A biometrical study of Micraster coranguinum and M. (Isomicraster) senonensis

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Abstract
The echinoid Micraster shows one of the best examples of an evolutionary series among the invertebrates. The broad lines which this evolution took were demonstrated by Rowe in 1899. In the present paper the work of Rowe is extended and developed, using statistical methods where these are appropriate. In addition to the species considered by Rowe, the paper deals with the closely related species M. (Isomicraster) senonesis , and with the zones of the English Chalk above that of M. coranguinum. A sample of 516 specimens of M.coranguinum and M. (Isomicraster) senonesis was used for the biometrical studies. These specimens were collected by Rowe from Northfleet, Kent, and are now in the British Museum. Ten measurements, or as many of these as the state of preservation allowed, were made upon each specimen. Tests were made upon each of these ten sets of measurements to determine their accuracy and reproducibility. After a consideration of the results of these tests, three of the ten measurements were rejected. A discussion is given of the effects of errors of measurement on the interpretation of results. The validity of applying the conclusions reached from the study of a sample of fossil material to the original living community is dealt with in two stages: (a) a study of the degree to which the sample of fossils is representative of the fossil population, and (b) a discussion of the degree to which the fossil population is representative of the living population from which it was derived. Rowe’s sample shows a bias due to unrepresentative collection. The larger specimens are overrepresented, while M. senonensis , and the transitional forms between that species and M. coranguinum, are similarly over-represented with respect to M. coranguinum itself. Such bias is probably common to most palaeontological samples. The bias due to an ‘unrepresentative fossilization’ of the living population is shown to be much reduced if samples are compared on the basis of estimates of ‘shape’ parameters defined in terms of allometric growth, rather than, as is usual, on the basis of ‘size’ parameters such as means. Seven shape characters were considered, each defined by a pair of variates. These characters were: relative breadth, defined by the variates length and breadth; relative height, defined by the variates length and total height; relative height of the apical system, defined by the variates length and height of the apical system; degree of projection of the labrum, measured by the variates length and length to the tip of labrum; relative depth of anterior groove, measured by length and length to bottom of groove; degree of development of sub-anal fasciole, defined by length and area of fasciole; and degree of the posterior rise of the test, defined by the height and the apical height. In six of these seven characters the ranges of variation of the two species ( M. coranguinum and M. senonensis ) overlap. The exception is when the character ‘degree of development of the sub-anal fasciole’ is considered. On this basis the two species can be sharply separated— M. coranguinum having a well-developed fasciole, while in M. senonensis this is vestigial or absent. The intergrading between the two species in respect of the other six characters is possibly due to hybridization. A biometric study has also been made to investigate the differences between the anterior, unpaired, ambulacra in the two species. Here again there is complete intergrading. The study throws some light on the formation of a petal. It is suggested that the differences between the species are due to their occupying different ecological niches. These differences are not considered great enough to warrant placing the two species in separate subgenera. The sample was divided between the two species on the basis of the area of the sub-anal fasciole; and the equation of allometric growth ( y = Bxa ) was fitted by the method of Kermack & Haldane (1950) to the pairs of variates defining each of the seven characters under consideration. In none of these seven cases did the estimates of a differ significantly for the two species. If, then, the values of a are assumed to be identical in both cases, the estimates of B so obtained differ significantly in the two species for five of the six pairs of variates considered. These differences between the two species would normally be obscured by the effects of allometric growth, M. senonensis being, on the average, larger than M. coranguinum . The changes in shape which occur during growth are important in elucidating the systematics of the genus. They show, for instance, that M. praecursor Rowe is a synonym for M. cortestudinarium Gold., the former author having been misled by the effects of allometric growth. His division of M. coranguinum into a narrow and a broad variety is indefensible for the same reason. The shape changes due to allometric growth are different from those seen in evolution. These echinoids do not show recapitulation. By the technique of partial correlation, it has been demonstrated that the seven characters under consideration are so related, in M.coranguinum , that specimens which are more advanced than the general level of evolution of their time in some of them will be behind it in others. The result is that the sum total of the characters in any individual ‘average up’ to a level of evolution characteristic of the time at which the individual lived. A discussion is given of the evolution and systematics of the English members of the genus Micraster . In southern England the main line of evolution of this genus passes through the lineage M. leskei— M. cortestudinarium— M. coranguinum. M. corbovis forms a branch phylum in the Turonian, and M. senonensis in the Senonian. In the north of England (the Northern Faunal Province of Wright 1952), the evolution of the genus follows a parallel, but not identical, line to that followed in the south. The northern forms culminate in M. glyphus and M. (Isomicraster) stollyei of the mucronata chalk. The study of the whole genus gives no grounds for assuming that these echinoids followed undeviating evolutionary trends, as some authors have supposed.

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