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Scientists solve 320-million-year mystery of reptile bone armor

Ancient reptile armor didn’t evolve just once — and Australian goannas may have pulled off evolution’s ultimate comeback.

Date:

May 20, 2026

Source:

The Conversation

Summary:

Reptiles have been growing armor in their skin on and off for hundreds of millions of years, but scientists never fully understood how it evolved. A massive new evolutionary study shows these skin bones appeared independently in multiple lizard groups rather than coming from a single armored ancestor. Even more astonishing, Australian goannas lost this armor long ago — then evolved it back again millions of years later.

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Rosenberg’s goanna (WAM R95408) with skin bones visible in purple. Credit: Roy Ebel

Our bones did not begin deep inside the body. They started in the skin, not long after the first complex animals took shape.

Ever since, skin bones have remained a recurring motif in evolution. Yet we still know surprisingly little about them. Why do they keep reappearing in groups as varied as turtles, crocodiles, lizards, snakes and even dinosaurs? And was there a single ancestor with skin bones that gave rise to them all?

In a new study published in the Biological Journal of the Linnean Society, we explored this question. We combined fossil evidence with modern computational tools to reconstruct 320 million years of reptile skin bone evolution.

What we found concludes a centuries-long debate: skin bones have indeed independently evolved across multiple lizard lineages. In the process, we also traced a unique evolutionary comeback in one of their most iconic groups – goannas.

When bones were superficial

The oldest skin bones in the fossil record may date back 475 million years. At that time, some of the earliest vertebrates evolved an elaborate bony exoskeleton.

This may seem counterintuitive, since vertebrates are literally defined by the fact that they have backbones. However, their bony internal skeleton didn’t evolve until 50 million years later.

Throughout evolutionary history, the skin’s ability to form bony tissue has resurfaced again and again. Fish scales are one example.

Another example is osteoderms – the skin bones of land-dwelling animals. After they left the water in the distant past, osteoderms may have helped animals adapt to terrestrial life.

Beyond that, the picture becomes less clear. Osteoderms vanished in most lineages, yet they kept reappearing, especially in reptiles. To understand how this happened, we needed to piece together a complex evolutionary puzzle.

A story told by bones

Imagine arriving at the scene of a bank robbery long after it happened. There’s no perfect witness. You speak to dozens of people – one saw the getaway car, another noticed the robber’s jacket. Someone else heard the alarm.

Each story is incomplete, and some even contradict one another. But as you collect more accounts, certain details begin to align. Eventually, a coherent picture emerges.

That is how we approached the mystery of skin bones in reptiles. Our witnesses were 643 living and extinct species. Each was related to the others in some way and offered a unique perspective. We kept investigating until their stories began to converge.

We found that most lizards first evolved osteoderms during the Late Jurassic and Early Cretaceous, more than 100 million years ago. At that time, some of the most iconic dinosaurs roamed the Earth, including the towering Brachiosaurus, the fierce Allosaurus, and the plate-backed Stegosaurus.

The climate and ecosystems were changing rapidly, creating new challenges and opportunities. Armour may have helped lizards survive predators, cope with harsh environments, or move into new habitats.

After those early bursts of osteoderm evolution, the pace slowed, and most groups have held onto their skin bones ever since.

With one major exception.

The goanna comeback

The ancestors of monitor lizards, also known in Australia as goannas, lost osteoderms entirely – likely because their active lifestyle and efficient bodies functioned better without the additional weight.

But when their descendants reached Australia about 20 million years ago, something remarkable happened: they grew them back.

We can pinpoint this re-evolution to the Miocene period, when Australia’s climate was becoming drier. Skin bones may have helped reduce water loss and likely offered protection in open, arid landscapes.

Strikingly, goannas are the only known lizard lineage to reacquire osteoderms after losing them. This challenges Dollo’s law, which holds that once a complex trait disappears, it cannot re-evolve.

Settling a century-old debate

Early in the 20th century, researchers assumed lizards inherited osteoderms from a common ancestor.

Later that view gave way to the idea tha