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PLoS One
2018 Jan 01;133:e0194575. doi: 10.1371/journal.pone.0194575.
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Diversification rates indicate an early role of adaptive radiations at the origin of modern echinoid fauna.
Boivin S
,
Saucède T
,
Laffont R
,
Steimetz E
,
Neige P
.
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Evolutionary radiations are fascinating phenomena corresponding to a dramatic diversification of taxa and a burst of cladogenesis over short periods of time. Most evolutionary radiations have long been regarded as adaptive but this has seldom been demonstrated with large-scale comparative datasets including fossil data. Originating in the Early Jurassic, irregular echinoids are emblematic of the spectacular diversification of mobile marine faunas during the Mesozoic Marine Revolution. They diversified as they colonized various habitats, and now constitute the main component of echinoid fauna in modern seas. The evolutionary radiation of irregular echinoids has long been considered as adaptive but this hypothesis has never been tested. In the present work we analyze the evolution of echinoid species richness and morphological disparity over 37 million years based on an extensive fossil dataset. Our results demonstrate that morphological and functional diversifications in certain clades of irregular echinoids were exceptionally high compared to other clades and that they were associated with the evolution of new modes of life and so can be defined as adaptive radiations. The role played by ecological opportunities in the diversification of these clades was critical, with the evolution of the infaunal mode of life promoting the adaptive radiation of irregular echinoids.
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Fig 1. Phylogeny of main Jurassic echinoid clades.Phylogenetic relationships between clades and estimated times of divergence (in millions of years) are based on Kroh and Smith [73]. Clades are a mix of higher taxonomic ranks. Crosses indicate extinct clades. The green area corresponds to the studied time interval. Ages in millions of years Before Present (after Gradstein et al. [74]).
Fig 2. Landmark position.Landmarks (red dots) are positioned at the junction between the perradial suture lines (central line of each ambulacrum) and the peristome edge (five inner landmarks) and between the perradial suture lines and the ambitus (five outer landmarks). Roman numerals refer to the echinoid’s five ambulacra.
Fig 3. Taxonomic diversity and disparity of Jurassic echinoids.Individual contribution of echinoid clades to total taxonomic diversity and disparity levels for the studied time period. Colored areas show the values for irregular echinoid clades. Areas in white and grey are for regular echinoid clades. Poly: Polycidaridae, Cid: Cidaridae, Aulo: Aulodonts, Hemi: Hemicidaridae, Ps: Pseudodiadematidae, Cal: Calycina, Ech: Echinacea, Orth: Orthopsidae, Eo: Eognathostomata, Deso: Desorellidae, Neo: Neognathostomata, Atelo: Atelostomata.
Fig 4. Morphospace plot of echinoid disparity.Black symbols correspond to the specimens of regular echinoids, yellow, blue, green, and red symbols represent irregular echinoids. Outlines shown for representative specimens among Cidaridae, Eognathostomata, Desorellidae, Neognathostomata, and Atelostomata. Poly: Polycidaridae, Cid: Cidaridae, Aulo: Aulodonts, Hemi: Hemicidaridae, Ps: Pseudodiadematidae, Cal: Calycina, Ech: Echinacea, Orth: Orthopsidae, Eo: Eognathostomata, Deso: Desorellidae, Neo: Neognathostomata, Atelo: Atelostomata.
Fig 5. Richness and disparity levels of main Jurassic echinoid clades in relation with their feeding strategies and modes of life.The spindle diagram shows taxonomic diversity and disparity levels for the 12 Jurassic echinoid clades. For each clade, taxonomic diversity is expressed as species richness for each geological stage, the wider the rectangles, the higher the number of species. Colors correspond to the tested disparity values relative to other clades: unusually high (red), normal (orange), unusually low (yellow), not tested due to lack of data (white). Main feeding strategies and modes of life were interpreted for each clade. Ages in million of years Before Present (after Gradstein et al. [73]).
Fig 6. Biplot of species richness and disparity.Species richness (expressed in number of species) is plotted against disparity (expressed as the Mean Pairwise Distance) for each clade and each time interval. The four clades of irregular echinoids are plotted separately from the others (respectively A and B).
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