New creature called an archosaur sheds light on how dinosaurs developed armor

Researchers have unveiled a novel species of reptile that existed close to the time when dinosaurs first began to appear. The unique creature, an archosaur boasting bony plates along its backbone, indicates that armor was an evolutionary trait that came and went several times in the lineage of dinosaurs and pterosaurs. 

This and many other findings have been revealed in the research, which is published in the Zoological Journal of the Linnean Society.

The study was led by Sterling Nesbitt, an associate professor of geosciences at Virginia Tech and a research associate in the American Museum of Natural History’s Division of Paleontology. 

“We are just starting to understand that there were many dinosaur-like creatures across the planet well before dinosaurs evolved,” said Nesbitt. “Dinosaurs were latecomers to the Triassic reptile party. They showed up well after many dinosaur-looking reptiles were established across our planet.”

What are archosaurs?

Archosaurs are an intriguing set of reptiles split into two prime branches: the bird-line, which encompasses pterosaurs and dinosaurs, including those still existing today (birds); and the crocodilian line, consisting of crocodiles, alligators, caimans, and gharials. 

The newly introduced archosaur species, designated Mambachiton fiandohana, claims the title of being the earliest diverging member of the bird-line archosaur evolution. 

The fossil, approximately 235 million years old, was discovered in Madagascar in 1997 by a team led by John Flynn, the Museum’s Frick Curator of Fossil Mammals. The team included scientists and students from the University of Antananarivo in Madagascar.

Flynn, who worked at the Field Museum at the time, highlighted the importance of the fossil: “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 emphasized the invaluable contributions of the Madagascar-U.S. research and education partnership to the advancement of scientific knowledge over a span of 25 years.

Archosaur named Mambachiton fiandohana

Mambachiton fiandohana, a four-legged, long-tailed archosaur, and a precursor to dinosaurs and pterosaurs, stood 4–6 feet (1.5–2 meters) tall, weighing a significant 25–45 pounds (10–20 kilograms). Surprisingly, this species had a comprehensive set of bony plates, known as osteoderms, covering its backbone. 

This bony plate feature, though common in crocodilians and their kin, is seldom found in bird-line archosaurs, with the notable exception of dinosaurs like stegosaurs, ankylosaurs, titanosaur sauropods, and at least one theropod.

Mambachiton conclusively shows that the bird-line archosaur group ancestrally donned armor. As the lineage evolved towards dinosaurs and pterosaurs, this armor was lost. However, it resurfaced several times (independently) in the dinosaur lineage.

Loss and re-evolution of armor

Study co-author Christian Kammerer is a former Gerstner Scholar at the Museum and a research curator in paleontology at the North Carolina Museum of Natural Sciences.

“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 Kammerer.

Project co-leader Lovasoa Ranivoharimanana of the University of Antananarivo said: “Mambachiton demonstrates that retention of ancestral features or acquisition of new traits depend on interactions within the ecosystem.” He explains that if a character is necessary, it will persist, but if it loses its utility, it will fade away.

The research team also included Emily Patellos from the University of Southern California and Virginia Tech, and André Wyss from the University of California, Santa Barbara, whose collective efforts brought this historical snapshot of Triassic life into focus.

More about dinosaur armor

Dinosaur armor generally refers to the bony plates, spikes, or other structural adaptations that certain dinosaurs possessed for defense against predators or for other purposes. Here are a few examples:

Ankylosaurus

One of the most well-known “armored” dinosaurs, the Ankylosaurus was a large quadrupedal dinosaur covered in hard, bony plates. It also had a heavy club at the end of its tail that it could swing as a weapon.

Stegosaurus

Although the Stegosaurus’ iconic plates were probably more for display and thermoregulation than armor, they could have provided some protection. Its tail ended in four long, sharp spikes that could be used for defense.

Triceratops

While not armored in the same sense as Ankylosaurus or Stegosaurus, the Triceratops had a large bony frill protecting its neck and three long horns on its face, which could have been used both for defense and for intra-species combat.

Euoplocephalus

This dinosaur was similar to the Ankylosaurus, with a body covered in armored plates and spikes. It also had a club-like tail.

Gastonia

A herbivorous dinosaur that was heavily armored, the Gastonia had rows of spikes along its body and large spines projecting from its hips.

These are just a few examples, but there were many other dinosaurs with varying degrees and types of “armor.” The shape, size, and arrangement of this armor could vary widely, and it would have been used for a variety of purposes depending on the species. 

While we tend to think of dinosaur armor as being used for defense against predators, it could also have been used for other purposes like heat regulation or attracting a mate.

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