New insights concerning homology of the oral region and ambulacral system plating of pentaradial echinoderms

Abstract

Abstract.-Universal elemental homology (UEH) underpins recent understanding of peristomial and ambulacral elemental homology of pentaradiate echinoderms by providing a uniform set of terminology to construct phylogenetic characters. Variation in the expression of these elements provides evidence for phylogenetic relationships. Two nonhomologous sets of plates border the peristome and are associated with two nonhomologous sets of floor plates forming the ambulacral food groove. Some edrioasteroid-grade and eocrinoid-grade echinoderms have ambulacral systems formed from oral frame plates and adradial floor plates, whereas most blastozoans and crinoids bear oral plates and abradial floor plates. These plates are expressed in a variety of ways among echinoderms, but nearly all can be reconciled with the underlying model. Arguments against UEH are methodologically flawed and confuse many terms and interpretations.

A re‐interpretation of the ambulacral system of Eumorphocystis (Blastozoa, Echinodermata) and its bearing on the evolution of early crinoids

“…Universal elemental homology (UEH) identifies ambulacral homology using the Carpenter system (Carpenter ), which focuses on the plate types bordering the peristome and the ambulacral system (Sumrall , ; Sumrall & Waters ; Kammer et al . ).…”

“…; Mooi & David , , ; David et al . ) is much coarser and differentiates the echinoderm skeleton into two large categories: axial (skeletal parts associated with the mouth and ambulacral system) and extraxial (the body wall); UEH refers only to elements within the axial system (Sumrall ).…”

“…() also asserted that superficial similarities shared by blastozoans and crinoids (e.g. homoplasy) are probably related to plesiomorphic pentaradial patterns of stemmed echinoderms, an argument that is not rooted in phylogenetic understanding, as it confuses the definitions of homology and homoplasy (Sumrall ). In contrast, phylogenetic arguments are rooted in the discovery of suites of synapomorphies providing support for nodes within proposed evolutionary trees.…”

Redescription of Macurdablastus and redefinition of Eublastoidea as a clade of Blastoidea (Echinodermata)

“…Although this taxon bears the typical plating of blastoids, differences in the size and shape of the lancet plates and in the nature of the respiratory structures differentiate it from other eublastoids, coronoids and Lysocystites . The conservative plating and increased understanding of homologous elements through universal elemental homology allow for precise examination and testing of morphological hypotheses through time (Sumrall , ; Sumrall & Waters ).…”

Progress in echinoderm paleobiology

“…This hypothesis focuses on the elements associated with the oral region, identifying possible homologies at the level of specific plates. Two papers, Paul (2017) and Sumrall (2017), deal with the homology of plates associated with the oral area in early pentaradial echinoderms. The former contribution describes and identifies homology in various 'cystoid' groups and represents a seminal work for understanding homology among these fossil taxa.…”