November 4, 2010
The traditional, simplified recipe for how to make a fossil goes something like this: take a dead animal, keep it safe from scavengers, cover it up with sediment, add a heaping dollop of time and presto!, you have a petrified skeleton. The second step is often cited as being especially important—a skeleton can’t enter the fossil record if it is destroyed—but sometimes predator kills and scavenged carcasses do make it into the fossil record. This year alone there has been a report of Tarbosaurus scavenging a hadrosaur carcass and a study confirming that Tyrannosaurus picking at the deceased of its own kind. Now paleontologist Lucas Ernesto Fiorelli has reported on a collection of Cretaceous crocodile bones that may be theropod dinosaur table scraps.
The crocodile in question, described in 1991 but not yet named, was found on the campus of the University of Comahue in Neuquén, Argentina. There was not very much of it left. A few pieces of skull, some vertebrae, a smattering of limb fragments and a nearly complete tail were all that remained. Based on the geology of the area, this animal lived along rivers or streams that skirted along huge sand dunes in a hot, seasonal environment, and its anatomy shows that it belonged to a group of extinct crocs called the peirosaurid crocodyliforms. These animals were more slender than their modern cousins and adapted to a more terrestrial lifestyle.
As described by Fiorelli, there are about 70 punctures and bite marks on the preserved remains of the animal, present on almost every skeletal element except the skull. Particularly noteworthy is the distribution of bitemarks along the preserved tail of the animal, which appears to have been crushed by the powerful bite of a large predator. The question is what left the bite marks.
Fiorelli rejects the hypothesis that this animal was the victim of aggression by another crocodile. When competing for dominance, modern crocodylians display and bite each other, but the amount of trauma indicated by the bite marks on this individual is inconsistent with such behavior. Additionally, while the crocodile was about 10 to 12 feet long, the animal that left the bitemarks appears to have been considerably larger, suggesting that the injuries were probably not caused by a member of the same species.
The idea that the injuries were caused by other crocodile species that have been found from the same deposits were also ruled out by Fiorelli. One, Notosuchus, may have been primarily herbivorous, and Fiorelli states that its contemporary Comahuesuchus just didn’t have the jaw power to do the kind of damage seen on the other crocodile bones. Likewise, even though two other genera of prehistoric crocodiles called baurusuchids were certainly predators, the pattern of bite marks on the victim’s skeleton indicate an animal with a much larger skull. As hypothesized by Fiorelli, a large theropod dinosaur is the most likely culprit, though the specific species of this predator cannot be ascertained. Both abelisaurids and carcharodontosaurids—two groups of diverse theropods common in the Cretaceous of South America—have been found from the geologic formation the crocodile skeleton came from, but so far, no teeth or other remains have been found in proximity to the skeleton to conclusively close the case.
Lucas Ernesto Fiorelli (2010). Predation bite-marks on a peirosaurid crocodyliform from the Upper Cretaceous of Neuquén Province, Argentina Ameghiniana, 47 (3)
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