Taphonomy of a dinosaur breeding colony in southern Patagonia

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

Los Angeles

Congreso; 73rd Annual Meeting Society of Vertebrate Paleontology; 2013

The Late Triassic/Early Jurassic Laguna Colorada Formation of Patagonia contains a unique fossil assemblage dominated by the basal sauropodomorph. Mussaurus patagonicus. This taxon was originally described from several well-preserved post-hatchling specimens associated with egg remains found at the Laguna Colorada type section. Our recent expeditions to this locality have yielded 25 new specimens of this taxon, comprising skeletons of six different ontogenetic stages along with several complete ?nests? of un-hatched eggs. Detailed sedimentological investigation shows the skeletal remains and eggs occur in three distinct horizons within a 3 m- thick bed of mottled light reddish- brown/olive-grey massive siltstone. The bones are encrusted in brown weathering calcareous siltstone similar to the numerous spherically-shaped calcareous nodules that occur in the same horizons. The latter are interpreted as palustrine carbonate precipitated in loessic parent material around a floodplain pond under a seasonally warm climate. The first Mussaurus hatchlings described from this site comprise 8 closely- associated and notably small individuals (femoral length 3 cm). Their proximity to unbroken eggs and eggshell fragments clearly suggests an aggregation of nestlings rather than un-hatched embryos, as their body size largely exceeds that of all the associated eggs, and the lack of size variation among them suggests they are from the same brood. A new aggregation of at least 11 articulated juvenile skeletons (femoral length 12 cm) was found in the vicinity of the type hatchlings. Taphonomic assessment of this specimen rules out any post-mortem transport of the carcasses and suggests these are the result of synchronous death and burial of behaviourally aggregated individuals. These specimens are all the same size and histological data indicates that these individuals died together before reaching the first year of life. Our latest field studies located several clusters of up to 24-27 un-hatched dinosaur eggs close to the Mussaurus juvenile and nestling aggregations, in the same horizons. The eggs clusters lie in 2 layers within elongate depressions or tunnels that appear to have been deliberately excavated in the loess. We propose that the fertilized but un-hatched eggs (at least one with an ossified embryo) were all laid at the same time, possibly by more than one female Mussarus, and left half exposed to incubate. The un-hatched clutches appear to have been asphyxiated by rapid deposition of a thick layer of aeolian silt loaded with volcanic ash.