An Updated Classification of the Recent Crustacea

THE FOSSIL RECORD AND CLASSIFICATION
OF THE CRUSTACEA
No understanding of crustacean diversity and evolution
would be complete without knowledge of the
fascinating fossil history of the group. And many
exciting discoveries that bear on crustacean origins,
relationships, and classification have surfaced since
the Bowman and Abele treatment. A recent example
is the intriguing find of a serolid-like sphaeromatoid
isopod from the Solnhofen of Germany
(Brandt et al., 1999), pushing back the origin of
sphaeromatoid isopods to at least the Late Jurassic.
Although a thorough review of such discoveries is
beyond the scope of this report (see papers in Edgecombe,
1998, and reviews by Delle Cave and Simonetta,
1991; Bergström, 1992; Schram and Hof,
1998; Walossek and Müller, 1997, 1998; Wills,
1998; Wills et al., 1995; Fortey et al., 1997; Fryer,
1999c), we feel the need to mention especially the
stem and crown group crustaceans of the ‘‘Orsten’’
fauna of Sweden (Orsten-type fossils have also been
found on other continents; see review by Walossek,
1999). These works include papers by Müller
(1982, Hesslandona; 1983, crustaceans with soft
parts), Müller and Walossek (1985, Skaracarida;
1986a, Martinssonia; 1986b, various arthropod
larvae; 1988, the maxillopod Bredocaris), Walossek
and Müller (1990, stem line crustacean concept;
1992, overview of the Orsten fauna; 1994, possible
pentastomids; 1997, 1998, overviews), Walossek
and Szaniawski (1991, Cambrocaris), Walossek et
al. (1994, possible pentastomids), and Walossek
(1993, 1995, the branchiopod Rehbachiella; 1999,
overview of Cambrian crustaceans). These publications
include detailed descriptions of several new
taxa that have in many ways altered our view of
primitive crustaceans and the timing of crustacean
evolution.
The Burgess Shale crustaceans have been reexamined
recently by Briggs et al. (1994), and the
remarkable fossil arthropods from the Lower Cambrian
Chengjiang fauna of southwest China have
been summarized by Hou and Bergström (1991,
1997). Included in the Chengjiang fauna are no unequivocal
crustaceans (Waptia being the only remote
possibility), but several fossils seem to have a
bearing on our understanding of crustacean evolution.
Other recent studies of Chinese fossil crustaceans
have included papers on conchostracans (e.g.,
Shen, 1984, 1990; Zhang et al., 1990; see also Orr
and Briggs, 1999, for Carboniferous conchostracans
from Ireland), and Lower Cambrian crustaceans
are known from other sites around the world
as well (e.g., see Butterfield, 1994). Studies of bradoriid
and phosphatocopid arthropods (once
thought to be ostracodes) (see Siveter and Williams,
1997) have even shed light on our understanding
of the evolution of the crustacean circulatory system
(Vannier et al., 1997). The phosphatocopids
are now thought to be close to the ‘‘stem-line’’ crustaceans
(and possibly the sister taxon to Crustacea;
see Walossek, 1999) rather than relatives of any of
the crown-group crustaceans such as ostracodes or
maxillopods, which had been suggested previously
(e.g., see reviews by Walossek and Müller, 1992,
1998). At least two major groups, and possibly
many more unknown to us, remain enigmatic as to
whether they belong in the Crustacea or not: Thylacocephala
(see Pinna et al., 1982, 1985; Secretan,
1985 [as Conchyliocarida]; Rolfe, 1985, 1992;
Schram et al., 1999) and Cycloidea (see Schram et
al., 1997; Schram and Hof, 1998), although cycloids
were probably allied to the maxillopodans
(Schram et al., 1997). Schram and Hof presented,
as part of the Fourth International Crustacean Congress
(ICC-4) in Amsterdam, evidence that the Thylacocephala
are indeed crustaceans; they further
postulate the inclusion of the Thylacocephala in the
Thecostraca on the basis of the presence of lattice
organs. Their paper, entitled ‘‘At last: the Thylacocephala
are Crustacea,’’ was a late addition and
therefore is not included among the published abstracts
of the ICC-4 Congress, but since then, the
information has been submitted (Lange et al., in
press). However, Schram et al. (1999) are more
cautious and stopped short of declaring that thylacocephalans
were crustaceans. The Permian ‘‘pygocephalomorph’’
crustaceans and their relationship
to extant mysidaceans was examined recently
by Taylor et al. (1998). A thorough review of most
of the above contributions is presented by Schram
and Hof (1998). Many other papers on crustacean
fossils continue to add to our knowledge of the history
of the group (e.g., Brandt et al., 1999, on the
Late Jurassic origin of sphaeromatoid isopods).
In light of these remarkable finds, it is understandable
that a number of colleagues have suggested,
some rather strongly, that we incorporate
fossil taxa into the current classification. We have
opted not to do so, primarily because we are less
familiar with the fossil crustacean literature (and
with workers in that field) than we are with the
literature on extant groups. Thus, the opportunities
for us to inadvertently perpetuate or create errors
would have been much greater had we attempted
this task. Also, if the currently proposed classification
proves to have merit, it should not be difficult
for more paleontologically inclined carcinologists
to, at some point, add these fossil taxa to the existing
framework. We hope that the classification is
of some use to paleontologists and that, at some
point, we can incorporate fossil taxa into this
scheme. Relatively recent lists of crustacean fossil
taxa can be found in Whatley et al. (1993, ostracodes)
and Briggs et al. (1993, all other crustacean
groups) (both in M. J. Benton, editor, The Fossil
Record 2, Chapman and Hall, 1993). However,
since our knowledge (and time) is limited, we have
decided to include only extant taxa for now.
A NOTE ON THE APPENDICES
APPENDIX I. COMMENTS AND OPINIONS
After receiving and considering the input from various
workers around the world, we then asked the
10 � Contributions in Science, Number 39 General Introduction