Armored archosaur fossil reveals boomerang trait in dinosaur evolution

Researchers have announced the discovery of a new armored reptile species that lived around the same time as the earliest dinosaurs. This ancient archosaur had bony plates on its backbone, indicating that armor was a recurring feature in the evolution of dinosaurs and pterosaurs. While their ancestors were armored, this characteristic was lost and then independently reappeared multiple times in later specialized dinosaurs like ankylosaurs and stegosaurs. The study has been published in the Zoological Journal of the Linnean Society.

The lead author of the study, Sterling Nesbitt, who is an associate professor of geosciences at Virginia Tech and a research associate in the American Museum of Natural History’s Division of Paleontology, stated, “We are just starting to grasp that there were numerous dinosaur-like creatures present on Earth long before dinosaurs came into the picture. Dinosaurs were not the first ones to arrive at the Triassic reptile party; they appeared after many dinosaur-like reptiles had already established themselves across the planet.”

Archosaurs, which are reptiles, are divided into two major branches: the bird-line, consisting of pterosaurs and dinosaurs, including modern-day birds; and the crocodilian line, comprising crocodiles, alligators, caimans, and gharials. The newly described species, Mambachiton fiandohana, belongs to the earliest diverging member of the bird line of archosaur evolution.

Reconstruction of Mambachiton fiandohana. Scale bar = 25 cm. Credit: Nesbitt, et al.

In 1997, a team of researchers, led by John Flynn, the Frick Curator of Fossil Mammals at the Museum, made an exciting discovery in Madagascar. They found a 235-million-year-old fossil that sheds light on the early Triassic period when dinosaurs were just emerging. The research was a collaboration between the Field Museum and the University of Antananarivo in Madagascar.

The newly discovered species, named Mambachiton, was a four-legged, long-tailed creature that served as a precursor to both dinosaurs and pterosaurs. It was estimated to be around 4 to 6 feet long, weighing between 25 to 45 pounds. What surprised the researchers was that this creature had bony plates called osteoderms along its backbone. Although common in crocodilians, osteoderms were rare in bird-line archosaurs, except for certain dinosaurs like stegosaurs, ankylosaurs, and others.

This finding definitively proves that the bird-line archosaur group had armor as an ancestral trait. However, this armor was lost in the evolutionary path of dinosaurs and pterosaurs, only to independently reappear later in the lineage of dinosaurs multiple times. The discovery highlights the significance of the southern hemisphere’s fossil record in understanding the early Triassic era and underscores the value of the long-standing research partnership between Madagascar and the United States in advancing scientific knowledge.

The fossilized cervical vertebra and overlaying osteoderms of the new archosaur species Mambachiton fiandohana shown in a photograph (top), line drawing (middle), and computed tomography (CT) rendering model with the osteoderms and matrix removed (bottom). Scale bar = 5 cm. Credit: Nesbitt et al.

According to co-author Christian Kammerer, the loss and subsequent re-evolution of armor in dinosaurs played a crucial role in their evolution. This process freed them from certain biomechanical constraints inherited from their ancestral archosaurs. As a result, dinosaurs were able to undergo various locomotor shifts and diversify into a wide range of ecological niches and body forms. Kammerer, who is a former Gerstner Scholar at the Museum and a research curator in paleontology at the North Carolina Museum of Natural Sciences, emphasizes the significance of this adaptive trait in shaping the course of dinosaur evolution.

Project co-leader Lovasoa Ranivoharimanana, from the University of Antananarivo, adds that Mambachiton’s discovery underscores how the retention of ancestral features or the acquisition of new traits depends on interactions within the ecosystem. When a particular characteristic is crucial for survival, it tends to be retained, but if it becomes obsolete or no longer serves a purpose, it gradually disappears over time. This dynamic interplay with the environment played a role in determining the evolutionary path of dinosaurs and other ancient creatures.

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