Abstract
Living reef fishes are one of the most diverse vertebrate assemblages on Earth. Despite its prominence and ecological importance, the origins and assembly of the reef fish fauna is poorly described. A patchy fossil record suggests that the major colonization of reef habitats must have occurred in the Late Cretaceous and early Palaeogene, with the earliest known modern fossil coral reef fish assemblage dated to 50 Ma. Using a phylogenetic approach, we analysed the early evolutionary dynamics of modern reef fishes. We find that reef lineages successively colonized reef habitats throughout the Late Cretaceous and early Palaeogene. Two waves of invasion were accompanied by increasing morphological convergence: one in the Late Cretaceous from 90 to 72 Ma and the other immediately following the end-Cretaceous mass extinction. The surge in reef invasions after the Cretaceous–Palaeogene boundary continued for 10 Myr, after which the pace of transitions to reef habitats slowed. Combined, these patterns match a classic niche-filling scenario: early transitions to reefs were made rapidly by morphologically distinct lineages and were followed by a decrease in the rate of invasions and eventual saturation of morphospace. Major alterations in reef composition, distribution and abundance, along with shifts in climate and oceanic currents, occurred during the Late Cretaceous and early Palaeogene interval. A causal mechanism between these changes and concurrent episodes of reef invasion remains obscure, but what is clear is that the broad framework of the modern reef fish fauna was in place within 10 Myr of the end-Cretaceous extinction.
Highlights
Despite the highly fragmented distribution of reefs globally, reef fish faunas are remarkably similar; characteristic lineages are abundant on reefs around the world, such as wrasses, damselfishes, tangs and butterflyfishes on coral reefs [1] and porgies, rockfishes and wrasses on temperate rocky reefs [2,3]
The end-Cretaceous mass extinction (Cretaceous–Palaeogene; K–Pg) did not result in a decline of reef volume, but it did result in the loss of diversity [13,14]: rudists went extinct at, or close to, the K–Pg (66 Ma) [10] along with an estimated 45% of all scleractinian coral species [13]
We estimate fewer reef transitions from the empirical data than the non-phylogenetic null model [43], indicating reef lineages are, on average, phylogenetically clustered. This result is supported by the net relatedness index (NRI), with negative values of the standardized effect size and the low p-values confirming that reef lineages are more closely related than expected by chance
Summary
Despite the highly fragmented distribution of reefs globally, reef fish faunas are remarkably similar; characteristic lineages are abundant on reefs around the world, such as wrasses, damselfishes, tangs and butterflyfishes on coral reefs [1] and porgies, rockfishes and wrasses on temperate rocky reefs [2,3]. To avoid an arbitrary threshold for assigning a habitat to each family, we used the percentage of extant species in each family that are reef-associated as given by FISHBASE [21] (this includes fishes living on or near any shallow-water, consolidated waveresistant structure) using functions in the R package rfishbase [34] These percentages were used to specify a binomial distribution from which we sampled 100 times for each family, generating 100 habitat datasets. With the morphological PCA axes, we estimated DTT on the 1000 reef and non-reef trees separately using the methods of Harmon et al [46] implemented in the R package geiger [40] This method estimates the average squared Euclidean distance between species across the entire phylogeny and for every node in that phylogeny (sub-clade). See the electronic supplementary material, Results and methods
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