Cephalochordate

Rezentes lancelet ( Branchiostoma lanceolatum )

The acrania ( Cephalochordata ( Gr. " head chordates " ), Syn: Acrania of ( Altgr. Ακράνια. "Skull Loose " ) ) are a sub-tribe of the chordates. The fossil acrania were an extremely successful group, today these chordates are only three genera of lancelets ( Amphioxiformes ) represented.

Features

The animals are of slender fish -like shape and taper at both ends. A bony skull, a real brain, a spine and extremities are missing. In the mouth, they have Mundcirren which protect the mouth opening.

Skull lots have characteristic features ( apomorphies ) that identify them as chordates, including the notochord, a dorsal neural tube and a muscular tail postanalen. You possess an appendix, which is probably homologous to the liver of vertebrates.

System

The acrania are probably the closest relatives of vertebrates ( Vertebrata ), also known as Skull Animals ( Craniata or Craniata ) are called ( Notochordata - Urochordata hypothesis). However, another hypothesis, the acrania basal and sees the tunicates as the closest relatives of vertebrates ( Olfactores - Cephalochordata hypothesis).

Skull lots in the fossil record

The body of acrania are due to lack of cartilaginous or knöchener hard parts rarely get, so their evolutionary history is documented only by bad or doubtful fossil finds. Probably the oldest known form is pikaia from the middle Cambrian about 530 million years ago from the Canadian known for its exceptional soft-bodied Burgess Shale.

The best known fossil acrania are the major index fossils as conodonts with more than 3,000 species described. Your exact systematic position is not clarified, many editors put them to the vertebrates.

Evidence

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