Xenopus in Space and Time: Fossils, Node Calibrations, Tip-Dating, and Paleobiogeography

Abstract

Published data from DNA sequences, morphology of 11 extant and 15 extinct frog taxa, and stratigraphic ranges of fossils were integrated to open a window into the deep-time evolution of Xenopus. The ages and morphological characters of fossils were used as independent datasets to calibrate a chronogram. We found that DNA sequences, either alone or in combination with morphological data and fossils, tended to support a close relationship between Xenopus and Hymenochirus, although in some analyses this topology was not significantly better than the Pipa + Hymenochirus topology. Analyses that excluded DNA data found strong support for the Pipa + Hymenochirus tree. The criterion for selecting the maximum age of the calibration prior influenced the age estimates, and our age estimates of early divergences in the tree of frogs are substantially younger than those of published studies. Node-dating and tip-dating calibrations, either alone or in combination, yielded older dates for nodes than did a root calibration alone. Our estimates of divergence times indicate that overwater dispersal, rather than vicariance due to the splitting of Africa and South America, may explain the presence of Xenopus in Africa and its closest fossil relatives in South America.

A Late Miocene Pipine Frog from the Urumaco Formation, Venezuela

“…Cretaceous (Cannatella, 2015;Gómez, 2016), but the contribution of palaeontology to the evolutionary history of the South American Pipa is virtually null, since the only fossils putatively referred to this genus come from an undetermined Quaternary of Brasil and were reported (not figured or described) as identical to that of extant Pipa (Liais, 1872). This is quite surprising for a clade of anurans that is morphologically highly derived, with a well-known osteology (e.g., Trueb et al, 2000) and that is therefore potentially easily identifiable.…”

“…The extant pipid genera can be grouped into two clades, Pipinae and Xenopodinae, whose content varies according to different phylogenetic analyses. The molecular approach favours in most cases the inclusion of the sole Pipa in the Pipinae whereas the Xenopodinae host Xenopus (and, if not considered a synonym of the former, also Silurana) plus the Hymenochirini: Hymenochirus and Pseudhymenochirus (see literature cited in Cannatella, 2015). Conversely, the morphological approach generally suggests the inclusion of Pipa and the Hymenochirini in the Pipinae and Xenopus (plus Silurana) in the Xenopodinae (Gómez, 2016, and literature therein).…”

Molecular Phylogenies indicate a Paleo-Tibetan Origin of Himalayan Lazy Toads (Scutiger)

“…The upper 95% CI value was set to 252 Mya, corresponding to the maximum age of the salientian Triadobatrachus 102 . (2) A minimum age of 148 Mya for the Rhinophrynidae-Pipidae split, based on the fossil Rhadinosteus parvus attributed to Kimmeridgian age 103 and considering the lower range specification by Canatella 104 . (3) A minimum age of 50.3 Mya of the MRCA of Pelobatoidea, based on the fossil Scaphiopus guthriei 105, 106 .…”

First Record of the Anuran Family Rhinophrynidae from the Oligocene of Eastern North America

“…Among North American frogs, the family Rhinophrynidae is among the most morphologically distinctive. This family and the Pipidae together comprise the Pipoidea, which originated in the Jurassic and is among the earliest diverging lineages of extant anurans (Cannatella, 2015;Feng et al, 2017). The single extant species Rhinophrynus dorsalis is today restricted to southernmost Texas, Mexico, and Central America, where it occurs in semiarid and arid areas with loose soils conducive to digging (Dodd, 2013); it has been reported from the Pleistocene of Mexico (Holman, 1969).…”