The phylogeny and classification of post-Palaeozoic echinoids

Abstract

The relationships of post-Palaeozoic echinoids at family level are established through phylogenetic analysis of 169 taxa and 306 skeletal characters (excluding pedicellariae). Previous phylogenetic analyses of echinoids have either examined specific subgroups in detail or have looked at a relatively small number of taxa selected from across the class, with sparse sampling potentially affecting the reliability of results adversely. Our new analyses represent a compromise between encompassing the diversity of form that exists, while keeping the number of taxa to a level that does not make rigorous analysis impossibly time-consuming. In constructing the taxon-character data matrix we have encountered a surprising lack of primary data on plating pattern, lantern, and girdle structure for many supposedly "well-known" taxa. A well-resolved phylogenetic hypothesis was obtained and is used as the basis for a formal classification. Characters generally have a high retention index (>0.7) but low consistency index (<0.25) suggesting that, although characters are largely retained after they first evolve, most also undergo occasional reversal or convergence. Although parts of the resulting trees are only weakly supported (e.g. the precise sister group of the Irregularia), other parts are unambiguously resolved. Not unexpectedly, deep nodes are often not supported by unique apomorphies and higher taxa acquire their characteristic set of features over time. Diagnoses based on crown group taxa thus often fail to encompass fossil stem-group members adequately. Establishing the relationships of taxa at the root of large groups is hampered by limited character resolution. The influence of fossil taxa on the topology was explored by comparing the tree topologies obtained with and without their inclusion. We show that removal of fossils from stem groups makes no difference where their crown group is morphologically conservative, but has a major influence where extant sister groups are separated by large morphological gaps. Completeness of the echinoid record and its match to the stratigraphical record of first occurrences is tested using various metrics and found to be highly congruent, with irregular echinoids showing a higher congruence than regular ones.

Evolutionary rewiring of gene regulatory network linkages at divergence of the echinoid subclasses

“…One uncertainty that could affect dynamic interpretation of the results is the possibility that the differences we observe between the test species of this work, Sp and Et, are actually in part the sum of changes that occurred only graduallythat is, during the Mesozoic (Triassic, Jurassic, and Cretaceous), subsequently to the split from which emerged the modern euechinoid and cidaroid subclasses. This would require, however, that the specific circuitry features we investigated vary among modern euechinoid orders that arose during the Mesozoic (18). However, although indeed incomplete, the evidence so far limits this possibility.…”

“…Since the Triassic, a curious and perhaps profound difference in evolutionary flexibility distinguishes euechinoid and cidaroid subclasses. The euechinoids have radiated prodigiously, diversifying into nearly 1,000 species of highly various morphology, whereas the cidaroids, comprised of only ∼100 species, have retained extremely conservative morphologies seemingly not far removed from their ancestral forms (18,20). For example, during the Mesozoic the euechinoids evolved diverse clades displaying irregular morphology, such as sand dollars and heart urchins, whereas no such deviations from the ancestral symmetrical globular form have arisen in the cidaroid subclass.…”

“…Within these constraints, the fossil record displays a remarkable variety of early Paleozoic echinoid morphology. However, in the late Paleozoic, there arose an echinoid branch that is clearly ancestral to both the modern euechinoid and cidaroid subclasses, known as the archaeocidaroid lineage (18). A new high-resolution paleontological analysis (19) indicates that the last common archaeocidarid ancestor of both modern echinoid subclasses existed at the latest ∼268 my ago-i.e., at least 16 my before the Permian/Triassic extinction event, which terminated the Paleozoic and many of its canonical denizens.…”

“…This fact generally biases the likelihood that novel features arising since divergence occurred in the euechinoid rather than the cidaroid lineage. Nonetheless, both subclasses display evolutionary innovations-i.e., subclass-specific, shared derived characters (apomorphies) with respect to the (fossilized) skeletal characters of their archaeocidarid ancestor, just as both display plesiomorphic morphological characters (9,18,19,21).…”

The benthic macrofauna from the Lower Maastrichtian chalk of Kronsmoor (northern Germany, Saturn quarry): taxonomic outline and palaeoecologic implications

“…Echinoids are important elements in chalk faunal communities (e.g., Ernst 1970Ernst , 1972. The classification applied herein follows Kroh and Smith (2010).…”