Although reptiles once ruled the earth, in today’s “Age of Mammals” reptiles are primarily smaller than the mammals with which they compete for food and are more often than not found in those habitats rejected by mammals. This is especially true for large herbivorous lizards, who exist only in mammal-free regions. One hypothesis is that competition for food resources with herbivorous mammals limits both the scope and size of modern lizards. Examination of a recently discovered extinct species, however, points toward climate, not competition, as the limiting factor in populations of extant lizards.
Dental fossils of the newly discovered lizard Barbaturex morrisoni* were found in the Pondaung Formation in central Myanmar. From the size of the teeth alone researchers were able to determine an average snout to vent length of 98.1 cm, which places B. morrisoni in the category of “large lizards” (greater than 100 cm, or ~ 3.2 ft). Based on the relationship between body size and length of living lizards, B. morrisoni is expected to have had an average body mass of 26.7 kg (~ 60 lbs). To maintain this body mass, which is larger than any living herbivorous lizard, the minimum annual temperature of B. morrisoni’s world would have had to be 27.0 – 28.4ºC, or 2-5ºC greater than average temperatures in today’s world.
Higher temperatures can encourage increased body size in a number of ways, including increased plant growth, which provides more food, and elevation of temperature dependent metabolic processes, this increasing the rate at which lizards can process food and use it as fuel for growth. This evidence suggests that modern lizard size is being controlled by climatic limits, and that appearance of competition from herbivorous mammals may be a result of coincidence and artifacts of modern climatic conditions.
*This lizard’s name has an interesting etymology. Barbaturex derives from “Barbatus” (bearded) + “rex” (king), the latter word reflecting the lizard’s large size. Morrisoni is in reference to The Doors lead singer Jim Morrison, who was sometimes referred to as The Lizard King.
Find the complete study in Proceedings of the Royal Society B.
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