291-9 The Challenges and Possiblities of Homoplasy in Cephalopod Phylogenetics

Session: Cephalopods Through Time: Insights into Evolution, Ecology, and Environmental Reconstruction

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Phylogenetic analyses of morphological datasets are rooted in the idea that coded character states reflect evolutionary processes. Characters that show homoplasy (independent acquisition or reversal of states) are traditionally deemed to be less evolutionarily informative than characters that evolve in a more straightforward, parsimonious way. However, homoplasy can still reflect evolutionary changes. Taxonomic groups with relatively simple morphologies, such as the shells of ammonoids and nautiloids, show extensive homoplasy in phylogenetic analyses. Can an exploration of this homoplasy and its distribution in a dataset tell us anything about evolution, such as the developmental regulatory constraints or selective pressures a clade experienced? We compiled separate discrete morphological datasets for three clades of Cretaceous ammonoids, the acanthoceratid genus Metoicoceras and its close relatives, the morphologically conservative Desmoceratoidea, and the straight-shelled heteromorph family Baculitidae. Characters were partitioned into three categories: shell shape and size, ornamentation, and suture patterns. Bayesian phylogenetic analysis generated maximum clade credibility trees for the latter two groups, while a most parsimonious tree was determined for the Metoicoceras clade. Standard homoplasy metrics were calculated for individual characters and multiple tree-level homoplasy indices were computed for each clade, including the relative homoplasy index, which permits comparisons across clades and datasets. Overall, ornamentation characters showed less homoplasy than shell shape or suture characters in the two planispiral ammonoid clade but more homoplasy than the other partitions in the baculitids. However, for the Metoicoceras clade, the developmental timing of disappearance of ventrolateral tubercles showed the most homoplasy, and in desmoceratoids, developmental loss and gain of constrictions was notably plastic, suggesting that shifts in developmental regulation of shell ornamentation were important for independently generating repeated morphological variations in both clades. At the clade level, the Metoicoceras clade showed much less homoplasy than the desmoceratoids or baculitids, which had similar homoplasy indices. For the latter two groups, the homoplasy excess ratio (HER) was negative, indicating that the datasets had even more homoplasy than would be expected from a random dataset. This pattern may indicate active selection that drove the repeated evolution of similar morphologies in both of these ammonoid clades.

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

doi: 10.1130/abs/2025AM-9062

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