Back-casting sociality in extinct species: new perspectives using mass death assemblages and sex ratios

Despite 150 years of interest in the ecology of dinosaurs, mammoths, proto-hominids and other extinct vertebrates, a general framework to recreate patterns of sociality has been elusive. Based on our recent discovery of a contemporary heterospecific mass death assemblage in the Gobi Desert (Mongolia), we fit predictions about gender-specific associations and group living in extant ungulates to extinct ones. We relied on comparative data on sex-ratio variation and body-size dimorphism, basing analyses on 38 additional mass mortality sites from Asia, Africa, Europe and North America that span 50 million years. Both extant and extinct species died in aggregations with biased adult sex ratios, but the skew (from 1:1) was greater for extinct dimorphic taxa, suggesting that sociality in these extinct species can be predicted from spatial and demographic traits of extant ones. However, extinct rhinos, horses and zebras were inconsistent with predictions about adult sex ratios, which underscores the inherent difficulty in back-casting historic patterns to some monomorphic taxa. These findings shed light not only on the sociality of extinct species but provide a sound, although limited, footing for interpretation of modern death assemblages within the context of the emerging science of taphonomy and palaeobehaviour.