Pachypterida

Pachypterida (meaning 'thick fin') is the clade containing all multicellular organisms with thick and muscular fins capable of powerful darting attacks.

Pachypterid evolves from hexapterids that are more specialized in feeding on small nektons. Early pachypterids slowly and quietly stalk their target, when within their reach, darting with a surprising speed and catches prey with their powerful jaw. The secret of their speed is their fins and body. Thanks to their gigantic, robust fins reinforced with thick muscle fibers and the hydrodynamic body shape, they are able to dart at their targets faster than they could react.

Some species, however, are more on generalist, feeding on all sorts of objects it could digest. This includes phyllostomes' appendages and carcasses in addition to the normal diets. Some others, however, forsook the ambush technique, give chase to their prey then use their powerful fins to perform a lunging attack to catch the prey items.

Over time, the core muscle fibers become mineralized, increasing their durability and strength, which in turn making their darting attacks even more powerful.

Anatomy
Similar to all organisms within this lineage, pachypterids have bilaterally symmetrical bodies. Unlike their cousins, notaspids, however, their pelvic fins move away from each other, one pair moves toward the pectoral fins while the other pair moves toward the caudal fin, to the point of locating over the gonopore and anus openings.

Fins Arrangement
Unlike notaspids, pachypterids' fins are greatly developed for a "short" distance burst, covering nearly as much as four times the creature's body length. They also evolve to keep their fins very close to their body, both when swimming normally and after the initial burst so that the drag is reduced when darting at their prey. They can also turn and rotate their fin at a very large angle.

Gills
Unlike their ancestors, pachypterids have only one gill slit covering the gills supported by operculum-like muscles.

Mouth
To effectively catch their prey, pachypterids evolve an elongated mouth reinforced with muscle fibers and jaws lined with hard setae. However, some species may lack setae. They then swallow the prey whole.

Nervous System and Senses
Their nervous systems also need to improve to cope with this lifestyle. Their eyes become bigger and more complexed to better see in darkness as they normally reside in the deeper water. This also makes them able to hunt at night. Their nerve ganglions also develop further as they have to patiently stalk the prey.

Behavior
Pachypterids are intelligent creatures. They learn to swim quietly to stalk their preys and dart out quickly on their own. They need to learn this quickly as it is the only efficient hunting method in the environment filled with creatures with an acute sense of sight. They sometimes have to give chase a bit on their preys, leading some of them to pursue their preys instead of ambushing them. They also hunt at night, emerging from the deep sea in search of stray creatures within the darkness of the night.

Adult pachypterids live in a group of around ten individuals. They will fiercely protect their territory from other groups.

Reproduction
Like their ancestors, pachypterids spawn in the shallow water within their territory, though the young are left to take care of themselves since their first second after hatched. Sometimes, a rogue male barge into the other's territory during the spawning and release its sperm alongside the natives before quickly leaving the scene.

Larvae
Similar to notaspids, pachypterid larvae live in the shallow water within their parents' territory, feeding on planktons. They also tend to group together into a small school within their parents' territory for protection. Once the young have grown to a certain size, they will leave their parents' territory and form their own territory with the youngs from the same or other groups or risk getting eaten by their parents. Sometimes, however, some of the young stay with their parents.

Competitors and Predators
Like all creatures during the same period, pachypterids have no other competitor but themselves in their niche. They generally hunt other hexapterids, contributing to one of the factors that cause the hexapterid to went extinct. The young, however, have quite a hard time surviving until adulthood since they are too small that they easily fall prey to other creatures.

Diets
Pachypterids feed on small nektons, such as the juveniles of other hexapterid species. However, hexapterid lineages are well-known for a keen sense of sight, so the pachypterids have to approach them without showing themselves. By stalking prey from below, then dart out when within range.

However, larvae of pachypterids are too small and their jaws are not strong enough to efficiently kill prey larger than themselves. The larvae have to resort to foraging the sea for planktons, eggs, and larvae of other species. They sometimes feed on a corpse of other creatures, or even tear phyllostome appendages off or try going after a juvenile notaspids. This, however, is also done by the adults when they can't find a prey.

Distribution and Habitats
Pachypterids are widespread since their emergence like the notaspids. However, they generally reside in the deeper water, where they can quietly stalk their prey at any time.