An Updated Classification of the Recent Crustacea

elationship, although the view is certainly not universally
shared (e.g., see the exchange between Olesen,
1998, 2000, and Fryer, 1999, 2001), and there
is a large body of evidence suggesting that Diplostraca
is nonmonophyletic. Additionally, there is
considerable doubt concerning the monophyly of
some of the groups we have included within it, such
as the Cladocera. Fryer (1987a, 1995, 1999a, b)
discusses the great morphological differences
among the four groups traditionally placed in the
‘‘so-called Cladocera’’ and highlights the trenchant
differences among these taxa and the difficulty in
reconciling these forms within one taxonomic category.
We should also point out that the ‘‘secondary
shield’’ mentioned as unifying these taxa (e.g., by
Walossek, 1993; Olesen et al., 1997; Olesen, 1998)
is, according to Fryer (1996b, 1999b), simply nonexistant,
a misunderstanding of the nature of the
crustacean carapace. Other characters that supposedly
unite the ‘‘diplostracan’’ groups are similarly
called into question by Fryer in a series of papers
(1987a–c, 1995, 1996a, b, 1999b). In particular,
after considerable work in attempting to reconstruct
a primitive anomopod from which extant anomopods
could have been derived and by so doing
highlighting the great difficulties of any such exercise,
Fryer (1995) argued against attempting to
force such disparate taxa as Leptodora, Bythotrephes,
and the superficially similar ctenopods into a
taxon with the Anomopoda, stating (pers. comm.)
that ‘‘when those who make these proposals can
support them by evolutionary series that involve
animals that would work, I’ll pay more attention
to them.’’
Within the Diplostraca, we have removed the
‘‘Conchostraca’’ (following to some extent the suggestions
of Fryer, 1987c, and Olesen, 1998) in recognition
of (1) the distinct nature of the Laevicaudata
(Lynceidae), (2) the stark differences that separate
Cyclestheria hislopi (sole member of the Cyclestheriidae)
from all other conchostracans, and
(3) Cyclestheria’s possible affinities to the cladocerans
on morphological and molecular grounds (see
Martin and Cash-Clark, 1995; Olesen et al., 1997;
Olesen, 1998; Spears and Abele, 1998, 2000). The
fact that Cyclestheria differs significantly from other
spinicaudate conchostracans, and probably to
the extent that it should not be placed among them,
has also been highlighted (Martin and Cash-Clark,
1995; Olesen et al., 1997; Olesen, 1999; Negrea et
al., 1999). Thus, our resulting classification within
the Diplostraca differs slightly from, and is in some
ways a compromise between, the classification suggested
by Olesen (1998) based on morphological
characters and that suggested by Spears and Abele
(2000) based on molecular data and is easily reconciled
with the phylogeny proposed by Negrea et
al. (1999). Our arrangement does not agree with
the somewhat preliminary findings of Hanner and
Fugate (1997) based on a relatively small segment
of the genome.
Removal of Cyclestheria from the Spinicaudata
and placing it on an equal footing with the remaining
Spinicaudata and with the Cladocera necessitated
the creation of a separate suborder, the Cyclestherida,
which we are crediting to Sars (1899)
in keeping with ICZN article 50.3.1. Negrea et al.
(1999) used the same spelling to refer to an order
(Cyclestherida) within their superorder Conchostraca,
thus indicating a closer affinity of Cyclestheria
to the conchostracans rather than the cladocerans.
We have not taken the bolder step of actually
including the Cyclestheriidae among the Cladocera,
although there is apparently evidence for
this as well. Spears and Abele (1999a, b, 2000) note
that, not only do 18S rDNA sequence data support
the close relationships of Cyclestheria and the cladocerans,
the two groups also share certain hypervariable
regions of the gene that are not found in
other branchiopods, and these are potential synapomorphies.
Ax (1999) first suggested the term
‘‘Cladoceromorpha’’ for the clade containing Cyclestheria
plus Cladocera. Papers by Crease and
Taylor (1998) and Taylor et al. (1999) appear to
offer additional molecular support, and the phylogeny
suggested by Negrea et al. (1999:196) supports
such a clade as well, although their resulting classification
of the Branchiopoda into five superorders
does not.
Sassaman (1995) presented fascinating insights
into possible phylogenetic models for the conchostracan
families based on the evolution of unisexuality
in the group; he views lynceids as the sister
group to all other families, while noting at the same
time the unusual nature of the cyclestheriids, which
he posits as the sister group to the remaining ‘‘spinicaudatan’’
families. Thus, in many ways, Sassaman’s
(1995) phylogeny is consistent with our classification.
Within the former ‘‘conchostracan’’ groups, the
spelling of the Lynceidae has been corrected (from
Lyncaeidae, a typographical error in Bowman and
Abele, 1982), and authorship for the family is now
credited to Baird, 1845 (L. Holthuis, pers. comm.).
Mark Grygier points out (pers. comm.) that ICZN
Opinion 532 attributes the family name to Sayce,
1902; however, there are clearly earlier uses of the
family name Lynceidae (e.g., see review by Martin
and Belk, 1988), and we are crediting the family
name to Baird as noted above.
Although the genera Imnadia and Metalimnadia
at times have been suggested to represent distinct
families (the Imnadiidae Botnariuc and Orghidan
and the Metalimnadiidae Straskraba; see Marincek
and Petrov, 1991; Roessler, 1991, 1995a, b; Orr
and Briggs, 1999:8), most workers (e.g., Martin,
1992; Sassaman, 1995) consider them members of
the family Limnadiidae, as do we. Roessler’s (1991)
erection of the family Paraimnadiidae was based on
a species he described as Paraimnadia guayanensis,
a junior synonym of Metalimnadia serratura (see
Orr and Briggs, 1999). We also include among the
limnadiids the genus Limnadopsis and agree with
18 Contributions in Science, Number 39 Rationale