For an extended period, monitor lizards were considered an anomaly among their kin. Within a group where bony plates, concealed beneath the skin, are remarkably common, monitor lizards were presumed to lack them entirely. No armor, just scales.
This assumption was debunked by recent studies that revealed hidden bones in dozens of monitor lizard species. Consequently, scientists embarked on an investigation to determine the true origin of this concealed armor and how many separate times it evolved over 320 million years.
These bony plates are known as osteoderms, and they appear in the most unexpected corners of the animal kingdom. Crocodiles sport them as armor on their backs, while armadillos, frogs, and certain turtles also possess them.
Lizards and snakes – collectively referred to as squamates – form a particularly fertile group for examining these structures. Some species exhibit dense, overlapping plates that nearly interlock. Others, conversely, feature only sparse, worm-like fragments scattered across their skin.
Roy Ebel, an evolutionary biologist at the Australian National University (ANU), spearheaded the team endeavoring to untangle this evolutionary puzzle. Collaborating with colleagues from Museums Victoria in Melbourne, he set out to map when osteoderms first emerged and with what frequency they evolved. The findings of this study have been published in the Biological Journal of the Linnean Society.
The team constructed an extensive phylogenetic map—essentially, a family tree—encompassing 643 species. While the majority consisted of extant lizards and snakes, 70 species were drawn from the fossil record and other extinct lineages.
Similar trees had been compiled previously, but none had systematically tracked the presence of bony plates to this degree. Ebel’s group drew upon a recent catalog that covered nearly all major lizard groups, including monitor lizard species long thought to be entirely devoid of these features.
The analysis then proceeded backward along the tree, charting which branches possessed bony armor and when. In total, the analysis spans 320 million years of reptilian history—predating the emergence of the first dinosaurs.
For a considerable time, researchers grappled with two competing theories regarding the origin of these structures. One posited that osteoderms appeared early in lizard evolution and subsequently disappeared multiple times. The other suggested that various lineages developed them independently.
Ebel’s reconstruction strongly favors the latter explanation. Osteoderms arose at least 13 separate times throughout lizard and snake evolution. Not through a single, ancient inheritance, but rather, different branches fashioned their own bony armor without a shared, proximate ancestor.
The majority of these occurrences took place approximately 140 million years ago, during the Late Jurassic and Early Cretaceous periods. Continents were drifting apart, the climate was warming, and the first flowering plants were beginning to establish themselves.
Thereafter, the trait tended to persist. Once a lineage acquired bony armor, it almost invariably retained these structures, even through more than 100 million years of subsequent evolution.
A notable exception exists. Among monitor lizards—long-tailed predators possessing sharp talons—the research team reconstructed a complete loss of osteoderms around 72 million years ago. The ancestral armor vanished.
The likely reason is tied to monitor lizard hunting strategies. Unlike most other reptiles, they pursue their prey with relentless speed. Their velocity and endurance are supported by a physiology more akin to mammals than to typical reptiles. A heavy, bony shell would be incompatible with such a lifestyle.
It is plausible that natural selection actively suppressed the presence of armor. This characteristic was lost in the lineage leading to modern monitor lizards and remained absent for tens of millions of years. Until circumstances changed.
Approximately 17 million years ago, during the Miocene epoch, osteoderms began to reappear. This time, their emergence was observed in monitor lizards, primarily in Australia and Papua New Guinea, who reacquired structures lost by their distant ancestors.
These events coincide with a period of dramatic change in the region’s history. Shortly before, the collision of the Australian and Asian continental plates altered habitats and created corridors for animal dispersal. Australia was drying out, with arid zones expanding across its interior.
Analysis of osteoderm function suggests that water retention is one of the most compelling arguments for the skin’s armored role. Protection from predators and calcium storage may also benefit from these same structures.
The spread of arid environments throughout Australia aligns so precisely with the appearance of new species that it is difficult to ignore. The bony carapace may have aided these monitor lizards in conserving moisture as the continent grew drier.
The evolutionary trajectory of the skin’s armor now possesses a clear timeline. Osteoderms independently arose on 13 occasions across the entire family tree of lizards. Two peaks of emergence stand out: one around 140 million years ago, and another in the Miocene in Australia and Papua.
The long-debated question of whether bony armor was inherited or repeatedly developed has now been definitively answered. Different lineages constructed analogous structures independently, lacking a recent common ancestor. This debate had persisted for over a century.
Biologists now have a clearer map to investigate the reasons behind the evolution of bony armor in specific times and places. Climate, hunting styles, and habitat—all have left their imprint across time. What was once perceived as a disconnected trait now boasts a more coherent evolutionary narrative.
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