ICHNOLOGY OF SAUROPODOMORPH NESTS FROM PATAGONIA INDICATES EARLY JURASSIC ORIGIN OF HERD-LIVING AND BREEDING SITE FIDELITY

SMITH, ROGER; MARSICANO, CLAUDIA; POL, DIEGO; MANCUSO, ADRIANA CECILIA

Mendoza

Congreso; 4° International Palaentontological Congress; 2014

IPA

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 (+/-2mm). 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 100My earlier than previously thought.