Flightless rails (Aves; Rallidae) from the early Miocene St Bathans Fauna, Otago, New Zealand.

While known for over a decade to exist, fossil rails of the early Miocene (19–16 Ma) St Bathans Fauna, from the South Island of New Zealand, have not previously been described taxonomically or studied in detail. Here we use qualitative osteological features and analyse measurements from wing and leg bones to determine the number of taxa represented, their flight ability, and the presence and nature of sexual dimorphism within the identified taxa. We describe two new rail species in monospecific genera from the St Bathans Fauna: Priscaweka parvales gen. et sp. nov., which is extremely common, and Litorallus livezeyi gen. et sp. nov., a distinctly larger, uncommon species. Priscaweka parvales exhibited a significant degree of sexual dimorphism and was tiny, being the size of the extinct Chatham Island Rail Cabalus modestus. Both newly described species exhibit skeletal proportions and osteological features that indicate they had reduced wings and were flightless. These observations reveal that flightless rallid species have been present in New Zealand for millions of years. The distinctiveness of the St Bathans rails from their closest geographical and chronological neighbours suggests some hidden diversity of volant rails in Australia's fossil record. However, the combined data from Australasian and European records reveal no evidence for a diverse early Miocene crown rallid fauna as predicted by some molecular studies. A subsequent, middle Miocene radiation for crown rallids seems more likely, and appears to have produced the high taxonomic diversity seen in Holocene Australasian rail faunas.