Exploring rates of change and modes of evolution in blastozoan echinoderms

Over the past half-century, paleobiologists have advanced the estimation of phylogenetic relationships among fossil taxa to explore evolutionary patterns in deep time. This study employs phylogenetics, divergence time calculations, and character rate evolution within three blastozoan echinoderm clades: Diploporita, Eublastoidea, and Paracrinoidea. Utilizing Reversible Jump Markov Chain Monte Carlo (rjMCMC) and Fossilized Birth-Death (FBD) models, we investigated evolutionary rates through anatomical subunit partitioning. Results suggest similar rates among the three groups, though Paracrinoidea exhibits elevated rates in several anatomical subunits. The inferred trees largely agree with other recently published analyses and highlight the need to revise echinoderm taxonomy. We tested different clock models for each group and found that model choice had strong effects on resulting trees; our findings suggest that linked clocks had more support than unlinked clocks. These findings indicate a need to carefully consider model choice and rates of evolution when conducting these types of analyses.

Methods
Recent work suggests variation in rates of evolution of echinoderm traits (Novack-Gottshall et al. 2022). In prior work, it has been hypothesized that “functional” traits like feeding characters may evolve at faster rates than other traits, a hypothesis not limited to echinoderms (e.g., Foote 1995; Ciampaglio 2002; Smith and Hopkins 2015; Wright 2017), but seen in animals in general (Wagner 1995; Sánchez-Villagra and Williams 1998). The morphological diversity seen in blastozoan echinoderms has frequently hindered our abilities to establish homology amongst characters, let alone examine rates of evolution. However, with the development of echinoderm-specific homology schemes (Mooi and David 1997; Sumrall 2010) and the application of these schemes to many blastozoan groups, these questions of evolution questions can be addressed. Both homology schemes have strengths and weaknesses (Sumrall et al. 2023), but, because this particular study is focused on Paleozoic blastozoans, and the modeling of morphological shifts of the oral and ambulacral systems, the morphological character datasets utilized in this particular study (i.e. Sheffield and Sumrall 2018; Bauer 2020; Limbeck et al. 2024) were developed predominantly using UEH with some influence from EAT. We explored anatomical subunit partitioning in this study using three groups of blastozoans, each of which is variable in its temporal and geographic patterns and in its body plans (Table 1). Each of these groups — Eublastoidea (Bauer 2021), Diploporita (Sheffield and Sumrall 2019a), and Paracrinoidea (Limbeck et al. 2024) — has recently published phylogenetic analyses from which we have pulled character data to build the models tested herein.

Character groups were used to partition the data in downstream analyses. The three character matrixes were compiled separately for different studies so for this work, the types of characters were considered and grouped based on the following seven anatomical features: (1) Respiratory: characters that are describing part of the respiratory structures; (2) Oral Plate Circlet: characters that include the plates from the oral plate circlet series as defined in Sumrall and Waters (2012); (3) Feeding: characters related to the ambulacral and brachiolar structures; (4) Periproct Area: characters related to the anus opening and plates of the anal area; (5) Reproduction: characters related to the gonopore (i.e. the genital pore); (6) Theca: characters that broadly represent the thecal body and do not fall into the other assigned character groups; and (7) Attachment structure: characters related to the stem or holdfast (Table 2; refer to Fig. 1 for examples of these characters for each set).