Ontogenetic changes in the carapace of Tyrrhenocythere amnicola (Sars), a hemicytherid ostracode

—Detailed description and illustration of the carapace of 7)rrhenocythere amnicola (Sars, 1888) at eight different growth stages show morphological changes during ontogeny and lay groundwork for a better understanding of its relationships to similar ostracodes. Regression of log size on developmental stage indicates that growth rate is constant; the area enclosed by the outline of the carapace increases by a factor of 1.74 from one instar to the next. Biorthogonal analysis graphically shows the amount and direction of change of shape in homologous regions of the carapace between successive instars. Eigenshape analysis and qualitative evaluation indicate that only adults, not juveniles, are sexually dimorphic in shape. Both eigenshapc and biorthogonal analyses indicate that change of shape during growth is primarily monotonic and is concentrated posteriorly. More change occurs early than late, and the greatest change occurs between instars A-6 and A-5. Most recent ostracode species have nine growth stages or instars if one includes the initial larval stage and the adult. Unfortunately, juvenile instars are often poorly preserved, and most taxonomic descriptions cover only adults. Study of juvenile instars has too often been Manuscript received April 30, approved August 29, 1986. limited simply to recognizing them as immature so as not to mistake them for new species. Knowledge of ontogeny is important in understanding evolutionary processes and phylogenetic relationships, however, especially those that are indicated by heterochrony of the carapace (Gould, 1977). Ostracodes are ideal candidates for study of 2 The University of Kansas Paleontological Contributions—Paper 118 heterochrony. Such studies require determination of the developmental stage of an organism, which is possible with ostracodes because they molt a fixed number of times in life. A prerequisite to the search for heterochrony among ostracodes is detailed descriptions of the ontogenies of individual species, including quantification of changes in size and shape. We have studied ontogenetic changes and morphological variability in the carapace of Tyrrhenocythere amnicola (Sars, 1888) (Maness and Kaesler, 1985), a brackish-water species of the Mediterranean area that is used in biostratigraphy and paleoenvironmental analysis. T amnicola is a member of the Hemicytheridae, an important Cenozoic, predominantly marine family. The ontogenies of hemicytherid species are poorly known, and the ontogeny of T amnicola may be representative of hemicytherid ontogeny in general. While the immediate goal was a better understanding of the ontogeny of T amnicola, this study also lays groundwork for a better understanding of this species's phylogenetic relationship to similar ostracodes. Moreover, it demonstrates some of the methods that are useful in studying ontogeny of ostracodes. This study focused on taxonomically useful features of the carapace: muscle-scar pattern, shape, hinge, surface ornamentation, marginal area, and normal-pore canals. Hazel (1967) used muscle scars and shape to delineate subfamilies of Hemicytheridae and primarily shape, muscle scars, hinge, and surface ornamentation to delineate genera. Tyrrhenocythere is distinguished primarily by bundled radial-pore canals on its marginal area (Ruggieri, 1955). Krstie (1977) recognized three groups of Tyrrhenocythere species based on shape and surface ornamentation. Note that shape, particularly shape of the outline, is used at several taxonomic levels in the identification of T amnicola, as it is in the identification of ostracodes in general. Qualitative descriptions, however, are inadequate for rigorous comparisons of shape, because although the human eye and brain discriminate shapes remarkably well, description fails when differences are subtle, "sample sizes large, variation multidimensional, and groups ill defined" (Scott, 1980:758). To avoid these problems in the present study of ontogenetic differences in shape, we used quantitative methods to describe changes of shape. Shape analysis has recently stirred interest of scientists in diverse fields, and many techniques have been developed. Researchers must decide which technique to use, depending on their objectives and the questions to be answered (Kaesler and Foster, in press). Our objectives in this study of the shape of T amnicola were (1) to show graphically how shape changes during ontogeny and (2) to archive a representative shape for each instar in such a way that shape and shape change of this species are available for comparison with those of other ostracodes. In addition, we wanted to know (3) whether change of shape is unidirectional, (4) at what stage or stages shape changes the most, and (5) if sexual dimorphism is apparent in early instars.