Acta
Palaeontologica Polonica
26, 161-191(1981). Jerzy Dzik Abstract.
Cephalopods
are postulated to have evolved from planktic monoplacophorans possibly related
to the circothecid hyoliths. It is suggested that secretion of a liquid
characterizes by a density lower than seawater preceded development of a
gas-containing phragmocone. Phragmocone formation was due to a prolongation of
the functioning of the larval hydrostatic apparatus in the adult stages.
Attachment of the larval pedal retractor to the apex of the larval shell was a
reason why the soft tissue (siphon) remained in the apical parts of the shell
and was subsequently surrounded by diaphragms (septa). A possible mode of
producing of the light liquid is removal of salt by an ionic pump. The origin
of the rhythmic alteration in the septa and liquid secretion was the next
step. An osmotic pressure caused by removal of salt from the liquid resulted
in slow removal of cameral liquid from older chambers and diffusion of gas
from the liquid under conditions of lowered pressure. A model for the
evolutionary transformation of the monoplacophoran operculum into the
cephalopod lower jaw is presented. Externally calcifying aptychi of the
Palaeozoic orthoceratids and the Mesozoic ammonites were produced in the same
way as opercula of Recent gastropods Natica and Turbo. The early
evolution of the class is reviewed. |