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Eggshell Taphonomy at Modern Gull Colonies and a Dinosaur Clutch Site

¹ Biology Department, Andrews University, Berrien Springs, MI 49104-0410
² Department of Geology and Geophysics, The University of Calgary, 2500 University Dr. NW, Calgary, Alberta T2N 1N4, Canada
³ Biology Department, Andrews University, Berrien Springs, MI 49104-0410

Eggshell taphonomy often is acknowledged to be worthy of investigation, but few studies have examined factors influencing eggshell preservation. In this study eggshell weathering, 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 dinosaur 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 relatively slowly, suggesting that colony resident activity is responsible for the relatively rapid disappearance of eggshell on the colony surface; (2) depredated eggshells could be distinguished from hatched eggshells due, in part, to differences in fracture patterns; (3) both gull and non-avian dinosaur 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 dinosaur eggshell fragments within nest areas were oriented concave up more commonly than concave down, in contrast to transported eggshell fragments; (5) chicken eggs experimentally 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 following compression beneath simulated sediment loads. Eggshell weathering, fragmentation, orientation, and dispersion patterns should be characterized during the excavation of fossilized eggs and nest sites.

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