Finally! An Ancient Fish That Understood Life's Terrors | Defector

"If life in the oceans seems scary now for anything small and soft-bodied, they were undoubtedly scarier 518 million years ago in the Cambrian period. For eons, the oceans belonged to filter feeders, which raked in plankton and other animalcules rather peacefully (for everyone except for the plankton, of course.) But over time, the oceans gave rise to large, carnivorous predators, which meant little fish needed to adapt if they were going to make it out of the Paleozoic period alive."

"Myllokunmingiids, the earliest fossils that look anything like fish, were discovered in China and hail from around 518 to 530 million years ago. One species of myllokunmingiid called Haikouichthys was, scientifically speaking, just a little guy, topping out at about an inch long. Like a modern fish, Haikouichthys had a head, a modest array of slits that looked like gills, and a distinct muscle-bound spine. Unlike modern fish, it lacked a jaw."

The Cambrian oceans transitioned from dominance by passive filter feeders to ecosystems containing large, carnivorous predators, creating strong selective pressure on small animals. Myllokunmingiids represent some of the earliest fish-like fossils from about 518–530 million years ago. Haikouichthys measured about an inch long, had a head, gill-like slits, a muscle-bound spine, limited fins, and lacked a jaw, possessing a conical mouth opening similar to lampreys or hagfish. Exquisitely preserved specimens show two larger eyes outside the head and two smaller median eyes, with all four eyes containing melanosomes involved in coloration and light absorption.

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