Chondrichthyan fish or “true sharks” with cartilaginous skeletons, first emerged during the Silurian period around 450 mya. When they diverged from the Osteichthyes (fish with bony skeletons). Fossil evidence from Silurian deposits show teeth, denticles and fin spines. However, the oldest complete articulated skeleton from a shark found to date, is from the early Devonian period almost 409 mya. Fossils like this are rare because cartilage usually decays quickly leaving little time for fossilisation to occur.
The Devonian ended with extinction events that saw the loss of around 70-80% of marine species. During the Carboniferous period (300-360 mya) shark forms diversified greatly, with species evolving and adapting to their environment and their food source.
The Bear Gulch Limestone deposits of Montana and North Dakota have provided evidence of more than 65 species of shark from around 320 mya. However, this “Golden age of Sharks” wasn’t too last, with the Great Permian extinction (c. 250 mya) wiping out 90-95% of all marine species. The Chondrichthyan lineages that survived were likely deep-water species.
There are several theories for the causes of extinction events including meteor strikes, volcanic eruptions, extensive low dissolved oxygen concentrations, sea floor methane emissions and the formation and breaking apart of continents which may have caused large scale atmospheric and oceanic impacts.
By the late Triassic / early Jurassic (c.200 mya) modern sharks (neoselachians) arose, with similar species to those still around today having evolved in the Cretaceous period (145 mya). ‘Modern’ sharks possess several physiological traits which ultimately enhanced their feeding (e.g.: tooth morphology, increased jaw gape) and their swimming efficiency (fin flexibility, body shape etc).
Different body shapes and features are among the characteristics used to classify the nine separate orders of sharks which exist today:
Squatiniformes (Angelsharks)
Pristiophoriformes (Sawsharks)
Echinorhiniformes (Bramble Sharks)
Squaliformes (Dogfish)
Hexanchiformes (Frilled and Cow Sharks)
Carcharhiniformes (Ground Sharks)
Lamniformes (Mackerel Sharks)
Orectolobiformes (Carpet Sharks)
Heterodontiformes (Bullhead Sharks)