INVESTIGADORES
POL Diego
congresos y reuniones científicas
Título:
Ichnology of sauropodomorph nests from Patagonia indicates Early Jurassic origin of herd-living and berrding site fidelity.
Autor/es:
SMITH, R.; MARSICANO, C.A.; POL, D.; MANCUSO, A.C.
Lugar:
Mendoza
Reunión:
Congreso; 4th International Paleontological Congress; 2014
Resumen:
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The Late Triassic/Early Jurassic Laguna Colorada
Formation of Patagonia contains a unique sub- polar fossil assemblage dominated
by the basal sauropodomorph Mussaurus patagonicus. This taxon was
originally described from 8 well-preserved post-hatchling specimens associated
with unbroken eggs and shell fragments found at the Laguna Colorada type
section. A new detrital zircon date for the egg-bearing interval at this site
of ±192.57 Ma is contemporaneous with the Massospondylus eggs with
embryos recently described from South Africa. Our collecting at the original Mussarus
site yielded 25 partial Mussaurus skeletons of six different
ontogenetic stages, from embryonic to adult, associated with several complete
?nests? of un-hatched eggs. The taphonomic signature is of an attritional
accumulation of a stable (non-migratory) population living at this site
year-round. The skeletons and egg clusters occur in three distinct horizons
within a 3 m-thick sequence of mottled light reddish- brown/olive-grey massive
siltstone. The host sediment is interpreted as wind transported loessite
deposited around a floodplain lake under a seasonally dry climate. Eggs are all
of similar dimensions and slightly ovoid in shape. The shell lacks discernable
columnar structure, and is notably thin (±2 mm). Pores are evident on the outer
surface as small inverted cones linked by shallow sinuous grooves. Pore
densities do not vary within or between eggs. From compression breakage
patterns of complete eggs and stacked shell fragments of hatched eggs, we infer
that eggs were left open to incubate. The spacing of the nests suggests a
common breeding ground rather than a colony, and the repeated occurrence of
nests and skeletons through the 3 m-thick interval indicates year-on-year
breeding at this site. Field observations and X-ray imaging of 5 complete nests
confirms that 24-27 eggs are arranged in 2 or 3 layers within elongate
depressions or trenches with an irregular bottom profile. These depressions
appear to have been purposely excavated into semi-consolidated loess and as
such they qualify as nest structures. The attitude and juxtapositions of the
eggs within the nest hollows suggest that they were laid in repeated episodes
of 3-4 at a time. To date the oldest record of nest digging and site fidelity
has been from Late Cretaceous titanosaur sauropods. Our new ichnological
evidence from Laguna Colorada Formation strongly indicates that sauropodomorphs
of sub-polar Gondwana were practicing herding, nest digging and breeding site
fidelity as far back as the Early Jurassic, at least 100 Ma earlier than
previously thought.