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

Lamsdell JC, Briggs DE, Liu HP, Witzke BJ, McKay RM - BMC Evol. Biol. (2015)

Bottom Line: 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 relatively derived position of megalograptids within the eurypterids indicates that most eurypterid clades were present by the Middle Ordovician.The high degree of appendage specialization in eurypterids is only matched by arachnids within chelicerates, supporting a sister taxon relationship between them.

View Article: PubMed Central - PubMed

Affiliation: Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, CT, 06511, USA. james.lamsdell@yale.edu.

ABSTRACT

Background: Eurypterids 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.

Results: The 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.

Conclusions: The 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.

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Evolutionary tree of the major clades of Eurypterina. Stratigraphic ranges of clades are shown by triangles, those of individual genera and species by boxes. Dashed black lines represent inferred ghost ranges. Solid gray lines show the tree topology
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Fig22: Evolutionary tree of the major clades of Eurypterina. Stratigraphic ranges of clades are shown by triangles, those of individual genera and species by boxes. Dashed black lines represent inferred ghost ranges. Solid gray lines show the tree topology

Mentions: Pentecopterus decorahensis is the oldest described eurypterid, predating Brachyopterus stubblefieldi from the Sandbian of Avalonia [7] by some 9 million years. Eurypterids from the Tremadocian Fezouata formations of Morocco await investigation [76]. Several eurypterid clades have already been identified with long ghost ranges extending into the Upper Ordovician [9] but the placement of megalograptids within Carcinosomatoidea increases the number of clades with ranges that must have extended into the Middle Ordovician (Fig. 22). The inferred appearance of a large number of morphologically diverse eurypterid clades during the Darriwilian indicates that the eurypterids radiated explosively during the early stages of the Ordovician or that they originated during the Cambrian and underwent a period of cryptic evolution prior to the Ordovician radiation. The former scenario is supported by the observation that clades commonly reach their maximum disparity early in their evolution [77] and rates of morphological change are greater during this interval [78]. However, chasmataspidids, which form the sister group to the clade comprising eurypterids and arachnids [34], may range back to the Upper Cambrian [79], suggesting that eurypterids may also originate prior to the Ordovician.Fig. 22


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

Lamsdell JC, Briggs DE, Liu HP, Witzke BJ, McKay RM - BMC Evol. Biol. (2015)

Evolutionary tree of the major clades of Eurypterina. Stratigraphic ranges of clades are shown by triangles, those of individual genera and species by boxes. Dashed black lines represent inferred ghost ranges. Solid gray lines show the tree topology
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC4556007&req=5

Fig22: Evolutionary tree of the major clades of Eurypterina. Stratigraphic ranges of clades are shown by triangles, those of individual genera and species by boxes. Dashed black lines represent inferred ghost ranges. Solid gray lines show the tree topology
Mentions: Pentecopterus decorahensis is the oldest described eurypterid, predating Brachyopterus stubblefieldi from the Sandbian of Avalonia [7] by some 9 million years. Eurypterids from the Tremadocian Fezouata formations of Morocco await investigation [76]. Several eurypterid clades have already been identified with long ghost ranges extending into the Upper Ordovician [9] but the placement of megalograptids within Carcinosomatoidea increases the number of clades with ranges that must have extended into the Middle Ordovician (Fig. 22). The inferred appearance of a large number of morphologically diverse eurypterid clades during the Darriwilian indicates that the eurypterids radiated explosively during the early stages of the Ordovician or that they originated during the Cambrian and underwent a period of cryptic evolution prior to the Ordovician radiation. The former scenario is supported by the observation that clades commonly reach their maximum disparity early in their evolution [77] and rates of morphological change are greater during this interval [78]. However, chasmataspidids, which form the sister group to the clade comprising eurypterids and arachnids [34], may range back to the Upper Cambrian [79], suggesting that eurypterids may also originate prior to the Ordovician.Fig. 22

Bottom Line: 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 relatively derived position of megalograptids within the eurypterids indicates that most eurypterid clades were present by the Middle Ordovician.The high degree of appendage specialization in eurypterids is only matched by arachnids within chelicerates, supporting a sister taxon relationship between them.

View Article: PubMed Central - PubMed

Affiliation: Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, CT, 06511, USA. james.lamsdell@yale.edu.

ABSTRACT

Background: Eurypterids 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.

Results: The 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.

Conclusions: The 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.

Show MeSH
Related in: MedlinePlus