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(See a gallery of prehistoric fish pictures and profiles, a list of 10 recently extinct fish, and a slideshow of 10 Prehistoric Fish Everyone Should Know.) Although most paleontologists wouldn't recognize them as true fish, the first fish-like creatures to leave an impression on the fossil record appeared during the middle Cambrian period, about 530 million years ago.
The most famous of these, Pikaia, looked more like a worm than a fish, but it had four features crucial to later fish (and vertebrate) evolution: a head distinct from its tail, bilateral symmetry (the left side of its body looked like the right side), V-shaped muscles, and most importantly, a nerve cord running down the length of its body.
A new addition is Bonnerichthys, yet another large, Cretaceous fish with a tiny, protozoan diet.
Bear in mind, though, that for every "dino-fish" like Leedsichthys there are a dozen smaller prehistoric fish of equal interest to paleontologists.
Compared to dinosaurs, mammoths and saber-toothed cats, fish evolution may not seem all that interesting--until you realize that if it weren't for prehistoric fish, dinosaurs, mammoths and saber-toothed cats would never have existed.
The first vertebrates on the planet, fish provided the basic "body plan" subsequently elaborated on by hundreds of millions of years of evolution: in other words, your great-great-great (multiply by a billion) grandmother was a small, meek fish of the Devonian period.
The osteichthyans, meanwhile, split into two further groups: the actinopterygians (ray-finned fish) and sarcopterygians (lobe-finned fish). Well, you do: the lobe-finned fishes of the Devonian period, such as Panderichthys and Eusthenopteron, had a characteristic fin structure that enabled them to evolve into the first tetrapods--the proverbial "fish out of water" ancestral to all land-living vertebrates, including humans.
The ray-finned fish stayed in the water, but went on to become the most successful vertebrates of all: today, there are tens of thousand of species of ray-finned fish, making them the most diverse and numerous vertebrates on the planet (among the earliest ray-finned fish were Saurichthys and Cheirolepis).
Their Silurian descendants shared the same body plan, with the important addition of forked tail fins, which gave them more maneuverability.
If the "-aspis" fish were the most advanced vertebrates of their time, why were their heads covered in bulky, un-hydrodynamic armor?
The answer is that, hundreds of millions of years ago, vertebrates were far from the dominant life forms in the earth's oceans, and these early fish needed a means of defense against giant "sea scorpions" and other large arthropods.
By the start of the Devonian period--about 420 million years ago--the evolution of prehistoric fish veered off in two (or three, depending on how you count them) directions.
Haikouichthys is considered by some experts--at least those not overly concerned by its lack of a calcified backbone--to be the earliest jawless fish, and this inch-long creature had rudimentary fins running along the top and bottom of its body.Tags: Adult Dating, affair dating, sex dating