Xenotarsosaurus
Xenotarsosaurus (ZEE-no-TAR-so-SAWR-us; “Strange Ankle Lizard”)
Xenotarsosaurus is a genus of abelisaurid theropod dinosaur that lived during the Late Cretaceous period, approximately 95 million years ago (Cenomanian-Turonian stages), in what is now Argentina, South America. The name Xenotarsosaurus comes from the Greek words “xenos” (strange), “tarsos” (ankle), and “sauros” (lizard), referring to the unusual fused ankle bones found in its fossils. It was first described by paleontologists Ricardo Martínez, Olga Giménez, Jorge Rodríguez, and Graciela Bochatey in 1986 based on limited fossil remains discovered in the Bajo Barreal Formation.
Description and Classification
Xenotarsosaurus was a carnivorous dinosaur that walked on two legs (bipedal). Based on the incomplete fossil material, which includes vertebrae and parts of a hind limb, scientists estimate it was a medium-sized theropod, possibly reaching lengths of around 4 to 6 meters (13 to 20 feet). Like other abelisaurids, such as Carnotaurus and Abelisaurus, Xenotarsosaurus likely had a short, deep skull (though no skull material has been found for this specific genus), very small and stubby forelimbs, and powerful hind limbs built for running.
Xenotarsosaurus belongs to the family Abelisauridae, a group of ceratosaurian theropod dinosaurs that were common predators in the southern continents (Gondwana) during the Cretaceous period. The specific features of its ankle bones help place it within this group, possibly as an early or basal member, showing some of the key traits that define abelisaurids.
Distinguishing Features
Xenotarsosaurus is distinguished by several features, primarily related to its hind limb anatomy:
- The most notable feature is the fusion of its upper ankle bones (astragalus and calcaneum) with each other and also with the lower end of the shin bone (tibia). This created a very rigid and strong ankle structure, which is characteristic of abelisaurids.
- It possessed relatively stocky and robust hind limbs for its body size, indicating it was a powerful terrestrial predator.
- Specific details on its vertebrae (backbones) also show differences from other contemporary theropods, though these are more technical.
Paleoenvironment and Diet
Fossils of Xenotarsosaurus were found in the Bajo Barreal Formation in Chubut Province, Argentina. During the Late Cretaceous, this area was a warm, semi-arid environment with river systems and floodplains. This ecosystem supported a variety of other dinosaurs, including large sauropods like Epachthosaurus, which could have been potential prey for theropods. Other carnivorous dinosaurs, including different types of abelisaurids and possibly carcharodontosaurids, also lived in this region.
As a carnivorous abelisaurid, Xenotarsosaurus would have hunted other animals. Its diet likely consisted of small to medium-sized dinosaurs and other vertebrates available in its habitat. Abelisaurids are generally thought to have been ambush predators or hunters of slower-moving prey, using their powerful jaws and serrated teeth.
Significance and Ongoing Research
Xenotarsosaurus is significant because it helps paleontologists understand the early evolution and diversity of abelisaurid dinosaurs in South America. The unique fused ankle bones are a key characteristic that links it to this important group of Gondwanan predators. Its discovery adds another piece to the puzzle of the dinosaur fauna that thrived in Patagonia during the Late Cretaceous period.
Due to the limited amount of fossil material discovered so far, much about Xenotarsosaurus remains unknown. Future discoveries of more complete skeletons would provide valuable information about its full anatomy, size, behavior, and precise relationships to other abelisaurids like Majungasaurus or Skorpiovenator. Ongoing research on the Bajo Barreal Formation and comparative studies with other theropods continue to refine our understanding of Xenotarsosaurus and the world it inhabited.