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Evolutionary Lability of Integration in Cambrian Ptychoparioid Trilobites

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Abstract

Phenotypic integration can influence evolutionary rate and direction by channeling variation into few dimensions. The extent to which that channeling serves as a constraint over macroevolutionary timescales is determined in part by the evolutionary lability of phenotypic integration. Evolutionary change in patterns of pleiotropy, potentially reducing that constraint, is thought to be more readily achieved when pleiotropy is structured by variation arising in parallel along different developmental pathways rather than by variation arising from direct interactions within and between those pathways. Herein we test two predictions that follow from that hypothesis: (1) that clades undergoing dramatic diversification are characterized by integration that is weakly influenced by direct interactions; and (2) that the structure of integration arising from direct interactions is more conservative than that arising from parallel variation. We examine integration of the cranidium of two Cambrian ptychoparioid trilobites, Crassifimbra walcotti and Eokochaspis nodosa, comparing them to each other and to a previously studied species, C.? metalaspis. Shape variation is decomposed into components representing variation among individuals and variation due to direct interactions. In all three species, variation among individuals was only weakly influenced by direct interactions, suggesting that integration was unlikely to have been a long-term constraint on the Cambrian diversification of ptychoparioids. Phenotypic integration of E. nodosa is no more similar than expected by chance to either Crassifimbra species, but the component due to direct interactions is more similar than expected by chance to that of C.? metalaspis. Conversely, the two Crassifimbra species are generally similar (although not identical) in phenotypic integration, but markedly differ in their structure of direct interactions. Integration arising from direct interactions was therefore not immune to restructuring over even short evolutionary timescales, and was not always more conservative than that arising from parallel variation.

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Acknowledgments

H. D. Sheets fixed a bug in the SemiLand software at short notice and provided helpful technical comments regarding size-standardization. Some of the material analyzed herein was collected by A. R. (Pete) Palmer. Assistance to MW in the field was generously provided by N. C. Hughes, R. R. Gaines, L. McCollum, and M. McCollum. Code for performing the bootstrapping procedure was devised by A. Haber as part of her Ph.D. thesis (University of Chicago); her generosity in providing the code is greatly appreciated. We thank two anonymous reviewers for their comments.

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  1. Department of the Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, IL, 60637, USA

    Mark Webster

  2. Museum of Paleontology, University of Michigan, Ann Arbor, MI, 48109-1079, USA

    Miriam L. Zelditch

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Appendix

Appendix

Phylogenetic Relatedness of the Species

Crassifimbra walcotti is very closely related to Crassifimbra? metalaspis (see comments in Webster 2011), and their coeval stratigraphic occurrence in the uppermost Dyeran suggests very recent common ancestry. (The provisional assignment of the latter species to Crassifimbra [and hence the use of the “?” in the name] reflects the needs for a full systematic revision of that genus and other poorly diagnosed antagmine ptychoparioid genera, as discussed by Webster 2011.) Eokochaspis nodosa occurs in slightly younger sediments (basal bed of the Delamaran). Phylogenetic analysis places E. nodosa as a member of a kochaspid clade separated from the two species of Crassifimbra by nine internal nodes, although branch lengths were very short (Sundberg 2004; Webster 2011; unpublished data). A new species representing the oldest kochaspid (to be described elsewhere) occurs at very low abundance in collections with C.? metalaspis. Phylogenetic analyses and stratigraphic data therefore suggest that E. nodosa shared a last common ancestor with Crassifimbra in the late Dyeran.

Assigning absolute time to these phylogenetic branches is necessarily speculative because the strata sampled here are inappropriate for radiometric dating and there are very few radiometric dates compiled worldwide for strata of this age interval. However, the base of the Delamaran is approximately 510 million years old, and beds from New Brunswick dated as 511 ±1 million years old approximately correlate to the upper Dyeran (Landing et al. 1998; Shergold and Cooper in Gradstein et al. 2004). Given that the oldest known occurrences of C. walcotti, C.? metalaspis, and the kochaspid clade are stratigraphically coeval in the uppermost Dyeran, we infer that (1) there is on the order of just a few tens of thousands to hundreds of thousands of years between the sampled occurrence of the two Crassifimbra species and their last common ancestor; (2) branch length between the last common ancestor of the two Crassifimbra species and the last common ancestor of [Crassifimbra + the kochaspid clade] is also on the order of just a few tens of thousands to hundreds of thousands of years; and (3) there is ≈1 million years separating E. nodosa from its last common ancestor with Crassifimbra. Such estimates are consistent with available phylogenetic and stratigraphic data (Sundberg 2004; Webster 2011; unpublished data), and apply minimal insult to the fidelity of the trilobite fossil record.

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Webster, M., Zelditch, M.L. Evolutionary Lability of Integration in Cambrian Ptychoparioid Trilobites. Evol Biol 38, 144–162 (2011). https://doi.org/10.1007/s11692-011-9110-2

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