Testing the plateau: a reexamination of disparity and morphologic constraints in early Paleozoic crinoids

Abstract

Studies of crinoid morphology have been pivotal in understanding the constraints on the range of morphology within a clade as well as the patterns of disparity throughout the Phanerozoic. Newly discovered and described faunas and recent study of early Paleozoic crinoid diversity provide an ideal opportunity to reanalyze Ordovician through Early Silurian crinoid disparity with more complete taxonomic coverage and finer stratigraphic resolution. Using the coarse stratigraphic binning of Foote (1999), the updated morphologic data set has a similar disparity pattern to those previously reported for the early Paleozoic. However, with the more resolved stratigraphic binning used by Peters and Ausich (2008), a significant difference exists between the original and current data sets. Both data sets have a pronounced disparity high during the late Middle Ordovician. However, the updated disparity curve has a much higher initial disparity during the Early Ordovician and a pronounced rise in disparity during the Silurian recovery. Examination of differential sampling, proportions of the crinoid orders through time, and methods of coding characters indicate these factors have little effect on the pattern of crinoid disparity. The Silurian morphospace expansion occurs primarily within disparids and coincides with the origination of the myelodactylids. These findings corroborate the rapid expansion of morphospace during the Ordovician. However, crinoid disparity did not remain static and, although less frequent than during the initial radiation, new body plans evolved following the Ordovician Extinction (e.g., the myelodactylids). These results are consistent with the hypothesis of ecology constraining the limits on morphologic disparity at the class level.

Comparing taxonomic and geographic scales in the morphologic disparity of Ordovician through Early Silurian Laurentian crinoids

“…Average biofacies disparity can be compared with regional disparity (Deline and Ausich 2011) and taxonomic diversity at both the local and regional scales (Peters and Ausich 2008) (Fig. 2).…”

“…Global samplestandardized generic crinoid taxonomic diversity is also shown to indicate that the Middle-Late Ordovician extinction event is largely an artifact of sampling, modified from Peters and Ausich (2008) and explanation of the standardization methods contained within. B, early Paleozoic crinoid disparity from Laurentia based on a character-based analysis of 479 crinoid species previously analyzed by Deline and Ausich (2011). C, Average uncorrected species richness within biofacies from Peters and Ausich (2008).…”

“…As discussed previously (see Deline 2009 andDeline andAusich 2011 for detailed examples), the coding scheme here differs from that of Foote (1999) in the treatment of unpreserved (missing due to lack of preservation) and inapplicable (missing because of the absence of a previous character, e.g., arm branching for a crinoid lacking arms) characters. Both greatly increase the amount of missing data in the analysis and can produce artifacts in the results.…”

“…However, if within-genus variation is playing a large role in the disparity curve, as is observed in the late Middle Ordovician, disregarding this variation could reduce the estimated disparity during that interval. Similarly, the Silurian peak is dampened by the consolidation of 11 species into two genera within the myelodactylids, which created the Silurian peak in the regional species curve (Deline and Ausich 2011). Taxonomic scale has little effect at the biofacies level (Fig.…”

Forbidden phenotypes and the limits of evolution

“…This kind of variation has been documented in Early Cambrian lapworthellids [26], segment number in early trilobites [27][28][29], plate numbers and positions in early pelmatozoan echinoderms [30][31][32] and the pattern of tertiary and higher order veins in the leaves of early angiosperms [33].…”