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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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Result of the phylogenetic analysis. Single most parsimonious tree. Bootstrap branch support values are shown above the node in italics, with Bremer support values in bold within parentheses. Jackknife values are displayed beneath each node. The position of Pentecopterus decorahensis is highlighted in red. Arachnids are not included in the analysis
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Fig21: Result of the phylogenetic analysis. Single most parsimonious tree. Bootstrap branch support values are shown above the node in italics, with Bremer support values in bold within parentheses. Jackknife values are displayed beneath each node. The position of Pentecopterus decorahensis is highlighted in red. Arachnids are not included in the analysis

Mentions: The phylogenetic analysis, as detailed in the methods section, yielded a single most parsimonious tree (Fig. 21) with a length of 475 steps, an ensemble Consistency Index of 0.429, ensemble Retention Index of 0.796, and Rescaled Consistency Index of 0.341. The topology is predominantly congruent with that retrieved from previous analyses of Stylonurina [68–70] and Eurypterina [9, 26, 45], with the exception of the position of Alkenopterus, which is retrieved here as a basal member of the Eurypterina as suggested by Poschmann [46]. Pentecopterus decorahensis is resolved as the basalmost member of the Megalograptidae, united with Echinognathus and Megalograptus by the shared possession of two or more pairs of spines per podomere on prosomal appendage IV, a reduction of all spines of appendage V except the pair on the penultimate podomere, an ornamentation of angular scales across the posterior margin of the dorsal tergites, longitudinal rows of large scales on the tergites, and an ornamentation consisting predominantly of guttalate scales. Pentecopterus is separated from the other genera in the Megalograptidae (Echinognathus and Megalograptus) by the presence of only one pair of spines on the third podomere of appendage III, a single terminal spine on each prosomal appendage, and the absence of dense cuticular ornamentation.Fig. 21


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)

Result of the phylogenetic analysis. Single most parsimonious tree. Bootstrap branch support values are shown above the node in italics, with Bremer support values in bold within parentheses. Jackknife values are displayed beneath each node. The position of Pentecopterus decorahensis is highlighted in red. Arachnids are not included in the analysis
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

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

Fig21: Result of the phylogenetic analysis. Single most parsimonious tree. Bootstrap branch support values are shown above the node in italics, with Bremer support values in bold within parentheses. Jackknife values are displayed beneath each node. The position of Pentecopterus decorahensis is highlighted in red. Arachnids are not included in the analysis
Mentions: The phylogenetic analysis, as detailed in the methods section, yielded a single most parsimonious tree (Fig. 21) with a length of 475 steps, an ensemble Consistency Index of 0.429, ensemble Retention Index of 0.796, and Rescaled Consistency Index of 0.341. The topology is predominantly congruent with that retrieved from previous analyses of Stylonurina [68–70] and Eurypterina [9, 26, 45], with the exception of the position of Alkenopterus, which is retrieved here as a basal member of the Eurypterina as suggested by Poschmann [46]. Pentecopterus decorahensis is resolved as the basalmost member of the Megalograptidae, united with Echinognathus and Megalograptus by the shared possession of two or more pairs of spines per podomere on prosomal appendage IV, a reduction of all spines of appendage V except the pair on the penultimate podomere, an ornamentation of angular scales across the posterior margin of the dorsal tergites, longitudinal rows of large scales on the tergites, and an ornamentation consisting predominantly of guttalate scales. Pentecopterus is separated from the other genera in the Megalograptidae (Echinognathus and Megalograptus) by the presence of only one pair of spines on the third podomere of appendage III, a single terminal spine on each prosomal appendage, and the absence of dense cuticular ornamentation.Fig. 21

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