A recent study has provided insights into the evolutionary history of osteoderms, or skin bones, in lizards. The research indicates that these bony structures evolved independently across multiple lizard lineages and were later reacquired by goannas after an ancestral loss. This finding contributes to understanding the complex nature of evolutionary processes.
Origins and Recurrence of Skin Bones
Bones are understood to have originated in the skin of early complex animals, with the earliest recorded skin bones dating back 475 million years. This predates the evolution of the internal bony skeleton in vertebrates by 50 million years. The capacity of skin to form bony tissue, exemplified by fish scales and osteoderms in land animals, has recurred throughout evolutionary history. Osteoderms have appeared in diverse groups including turtles, crocodiles, lizards, snakes, and dinosaurs. While they largely disappeared in many lineages, they frequently reappeared, particularly within reptiles, potentially aiding adaptation to terrestrial life.
The specific reasons for their recurrence and whether they shared a single common ancestor have been subjects of scientific inquiry.
Research Methodology
A study published in the Biological Journal of the Linnean Society investigated these phenomena by combining fossil evidence with modern computational tools. Researchers analyzed 643 living and extinct species to reconstruct 320 million years of reptile skin bone evolution.
Independent Evolution in Lizards
The study found that osteoderms independently evolved across multiple lizard lineages. Most lizards initially developed osteoderms during the Late Jurassic and Early Cretaceous periods, over 100 million years ago. This era was characterized by shifts in climate and ecosystems. The presence of these dermal bones may have offered protection against predators, aided survival in challenging environments, or facilitated movement into new habitats. Following these initial evolutionary phases, most groups retained their skin bones.
The Goanna Reacquisition
A notable exception was identified in the ancestors of monitor lizards, commonly known as goannas. These ancestors lost their osteoderms, a loss possibly associated with an active lifestyle where additional weight could have been disadvantageous.
Approximately 20 million years ago, as goanna descendants reached Australia during the drier climate of the Miocene period, they reacquired these skin bones. This re-evolution may have contributed to reducing water loss and provided defense in arid, open landscapes.
Goannas are currently the only known lizard lineage identified to have reacquired osteoderms after an ancestral loss. This observation is considered inconsistent with Dollo's law, a principle suggesting that complex traits, once lost, are unlikely to re-evolve.
Resolution of Evolutionary Debate
Historically, researchers initially hypothesized in the early 20th century that lizards inherited osteoderms from a common ancestor. Later perspectives proposed independent evolution among specific groups. This study provides a chronological framework, confirming that osteoderms evolved independently multiple times across various lizard lineages over millions of years. This clarification is expected to support future scientific investigation into the genetic and developmental mechanisms underlying osteoderms.
The pattern observed in goannas is consistent with other evolutionary adaptations found in Australia and indicates that evolution often follows a non-linear path in response to changing environmental conditions.