and horn corals, they are replaced by the scleractinians, forms that will persist to our time.
As interesting as the invertebrates and even the fish are of this world, it is the larger vertebrates in the sea that really give pause. Diverse reptilian stocks have obviously returned to the ancestral ocean. Some, the stocky placodonts, resemble large, clumsy seals as they swim down and root through the clam beds with their peg-like teeth. But the stars of the show are the ichthyosaurs, reptiles that have so thoroughly evolved for a swimming habit that they have lost their legs entirely and are the most fish-like of originally terrestrial vertebrates that the world has ever known or will know until the eventual evolution of dolphins, far, far into the future from this Triassic time. While many are small, there are larger forms too: Mixosaurus is 30 feet long but it is dwarfed by the monstrous Shonisaurus, some of which reach a length of 60 feet. This huge ichthyosaur rivals sperm whales for the title of world’s largest aquatic predator of all time, and it preys at will on hosts of fish and smaller reptiles. It has dinner-plate-sized eyes, the largest eyes ever evolved either before or after this.
The marine world, at least, despite its strange and frightful beasts, strikes a sense of some familiarity. But even here we begin to notice strange behavior: the diving reptiles, like the placodonts, come to the surface frequently to breath, as do even the ichthyosaurs. The low oxygen takes its toll.
The oxygen story for the Triassic is stunning. Oxygen dropped to minimal levels of between 10 and 15 percent and then stayed there for at least 5 million years, from 245 million to 240 million years ago.
The officially designated early Triassic time interval was from 250 million to about 245 million years ago. During this time there was little in the way of recovery from the Permian extinction. There is also a very curious record of large-scale oscillations in carbon isotopes from this time, indicating that the carbon cycle was being perturbed in what looks like either methane gas entering the oceans, or a succession of small-scale extinctions taking place. All evidence certainly paints a picture of a stark and environmentally challenging world for animal life.