EARLY CAMBRIAN MICROBURROW NESTS AND THE ORIGIN OF PARENTING SKILLS

Enigmatic Early Cambrian limestone cavities filled with homogenous matrix and penetrated by microburrows occur in archaeocyathan carbonates of Mexico (northwestern Sonora) and Nevada (Barrel Springs). Matrix was formed by closely packed pellets 0.25-0.5 mm in diameter. Pellets disaggregated to form lime mud penetrated by microburrows. The microburrow swarms occur within shelter porosity spaces that developed early diagenetic fibrous rims. Microburrow swarms also occur inside dead archaeocyath skeletons. As the microburrows did not collapse during diagenesis (and are now preserved by sparry calcite), the burrow complex represents a network of microburrows with stabilized walls that allowed pore water to pass through. I propose here that the microburrow networks created a microbial garden (or “mushroom farm”), where microbes would break down refractory organic matter derived from the pellets. A comparable mushroom farm function has been proposed for the Cambrian graphoglyptid burrow Protopaleodictyon. The animal that formed the pellets was too large to form the microburrows; therefore, the microburrow network may represent a hatchery or nursery for progeny of the pellet makers. If so, the microburrow complex provided food and shelter for the hatchlings. Initial pore space consisted of the spaces between pellets. This initial porosity was subsequently replaced by the microburrows themselves. Microburrow diameter appears to increase from the core of the nest to its outer edges, suggesting that the eggs were deposited near the center of the shelter space. This therefore represents the earliest fossil evidence for nesting behavior. Such sophisticated behavioral programming is remarkable for animals living close to the Proterozoic-Cambrian boundary, and the discovery contributes to the perceived magnitude of the Cambrian explosion.