An Updated Classification of the Recent Crustacea

(Müller and Walossek, 1985), the Orstenocarida
(Müller and Walossek, 1988), and the Mazon
Creek Cycloidea (Schram et al., 1997). A relatively
recent and widely used text on invertebrates (Brusca
and Brusca, 1990) recognizes the Maxillopoda
(including the Ostracoda), and that text is often cited
in other listings of crustaceans (e.g., the Tree of
Life web project; see URL http://ag.arizona.edu/
tree/eukaryotes/animals/arthropoda/crustacea/
maxillopoda.html), whereas another recent text
(Gruner, 1993) treats the various maxillopod
groups separately.
While it is clear that there is not a single ‘‘good’’
character shared by the various maxillopod groups
(see especially Boxshall, 1992), it is also true that
some of them seem closely related on morphological
and molecular grounds. Furthermore, even
some of the more vocal opponents to the Maxillopoda
will argue from time to time that there
seems to be a core group of taxa that ‘‘hang together
well’’ (although the members of this core
group change depending on the speaker). The question
as to which groups are and which are not
‘‘true’’ maxillopods and whether any of the constituent
groups should remain allied in a classification
has not been, in our opinion, satisfactorily
answered.
Although the issue is still unresolved, we have
found it useful to continue to recognize the Maxillopoda,
and refer the reader to discussions of morphological
characters seeming to unite the maxillopodan
groups (see above). At the same time, we
caution readers that acceptance of the Maxillopoda
as monophyletic and acceptance of the constituent
groups are not universal and nowhere near as finalized
as envisioned by Walossek (1993; see review
of this work by Martin, 1995) or by Walossek
and Müller (1994). In the latter paper, Walossek
and Müller state that the ‘‘interrelationships of the
majority of maxillopod taxa, particularly of the
thecostracan lineage, are well-founded on morphological,
ontogenetic, and fossil data.’’ This could
hardly be further from the truth. We have followed,
for the most part, the treatment by Newman (1992)
for higher classification of the Maxillopoda and his
subsequent work (especially Newman, 1996) for
lower taxonomic divisions. We differ from Newman’s
treatment in not using the ‘‘superclass’’ rank,
in an attempt to be consistent with our other uses
and categories. This necessitated the creation of
some lower level taxonomic names (superorders,
infraorders, etc.) that unfortunately add to the clutter
of this already confusing assemblage. We also
differ from Newman’s treatment in that we have
treated the Rhizocephala as members of the cirripedian
line (see below), as suggested by J. Høeg
(pers. comm.) and others (see below).
Published and unpublished hypotheses of relationships
within the Maxillopoda are numerous. As
one example, Walossek and Müller (1998) feel that
there are two rather clear lines and presented character
states for each. The first is the ‘‘copepod line,’’
including the copepods, mystacocarids, and the extinct
Skaracarida (which is in keeping with the
analysis of maxillopod orders by Boxshall and
Huys, 1989a). The second is the ‘‘thecostracan line’’
that includes the tantulocarids, ascothoracidans, facetotectans,
acrothoracicans, and cirripeds. However,
this division does not appear to have much
neontological (e.g., Høeg, 1992a) or molecular
(Spears et al., 1994; Spears and Abele, 1997) support.
Some of the major areas of disagreement in
the various maxillopod hypotheses include whether
the ostracodes should be included vs. excluded,
where the Facetotecta belong, where the Tantulocarida
belong, and the placement (and subdivision)
of the cirripedes. We have attempted to list the
more salient of these efforts in the individual sections
that follow. For an overview of maxillopod
classification and phylogenetic studies, we refer
readers to Grygier (1987a, b), Newman (1987),
Boxshall and Huys (1989a), Boxshall (1992), Huys
et al. (1993), Spears et al. (1994), and Spears and
Abele (1997).
SUBCLASS THECOSTRACA
Spears et al. (1994) concluded, based on 18S rDNA
sequence data, that the Thecostraca, as recognized
by Grygier (1987a; see also Grygier, 1987b) and
Newman (1987, 1992) on morphological grounds,
is a monophyletic assemblage. Furthermore, within
the Thecostraca, Spears et al. (1994) recognized
two major subdivisions, one containing the Ascothoracida
and a second (a modified ‘‘Cirripedia’’)
containing the Acrothoracica, Rhizocephala, and
Thoracica. Although we have maintained the Thecostraca,
we have not divided the group as suggested
by Spears et al., treating instead the Facetotecta
(which was not treated by Spears et al.),
Ascothoracida, and Cirripedia (now including the
Acrothoracica, Rhizocephala, and Thoracica) as
taxa of equivalent rank (infraclasses in the current
scheme) within the Thecostraca. Huys et al. (1993)
recognized the Thecostraca (without the tantulocarids)
and postulated a sister-group relationship
between the Tantulocarida and Thecostraca, noting
that ‘‘inclusion of the Tantulocarida in the Thecostraca,
as proposed by Newman (1992), would significantly
dilute the otherwise robust concept of the
Thecostraca.’’ Jensen et al. (1994b) described cuticular
autapomorphies (details of the lattice organs;
see also Høeg et al., 1998) that also support the
Thecostraca as a monophyletic assemblage.
INFRACLASS FACETOTECTA
Surely one of the biggest remaining mysteries of
crustacean classification is the taxon Facetotecta.
Credited to Grygier (1985, corrected from 1984 in
Bowman and Abele by M. Grygier, pers. comm.;
see also Grygier, 1987a, b, 1996a), the taxon currently
contains no further taxonomic divisions other
than a single genus, Hansenocaris Itô, to accommodate
the curious ‘‘y-larvae.’’ The group consists
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