Abstract

BackgroundFew studies on eurypterids have taken into account morphological changes that occur throughout postembryonic development. Here two species of eurypterid are described from the Pragian Beartooth Butte Formation of Cottonwood Canyon in Wyoming and included in a phylogenetic analysis. Both species comprise individuals from a number of instars, and this allows for changes that occur throughout their ontogeny to be documented, and how ontogenetically variable characters can influence phylogenetic analysis to be tested.ResultsThe two species of eurypterid are described as Jaekelopterus howelli (Kjellesvig-Waering and Størmer, 1952) and Strobilopterus proteus sp. nov. Phylogenetic analysis places them within the Pterygotidae and Strobilopteridae respectively, both families within the Eurypterina. Jaekelopterus howelli shows positive allometry of the cheliceral denticles throughout ontogeny, while a number of characteristics including prosomal appendage length, carapace shape, lateral eye position, and relative breadth all vary during the growth of Strobilopterus proteus.ConclusionsThe ontogeny of Strobilopterus proteus shares much in common with that of modern xiphosurans, however certain characteristics including apparent true direct development suggest a closer affinity to arachnids. The ontogenetic development of the genital appendage also supports the hypothesis that the structure is homologous to the endopods of the trunk limbs of other arthropods. Including earlier instars in the phylogenetic analysis is shown to destabilise the retrieved topology. Therefore, coding juveniles as individual taxa in an analysis is shown to be actively detrimental and alternative ways of coding ontogenetic data into phylogenetic analyses should be explored.

Highlights

  • Few studies on eurypterids have taken into account morphological changes that occur throughout postembryonic development

  • Two species can be recognised from the locality: the pterygotid Jaekelopterus howelli (Kjellesvig-Waering and Størmer, 1952) which is known from the younger section at Beartooth Butte [6], and Strobilopterus proteus sp. nov

  • Multiple instars of both species are recognisable at the Cottonwood Canyon locality, and these represent a unique opportunity to study the postembryonic development of extinct chelicerate species

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Summary

Introduction

Few studies on eurypterids have taken into account morphological changes that occur throughout postembryonic development. Two species of eurypterid are described from the Pragian Beartooth Butte Formation of Cottonwood Canyon in Wyoming and included in a phylogenetic analysis. Both species comprise individuals from a number of instars, and this allows for changes that occur throughout their ontogeny to be documented, and how ontogenetically variable characters can influence phylogenetic analysis to be tested. Multiple instars of both species are recognisable at the Cottonwood Canyon locality, and these represent a unique opportunity to study the postembryonic development of extinct chelicerate species. What is largely unknown is what effect including such ontogenetic species into phylogenetic analyses would have, and so the instars of Strobilopterus proteus are used in a brief case-study of this possibility

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