Cole—Phylogeny and evolution of Ordovician camerate crinoids
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clades of two taxa (Anthracocrinus, Strimple and Watkins, 1955 and Gustabilicrinus Guensburg, 1984; Ptychocrinus Wachsmuth and Springer, 1885 and Euptychocrinus Brower, 1994) and one clade of three taxa (Atactocrinus Weller, 1916, Maquoketocrinus Slocom, 1924, and Ortsaecrinus Gil et al., 1999) resolved (Supplemental Data 3). The subsequent reweighted analysis, which used RC to
reweight characters based on the trees recovered in the initial analysis, produced a single most parsimonious tree (Fig. 2) with a length of 70.152 and CI and RI values of 0.319 and 0.507, respectively. The four stratigraphic congruence metrics calculated for the recovered tree all proved significant, indicating the fit of the recovered tree to stratigraphy based on the observed stratigraphic ranges of taxa was significantly higher than for randomly generated trees (Table 4). The per-interval preservation probability of camerate taxa
was estimated to be 0.51, and the per-lineage-million-year sampling rate was calculated to be 0.12. The estimated sampling probability of Ordovician camerates is comparable to that of previous estimates for crinoids. For example, Foote and Raup (1996) calculated a per-interval preservation probability of 0.5 (equivalent to a sampling rate of 0.1) for 395 globally distributed Ordovician–Devonian crinoid genera that included both camerates and non-camerates (see Bapst and Hopkins, 2016). These estimates suggest the record is more than 70% complete at the genus level (Foote and Raup, 1996). Thus, the taxon sampling employed within this study is interpreted herein as a highly representative sample of the crinoid fossil record.
Figure 1. Strict consensus of 12 most parsimonious trees recovered from the preliminary phylogenetic analysis. Support for nodes is given by bootstrap values (above) and Bremer support (below). Labels at nodes are given for major clades; (R) = taxa traditionally placed in family Reteocrinidae.
Monte Carlo simulation (Fig. 4), and a p-value was calculated for each of the observed metrics (Table 4). Finally, to assess the relative completeness of taxon sampling
in this analysis relative to the crinoid fossil record, preservation probability and sampling rate were calculated for Ordovician camerates using methods outlined by Foote (1997). This approach uses the distribution of taxonomic ranges to infer maximum likelihood estimates of per-interval sampling probability, thereby minimizing potential bias of finite sample size (Foote and Raup, 1996). All Ordovician camerate genera except Schizocrinus, Delgadocrinus, and Habrotecrinus were included in this analysis. Discrete time intervals were used based on stratigraphic ranges of genera tabulated from Webster (2003) updated to match current divisions of the International Chronostratigraphic Chart.
Results
Phylogenetic and other analyses.—The initial equally weighted parsimony analysis recovered 801 most parsimonious trees of 634 steps in length. The topology resulting from the strict consensus of these trees was largely unresolved with only two
Tree topology and clades recognized.—For the reweighted parsimony analysis, the following results are presented in terms of the clades recovered, their relative placement within the tree, and diagnostic characters shared by taxa composing those clades. The major clades recovered are designated by letters at nodes in Figure 2. For each clade, unique combinations of traits are given that are diagnostic of their constituent taxa. It should be emphasized that these character combinations are comprised of both plesiomorphic and shared derived traits but are useful for diagnosing the taxa descended from each node. Characters in these combinations that represent synapomorphies (shared derived characters) are noted. Two major clades were recovered that broadly correspond
to the orders Monobathrida and Diplobathrida, with several exceptions. All diplobathrid genera form a single clade except for Rosfacrinus Le Menn and Spjeldnaes, 1996 and the four genera assigned to family Reteocrinidae (Reteocrinus, Quechuacrinus, Gaurocrinus,and Cnemecrinus). This diplobathrid clade is represented in Figure 2 by Node G and all of its descendents. All monobathrid genera form a single clade with the exception of Adelphicrinus Guensburg and Sprinkle, 2003 and Eknomocrinus. This monobathrid clade is represented in Figure 2 by Node D and all of its descendents. Most of the genera that do not fall within the monobathrid or diplobathrid clades are positioned near the base of the tree as taxa that are stem to the monobathrid and diplobathrid
clades.These include Cnemecrinus, Eknomocrinus, Adelphicrinus, Reteocrinus, and Quechuacrinus. With Eknomocrinus designated as the outgroup, Adelphicrinus and Cnemecrinus are, respectively, the most basal lineages. If Cnemecrinus is designated as the outgroup, a clade composed of Adelphicrinus and Eknomocrinus is
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