FOSSIL 2: GASTROPODA AND CEPHALOPODA

The next types of fossils are those of gastropoda and cephalopoda. When describing fossils of these two groups, additional terms are often used: trochospiral and planispiral.

Trochospiral is usually associated with gastropoda and refers to a three-dimensional coiling of the shell, meaning the shell has height and spirals from top to bottom. There are two types of coiling directions in trochospiral forms: dextral and sinistral. A dextral coil spirals in a clockwise direction, while a sinistral coil spirals counterclockwise.

To make it easier to understand, you can look at the position of the aperture (the opening where the gastropoda emerges). If the aperture is on the right side, the coiling is dextral; if it’s on the left side, the coiling is sinistral.

Sinistral (left) and Dextral (right)

Planispiral is typically found in cephalopoda and refers to a two-dimensional shell coiling (lacking height). In planispiral forms, there are three types of coiling patterns: involute, convolute, and evolute.

GASTROPODA

A. Definition

Etymology: Gaster = stomach and Podos = foot
Terminology: Gastropoda are soft-bodied animals that move using a muscular “foot” located on their underside (belly).

B. Characteristics

Possess a chewing organ located in the head region, known as the mantle cavity.
Their shells are composed of calcium carbonate (CaCO₃).
The shell typically has a conical shape.

C. Taxonomic Level

Class: Gastropoda

D. Order Classification

a. Subclass Protogastropoda, consisting of the orders:

Cynostraca
Cochliostracea

b. Subclass Prosobranchia, consisting of the orders:

Archaeogastropoda
Mesogastropoda
Neogastropoda

c. Subclass Opisthobranchia, consisting of the orders:

Pleurocoela
Pteropoda

d. Subclass Pulmonata, consisting of the orders:

Basommothopora
Stylommothopora

E. Morphology

F. Index Fossil

Vicarya verneulli vicassa: Middle Miocene (N13) and found in the North Pacific region.

Turricula flammea atjehensis: Plio-Pleistocene, serves as evidence of Java-Sumatra tectonics during the Tertiary period.

Babylonia gracilis & Babylonia pangkaensis: Plio-Pleistocene.

Babylonia gracillis

Babylonia pangkaensis

G. Oostingh’s Biozonation

In tropical regions, gastropods thrive exceptionally well, and certain gastropod species are characteristic of specific layers. This fact was used by Oostingh to establish the Neogene stratigraphy of Java Island. The biozones are as follows:

a. Rembang Stage (Early Miocene)