Journal of Paleontology, 91(4), 2017, p. 582–603 Copyright © 2017, The Paleontological Society 0022-3360/17/0088-0906 doi: 10.1017/jpa.2016.151
Testing for homologies in the axial skeleton of primitive echinoderms
Christopher R. C. Paul School of Earth Sciences, University of Bristol, Bristol, UK ⟨
glcrcp@bris.ac.uk⟩
Abstract.—The extraxial axial theory is used to investigate homology of ambulacral and oral plating because it pre- dicts 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 indepen- dently 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 ambu- lacral 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.
Introduction
There has been great progress in the study of fossil echinoderms recently, in terms of new taxonomic discoveries (e.g., helico- cystids; Smith and Zamora, 2013), in teasing out evolutionary relationships (e.g., Sumrall and Wray, 2007; Sumrall and Waters, 2012; Zamora and Smith, 2012; Zamora and Rahman, 2015), and in theoretical concepts, such as the extraxial-axial theory (EAT; Mooi et al., 1994; David and Mooi, 1998) or universal elemental homology (UEH; Sumrall, 2008, 2010). Establishing homologies is an essential first step in phylogenetic analysis (e.g., Kammer et al., 2013). Over 50 years of research has taught me that to every generalization about fossil echino- derms there is an exception. Hyman (1955) was right that echinoderms are a group ‘especially designed to puzzle the zoologist’ (p. vi). Nevertheless, since science advances by test- ing hypotheses, these exceptions, whether teratological indivi- duals or anomalous taxa, can often be used to test hypotheses about homology or phylogenetic relationships. Whether such exceptions ‘prove the rule’ or are the ‘ugly fact that slays the beautiful hypothesis’ or even just annoying anomalies that are hard to explain, they cannot be ignored. Choosing only exam- ples that fit hypotheses does not test them. ‘Anomalous’ speci- mens test hypotheses of homology because they challenge us to decide which parts are homologous and which not and how the differences arose. This, in turn, may lead to further hypotheses of evolutionary relationships.
Here I confine discussion to axial skeletons of primitive
echinoderms because the EAT hypothesizes that axial skeletal elements are only added and radial water vessels only branch terminally (though see the apparent exceptions to the second assumption posed by the Ordovician fossil sea urchin Neobothriocidaris Paul, 1967c in the following). The enormous diversity of early echinoderm ‘designs’ also poses a special challenge to recognizing homologies and to phylogenetic analysis. Extraxial skeletal elements are even more diverse. Plates are added almost anywhere, making it more difficult to recognize homologous elements. However, repeated extraxial designs do enable us to recognize distinctive (and possibly monophyletic) taxonomic groups. Discussion is organized into four somewhat arbitrary
sections, recognizing: (1) homologous ambulacra, (2) homologous oral plating, (3) homologous ambulacral plates, and (4) homologous cover plating, both oral and ambulacral. Most of the taxa discussed arereferredtoinformallyas ‘cystoids,’ which is shorter than ‘noncrinoid pelmatozoans’ and equates more or less to blastozoans but does not imply a formal taxonomic group.
Homology versus analogy
‘Homology’ may be defined as ‘the same structure modified for different functions,’ such as the forelimbs of ichthyosaurs, dinosaurs, and pterosaurs. Analogy involves different structures that perform the same function, such as the wings of birds and
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