HETEROCHRONY (PERAMORPHOSIS, PREDISPLACEMENT) IN A HOLLINELLID OSTRACODE SPECIES FROM THE PERMO-CARBONIFEROUS OF KANSAS

Carboniferous genera of the ostracode family Hollinellidae have been considered by many workers to have true trimorphic species because dimorphism with respect to adventral structures (frills) appears only in the adult stage. All preadult molt stages lack frills. Bless (1969) reports a single exception to this condition from the Namurian (probably an early Pennsylvanian age equivalent) of Spain where frills occur in the last juvenile stage of a hollinellid ostracode.

Hollinellid ostracodes have been recovered from strata of the Council Grove Group of Kansas that span the Permo-Carboniferous boundary. Adults and juvenile specimens of a species of Hollinella (Hollinella) from the Janesville Shale and Foraker Limestone (uppermost Pennsylvanian) of Kansas number in the hundreds per kilogram. Most are preserved as valves rather than complete carapaces, which facilitates study of internal as well as external features. This species follows Brook’s rule for arthropod growth stages closely; individuals can be assigned to molt stages with confidence. The lithofacies and faunal changes that occur throughout the two lithic units indicate that they are parts of two relatively minor cyclothems, each of which represents both marine and non-marine environments. The hollinellid ostracodes occur within nearshore marine parts of the two sequences.

The Kansas hollinellid species is remarkable in that marginal adventral structures occur not only in the A-1 instar (similar to the Spanish Namurian species), but also in some specimens of A-2 and A-3 instars. Further, almost all A-1 instars have a single type of frill that is not incurved (?technomorphs), although one specimen has a frill that is incurved (?heteromorph); this suggests that precocious sexual dimorphism occurs in this species. Some A-3 or smaller instars have tubulous rather than completely calcified frills.

The evolutionary development in late juvenile growth stages of a feature that was present only in adults of an ancestor represents a type of heterochrony known as predisplacement. Evolution by the process of predisplacement in this species is interesting because that variety of evolutionary development is relatively uncommon.