Our bones did not start deep inside the body. he started in the skinShortly after the first complex animals took shape.
Since then, the bones of the skin have remained recurring motif In development. Yet we still know surprisingly little about them. Why do they keep reappearing in groups as diverse as turtles, crocodiles, lizards, snakes and even dinosaurs? And was there a single ancestor with skin and bones that gave rise to them all?
In a new study published in Biological Journal of the Linnean SocietyWe 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 actually evolved independently in several lizard lineages. In the process, we also discovered a unique evolutionary return to one of their most iconic groups – the goannas.
when the bones were superficial
Oldest skin bones in fossil record may date back to earlier times 475 million years. At that time, some early vertebrates evolved broad bone exoskeleton.
This may seem counterintuitive, since vertebrates are literally defined by the fact that they have a backbone. However, their bony internal skeleton did not develop until 50 million years later.
Throughout evolutionary history, the skin’s ability to form bone tissue has emerged repeatedly. Fish scales are an example of this.
Another example is osteoderms – the skin bones of land-dwelling animals. After leaving the water in the distant past, osteoderms may have Helped animals adapt to terrestrial life.
Beyond that, the picture becomes less clear. Osteoderms became extinct in most lineages, yet they keep reappearingEspecially in reptiles. To understand how this happened, we need to piece together a complex evolutionary puzzle.
A story told by bones
Imagine that you arrive at the scene of a bank robbery long after it has occurred. There is no perfect witness. You talk to dozens of people – one saw the getaway car, another saw the robber’s jacket. Someone else heard the alarm.
Each story is incomplete, and some even contradict each other. But as you collect more accounts, some details start to align. Eventually, a coherent picture emerges.
In this way we reached the mystery of the skin bones in reptiles. Our witness was 643 living and extinct species. Each was related to the other in some way and offered a unique perspective. We kept investigating until their stories started matching up.
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 earthWhich includes Vishal brachiosaurusTerrifying Allosaurusand plate-backed Stegosaurus.
climate and ecosystem were changing rapidlyCreating new challenges and opportunities. The armor may have helped the lizards escape predators, cope with harsh environments, or move into new habitats.
After those initial bursts of osteoderm evolution, the pace slowed, and most groups have retained their skin bones since then.
With one big exception.
goanna return
The ancestors of monitor lizards, also known as goannas in Australia, lost osteoderms entirely – possibly because their active lifestyles and efficient bodies functioned better without the extra weight.
But when their descendants reached Australia 20 million years agoSomething remarkable happened: They grew them back.
We can point this regrowth back to the Miocene period, when Australia’s climate was becoming drier. May help reduce skin blemishes Water damage and possibly offer protection In open, dry landscapes.
Surprisingly, goannas are the only known lineage of lizards that regrow osteoderms after losing them. this is the challenge Dollo’s lawWhich holds that once a complex trait disappears, it cannot evolve again.
Settlement of a century old debate
In the early 20th century, researchers believed that lizards inherited osteoderms common ancestor.
That view later gave rise to the idea that these were bone plates developed independently among select groups. argue about underlying evolutionary mechanisms Even at the molecular level, these discussions proceeded without fixing the origin of osteoderms in a clear evolutionary timeline for today’s reptiles.
Our study provides this foundation, and we are proud that it is published in the same journal in which Charles Darwin first shared his groundbreaking ideas. In many ways, our work is a synthesis of past and present.
Fossil evidence helped us solve a long-standing question, but only modern computing made it possible to condense thousands of evolutionary scenarios, each informed by trait data from hundreds of species, into a single, coherent story.
The evidence is clear: Osteoderms evolved independently multiple times in different lizard lineages over hundreds of millions of years. Now that we know this, scientists will be able to investigate the genetic and developmental mechanisms behind them.
Among lizards, goannas are known to be the only lineage that lost this armor, but regained it in a remarkable evolutionary twist. This pattern fits seamlessly among other evolutionary oddities found in Australia, where marsupials rule and mammals lay eggs.
It also shows that evolution rarely follows a straight path, but rather meanders among the constantly changing conditions on our planet.