Conceived and designed the experiments: CRC BSL. Performed the experiments: CRC. Analyzed the data: CRC. Contributed reagents/materials/analysis tools: CRC BSL. Wrote the paper: CRC BSL.
The authors have declared that no competing interests exist.
Sphaerexochinae 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.
Cladistic 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.
The genus
The Cheiruridae are a diverse family of phacopine trilobites with a long geologic history spanning the latest Cambrian to the Middle Devonian. Although the group is believed to be monophyletic, the individual species level relationships are largely unknown due to a paucity of phylogenetic studies within the group
The end Ordovician mass extinction event is considered to be the second largest mass extinction in the history of life and is classically interpreted as being caused by a brief, unstable icehouse during otherwise greenhouse conditions
Morphological terminology follows Whittington et al.
From Treatise on Invertebrate Paleontology, courtesy of ©1997, The Geological Society of America and The University of Kansas.
From Treatise on Invertebrate Paleontology, courtesy of ©1997, The Geological Society of America and The University of Kansas.
A total of thirty-one taxa were included in this analysis.
(Relevant material examined is listed where appropriate. In instances where museum material was not examined, species were coded using photographs from scientific publications.)
The characters used in phylogenetic analysis are listed below in approximate order from anterior to posterior position on the organism. A complete character matrix is given in
Cephalon
S1; 0: contacts S0, 1: does not contact S0.
Space between the proximal edges of both L1 lobes measured transversely (dorsal view); 0: wide (distance between the proximal edges of L1/posterior glabellar margin transverse width = 0.5), 1: narrow (distance between the proximal edges of L1/posterior glabellar margin transverse width = 0.33).
Point of maximum glabellar convexity (lateral view); 0: medial, 1: anterior.
S2 and S3; 0: strongly incised, 1: weakly incised, 2: indistinct or absent.
Genal spines; 0: present; 1: absent or reduced to small thorn-like projections.
Angle formed by the intersection of the anterior and lateral glabellar margins, in anterior view; 0: relatively broad (115–120 degrees), 1: relatively narrow (105–110 degrees).
Tubercle on center of L0; 0: present, 1: absent.
Shape of S1 close to the lateral glabellar margins; 0: S-shaped, 1: straight.
Border of librigena; 0: wide (ratio of exsagittal width of librigena to border width is 0.4–0.5); 1: narrow (ratio of exsagittal width of librigena to border width is 0.2–0.33).
L0; 0: wide (maximum glabellar width (tr.)/L0 (tr.) is 1.2–1.4), 1: narrow (maximum glabellar width (tr.)/L0 (tr.) is 1.6–1.8).
S1; 0: strongly incised, 1: weakly incised to indistinct.
Anterior glabellar margin (in anterior view); 0: roughly straight, 1: strongly convex.
Glabella between S0 and S1 (in lateral view); 0: curves uniformly with the rest of the glabella, 1: inflates dramatically.
S1 orientation; 0: runs roughly transverse, 1: curves posteriorly.
Shape of medial part of S0; 0: straight, 1: concave anteriorly.
Lateral margins of the glabella immediately anterior of S1 (in dorsal view); 0: roughly parallel, 1: strongly converging, 2: strongly diverging.
Border furrow on librigena; 0: pencil thin (ratio of exsagittal width of librigena to border furrow width is 0.1), 1: narrow (ratio of exsagittal width of librigena to border furrow width is 0.15–0.22), 2: wide (ratio of exsagittal width of librigena to border furrow width is 0.27–0.33).
Hypostome
Middle body furrow; 0: does not intersect or only faintly contacts outer border furrow, 1: prominently intersects outer border furrow.
Middle body furrow of hypostome; 0: prominently intersects entire middle body, 1: restricted to the lateral edges of the middle body.
Posterior margin; 0: possesses a strongly concave pocket, 1: is straight or with concave pocket strongly reduced to absent.
Pygidium (Note, all measurements of the terminal axial piece use the notch on the lateral edges of the terminal axial piece as the anteriormost point of the axial piece if the segment has been fused to the axial ring.)
Pleural spines; 0: terminate close to each other, forming a pygidial shield, 1: separate from each other distally.
Inter-pleural furrows; 0: wide, 1: narrow (pencil thin).
Anteriormost set of pleural spines; 0: has proximal “kink” associated with a 60–80 degree angle change and a long crescent shaped notch on the anterior side of the spine, 1: gradually curves proximally, with the notch absent or reduced.
Distal pleural tips; 0: flat, 1: rounded, 2: subtriangular.
