30-3 Crushed and Collapsed: Taphonomic and Paleoecological Clues to Herbivory and Detritivory in Paleozoic coal balls

Session: Laws of the Grave: Advances in Taphonomy Across the Paleontologic Record (Posters)

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Abstract:

Mid-Pennsylvanian coal balls from the Williamson No. 3 mine of Iowa contain the earliest known swamp community dominated by seed plants. Coal balls, i.e. concretions of permineralized peat, enable us to reconstruct the paleoecology and taphonomy of ancient swamp communities, addressing questions of evolutionary paleoecology.

Medullosan seed ferns, which were small trees and vines, are one of the most common seed-plant groups in mid-Pennsylvanian coal balls, and the most common medullosan fossil is Myeloxylon, the branch system of medullosan compound leaves. Coal-ball Myeloxylon fall into four taphonomic categories: nearly complete uncrushed (15%); large uncrushed fragments (25%); nearly complete, collapsed (25%); and fragments of hypoderm (35%). Myeloxylon has many resin ducts and is often preserved as charcoal, leading to the hypothesis that medullosans were fire prone. Myeloxylon debris burned more often than cordaitean stem wood (another common seed-plant group in the assemblage: 57% burned Myeloxylon vs 42% burned cordaitean), suggesting that medullosans may have been fire prone. The high percentage of large fragments or nearly complete Myeloxylon, which were also burned (10 of 12), suggests that burning shielded Myeloxylon from further decomposition.  

Myeloxylon have an exterior hypoderm with sclerenchymatous fiber bundles surrounding interior parenchyma with scattered vascular strands. Two exceptional Myeloxylon specimens preserve intact frass-filled, arthropod tunnels, 4-5 mm wide, in the parenchymatous interior.  Arthropods apparently targeted soft parenchyma cells and avoided hypoderm fiber bundles. Crushed and collapsed Myeloxylon with intact hypoderm could result from similar large tunnels made by detritivores or herbivores. In peat, roots typically grow into tunneled plant debris; collapsed Myeloxylon penetrated by roots may signal the presence of former tunnels. Detritivores probably targeted unburned Myeloxylon: 31% of tunneled specimens were burned compared to 57% overall. The most common Myeloxylon fossils are fragments of the exterior hypoderm, with fiber bundles that may have been difficult for detritivores to process.

In the Late Pennsylvanian, spore-reproducing plants (first lycopsids, then tree ferns) replaced seed plants as the dominant group in peat swamps, although medullosans persisted in swamps in low abundance into the Permian. Of the three plant groups, medullosan peat has larger coprolites and arthropod tunnels, signaling higher rates of nutrient recycling. Understanding medullosan taphonomy and paleoecology will help elucidate why lycopsids and tree ferns replaced seed plants as the dominant group in Pennsylvanian peat swamps.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10061

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