, from forest floors to tree tops.
Giant predatory short-skulled amphisbaenians prey on large surface-dwelling animals [adjacent image, from Digimorph, shows the 10 mm skull of the predatory amphisbaenian Anops kingii. Imagine enlarging this animal so that its skull was 30 cm long]. Crotaphytid iguanians in the Americas evolve erect gaits and endothermy, and become long-legged giant cursorial predators. Across North America and Eurasia, diverse venomous helodermatids and big geckos skulk around at night. Big terrestrial and scansorial anoles dominate the American tropics and big bizarre iguanians are spread across the islands of the Pacific.
The lack of amphibious and aquatic mammals means that several squamate lineages take to the seas, with Mediterranean lacertids, Australasian skinks, Pacific iguanines and American anguids all evolving amphibious and aquatic forms. The seas are filled with untold billions of sea snakes, and not just small ones, but gian tones too. Boids and their kin never go through an Oligocene-Miocene 'dark period' and Eurasia and North America are full of sandboas and related forms, and colubroids never really took off because rodents didn't; instead small, gracile boas and pythons undergo aradiation and mostly prey on tiny diverse lacertids and geckos.
Varanids in SE Asia and Australasia, and iguanines in South America,evolve endothermy and enhanced encephalisation. Short-faced arboreal iguanines evolve increasingly complex societies and cultures from the Miocene onwards.
Now, a little background and building on this staement.
_Prehistory and other tetrapods:
Turtles are mostly similar to the present arrangement, sea turtles are slightly more diverse, and land tortoises less so.
Sebecosuchians have a Sparassodont-like boom in south america during the earlier part of the tertiary. These are largely extinguished with the faunal interchange in the later trtiary, out-competed by large teeids and monitors. Present crocodylians are even less diverse than in our timeline, restricted to typical aquatic lurkers and opportunists. Gracile snouted piscivores are absent, that having gone to champsosaurs.
Birds remain conservative, the advent of large terrestrial varanoids suppresses the development of game-birds and waterfowl. Game-birds larger than quails are absent, waterfowl are mainly present as small, dabbling ducks. Presybyornithids manage to persist into the present, hinging on the lack of filter feeding in most squamates.
Ratites decline into near extiction during the oligocene, except for in new zealand, where they produce flightless probers not unlike kiwis. Most seed and nectar eating birds are basically passerines. Piciforms exist as small barbet-like frugivores and some small woodpeckers, kingfishers do not produce kookaburras but remain as small-fish specialists. Crows and magpies are absent. Gruiforms onlyproduce small rails and crakes, chaardriforms remain coastal probers.
Petrels, terns and gulls become mainly specialised for oceanic piscivory and plankton eating on the wing. Storks and herons are mainly absent. Swifts and swallows are everywhere in the air. Cuckoos exist as typical nest parasites. Small loons, cormorants, and grebes are sparse but present. Humming birds are basically the same. Owls, eagles and falcons only exist as very small mammal catchers and bird chasers in flight. Parrots remain small but diverse. Pigeons and sangrouse are everwhere as arid seed eaters.
Mammal groups are severely lacking in diversity, most groups neverarise. Shrews and other tiny insectivores of Euarchodont, Cimolestan, Leptictid, Paleanodont (including mole-like diggers), Afrothere(including elephant shrews), Xenarthran and Primate affinities makeup the bulk of the group. Multituberculates get a foot hold and are widespread but low-diversity (and slow breeding) small herbivoresand granivores while glires remain as Anagale-like generalists, not producing rodents or lagomorphs.
"Condylarths" have a minor vogue ashyrax like herbivores before declining into extinction in themiocene. Marsuipials and other metatheres are widespread worldwideas small, arboreal insectivores and omnivores, like some neotropical opossums. The only group that is anything like the presnt day are an minor radiation of small pangolins in indonesia.
Fish are almost exactly the same, with the exception that teleosts have not produced as many advanced forms like puffers, triggerfishes, or sygnathids. Sharks are much the same, but with some freshwater species. Lungfish, Bowfins and Garpikes choke up many slow moving freshwater ecosystems. Amphibians experience similarly conservative advances. The diversity of frogs and salamanders are similar, Caecelians have slightly higer diversity as aquatic swimmers.
Up next, we'll work out the ruling lizards, group-by-group.