Joaquinraptor casali life reconstruction. Credit: Andrew McAfee, Carnegie
The fossilised remains of a new species of theropod dinosaur with a crocodile leg bone pressed against its lower jaws has been unearthed from the Lago Colhué Huapi Formation in Patagonia, Argentina.
The finding suggests the species, Joaquinraptor casali, may have been an apex predator in the region.
It was a member of a family of carnivorous theropod dinosaurs known for their powerful forelimbs equipped with large, sharp claws: Megaraptora.
Megaraptorid thumb claw from the Laguna Palacios Formation of Argenitna. Credit: Matt Lamanna
“The preservation of a crocodyliform humerus between the dentaries of the new theropod may provide information on megaraptoran dietary preferences and feeding strategies,” write the authors of a study presenting the findings in Nature Communications.
“Megaraptorids appear to have been the apex predators in central and southern Patagonian palaeoecosystems approaching the end of the Cretaceous, in contrast to more northerly areas of South America where these niches were occupied by other non-avian theropod groups.”
Megaraptoran specimens have been found across Asia, Australia and South America, but their fragmentary, incomplete nature has limited palaeontologists’ understanding of their evolution.
The newest fossil is well-preserved and partially articulated, with much of the skull, fore- and hind-limbs, ribs and vertebrae intact.
Joaquinraptor casali skeleton in the field. Credit: Marcelo Luna, Lab. Paleontología de Vertebrados -UNPSJB
According to the authors, J. casali is one of the geologically youngest, if not the youngest, megaraptorans discovered yet. Their phylogenetic analysis supports the placement of Megaraptora within the clade Coelurosauria as the sister group of Tyrannosauroidea.
J. casali lived during the latest part of the Cretaceous Period between 70 to 66 million years ago, before going extinct along with other non-avian dinosaurs in the Cretaceous mass extinction.
“Because this taxon is also one of the most completely represented megaraptorans, it adds significantly to our understanding of the morphology, phylogenetic relationships, and palaeobiology of this enigmatic large clawed theropod group,” the authors write.
The bones indicate the specimen was likely a sexually mature adult at least 19 years old at death. They estimate it would been about 7m in length and weighed more than a tonne, and it may not yet have been fully grown.
Sediments indicate that it lived in a warm, humid floodplain close to the sea.
Close up view of Joaquinraptor casali bones in the quarry. Credit: Marcelo Luna, Lab. Paleontología de Vertebrados -UNPSJB
“The distinctive megaraptorid forelimb may have played an important role in the apparent adaptability of these theropods, perhaps allowing them to exploit resources in varying ways and/or to feed on an array of vertebrate taxa,” write the authors.
This may have included crocodyliforms, as a right humerus was found lodged between the closely associated left and right lower jaw bones. The tops of several tooth crowns were in direct contact with the bone, which also showed potential tooth marks.
“This humerus is the only nonmegaraptorid vertebrate fossil yet identified from the site. As such, this discovery may constitute direct evidence regarding prey selection within Megaraptoridae,” the authors suggest.
They caution that the presence of the leg bone could also be the result of other behaviours, such as interspecies combat or competition for food.
It may also be a simple coincidence, having been washed into position by water currents.
“Detailed … studies are currently underway to assess the probable cause and potential significance of this association,” the authors conclude.