Abstract
The extant diversity of the avian clade Palaeognathae is composed of the iconic flightless ratites (ostriches, rheas, kiwi, emus, and cassowaries), and the volant tinamous of Central and South America. Palaeognaths were once considered a classic illustration of diversification driven by Gondwanan vicariance, but this paradigm has been rejected in light of molecular phylogenetic and divergence time results from the last two decades that indicate that palaeognaths underwent multiple relatively recent transitions to flightlessness and large body size, reinvigorating research into their evolutionary origins and historical biogeography. This revised perspective on palaeognath macroevolution has highlighted lingering gaps in our understanding of how, when, and where extant palaeognath diversity arose. Towards resolving those questions, we aim to comprehensively review the known fossil record of palaeognath skeletal remains, and to summarize the current state of knowledge of their evolutionary history. Total clade palaeognaths appear to be one of a small handful of crown bird lineages that crossed the Cretaceous-Paleogene (K-Pg) boundary, but gaps in their Paleogene fossil record and a lack of Cretaceous fossils preclude a detailed understanding of their multiple transitions to flightlessness and large body size, and recognizable members of extant subclades generally do not appear until the Neogene. Despite these knowledge gaps, we combine what is known from the fossil record of palaeognaths with plausible divergence time estimates, suggesting a relatively rapid pace of diversification and phenotypic evolution in the early Cenozoic. In line with some recent authors, we surmise that the most recent common ancestor of palaeognaths was likely a relatively small-bodied, ground-feeding bird, features that may have facilitated total-clade palaeognath survivorship through the K-Pg mass extinction, and which may bear on the ecological habits of the ancestral crown bird.
Highlights
Crown birds (Neornithes) comprise roughly 11,000 extant species [1]
We hope that this review provides both a solid base of information for those interested in the evolution and fossil record of palaeognaths, and helps inspire further work clarifying the evolutionary history of these remarkable birds
The present review affirms that the palaeognath crown group has a reasonably thorough fossil record from the late Oligocene-early Miocene onwards, with the exception of early elephant birds and early representatives of the New Zealand ratites, whose fossil record remains sparse until the Pleistocene [392,397,409]
Summary
Crown birds (Neornithes) comprise roughly 11,000 extant species [1]. They are divided into the reciprocally monophyletic Palaeognathae and Neognathae, with the latter including the hyperdiverse clade Neoaves [1]. The extinct lithornithids (Lithornithidae), known from the Paleocene and Eocene of Europe and North America, were apparently volant and appear to represent the oldest and most stemward known total-clade palaeognaths [49,62,63,64,65] They appear to have been more capable long-distance fliers than extant tinamids are [62,65], and, as the earliest known palaeognaths in the fossil record, they may provide the best models for informing reconstructions of the dispersive ancestral palaeognaths that gave rise to extant palaeognath diversity. Lithornithids were small bodied, presumably volant birds that were first recognized as palaeognaths by Houde and Olson [79], and described in detail as a clade by Houde [62] Far, they are only known from Europe and North America, contrasting with the Gondwanan distribution of extant palaeognaths. The identity of this fossil remains uncertain, and more material needs to be recovered from both this formation and other contemporaneous localities to clarify which groups of total-clade palaeognaths persisted across the K-Pg boundary
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