INTRODUCTION TO FOSSILS

 

This set of fossils is incomplete but includes the following items. The notes accompanying the specimens include diagrams.

A brachiopod. Rhynchonella octoplicata. Cretaceous.
Brachiopods are animals which secrete a calcareous bivalve shell for protection. The two valves usually differ in size and shape but each one is equilateral. They are held together by muscles internally and a series of teeth below the beak. The ventral valve has a foramen just below the beak visible on the broken specimen. Through this, a stalk passed which fixed the animal to its substrate. Rhynchonella spp. Have existed from Ordovician times to present day, when they are to be found in deep water off British coasts.
 
A lamellibranch. Glycimeris glycimeris. Pliocene.
The specimen is one valve of a bivalve shell of a mollusc. The two are held together in life by the teeth, ligament and a pair of muscles, the scars of which can be seen internally.
This animal lived just before the Ice Ages in a shallow cool sea. Animals of the same species can be found alive today around British coasts.
 
A lamellibranch. Trigonia gibbosa. Jurassic.
The specimen is an internal cast of an animal which has lived from Jurassic times to the present day and is to be found in Australian seas.
 
A belemnite.
D’Orbigny’s restoration of a belemnite shows the position of the guard and phragmocone, which are the only parts of the internal skeleton preserved in this specimen.
Belemnites were most abundant during Jurassic and Cretaceous times and are now extinct. Their nearest relatives at the present time are squids and cuttlefish.
 

An ammonite. Hildoceras sp. Jurassic.
The specimen is the coiled shell of a mollusc, which is divided into chambers by internal septa – the positions of which are marked externally by a suture line. The animal lived in the last formed and biggest chamber, and probably used tentacles for slow swimming. The other chambers were in contact with each other. Filled with gas, they may have been used for buoyancy. Suture lines are important in identifying ammonites and become more complex in form towards the end of their period of existence. Their range is from the Devonian to Cretaceous and they are now extinct. Their nearest relatives are four species of Nautilus, which live on the edge of the Indian Ocean.
 
An echinoid (sea urchin). Echinocorys sp. Cretaceous.
The specimen shows the series of interlocking calcareous plates which form the globular skeleton. 5-rayed symmetry, characteristic of all echinoderms, is shown by five sets of pores, through which tube feet passed in life for use in movement and respiration. Sea urchins range from Ordovician to present day, when they are to be found most abundantly in a shallow warm sea on a rocky bottom. This species is one of many found in the chalk and it is probable that the sea in which this was laid down was also shallow and warm.
 
A crinoid. Pentacrinus sp. Jurassic.
The specimen is of stalks only of sea lilies showing the pentagonal segments of which they are made.
Food was directed to the mouth down the length of the arms by cilia. Pentacrinus is found only in the Jurassic period, but other crinoids are alive today, living in deep water. Some, the Feather Stars, break loose and swim freely as adults.
 
A graptolite. Diplograptus sp. Silurian.
Graptolites were colonial animals either free-floating or attached to floating weeds in the sea. The soft parts of the animal are entirely unknown and thus their affinities with other groups of animals are dubious. They are found over a wide geographical range but only during the Ordovician and Silurian periods. They are nearly always preserved in black shale, suggesting that they lived some distance from shore where fine sediment was being slowly deposited in calm water.
 
A shark’s tooth. Otodus sp. Cretaceous.
A sharply pointed tooth indicating that it came from a fish-eating shark.
 
A pteridosperm. Mariopteris sp.
The specimen shows part of the leaves of two species of seed ferns – plants which show structural characteristics of both ferns and gymnosperms. Such plants were abundant in the coastal swamps of the Carboniferous period and have since been compressed to form coal. These leaves fell into mud providing substrate for the trees and have thus been preserved in some detail.
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