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Theropod nesting trace with eggs from Upper Cretaceous of Montana



Ben Creisler
bcreisler@gmail.com

A new paper:

Frankie D. Jackson, Rebecca J. Schaff, David J. Varricchio, and James
G. Schmitt (2015)
A theropod nesting trace with eggs from the Upper Cretaceous
(Campanian) Two Medicine Formation of Montana.
PALAIOS 30(5): 362-372
doi: http://dx.doi.org/10.2110/palo.2014.052
http://www.bioone.org/doi/abs/10.2110/palo.2014.052

A nesting trace preserved in alluvial floodplain deposits in the Upper
Cretaceous Two Medicine Formation at the Willow Creek anticline in
north-central Montana contains four crushed theropod eggs referable to
the oospecies Continuoolithus canadensis. These eggs immediately
overlie the lower surface of a 35-cm-long × 7-cm-thick, dark-green
mudstone lens, surrounded by reddish-purple mudstone. The long axes of
three eggs are parallel to one another and to the lower boundary of
the lens, whereas the fourth egg lies at a 30° angle to the others. A
thin, 1-cm-thick organic horizon overlies the eggs, suggesting they
were buried with some vegetation. Geometric modeling of the slightly
asymmetrical C. canadensis eggs yields a volume and mass of
approximately 194 cm3 and 205 g for each egg. This method provides a
more accurate estimation for the surface area than allometric
equations that are based on modern bird eggs because of the elongate
shape of many non-avian theropod eggs. Pore density and water vapor
conductance (GH2O) calculated from one egg in the trace and five
additional C. canadensis eggs from the Willow Creek anticline vary
across three regions. High, moderate, and very low GH2O characterize
the equatorial zone, blunt, and tapering poles, respectively. The
average GH2O for all eggs exceeds that of an avian egg of similar mass
by 3.9×, thus supporting sedimentologic evidence of substrate burial
during incubation.
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