The fossil record of spiders revisited: implications for calibrating trees and evidence for a major faunal turnover since the Mesozoic

MAGALHAES, I.; AZEVEDO, G.; MICHALIK, P.; RAMIREZ, M.J,

BIOLOGICAL REVIEWS

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2020 vol. 95 p. 184 - 217

1464-7931

Studies in evolutionary biology and biogeography increasingly rely on the estimation of dated phylogenetictrees using molecular clocks. In turn, the calibration of such clocks is critically dependent on external evidence(i.e. fossils) anchoring the ages of particular nodes to known absolute ages. In recent years, a plethora of newfossil spiders, especially from the Mesozoic, have been described, while the number of studies presenting datedspider phylogenies based on fossil calibrations increased sharply. We critically evaluate 44 of these studies,which collectively employed 67 unique fossils in 180 calibrations. Approximately 54% of these calibrationsare problematic, particularly regarding unsupported assignment of fossils to extant clades (44%) and crown(rather than stem) dating (9%). Most of these cases result from an assumed equivalence between taxonomicplacement of fossils and their phylogenetic position. To overcome this limitation, we extensively review theliterature on fossil spiders, with a special focus on putative synapomorphies and the phylogenetic placementof fossil species with regard to their importance for calibrating higher taxa (families and above) in the spidertree of life. We provide a curated list including 41 key fossils intended to be a basis for future estimations ofdated spider phylogenies. In a second step, we use a revised set of 23 calibrations to estimate a new datedspider tree of life based on transcriptomic data. The revised placement of key fossils and the new calibratedtree are used to resolve a long-standing debate in spider evolution ? we tested whether there has been a majorturnover in the spider fauna between the Mesozoic and Cenozoic. At least 17 (out of 117) extant families havebeen recorded from the Cretaceous, implying that at least 41 spider lineages in the family level or above crossedthe Cretaeous?Paleogene (K?Pg) boundary. The putative phylogenetic affinities of families known only fromthe Mesozoic suggest that at least seven Cretaceous families appear to have no close living relatives and mightrepresent extinct lineages. There is no unambiguous fossil evidence of the retrolateral tibial apophysis clade(RTA-clade) in the Mesozoic, although molecular clock analyses estimated the major lineages within this cladeto be at least ∼100 million years old. Our review of the fossil record supports a major turnover showing that thespider faunas in the Mesozoic and the Cenozoic isare very distinct at high taxonomic levels, with the Mesozoicdominated by Palpimanoidea and Synspermiata, while the Cenozoic is dominated by Araneoidea and RTAcladespiders.