Juvenile Justice System

Sharks

Shark-like chondrichthyans were discovered in the Ordovician period, 450-420 million years ago, before land vertebrates and a variety of plants colonized the continents. [10] Only scales have been recovered from the first presumed elasmobranchians, and not all paleontologists agree that they are from true sharks, with some suspecting that they are from thelodont agnathans. [11] The oldest generally accepted "shark" scales date back to the Silurian period, approximately 420 million years ago. [11] Those animals looked very different from modern sharks. [12] At the moment, the most common shark tooth is the cladodont, a thin tooth with three tines like a trident, presumably to aid in fish catching. The majority of modern sharks can be traced back to around 100 million years ago.

Cladoselache, a shark-like fish that lived about 370 million years ago[12], has been discovered in Paleozoic strata in Ohio, Kentucky, and Tennessee. These rocks were part of the soft bottom sediments of a large, shallow ocean that stretched across much of North America at the time. Cladoselache was only about 1 metre (3.3 ft) long with stiff triangular fins and slender jaws. [12] Its teeth had several pointed cusps that wore away with use. Cladoselache did not replace its teeth as frequently as modern sharks, based on the small number of teeth discovered together. Its caudal fins resembled those of great white sharks and pelagic shortfin and longfin makos.