An Updated Classification of the Recent Crustacea

mipedes occupy the most basal position among the
extant crustaceans. These arguments are perhaps
best summarized in Schram and Hof (1998) and in
Wills (1997), where remipedes come out at the base
of all other Crustacea groups following cladistic
analyses of large datasets. Moura and Christoffersen
(1996) take an opposing stance, suggesting that
remipedes are an apical group of crustaceans that
are possibly the sister group to terrestrial mandibulates.
To us, the evidence (morphological, molecular,
and developmental) for branchiopods being
basal appears stronger (see earlier comments on
primitive crustaceans). Emerson and Schram (1990;
see also Emerson and Schram, 1991) have suggested
that crustacean biramous limbs may have arisen
from fusion of adjacent uniramous limbs, and this
has a bearing on the placement of remipedes relative
to other crustacean groups (discussed further
in Schram and Hof, 1998). Spears and Abele
(1997) also discussed possible affinities between remipedes
and cephalocarids, some of which may be
artifactual as a result of long branch attractions.
Within the Remipedia, the order Nectiopoda was
erected by Schram (1986) to separate extant remipede
families from some fossils that appear remipedian
(and that are treated as the fossil order Enantiopoda).
One additional family, the Godzilliidae,
was added by Schram et al. (1986). Yager and
Humphreys (1996) reported the first species from
Australia and the Indian Ocean and presented a key
to the world species known at that time. Cals
(1996) reviewed the biology of the group and presented
a table comparing the characteristics of the
two currently accepted families, Speleonectidae and
Godzilliidae; more recently, Yager and Carpenter
(1999) and Carpenter (1999) have added to what
is known of the natural history of speleonectids.
CLASS CEPHALOCARIDA
Our classification differs from that of Bowman and
Abele (1982) only in recognizing a single family,
Hutchinsoniellidae, rather than two families. The
family Lightiellidae proposed by Jones (1961) is
thought to differ only slightly and insignificantly
from the characters established for the former family
(R. Hessler, pers. comm.). Our placement of the
cephalocarids here, between the remipedes and
maxillopods, to some degree reflects the summary
finding of Spears and Abele (1997) that remipedes
and cephalocarids may constitute a clade that is the
sister group to one of the maxillopodan groups (the
Copepoda) (e.g., Spears and Abele, 1997, figs. 14.4,
14.7, and accompanying text), although Spears and
Abele (1997) also note that this arrangement is not
well supported by their bootstrap analysis. The
placement of cephalocarids and remipedes together,
and adjacent to the maxillopods, in some ways also
supports Itô’s (1989) morphology-based suggestion
of a remipede � cephalocarid � copepod clade.
Hessler and Elofsson (1996) recently reviewed
what is known of cephalocarid biology and phylogeny.
CLASS MAXILLOPODA
The Maxillopoda continues to be a terribly controversial
assemblage concerning both the number of
constituent groups and the monophyly of the entire
taxon. We were tempted to abandon, once and for
all, the concept of a monophyletic Maxillopoda, as
there seems very little in the way of morphological
or molecular evidence uniting the disparate groups
(Wilson, 1992; Spears and Abele, 1997; Shultz and
Regier, 2000). Ostracodes in particular have been
placed sometimes within the Maxillopoda (e.g., see
Boxshall and Huys, 1989a) and sometimes in their
own class, and the issue remains unresolved despite
much debate (e.g., see Boxshall et al., editors, Acta
Zoologica, vol. 73(5), 1992). It is certainly no secret
that the characters used in defining the group
do not hold for many of the taxa traditionally
thought of as being ‘‘maxillopodan.’’ Abandoning
the Maxillopoda seems to have been implied in
tome VII fascicule II of the Traité de Zoologie
(1996), as only the constituent groups are treated
with no mention of maxillopod affinities or relationships
(e.g., see Grygier 1996a, b), and Gruner
(1993) similarly did not recognize the Maxillopoda.
Boxshall (1983) and others have argued against
recognition of the Maxillopoda on morphological
grounds, although Boxshall has also continued to
employ it from time to time (e.g., in Huys et al.,
1994). Yet other workers (e.g., see Newman, 1983;
Grygier, 1983a; Walossek, 1993; Wills, 1997; Walossek
and Müller, 1998) have argued, some quite
forcefully, that there is merit to recognition of the
Maxillopoda as a natural (monophyletic) assemblage,
despite the fact that there seem to be exceptions
to every synapomorphy proposed. In fairness,
so many maxillopodan taxa are so small and/or
modified as parasites that it should come as no surprise
to find exceptions to groundplans. Removal
of the Maxillopoda as a class would raise the number
of crustacean classes from six to nine once the
maxillopodan subclasses were elevated (each to the
level of class).
The somewhat controversial history of the concept
of the Maxillopoda (whether it is monophyletic,
and if so, which groups should be included,
and what the relationships are within the group and
of the group to other crustaceans) is reviewed on
morphological grounds by Grygier (1983a, b,
1985, 1987a–c), Müller and Walossek (1988),
Boxshall and Huys (1989a), Huys (1991), Newman
(1992), Schram et al. (1997), Schram and Hof
(1998), and papers cited therein, and on molecular
grounds by Abele et al. (1992), Spears et al. (1994),
and Spears and Abele (1997). Some of the fossil
discoveries since the Bowman and Abele classification
have a bearing on our understanding of the
monophyly and definitions of the Maxillopoda as
well, such as the description of the Skaracarida
20 � Contributions in Science, Number 39 Rationale