Phytosaur and Synapsid
After the Permian-Triassic extinction, the most important extinction to the dominance of the dinosaurs was the Triassic-Jurassic extinction. While there are many competing theories as to the cause of this extinction, its impact was undeniable. The pseudosuchians and phytosaurs (which were arguably the dominant tetrapods at the end of the Triassic) went extinct or into severe decline except for the Crocodiliformes; Conodontophora, non-ammonitid ammonoids, and dicynodonts went extinct entirely; and numerous other groups were decimated. This great turnover in terrestrial fauna allowed the dinosaurs (in direct competition in many cases with their distant crocodile-line relatives) to come to prominence, and most of the groups that defined the "Mesozoic Marine Revolution" evolved after this event. What would have happened if the Triassic fauna had lived to see the Cretaceous-Paleogene Mass Extinction? What if these great groups had not gone extinct?

Chronology and ClimateEdit

Triassic Period (251 to 182 million years ago): The Triassic extends into what is known in our chronology as the Early Jurassic. During the extended portion of the "para-Triassic" (201 to 182 million years ago), Pangea began to break up. The split between Laurasia in the north and Gondwana in the south created a tropical ocean that led to a climate much wetter than in the earlier epochs of the Triassic. Deserts receded, and diversity increased. The end of the Triassic in this world is marked by a minor extinction (in our timeline known as the "Toarcian Turnover") that principally affected life in the oceans.

Jurassic Period (182 to 145 million years ago): The Jurassic is somewhat shortened from that in our timeline, but the climate was more-or-less the same as in ours. Seasonal climates reigned supreme away from the true tropics. Pangea continued to break apart as the North Atlantic began to form. The end of this period is marked (as in our timeline) by a minor faunal turnover before the Cretaceous.

Cretaceous Period (145 to 66 million years ago): The Cretaceous began as a relatively cool, wet time in the history of the Mesozoic. Jungles dominated the tropics, and glaciers were even present in some of the high latitudes. However, the period progressively grew warmer and drier with increased volcanism. This is correlated with the continued breakup of Pangea. The end of the Cretaceous is marked (as in our timeline) by a mass extinction caused by a meteorite impact that led to the extinction of 65% of animal life on Earth.

Paleogene Period (66 to 34 million years ago): The Cenozoic era began with life rebounding from one of the largest mass extinctions in history. Unlike the Permian Mass Extinction, the continents were all comparatively isolated, leading to a very different configuration of animal and plant life. The climate began as very warm and wet, with jungles dominating even at high latitudes. However, the world became colder and drier as Antarctica moved into its polar position.

Neogene Period (34 million years ago to present): The Neogene continued the trend of increasing glaciation as well as the continents moving into their present-day positions. South America finally collided with North America just a few million years ago, and frequent land bridges opened between Eurasia and North America. The period of cooling culminated in the Last Ice Age, when glaciers extended as far south as what is the United States in our timeline. This debatably ended around 15,000 to 10,000 years ago, when the glaciers receded to their present day positions.

Present Day: The climate of the present in the parallel world is very similar to that of our timeline. The tropics are somewhat drier (the result of differing flora), and the world is slightly cooler (the result of the lack of human-induced global warming). In addition, some local ecosystems are radically altered due to the dominance of different organisms.


Without the Triassic Mass Extinction, many more groups evolved and flourished through the Mesozoic, so it was a very different world when the meteor hit. Through the Mesozoic, the Pseudosuchia dominated the role of the large predators. Birds (in their home Earth form) were never to evolve, and the most diverse dinosaur groups remained small, bipedal forms (with the exception of a few para-sauropods evolving in the Jurassic from prosauropods). Pterosaurs were much more diverse towards the end of the Mesozoic, and true squamates (lizards and snakes) were limited to some arboreal forms. Of course, all life was changed. Brachiopods became more diverse, and many groups of plants that never got the chance to diversity flourished. Of course, the K-Pg extinction culled diversity in a big way, and the world of today is as dissimilar from the Mesozoic as our home Earth's is.

Pterosaurs rule the skies, while the seas are dominated by various marine reptiles and unusual fish. On land, archosaurs of one form or another fill most every niche, from armored aetosaur grazers and prosauropod browsers to feathered neopoposaurs and running crocodylomorphs. Freshwater environments are patrolled by monster amphibians in Australia and true freshwater sharks in the Americas, while giant bowfin are the apex predators of Eurasia's waterways. Jawless conodonts are the scavengers of the oceans, ammonids are the pelagic filter-feeders, and brachiopods are the most common shellfish above the substrate.

Suggested OrganismsEdit

  • Common Unhyrax -- Ungulatacynodon commonus -- This small herd animal is actually a cynodont, not a crown group mammal. Their cheek teeth developed ridges which enables them to masticate fibrous material and their incisors are much longer, while their canines are used only for sexual display. They can run for long distances, and they developed hooves (indeed they are unguligrade) which absorb force while locomotion.
  • Sabrefang -- Machairidens piscivora -- In the absence of the T-J extinction, the cynodonts experienced a radiation, akin to the radiation of crown mammal groups during the late Cretaceous in our timeline. Eventually, one species became semiaquatic, using their elongated canines to stab their prey (which are fish). They possess a sleek bodyframe similar to archaeocetes and fin-like limbs. They only gather in land for breeding purposes.


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