Palaeotemperatures can be estimated from characteristics of fossils if their living relatives represent the full evolutionary potential of the larger taxon to which the fossil belongs. By drawing on observations1,2 that the body size of poikilotherms decreases globally with ambient temperature, Head et al.3 used the 13 m length of the newly described fossil boid Titanoboa cerrejonensis to estimate that the Palaeocene neotropical mean annual temperature (MAT) was 30–34 °C. I question the validity of this palaeotemperature estimate by using the same data and approach as Head et al.3 to show that Varanus (Megalania) prisca4, a large extinct lizard that lived in eastern Australia during the Late Pleistocene, was 3–4 times longer than predicted by the largest lizard species in the tropics today. This suggests that the scarcity of large predatory reptiles today may primarily be a function of competition with mammalian carnivores, rather than a function of modern temperatures.
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