Data from: Lungless tadpoles breathe fresh air into hypotheses for tetrapod lung loss and trait regain

The environmental factors associated with adaptive trait loss and the
extent to which lost traits can be regained have been subject to much
speculation and debate in evolutionary biology. We use tadpole lungs to
test if previously proposed environmental factors, such as a stream
habitat, are associated with larval lung loss and whether lungs can be
regained following loss. We assembled a dataset of lung presence for the
larvae of 529 anurans, finding 28 instances of larval lung loss, and
developed a methodological framework to test the evolutionary associations
between lung loss, general habitat type, terrestriality, and stream
specialization, finding strong support for the final two factors in
adaptive lung loss. The likelihood of regain is thought to depend on
whether developmental pathways are preserved over time; accordingly, we
predicted larval lung loss to be highly reversible. And yet, we found that
larval lungs were never regained, despite lungless tadpoles evolving to
live in habitats that favor lung use and all lungless tadpoles developing
into lunged frogs. Traditional explanations of irreversibility do not
easily explain why stage-specific trait loss would be irreversible,
prompting us to examine alternative explanations for natural patterns of
irreversibility observed across the tree of life.