Lizard Armor Evolved Multiple Times, Not Inherited Once, New Study Reveals
For decades, biologists believed the bony plates found in some lizards were an ancient trait passed down through generations. They thought early lizards carried this armor and later groups either kept it or lost it. A groundbreaking new study now turns this idea on its head.
Research led by the Museums Victoria Research Institute presents a completely different pattern. The study argues that most of these bony plates, known as osteoderms, appeared much later in evolutionary history. They did not come from a single origin but evolved repeatedly over time.
Armor Reappeared After Long Absence
The work draws on multiple sources of evidence. Researchers examined fossils, modern CT scans, and genetic relationships across hundreds of species. Their findings suggest the earliest lizards almost certainly lacked osteoderms entirely.
This trait remained absent for tens of millions of years after lizards first appeared. Armor did not gradually fade from a common starting point. Instead, it arrived late and unevenly across different lizard lineages.
The study reconstructs evolutionary histories across squamate reptiles, which include both lizards and snakes. This comprehensive approach provides strong evidence against the traditional view of armor inheritance.
Multiple Origins Reshaped Lizard Evolution
The research team examined an impressive range of species. They looked at 643 living and extinct species across roughly 320 million years of evolutionary history. Using CT scan data, fossil evidence, and molecular relationships, they tracked where osteoderms were present or missing.
Their results reveal a surprising pattern. Bony plates evolved at least thirteen times in modern lizard groups. Many of these gains occurred during a narrow window around the Late Jurassic and Early Cretaceous periods.
This was about 140 million years ago, as major lizard lineages began to diversify significantly. The pattern clearly points to repeated innovation rather than simple inheritance from a common ancestor.
Monitor Lizards Show Loss and Return
Monitor lizards provide one of the most fascinating examples within these findings. Evidence suggests their ancestors lost osteoderms around 72 million years ago. This loss likely coincided with a shift toward a more active hunting style.
Early monitors, including the ancestors of Australian goannas, appear to have traded protection for movement. Heavy armor would have slowed them down during pursuit hunting. Speed and flexibility became more important than defensive plating.
Much later, during the Miocene epoch, osteoderms returned in several Australian and Papuan monitor lineages. These later forms were lighter and looser than earlier versions. They offered some protection without limiting motion significantly.
Environment Shaped Armor's Comeback
The study suggests changing environments played a crucial role in armor's return. As monitor lizards spread into drier regions of Australia, osteoderms may have offered new advantages beyond simple defense.
These bony plates can help reduce water loss in arid conditions. They can store calcium and support the body during climbing activities. In this sense, armor may have been reselected for different functions than before.
This makes monitor lizards one of the clearest examples of evolutionary re-innovation seen in modern reptiles. They demonstrate how traits can disappear and then reappear when environmental conditions change.
Bony Plates Serve Multiple Functions
Osteoderms are not part of the skeleton itself. They form within the skin and vary widely in shape and placement across different species. In crocodiles, they help regulate blood chemistry during long periods underwater by releasing calcium.
In lizards, their roles are less well understood but appear equally varied. They can act as thermal regulators, water retention structures, or simple protection against predators. Despite their importance, their developmental origins remain unclear.
This new study provides a valuable framework for exploring how such structures form again and again in different lineages. It opens new avenues for understanding evolutionary processes.
Similar Looks Hide Different Histories
The findings help explain why unrelated lizards can look remarkably similar. The Gila monster of North America and Australia's shingleback lizard both appear heavily armored. Yet their similarities arose through completely different evolutionary paths.
This mirrors other repeated innovations in nature. Flight evolved separately in insects, birds, and bats. Similarly, armor evolved independently in different lizard groups facing similar environmental pressures.
By mapping when and where osteoderms appeared, researchers have added significant depth to the story of lizard evolution. The picture that emerges is uneven and complex. Armor comes and goes across evolutionary time. Sometimes it returns after long absences. Sometimes it arrives quietly, shaped by pressures that only become clear much later.
The study fundamentally changes how scientists read the lizard fossil record. It also transforms how they think about adaptation and evolutionary flexibility over deep time periods. Australia's monitor lizards now stand at the center of a broader story about loss, return, and innovation in the natural world.
