Geological and biological history of Zebia

Geological history

The break-up of PanZebia

Zebia, circa 350 million years ago. Almost all landmasses are comprised in PanZebia, a large supercontinent with its center just north of the equator, near the current position of Trita's northeastern tip. It is mostly oriented in north-to-south direction, with a huge bay in the northwestern corner. Much of PanZebia is covered by shallow oceans as there is neither land nor ice at either pole at the time, and sea levels are ~150m higher than currently. A perfect environment for amphibians, of which there is an amazing variety.

The northernmost part of PanZebia soon splits off and moves north by northwest. It will be called Proto-Pempta. Around 30 MYA later it is sufficiently near the north pole for an ice cap to form, which causes a drop in sea level and enhanced evolutionary pressure on the amphibians on PanZebia. Some of them become capable of surviving in drier environments and evolve into primitive reptiles.

About 300 MYA ago, the southeastern edge of PanZebia breaks away and moves east by southeast. This will later become the southeastern portion of Protera, but for now it is a small continent of its own, called Southern Proto-Protera.

Around 280 MYA, the rest of the supercontinent splits in two halves, starting in the northwest. The eastern portion, which encompasses today's Deutera, Pempta, southwestern Protera, and northern Trita, slowly moves north, rotating clockwise. It will be called Borea for now. The western portion, a long but rather narrow stretch of land consisting of today's Tetarta, northeastern Protera, and southern Trita, moves west by southwest and turns counter-clockwise. It will go by the name of Austra. All continents except Arctica carry both amphibians and reptiles, although only Austra has diapsids (the ancestors of dinosaurs and birds), most of them in the southern regions.

The age of small continents

Soon after this final split of PanZebia, around 250 MYA, the southern portion of Borea breaks away and heads southeast, moving towards the equator and the southern hemisphere. This will eventually be the southwestern portion of Protera, and go by "Western Proto-Protera" for now. It carries several species of synapsid reptiles that are at this time rather closely related to groups on both Austra and Borea, but will evolve into a separate class during 180 MYA of isolation, a class that doesn't exist on Earth. These animals will end up on Protera and Tetarta.

The rest of Borea continues to the northwest and collides with the small polar continent of Proto-Pempta around 210 MYA. However, these continents do not completely fuse, but rather slide alongside in opposite directions. The rock shapes resulting from this slide are rather bizarre. Finally, around 60 MYA these continents separate for good; Pempta now dropping the "Proto-" from its name and drifting southeast.

This implies that the flora and fauna of Deutera and Pempta is roughly similar at the order level, but distinct in further subdivisions. As for animals, there are synapsid reptiles in all kinds of shapes, filling most of the available ecological niches. As the continents have both been near the north pole for a long time, fur and feather-like scales to save body warmth was evolved early on. Many of these reptiles look rather similar to certain types of mammals, but they belong to a completely different class.

In the meantime, the southeastern third of Borea proper has split off (around 180 MYA), moving south under the name of Proto-Trita. Accordingly, Trita has a few synapsid subclasses (but not orders or families) shared with Deutera and Pempta, but not with the other continents. From this time on, the rest of Borea is called Deutera.

Formation of the modern continents

In the southern hemispere, Austra collides with Southern Proto-Protera around 160 MYA, leaving behind the cleft to the east of the inland sea. In most other places, no clear plate boundary is discernible. The CCW rotation of Austra accelerates because of this collision. Its southeastern tip breaks off around 140 MYA and moves east, colliding with Proto-Trita by 100 MYA.

By 80 MYA, Western Proto-Protera joins Austra, forming modern Protera.

The splitting of Austra occurs shortly after the emergence of mammals, and also after the divergence of archosauria vs. lepidosauria. Accordingly, all southern hemisphere continents have both mammals and dinosaurs. However, the course of evolution is different from one continent to the other. On Protera there are rather few dinosaurs, and they never reach the size they had on Earth, due to evolutionary pressure from other types of reptiles (e.g. lizards) and mammals. There has been no single mass extinction event, so several archaic clades of animals (both reptile and mammal) survive to date with a few rare species. In general, Protera has been dominated by mammals for at least 90 MYA, about 150% the time this has been the case on Earth. This leads to a lot of diversity.

On Trita, on the other hand, dinosaurs had the upper hand and soon became the dominant group of animals. They did not become quite as huge as a T-Rex or a brontosaur, given that the continent is rather small and Zebia has slightly more gravity than Earth. But then, elephant-sized is still big, and there may have been larger ones... And, like on Earth, they evolved into birds at a point. About 30 MYA the first birds managed to cross the ocean to Protera, and about the same time the dominance of dinosaurs began to wane. Mammals had their chance, and succeeded in taking it. Today, Trita has large species of all three classes. Dinosaurs have become rare and are not bigger than, for instance, a hippopotamus, at their largest today, but there's still a stunning variety of them, compared to relatively few species of mammals with high numbers each.

Where there was once a spreading sea floor between Deutera and Protera there is now an ocean plate being subsumed on almost all sides. Also gone without a trace are the spreading zones that separated Trita and Deutera and Western ProtoProtera from PanZebia. Most of the current spreading zones are new. Part of northern Protera splits off beginning 40 MYA, creating Tetarta. A smaller zone north of Trita creates the volcanic island chain north of that continent. Both of these zones push Protera and Trita further south, creating the circumpolar mountains.

Biology

Neither Deutera nor Pempta have native mammals. Both continents do have a few species of birds, evolved as their ancestors flew across the ocean. All in all, the fauna of the northern hemisphere is dominated by synapsid reptiles, most of them with fur. Many of them look rather similar to mammals, but they belong to a class that did not survive on Earth (except for those that became mammals in the first place), and there are also lots of strange creatures among these synapsids. The only continent that was ever dominated by dinosaurs is Trita, and this is also where birds evolved. Some smaller dinosaurs may still exist there, but Trita also has several native mammalian species. Nevertheless, it is Protera that has the greatest variety of completely different types of animals, and it is Protera where mammals originated.