At approximately 24,500 species, fish make up the largest group of vertebrates alive today. They come in a variety of different forms, from the diminutive minnow to the immense whale shark, and dominate rivers, lakes, and oceans worldwide. Collectively, fishes possess a hardened skull, limbs in the shape of fins that lack digits, and the ability to breathe oxygen from water by use of gills. There are currently five recognized classes of fish: the Osteichthyes (bony fishes), the Chondrichthyes (the cartilaginous fishes, such as sharks and rays), the Agnathans (jawless fishes), the extinct Acanthodians (“spiny sharks”), and the extinct Placoderms (the armored fishes). The common ancestor of all fishes appears to have evolved in the Cambrian period (approximately 530 million years ago) in the form of a soft-bodied fish that possessed an early, flexible version of a spine called a notochord. Some animals today possess a notochord, such as tunicates, which are invertebrates that are commonly associated with vertebrates because of this variation of the spine. The coral-like tunicates are tadpole-like in their larval stage, with some species retaining their notochords into adulthood. These varieties of tunicates highly resemble the earliest fossil fishes, such as the 516-530 million year old Haikouichthys, and help us visualize what the common ancestor of fishes may have looked like. These undulating and jawless forms would eventually evolve into the thousands of different species we know today, as well as countless extinct forms, and new extant varieties that are still being discovered.