Notaspida

Notaspida (meaning 'back shield') is the clade containing all multicellular organisms with hard exoskeletons, mainly on their back.

Notaspid evolves from hexapterids that are more specialized in feeding on planktons. This feeding method grants them virtually inexhaustible food sources, easily making them becomes even larger due to enormous energy intake. However, due to their rather slow speed, they are an easy target for predators, especially at a young age. They thus evolve hard chitin scales covering their body, which fused into hard armor plates over their back.

Some of them becoming more keen on larger, chunky food such as phyllostomes' arms, opening them up to a new niche that none have ever utilized yet.

Anatomy
Similar to their ancestors, their bodies are bilateral symmetry, with the four pelvic fins move closer to the front of the body and align with the pectoral fins.

Armor Plates
To protect themselves from getting eaten, notaspids evolve hard scales made off chitin covering their entire body. These scales, over time, fused together, becoming armor plates. These armor plates offer even more protection in exchange for somewhat limited movement. Later species evolve smaller, overlapping armor plates, which in turn increasing movement possibilities.

Appendages
Notaspids have two appendages near the mouth lined with tightly-spaced setae to filter through the water. The appendages are then moved over the mouth so that the food particles are swallowed. The mouth also widens to take in more food particles and is capable of open and close to some degrees.

Tail
Although they don't need great speed to hunt their prey, notaspids have a large tail with powerful muscles. This tail is capable of powerful speed burst and strong physical attacks. The size of the tail makes it easily stun any predators that dare to attack it. The speed burst from the tail could easily leave the predators behind which is used by the juveniles.

Nervous System and Sense
Notaspids retain all of the nervous organs their ancestors, hexapterids, have. Their eyes, however, changed. In addition to their main frontal eyes, they have a pair of smaller, fully functional eyes located on the side of the head near the main eyes. These additional eyes grant them a larger field of view, enable them to see behind without having to turn their head and react quickly.

Behavior
Notaspids are generally a docile creature, minding their own business yet they are very mindful of their environment. They swim slowly while filtering planktons out of water. However, if they feel threatened, they will swim away with incredible speed, sometimes striking the threats with their tail before leaving. They are so fast that nothing at that time can chase a fleeing notaspid.

Male notaspids are nomadic, venturing all over the ocean, while the females have their own territory near the shallow water, though they are not territorial and won't mind having their territory overlapped by another's.

Reproduction
Unlike their ancestors, during mating season, instead of flocking at the shallow water, male notaspids will seek out the females. When met, they then move to the shallow before proceeding to spawn. The male then leaves the female's territory.

Larvae
The larvae of notaspids develop very fast. The juveniles become resembling the adult form within only a few months after fertilization. The larvae live in shallow water and they are capable of digging into the sand using their fins. They gradually move to the deeper water as they grow bigger.

Competitors and Predators
Similar to their ancestors, competitors of notaspids are themselves. They are the megafauna of the sea. Their immense size and body armors make nothing able to mortally harm them. However, this does not stay true to the larvae. The larvae are very small compared to the adults, thus they are easily killed. The larvae are generally hunted by their cousins, the pachypterids.

However, there soon emerges a creature capable of hunting them.

Diet
Notaspids feed on billions of planktons in the water. Due to the enormous energy intake, their bodies grow to a very large size, dwarfing all other species on the planet. As they are taking the same niche as their ancestors yet they do better, their ancestors, the hexapterids become extinct within several million years after their emergence.

Distribution and Habitats
Notaspids can be found throughout the ocean. They normally stay in the shallow water, filtering planktons as they go. However, they also reside in deeper water, where they generally sleep for about a quarter of a day.