Burrowing into marsupial paleobiology

I can usually get through an entire scientific paper (in English) without having to look up the definition of a word. So when I recently read a paper that had a word in the first sentence I’d never heard of, it caught my attention.

The word in question was crotovine, which is more commonly written in English as crotovina or krotovina, the latter being the original Russian spelling. However you spell it, the term refers to an animal burrow that subsequently fills with sediment. According to this book on soil biology, it was originally used in reference to burrows of marmots (large, terrestrial squirrels of the genus Marmota that we in the Midwest call woodchucks or ground hogs), but it is now used to refer to filled burrows of many vertebrates and invertebrates alike.

The study caught my interest because it was about a crotovina from the east coast of Argentina near the city of Mar del Plata, which is about 400 km south of Buenos Aires. The sea cliffs in this area, particularly near the town of Chapadmalal, are famous for the fossils of mammals and other animals that have been found there, which are between about 5 and 3 million years old (Pliocene). In fact, they are considered the most characteristic fossils of this age and are the basis of what is known as the Chapadmalalan South American Land Mammal Age (SALMA).

The outcrops in the Chapadmalal area have long been known to be filled with the remains of ancient animal burrows in addition to animals themselves. Many of these are relatively small and were probably made by correspondingly small animals, including extinct relatives of tuco-tucos (family Ctenomyidae; the South American equivalents of North American pocket gophers), and small notoungulates of the family Hegetotheriidae (Paedotherium spp.). However, somewhat larger burrows have also been identified and attributed to armadillos and even ground sloths (in slightly younger strata).

Diagram of a rather complex crotovina from the sea cliffs between Mar del Plata and Miramar, Argentina, produced by the rodent Actenomys. The diagram on the left shows the gross structure of the crotovina; the one on the right shows the five different types of sediments that filled the ancient burrow, which indicate that it represents ten distinct overlapping tunnels. Diagrams modified from Genise (1989:figs. 2-3).

How does one know which animal dug a particular burrow? Finding remains inside can be an important clue. However, many burrow-dwelling animals simply occupy burrows dug by others, sometimes even living there at the same time. Remains can also wash in from the outside. The size and other characteristics of the burrow can usually be used to narrow down at least its original excavator.

The striking aspect of the crotovina studied by Marcos Cenizo and colleagues is that the remains of a variety of animals were preserved in this ancient burrow rather than just one. These extinct species include relatives of modern opossums (Thylophorops chapadmalensis), armadillos (Ringueletia simpsoni), raccoons (Cyonasua lutaria), viscachas (Lagostomus incisus), tuco-tucos (Actenomys sp.), guinea pigs (Microcavia sp.), and even a Pacman frog (Ceratophrys sp.). Although all of these animals were probably diggers (based on their modern relatives), the burrow in which they were found was most likely excavated by an armadillo, based on the burrow’s relatively large size (nearly 1 meter or 3 feet in greatest diameter). The simplest explanation is that it was produced by Ringueletia simpsoni, the same species whose remains it preserves.

But the story doesn’t stop there. Sometime after the armadillo died, the cave was flooded – probably from rising groundwater levels – which left a thin layer of clay on the bottom of the burrow. Then other inhabitants took up residence, as often occurs in many modern ecosystems. The Microcavia and Actenomys specimens were found in coprolites (ancient fecal material), indicating that they were more likely prey than residents, though it is possible they were the latter before the former. These small rodents could have been preyed upon by either Thylophorops or Cyonasua, as both of these animals were opportunistic carnivores large enough to kill these small rodents. (They were similar in size to a modern Virginia opossum or a house cat). The Lagostomus and frog remains show no obvious signs of predation, so these animals could have been residents of the burrow at one time or another or had their remains washed into it sometime later.

Mammalian predator–prey relationships and reoccupation of burr
Images of the crotovina studied by Cenizo et al. including: (A) its appearance in the field; (B) a cartoon highlighting its extent; and (C) a diagram of the locations of fossils within it. In C, letters correspond to the animals listed in the key. The purple shape is where more than 40 osteoderms and other bones of the armadillo Ringueletia were collected. The two grey ellipses represent coprolites. Modified from Cenizo et al. (2016:fig. 2).

The specimens identified as Thylophorops add an additional level of intrigue to the story: they belong to two individuals rather than just one, and the snout of one specimen shows signs of having been bitten. These tooth marks could have been inflicted by Cyonasua, another Thylophorops, or a third predator not represented by fossil remains. The size and position of the bite marks, as well as the fact that modern opossums sometimes kill other members of the same species in violent interactions – particularly in confined spaces – suggest that one Thylophorops probably inflicted the damage on the other. Perhaps one Thylophorops was living in the burrow when another decided it looked too inviting to pass up…

Mammalian predator–prey relationships and reoccupation of burr
A snout of the small marsupial Thylophorops in left lateral (upper left), palatal (lower left), and dorsal (upper right) views, with inset box (lower right) showing the pitting and scoring interpreted as tooth marks. Modified from Cenizo et al. (2016:fig. 3).

Assuming this scenario is correct, this study presents an interesting contrast with another study of extinct marsupials (more or less*) that was published a few years ago in Nature by Sandrine Ladevèze and colleagues. Like the Chapadmalal study, this one described fossil remains found in an ancient burrow. But quite unlike the Chapadmalal study, it concluded that this shrew-sized, much more distant relative of modern opossums known as Pucadelphys andinus was social and peacefully coexisted in groups of at least 35 individuals – the number of specimens found in a single burrow (technically a crotovina) at a 64 million-year-old site in central Bolivia. Such behavior is unknown among modern opossums and highlights the perils of extrapolating what we know about animals alive today to those that lived in the past, particularly those that are not closely related to living species.

Both of these studies do an excellent job of illustrating one of the most fundamental principals of paleontology: the context in which fossils are found is just as important as the fossils themselves.

*This species may have branched off the marsupial evolutionary tree before the various groups of living marsupials diverged from one another. If so, then it is technically a metatherian but not a marsupial.

References cited:


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