Large archosaur burrows are rarely interpreted from the geologic record, a circumstance that may be attributable to a lack of search images based on modern examples, rather than actual rarity. To test this idea, we measured, imaged, and mapped den structures of the American alligator (Alligator mississippiensis) on St. Catherines

Island (Georgia, USA). St. Catherines is an undeveloped barrier island on the Georgia coast, consisting of Pleistocene and Holocene sediments. Alligators dug most dens along the edges of freshwater ponds in loosely consolidated Holocene or Pleistocene sand. Adult female alligators use dens to protect offspring, but burrows also aid in thermoregulation or serve as refugia for alligators during droughts and fires. Some dens are evidently reused and modified by different alligators after initial construction. Drought conditions along the Georgia coast have exposed many abandoned dens, thus better allowing for their study while increasing researcher safety. Den entrances have half-moon cross sections, and based on one sample (n = 20), these range from 22-115 cm wide (mean = 63 +/- 23 cm) and 14-55 cm high (23 +/- 9 cm). In addition to field descriptions, we applied geographic information systems (GIS) and groundpenetrating radar (GPR) to help define the ecological context and subsurface geometry of these structures, respectively. GIS gave spatial data relatable to alligator territoriality, substrate conditions, and proximity to potential nest sites. GPR produced subsurface images of active dens, which were compared to abandoned dens for a sense of taphonomic history. Most den entrances are southerly facing, with tunnels dipping to the northwest or northeast. From entrances, tunnels slope at about 10-15°, turn right or left within a meter, and lead to enlarged turn-around chambers. Collapsed dens in formerly ponded areas (secondarysuccession maritime forests) provided further insights into subsurface forms of these structures. These features are: 3.1-4.6 m long; 30-40 cm deep, relatively narrow at either end (35-60 cm), and 1.2-1.6 m wide in their middles. Expansive areas were probable turn-around chambers, and total volumes of collapsed dens accordingly reflect maximum body sizes of their former occupants. One sampled area (8,100 m2), an almost dry former pond, had 30 abandoned dens, showing how multiple generations of alligators and fluctuating water levels can result in dense concentrations of alligator burrows over time. In summary, the sheer abundance, distinctive traits, and sizes of these structures on St. Catherines and elsewhere in the Georgia barrier islands give paleontologists excellent search images for seeking similar trace fossils made by large semi-aquatic archosaurs.