Between 23 and 16 million years ago, just outside of where the city of Lisbon, Portugal sits today, there lived a unique mix of mammals which would have seemed both strange and familiar. From bones and footprints left in fossilized feces, paleontologists have found that rhinoceros, deer, horses, antelope, and elephants browsed and grazed in the ancient ecosystem, and many were preyed upon by archaic carnivores such as the fearsome amphicyonids (popularly known as "bear-dogs"). That such confrontations occurred can readily be inferred by the presence of large predators and prey in the same place, but direct evidence of interaction is rare. It does not require a stretch of the imagination to envision a large bear-dog grappling a fleeing antelope or rhinoceros to the ground, but how do we really know that such events took place?

Without a time machine, it can be extremely difficult to tease out the relationships between extinct organisms, but every now and then paleontologists find a rare specimen which records the interaction of two species. One such specimen, a bit of the left lower jaw from the rhinoceros Iberotherium rexmanueli, was described by scientists Miguel Antunes, Ausenda Balbino, and Léonard Ginsburg in 2006. Although rather plain-looking at first sight, the specimen is remarkable for exhibiting a series of pits and scratches that were most probably made by the bear-dog Amphicyon giganteus.

The Iberotherium specimen in question is the middle portion of the animal's lower jaw; the front of the jaw is missing and the rear portion was accidentally broken off during collection. There are a number of small pits and perforations along the jaw which indicate that a carnivorous mammal stripped the flesh off it, with some of the most conspicuous damage being seen towards the front of the bone. Here, near the area of the left jaw which would have met with the front of the right jaw (called the symphysis), there are three indentations on both sides which were probably made when the carnivore gripped the jaw with its incisors.

The hypothesis that Amphicyon giganteus was probably the offending carnivore came out of a process of elimination. Although the large carnivore Dinocyon also lived around the same time, its remains have not been found from the same locality, and while the bear-dog species Amphicyon major also prowled nearby, its presence in the Lisbon beds has not been confirmed. Likewise, the size of the bite marks indicated the activity of an animal the approximate size of a brown bear, and other contemporary candidates were not large enough to fit the profile. Hence the scientists were left with only one valid candidate - Amphicyon giganteus.

But, even if we can be confident in the identification of Amphicyon giganteus as the predator in question, does the tooth-marked jaw represent a predation event or scavenging? It is impossible to know for sure. Antunes and colleagues propose that the pattern of toothmarks on the jaw suggest that the Amphicyon was holding onto and biting the rhinoceros mandible in a manner similar to how we eat corn on the cob. Hence they hypothesize that this means that the carnivore was eating a largely-intact carcass "on the spot." If true, this means that the Amphicyon either killed the rhinoceros - perhaps preying upon an individual weakened by droughts which frequently occurred during the time - or was lucky enough to happen upon an intact Iberotherium which had been killed in a flood.

Yet, as a large carnivore, Amphicyon would have also been capable of tearing off portions of a carcass - such as a head or jaws - and carrying them away to consume in relative peace. This hypothesis could also apply to either a hunting or scavenging scenario, but without the rest of the Iberotherium skeleton, it is impossible to tell exactly what happened. The toothmarks indicate that an Amphicyon fed on the rhinoceros jaw, but they can't tell us how the rhinoceros died in the first place.

References:

ANTUNES, M., BALBINO, A., & GINSBURG, L. (2006). Ichnological evidence of a Miocene rhinoceros bitten by a bear-dog (Amphicyon giganteus) Annales de Paléontologie, 92 (1), 31-39 DOI: 10.1016/j.annpal.2005.10.002

ANTUNES, M., BALBINO, A., & GINSBURG, L. (2006). Miocene Mammalian footprints in coprolites from Lisbon, Portugal Annales de Paléontologie, 92 (1), 13-30 DOI: 10.1016/j.annpal.2005.09.002

Top image from: Christine Argot (2010). Morphofunctional analysis of the postcranium of Amphicyon major (Mammalia, Carnivora, Amphicyonidae) from the Miocene of Sansan(Gers, France) compared to three extant

carnivores: Ursus arctos, Panthera leo, and Canis lupus Geodivertistas, 32 (1), 65-106