The squamates (lizards, snakes, and relatives) today comprise more than 10,000 species, and yet their sister group, the Rhynchocephalia, is represented by a single species today, the tuatara (Sphenodon punctatus). The explosion in squamate diversity has been tracked back to the so-called Cretaceous Terrestrial Revolution, 100 million years ago, the time when flowering plants began their takeover of terrestrial ecosystems, associated with diversification of coevolving insects and insect-eating predators such as lizards, birds, and mammals. Squamates arose much earlier, but their long pre-Cretaceous history of some 150 million years is documented by sparse fossils. New research provides evidence for an initial radiation of squamate morphology in the Middle and Late Jurassic epochs (174 to 145 million years ago), and shows that they established their key ecological roles much earlier than had been assumed, and they have not changed them much since.
Retinosaurus hkamtiensis, which was approximately 3.5 cm (1.4 inches) in length, prior to being trapped in tree resin 110 million years ago. Image credit: Stephanie Abramowicz.
Squamates (lizards, snakes and worm lizards) are all cold-blooded, and their skins are covered by horny scales.
They are key parts of modern terrestrial faunas, especially in warmer climates, with an astonishing diversity of more than 10,000 species.
However, understanding the evolutionary paths that forged their success are still poorly understood.
Establishing the timing and mode of radiation of squamates is key for not only understanding the dynamics of terrestrial ecosystems in the Mesozoic, but also for deciphering how the group achieved an astonishing diversity of more than 10,000 species, only rivalled by birds among tetrapods.
“Even though Jurassic squamates are rare, reconstructed evolutionary trees show that all the main specializations of squamates evolved then, and it’s possible to distinguish adaptations of geckoes, iguanas, skinks, worm lizards, and snakes some 50 million years earlier than had been thought,” said Professor Michael Benton, a researcher in the School of Earth Sciences at the University of Bristol.
“But how could the scarce Jurassic fossils suggest an early burst in evolution? The key is in their anatomy.”
Phylogeny, morphospace, disparity, and evolutionary rates of lepidosaurs. Image credit: Bolet et al., doi: 10.7554/eLife.66511.
According to the authors, few Jurassic squamates do not show primitive morphologies as would be expected, but they relate directly to the diverse modern groups.
“Instead of finding a suite of generalized lizards on the stem of the squamate tree, what we found in the Jurassic were the first representatives of many modern groups, showing advanced morphological features,” said Dr. Arnau Bolet, a researcher in the Institut Català de Paleontologia Miquel Crusafont at the Universitat Autònoma de Barcelona and the School of Earth Sciences at the University of Bristol.
The observed times of divergence, morphospace plots and evolutionary rates, all suggest that the Jurassic was a time of innovation in squamate evolution, during which the bases of the success of the group were established.
“The apparent sudden increase in diversity observed in the Cretaceous period could be related to an improved fossil record, capable of recording a larger number of species, or to a burst of origins of new species related to the new kinds of forests and insects,” the researchers said.
Their paper was published online in the journal eLife.
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Arnau Bolet et al. 2022. The Jurassic rise of squamates as supported by lepidosaur disparity and evolutionary rates. eLife 11: e66511; doi: 10.7554/eLife.66511
