Hydrobiologia 298: 1-13, 1995.
D. Belle, H. J. Dumont & G. Maier (eds), Studies on Large Branchiopod Biology and Aquaculture 1/.
©1995 Kluwer Academic Publishers.
1
The Upper Cambrian Rehbachiella, its larval development, morphology and
significance for the phylogeny of Branchiopoda and Crustacea
Dieter WaloBek
Zoologisches Institut der Universitiit, Olshausenstraj3e 40 (Biologiezentrum), D-24 098 Kiel, Germany
Key words: Ontogeny, morphology, morphogenesis, SEM, phylogeny, Branchiopoda, Crustacea
Abstract
The monograph on the 500 million-year-old Rehbachiella from the Upper Cambrian 'Orsten' of Sweden, published
in Fossils & Strata 32 by Walossek (1993) comprises a detailed description of its larval sequence and a discussion
of functional and comparative aspects of its morphology and ontogeny. A particular attempt was made to clarify
the status of Branchiopoda and the phylogenetic position of Rehbachiella as an early branchiopod with particular
respect to structural and functional identity of the postmandibular locomotory and feeding apparatus apomorphic
to the Branchiopoda. The data provided by Rehbachiella and other 'Orsten' fossils also aided examination of the
ontogenetic patterns of Crustacea and discussion of the morphology and evolution of Crus- tacea.
Introduction
In Sweden, Upper Cambrian limestone nodules
(orsten) embedded in shales yielded rich arthropod faunas summarized in Muller & Walossek (1991). Unusually the complete bodies of the arthropods are preserved in three dimensions. In various cases, complete
series of ontogenetic stages are preserved. Since phosphate has impregnated the integument of the fossils,
it is possible to extract them from the rock by careful etching with dilute acetic acid. The members of
these marine arthropod assemblages lived obviously
mainly as benthos, in or on a soft sediment rich in
organic matter, the 'flocculent zone'. The vast majority of these are crustaceans and related forms. Recently
Walossek & Muller (1994) have also identified representatives of non-euarthropodan groups, such as stemgroup representatives of the Pentastomida, parasites
which today exclusively infest land tetrapods. Fossils
of 'Orsten' -type preservation are known now also from
other regions, such as Australia, England, Canada and
Siberia, and other periods, ranging from the Lower
Cambrian to the Lower Ordovician.
The crustaceans in the Swedish 'Orsten' are represented by member of both the stem group and crown
group (cf. Walossek & Muller 1990, 1992 for discussion of characters). The paper on Rehbachiella kinnekullensis Muller, 1983, I published in 1993 as no. 32
of Fossils & Strata is the third monograph on 'Orsten'
crown-group Crustacea. The monograph is 202 pages
long and includes 54 text figures, a coloured scheme
of the possible evolution from the euarthropod type of
limb toward the crustacean limb(s), and 34 plates with
SEM pictures of the different instars. In the following short account, I will summarize some of the major
aspects of the monograph on Rehbachiella.
Larval development
My reconstruction of the larval sequence of
Rehbachiella was based on more than 130 specimens
from different developmental stages. The recognized
series comprises no less than 30 successive stages. It
starts with a nauplius 160 /Lm in length, while the last
definitely recognizable stage was about 1.7 mm long
and had 13 trunk segments, 12 of which bore limbs
(Figs. 1, 2). The immature state of the last stage is
evident from its still undeveloped posterior limbs, the
growth data (see also Fig. 5), and fragments of much
larger, unassignable specimens.
Some features of the Rehbachiella nauplii are special early larval characters such as the dorsocaudal
spine above the anus, known from many crustacean
D. Belle, H. J. Dumont & G. Maier (eds), Studies on Large Branchiopod Biology and Aquaculture 1/.
©1995 Kluwer Academic Publishers.
1
The Upper Cambrian Rehbachiella, its larval development, morphology and
significance for the phylogeny of Branchiopoda and Crustacea
Dieter WaloBek
Zoologisches Institut der Universitiit, Olshausenstraj3e 40 (Biologiezentrum), D-24 098 Kiel, Germany
Key words: Ontogeny, morphology, morphogenesis, SEM, phylogeny, Branchiopoda, Crustacea
Abstract
The monograph on the 500 million-year-old Rehbachiella from the Upper Cambrian 'Orsten' of Sweden, published
in Fossils & Strata 32 by Walossek (1993) comprises a detailed description of its larval sequence and a discussion
of functional and comparative aspects of its morphology and ontogeny. A particular attempt was made to clarify
the status of Branchiopoda and the phylogenetic position of Rehbachiella as an early branchiopod with particular
respect to structural and functional identity of the postmandibular locomotory and feeding apparatus apomorphic
to the Branchiopoda. The data provided by Rehbachiella and other 'Orsten' fossils also aided examination of the
ontogenetic patterns of Crustacea and discussion of the morphology and evolution of Crus- tacea.
Introduction
In Sweden, Upper Cambrian limestone nodules
(orsten) embedded in shales yielded rich arthropod faunas summarized in Muller & Walossek (1991). Unusually the complete bodies of the arthropods are preserved in three dimensions. In various cases, complete
series of ontogenetic stages are preserved. Since phosphate has impregnated the integument of the fossils,
it is possible to extract them from the rock by careful etching with dilute acetic acid. The members of
these marine arthropod assemblages lived obviously
mainly as benthos, in or on a soft sediment rich in
organic matter, the 'flocculent zone'. The vast majority of these are crustaceans and related forms. Recently
Walossek & Muller (1994) have also identified representatives of non-euarthropodan groups, such as stemgroup representatives of the Pentastomida, parasites
which today exclusively infest land tetrapods. Fossils
of 'Orsten' -type preservation are known now also from
other regions, such as Australia, England, Canada and
Siberia, and other periods, ranging from the Lower
Cambrian to the Lower Ordovician.
The crustaceans in the Swedish 'Orsten' are represented by member of both the stem group and crown
group (cf. Walossek & Muller 1990, 1992 for discussion of characters). The paper on Rehbachiella kinnekullensis Muller, 1983, I published in 1993 as no. 32
of Fossils & Strata is the third monograph on 'Orsten'
crown-group Crustacea. The monograph is 202 pages
long and includes 54 text figures, a coloured scheme
of the possible evolution from the euarthropod type of
limb toward the crustacean limb(s), and 34 plates with
SEM pictures of the different instars. In the following short account, I will summarize some of the major
aspects of the monograph on Rehbachiella.
Larval development
My reconstruction of the larval sequence of
Rehbachiella was based on more than 130 specimens
from different developmental stages. The recognized
series comprises no less than 30 successive stages. It
starts with a nauplius 160 /Lm in length, while the last
definitely recognizable stage was about 1.7 mm long
and had 13 trunk segments, 12 of which bore limbs
(Figs. 1, 2). The immature state of the last stage is
evident from its still undeveloped posterior limbs, the
growth data (see also Fig. 5), and fragments of much
larger, unassignable specimens.
Some features of the Rehbachiella nauplii are special early larval characters such as the dorsocaudal
spine above the anus, known from many crustacean