Probing the cassiduloid origins of clypeasteroid echinoids using stratigraphically restricted parsimony analysis

Abstract

Clypeasteroid echinoids are a familiar and easily defined clade with a cryptic origin. They first appear in the late Paleocene and are believed to have arisen from cassiduloid ancestry, but identifying sister-group relationships more precisely has proved difficult. Two factors are responsible for this problem, the extreme morphological conservatism of cassiduloids, which has given rise to high levels of character exhaustion, and the origin of crown-group clypeasteroids through paedomorphosis. Previous analyses, based on extant representatives alone or including all Mesozoic to Recent genera, have proved unsatisfactory.Here a parsimony analysis is undertaken using a restricted set of all stem-group clypeasteroids and cassiduloid taxa that existed immediately prior to the appearance of crown-group clypeasteroids. Inclusion of Togocyamus, the fossil taxon lying closest to the origin of crown-group clypeasteroids, is phylogenetically uninformative because that taxon is highly paedomorphic and has only generalized juvenile characteristics. However, earlier stem-group plesions provide critical data that identify Apatopygidae as extant sister group to the Clypeasteroida. Stratigraphically restricted analyses cannot eradicate the problems that arise from character exhaustion, but can minimize these with respect to specific phylogenetic questions.

Trait-based diversification shifts reflect differential extinction among fossil taxa

“…However, biogeographic differentiation should encourage the subclade divergence and thus, should generate more DSC than null models. Other phylogenetic datasets deliberately target the oldest members of clades to unravel subclade relationships because of a concern that homoplasy among late-appearing members of subclades will confound relationships among those subclades (48)(49)(50). Deliberately targeting early members of subclades should elevate DSC.…”

Phylogeny and origin of Jurassic irregular echinoids (Echinodermata: Echinoidea)

“…Fossil taxa are of extreme interest when trying to resolve phylogenies because they bear morphological information of phylogenetic significance that is often absent in extant taxa (Benton, Wills & Hitchin, 2000;Wagner, 2000;Smith, 2001). Moreover, extant taxa are sometimes so different morphologically, when evolution has been rapid enough to accumulate large numbers of apomorphies along a given lineage, that comparisons only among extant forms can be misleading.…”

“…At other times, they are evidence that hypotheses of homology among the characters in question require reassessment. California Berkeley, 1966;Kier, 1974;Saucède, Mooi & David, 2003;Solovjev & Markov, 2004), and more generally for numerous traits within the Cassiduloida (Kier, 1962(Kier, , 1966Suter, 1994;Smith, 2001;Wilkinson, Suter & Shires, 1996). W. Mintz, unpub.…”

“…Atlasaster jeanneti Lambert, 1937Sinemurian Collignon 'Plesiechinus' hawkinsi Jesionek-Szymanska, 1970 Upper Sinemurian Plesiechinus ornatus (Buckman, 1845) in Strickland & Buckman, 1845 an interval of time (Sanderson & Donoghue, 1989;Suter, 1994;Smith, 2001) or if the selected terminal taxa are too distant from the origination time of the group. These problems foster the phenomena of long branch attraction and character exhaustion (Wagner, 1995(Wagner, , 2000.…”

Desenvolvimento e ciclo reprodutivo da bolacha-do-mar Clypeaster subdepressus (Echinodermata: Echinoidea) de São Sebastião, SP

“…A inclusão do registro fóssil nas análises morfológicas indica que esta ordem originou-se de uma linhagem da ordem Cassiduloida (Mooi, 1990;Smith, 2001). A análise filogenética sugere que durante a evolução de Clypeasteroida houve retenção de caracteres juvenis na fase adulta e alterações morfológicas associadas à ocupação de um novo ambiente (Mooi, 1990).…”