Episode 1: New Niches Opened Up

With new lifeforms emerge throughout the sea, opening up many new niches for which creatures can exploit.

Pelagic Zone
The ocean offers a massive food source in the form of billions of autotrophic microbes and many types of autotrophic multicellular organisms at the foundation, which is eaten by other creatures of various sizes and biology. These creatures are subsequently eaten, with one creature standing at the top of the oceanic food web.

One member of a very small hexapterid managed to survive through the extinction of their larger cousins. From them being very small means they do not need as much energy as their ancestor. They forage for planktons in the open water near some rock formations. Over time, they start exploiting this food source and live permanently in the open water. Due to their size, their population faces a massive mortality rate as even only a single notaspid passing by means the death of thousands of these creatures. To counter this dire situation, they further develop their sexual organ to produce massive amounts of gametes to keep the population high enough. As their size becomes so small that even they no longer able to resist the water current, their fins are repurposed to instead push the water toward the mouth while the tail fin becomes vestigial, virtually swimming in reverse. The gills opening move toward the dorsal side and eventually joined together. They also grow a pair of appendages lined with setae like notapids.

This new lifestyle coupled with their bizarre body earn their name Xenopterida ("Strange Fins").

During their time since emergence, pachypterids are a very adaptive creature, capable of sustaining themselves with many types of food sources. One clade specialized in feeding on the reemerging xenopterids and very occasionally the planktons, they sport a similar body structure as a normal pachypterid, albeit they are significantly smaller and their snout is shorter. Using the same strategy as their prey, they reproduce at a very surprising rate. They also travel from places to places in a massive school consists of hundreds or even thousands of individuals.

This significant change of body size gives their name Micrognatha ("Small Jaw").

On the other hand, one clade of pachypterid, who specialized in feeding on larger nektons, evolves to take on even larger prey. They become larger and longer, their tail stiffens with muscle fibers supported by the "bony core" present in pachypterids. From their tail become stronger, able to provide enough thrust from the tail alone, their six fins shrunk in size, with the two back fins becoming very small while the four front fins are still large enough to provide a burst alongside the tail. From feeding on larger prey, their jaw widens and becomes thicker. In addition, the jaw grows sharp, bony, tooth-like extensions to effectively wound the giant prey.

From these changes in their prey of choice, they become fearsome predators of the sea known as Carcharognatha ("Serrated Jaw").

Megafaunas of the Ocean
With the virtually infinite sources of planktons in the ocean, the filter-feeders become even larger than they have ever been.

From their originally large size, this clade of notaspids becomes even larger thanks to their massive energy intake possible by their size. They are so big that they reached the limit where their gills can support them with enough oxygen and none but one predator dares to attack them. They also sport a massive chitinous skute comprised of several smaller pieces.

From their massive size and carapace, they earned their name Baraspida ("Heavy Shield").

At the same time, another clade of notaspids evolves differently, leading them to fill in the missing niche. Their shells become articulated with each piece completely separated from each other, enabling them to move more freely. In this clade, some members' shells become mineralized, increasing the hardness and durability of the shells. This enabled them to tackle large faunas in the ocean. Their appendages are now lined with hard setae instead of baleens to tear apart their prey. Due to them used to be a filter-feeder, they have mild gastric acid, causing them to occasionally face indigestion. They develop another pair of appendages with a pincer on each end to tear the meat into smaller pieces before being consumed.

From their shells become mineralized, having a bone-like property, and will be called Osteaspida ("Bony Shield").

During this period, a massive creature from a different origin also emerges. In the northern hemisphere where ichthyoidmorphs are abundant, some of them grow larger and larger, rivaling that of the gigaspid. They also develop a different, more efficient method of performing Aethersynthesis, which makes them able to grow so large. Instead of passively intake the needed gases, they develop a massive mouth-like part capable of open and close. By opening up and gulping the air, sending it to the core in a one-way tract, Aethersynthesis can occur rapidly as the decayed Aether Oxide releases their components, which are used in another cycle of Aethersynthesis and respiration respectively, though some of the Aether is used to make their Calcium Aethide armor. Their dorsal fins shrunk in size and the trachea system disappeared due to not being used anymore. Their tail, however, grows in length and, like the carcharognathans, stiffens with muscle fibers to provide enough thrust to move the creature.

The creature's massive yet slow lifestyle earning their name Cetioidmorpha ("Whale-like Form").

Benthic Zone
On the seafloor, the situations are quite different. The autotrophic microbes which are the foundation of marine food chains are not as high as at the water surface. However, large amounts of detritus falling from above, the marine snow, and sometimes carcasses of various creatures provide enough nutrients to sustain the benthic life.

Phyllostome lineages are the most abundant benthic organism found on the planet. They normally feed on small food particles floating in the water. However, as the motile creature diversify, giving birth to creatures of various sizes, some of the phyllostomes find a way to exploit this. By using their already sensitive appendages to wrap around a passing by small creature and holding them in place. When the creature dies of starvation and starts decaying, the phyllostomes then absorb the nutrients through their appendages. Over time, they evolve the appendages into grasping tentacles lined with hard setae to hold the prey more effectively. They also evolve a body cavity that produces gastric acid to digest the prey, the leftover is then expelled out of the same opening.

By these changes in lifestyle, they climb the trophic level and become a predator known as Onychochirida ("Clawed Hand").

On the other hand, some creatures evade the dangerous open water and seek shelter on the seafloor. Coming from the same evolutionary line as the osteaspids, these small creatures are only several centimeters long. Their fins become strong limbs covered in jointed chitinous shells used for digging into the sand. Being their close relative, they have several similar traits as the osteaspids, such as the second pair of appendages that act as mandibles and the articulated shells. Unlike the osteaspids, however, they are a generalist, feeding on anything they can find, ranging from piles of detritus to the leaf of polyphyllophytes and bulbochemophytes' gametophytes and sometimes carcasses. They can tear through the tough polyphyllophytes thanks to their mandibles and with the help of their strong pedipalps lined with small spikes. These creatures can curl up to protect their underbelly which only covered in soft shells while using their legs to hold onto themselves. They can also live nearly everywhere, from shallow to deep water even deeper than the large organisms in the ocean. Those that live in the shallow water may find themselves in the intertidal zone, which can lead to land colonization.

These creatures are given the name Helicaspida ("Twisting Shield") from their ability to curl up into a ball.