As mentioned earlier the Coleoidea or Dibranchia group of cephal possesses an internal shell though in most cases a shell is a Belemnites, an extinct coleoidea and a fossilised representative of group, possesses an internal shell consisting of three parts known as guard or rostrum (ii)phragmocone and (iii) pro-ostracism.
The go commonly preserved as fossil. It is the solid conical part of the shell f at its apex. The phragmocone is found inside a conical hollow space in guard and is divided into chambers traversed by the siphuncle. Calcareous plate, either oval or elongated in shape projected from a phragmocone is known as pro-ostracism. The phragmocone and the ostracism are seldom preserved as fossil.
The surface of the shell may be smooth or may be traversed by trams ribs, which may be bifurcating, trifurcating or multiplication. Spine tubercles also constitute some of the ornamental features of the surface the shell.
The Cephalopods are exclusively marine animals largely adapted to w seas. Except Ammonoidea, representatives of all other groups Cephalopods are found living today. Some of them live in bottom of sea while others found swimming in warm and shallow sea water.
The fossil record indicates Nautiloids to be the most ancient form which came to live on this earth much before the Ammonoids. Nautiloid fuss’ are found preserved in rocks formed from Early Paleozoic times, till t present day and in the passage of time they have progressively evolve though they attained their culmination during Silurian.
During La Paleozoic the earlier Nautiloids lost their importance though dui Mesozoic, Nautilus type of nautiloids appeared which attained worldwide distribution during Cretaceous time and are found to occur in the mode seas as well.
The Ammonoids, however, appeared during late Paleozoic times though the Mesozoic era witnessed their culmination and subsequent extinction. Some of the Cretaceous ammonoids were extremely large in size. They were totally extinct at the end of Cretaceous period and do not occur as fossils within the rocks formed during later periods.
The Coleoidea group appeared during early Mesozoic era with the Belemnites attaining their abundance during Jurassic and Cretaceous periods. This group became less important during Tertiary and represented today by octopuses, squids and cattle-fishes.
Description of some important fossils
Shell globular, the last whorl covers all earlier whorls, large body chamber simple nautiloid suture line. Age -Triassic to Recent.
Shell discoid, planispiral, ceratitic suture, fine ribs present. Age – Triassic.