On trends and patterns in macroevolution: Williston\u2019s law and the branchiostegal series of extant and extinct osteichthyans

Eduardo Ascarrunz,Michel Laurin,Ricardo Betancur-R,Marcelo R. Sánchez-Villagra

doi:10.1186/s12862-019-1436-x

Abstract

BackgroundThe branchiostegal series consists of an alignment of bony elements in the posterior portion of the skull of osteichthyan vertebrates. We trace the evolution of the number of elements in a comprehensive survey that includes 440 extant and 66 extinct species. Using a newly updated actinopterygian tree in combination with phylogenetic comparative analyses, we test whether osteichthyan branchiostegals follow an evolutionary trend under ‘Williston’s law’, which postulates that osteichthyan lineages experienced a reduction of bony elements over time.ResultsWe detected no overall macroevolutionary trend in branchiostegal numbers, providing no support for ‘Williston’s law’. This result is robust to the subsampling of palaeontological data, but the estimation of the model parameters is much more ambiguous.ConclusionsWe find substantial evidence for a macroevolutionary dynamic favouring an ‘early burst’ of trait evolution over alternative models. Our study highlights the challenges of accurately reconstructing macroevolutionary dynamics even with large amounts of data about extant and extinct taxa.

Highlights

The branchiostegal series consists of an alignment of bony elements in the posterior portion of the skull of osteichthyan vertebrates
Our results show that fossil sample size (Fig. 4) had a greater effect than extant sample size (Additional file 4: Figure S2) on Akaike weights and model parameter estimations
We found strong support for the Early Burst (EB) model to describe the macroevolutionary pattern of branchiostegal ray numbers

Summary

Introduction

The branchiostegal series consists of an alignment of bony elements in the posterior portion of the skull of osteichthyan vertebrates. The evolution of the number of skull bones has been postulated to follow a general trend towards the reduction in the number of individual parts, resulting from losses and fusions of bones. This simplification trend is known as “Williston’s law” [1], and it has recently been studied most intensively in tetrapod dermal skull bones [2,3,4]. In synapsid stem-mammalian lineages, a pattern of reduction in the number of skull and lower jaw bones (through either loss or fusion) during approximately 150 million years has been described [2]. Variation in the number of rays, is not uniformly distributed and clade-specific patterns have been documented [29, 30]

Methods

Results

Conclusion