6
setae (Fig. 2d). I assume that the late larval stages of
Rehbachiella moved their limbs in a metachronic beat
and that particle transport toward the mouth occurred
mechanically within the depth of the ventral sternitic
food groove, and not in the inter-limb space more distally.
The growth process of the body of RehbachieLLa reveals a striking similarity to that of the extant
anostracan Artemia salina. While the increase in total
length is only insignificantly greater in Artemia, the
length of the postcephalic trunk (without furca) is virtually identical. The only major differences concern
the head region and the posterior part of the thrunk. In
Rehbachiella the development of the head ceases until
all cephalic limbs are developed. This results in some
defletion of the growth curve for several stages, most
obviously in the series of larger-sized larvae (arrow in
Fig. 5a). Again, in Artemia the two genital segments
(tsI2, 13) appear in the same way as all preceding segments but do not elongate subsequently. The effect is a
sudden bend in the growth curve for the thrunk length
of Artemia (arrow in Fig. 5b), while these segments
continue to elongate in Rehbachiella.
The status of Branchiopoda and the phylogenetic
position of Rehbachiella
Branchiopoda possess a postcephalic filter-feeding
apparatus which is specific to this group and different in
its morphology as well as function from postmandibular locomotory and feeding systems of all other Crustacea. This is particularly true for the Phyllocarida.
The limbs of this group are also often termed 'phyllopodous' but not only are they different in detail from
those of Branchiopoda, but also the feeding currents
operate differently. Furthermore, Phyllocarida lack the
sternitic food groove, an essential character in the feeding system of Branchiopoda. I regard the unique locomotory and feeding system of Branchiopoda - in fact
a whole complex of characters - as an apomorphy
of this taxon which supports the assumtion that it is
monophyletic (character 2 in Fig. 6). Secondary modifications from the ground-pattern filter feeding system
occur in a different manner, for example, in the feeding
system of Notostraca, in the minute Cladocera, and
the Devonian Lepidocaris rhyniensis (see also Fryer
1985). Since the filter apparatus of Rehbachiella corresponds in all structural and functional aspects with
that of the Branchiopoda, this has led me to affiliate
Rehbachiella with the Branchiopoda.
Another feature of interest is the osmoregulatory
'neck organ' of Branchiopoda (cf. e.g., Rieder, 1984),
particularly evident in their larvae, which is also developed in the early larvae of Rehbachiella. I assume that
this organ originated from a more primordial sense
organ at the same location distal to the third limbbearing head segment, as developed in Malacostraca
(see e.g., Laverack & Sinclair 1994) and even in Trilobita. Cephalocarida lack such an organ. Extant Maxillopoda have sensory structures in a corresponding position, but comparative studies are still wanting. On the
other hand, Muller & Walossek (1988) have described
a very similar structure from the Upper Cambrian Bredocaris admirabilis. The affinities of this fossil are
clearly with the thecostracan clade of the Maxillopoda, an assumption which is established on the special
postembryonic development in which limb growth during the first phase is suppressed and all thoracopods
simultaneously appear at a single moult (Muller &
Walossek 1988; Walossek & Muller 1992; Walossek
1993). Although this is the only evidence at present,
I propose the common possession of the 'neck organ'
as a synapomorphy of Branchiopoda and Maxillopoda
(character 1 in Fig. 6).
Unlike Fryer (1987b), I recognize two major lines
within Branchiopoda, Anostraca and Phyllopoda. The
monophyly of the Phyllopoda is founded on the internalization of the compound eyes, a feature described
already by Claus (1873); character 4 in Fig. 6). Phylllopoda embrace the Notostraca, Kazacharthra and
Onychura. The Notostraca, known since the Carboniferous, and the Upper TriassiclLower Jurassic
Kazacharthra are recognized as sister taxa particularly
by their loss of the filtratory habit of the anterior trunk
limbs, flattening of the anterior body, loss of the ventral
filter groove, a plate-shaped labrum, cirriform furcal
rami, and polymetamerism of the trunk, which supports the validity of Calmanostraca as a monophyletic
taxon (character 6 in Fig. 6).
Onychura develop a secondary shield behind the
original head shield during ontogeny which originates
from tergal outgrowths of the maxillary or first trunk
somite. This structure is clearly not homologous to
the similarly large, bivalve head shields of other Crustacea, and is even unique among Arthropoda (apomorphy 7 in Fig. 6). Onychura embraces the Conchostraca
(Spinicaudata and Laevicaudata) and Cladocera. Fryer
(1987a) has questioned the validity of the latter taxon as a monophylum, but I have not reviewed this in
detail.
setae (Fig. 2d). I assume that the late larval stages of
Rehbachiella moved their limbs in a metachronic beat
and that particle transport toward the mouth occurred
mechanically within the depth of the ventral sternitic
food groove, and not in the inter-limb space more distally.
