Evolution Within a Bizarre Phylum: Homologies of the First Echinoderms

SYNOPSIS. The Extraxial/Axial Theory (EAT) of echinoderm skeletalhomologies describes two major body wall types: axial and extraxial.The latter is subdivided into perforate and imperforate regions.Each of the regions has a distinctly different source in earlylarval development. Axial skeleton originates in the rudiment,and develops in association with the pentaradially arrangedhydrocoel according to specific ontogenetic principles. Perforateand imperforate extraxial regions are associated with the leftand right somatocoels respectively, are not governed by ontogeneticprinciples of plate addition, and are products of the non-rudimentpart of the larval body. The morphology of even the most bizarreof the earliest echinoderms can be explored using the EAT. Amongthese, edrioasteroid-like taxa best fit the idea that formsexpressing archimery in the sequential arrangement of axial,perforate extraxial, and imperforate extraxial regions are thefirst echinoderms. Metamorphosis is especially marked in cladesthat have a high axial to extraxial skeleton ratio because structuresdeveloping from the non-rudiment part are suppressed in favorof the developing axial elements during this process. However,inearly echinoderms, extraxial skeleton makes up a far largerproportion of the body wall than axial, implying that metamorphosiswas not as significant a part of the developmental trajectoryas it is in more recently evolved taxa. Echinoderm radiationconsists of a succession of apomorphies that reduced the expressionof extraxial components but increased the influence of axialones, with a concomitant increase in the prominence of metamorphosis.     

Notograptidae, sister to Acanthoplesiops Regan (Teleostei: Plesiopidae: Acanthoclininae), with comments on biogeography, diet and morphological convergence with Congrogadinae (Teleostei: Pseudochromidae)

The Notograptidae contains one genus, Notograptus Günther, and five nominal species from northern Australia and southern New Guinea. Morphological evidence places Notograptus among acanthoclinine plesiopids (continuous free margin of lower lip; head naked; dorsal and anal fins with many spines and few segmented rays; no extensor proprius; reduced number of caudal-fin rays) and supports a sister relationship with Acanthoplesiops (symphyseal flap on lower lip; reduced hypural 5; reduced hypurapophysis). This hypothesis resolves the relationships within Acanthoplesiops , clarifying the polarity of autogenous middle radials of dorsal- and anal-fin pterygiophores. The proposed relationships among acanthoclinines are: Acanthoclinus ( Belonepterygion ( Beliops ( Notograptus ( Acanthoplesiops hiatti ( A. indicus ( A. psilogaster ( A. echinatus ))))))). The distribution of Notograptus compliments that of its proposed sister clade in that Acanthoplesiops is unknown from northern Australia or southern New Guinea. There are repeated geographical patterns among several groups suggesting that Australia is a basal area to a broader Indo-Pacific region. Similarity between the Congrogadinae (Pseudochromidae) and Notograptus has long been noted, both having a loosely connected suspensorium and elongate body which were mistakenly considered indicators of relationship; we add reduced branchial arches, straight, tube-like gut and highly expandable anus. We examine these similarities as an indication of a shared specialized feeding habit. Notograptus is an alpheid shrimp predator, able to swallow its large prey whole. Most species of congrogadines eat whole, large crustaceans. This is probably an example of convergent adaptation to a particular selective regime.  © 2004 The Linnean Society of London, Zoological Journal of the Linnean Society , 2004, 141 , 179–205.     

Evolution to the extreme: origins of the highly modified apical system in pourtalesiid echinoids

The apical system of the genus Pourtalesia displays a plate architecture that falls so far outside that typical of other echinoids that plate homologies remain problematic. A new approach using the Extraxial–Axial Theory (EAT) that develops homologies for the Echinodermata is proposed. The exploration of apical plate patterns throughout ontogenetic sequences shows that the typical holasteroid pattern found in the youngest specimens undergoes a series of disturbances that result in a multiple disjunction accompanied by isolation or disappearance of certain genital plates. We propose a new interpretation of the apical architecture of the genus that agrees with: (1) the plate addition processes as predicted by the EAT; (2) patterns observed in other genera of the Pourtalesiidae as well as in its sister‐group (plexechinids); and (3) the patterns known from Palaeocene holasteroids. In the context of the EAT, the genus Pourtalesia appears to represent the extreme in a reduction of the extraxial part of the body wall. © 2004 The Linnean Society of London, Zoological Journal of the Linnean Society, 2004, 140 , 137–155.