23-5 Reign of Trilobites in Upper Mississippi Valley: Phylogeny of the late Cambrian trilobite family Dikelocephalidae Miller, 1889

Session: Phylogenetic and Computational Approaches in Paleobiology and Paleoecology, Part I

Presenting Author:

Authors:

Abstract:

In an age of genomics, the degree to which phylogenetic relatedness can be resolved with confidence is commonly sufficently secure allowing systematists to explore other evolutionary questions. To what extent can similar claims be made for the skeletonized record of the long extinct groups? Dikelocephalidae are not atypical of late Cambrian trilobite families in having certain distinctive and plausibly apomorphic features common to all or most putative members. They are collectively restricted to relatively shallow water facies within a single paleocontinent, and to a relatively short stratigraphic interval likely some 1.5 to 3.5 million years in duration. As such, they provide a suitable candidate for evaluating the question posed. We explored evolutionary relationships using multiple outgroup taxa, and varied ingroup composition based on the availability of character information and inclusion of stem taxa of other derived clades. The phylogenetic analyses conducted here also used two datasets, one using discretized characters only, the other mixed character matrix of continuous and discrete characters. Results to date include the following: 1) support levels for parsimony-based evaluations, while similar to other studies of trilobite phylogeny, are notably low compared to those of living clades, 2) including apparent stem taxa of other clades that may root among Dikelocephalidae reduces support levels markedly, 3) mixed matrix datasets yielded more strongly supported relationships than those based on discretized characters, 4) quite commonly within Dikelocephalidae clades of relatively small "osceolimorph" trilobites with elongated glabella and large eyes very close to the glabella are distinguished from "dikelomorph" trilobites with a more square glabella and relatively smaller eyes, some species of which grow to large sizes, 5) almost all analyses support a sister-taxon relationship between Walcottaspis vanhornei and a particular Dikelocephalus morph, 6) Bayesian analyses incorporating information on stratigraphic occurrence are broadly consistent with parsimony-based results. Our analyses suggest that only situation 5 is sufficiently well supported across various datasets and methods and permits further consideration of evolutionary tempo and mode in phylogenetic context within this group.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8347

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