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.
Keywords: Echinoids; phylogeny; cladistics; evolution; classification
