the development of segments relative to Anostraca
and Rehbachiella, development of the trunk limbs
of Cephalocarida is extremely delayed: at a TS13
stage no more than two trunks limbs have appeared,
one still being a bud. Limb development progresses more gradually after all 19 trunk segments have
appeared, but here also stages are skipped which results
in rapid functio- nality of the limbs. It should be noted
that Sanders (1963) has omitted the last, ninth thoracic limb, an egg carrier, from his original Table 27.
Accordingly, the tagmosis of the Cephalocarida should
be 5 + 9 + 10 + telson rather than 5 + 8 + 11, as is commonly stated.
Discussion of limb morphology of Branchiopoda
and Crustacea
Structural homology exists for the limb parts of Branchiopoda and all other crown- group Crustacea. Such
a limb comprises a soft body-limb joint of different
sizes, a proximal endite as the structural homologue
of the coxa (see below) and a basipod which carries
two rami, endopod and exopod. Epipods are developed only in Malacostraca and Branchiopoda. They
are lacking in the minute Cephalocarida, the possibly
neotenic Maxillopoda, and the Remipedia. Lack of
epipods in Rehbachiella may be due to its immature
status.
I have summarized the evidence from the Swedish
'Orsten' fauna and the limbs of various trilobites and
other early arthrpods in a colour figure (Walossek
1993: Fig. 54), presenting my ideas of the possible
evolution of the arthropod limb toward the crustacean
limb, or better, limbs (Fig. 7). There are two extremes.
A distinct coxal portion is developed only in specific limbs. It is best developed in the second antennae
and mandibles, and in the maxillulae and postcephalic
limbs of Malacostraca. The palp with the basi pod and
the rami are eventually reduced in Branchiopoda and
Cephalocarida.
A coxal portion is not obvious in the postmandibular limbs of the entomostracan taxa. These taxa exhibit
a separate endite at the inner proximal edge of the
basis, which forms the major portion of the limb stem.
Walossek & Muller (1990) showed that such a 'proximal endite', often called 'gnathite' or 'arthrite' represents the precursor of the coxa but, on the other hand, is
apomorphic to the crustacean line; it is found in several
stem-group Crustacea of the Swedish 'Orsten'. Basically this 'proximal endite' is retained as plesiomorphy
11
in the phyllopodous trunk limb of Branchiopoda, as
noticed already by CaIman (1909). However, it is again
modified as a brush organ for transporting the food particles anteriorly deep in the sternal food groove, while
the basi pod has enlarged to form the main body of the
branchiopod limb. Again, it is (more or less) this endite
alone which in the maxillulae and maxillae is retained
and still operates according to its original function.
The scheme emphasizes the structural identity of
the crustacean second antennae and mandibles, best
recognizable in the naupliar limbs of Cirripedia. It is
apparent that terminological misinterpretations have
largely disguised the homology between all the limb
types of Euarthropoda. This is particularly true for
the trilobite basis, which has been termed 'coxa', and
this has led to the belief that the rami of this group
are not homologous with those of the Crustacea. The
basic type of euarthropod limb, as reconstructed here,
has been known for almost 20 years (cf. Muller &
Walossek 1987, for references) and has been confirmed
recently by the find of the oldest euarthropod limb
known so far from lower Cambrian sediments in China
(Chen et at. 1991: Fig. 7).
It is obvious that the possibility of studying the morphogenesis of structures in a three-dimensionally preserved fossil such as Rehbachiella facilitates homologization. As an example, the mandibular growth shows
up as a continuous process of enhancing the coxal portion, so challenging distinction between 'larval' and
'adult mandibles' as proposed by Schrehardt (1986).
Again, Walossek & Muller (1990) concluded that the
second maxilla, as the fifth limb of the head could not
have specialized before the branching of the crowngroup taxa of Crustacea. This interpretation is founded
on the long known facts that the extant Cephalocarida
have the second maxilla like a trunk limb and that the
same it true of the 'Orsten' thecostracan maxillopod
Bredocaris. To this can be added that Rehbachiella
also and possibly Lepidocaris as fossil branchiopods
reveal this plesiomorphic state.
This implies that it is no longer possible to 'define'
Crustacea by the existence of two postmandibular
mouthparts, the first and second maxillae. Since stemgroup Crustacea reveal a stepwise specialization of the
crustacean mouthparts, the fate of all postantennular
cephalic limbs, particularly the mandibles to the second maxillae, may serve as tools - in the sense of
the 'terminus post quem non' - for interpreting the
evolution of the different crustacean lines, and hence
for the recognition of relationships between the various
crustacean taxa.
and Rehbachiella, development of the trunk limbs
of Cephalocarida is extremely delayed: at a TS13
stage no more than two trunks limbs have appeared,
one still being a bud. Limb development progresses more gradually after all 19 trunk segments have
appeared, but here also stages are skipped which results
in rapid functio- nality of the limbs. It should be noted
that Sanders (1963) has omitted the last, ninth thoracic limb, an egg carrier, from his original Table 27.
