An Updated Classification of the Recent Crustacea

of small (250–620 micrometers) nauplii with a
vaulted and ornamented cephalic shield, sometimes
with complex honeycomb patterns, followed by a
relatively long and ornamented trunk region. The
intriguing possibility that these planktonic forms
may be larval tantulocaridans (which would result
in tantulocaridans being classified under the Facetotecta)
has also been suggested (M. Grygier and
W. Newman, pers. comm.), based in part on the
fact that there are still gaps in the known life cycle
of tantulocarids following the work of Boxshall
and Lincoln (1987) and Huys et al. (1993). As Grygier
(pers. comm.) points out, ‘‘there is a hole in the
tantulocaridan life cycle where y-larvae might fit
(i.e., the progeny of the supposedly sexual males
and females), but it would be a very tough fit.’’
Newman (pers. comm.) succinctly describes the
current state of our knowledge: ‘‘They [facetotectans]
are the larvae of some very small, parasitic
maxillopodan, and if not tantulocarids, they are the
last survivors of some other great free-living radiation
close to them.’’ A recent review of the Facetotecta
was provided by Grygier (1996a).
INFRACLASS ASCOTHORACIDA
The Ascothoracida have been treated in the past
sometimes as an order (e.g., by Newman, 1992),
but that rank is changed to infraclass here to accommodate
the constituent taxa that have been elevated
to (or treated as) orders by Grygier (1987a,
b) and Newman (1987, 1996), whose classifications
we follow (see also Grygier, 1983a, b, 1987c,
1996b). Our classification thus includes two families,
Ascothoracidae Grygier, 1987, and Ctenosculidae
Thiele, 1925, that were not included in the
Bowman and Abele (1982) listing. Thus, the infraclass
currently consists of two orders, Laurida and
Dendrogastrida, each with three families.
INFRACLASS CIRRIPEDIA
Whether the Cirripedia should include the Rhizocephala
(e.g., Høeg, 1992a) or whether the Rhizocephala
are early offshoots of the cirripedian line
and not members of the crown group (as in Newman,
1982, 1987; Grygier, 1983a; Schram, 1986)
is not settled. However, there appears to be a growing
consensus that the Rhizocephala and the Cirripedia
form a monophyletic group. Høeg (1992a)
provides strong evidence based on larval morphology,
and Spears et al. (1994) support this with molecular
data. There is some evidence (both morphological
and molecular) that Cirripedia, with or
without the Rhizocephala, may be paraphyletic
(Newman, 1987; Spears et al., 1994). Our classification
treats the Cirripedia as one of three infraclasses
of the subclass Thecostraca. Included in our
Cirripedia are the Rhizocephala. This is more in
line with Høeg’s (1992a) view, where he suggested
that Cirripedia be defined as containing the Rhizocephala,
Thoracica, and Acrothoracica, than
with Newman’s (1992) view, although Newman
(pers. comm.) has indicated to us more recently that
he now agrees with placing the rhizocephalans
within the Cirripedia. Characters of the naupliar
and cypris larval stages argue for inclusion of the
rhizocephalans within the Cirripedia (Høeg,
1992a), and molecular evidence (in the form of
rRNA sequences) supports this (Spears et al.,
1994). A close relationship between Rhizocephala
and Thoracica is supported by 18S rDNA data as
well (Abele and Spears, 1997).
Although earlier molecular studies (Spears et al.,
1994) seemed to indicate that the Ascothoracida
might be the sister taxon to the Acrothoracica
(which we have included in the Cirripedia), further
analyses have not supported this arrangement
(Spears and Abele, 1997). Thus, our current arrangement
maintains the inclusion of the Acrothoracica
within the Cirripedia.
Treatment of the Iblomorpha as one of four thoracican
suborders (with no phylogenetic order implied)
is at least in keeping with the finding of Mizrahi
et al. (1998) that Ibla is not as different from
other thoracicans as some earlier workers had supposed
and should not be treated as near the base
of the stem of the Thoracica.
An extensive morphology-based cladistic analysis
of the Cirripedia Thoracica by Glenner et al.
(1995), reanalyzed with some characters rescored
by Høeg et al. (1999), supported the monophyly of
the Balanomorpha and Verrucomorpha and suggested
that several groups, among them the Pedunculata,
Scalpellomorpha, and Chthamaloidea, were
demonstrably paraphyletic. Yet other major questions
remained unresolved, and Glenner et al.
(1995) suggested that the fields of larval ultrastructure,
early ontogeny, and molecular sequencing
might be promising areas for future research. Anderson
(1994:326) presented a slightly different
classification, where the Cirripedia (which he treats
as a subclass within the class Thecostraca) comprises
five superorders (two of which, the Archithoracica
and Prothoracica, would be new taxa
coined by him), but this has not been followed by
many other workers. Naupliar evidence seems to
support, in general, the classification we have depicted
within the cirripedes based on adult morphology
(Korn, 1995). Høeg (1995) presents some
interesting alternatives based on evolution of the
sexual system of cirripedes and related groups,
where again thecostracans and tantulocaridans are
depicted as sister taxa.
A study of the brachylepadomorphs (Newman,
1987) led Newman to abandon thoughts of polyphyly
in favor of monophyly of the sessile barnacles
(Newman, 1991, 1993, 1996, and pers. comm.).
Thus, the Sessilia was resurrected to contain the
brachylepadomorphs, verrucomorphs, and balanomorphs,
as was the Penduculata for the pedunculate
barnacles. This has been challenged by Glenner
et al. (1995) (see above and see also the reanalysis
of the Glenner at al. data by Høeg et al., 1999).
A review of various bodies of information con-
22 � Contributions in Science, Number 39 Rationale