Chondrichthyes
Lamniformes There are seven families in this order.  They are commonly referred to as the mackerel sharks.  They include the goblin shark, basking shark, megamouth, the thresher, mako shark and great white shark.  They are distinguished by their large jaws and ovoviviparous reproduction. The Lamniformes contains the extinct Megalodon (Carcharodon megalodon), which like most extinct sharks is only known by the teeth (the only bone in these cartilaginous fishes, and therefore are often the only fossils produced).  A reproduction of the jaw was based on some of the largest teeth (up to almost 7 inches in length) and suggested a fish that could grow 120 feet in length.  The jaw was realized to be inaccurate, and estimates revised downwards to around 50 feet.