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Phylogenetic
analysis of Polyplacophora
Polyplacophora
is a distinctive molluscan clade whose members are commonly
referred to as chitons, all of which normally have eight shell
plates (valves) as adults. Because the chiton lineage is thought
to have diverged early from remaining molluscs and because the
overall appearance and habits of chitons have apparently not
changed much over their half a billion year history, this
“living fossil” group is of considerable interest to those
with interests in early molluscan evolution. While the
species-level taxonomy of chitons has perhaps been studied in
relatively more detail than many molluscan groups, there have
been few attempts to resolve high-level relationships within
chitons. This contribution is part of an ongoing effort by the
author to investigate the phylogenetic relationships among the
major chiton subclades. A somewhat revised and expanded
cladistic analysis of chiton morphology is presented, which
builds on the author’s previous investigations as well as data
in the literature. The results appear encouraging, and might
help decide between conflicting classification schemes that have
been proposed for chitons in recent decades. For example, the
analysis unambiguously supports a relatively derived, not basal,
position for “acanthochitonid” chitons. These chitons have a
divergent valve morphology that was first emphasized by Ashby
and Bergenhayn over sixty years ago, and more recently used by
some to justify a high rank for acanthochitonids (e.g., as
Acanthochitonina Bergenhayn, 1930). However, this analysis
suggests that acanthochitonids
share a number of derived features of gill placement, egg
hulls, and sperm morphology with particular other chiton
lineages, suggesting that the valve distinctions are more
recently acquired autapomorphies, Despite such progress here, in
agreement with many conclusions also made by some recent authors,
overall relationships of living chitons still remain relatively
poorly resolved. One possibility might be that the addition of
molecular sequence comparisons could help, but such data are
just now emerging, and presently provide little resolution. This
conclusion is based on an analysis presented herein of the five
near-complete 18S ribosomal DNA sequences, plus various
outgroups, all currently available in GenBank. Results from this
sequence analysis suggest that there are still too few sequences
available for this data set to be useful. Besides the limited
taxon sampling, all available outgroups are possibly too
divergent from chitons. Depending on which single or combination
of outgroups is used to root chitons, fundamentally different
alternative ingroup topologies within chitons are supported in
phylogenetic analyses. It is concluded that sequencing of more
taxa for this gene as well as other genes is needed, as well as
continued investigation of relatively poorly sampled, but
promising, morphological characters such as egg hull and sperm
morphology. |
