It is not rare to find in the fossil record of vertebrates cases in which fossils from one or few extinct species predominate over others represented in a bone assemblage. This type of findings has happened in deposits of several geological ages and environmental contexts.
A very common interpretation for such deposits is that a single event of mortality occurred (i.e. catastrophic mortality). In this case, what we have is a “family portrait,” because what we find is similar to what the population was at a given moment in the past and reflects the abundance of individuals in different age groups at a given time.
Another possible explanation for this sharp, uneven representation of species in a bone assemblage, most often not considered, is that multiple death events occurred to a single or few individuals of a species (i.e. attritional mortality). In this case, we have a “photo album,” because what we find is individuals who did not coexist in time and represent samples of different generations of the population.
Depending on the interpretation one assumes for the bone assemblage formation, different paleoecological conclusions can be drawn from the same bone assemblage. Since the prevailing view is of catastrophic mortality for such assemblages, the most common interpretation for the existence of a predominant extinct species is that it was gregarious. However, if one considers that attritional mortality was the major process, nothing can be concluded about the behavior of a predominant species.
In our paper “The dominance of an extant gregarious taxon in an attritional accumulation: Taphonomy and palaeoecological implications,” published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology, we added information to challenge the common notion that assemblages dominated by one or few extinct species means that these species were gregarious. We studied a not completely buried bone assemblage formed in the 65m deep pitfall cave Sumidouro do Sansão. This cave is located in the Serra da Capivara region, northeast Brazil, which is one of the richest archaeological and paleontological areas in the country.
We analyzed more than 900 bone/teeth pieces from this assemblage housed in the collection of Fundação Museu do Homem Americano (FUMDHAM), Piauí State, Brazil. By far, the most abundant species of the assemblage was Kerodon rupestris, an endemic-extant rodent from Brazil. This rodent is gregarious, and a habitat specialist that inhabits rocky outcrops. From a total of 38 individuals securely identified, 35 belong to Kerodon rupestris.
A male individual of K. rupestris. Image courtesy Elver Mayer
The emplacement of Kerodon rupestris remains was probably due to the pitfall entrapment of individuals. Besides the morphology of the cave that is suggestive of such process, our interpretation is supported by the fact that we found remains representing skulls and many bones from different parts of the skeleton, and that the skeletons are relatively complete and do not present extensive fragmentation. Based on the known gregarious behavior of the species and its adaptations for climbing, a very logical conclusion for our findings is of catastrophic mortality (e.g. lightning stroke the colony, since the cave is in an elevated position, killing several individuals at once). However, fortunately, we were able to obtain radiocarbon dating from four individuals of Kerodon rupestris. Our chronology reveals that ages range approximately from 8kyr BP to 4kyr BP. Thus, the bone assemblage was formed by the addition of individuals in multiple events, characterizing an attritional accumulation.
Skull of K. rupestris. Image courtesy Elver Mayer
Our finding was surprising because since these rodents inhabit rocky outcrops and are known for their climbing skills — we would not expect them to get trapped in a cave. However, the cave pitfall has steep walls and is deep, which may explain the imprisoning of those animals. In addition, this environment is part of their habitat, representing a regular risk of injuries and entrapment. Lastly, given the chronological range of the assemblage, a death every 115 years would be enough to account for the number of individuals found.
After death, each skeleton remained exposed on the bottom of the pit during an uncertain period of time until the recovery. This period was marked by the occurrence of post-depositional damage on the skeletons. They were disarticulated, their bones were fragmented and a few developed features due to weathering. Also, some bones were covered by incrustation and others perforated by invertebrates, probably for pupation purposes.
Scheme of assemblage formation. Figure courtesy Elver Mayer
Our study shows that even a species that is known to live in groups, such as Kerodon rupestris, can dominate bone assemblages formed by the subsequent death of individuals from different generations of the population. The main paleoecological implication of our finding is that the association of many individuals from the same species in a bone assemblage does not necessarily mean a single event of mortality, and it is not reliable to infer gregariousness for extinct species based solely on the disproportional abundance of one or few species. Thus, understanding the genesis of a fossil assemblage is mandatory to interpret the paleoecological information preserved in the fossil record.
These findings are described in the article entitled The dominance of an extant gregarious taxon in an attritional accumulation: Taphonomy and palaeoecological implications, recently published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology. This work was conducted by Elver Luiz Mayer from the Universidade Federal do Rio Grande do Sul, Leonardo Kerber from the Universidade Federal de Santa Maria, Ana Maria Ribeiro from the Universidade Federal do Rio Grande do Sul and the Fundação Zoobotânica do Rio Grande do Sul, and Alex Hubbe from the Universidade Federal da Bahia and the Instituto do Carste.