Life in the Early Carboniferous, 359 – 327 million years ago
| During the
Early Carboniferous the Mendip area was part of a broad, shallow,
tropical sea that stretched westwards into Pembrokeshire, and across
which thick successions of Carboniferous Limestone, now forming the
Mendip hills, was being deposited. A rich variety of life inhabited
this marine environment, particularly corals, brachiopods and crinoids,
which are the most commonly occurring fossils.
Crinoids inhabited shallow water and grew in dense clusters, sometimes
called 'crinoid gardens' because of their resemblance to plants.
Long stems were anchored to the sea bed, and held aloft a globose,
cup-like structure with radiating arms. The whole animal is formed
of many individual plates that usually become scattered when the
creature dies. The stem plates are common fossils and the main constituent
of crinoidal limestone.
Like modern-day coral reefs, the abundant remains of fossil corals
in the Carboniferous limestone suggest the former existance of warm,
clear, shallow and well-lit tropical seas. Corals have a variety
of branching and encrusting shapes that provide homes for other creatures
and act as a baffle to trap sediment. Different
kinds of fossil corals occur at different levels in the limestone,
allowing geologists to distinguish between older and younger beds.
Brachiopods have become all but extinct in modern seas and oceans,
but in the geological past they flourished at the shallow margins
of oceans, especially in the Carboniferous. At first they appear
little different from familiar modern-day sea shells, but they are
in fact quite distinct, with different shell and soft part anatomy.
Many brachiopods lived openly on the sea bed, but some such as Lingula,
occurring near the base of the Carboniferous Limestone, inhabited
burrows. Two important groups of brachiopods in the Carboniferous
are strongly radially ribbed forms, called 'spiriferids', and
large, less strongly ribbed forms with relatively plano-convex valves,
Life in the Late