Mussaurus and the social behaviour of early sauropodomorphs

Mussaurus patagonicus with neonate individuals. Credit: Jorge Gonzalez.

Sauropods were the largest terrestrial vertebrates. Their morphology is easy recognizable: a long, slender neck and a tail at the end of a large body supported by four columnar limbs. They were the first successful group of herbivorous dinosaurs, dominating most terrestrial ecosystems from the Late Triassic to Late Cretaceous. The group evolved from small, gracile, bipedal forms, and the acquisition of giant body size occurred during the Jurassic. The early sauropodomorph Mussaurus patagonicus was originally described from several well-preserved post-hatchling specimens associated with egg remains found at Laguna Colorada Formation (Late Triassic/Early Jurassic) in Argentina. The remains were briefly described by Jose Bonaparte in 1979. Now, the discovery of 80 individuals of Mussaurus patagonicus, ranging from embryos to fully-grown adults, and more than 100 eggs, provides the earliest evidence of complex social behaviour.

Nest with eggs of Mussaurus patagonicus. Image credit: Diego Pol.

The new findings comprise skeletons of six different ontogenetic stages ranging from embryos to adult individuals, that are either fully-articulated or partially disassociated. Their bones show a 0.5 cm-wide phosphatic halo related to the microbial decomposition of soft-tissue, which suggests that the skeletons were buried relatively rapidly at the same time. Histological analysis of the juvenile aggregation suggests a fast growth rate, while the absence of lines of arrested growth (LAGs) in their femora, tibia, humeri, and ribs, combined with their size, indicate that they were possibly members of a single brood. The eggs and nests of Mussaurus were found in three distinct horizons in the middle of the Laguna Colorada Formation. X-ray computed tomography reveals that the eggs were arranged in two or three layers within elongate depressions or trenches that appear to have been purposely excavated.

The research team lead by Diego Pol calculated the site’s age at 193 million years, predating previous records of social behavior in dinosaurs by at least 40 My. The researchers also suggest that sociality may have influenced the early success and the first global radiation of sauropods.

 

References:

Pol, D., Mancuso, A.C., Smith, R.M.H. et al. Earliest evidence of herd-living and age segregation amongst dinosaurs. Sci Rep 11, 20023 (2021). https://doi.org/10.1038/s41598-021-99176-1

Bonaparte, J. F. & Vince, M. E. hallazgo del primer nido de dinosaurios triásicos (Saurischia, Prosauropoda), Triásico Superior de Patagonia, Argentina. Ameghiniana 16, 173–182 (1979).

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