Abstract
Cichlid fishes are a key model system in the study of adaptive radiation, speciation and evolutionary developmental biology. More than 1600 cichlid species inhabit freshwater and marginal marine environments across several southern landmasses. This distributional pattern, combined with parallels between cichlid phylogeny and sequences of Mesozoic continental rifting, has led to the widely accepted hypothesis that cichlids are an ancient group whose major biogeographic patterns arose from Gondwanan vicariance. Although the Early Cretaceous (ca 135 Ma) divergence of living cichlids demanded by the vicariance model now represents a key calibration for teleost molecular clocks, this putative split pre-dates the oldest cichlid fossils by nearly 90 Myr. Here, we provide independent palaeontological and relaxed-molecular-clock estimates for the time of cichlid origin that collectively reject the antiquity of the group required by the Gondwanan vicariance scenario. The distribution of cichlid fossil horizons, the age of stratigraphically consistent outgroup lineages to cichlids and relaxed-clock analysis of a DNA sequence dataset consisting of 10 nuclear genes all deliver overlapping estimates for crown cichlid origin centred on the Palaeocene (ca 65–57 Ma), substantially post-dating the tectonic fragmentation of Gondwana. Our results provide a revised macroevolutionary time scale for cichlids, imply a role for dispersal in generating the observed geographical distribution of this important model clade and add to a growing debate that questions the dominance of the vicariance paradigm of historical biogeography.
Highlights
Cichlid fishes, along with Darwin’s finches and Caribbean Anolis lizards, represent a key vertebrate model system for understanding the evolutionary assembly of biodiversity [1,2]
Our three approaches to estimating a time scale of cichlid origin and diversification yield overlapping credible intervals (CIs) that diverge significantly from the predictions made by the Gondwanan vicariance biogeographic hypothesis, and are discussed in turn in §3a,b
If only the records of landmasses that are currently inhabited by cichlids are considered, the time of origin of the clade is estimated as 59.2 mean age (Ma)
Summary
Along with Darwin’s finches and Caribbean Anolis lizards, represent a key vertebrate model system for understanding the evolutionary assembly of biodiversity [1,2]. Imprecisely dated deposits are given their oldest plausible age This approach systematically biases analysis towards older age estimates for the time of clade origin, thereby providing a more generous test of the Gondwanan vicariance hypothesis. We calculated CIs based on the record of all cichlid fossils and estimated range extensions based on articulated cichlid remains alone combined with appropriate recovery potential functions generated from the subset of deposits that yield complete fish specimens This modified procedure is more conservative and reflects the very real possibility that the earliest cichlids might be rspb.royalsocietypublishing.org Proc R Soc B 280: 20131733. Two sets of calculations were completed: one using the oldest proposed minimum age for a clade and the other applying the youngest These paired analyses provide upper and lower estimates of CIs for divergence times given present understanding of both the fossil record and teleost interrelationships. We have not included any internal calibrations within Cichlidae, so that our relaxedmolecular-clock estimate of the evolutionary time scale for the group is truly independent of its fossil record, which contributes to our palaeontological estimates of divergence times (see §2)
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