Social Behavior in Palaeocene Metatherians

Textbooks often describe late Cretaceous or early Palaeocene mammals as small, nocturnal, and solitary (Figure 1). That description has recently been amended thanks to a remarkable suite of fossil metatherians from Bolivia.

sinodelphys
Figure 1. An artist’s reconstruction of the stem metatherian Sinodelphys from the early Cretaceous. (From Prothero and Buell Evolution: What the Fossils Say and Why It Matters, 2007)

Tiupampa, a fossil rich site in the heart of Bolivia, has begun to yield a treasure-trove of early mammalian fossils. Among them are large numbers of well-preserved skulls and post cranial material from several stem metatherians (marsupials). A recent report by Ladevese and colleagues (2011) in the journal Nature describes an exceptionally large, and extraordinarily preserved group of skulls and skeletons from Pucadelphys andinus, a stem-metatherian. To date, 35 individuals have been recovered from these early Palaeocene deposits. What makes this find so unusual is that the skulls, mandibles, and post cranial skeletal material is so well preserved and that they were recovered from an area of just a few square meters. The skeletons remain articulated suggesting that the animals were buried rapidly during a catastrophic event (Figure 2).

Pucadelphys
Figure 2. Skulls and partial skeletons of P. andinus from basal Palaeocene beds at Tiupampa (Bolivia). The lower plate depicts a fossil block and a reconstruction of the same two animals. (From Ladeveze et al. 2011)

The catastrophic burial coupled with the fact that 35 individuals of
P. andinus, including adults, sub-adults, and juveniles occur together within a few feet of one another suggests that these stem metatherians were social (or at least were very tolerant of other individuals). This population also exhibits fairly strong sexual dimorphism, with males 35% larger than females.

Sexual dimorphism in living metatherians appears connected with
semelparity, but P. andinus could not have been semelparous (because adult males, sub-adults and juveniles all co-occurred in the same population). Alternatively, sexual dimorphism may be related to male dominance over territorial females, but this is unlikely for the Palaeocene P. andinus populations since several adult males and females co-occurred in the population (suggesting that females were not territorial). Thus, it appears that the sexual size dimorphism in P. andinus was associated with male–male competition, and polygyny (where one male mates with multiple females during one breeding season).

Such gregariousness in stem metatherians contrasts sharply with the mostly solitary life styles of modern South American matatherians. Perhaps it is time to rethink the behavior of basal metatherians.
Ladeveze et al. (2011) conclude that “social interactions occurred in metatherians as early as the basal Palaeocene and that solitary behavior may not be plesiomorphic for Metatheria as a whole.”


References

Ladev├Ęze, S., de Muizon, C., Beck, R., Germain, D., & Cespedes-Paz, R. (2011). Earliest evidence of mammalian social behaviour in the basal Tertiary of Bolivia Nature DOI: 10.1038/nature09987