Two further places where sedimentary rock may be exposed are roadcuts and quarries, even though the solid rocks nearby are covered with glacial deposits and soil. Fossil collectors are opportunistic. Some of the best fossils are found where weather and well-beaten paths have not had a chance to degrade them; fresh exposures are most often the best. Limestone (calcium carbonate) and shale (fine-grained, layered rocks formed from clay and silt) and sandy silicates often replace the hard parts of ancient animals so well that the original shapes and structures survive many years of exposure. The advent of "acid rain" is certain to do more damage than centuries of normal weathering.
New species of fossilized invertebrate animals (in every phylum or group, in every class and subclass or order) are still being discovered by scientists and amateurs. The phyla or groups known to be represented by fossils from the Ordovician Period (425 to 500 million years ago), for which I have no examples, are Protozoa, Porifera, and local Coelenterata. [Corals from the Anthozoa Class in the Coelenterata Phylum may be found on Manitoulin Island.] Others, for which I shall supply photographs, are as follows:
BRYOZOA - Colonies of these tiny animals underwent extensive development during the Ordovician period. There are approximately as many living species (c.3000) as extinct.
BRACHIOPODA - They have shells of unequal valves, and the plane of symmetry runs through the valves rather than between them. About 15,000 species are extinct, but over 200 survive to recent times.
- Class Pelecypoda includes clams, mussels, oysters and scallops.
(Living: 11,000 and Extinct: 15,000)
- Class Gastropoda includes snails, slugs and conches.
(Living: 69,000 and Extinct: 15,000)
- Class Cephalopoda, Subclass Nautiloidea, has external chambered shells and the
only living representative is the Nautilus. Of the entire class, about 300 species
(Mainly squids, octopuses and cuttlefish) are living and around 10,000 are extinct.
ANNELIDA - Segmented worms originated in the Cambrian period. Around 36,000 species are living today, while about 1,000 are extinct.
ARTHROPODA - Subphylum Trilobitomorpha includes trilobites and related forms, all of which are extinct. Of the entire class other than insects, the ratio is 58,600 living to 4,400 extinct species.
ECHINODERMATA - Subphylum Pelmatozoa, Class Crinoidea called "sea lilies" are animals dating from Ordovician times to recent. Of the crinoids, some 800 are living and 5,000 are extinct.