Immature feathers preserved in Burmite provide evidence of rapid molting in enantiornithines

Fossil feathers are recorded in the greatest fidelity in amber. A new specimen of Cenomanian aged Burmese amber provides the first unequivocal evidence of immature feathers in the Mesozoic fossil record. The small, clear piece, identified as aerial amber, preserves a cluster of body feathers with their basal portions encased in keratinous sheaths. The feathers are identified as belonging to a young enantiornithine individual based on the combination of preserved feather morphotypes, degree of feather asymmetry in the single preserved remix, their diminutive size, and because only enantiornithines have so far been recovered from these amber deposits. The body feathers are all at approximately the same stage of development suggesting that immature enantiornithines rapidly molted body feathers. This interpretation is supported by evidence from lithic specimens of juveniles from the Lower Cretaceous Jehol group that preserve immature feathers throughout the body. This specimen contributes to evidence that indicates enantiornithines hatched flight ready, with fully developed remiges, but with sparse body plumage that was simultaneously replaced during the prejuvenal molt. Rapid molts indicate periodic increases in thermostatic costs due to the temporary loss of insulation. This information leads to an intriguing new hypothesis regarding the extinction of Enantiornithes: that periods of increased thermostatic cost during early ontogeny created untenable energetic budgets during the asteroid induced impact winter, which resulted in both decreased global temperature and resource scarcity. High energetic budgets are associated with extinction vulnerability and may have been a major factor underpinning the extinction of this clade.