Width (tr.) of terminal axial piece; 0: narrow (tr.) (transverse width of the anteriormost part of the axial piece ∼ three quarters of its length (sag.)), 1: wide (tr.) (transverse width of the anteriormost part of the axial piece ∼ two times its length (sag.)), 2: average (tr.) (transverse width of the anteriormost part of the axial piece ∼ its length (sag.)).
Pygidial convexity (posterior view); 0: vaulted, 1: nearly flat.
Pygidial dimensions; 0: wide and short (pygidial width (tr.) divided by length (sag.) is roughly 2.1–2.2), 1: long and narrow (pygidial width (tr.) divided by length (sag.) is roughly 1.6–1.8), 2: very long (pygidial width (tr.) divided by length (sag.) is roughly 1–1.3).
First axial ring; 0: wide (width (tr.) of axial ring divided by width (tr.) of pleural field ∼1.5–1.7), 1: narrow (width (tr.) of axial ring divided by width (tr.) of pleural field ∼1).
Posteriormost part of terminal axial piece in dorsal view; 0: rounded, 1: pointed.
Maximum convexity of terminal axial piece, in lateral view; 0: anterior, 1: medial, 2: posterior.
Interpleural furrows; 0: deep, 1:shallow.
Lateral margins of second set of pleural spines at approximate spine midpoint; 0: strongly curved, 1: weakly curved to straight.
Terminal axial piece size; 0: small (length (sag.) < length (sag.) of first axial ring), 1: large (length (sag.) >1.5 length (sag.) of first axial ring).
Distal tips of pleural spines; 0: hooked (i.e., sharply curved near distal ends), 1: straight.
Distal ends of the posteriormost pleural spines; 0: dramatically inflate laterally, 1: remain relatively the same size.
Angle the pygidial axial furrow along axial ring 1 and 2 forms with a sagittal line, 0: shallow (∼20°), 1: sharp (>30°).
Furrow on proximal end of first pleural spine; 0: visible in dorsal view, 1: not visible in dorsal view.
Lateral edges of terminal axial piece; 0: straight sided, 1: strongly curved.
Third axial ring; 0: fused completely to terminal axial piece, forming a notch, 1: partially fused (ring partly visible), 2: ring distinct (not fused).
The data were analyzed using TNT v1.1
The results from phylogenetic analysis were used in biogeographic analysis by applying Lieberman-modified Brooks Parsimony Analysis (LBPA)
The biogeographic areas used in this analysis are shown where
Tree graphics generated using FigTree v.1.1.2
The analysis generated 29 most parsimonious trees of length 115 steps, with CI (excluding uninformative characters) and RI values of 0.405 and 0.715 respectively. A strict consensus of these trees (
Part of the lack of resolution in Silurian
The LBPA yielded three most parsimonious geodispersal trees of length 69 steps (
We used the test of Hillis
Our analysis suggests that the genus
The topology suggests that there are at least two smaller clades within the genus
The tree topology suggests that the genus
Phylogenetic analysis reveals interesting patterns of character evolution within
Medial portion of S0 strongly concave anteriorly. Lateral margins of the glabella strongly converge anterior of S1. Border of the librigena narrow (tr.). Border furrow of the librigena pencil thin. For addition diagnostic criteria see the diagnosis of
Because the phylogenetic analysis indicates
See Lane
Some species that have traditionally been placed within the genus
The species that have been previously referred to as
The vicariance tree suggests a close relationship between N.W. Laurentia and Avalonia that is not replicated in the geodispersal tree. This relationship is governed by the condition of the basal node of the phylogeny, which was reconstructed as a combined E. Laurentia-N.W. Laurentia-Avalonia. The node is temporally constrained to the Early Ordovician, when Avalonia was separated from Laurentia and peripheral to Gondwana
The geodispersal tree also suggests a close relationship between E. Laurentia and the Yangtze block. This relationship is replicated in the vicariance tree, suggesting that the processes affecting geodispersal and vicariance between these two regions were the same, potentially implicating cyclical processes such as sea-level rise and fall. However, paleomagnetic and other faunal evidence suggest that these two regions were far apart
A phylogenetic analysis of the sphaerexochines suggests that the genus
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We thank Susan Butts from the YPM, Jessica Cundiff from the MCZ, Stefan Bengtson and Christiana Frauzen-Bengtson from the AR, David Bruton and Franz-Josef Lindemann from the PMO, Linda Wickström from the SGU, and Ivan Gogin from the VSEGEI for providing access to study material that was vital for the completion of this study, and also for loaning relevant material. Thanks to Jonathan Adrain for information on taxonomic composition of the sphaerexochines, to David Bruton for information on specimen repositories and providing relevant references, and to Brian Chatterton, Malte C. Ebach, and one anonymous reviewer for comments that greatly improved the quality of this manuscript.