Question

Australia's iconic emu and cassowaries comprise one family within the palaeognaths. The palaeognathous birds also include six other families (two of which recently became extinct):

the tinamous (from South America), the rheas (from South America), the kiwis (from New Zealand), the moa (from New Zealand), the elephant birds (from Madagascar), and the

ostrich (from Africa). For many years, it was though that these families were the result of Gondwanan vicariance. That is, it was thought their diversification was driven by the

isolation of a flightless ancestor on the separating fragments of the supercontinent Gondwana: Africa & Madagascar splitting off first, followed by New Zealand, followed by the

eventual separation of Australia, Antarctica, and South America.

a. What is ONE major implication for our understanding of palaeognath evolution of the disagreement between the molecular evidence and the phylogeny you drew ?

Answer 1

Palaeognaths, the sister group of all other living birds (neonates), was once considered to be vicariant relics from the breakup of the Gondwanan supercontinent. However, recent molecular studies instead argue for dispersal of volant ancestors across marine barriers. Resolving this debate hinges upon accurately reconstructing their evolutionary relationships and dating their divergences, which often relies on phylogenetic information from extinct relatives and nuclear genomes. Mitogenomes from the extinct elephant birds of Madagascar have helped inform the palaeognath phylogeny; however, nuclear information has remained unavailable. Here, we use ancient DNA (aDNA) extracted from fossil eggshell, together with target enrichment and next-generation sequencing techniques, to reconstruct an additional new mitogenome from Aepyornis sp. with 33.5X coverage. We also recover the first elephant bird nuclear aDNA, represented by 12,500 bp of exonic information. While we confirm that elephant birds are sister taxa to the kiwi, our data suggest that, like neonates, palaeognaths underwent explosive radiation between 69 and 52 Ma—well after the break-up of Gondwana, and more rapidly than previously estimated from mitochondrial data alone. These results further support the idea that ratites primarily diversified immediately following the Cretaceous-Palaeogene mass extinction and convergently evolved flightlessness. Our study reinforces the importance of including information from the nuclear genome of extinct taxa for recovering deep evolutionary relationships. Furthermore, with approximately 3% endogenous aDNA retrieved, avian eggshell can be a valuable substrate for recovering high-quality aDNA. We suggest that elephant bird whole genome recovery is ultimately achievable, and will provide future insights into the evolution of these birds.