Sphaerapus, A Poorly Known Invertebrate Trace Fossil From Nonmarine Permian and Jurassic Strata of North America

Abstract

In 1858, Edward Hitchcock named Sphaerapus larvalis and S. magnus for burrows from the nonmarine Lower Jurassic Turners Falls Formation in Massachusetts. Walpia hermitensis White, 1929, from the Lower Permian Hermit Shale, Arizona, is also a little known ichnotaxon. Examination of the type material indicates that Sphaerapus is a senior subjective synonym of Walpia. Sphaerapus is also a nomen oblitum, but Walpia has not been sufficiently used to replace its senior synonym as a conserved name; thus, we use the senior synonym Sphaerapus as the valid name of this ichnogenus, which is monospecific (S. larvalis = S. magnus = W. hermitensis). Numerous specimens of Sphaerapus from the Lower Permian of New Mexico and Texas, and a new record from the Lower Jurassic of Massachusetts show that it is a horizontal endichnion or exichnion having an unpacked tunnel with a cylindrical to ellipsoidal cross section, in some specimens branching, and lined with flattened, lenticular, non-coprolitic sediment pellets that are irregularly arranged and in some specimens imbricated. The pellets distinguish Sphaerapus from other endichnia. Sphaerapus resembles burrows of recent variegated mud-loving beetles and mole crickets, so Sphaerapus was likely produced by a hexapod similarly adapted for subsurface, compaction burrowing. Variation in pellet shape and arrangement in Sphaerapus reflects fluidity of the sediment burrowed, so this variation is a taphonomic artifact of no ichnotaxonomic significance. The latest Wolfcampian first appearance of Sphaerapus is not facies related, as similar facies lacking Sphaerapus are present in middle-late Wolfcampian strata. Therefore, the first appearance of Sphaerapus may reflect the evolutionary first appearance of a hexapod that moved through the sediment by compaction tunneling. However, the stratigraphically disjunct distribution of Sphaerapus may indicate that its preservation and fossilization were sensitive to paleoenvironmental and/or taphonomic factors that may also be involved in its first appearance.