The oldest described eurypterid: a giant Middle Ordovician (Darriwilian) megalograptid from the Winneshiek Lagerstätte of Iowa

Abstract

BackgroundEurypterids are a diverse group of chelicerates known from ~250 species with a sparse Ordovician record currently comprising 11 species; the oldest fully documented example is from the Sandbian of Avalonia. The Middle Ordovician (Darriwilian) fauna of the Winneshiek Lagerstätte includes a new eurypterid species represented by more than 150 specimens, including some juveniles, preserved as carbonaceous cuticular remains. This taxon represents the oldest described eurypterid, extending the documented range of the group back some 9 million years.ResultsThe new eurypterid species is described as Pentecopterus decorahensis gen. et sp. nov.. Phylogenetic analysis places Pentecopterus at the base of the Megalograptidae, united with the two genera previously assigned to this family by the shared possession of two or more pairs of spines per podomere on prosomal appendage IV, a reduction of all spines except the pair on the penultimate podomere of appendage V, and an ornamentation of guttalate scales, including angular scales along the posterior margin of the dorsal tergites and in longitudinal rows along the tergites. The morphology of Pentecopterus reveals that the Megalograptidae are representatives of the derived carcinosomatoid clade and not basal eurypterids as previously interpreted.ConclusionsThe relatively derived position of megalograptids within the eurypterids indicates that most eurypterid clades were present by the Middle Ordovician. Eurypterids either underwent an explosive radiation soon after their origination, or earlier representatives, perhaps Cambrian in age, remain to be discovered. The available instars of Pentecopterus decorahensis suggest that eurypterids underwent extreme appendage differentiation during development, a potentially unique condition among chelicerates. The high degree of appendage specialization in eurypterids is only matched by arachnids within chelicerates, supporting a sister taxon relationship between them.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-015-0443-9) contains supplementary material, which is available to authorized users.

Segmentation and tagmosis in Chelicerata

“…A segment-bysegment comparison of horseshoe crabs and scorpions was fundamental to Lankester's (1881) classic demonstration that Xiphosura are closer to Arachnida than to Crustacea. We also draw on historical (Pocock, 1893;B€ orner, 1904;Petrunkevitch, 1922;Millot, 1949;Zachvatkin 1952) and contemporary (van der Hammen, 1989;Kraus, 1998;Villpoux and Waloßek, 2003;Lamsdell, 2013;Lamsdell et al, 2015a) accounts of segmentation patterns across groups, and expand these here to include data from several recently described fossil taxa which appear to reveal key stages in the early evolution of the chelicerate body plan and limb arrangement.…”

An Evolutionary Model for the Neoproterozoic (Ediacaran)-Phanerozoic Biosphere. Part One: Concepts and Exploitation of Aquatic Environments

“…Eurypteroids were active, predaceous, benthonic animals occupying all aquatic environments and commonly reached a size of 10 -20 cm [12], although giant forms of two or more meters in length are known [13]. Eurypteroids had claw-shaped chelicerae that served for catching and crushing prey, and several pairs of oar-shaped appendages served for swimming.…”

Exceptionally preserved conodont apparatuses with giant elements from the Middle Ordovician Winneshiek Konservat-Lagerstätte, Iowa, USA

“…The Winneshiek fossil fauna includes a variety of new forms, and the invertebrate fossils are dominated by arthropods (Lamsdell et al, 2015a;Nowak et al, 2015;Briggs et al, 2015Briggs et al, [2016), including the oldest eurypterid Pentacopterus (Lamsdell et al, 2015b). Possible algae and jawless fish also occur in the fauna.…”