THE TRACE FOSSIL SCOYENIA GRACILIS IN THE UPPER DEVONIAN (FAMENNIAN) CATSKILL FORMATION, NORTH-CENTRAL PENNSYLVANIA: IMPLICATIONS FOR THE EARLY EVOLUTION OF THE LARVAL STAGE AND HOLOMETABOLOUS INSECTS

Burrows assigned to Scoyenia gracilis have recently been discovered in fluvial channel-bar and overbank deposits of the Upper Devonian (Famennian) Catskill Formation (Fm) at Canton, Powys Curve, Steam Valley, Tioga, and Trout Run in north-central Pennsylvania. The burrows occur in redbeds composed of fine-grained sandstone interbedded with mudstone often modified by pedogenesis. These specimens represent the earliest stratigraphic occurrence of this ichnotaxon bearing all the hallmarks of S. gracilis White 1929 from the type section of the Permian Hermit Shale, Grand Canyon, Arizona.

The architectural and surficial morphologies of S. gracilis are distinct. The burrows range from 2–5 mm in diameter, horizontal to vertical in orientation, unbranched, gently curved or loosely sinuous, and exhibit areas of widening and narrowing along their length. The burrow surface is completely covered in closely spaced, short, paired and tapered scratch marks that produce a ropelike appearance. The burrows contain backfill meniscae that often exhibit a helical pattern along the burrow length. Overall, these burrows are identical to S. gracilis in Mesozoic and early Cenozoic continental deposits in North America, Europe, and Africa.

The tracemaker of S. gracilis is thought to have been a larva analogous to those of the modern Tipulidae (Diptera; apodous, hemicephalus larvae) and/or members of the Scarabaeidae (Coleoptera; oligopod, scarabeiform) composed of a gelatinous, segmented, and elongate body with a reduced or full head shield with mandibles. Most importantly, the body of the Devonian tracemaker would have had paired setae or tubercules on each segment of the thorax and/or abdomen in order to produce the scratch marks on the circumference of the burrow wall. It also would have moved by peristalsis to produce the widening and narrowing of the burrow morphology.

If our observations and interpretations are accurate, then S. gracilis likely represents the earliest evidence for the evolution of the insects that possessed a geophilic (epigeal + subterranean) life history that involved the life stages of egg, larva, pupa, and adult. Such insects would belong to the supraordinal clade Holometabola (Endopterygota) within the infraclass Neoptera. This was a major ecological innovation for building terrestrial ecosystems.