Seas where animals and plants began!

The seas are the original environment of animal life, as well as the original environment for plants.

1. There are 4 lines of evidence that point to the greater age of the marine fauna over the terrestrial or freshwater faunas.

a. General composition of present-day fauna (animal life of a given area or time).

1). every single phylum contains representatives in the oceans, and most, if not all, are believed to have originated there. 5 of them, the Ctenophora/comb jellies, Echinodermata/spiny skinned animals, Phoronida/tube worms, Brachiopoda, and Chaetognatha/arrow worms are exclusively marine.

2). 45% of the classes are exclusively marine whereas only 3 classes (less than 5%) are exclusively non-marine.

3). vertebrates--fish are predominantly marine; amphibians are non-marine; reptiles, birds, and mammals are almost wholly terrestrial.

b. Similarity of body fluids of marine invertebrates to the composition of sea water, already discussed previously.

c. Life histories of invertebrates:

1). The early stages are markedly different from the mature stages in some animals.

2). Larval stages, which sometimes resemble the mature stages of other groups, or only the larvae of still other groups, are thought to reflect the structural similarities to the ancestral stock.

3). Tendency for some animal groups to leave the sea for the freshwater or terrestrial habitats, especially in crustaceans. Eriocheir, a crab, enters fresh water at a young stage, but when mature, returns to the sea to spawn. Cardisoma and Gecarcinus, both land crabs, go through a free-swimming larval stage in sea water. In these migratory species, the site of origin is the site of breeding, and there are many more examples of animal species returning to the sea to breed than vice versa.

a. Animal fossils that occur in the oldest known fossiliferous rocks of the earth’s crust are chiefly marine forms. Marine animals were already abundant and became fossilized in the Cambrian period (500 million years ago) when certain portions of the land now above sea level formed a part of the sea bottom along the coastlines of ancient seas.