Stonesharks, family Perdideruntpedes

“I’m just a simple guy, swimming in a sea of sharks” -Don Johnson

From the insects known as the stoneflies comes a new legacy. Perdideruntpedes, the stonesharks, the ones that would fill the void left by the fall of the fish, one of the world's most ancient lineages. Stoneflies are small, unassuming insects that lay their eggs underwater, which then hatch into aquatic nymphs, then pupate to come onto land. But through a process known as neoteny, in which an animal abandons its adult, or imago, stage and becomes a pure, adult larva. Through neoteny the stoneflies have adapted to an aquatic lifestyle, losing their limbs and moving out to sea.
The Stune, (Quiprospicit Terramaris) is a 2 foot long schooling stoneshark that inhabits the massive Atlantic ocean of the Microcene. Feeding on squid, krill, and other, smaller stonesharks, this animal is near the bottom of the food chain. But just because it isn’t gigantic or horrifying doesn’t mean it hasn’t had amazing changes since its start as a freshwater insect. While visually it looks like an ordinary fish, its segmented body is covered in chitinous plates, and its six legs have adapted into flattened paddles. It has six sets of mandibles, each adapted for a different purpose. The two large flat ones trap the food, while the four smaller ones grind the food up for consumption with its bristly mouth. Its antennae have moved to the bottom of the head and are used similarly to the lateral line on fish, used to sense movement in the water. The third to last segment of the body has flattened and elongated, becoming a tail fin used to power most of the movement. But from the bottom of the food chain, there's nowhere to go but up. And up is where this creature is headed.
In the freshwater swamps of North and South America lives the Shearhead, (Faciesfices Etterrores), an ambush predator that is the link between the stoneflies of the land and the stonesharks of the sea, it forms its own evolutionary group, the stonecrocs. With legs that terminate in large blunt claws, antennae that poke above the water’s surface, and a massive mouth like a set of gardening shears, this creature has evolved to fill the cushy niche once filled by crocodilians. When an unsuspecting animal comes to drink, it lashes out with its first pair of mandibles, equipped with hooked spines to pull the animal closer. Once in its grasp, it slashes the prey's face with two sets of massive curved mandibles that move independently. Instead of trying to drag it underwater like the crocodilians of today, this animal simply slashes the animal to pieces in one rapid attack, and then feeds off of the shredded remains. Its tail lacks the fin of its oceanic cousins, instead having a long, thick tail used to stabilize itself when attacking, and to act as a large paddle for slow swimming.
But the shearhead of the freshwater is no match for one of the apex predators of the Spinas Mundi seas, the Bulkhead (Ingensunum Capiteferro). This goliath is 20 feet long from jaws to tail, attaining the largest size any arthropod has achieved since the Sulurian eurypterids. With its only competition coming from large krakens, settlements of Sculptors, warheads, and other bulkheads, these animals are kings of the sea. With massive serrated mandibles that can exert 11,000 newtons of force, a massive streamlined body with inch thick armor, and antennae tuned to the slightest movement, this stoneshark has completely lost its link to the land. But in order to grow so large, it had to change its insides as much as its outside. Its less effective open circulatory system, a holdover that arthropods have kept since the beginning, has been replaced by a closed circulatory system, more reminiscent of vertebrates. The way the bulkhead breathes has changed as well, with feathery gills hidden in the breaks between armor plates, again shifting more towards the design that fish have taken.
The Warhead, (Altavenatur Insarcinis) has taken this vertebrate lifestyle one step further with the evolution of mandibles that can move vertically as well as horizontally, making them one of the first insects to do this since the hell ants of the Cretaceous period. This descendant of the bulkhead hasevolved into a fast and intelligent pack hunter, using its sophisticated attack strategies to take down even the largest and most powerful prey. Floating horizontally in the water column like a pod of sperm whales, the pack, which can be up to 50 strong, waits patiently for a prey animal to swim above them. Once it is above them they burst upward, each grabbing a chunk of flesh with their serrated mandibles and ripping it off as they race upward. Once the meat has been swallowed they curve around backwards towards the animal and continue the process until all the meat has been sheared off. This attack pattern creates a distinctive mushroom cloud shape, hence the name.
While the pacific ocean continues to shrink, the volcanic activity on the subducting seafloor spews massive amounts of minerals and nutrients from the sea floor, creating a final explosion of life for this doomed ocean. The huge amount of speciation triggered by this has left plenty of opportunities for life to become increasingly odd. And few are odder than the Yanagiba, (Iaponica Cultromaxilla), a species of stoneshark found only off the coast of Japan. This long, thin creature has massively extended its largest pair of mandibles, which are now almost the length of the rest of the body, and is edged with deadly serrations. It swims slowly into schools of smaller stonesharks, and then rapidly opens and closes its mandibles like a pair of scissors, slashing the prey to bits. Its other four arms act like nets to scoop up the bits of food and shovel them into its mouth.

Family: Perdideruntpedes
Distribution: Oceans, worldwide
Number of Species: 2,100, various subspecies
Ancestor: Stoneflies, Plecoptera
Average size: 7 feet long
Common traits: Grey exoskeleton, multi segmented body, six paddle-like legs, six pairs of independent mandibles, downward facing antennae.
Reproduction: The hundreds of eggs are fertilized externally and join the planktonic soup until they hatch and mature.

Art by Crustaceous Rex