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Troodon egg porosity and archosaur nest humidity



From: Ben Creisler
bcreisler@gmail.com


New new papers:


David J. Varricchio , Frankie D. Jackson , Robert A. Jackson, and
Darla K. Zelenitsky (2013)
Porosity and water vapor conductance of two Troodon formosus eggs: an
assessment of incubation strategy in a maniraptoran dinosaur.
Paleobiology 39(2):278-296. 2013
doi: http://dx.doi.org/10.1666/11042
http://www.bioone.org/doi/abs/10.1666/11042

Using tangential thin sections, we examined variation in porosity and
water vapor conductance across two eggs of Troodon formosus, a small
(50 kg) theropod dinosaur from the North American Upper Cretaceous,
testing two hypotheses of egg incubation: (1) full burial within
sediments or vegetation and (2) partial burial with exposed upper egg
portions. We divided and sampled the eggs in five zones, 1 through 5
from blunt top to more pointed bottom. A geometric model composed of a
hemisphere, cone, and paraboloid was used to estimate total and zonal
volumes and surface areas. The 138 × 67 mm idealized Troodon egg has a
volume, surface area, and mass of 296.4 cm3, 239.23 cm2, and 314.2 g,
respectively. Zonal surface areas and volumes highlight the strongly
asymmetric and elongate form of the Troodon egg. Geometric modeling
provides better estimates of volume and surface area where egg shape
diverges markedly from that of a typical bird egg. Porosity varies
significantly across both Troodon eggs, with zones 2 and 3 having the
largest pores and a majority (70–78%) of total conductance, whereas
zone 5 has very low conductance. Total water vapor conductance in the
two eggs are 31.85 and 40.62 mg H2O day− Torr−, values 76% and 97% of
those predicted for an avian egg of similar size. Low total
conductance compares favorably to values in extant birds and non-avian
reptiles that incubate in open nests, arguing against full burial
incubation. Together with nesting site evidence, low conductance
values favor partial burial and incubation by a Troodon adult.
Asymmetric egg shape concentrates volume, surface area, and
conductance near or at the point of subaerial exposure. Among
non-avian dinosaurs, the eggs of Troodon and troodontids are most
similar to those of modern birds in having an asymmetric shape, low
porosity, no ornamentation, and three structural eggshell layers.

====

Kohei Tanaka & Darla K. Zelenitsky (2013)
Relationships between nest humidity and nest types in living archosaurs.
Historical Biology (advance online publication)
DOI:10.1080/08912963.2013.772169
http://www.tandfonline.com/doi/full/10.1080/08912963.2013.772169


Nest humidity (P nest) is important for archosaur incubation because
it has a direct effect on the water vapour conductance (GH2O) of the
eggs. Estimates of GH2O  are commonly used to infer nest type in
extinct archosaurs, although it is unknown whether a relationship
exists between nest type and nest humidity or between nest type and
GH2O . In this study, the nests of 54 living archosaur species (i.e.
birds and crocodilians) were classified into three generalised nest
types, including covered, non-covered cup and non-covered scrape
nests, based on their nest architectures. P nest and ambient humidity
of the nesting habitat (P a) of these species were compared among
these three nest types. Statistical analyses show that P nest is
significantly higher in covered nests than in the non-covered nests,
including both cup and scrape types, indicating that the covered nests
are significantly more humid than the non-covered nests. Although P a
is lower than P nest in all species, P a is not significantly
different among the three nest types. These results suggest that
although P a has a fundamental affect on P nest (i.e. P nest ≥ P a),
the nest humidity is affected by the architecture of the nesting
materials, that is the covered nests are shown to retain more humidity
than the non-covered nests.