THE TAPHONOMIC SIGNATURE OF PENNSYLVANIAN SEED PREDATION

Pennsylvanian seeds and ovules pose an intriguing paleoecological problem. Thick, woody seed coats, variations in seed coat morphology, and the occurrence of drilled ovules suggest that insect and vertebrate seed predation occurred in the Pennsylvanian. In addition to drilling insects, paleopteran insects with long probosci and cibarial pumps may have sucked the contents of the megaspore through the micropyle without drilling the seed coat. Large ovule fragments are common in Pennsylvanian permineralized peat; these may result from germination, vertebrate seed predation, or other taphonomic processes. Possible Pennsylvanian vertebrate seed predators include early amniotes (captorhinids), and synapsids (ophiacodontids and edaphosaurids). Although the possible seed predators remain poorly known, the taphonomy of Pennsylvanian ovules provides insight into ancient seed predation.

The largest, strongest Pennsylvanian ovules belong to the medullosan genus, Pachytesta, with 16 described species. A taphonomic survey of permineralized peat from three localities in the mid-Moscovian Kalo Fm. of Iowa revealed 40 Pachytesta specimens for which the condition and contents of the seed cavity could be evaluated. Of these, 30% consist of single valves, separated along commissures, consistent with germination, although single valves could result from other taphonomic processes. Ovules with an intact seed chamber and megaspore or nucellus (12% of evaluated ovules) have not been eaten. Ovules with an intact seed chamber, a fragmented megaspore and no detritivore coprolites (10% of evaluated ovules) have a taphonomic pattern consistent with paleopteran seed predation. One third of the ovules with an intact seed chamber (12% of evaluated ovules) could not be assigned to either category (i.e. uneaten, or possibly predated). Thirty-five percent of evaluated ovules occur as groups of fragments. The predation status of fragmented ovules separated at the commissures (13% of evaluated ovules) could not be determined. Fragmented ovules with a nucellus or megaspore (7% of evaluated ovules) have not been eaten. Fragmented ovules without a nucellus or megaspore, cracked in the middle of the valve (between commissures) may have been eaten by vertebrates; 15% of evaluated ovules belong in this category.