Shallow waters on continental shelves were the incubation sites for marine vertebrate diversity. By the Ordovician period (485–443 MYA), armored jawless fish several centimeters long (the size of a minnow) were swimming by tail propulsion, feeding by drawing water into their mouth and forcing it out their gill slits. Their variety became overwhelming.
Efficacious swimming is nontrivial. Water is incompressible. A fish moves through water by shoving it askew. This is done by wiggling back and forth in a snake-like motion: pushing water aside by the forward motion of its head – first to the left, then the right. Adaptively, the curve of the body and tail synchronously shove water with the utmost efficiency.
Water tends to return to its original position: flowing back along a fish’s narrowing sides, closing in at the tail. This helps propel a fish forward and explains why fish are fashioned with their specific shape, with fins for stability, balance, and turning.
The Cambrian brought onto life’s stage a sizable number of phyla in a short time. The mid-Ordovician saw a 2nd great diversification. This came by serendipity, on the back of meteorite showers.
470 MYA, out Jupiter way, an asteroid broke up, spraying bits down on Earth. The impacts destabilized seabed communities, opening opportunities for invasive species to displace previous residents.
The random culling by bombardment resulted in greater species mixing. This increased competition and promoted rapid adaptation. While impact events are often a storm of unmitigated doom, the relatively lightweight spatter from space prompted marine life diversification.
The 1st jawed fish appeared in the early Silurian period, 430 MYA; a way to bite past the armor of the boxed happy meals swimming about. This was a seminal advance in fish evolution. Besides the skull, the evolution of jaws was accompanied by changes in the shoulder girdles and fins.
Fish populations proliferated during the Devonian. Ample oceanic oxygen spurred growth in numbers and variety, especially predators. Marine reefs were as important to sea life diversity then as they are now.
Armor-plated placoderms were among the 1st jawed fish. They were early bottom dwellers that diversified into a variety of habitats, including freshwater, achieving worldwide distribution. Most were predators, some quite husky: up to 11 meters long. Placoderms are the oldest viviparous vertebrate.
Placoderms went extinct during the Devonian–Carboniferous (Hagenberg) extinction event. Bony fish fared better, as did sharks.