Study of Fossils : Asteroceras (Ammonoid)

 Study of Fossils : Asteroceras (Ammonoid)

Fossil of Asteroceras (Ammonoid)

 Systematic Position: (Hyatt, 1867)

Kingdom: Animalia

    Phylum:Mollusca

        Class: Cephalopoda

             SubClass:Ammonoidea

               Order: Ammonitida

                  Family: Arietitidae

                          Genus: Asteroceros

Origin:

Asteroceras belong to the class Mollusca. They are cephalopods. and first appeared in the lower Devonian period.

 These fast-moving nektonic carnivores lived during the Triassic and Jurassic periods (from 205.6 to 189.6Ma).

 They are found in the Jurassic marine strata of Canada, China, Germany, Hong Kong, Hungary,Peru, and Turkey, in the Triassic of United States and at Lyme Regis in the Asteroceras obtusum zone of Upper Sinemurian age.

                                     Fossil of Asteroceras (Ammonoid)

Characters:

 A strongly ribbed evolute shell. Whorls are high,rapidly enlarging.

 Presence of smooth ribs, curved forward.

 The outer rim, i.e. venter, bears a large keel flanked on either side by a wide groove, or sulcus.

 Presence of Septa, the dividing walls that separate the chambers in the phragmocone, by the nature of their sutures where the septa joint the outer shell wall, and in general by their siphuncles.

Three major type of sutures are found in Ammonoidea:

 Goniatitic - numerous undivided lobes and saddles;typically 8 lobes around the conch. This pattern is characteristic of the Paleozoic ammonoids.

 Ceratitic - lobes have subdivided tips, giving them a saw-toothed appearance, and rounded undivided saddles. This suture pattern is characteristic of Triassic ammonoids and appears again in the Cretaceous "pseudoceratites".

 Ammonitic - lobes and saddles are much subdivided (fluted); subdivisions are usually rounded instead of saw-toothed. This suture type is characteristic of Jurassic and

Cretaceous ammonoids, but extends back all the way to the permian.

Evolutionary significance:

Ammonoid, are frequently found as fossils in marine rocks dating from the Devonian Period (began 419 million years ago) to the Cretaceous Period (ended 66 million years ago).

 They are excellent index fossils because of their wide geographic distribution in shallow marine waters, rapid evolution, and easily recognizable features. Their absence in fossil layer indicate K-T transition in the geological timescale.

 Ammonoid survival was closely tied to the availability of plankton in Paleozoic and Mesozoic seas. The sudden decline of plankton during the K–T extinction at the end of the Cretaceous brought about the demise of the remaining ammonoid groups.