PROBABLE POLYCHAETE WORM BROOD CAPSULES FROM THE PENNSYLVANIAN OF MIDCONTINENT, NORTH AMERICA

Black, organic-walled, microfossils have been recovered from Upper Pennsylvanian (Missourian and Virgilian) marine units in Iowa, Missouri, Kansas, Oklahoma, and a Permian limestone in Kansas. They are cylindrical in shape and usually inflated on one side with respect to the long axis of the cylinder. Longer forms are 1200-2200 microns in length and 300-500 microns in diameter, while shorter forms are 800-1200 microns long and 500-700 microns in diameter. Shoulders on each cylinder end slope to a narrow neck that connects one cylinder to the next, in a chain-like configuration. Most connections between vesicles show as a simple enlargement of the neck at the juncture. A small tubule, 30-50 microns long and 50-80 microns in outside diameter, is located on the inflated side of one of the cylinder shoulders, but sometimes occurs on the connecting neck. Exceptionally well-preserved specimens exhibit a flange, 150-250 microns in diameter and 1-4 microns thick, arranged around the distal end of the small tube. The outer surface is very smooth, shiny, and without ornamentation. The wall thickness is 6-10 microns and only rarely shows evidence of layered structure. These fossils are always associated with annelid worm jaw elements (scolecodonts), and except for the tubule, are of similar shape and size to brood capsules produced by some extant polychaete families (e.g. glycerids). Most specimens are found in the dark gray to black shale facies of the core shale of cyclothems, as defined by Heckel, in abundances of up to several thousand per kilogram, but can also rarely be found in the more offshore facies of the transgressive and regressive limestones. Even though there is no direct evidence, I suggest that these structures are the brood capsules of Paleozoic marine polychaete worms. Eggs in the capsule would mature into larvae (trochophore?), which at a certain growth stage (late larval or early adult) escaped from the brood capsule by the small flanged tubule.