Sharks and their kin are sometimes described as "living fossils," and
they are indeed part of an ancient clade of vertebrates. Very recently, fossil
denticles (scale-like bony pieces embedded in or on the skin) that resemble
chondrichthyan scales in minute detail have been found in the Late Ordovician of
Colorado (Sansom et al. 1996). More fossil scales that probably
belonged to unknown sharks have been found in the Silurian. Aside from
these finds, the oldest known complete,
identifiable cartilaginous fish date from the middle
Devonian. Sharks
and their relatives were diverse in the Paleozoic, but most of them were
not directly related to living sharks, belonging instead to side groups that
died out in the Permian or Triassic. Living sharks, rays, and skates
belong to a group known as the Neoselachii. This group may have appeared in
the Triassic or even as early as the Permian, but the oldest well-understood
neoselachian fossils are
Jurassic
in age. By the Cretaceous, modern-looking
sharks, sawfish, and skates -- such as this beautifully preserved
Cyclobatis longicaudatus, from the Upper Cretaceous of Lebanon --
had appeared.
Shark and ray teeth, and sometimes calcified vertebrae,
are common fossils in many Cretaceous and
Cenozoic
deposits. One of the more famous fossil sharks is the
Miocene
Carcharodon megalodon, with
serrated, triangular teeth (pictured on the background of this page)
ranging up to 17.5 cm (7 inches) in length. An
early reconstruction of Carcharodon from its teeth suggested that this
shark reached 30 meters (100 feet) in length. However, this reconstruction was
made only from the largest single teeth found, without taking into account
the fact that shark teeth taper in size from the center of the mouth to the
edges. A revised estimate of the size of Carcharodon puts its length
at "only" 12 meters (40 feet) -- about twice the size of the largest
great white sharks of today.
Click
here to view an image of the reconstructed jaws of Carcharodon,
courtesy of the Smithsonian Institution.
Various holocephalian fish (those with an upper jaw fused to the braincase)
appeared about the same time the earliest sharks did, in the Devonian. They
probably included the ancestors of the living chimaeras and ratfish, as
well as some other groups that are now extinct (traditionally classified
in the Iniopterygii), but their
relationships are not well understood.
We have pictures of hybodontid fossils from Ethiopia.
One of the most famous places to find fossil teeth of Carcharodon
and other sharks, dating from the
Miocene, is
Calvert County, Maryland, USA. Find out more about
Calvert County
sharks -- and other fossils -- from the
Maryland Geological Survey.
Got some shark teeth? You may be able to identify them using the
Key to Common Genera of
Neogene Shark Teeth, by Robert W. Purdy, available from the
department of
Paleobiology of the National Museum of Natural History.
Carroll, R.L. 1988. Vertebrate Paleontology and Evolution. W.H. Freeman and Company, New York.
Sansom, I.J., Smith, M.M., and Smith, M.P. 1996. Scales of thelodont and shark-like fishes from the Ordovician of Colorado. Nature 379 (15 February 1996): 628-630.