Accordingly, the tagmosis of the Cephalocarida should
be 5 + 9 + 10 + telson rather than 5 + 8 + 11, as is commonly stated.
Discussion of limb morphology of Branchiopoda
and Crustacea
Structural homology exists for the limb parts of Branchiopoda and all other crown- group Crustacea. Such
a limb comprises a soft body-limb joint of different
sizes, a proximal endite as the structural homologue
of the coxa (see below) and a basipod which carries
two rami, endopod and exopod. Epipods are developed only in Malacostraca and Branchiopoda. They
are lacking in the minute Cephalocarida, the possibly
neotenic Maxillopoda, and the Remipedia. Lack of
epipods in Rehbachiella may be due to its immature
status.
I have summarized the evidence from the Swedish
'Orsten' fauna and the limbs of various trilobites and
other early arthrpods in a colour figure (Walossek
1993: Fig. 54), presenting my ideas of the possible
evolution of the arthropod limb toward the crustacean
limb, or better, limbs (Fig. 7). There are two extremes.
A distinct coxal portion is developed only in specific limbs. It is best developed in the second antennae
and mandibles, and in the maxillulae and postcephalic
limbs of Malacostraca. The palp with the basi pod and
the rami are eventually reduced in Branchiopoda and
Cephalocarida.
A coxal portion is not obvious in the postmandibular limbs of the entomostracan taxa. These taxa exhibit
a separate endite at the inner proximal edge of the
basis, which forms the major portion of the limb stem.
Walossek & Muller (1990) showed that such a 'proximal endite', often called 'gnathite' or 'arthrite' represents the precursor of the coxa but, on the other hand, is
apomorphic to the crustacean line; it is found in several
stem-group Crustacea of the Swedish 'Orsten'. Basically this 'proximal endite' is retained as plesiomorphy
11
in the phyllopodous trunk limb of Branchiopoda, as
noticed already by CaIman (1909). However, it is again
modified as a brush organ for transporting the food particles anteriorly deep in the sternal food groove, while
the basi pod has enlarged to form the main body of the
branchiopod limb. Again, it is (more or less) this endite
alone which in the maxillulae and maxillae is retained
and still operates according to its original function.
The scheme emphasizes the structural identity of
the crustacean second antennae and mandibles, best
recognizable in the naupliar limbs of Cirripedia. It is
apparent that terminological misinterpretations have
largely disguised the homology between all the limb
types of Euarthropoda. This is particularly true for
the trilobite basis, which has been termed 'coxa', and
this has led to the belief that the rami of this group
are not homologous with those of the Crustacea. The
basic type of euarthropod limb, as reconstructed here,
has been known for almost 20 years (cf. Muller &
Walossek 1987, for references) and has been confirmed
recently by the find of the oldest euarthropod limb
known so far from lower Cambrian sediments in China
(Chen et at. 1991: Fig. 7).
It is obvious that the possibility of studying the morphogenesis of structures in a three-dimensionally preserved fossil such as Rehbachiella facilitates homologization. As an example, the mandibular growth shows
up as a continuous process of enhancing the coxal portion, so challenging distinction between 'larval' and
'adult mandibles' as proposed by Schrehardt (1986).
Again, Walossek & Muller (1990) concluded that the
second maxilla, as the fifth limb of the head could not
have specialized before the branching of the crowngroup taxa of Crustacea. This interpretation is founded
on the long known facts that the extant Cephalocarida
have the second maxilla like a trunk limb and that the
same it true of the 'Orsten' thecostracan maxillopod
Bredocaris. To this can be added that Rehbachiella
also and possibly Lepidocaris as fossil branchiopods
reveal this plesiomorphic state.
This implies that it is no longer possible to 'define'
Crustacea by the existence of two postmandibular
mouthparts, the first and second maxillae. Since stemgroup Crustacea reveal a stepwise specialization of the
crustacean mouthparts, the fate of all postantennular
cephalic limbs, particularly the mandibles to the second maxillae, may serve as tools - in the sense of
the 'terminus post quem non' - for interpreting the
evolution of the different crustacean lines, and hence
for the recognition of relationships between the various
crustacean taxa.