The early evolution of fishes is marked by the switch from using their gills as organs for filter feeding to using them as respiratory organs to absorb oxygen. Non-vertebrate chordates, such as lancelets, absorb oxygen from their aquatic environments through the surface of their bodies and use their gill slits for filter feeding. This type of respiration was adequate for these smaller organisms, but, as the vertebrates evolved, their body sizes increased, and they required more efficient oxygen systems. The filaments of fish gills provide a large surface area to allow increased oxygen uptake. The transition from filter feeding gills to oxygen transporting gills occurred with the evolution of the jawless fish.
The most primitive species of jawless fish alive today are the hagfish. The fossil record of hagfish is very sparce and traces back to the late Carboniferous period, about 330 million years ago. Fossilized species bear a striking resemblance to modern hagfish, indicating that they have remained relatively unchanged over a long period of evolutionary time. Even though the fossil record only dates back 330 million years ago, most scientists consider hagfish to be one of the earliest fish species to have evolved. The structural features of hagfish are likely very similar to those of the most recent common ancestor of all vertebrates. Until 1999, the earliest known fish fossil was of a jawless species, Arandaspis prionotolepis, which was a close relative of the hagfish. This fossil dates back 480 million years to the Ordovician period. Arandaspis prionotolepis was jawless, finless, and had armored plates covering the surface of its body. In 1999, Chinese paleontologists discovered 530 million year old fossils of two jawless fish species, which they named Myllokunmingia and Haikouichthys. These species appear to have had cartilaginous skulls.
Aucun commentaire:
Enregistrer un commentaire