The growth process of the body of RehbachieLLa reveals a striking similarity to that of the extant
anostracan Artemia salina. While the increase in total
length is only insignificantly greater in Artemia, the
length of the postcephalic trunk (without furca) is virtually identical. The only major differences concern
the head region and the posterior part of the thrunk. In
Rehbachiella the development of the head ceases until
all cephalic limbs are developed. This results in some
defletion of the growth curve for several stages, most
obviously in the series of larger-sized larvae (arrow in
Fig. 5a). Again, in Artemia the two genital segments
(tsI2, 13) appear in the same way as all preceding segments but do not elongate subsequently. The effect is a
sudden bend in the growth curve for the thrunk length
of Artemia (arrow in Fig. 5b), while these segments
continue to elongate in Rehbachiella.
The status of Branchiopoda and the phylogenetic
position of Rehbachiella
Branchiopoda possess a postcephalic filter-feeding
apparatus which is specific to this group and different in
its morphology as well as function from postmandibular locomotory and feeding systems of all other Crustacea. This is particularly true for the Phyllocarida.
The limbs of this group are also often termed 'phyllopodous' but not only are they different in detail from
those of Branchiopoda, but also the feeding currents
operate differently. Furthermore, Phyllocarida lack the
sternitic food groove, an essential character in the feeding system of Branchiopoda. I regard the unique locomotory and feeding system of Branchiopoda - in fact
a whole complex of characters - as an apomorphy
of this taxon which supports the assumtion that it is
monophyletic (character 2 in Fig. 6). Secondary modifications from the ground-pattern filter feeding system
occur in a different manner, for example, in the feeding
system of Notostraca, in the minute Cladocera, and
the Devonian Lepidocaris rhyniensis (see also Fryer
1985). Since the filter apparatus of Rehbachiella corresponds in all structural and functional aspects with
that of the Branchiopoda, this has led me to affiliate
Rehbachiella with the Branchiopoda.
Another feature of interest is the osmoregulatory
'neck organ' of Branchiopoda (cf. e.g., Rieder, 1984),
particularly evident in their larvae, which is also developed in the early larvae of Rehbachiella. I assume that
this organ originated from a more primordial sense
organ at the same location distal to the third limbbearing head segment, as developed in Malacostraca
(see e.g., Laverack & Sinclair 1994) and even in Trilobita. Cephalocarida lack such an organ. Extant Maxillopoda have sensory structures in a corresponding position, but comparative studies are still wanting. On the
other hand, Muller & Walossek (1988) have described
a very similar structure from the Upper Cambrian Bredocaris admirabilis. The affinities of this fossil are
clearly with the thecostracan clade of the Maxillopoda, an assumption which is established on the special
postembryonic development in which limb growth during the first phase is suppressed and all thoracopods
simultaneously appear at a single moult (Muller &
Walossek 1988; Walossek & Muller 1992; Walossek
1993). Although this is the only evidence at present,
I propose the common possession of the 'neck organ'
as a synapomorphy of Branchiopoda and Maxillopoda
(character 1 in Fig. 6).
Unlike Fryer (1987b), I recognize two major lines
within Branchiopoda, Anostraca and Phyllopoda. The
monophyly of the Phyllopoda is founded on the internalization of the compound eyes, a feature described
already by Claus (1873); character 4 in Fig. 6). Phylllopoda embrace the Notostraca, Kazacharthra and
Onychura. The Notostraca, known since the Carboniferous, and the Upper TriassiclLower Jurassic
Kazacharthra are recognized as sister taxa particularly
by their loss of the filtratory habit of the anterior trunk
limbs, flattening of the anterior body, loss of the ventral
filter groove, a plate-shaped labrum, cirriform furcal
rami, and polymetamerism of the trunk, which supports the validity of Calmanostraca as a monophyletic
taxon (character 6 in Fig. 6).
Onychura develop a secondary shield behind the
original head shield during ontogeny which originates
from tergal outgrowths of the maxillary or first trunk
somite. This structure is clearly not homologous to
the similarly large, bivalve head shields of other Crustacea, and is even unique among Arthropoda (apomorphy 7 in Fig. 6). Onychura embraces the Conchostraca
(Spinicaudata and Laevicaudata) and Cladocera. Fryer
(1987a) has questioned the validity of the latter taxon as a monophylum, but I have not reviewed this in
detail.