COMPARING MODELS OF CHARACTER EVOLUTION IN LATE CRETACEOUS AMMONOIDS USING A BAYESIAN PHYLOGENETIC APPROACH

Ammonoid cephalopods are famed for their rapid rates of evolution, however, certain morphological features in ammonoids may be more constrained than others. For example, suture patterns are important for distinguishing taxa and may therefore have been less prone to ecophenotypic variation or developmental plasticity than shell shape or ornamentation. We may also expect certain character suites to evolve in tandem due to genetic, developmental, or functional constraints. Hypotheses like these, involving differences in the tempos and modes of character evolution, have been difficult to test quantitatively. We used a Bayesian phylogenetic approach to evaluate three competing models of character evolution in Cretaceous ammonoids, focusing on the planispiral clades Acanthoceratidae and Desmoceratoidea and the heteromorph clade Baculitidae. Separate morphological datasets were constructed for each group and characters were divided into three partitions: shell shape, shell ornamentation, and suture pattern. We defined three different hypotheses of character evolution: 1) all three character partitions evolved at the same rate, 2) sutures evolved at a different rate than shell shape or ornament, and 3) all three character partitions evolved at different rates. Reversible jump MCMC analyses were implemented in RevBayes for each ammonoid clade, using a gamma distribution to draw rates of character evolution appropriate to each of the three hypothesized models. The relative support for each of the three competing models of character evolution was evaluated and rates of change among partitions were compared. The three ammonoid clades differed in their character evolution. For Acanthoceratoidea and Baculitidae, all three partitions showed different rates, though in acanthoceratoids, shell shape and ornament rates were more similar to each other and evolved faster than sutures, while in baculitids, suture patterns had the highest rate. In contrast, for Desmoceratoidea, the same evolutionary rate for all characters was best supported, consistent with this clade’s relative evolutionary conservatism. These results suggest that different ammonoid clades experienced different constraints on their morphological evolution, possibly due to differences in life modes, selective pressures, or developmental regulatory mechanisms governing shell growth.