Eggshell Taphonomy at Modern Gull Colonies and a Dinosaur Clutch Site

Abstract

Eggshell taphonomy often is acknowledged to be worthy of investigation, but few studies have examined factors influ-encing eggshell preservation. In this study eggshell weath-ering, fragmentation, dispersal, orientation, and predation were characterized in field and laboratory studies. Modern gull eggshell fragment dispersal and orientation patterns were compared with analogous patterns at a non-avian di-nosaur egg clutch site. Among other findings: (1) hatched and depredated gull eggshells protected by an exclosure, but exposed to two years of colony weathering, fragmented rel-atively slowly, suggesting that colony resident activity is re-sponsible for the relatively rapid disappearance of eggshell on the colony surface; (2) depredated eggshells could be dis-tinguished from hatched eggshells due, in part, to differenc-es in fracture patterns; (3) both gull and non-avian dino-saur eggshell fragments were most abundant close to nest centers, whereas this was not true for bones and mollusc shells in gull territories; (4) both gull and non-avian dino-saur eggshell fragments within nest areas were oriented concave up more commonly than concave down, in contrast to transported eggshell fragments; (5) chicken eggs experi-mentally placed in a gull colony were more likely to be taken by egg predators in areas of short or sparse vegetation than in areas of tall vegetation; and (6) the extent of fracturing was greater in fresh than in hollow chicken eggshells follow-ing compression beneath simulated sediment loads. Egg-shell weathering, fragmentation, orientation, and disper-sion patterns should be characterized during the excavation of fossilized eggs and nest sites.