Genetics, paleontology, and evolution / edited by Glenn L. Jepsen, Ernst Mayr, George Gaylord Simpson, for the Committee on Common Problems of Genetics, Paleontology, and Systematics, of the National Research Council.

Date:
1949
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Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Genetics, paleontology, and evolution / edited by Glenn L. Jepsen, Ernst Mayr, George Gaylord Simpson, for the Committee on Common Problems of Genetics, Paleontology, and Systematics, of the National Research Council. Source: Wellcome Collection.

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    same ecological type, for example inhabitants of sea floors of the same grade of sediment at the same depth of water, we find that genera may persist over considerable periods, species having shorter ranges, and it is then possible to pick out a series of species occurring in successive beds and to claim that they are ancestral to one another, the differences between them having been brought about by evolution. Such evolutionary series among invertebrate animals have repeatedly been brought forward and it is useful to consider on what basis the forms included in them have been selected. Probably the most perfect of such evolutionary series amongst inverte brates is that of Micraster in the English Chalk. Here we are dealing with an echinoid which lived burrowing in the mud of a sea floor which is now chalk. Chalk in general contains very little detrital material which has been derived from the land, pure chalk may have less than 1% of material other than calcium carbonate, and the only common constituent is silica as flint formed in situ, presumably from the skeletons of sponges and radiolaria which are themselves taken from sea water. The uniformity of chalk is probably due to the aridity and lack of river discharge from the neighboring lands, but it was clearly laid down in comparatively deep water and at a depth which varied. Specimens of Micraster are scattered through this chalk, more abundantly at certain horizons than at others, but they can be found in southern England in Cenomanian and Senonian times and clearly inhabited the area continuously. These animals, like all echinoids, have a test of very elaborate structure which is perfectly preserved in the fossils. They were first satisfactorily discussed by Dr. A. W. Rowe in a classical paper and he found it convenient to divide them into five species, each of which has a limited range in time, although these ranges overlap. Rowe was able to show that eleven apparently independ ent qualities of the test change with time, the direction of the change not running constantly in the same direction throughout the whole period, and that it is possible to date a series of Micraster found together rather accurately. In this particular case there is no possibility of confusing Micraster with any other form of echinoid found with it except for Epiaster , which by definition differs from Micraster by not possessing a sub-anal fasciole. In these circumstances it is natural to suppose that the differences between the earliest and the latest members of this series depend on an evolutionary change which has gone on in the area which is now southern England. It would be conceivable that the change had been brought about by immigration of new types into the area from somewhere else. The story as it is shown in north France appears to be identical with that in England. Micraster occurs also in north Germany and has been recorded from Italy, the Caucasus, and Madagascar, with very uncertain reliability.