Adelophthalmus pyrrhae: A new species of Eurypterid 'Sea Scorpion' from the Carboniferous of Montagne Noire, France, which may have been capable of breathing air.
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Arachnids are the second most successful terrestrial animal group after Insects and were one of the first Arthropod clades to successfully invade land. Fossil evidence for this transition is limited, with the majority of Arachnid clades first appearing in the terrestrial fossil record. Furthermore, molecular clock dating has suggested a Cambrian-Ordovician terrestrialization event for Arachnids, some 60million years before their first fossils in the Silurian, although these estimates assume that Arachnids evolved from a fully aquatic ancestor. Eurypterids or 'Sea Scorpions', the sister clade to terrestrial Arachnids, are known to have undergone major macroecological shifts in transitioning from marine to freshwater environments during the Devonian. Discoveries of apparently subaerial eurypterid trackways have led to the suggestion that Eurypterids were even able to venture on land and possibly breathe air. However, modern Horseshoe Crabs undertake amphibious excursions onto land to reproduce, rendering trace fossil evidence alone inconclusive.