What are fossils?
Fossils are remains of living beings or evidence of their biological activities preserved in different materials. This preservation occurs mainly in rocks, but it can also occur in materials such as sediments, ice, resins, soils and caves and the most cited examples are fossilized bones and stems, shells, eggs and footprints. They provide important data regarding biological evolution, dating and reconstruction of the Earth's geological history.
The appearance of fossils is rare. It happens that, sometimes, organic residues are quickly involved in a protective and impermeable material that preserves them from contact with the atmosphere, sea water and decomposing agents and thus end up becoming fossils, that is, when organic compounds that make up the dead organism are replaced by others more stable in the new conditions.
Fossils can be classified into two main categories:
(1) Somatofossils: type of fossil that occurs when some part of the living being is preserved. They are considered direct evidence of living beings. For example, fossils of teeth, shells, leaves, shells, logs, etc. They are the remains of living beings.
(2) Ichnofossils: type of fossil that occurs only with indirect evidence of living beings, that is, they result from their biological activities. For example, stromatolites, fossils of footprints, bite marks, eggs (from eggshells), excrement (coprolites), urinary secretions (uroliths), gastrolytes, tunnels, housing galleries, etc. They are traces of the passage of living beings.
Rudists
Rudists were marine bivalve mollusks of varying size and morphology. They had a shell attached to the substrate and were well adapted to the sessile way of life. These animals reached a large population in the second half of the Cretaceous period, mainly inhabiting shallow seas. They ceased to exist, however, due to the extinction that marked the end of the Mesozoic era.
The fossils of these bivalves, normally of large size, are quite evident due to the thick and massive aspect of the wall of their shells and the fact that they present a whitish color that stands out deep in the pinkish to reddish limestone. The ornamental rock to which these fossils are associated comes from quarries located in the region north of Lisbon.
Rudists are found in marble in longitudinal section (A), where the body is observed, or in cross section (B), where the whole myophore is observed.
Diagram on the palatal canals
The rudist visible at the zero point of the earthcache is of the caprine type. These rudist bivalves had the shell formed by two different valves from each other: one of them, the one that was buried in the vasous substrate was conical and the other - the free valve - was rolled up in "goat's horn". The mióforo (the "wall" of the rudistas) had paleal canals. The palatal channels are polygonal, when they are large, or pyriform, when they are small