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 pat terns of sociality has been elusive. Based on our recent discovery of a con temporary heterospecific mass death assemblage in the Gobi Desert (Mongolia ), we fit predictions about gender-specific associations and group living i n extant ungulates to extinct ones. We relied on comparative data on sex-ra tio variation and body-size dimorphism, basing analyses on 38 additional ma ss mortality sites from Asia, Africa, Europe and North America that span 50 million years. Both extant and extinct species died in aggregations with b iased adult sex ratios, but the skew (from 1:1) was greater for extinct dim orphic taxa, suggesting that sociality in these extinct species can be pred icted from spatial and demographic traits of extant ones. However, extinct rhinos, horses and zebras were inconsistent with predictions about adult se x ratios, which underscores the inherent difficulty in back-casting histori c patterns to some monomorphic taxa. These findings shed light not only on the sociality of extinct species but provide a sound, although limited, foo ting for interpretation of modern death assemblages within the context of t he emerging science of taphonomy and palaeobehaviour.