Note - this page talks in terms of evolution, the current accepted Scientific theory for the origins of life. It is important to understand that many people believe in other theories of creation.
There are 3 main theories :
- Creation of all life by a supreme being
- Creative Evolution
See the note at the bottom of the page for parents and educators that explains this site's policy on recognising both theories.
Fossils occur when an animal dies and gets buried quickly by mud and silt. As more and more mud falls on top of the dead animal it may get fossilized, over a period of time minerals leech into the organic matter until over millions of years there is nothing organic left and the remains of the animal are literally rock!
Good conditions for fossilisation happen when there is plenty of time for the organic matter to be replaced by minerals. So most of the soft tissues of sharks are lost to time since their skeletons, which are made of cartilage not bone, dissapear rapidly, most shark fossils consist of teeth or occasionally material such as spines, bits of spine and dermal denticles (the skin teeth that sharks have instead of scales).
However very occasionally a more complete (and occasionally intact) fossil is found giving us clues to the ancestors of modern sharks, often showing us how little modern sharks have changed.
The oldest fossils date back from the devonian era about 400 million years ago, during this time the seas were particularly rich, the chondricthyes (cartilaginous fishes) evolved at this time. Today there are over 1100 species of chondricthyes all evolved from the early proto sharks. What evolutionary pressure caused the chondricthyes to evolve will probably never be known.
Some Ancient Shark Info - Source "Sharks - A firefly guide - Andrea & Antonella Ferrari"
Cladoselache - Pronounced "Clay-dough-Selakki" (Probably ;-) )
- Grew to over 2m (6.5ft),
- Among the earliest of sharks,
- Had many of the structures associated with sharks today - Five gill slits, a tail with uneven lobes, strong spines infront of the two dorsal fins, made of the same stuff as teeth.
- Many teeth with a structure similar to many current shark species.
Its thought by some thinkers that Cladoselache might have been an agile swimmer with a tail similar in form to that of the great white or the mako sharks of today. Fossils have been found with smaller fish swallowed tail first, caught while they were swimming away fast, so those thinkers may be right.
Many people think that the carilaginous fishes are more primitive than the bony fishes, however these fossils come from a time when the seas were already full of bony fish and the cartilaginous guys had only just come on the scene. So rather than being more primitive it might be that the cartilaginous fish came from the bony fish!
So, what can we say ?
Let's look at a time line from 400 million years ago on the left to 2m years ago on the right
Pre-Historic -----------------------> Present
Click on the diagram to see a slightly bigger version.
400 Million Years ago Xenacanthidae - primitive chondricthyes that lived only in freshwater were already branching out. - devonian period
320 Million years ago - Carboniferous period - Hybodontidae evolve - They possess a number of modern characteristics and are likely to be the immediate ancestors of present day sharks. They became extinct about the same time as the dinosaurs
100 Million years ago - Lower Cretaceous - Modern sharks had already made an appearance, the teeth of a mako and a porbeagle shark have been found fossilized from this period.
60-65 Million years ago - fossilised teeth of Great White shark found in layers from this era.
2 Million Years Ago - Teeth of the Shark Carcharadon Megladon found not yet fully fossilised found from C. Megladon - This means that Megladon, a shark that probably looked like a great white but was 66ft (20m) long (the size of a Humpback Whale) was still possibly around as late as this. In evolution terms 2m years is the blink of an eye, though humans in their modern form have been around about 200,000 years only.
Some of ancient shark fossils found are pretty wierd, including one species that had a line of teeth in a spiral shape on the edge of the jaw.
So what's this all mean ?
It means that many present day sharks seem to have remained practically unchanged for 150 million years (based on fossil evidence) and that Sharks outlived the dinosaurs
There are still sharks around that are living fossils, such as the frilled shark. This shark lives deep around 1300m / 4200ft, the extreme unchanging environment is probably what allowed the fish to stay the same, keeping ancestral features like its long eel like body and a top jaw that is fixed to its skull rather than moveable like those of modern sharks. Other sharks also exhibit some of the ancient features, the Heterodontidae and hexanchidae families have 6 and 7 gills slits instead of the 5 that all other shark families have.
Another ancient shark we already mentioned is Carcharadon Megladon, this was a shark that has teeth almost exactly like those of the current day great white, but MUCH bigger, a GW tooth is the size of a zippo lighter, whereas a Megladon tooth is the size of a dinner plate!!
It was thought that Megladon therefore looked like a Great White and was related to the GW. Megladon is thought to have been about 15-20m (50-60ft) long , the size of a humpback whale!
The theory that GW and Megladon is related is now being looked at again. It was thought that GW evolved from megladon, becoming smaller due to a gradual reduction in the size of the prey. The new theory is that instead they were belonged to two different evolutionary branches, the megladon family died out while GW survived, who knows though ?
Another (less accepted) theory is that Megladon may still be alive in the deep ocean, feeding on whales ? Although the problem with this is that no marks have been found on whales dead or alive that would support this, though its true there are MANY animals out in the ocean large and small that we haven't yet discovered!