Bones did not originally form deep inside the body. They started in the skin soon after the first complex animals took shape. Ever since, ‘skin bones’ have appeared again and again in evolution. Turtles, crocodiles, lizards, snakes, and even dinosaurs all have this protective feature. Scientists have long been puzzled about how these skin bones evolved more than once and whether they came from a common ancestor.In a groundbreaking study, researchers tried to reconstruct 320 million years of reptile skin bone evolution using fossil evidence and modern computational tools. The researchers found an answer to a centuries-long debate: skin bones have indeed evolved independently across multiple lizard lineages. During the study, they also traced a unique evolutionary comeback in one of the most iconic groups – goannas. The findings are published in the Biological Journal of the Linnean Society.
The ancient origins
The earliest evidence of skin bones dates back approximately 475 million years. At that time, some of the earliest vertebrates evolved an elaborate bony exoskeleton. This might sound surprising because vertebrates are defined by having backbones. However, their internal bony skeleton evolved about 50 million years later. Throughout history, the capacity for skin to generate bony structures has surfaced repeatedly across different animal groups. For example, fish scales. Another example is osteoderms – the skin bones of land-dwelling animals. This recurring pattern suggests that skin bones provided genuine survival advantages.
Solving an ancient puzzle
Scientists studied 643 living and extinct reptile species to understand how skin bones became evident in many animals. They examined fossil records and combined them with computational analysis to determine whether all reptiles inherited osteoderms from a single ancestor or whether these structures evolved independently in different lineages.What they found was striking. Skin bones evolved multiple times separately across various lizard groups. Most lizards first developed osteoderms over 100 million years ago, during the Late Jurassic and Early Cretaceous periods. This was an era when dramatic climate and environmental shifts were common, and the armour also protected them from predators.
Goannas’ comeback
The ancestors of monitor lizards, also known in Australia as goannas, lost osteoderms entirely. Researchers think this was likely because their active lifestyle and efficient bodies functioned better without the additional weight. However, approximately 20 million years ago, when their descendants reached Australia, something extraordinary occurred: goannas re-evolved skin bones. The re-evolution likely took place in the Miocene period, when Australia’s climate was becoming drier. Skin bones may have helped the animals reduce water loss and also provided protection in open, arid landscapes. Interestingly, goannas are the only known lizard lineage to reacquire osteoderms after losing them. This discovery challenges Dollo’s law, which states that once a complex trait disappears, it cannot re-evolve.
Answers to a century-old debate
Researchers in the twentieth century assumed that all osteoderms descended from a shared ancestral trait. However, many later debated whether these structures evolved separately. These discussions, however, lacked clear evolutionary timelines. This new study provides that missing link by combining traditional fossil analysis with modern computing power. The findings suggest that evolution rarely follows a well-trodden path.
