NEOICHNOLOGY OF EXTANT EARTHWORM (ANNELIDA: OLIGOCHAETA) CASTS TO ESTIMATE EDAPHICHNIUM LUMBRICATUM TRACEMAKER BODY SIZES

Earthworms are considered ecosystem engineers because their burrowing and casting behaviors contribute significantly to soil development, maintenance, and stability. The pelleted ichnofossil Edaphichnium lumbricatum in the Paleogene Willwood Formation, Bighorn Basin, Wyoming, was likely produced by the subsurface foraging of geophagous earthworms and suggests well-developed ancient soil conditions. Typical specimens are composed of cylindrical, sometime branching structures filled with carbonate-cemented masses of ovoid or ellipsoidal pellets. Pellet sizes are generally positively correlated with burrow sizes and range from 0.5–11.6 mm long and 0.3–7.5 mm in diameter. Massed or loose pellets not obviously associated with burrows are also present. Approximately 2,526 individual pellets from 178 burrows in 48 Willwood Formation locations were measured for their length, diameter, and surficial pellet density.

In order to estimate the potential body sizes of the Edaphichnium tracemaker, specimens of such extant oligochaete species as Eudrilus eugeniae, Lumbricus terrestris, and Eisenia hortensis (African, Canadian, and European night crawlers, respectively) were allowed to burrow individually in soil filled enclosures. Surface casts were removed daily and their length and diameter recorded. After two weeks, the earthworms were euthanized in a 1:1 mixture of ethanol and water to facilitate consistent measurements of the body, anterior region, and clitellum lengths, and the diameters of the peristomium, clitellum, mid-body, and periproct. Of these, body length, mid-body diameter, and peristomium diameter of E. eugeniae had a significant linear relationship (R² >= 0.93) with the diameter of casts produced. This relationship suggests that the average 2.5 mm diameter Edaphichnium pellet was produced by earthworms ~178 mm long and ~5.8 mm in diameter. An earthworm estimated to be ~366 mm long and ~13 mm in body diameter would be inferred for the largest fossil pellets based on the data collected thus far.

These data can be used to speculate about ancient soil relationships, and may be extended to other areas of earthworm paleontology and evolution. Furthermore, diagnostic criteria are being collected to help distinguish between earthworm casts and the traces of other pellet producing biota.