House Rules...
How's that go?
You come in MY house, I make the rules.
Works for the oceans too, I'd guess.
I'd suspect that sharks don't have a lot of original thoughts:
(Cleveland Sharks?!!?)
The very earliest signs of sharks are minute fossil scales and teeth which are found in rocks from the late Silurian to early Devonian period {around 400 million years ago). It becomes more and more difficult, however, to identify shark scales in older rocks because they closely resemble those from jawless fishes called the lodonts, which lived at the same time. Only microscopic differences separate shark and the lodont scales, and the two kinds seem to become more and more alike the further one goes back.
A similar problem exists with ancient shark teeth, which did not seem to be present in rocks older than those from the mid-Devonian. It now seems that the reason for this was that scientists were not looking in the right places, and that early shark teeth were often very small. In 1986 teeth were found in Lower Devonian rocks from Spain which belonged to a group of sharks called Xenacanthids.
Although there is evidence of earlier sharks, the first complete fossils of shark-like fishes have been discovered in mid-Devonian rocks. Most frequently found are members of the genus Cladoselache, streamlined fish that grew to a length of about two meters.
Fortunately, complete specimens of Cladoselache have been preserved in the remarkable Cleveland shales, so quite a lot is known about them.
They had five pairs of gill slits, a fin spine and all the same fins as modern sharks, except for an anal fin. These spines, which become more common and elaborate in later sharks, and which still persist in some species today; were positioned in front of the dorsal fins and acted as cutwaters. Cladoselache had distinctive teeth with a large central cusp flanked by sever-al smaller points, and apparently they lived on small fish -the remains of which have been found in the stomachs of fossilized specimens. These sharks are not now considered to be the main line leading to the modern species.
Although sharks and shark-like fishes have a long history, the modern sharks (Cneoselachians) did not rise to dominance until after the Jurassic period, when, for some reason that is not yet clear, many of the more ancient forms had become extinct. Some Jurassic sharks are closely related to modern sharks, and this gives many present-day shark families histories which stretch back for 135 million years or more. The skates and rays, another group of cartilaginous fishes, also appeared in the mid-Jurassic, but they did not really come into their own until the Tertiary period, between 65 and 2 million years ago, when they were able to exploit a dramatic rise in the numbers of bivalve shellfish in the oceans.
The fossil record of modern sharks is fairly good, but again it normally consists only of hard parts such as teeth and scales. The appearance and relationships of present-day groups are well understood compared with the situation in the Paleozoic and Mesozoic eras. Oceans in the Cretaceous period (140 to 65 million years ago) were dominated by goblin, sand, probable, nurse, cow and angel sharks. Early sawfishes appeared and later evolved into the modern sawsharks. At the beginning of the Tertiary period, the gray nurse sharks were found in large numbers. All modern forms of sharks were present in seas of the Miocene epoch (25 to 5 million years ago), including the giant Carcharodon megalodon, perhaps the most awesome of all sharks, now extinct.
There has been an upsurge of interest in fossil sharks in recent years as more information has become available from new fossil discoveries, especially in the southern continents, and from a microscopic examination of existing fossil remains. In many cases these discoveries have helped scientists to understand some of the finds made last century~ There are still few definite answers about the origins and relationships of all the known fossil, and some modern, sharks but progress is being made.
