Abstract
BackgroundSphaerexochinae is a speciose and widely distributed group of cheirurid trilobites. Their temporal range extends from the earliest Ordovician through the Silurian, and they survived the end Ordovician mass extinction event (the second largest mass extinction in Earth history). Prior to this study, the individual evolutionary relationships within the group had yet to be determined utilizing rigorous phylogenetic methods. Understanding these evolutionary relationships is important for producing a stable classification of the group, and will be useful in elucidating the effects the end Ordovician mass extinction had on the evolutionary and biogeographic history of the group.Methodology/Principal FindingsCladistic parsimony analysis of cheirurid trilobites assigned to the subfamily Sphaerexochinae was conducted to evaluate phylogenetic patterns and produce a hypothesis of relationship for the group. This study utilized the program TNT, and the analysis included thirty-one taxa and thirty-nine characters. The results of this analysis were then used in a Lieberman-modified Brooks Parsimony Analysis to analyze biogeographic patterns during the Ordovician-Silurian.Conclusions/SignificanceThe genus Sphaerexochus was found to be monophyletic, consisting of two smaller clades (one composed entirely of Ordovician species and another composed of Silurian and Ordovician species). By contrast, the genus Kawina was found to be paraphyletic. It is a basal grade that also contains taxa formerly assigned to Cydonocephalus. Phylogenetic patterns suggest Sphaerexochinae is a relatively distinctive trilobite clade because it appears to have been largely unaffected by the end Ordovician mass extinction. Finally, the biogeographic analysis yields two major conclusions about Sphaerexochus biogeography: Bohemia and Avalonia were close enough during the Silurian to exchange taxa; and during the Ordovician there was dispersal between Eastern Laurentia and the Yangtze block (South China) and between Eastern Laurentia and Avalonia.
Highlights
The Cheiruridae are a diverse family of phacopine trilobites with a long geologic history spanning the latest Cambrian to the Middle Devonian
Kawina had previously been treated as closely related to Sphaerexochus on the basis of reduced triangular free cheeks, wide axis of the exoskeleton, eyes situated close to the axial furrow, rostral plate wide and short, S1 furrows deeper and longer than S2 and S3, a pygidial and thoracic doublure extending to the axial furrow, and a pygidium with two to three axial rings, a semi-circular outline, a pronounced terminal axial piece, and three pleural spines [4]
We suggest that Kawina be redefined as a monotypic genus including its type species K. vulcanus, and that all other species originally placed within Kawina and Cydonocephalus be placed within ‘‘Kawina’’, with the quote marks denoting paraphyly following the convention of Wiley [39]
Summary
One subfamily is the diverse, Ordovician-Silurian Sphaerexochinae; it is diagnosed by its possession of a wide axis, three pairs of glabellar furrows with S1 being longer and more incised than S2 or S3, eyes positioned close to the axial furrows, triangular free cheek, wide and short rostral plate, thoracic and pygidial doublure extending to the axial furrow, and a hypostome with small anterior wings and a gently inflated middle body and a shallow notch on the posterior border [4]. Kawina had previously been treated as closely related to Sphaerexochus on the basis of reduced triangular free cheeks, wide axis of the exoskeleton, eyes situated close to the axial furrow, rostral plate wide (transverse) and short, S1 furrows deeper and longer than S2 and S3, a pygidial and thoracic doublure extending to the axial furrow, and a pygidium with two to three axial rings, a semi-circular outline, a pronounced terminal axial piece, and three pleural spines [4]. Understanding these evolutionary relationships is important for producing a stable classification of the group, and will be useful in elucidating the effects the end Ordovician mass extinction had on the evolutionary and biogeographic history of the group
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