With its blade-like teeth and formidable claws, Majungasaurus crenatissimus was one of the world’s most fearsome predators, but new research reveals that it also possessed some of the animal kingdom's smallest and most peculiar arms.
The giant, cannibalistic dinosaur had such disproportionate arms that it could not have grasped anything or even scratched its own face, according to a new study published in the Journal of Vertebrate Paleontology.
Tyrannosaurus rex and other well known predatory dinosaurs also had reduced forelimbs. While Majungasaurus, which lived 66 million years ago in Madagascar, was not a close T. rex relative, some lifestyle factors might have caused them to evolve certain similarities.
"The evolution of short arms in predatory dinosaurs remains a mystery, but fossils like this are an important clue in understanding the process," co-author Matthew Carrano, a paleobiologist at the Smithsonian Institution, told Discovery News.
"Only by discovering the stops leading from 'normal' longer arms in the ancestral forms, to the short and bizarre ones in Majungasaurus and its close relatives, can we hope to explain the evolutionary sequence and its causes."
Carrano and co-author Sara Burch of Stony Brook University analyzed the arm and associated fossils for a new specimen of the Late Cretaceous dinosaur. They concluded that the arm proportions "are unlike anything we see in other theropods."
The forearm bones are only a quarter of the length of the upper arm bones, but would have been thick and muscular. The wrist bones, however, aren't even ossified, and the stubby fingers probably lacked claws.
"The bones of the hand are so small that the fingers may not have even been separated from each other, which means the hand could have been almost paddle-like," Burch told Discovery News.
The result was an arm that was part Popeye and part Barbie. The researchers have no idea what function the arms served.
The forelimbs might have just been stuck at a transitional point in their evolution. If non-avian dinosaurs did not all bite the dust at the end of the Cretaceous, Majungasaurus might have evolved into an enormous ostrich-like animal without arms, but with very strong legs.
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“It is possible that with such large heads full of teeth, some large predatory dinosaurs no longer needed to use their forelimbs to aid in capturing prey, so the forelimbs became smaller as they lost this function,” Burch explained.
During its lifetime, Majungasaurus was about 21 feet long and had a muscular neck, legs and tail. It held the distinction of being the top predator in its territory. Based on fossils bearing its tooth marks, Majungasaurus feasted on enormous long-necked sauropods and didn’t shy away from biting into members of its own species.
Jonah Choiniere, a postdoctoral fellow at the American Museum of Natural History, said he’s very interested "that this weird arm anatomy is showing up in dinosaurs in Madagascar as well as South America. This shows that whatever the use of the arms, the animals were widespread and successful."
Kristi Curry Rogers, an assistant professor in the biology and geology departments at Macalester College, said the new discovery sheds light on the "highly variable, and sometimes bizarre, anatomy of theropod dinosaurs."
"The tiny hands and unossified wrist bones of this new specimen introduce questions about how these hands and arms evolved and what they were used for," she told Discovery News.
She added, "It is especially interesting that Majungasaurus appears to lack claws — that’s not quite in line with the usual theropod suspects known for their fearsome teeth and claws, and it begs the question of what these strange dinosaurs were doing with their peculiar little hands."
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