Speculative island fauna of the mesozoic
I Pseudosauropod paraxenos, mid triassic - Native to an unnamed island off the south-western coast of Pangea, Pseudosauropod might be the first large dinosaur in history. Descended from small insectivores that dispersed there through drifting it inhabits an island never reached by pseudosuchians or synapsids, which allowed those small creatures to quickly fill the megafaunal niche and become large herbivores. Despite it's close resemblance to sauropodomorphs it is actually a basal theropod hovewer at this point in time it hasn't been long since those two groups split. Ultimately an evolutionary dead end, it is nevertheless a forecast of things to come.
II Melisodon nakamuri, late jurassic - While almost all non-mammalian synapsids vanished at the end of triassic, few non-mammal cynodonts managed to make it through into the jurassic. Among them was a lineage of the very last cynognathids, still clinging on in Antarctica. As time passed, some of them found themselves on an isolated island chain off the southernmost continent. On the islands they flourished and diversified into many different niches, thriving long after the extinction of their brethren on the mainland. They survived up until the jurassic-cretaceous boundary, when environmental changes ultimately led to their demise, making them the last non-Probainognathian synapsids to have ever lived. Melisodon, a relatively large omnivore of the end jurassic is one of the last species in the lineage.
III Adamatherium kynodontis, mid cretaceous - Animal hailing from a sizeable island in eastern Tethis Ocean. This land has no non-avian dinosaurs, however it was reached by several mammals from few different clades, that dispersed there by drifting. With the limitations posed by archosaurs largely gone, small furry creatures quickly evolved to fill almost all available niches on the island. From weird specialised omnivores, through megafaunal grazers to apex predators. This small kingdom became a "cenozoic before cenozoic", showing how quickly animals can evolve and flourish if given a chance. Adamatherium is a herbivorous multituberculate, and the largest animal on the island. Reaching the size of a cow, it is the largest mammal to have yet existed, and it will only be matched after several tens of millions of years.
IV Balearosaurus parvus, late cretaceous - This small iguanodont is a fascinating case of convergent evolution to Balearic Island cave "goat" (a real, bizarre animal killed off by humans just few millenia ago). It's story began when their small island separated from the landmass of future Spain, isolating few animals there from the outside world. Balearosaur's ancestors found themselves in a very small place, with no predators and almost no competition, but extremely limited resources, which demanded extreme adaptations. Over time their legs became shorter and weaker, with the animal losing it's ability to run, as it would be an unneeded waste of energy in this predator free environment. Lack of predator also caused another adaptation - forward facing eyes, an unnerving sight in a herbivore. Due to very limited access to calories they became much smaller from their ancestors, and developed an extreme adaptation - ectothermy. With slower metabolism and body heat controlled by external temperatures the animal can conserve more energy, making it perhaps the only cold blooded dinosaur in history. Yet eventually the island moved closer to larger landmasses again, meaning it became available for frequent visits by large azhdharids. With a sudden reappearance of predators Balearosaurus vanished in just a few generations.
V Contritodactylus maxima, late cretaceous - It is much rarer for flightlesness to evolve in pterosaurs that it is in birds, but with the right conditions it can still happen. Contritodactylus is a giant flightless tapejarid, native to a large but really isolated island in the Pacific. It's ancestors were lightweight frugivores, caught in a tropical storm and carried off course into the fated land. Here they found themselves in a place with an abundance of food but basically no land predators big enough to pose a serious threat, maybe with an exception of some large lizards. The island was also the only food source within a flying range. Those conditions caused many descending species to lose their ability to fly, and fill land dwelling niches. Contritodactylus is a specialised browser, with it's massive beak used to strip branches of fresh leaves, pick and crush fruit and crack open nuts and seed pods. It is also the heaviest pterosaur on the planet at that particular time period.
VI Oculiavis tetraplis, late jurassic - Close relative of the famous Archeopteryx, this flying dinosaur lives on a smaller tropical island. Their round, large faces might help them in locating their prey by hearing, since a large portion of Oculiavises diet consists of insect larvae hiding beneath the tree barks. However it is believed by some that the actual main function of this dinosaur's face is recognizing different individuals and intraspecific interaction. Indeed the most unique aspect of this animal is it's intelligence, with Oculiavis regurarly using tools to access their food and displaying suprisingly complex social interactions.
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