235 million-year-old fossil reveals new species of ancient armored reptile

Identification of the new species 

Mambachiton fiandohana is the name given to this newly identified species. 

The creature possessed four legs and a long tail. The fossil remains indicate that Mambachiton was 4–6 feet (1.5–2 meters) long, weighing between 25–45 pounds (10–20 kilograms). 

The bones were discovered in Madagascar in 1997 by a team led by the Museum's Frick Curator of Fossil Mammals, John Flynn.

“This discovery documents the importance of the southern hemisphere fossil record in understanding this important period of the Triassic when dinosaurs were first appearing,” Flynn said. “This time interval is really poorly known elsewhere in the world, showing the tremendous value of our quarter-century-long Madagascar-U.S. research and education partnership to advancing scientific knowledge.”

Adding to the evolutionary understanding of armored dinosaurs 

One intriguing aspect of this ancient reptile revealed by the fossil was the presence of several bony plates covering its backbone, known as osteoderms.

The authors suggest this new species was the archosaur group's first diverging member. Archosaurs are a kind of prehistoric reptile with two primary evolutionary branches. 

First, there is the bird line, which encompasses pterosaurs, dinosaurs, and modern birds. The second is the crocodilian line, which includes crocodiles, alligators, caimans, and gharials. 

The presence of osteoderms has been recorded mostly in descendants of crocodile-line archosaurs, while it is considered uncommon in bird-line archosaurs. Although certain dinosaurs, such as stegosaurs, ankylosaurs, titanosaurs, and sauropods, had this scaled-backbone trait. 

Following a comprehensive investigation, the scientists concluded that Mambachiton was most likely the ancestrally armored bird-line archosaur clade. 

This armor trait may have been lost later in the development of dinosaurs and pterosaurs. Surprisingly, it reappeared in the dinosaur lineage a few million years later, as this characteristic has been observed in a variety of armored dinosaur fossils.  

"The loss and re-evolution of armor is an important aspect of the story of dinosaur evolution—freeing them from some of the biomechanical body constraints of the ancestral archosaurs and potentially contributing to some of the locomotor shifts as dinosaurs diversified into a dizzying array of different ecology and body forms," said co-author Christian Kammerer, a former Gerstner Scholar at the Museum and a research curator in paleontology at the North Carolina Museum of Natural Sciences.

The research has been published in the Zoological Journal of the Linnean Society. 

Study abstract:

Understanding the evolution of the earliest avemetatarsalian (bird-line) archosaurs and inferring the morphology of the last common ancestor of Archosauria are hampered by a poor fossil record in critical temporal intervals. Here we describe an early-diverging avemetatarsalian from the ?Earliest Late Triassic (~235 Ma) ‘basal Isalo II’/Makay Formation of Madagascar, which helps bridge these gaps. This taxon, Mambachiton fiandohana gen. et sp. nov., is represented by well-preserved postcranial material and possibly a postfrontal bone. Features of the neck region include anteroposteriorly elongated vertebrae with laterally expanded dorsal ends of the neural spines with three pairs of osteoderms per cervical vertebra, lying dorsal to those vertebrae. Inclusion of Mambachiton in a phylogenetic analysis of archosauromorphs recovers it at the base of Avemetatarsalia, outside of the aphanosaur + ornithodiran clade. This new specimen indicates that osteoderms were present in the earliest avemetatarsalians, but were lost in more crownward lineages. The plesiomorphic morphology of the taxon also underscores the difficulty of identifying early avemetatarsalians from incomplete skeletons. This early-diverging avemetatarsalian occurring together with a lagerpetid and silesaurid in the ‘basal Isalo II’/Makay Formation of Madagascar documents the co-occurrence of multiple non-dinosaurian avemetatarsalian clades in Gondwana near the Middle–Late Triassic transition.