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A Study of Pseudobythocypris pediformis, a New Name for an Old Ostracod

Robert H. Shaver
The American Midland Naturalist
Vol. 59, No. 1 (Jan., 1958), pp. 120-137
DOI: 10.2307/2422380
Stable URL: http://www.jstor.org/stable/2422380
Page Count: 18
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
A Study of Pseudobythocypris pediformis, a New Name for an Old Ostracod
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Abstract

Ostracod paleontologists should exploit to full advantage two methods of study in quest of more useful taxonomy based as nearly as possible on the biologic species concept. These methods, which involve complete analysis of shell morphology and quantitative observation of molt series, are applied to the common Carboniferous ostracod, Bythocypris pediformis Knight. B. pediformis is one of possibly more than 200 species of the extravagantly artificial genus Bythocypris Brady, reported from Ordovician to Recent strata. Inadequacy and confusion of early concepts of B. pediformis and related species further indicate need for study. Shell morphology of B. pediformis includes dimorphism, complex adductor muscle scars, and simple ridge-and-groove margins without duplicature. These contrast so boldly with morphology in the family Bairdiidae and the Recent type species of Bythocypris, that a new genus, Pseudobythocypris, is introduced for 34 Carboniferous and Permian species and is transferred to Healdiidae. Measurements of 271 specimens in a molt series of P. pediformis are the basis for several useful concepts: (1) An assumption of nine molt stages is supported by a graphical representation, by a growth factor of 1.23 which is used to determine theoretical means of uncertain stages, by comparisons with average lengths which several control species attain each stage, and by use of standard deviations which confirm assignment of more than 95 per cent of the specimens to stages as shown on the graph. (2) P. pediformis exhibits allometric growth; a single dimension increases each molt stage by a constant percentage with some modification at sexual maturity. Measures of growth and resulting morphologic changes are useful, but difficulty arises in use of the biogenetic law for taxonomic purposes and reconstruction of ancestral morphology. (3) Variation in size amounts to as much as 22 per cent of a mean dimension in adults of one population and effects similar variation in form ratios. Magnitude of variation is uniform during growth as shown by a nearly common coefficient of variation for each molt stage; significant departure only in the adult stage is interpreted as verification of dimorphism. Magnitude of variation and dimorphism require conservative recognition of species of this type. (4) Computations using standard deviations and standard errors of means show that 15 and 6 specimens per instar of P. pediformis are required to assure a 99 per cent probability that computed mean lengths are accurate within 3 and 5 per cent respectively; 29 and 10 specimens are required for the same accuracies in height. Species of ostracods may have singular variations, possibly of taxonomic value, but 10 to 15 adult specimens would impart reliability to the statements of dimensions and form ratios commonly used to characterize a species as observed in one population. Additional quantitative studies of species or populations which are separated by time or space, are needed for further understanding of speciation in ostracods of the pediformis type.

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