Marking the end of the Paleozoic Era, the Permian Period witnessed several important evolutionary firsts—most notably the diversification of the two primary groups of amniote vertebrates, which, today, includes the mammals, birds, and reptiles. In the Permian, however, the amniotes were represented by early representatives of the synapsids and sauropods. The synapsids are often described as mammal-like reptiles, like our saber-toothed gorgonopsids that could reach the size of modern bears, though they may more appropriately be considered stem mammals. The sauropods, on the other hand, were indeed reptiles, like our swimming tangasaurid, eventually giving rise to true dinosaurs in the Mesozoic Era and the reptiles and birds we know today. Also swimming up-close and personal is the strange, shark-like Helicoprion, known for its unique, spirally arranged tooth whorls. These fish had cartilaginous skeletons like modern sharks but were huge—reaching over 10 meters in length, about the size of modern basking sharks (for comparison, great whites today are only about 6 meters at their largest). In the backdrop, the large interior continental rainforests of the Carboniferous Period were collapsing, being progressively replaced by early conifers and seed ferns, leaving the swamp-native lycopod trees to restricted equatorial coastal environments along the Paleo-Tethys Ocean coast. As can be seen in the flora of the background, cycads, a group of woody gymnosperms that resemble palms and ferns (but are related to neither), make their first appearance at the very end of the Carboniferous or earliest Permian, but remain rare until several tens of millions of years later. While life had been flourishing throughout the Permian, that was soon to change as the period drew to a close. The end-Permian mass extinction, also known as the Great Dying, was the most severe extinction event that the Earth has witnessed—with over 80% of marine species and 70% of terrestrial vertebrate species ceasing to exist.