Are homalozoans echinoderms? An answer from the extraxial-axial theory

Abstract

Homalozoans include four classes of non-pentamerous Paleozoic echinoderms: Homostelea (cinctans), Ctenocystoidea (ctenoid-bearing homalozoans), Homoiostelea (solutes), and Stylophora (cornutes and mitrates). Their atypical morphologies have historically made it difficult to relate them to other classes. Therefore, their systematic positions have been represented by two hypotheses (H): as stem taxa to echinoderms (H1) or as stem taxa to chordates (H2). These conclusions rest on previous inability to recognize synapomorphies with more crownward echinoderms, resulting in a forcing of the homalozoans down the phylogenetic tree that is more artifactual than evolutionary. The Extraxial-Axial Theory (EAT) identifies body-wall homologies, common ontogenetic patterns, and major events in bodyplan evolution. Therefore, the EAT can identify synapomorphies among even the most disparate of echinoderms. Application of the EAT undermines both H1 and H2 and strongly suggests that the bizarre asymmetry of homalozoans is a derived characteristic, and not indicative of plesiomorphic morphology for either chordates or echinoderms. Each of the four homalozoan clades and their major features are reexamined using the EAT. New findings are presented concerning homologies of thecal body wall, but we focus on stems, arms, and brachioles, which are recognized as very distinct products of independent evolutionary events. The results support a new interpretation (H3) of homalozoans as a polyphyletic assemblage that can be parsed out into other, clearly echinoderm clades. The Homoiostelea and Homostelea share the blastozoan synapomorphy of a brachiole. The enigmatic Ctenocystoidea also seem to have brachioles. The Stylophora have an arm as in crinoids. H3 is also more congruent with the known fossil record. Although they are stratigraphically early echinoderms, homalozoans are not indicative of the plesiomorphic morphology of the phylum.

Cambrian cinctan echinoderms shed light on feeding in the ancestral deuterostome

“…Our findings are broadly in agreement with previous interpretations of the earliest fossil stem-group echinoderms (Ctenoimbricata, ctenocystoids and cinctans) as pharyngeal filter feeders [14 -16], and argue against their interpretation as passive tentaculate feeders [19,20]. Among modern deuterostomes, active suspension feeding with pharyngeal gill slits is documented in enteropneust hemichordates, urochordates, cephalochordates and larval lampreys, while suspension feeding with tentacles characterizes crinoids and pterobranch hemichordates.…”

“…Cinctans are the best understood of these groups in terms of their anatomy and functional morphology, and so have the greatest potential for elucidating deuterostome evolution; however, their mode of feeding is controversial. It is widely accepted that cinctans were sessile epibenthic suspension feeders with an anterolateral mouth and one or a pair of marginal grooves [7,14,[17][18][19][20], but it is debated whether they were passive suspension feeders with a system of tentacles, analogous to crinoids [19,20], or active pharyngeal filter feeders, similar to urochordates [14,21].…”

The oldest echinoderm faunas from Gondwana show that echinoderm body plan diversification was rapid

“…Given that the rock record shows a gradual increase in diversity, it has been traditional to simply take this as an accurate record of echinoderm evolution (for example, refs 10,11). As these earliest echinoderm faunas comprise only ambulacral-bearing clades with radiate symmetry (helicoplacoids, stromatocystitids and gogiids), this has also been taken as evidence that the primitive echinoderm body plan was radiate and that asymmetric forms are derived (for example, refs [12][13][14]. However, against this the most careful of molecular clock studies 7,15 place the divergence of echinoderms from their sister group, the hemichordates, in the late Ediacaran at ca.…”

Evolutionary implications of a new transitional blastozoan echinoderm from the middle Cambrian of the Czech Republic

“…Primary homologies have been identified independently among the three morphological modules (aboral region, body wall, and feeding system; Table 1, Appendix), whereas mechanistic homologies were used to describe the organization of each module, independently from any terminological influence (David et al, 2000;Nardin et al, 2009;Zamora and Rahman, 2014).…”

“…Coding has been based on new observations of specimens of the 13 considered taxa and on a reinterpretation according to the extraxial-axial theory framework (Mooi and David, 1998;David et al, 2000;Nardin et al, 2009;Lefebvre et al, 2015).…”