The early diversification of ray‐finned fishes (Actinopterygii): hypotheses, challenges and future prospects

Abstract

Actinopterygii makes up half of living vertebrate diversity, and study of fossil members during their Palaeozoic rise to dominance has a long history of descriptive work. Although research interest into Palaeozoic actinopterygians has increased in recent years, broader patterns of diversity and diversity dynamics remain critically understudied. Past studies have investigated macroevolutionary trends in Palaeozoic actinopterygians in a piecemeal fashion, variably using existing compendia of vertebrates or literature‐based searches. Here, we present a comprehensive occurrence‐based dataset of actinopterygians spanning the whole of the Palaeozoic. We use this to produce the first through‐Palaeozoic trends in genus and species counts for Actinopterygii. Diversity through time generally tracks metrics for sampling, while major taxonomic problems pervading the Palaeozoic actinopterygian record obscure diversity trends. Many described species are concentrated in several particularly problematic ‘waste‐basket’ genera, hiding considerable morphological and taxonomic diversity. This taxonomic confusion also feeds into a limited understanding of phylogenetic relationships. A heavy sampling bias towards Europe and North America exists in both occurrence databases and available phylogenetic matrices, with other regions underrepresented despite yielding important data. Scrutiny of the extent to which spatial biases influence the actinopterygian record is lacking, as is research on other forms of bias. Low richness in some time periods may be linked to geological biases, while the effects of taphonomic biases on Palaeozoic actinopterygians have not yet been investigated. Efforts are already underway both to redescribe poorly defined taxa and to describe taxa from underrepresented regions, helping to address taxonomic issues and accuracy of occurrence data. New methods of sampling standardisation utilising up‐to‐date occurrence databases will be critical in teasing apart biological changes in diversity and those resulting from bias. Lastly, continued phylogenetic work will enable the use of phylogenetic comparative methods to elucidate the origins of actinopterygian biogeography and subsequent patterns of radiation throughout their rise to dominate aquatic faunas.

Sampling biases obscure the early diversification of the largest living vertebrate group

“…Palaeozoic actinopterygian occurrences are overwhelmingly geographically biased towards the Northern Hemisphere: fewer than 9% of known localities (52/586) are from the Southern Hemisphere [9]. Certain regions are notably underrepresented throughout the Palaeozoic, such as the northern, eastern and southern Palaeotethys (present-day Middle East, south and east Asia, north Africa) and the northern coastline of Laurussia ( present-day Siberia, Kazakhstan and interior of Asia).…”

“…Global occurrences of Palaeozoic Actinopterygii [9], incorporating information on stratigraphy, lithology, geography, age and taxonomy compiled from the literature, were screened for taxonomically indeterminate occurrences. Scale- and teeth-based occurrences were also removed as it is often difficult to assign them definitively to a taxon.…”

“…Much of this imbalance is driven by 'waste-basket' genera erected by monographic descriptions [72][73][74][75], despite a wide range of varied morphologies and extensive temporal and geographical ranges within genera [9,76,77]. Palaeozoic taxa typically lack characters that can be easily attributed from isolated remains and rely on well-preserved material in order to be confidently identified.…”

“…One explanation for these long-tails is the fact that the literature is biased towards descriptions of new taxa rather than occurrences of already-known taxa [45], resulting in an increase of doubletons and singletons. Given that the rate at which new Palaeozoic actinopterygians are being described continues to rise [9], it is unlikely that this trend will change (also meaning that rarefaction curves will not asymptote soon, causing problems for diversity estimation; electronic supplementary material, figure S4).…”

Fitting fangs in a finite face: A novel fang accommodation strategy in a 280‐million‐year‐old ray‐finned fish

“…The late Paleozoic (359.3–251.9 million years ago (mya) (Aretz et al, 2020 ; Henderson, Dunne, & Fasey, 2022 )) represents an important interval for understanding the evolution and diversification of actinopterygians, the ray‐finned fishes. During this time, ray‐finned fishes first evolved a series of innovations that have arisen many times since, including body elongation (Lund & Poplin, 2002 ), body deepening (Gill, 1925 ; Sallan & Coates, 2013 ), and various changes to feeding ranging from adaptations for durophagy (Friedman et al, 2018 ) to changes in jaw articulation (Argyriou et al, 2022 ).…”