Xiphosura
Evolutionary history

The oldest known stem-Xiphosuran, Lunataspis, is known from the late Ordovician of Canada, around 445 million years ago. No xiphosurans are known from the following Silurian. Xiphosurida first appears during the late Devonian. A major radiation of freshwater xiphosurids, the Belinuridae is known from the Carboniferous, with the oldest representatives of the modern family Limulidae also possibly appearing during this time, though they only appear in abundance during the Triassic. Another major radiation of freshwater xiphosurans, the Austrolimulidae, is known from the Permian and Triassic. All extant species are estimated to have diverged from a common ancestor that lived about 135 million years ago in the early Cretaceous. As a group they have never showed much diversity in regard of species. Less than 50 fossil species are known from the Carboniferous period, when they were at their most diverse.
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