Monitor lizards, commonly referred to as goannas in Australia, are among the continent's most iconic reptiles. These remarkable creatures have not only endured the mass extinction that wiped out non-avian dinosaurs but have also evolved into the largest living lizards on our planet. Today, they roam through Australia’s forests and scrublands, flicking their tongues to gather sensory information from their environment. A groundbreaking study published in the Zoological Journal of the Linnean Society delves deeper into the anatomy of these lizards, revealing hidden bone structures that could explain their evolutionary success.
The skin serves as a vital organ for survival in many animals. In particular, some species possess a layer of bone plates embedded within their skin tissue, known as osteoderms. These structures are reminiscent of the armor-like plates found in crocodiles and armadillos. Osteoderms vary significantly in size—from microscopic structures to the massive back plates of the stegosaurus, showcasing their evolutionary diversity. Despite their long history, our understanding of osteoderms remains in its infancy. They have been identified in animal lineages that diverged up to 380 million years ago, indicating that osteoderms evolved independently across various species, similar to the evolution of flight in birds, pterosaurs, and bats.
While the functionality of flight is well understood, the advantages of osteoderms are less clear. The most apparent role may be defensive, acting as a protective layer against injuries. However, osteoderms could serve multiple functions. In crocodiles, for instance, they play a crucial role in heat regulation, assist in movement, and even provide calcium during egg-laying. The complexity of these functions has made it challenging to pinpoint the evolutionary reasons behind the development of osteoderms.
To further investigate this intriguing topic, researchers initiated a large-scale study focusing on osteoderms in lizards and snakes. The study brought together an international team of specialists and examined specimens from scientific collections, including the Florida Museum of Natural History, the Natural History Museum in Berlin, and Museums Victoria. Researchers faced several challenges in their study. The presence of osteoderms can significantly vary among individuals of the same species, and there is no guarantee that these structures are well-preserved in all specimens. Moreover, since osteoderms are hidden deep within the skin tissue, traditional methods of discovery often involved damaging the specimens.
To overcome these challenges, researchers employed micro-computed tomography (micro-CT), an advanced imaging technique that provides high-resolution images similar to a medical CT scan. This allowed the team to examine even the smallest anatomical structures without harming the specimens. Utilizing computer-generated 3D models, the researchers digitally explored the bodies of lizards and snakes worldwide, processing almost 2,000 samples in their quest to locate osteoderms. They also developed a technique called radiodensity heatmapping, which visually highlights the locations of these bone structures, creating a comprehensive catalogue for future research.
The findings from this study were unexpected. Contrary to previous beliefs that only a few lizard families possessed osteoderms, the results indicated that nearly half of all lizards have these structures in some form. One of the most surprising discoveries pertained to goannas. For over 200 years, scientists have studied monitor lizards, initially believing they lacked osteoderms, except in rare cases like the Komodo dragon. This study revealed undocumented osteoderms in 29 Australo-Papuan species, increasing their known prevalence fivefold.
This newfound knowledge about osteoderms in Australian goannas opens up exciting avenues for further research. Given that goannas have a fascinating biogeographic history—having arrived in Australia approximately 20 million years ago and adapting to a harsh environment—understanding the timing and role of osteoderms in their evolution could be pivotal. If these bone structures emerged as a response to environmental challenges, it would provide essential insights into the function and evolution of osteoderms in reptiles.
In conclusion, the revelations from this study not only contribute significantly to our understanding of monitor lizards but also enhance our comprehension of the evolutionary forces that shaped Australia’s unique reptiles. As research progresses, the role of osteoderms in the evolutionary narrative of goannas and other species will undoubtedly continue to unfold.
