The Evolution and Fossil Record of Palaeognathous Birds (Neornithes: Palaeognathae)

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.

Reconstructing locomotor ecology of extinct avialans: a case study of Ichthyornis comparing sternum morphology and skeletal proportions

“…Flight itself is less well-estimated by sternum shape, possibly because 'flightless' birds are enormously disparate. They range from flightless ratites that have lost numerous skeletal specializations for flight [62], to penguins, which use their highly specialized forelimb apparatus and sternal keel for wing-propelled diving, to numerous additional taxa from across Neornithes that lost flying ability relatively recently, including flightless auks such as the great auk P. impennis, the Titicaca grebe R. microptera, and the Auckland teal Anas aucklandica [63][64][65]. Flightlessness may not be clearly reflected in the sternal features of taxa in the latter category, emphasizing the challenge of estimating flight capacity from shape-based morphometric analyses alone.…”

Microstructural and crystallographic evolution of palaeognath (Aves) eggshells

“…If cladogenesis is indeed related to the evolution of autapomorphic microstructures (see also ostrich-style and casuariid eggshells below), it may have implication for Lithornithidae monophyly ( Widrig and Field, 2022 ). The monophyly of Lithornithidae is supported by recent views ( Nesbitt and Clarke, 2016 ; Yonezawa et al, 2017 ) but there are also different results that support lithornithid paraphyly ( Houde, 1988 ; Worthy et al, 2017 ).…”

“…Extant Palaeognathae are usually larger than their sister-clade Neognathae and are flightless except for the poorly flighted tinamous ( Yonezawa et al, 2017 ; Altimiras et al, 2017 ). However, the Palaeogene palaeognaths Lithornithidae might have been a fully volant clade ( Torres et al, 2020 ; Widrig and Field, 2022 ). Proportional to their body size, absolute sizes of eggs and eggshells of Palaeognathae are usually large and thick, respectively ( Grellet-Tinner, 2006 ; Birchard and Deeming, 2009 ; Legendre and Clarke, 2021 ).…”

Microstructural and crystallographic evolution of palaeognath (Aves) eggshells

“…Considering that ancestral kiwi might have been a volant clade (Worthy et al, 2013), the disproportionally large size of egg and peculiar microstructure of kiwi might have appeared when the cladogenesis of (flightless and extremely precocial) Apterygidae took place (Worthy et al, 2013). If cladogenesis is indeed related to the evolution of autapomorphic microstructures (see also ostrich-style and casuariid eggshells below), it may have implication for Lithornithidae monophyly (Widrig and Field, 2022). The monophyly of Lithornithidae is supported by recent views (Nesbitt and Clarke, 2016; Yonezawa et al, 2017) but there are also different results that support lithornithid paraphyly (Houde, 1988; Worthy et al, 2017).…”