Fossil Assemblages Life and Death Assemblages An early

Fossil Assemblages Life and Death Assemblages

An early attempt at a life assemblage – the Jurassic Seas! The Henry de la Beche diorama. Illustration from WHC bid document.

Life assemblage • Also known as a biocoenosis. • An assemblage or group of fossil remains found where they lived.

How to recognise a life assemblage. • Organisms can be found in position of growth e. g. coral • • reefs. Trace fossils preserved will indicate a life assemblage, as any transport would destroy the track, trail or burrow in the sediment. A variety of organisms ( different species) may be present depending on the quality of the environment of deposition, but all from the same community. Both mature and juvenile forms can be found together. The fossils are generally in good condition and have not been abraded.

A modern example of a life assemblage A coral reef with a variety of hard and soft corals.

Statistical determination of type of assemblage • Measurements of say an assemblage of brachiopods. • Such a pattern would suggest a life assemblage with a l e n g t h range of ages of individuals. A death assemblage would have a cluster of similar size shells Best fit line width

Silurian Wenlock Limestone life assemblage. A complex community with a diverse fauna. There is some transported debris because it is a high energy environment. Illustration from Ecology of fossils by Mc. Kerrow.

Wenlock Limestone from Ironbridge, Shropshire. Fauna of brachiopods (Atrypa, Chonetes) bryozoa including Fenestella, Tentaculites and trilobite Dalmanites (only pygidium here).

Jurassic life assemblage • There a number of benthonic elements with in-fauna and epifauna as well as some nektonic organisms that would provide a death assemblage characteristic. Illustration from Ecology of fossils by Mc. Kerrow.

Death assemblage • Also known as a thanatocoenosis. • An assemblage or group of fossils formed because they were brought together after death by sedimentary processes rather than because they shared the same habitat during life.

How to recognise a death assemblage • Fossils preserved are often of similar size as they are • • sorted by mass during transport. The fossils are often abraded or even broken up (comminuted). They can contain fossils from several diverse living environments.

A modern example of a death assemblage Countless juvenile gastropod shells preserved in a shell bank along the Fleet shore north of the Moonfleet Hotel.

Another example of a modern death assemblage. Death assemblage of modern bivalve shells on the beach at Orcombe Rocks, Exmouth. Many of the shells are damaged.

A local death assemblage Forest Marble along the Fleet Shore near the Moonfleet Hotel with fragments of bivalves, brachiopods, crinoids etc.

Graptolite death assemblage from the Lower Palaeozoic • Because graptolites were planktonic organisms that floated, they sank to the sea floor on death and so form a death assemblage. They are often preserved in anaerobic environments The ecology of fossils by Mc. Kerrow.

Didymograptus murchisoni from Abereiddi Bay, Pembrokeshire

Jurassic death assemblage • As conditions on the sea • floor were unfavourable, the benthonic epifauna and infauna was limited. Much of the preserved fauna came in from nektonic or planktonic organisms and some attached to floating wood.

Ammonite Graveyard, Monmouth Beach, Lyme Regis.