PIT-TRAPPING PREDATION STRATEGY: THE FIRST RECORD OF ANT-LION PITS IN THE FOSSIL RECORD

Trapping is a specialized feeding behavior that utilizes external resources to help catch prey and is recognized in the fossil record only rarely. Trapping traces that have been reported include spider webs, scorpion pits, and burrows attributed to cerianthid anemones, echiuran worms, and polychaete worms. Ant-lion traps have not yet been identified from the fossil record although a burrow of similar morphology has been reported from the Devonian, which is far earlier than the ant-lion body fossil record.

Trace fossils consisting of small conical pits are reported from the Middle Permian Coconino Sandstone of northern Arizona. These occur singly or in groups at the foot of dune faces and on eroded bounding surfaces. As with modern ant-lion pits these show raised rims and in some cases a surrounding pad of sand that appears to have been thrown out during excavation, indicating that they represent primary structures and are not a diagenetic artifact. They vary in size but average about 15 mm in diameter and 6 mm deep.

The architecture of these traces most closely resembles that of the traps constructed by modern ant-lion larvae (Myrmeleodontidae), which are known as body fossils since the Early Permian. Modern ant-lions are concentrated in arid sandy regions and their larvae construct pits by scuttling backwards in a tightening spiral while displacing sand to the side, creating a conical depression. They then lie in wait at the bottom of the pit, capturing small prey that slide down the loose sides. Ant-lions could not have been feeding on ants in the Permian as ants are only known in the fossil record from the Albian. However, they may have fed on other prey or alternatively the pits may represent ant-lion predation strategy by another insect taxon. The Coconino Sandstone is well known for its vertebrate and invertebrate trace fossils and likely represents a diverse, in-situ fauna in which it would be quite reasonable to find such trapping behavior.