Testing for homologies in the axial skeleton of primitive echinoderms

Abstract

Abstract.-The extraxial axial theory is used to investigate homology of ambulacral and oral plating because it predicts terminal branching and terminal addition of plates in the axial skeleton, although exceptions to the former may occur in some Paleozoic echinoderms. The variety of morphological designs and anomalous individuals also provide tests of plate homology. Homology of ambulacra is generally accepted, with the hydropore and/or single gonopore in Carpenter's CD interray. In the 2-1-2 ambulacral pattern the unbranched ambulacrum is always in Carpenter's A ray. All ambulacral morphology requires just three instructions: 'grow,' 'branch,' and 'stop.' The range of variation in echinoderms with fewer than five ambulacra implies that both the 'branch' and 'stop' instructions acted independently in all five rays. Numbers of ambulacra may or may not correlate with numbers of orals. Two basic patterns of 'cystoid' oral plating occur; with a single radial (circum-oral, CO) plate from each ambulacrum plus a sixth in the CD interray, and with all six interradial peri-oral (PO) plates, with two in the CD interambulacrum. Five 'orals' may involve loss of PO3 or PO6. Erect ambulacral structures are lost first in taphonomy and so poorly known. All ambulacral skeletal elements bear the same topological relationship to ambulacral soft tissues. Where branched ambulacra occur, the trunk or flooring plates are often modified first brachiolars or pinnulars. Both brachioles and pinnules may arise from facets developed on one or two flooring plates. Terminal addition of plates, spacing of brachioles/pinnules, and lack of musculature to open cover plates all suggest that 'cystoids' had extensions of the water vascular system in their ambulacra.

A new Upper Ordovician aristocystitid diploporite genus (Echinodermata) from the Llanddowror district, South Wales

“…and B . dichotomus is the result of convergence, and the differences in the ambulacral system are much more significant (see Paul, ). It is also true that less densely crowded diplopores occur basally on the theca are more irregularly rounded and sometimes lack the central tubercle (Figure b below, 8).…”

“…Firstly, most aristocystitid genera have four ambulacra. Aristocystites itself has just two (Barrande, ; Parsley, ; Paul, ), Triamara has three (Paul, ; Tillman, ), and Oretanocalix (Gutiérrez‐Marco, ) has five. So it seems likely that Binocalix probably had four.…”

“…All aristocystitid genera where the oral plating and ambulacra are known have a mouth elongated at right angles to the oro‐anal axis within a broad ambulacral tract (Paul, , p. 57), lined with oral pores on either side and covered by two series of cover plates (Figure b,c). The mouth is surrounded by eight plates interpreted by Paul (, fig. 7, p. 589) as four, central, peri‐oral plates (PO1, PO2, PO4, and PO5) and four distal circum‐oral plates (CO1, CO2, CO4, and CO5; Figure ).…”

Prokopius, a new name for “Hippocystis sculptus” Prokop, 1965, and the status of the genus Hippocystis Bather, 1919 (Echinodermata; Diploporita)

“…Subsequently, Chauvel (1966, p. 38) introduced a new subfamily, the Hippocystinae, and Parsley (1990) has given a thorough, modern description of A. bohemicus and interpreted its mode of life. Furthermore, Makhlouf et al (2017) and Paul (2017) have introduced a new interpretation of the homologies of the oral plating in aristocystitid diploporites, which is thought to characterize the entire family (see below).…”

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.…”