BORED FUNGI: PROTOTAXITES-ARTHROPOD INTERACTIONS DURING THE DEVONIAN AND IMPLICATIONS FOR EARLY PENETRATION OF VASCULAR PLANT WOOD

The enigmatic fossil fungus Prototaxites was a massive, indurated, columnar thallus up to 8 m in height and 1 m in diameter, occurring from mid Late Silurian to latest Devonian in mostly perimarine Euramerican biotas dominated by bryophytes, primitive vascular plants and arthropods. Although Prototaxites was nutritionally heterotrophic and a probable saprobe on thick accumulations of plant litter, little is known of its ecological relationships with other organisms. However, borings consistent with an arthropod origin are known and exhibit characteristic tunnel geometry, evidence of host fungal response to tunneling, and distinctive internal contents. Two specimens of bored Prototaxites are known: one from the Early Devonian (Emsian) of Gaspé, Quebec, and other from the Late Devonian (Famennian) of Kettle Point, Ontario.

The network of borings in the Kettle Point specimen exhibits several features involving arthropod penetration of live fungal tissue, perhaps analogous to extant beetle borings in hardened bracket fungi. An extensive, rarefied tissue zone, reminiscent of necroses in plant tissues, surrounds the borings. This contrasts to an inner, thin but variably thickened, opaque region of hyphal mat lining the tunnel, analogous to plant callus and indicating host response. Small coprolites, 100-200 mm in diameter, bear degraded tubular (hyphal) structures that occasionally protrude above their smooth surfaces, suggestive of digestive processing. Each coprolite is enveloped by a narrow evacuated zone, suggesting in situ shrinkage. Reconstruction of the tunneling network is ongoing, but preliminary observations reveal a 3-D, unoriented network interrupted occasionally by more expansive galleries.

The arthropod culprit remains unidentified. Devonian possibilities include oribatid mites, albeit the disparity between small coprolite size and a much larger tunnel diameter is incongruous. These networks also are inconsistent with known myriapod life-habits, and any similarity to holometabolous larval or adult insect borings is countered by a 55 m.y. gap before the earliest occurrences of such derived insects. Identity aside, the borer of chitin-bearing fungal tissue may have switched to lignified woody plant tissue during the Late Devonian.