Variation and Predaton of the Pennsylvanian Gastropod Microdoma Conicum Meek and Worthen

Our study of a large population of the Pennyslvanian gastropod Microdoma conicum Meek and Worthen, which appears restricted to the Vanport marine unit of the Allegheny Group in Ohio, showed extreme variation in ornamentation of the shell ranging from essentially smooth to coarsely nodose. Smooth and lightly ornamental forms were more common than previously known. Nearly 38% of the population showed indications of external biological activity of one or a combination of 3 types: (1) small hemispherical pits, 0.1-0.2 mm in diameter, which did not penetrate through the shell material and were formed by an unknown passive epifaunal element while the host was alive; (2) borings, 0.2-0.3 mm in diameter, which passed through the shell and possibly were made by another gastropod (3) shell damage along the margin of the aperture at various stages of growth, which may have been caused by one of the associated faunal elements such as echinoids, fish, or nautiloid cephalopods. OHIO J. SCI. 80(2): 59, 1980 Our study had 3 main purposes: (1) to illustrate variation in shell ornamentation; (2) to analyze indications of predation and epifaunal relationships; (3) to indicate stratigraphic occurrences of this species. Microdoma conicum Meek and Worthen is a distinctive and easily recognized taxon having considerable variation in the ornamentation of the shell. Although Knight (1933) has pointed out some variation in this species, our collections indicated that it is more extensive than has been previously considered. Numerous specimens also showed evidence of predation and attachment of epifaunal elements for which various interpretations could be postulated. The stratigraphic range, which is restricted in Ohio, was compared with other known occurrences in North America. MATERIALS AND METHODS Several bulk samples of the Vanport shale lying immediately over the Vanport Limestone were made at a small abandoned drift mine on the north side of Jackson County Rt. 58 along Buffler Run, 2.4 km east of Ohio Rt. 124, S center sec. 24, T. 8 N., R. 16 W., Milton Tp., Jackson Co., Ohio, Mulga 73^' Quadrangle. The shale was processed by boiling in Quaternary 0 to release the fossil content. After dryiManuscript received 13 December 1978 (#78-74). ing, the residue was separated by screening and then picked over under a binocular microscope. Among a great variety of other taxa, several hundred specimens of Microdoma conicum. were present. These specimens ranged from juvenile forms of only a few whorls to gerontic growth stages. For the purposes of this study, 590 specimens having a length of 3 mm or larger were selected for analysis. The smaller specimens were eliminated because they did not possess the extent of development of the ornamentation to enable us to make a valid comparison with the larger specimens nor did they show evidence, like the larger forms, of epifaunal attachment or predation. The specimens selected were sorted into 3 categories on the basis of degree of development of ornamentation; (figs. 5, 10) smooth or fine (figs. 1, 2, 4), intermediate (figs. 3, 6), and coarse (figs. 5, 10). Since the population showed considerable variation in the development of ornamentation, these categories were somewhat subjective in nature. The specimens were further categorized on the basis of the presence or absence of epifaunal attachments, the presence of shell damage, and a combination of these factors (table 1). Several of the specimens showing indications of epifaunal attachment were sectioned or acidized to determine the nature of the attachment or boring in relation to the shell wall. Examination was made of other faunal elements present in the sample to determine whether they also showed the presence of predation or epifaunal attachments. The type specimens of Cyclopuncta girtyi Elias were borrowed from the University of Oklahoma for examination and comparative purposes.