SpecTember 16: Erased Chapter

The so-called "Cambrian Explosion" is often thought of as the first time life on Earth became truly complex and diverse, but as with so many other things commonly assumed, this was not the case. The Precambrian was longer than the rest of Earth's history put together, and given how long life existed during that time, it was perhaps inevitable that complex life would arise more than once before its current incarnation. One of the best-known examples of Precambrian complex life is the bizarre fossil animal Sprigginia, which resembles a cross between a worm and a trilobite. What the fossil record did not show, however, was that Sprigginia had larger relatives. Much larger. Eonatator giganteus was a distant relative of Sprigginia, which grew to the size of a dolphin. 

It was apparently a filter-feeder, having its limbs arranged in a basket-like structure to sieve plankton from the water. If so, this would have made it the first animal ever to occupy such a niche, pre-dating baleen whales by hundreds of millions of years. Equally remarkable were the hydro-dynamic winglike protrusions on either side of its body. Unlike its limbs, these were not flexible, and seemed to have functioned solely as stabilizers, keeping the animal steady as it glided through the water. Further stability was added by a flat, fan-shaped fin at the tip of the "tail", which may have also served as a unit of propulsion. At up to eight feet long and weighing an estimated one hundred pounds when alive, Eonatator would have been the first animal to reach truly megafaunal sizes.