Abstract
The phylogenetic relationships between major groups of plesiomorphic pentaradial echinoderms, the Paleozoic crinoids, blastozoans, and edrioasteroids, are poorly understood because of a lack of widely recognized homologies. Here, we present newly recognized oral region homologies, based on the Universal Elemental Homology model for skeletal plates, in a wide range of fossil taxa. The oral region of echinoderms is mainly composed of the axial, or ambulacral, skeleton, which apparently evolved more slowly than the extraxial skeleton that forms the majority of the body. Recent phylogenetic hypotheses have focused on characters of the extraxial skeleton, which may have evolved too rapidly to preserve obvious homologies across all these groups. The axial skeleton conserved homologous suites of characters shared between various edrioasteroids and specific blastozoans, and between other blastozoans and crinoids. Although individual plates can be inferred as homologous, no directly overlapping suites of characters are shared between edrioasteroids and crinoids. Six different systems of mouth (peristome) plate organization (Peristomial Border Systems) are defined. These include four different systems based on the arrangement of the interradially-positioned oral plates and their peristomial cover plates, where PBS A1 occurs only in plesiomorphic edrioasteroids, PBS A2 occurs in plesiomorphic edrioasteroids and blastozoans, and PBS A3 and PBS A4 occur in blastozoans and crinoids. The other two systems have radially-positioned uniserial oral frame plates in construction of the mouth frame. PBS B1 has both orals and uniserial oral frame plates and occurs in edrioasterid and possibly edrioblastoid edrioasteroids, whereas PBS B2 has exclusively uniserial oral frame plates and is found in isorophid edrioasteroids and imbricate and gogiid blastozoans. These different types of mouth frame construction offer potential synapomorphies to aid in parsimony-based phylogenetics for exploring branching order among stem groups on the echinoderm tree of life.
Highlights
Recent attempts to place early Paleozoic echinoderms into a comprehensive phylogenetic hypothesis have been hampered by the lack of a rigorous framework of underlying homologies with which to construct phylogenetic character matrices
Many advances have been made during the past few decades, notably the Axial-Extraxial Theory (EAT) [1,2], that hypothesizes homology based on presumed embryonic origins of body regions and organ systems
Because EAT homologies are relatively coarse and difficult to reconcile with many Paleozoic taxa, this scheme has the greatest utility in phylogeny inference only for the most phylogenetically deep character states [3,4]
Summary
Recent attempts to place early Paleozoic echinoderms into a comprehensive phylogenetic hypothesis have been hampered by the lack of a rigorous framework of underlying homologies with which to construct phylogenetic character matrices. By combining ambulacral homology using the Carpenter system [5] with the homology of plate types in the peristomial border and ambulacral system, UEH identifies homology at a fine scale. This allows high precision in character description for subsequent analysis of echinoderm phylogeny. Such an approach is needed to infer phylogenetic relationships among extinct stem-group taxa within the echinoderm tree of life. These are the plesiomorphic pentaradial echinoderms: edrioasteroids, blastozoans, and crinoids (including extant Articulata). Excluded from this analysis are the extinct nonpentaradial echinoderms such as homalozoans and helicoplacoids, plus all eleutherozoans, which include all living echinoderms except the articulate crinoids
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
