An Updated Classification of the Recent Crustacea

ancestral crustacean is of course not completely settled,
and there are published arguments for presenting
either the Cephalocarida or the Remipedia
as the most primitive group of living crustaceans.
There have also been, from time to time, hypotheses
presented where other groups of crustaceans
have occupied a basal position (e.g., McKenzie,
1991, postulated a bradoriid ostracode origin for
all other crustaceans).
In favor of depicting remipedes as the most primitive
class are the works of Schram (1986), Brusca
and Brusca (1990), Briggs et al. (1993a), Schram
and Hof (1998), Wills (1997), and Wills et al.
(1998), all based on cladistic analyses of morphological
characters from extant and extinct forms.
Also supporting this view is the phylogeny presented
by Jamieson (1991a) based on sperm ultrastructure,
in which the Remipedia is the most basal of
the crustacean groups. (It should be noted, however,
that Jamieson’s study is not purely independent
of other phylogenies in that his figure is actually
an overlay of the various sperm types on top
of the classification offered by Schram in 1986.)
Thus, there are workers at several independent laboratories
whose studies have indicated that remipedes
occupy the most basal position among the
crustaceans, and several textbooks have followed
this arrangement as well (e.g., Hickman et al.,
1996: 401, figs. 20–30; Brusca and Brusca, 1990).
Molecular evidence concerning where remipedes
belong has been maddeningly difficult to obtain.
Regier and Schultz (1998b) could not say with certainty
(using EF-1), and Spears and Abele (1997)
were equally unsure (using 18S rDNA). Emerson
and Schram (1990, 1997) have also suggested that
crustacean biramous limbs arose from fusion of adjacent
uniramous limbs, and this has a bearing on
the placement of remipedes relative to other crustacean
groups as well (discussed further in Schram
and Hof, 1998, but see Spears and Abele, 1997). It
should also be pointed out that at least one publication
(Moura and Christoffersen, 1996) suggests
that the Remipedia are a derived assemblage that
may be the sister group to the Tracheata (terrestrial
mandibulates).
In support of cephalocarids occupying the most
basal position among extant crustaceans are some
surely primitive external morphological features.
These features include the flattened and ‘‘Orstenlike’’
limbs, the lack of differentiation of the second
maxilla (also shared with some of the Orsten crustaceans),
and relatively anamorphic development.
Hessler (1992) reviewed early considerations of the
placement of the cephalocarids with respect to other
crustaceans. He concluded, based on the morphology
of some of the Upper Cambrian ‘‘Orsten’’
fauna of Sweden and in comparison with remipedes
and other crustaceans, that the argument for placing
cephalocarids at the base of the crustacean lineage
is still strong (see also Walossek, 1993; Moura
and Christoffersen, 1996). In Hessler’s words,
‘‘among living crustaceans, cephalocarids still best
personify what the ur-crustacean must have looked
like.’’ Hessler (1992) also made the point, with
which we agree, that remipedes are quite specialized,
and he found it ‘‘impossible to accept the
claim that the Remipedia better approximates the
ur-crustacean.’’ However, cephalocarids face problems
as primitive crustaceans as well. Schram and
Hof (1998) point out some cephalocarid features
they consider highly derived, and molecular studies
(e.g., Spears and Abele, 1997; Regier and Schultz,
1998b) and spermatological data (especially lack of
a flagellum; see Jamieson, 1991a) do not place cephalocarids
basal to other crustacean taxa (although
in fairness, the EF-1 data of Regier and
Shultz do not decisively place cephalocarids elsewhere,
either). We have not followed the suggestion
of Hessler (1992) to revive the taxon Thoracopoda
to include the cephalocarids, branchiopods, and
malacostracans (based on their shared possession of
an epipod on the trunk limbs).
What Are the Relationships Among the Classes?
This question is closely related to the issues raised
above. In fact, most of the competing phylogenetic
hypotheses for class-level relationships have already
been alluded to in earlier sections (e.g., in the sections
‘‘Cladistics and Classification of the Crustacea’’
and ‘‘Molecular Systematics and Classification
of the Crustacea,’’ and under the above three questions
on crustacean monophyly, number of classes,
and most primitive class). Rather than attempt a
discussion of the many competing hypotheses for
the relationships within and among the various
classes, we have opted to treat each group individually
below. We also refer the reader to the reviews
by Wills et al. (1998) and Schram and Hof (1998),
both in Edgecombe (editor, 1998, Arthropod Fossils
and Phylogeny), and to the review of 18S rDNA
studies by Spears and Abele (1997).
Concerning authorship of the name Crustacea,
although most workers credit Pennant (1777), Lipke
Holthuis, in a detailed and well-researched footnote
to his FAO volume on marine lobsters (Holthuis,
1991), noted that the first usage was actually
that of Brünnich in 1772. We have followed Holthuis’
(1991) suggestion and have credited Brünnich
(1772) with authorship of this taxon.
CLASS BRANCHIOPODA
Virtually all evidence points to the fact that the
branchiopods are a strongly supported monophyletic
group, despite the staggering diversity of extant
forms (e.g., see Martin, 1992). Lines of evidence
indicating branchiopod monophyly include
sperm morphology (Wingstrand, 1978), larval
characters (e.g., Sanders, 1963), feeding apparatus
(Walossek, 1993), adult characters (e.g., Negrea et
al., 1999), and 18S rDNA sequence data (Spears
and Abele, 1997, 1998, 1999a, b, 2000). However,
the group’s tremendous morphological diversity
and age (see Fryer, 1987a–c, 1999; Martin, 1992;
Contributions in Science, Number 39 Rationale 15