541 million years ago the
Cambrian Explosion produced a wide range of sophisticated bilaterally
symmetrical organisms with biomineralized skeletons, which apparently
all appeared more-or-less simultaneously. In contrast the Ediacaran
Fauna, which is found in strata between 550 and 543 million years old
in several parts of the world, comprises a number of radially
symmetrical or unsymetriacl forms, including about ten biomineralized
genera, which are generally thought to have been Poriferans
(Sponges), Cnidarians (the group that includes modern Jellyfish,
Corals and Sea Anemones) or even colonial microorganisms.
On such Ediacaran fossil
is Namacalathus hermanastes,
a colonial, reef-dewlling organism discovered in rocks of the Nama
Group in Namibia in the 1990s and subsequently reported in the Byng
Formation of the Canadian Rocky Mountains, the Birba Formation of
Oman, the Kolodzha and Raiga formations of West Siberia and the
Anastas’ino Formation of the Altay-Sayan Foldbelt (also Siberia).
Namacalathus comprises
cup-shaped, stemmed, biomineralized organisms reaching about 5 mm in
height, with a hexaradial cross section and apparent budding growth
pattern.
In
a paper published in the journal Proceedings of the Royal SocietySeries B: Biological Sciences on 4 November 2015, Andrey Zhuravlev
of the Department of Biological Evolution at the Lomonosov MoscowState University and Rachel Wood and Amelia Penny of the School of GeoSciences
at the University of Edinburgh, discuss the microstructure of the
biomineralized cups of a number of exceptionally well-preserved
Namacalathus specimens
from the Nama Group in Namibia, and discuss the implications of this
microstructure for the taxonomic affiliations of Namacalathus.
Zhurayev
et al. observe that
the exoskeleton of Namacalathus
has a triple-layered structure, with two thin inner and outer layers
encasing a thicker inner layer comprising irregulatly arranged
rod-like crystals. In places dolomite microcrystals have formed
within this layer, suggesting that it originally comprised
high-magnesium calcite.
Secondary
emission SEM image of etched and polished transverse section of
Namacalathus
hermanastes skeletal
wall ultrastructure from the Nama Group, Namibia. Tripartite organization. (M) internal (middle) layer of rod-like microdolomite crystals; (O), external outer foliated layers. (I) inner foliated layers.. Scale bar, 100 mm.
Zhurayev et al. (2015).
High-magnesium
calcite is found in a wide range of modern and fossil organisms,
including Calcareous Algae, Foraminiferans. Sponges, Corals, Annelid
Worm Tubes, Molluscs, Brachiopods, Bryozoans, Tentaculitoids
(Lophophorate Animals that went extinct in the Jurassic and
Tommotiids (small shelly fossils from the beginning of the Cambrian).
However a triple-layered laminated microstructure is much rarer,
being found only in Molluscs, Brachiopods, Bryozoans, Tentaculitoids
and Tommotiids.
The
Namacalathus specimens
also show a curious pattern of sediment infill, with the sediment
around the opening at the top of the cup matching the surrounding
sediment, but with sediment with s distinctive texture filling the
rest of the shell. This implies a soft tissue barrier to sediment
ingress, which has not been directly preserved. This is at odds with
a Molluscan origin for the shell, as Molluscs tend to fill their
shells, and in the case of filter feeding Molluscs (the only
ecological pattern that would be expected in a shelly organism living
attached to a reef structure), often have muscular syphons which
extend beyond the shell. Lophophorate Animals such as Brachiopods,
Bryozoans and the extinct Tentaculitoids, have a different
structure, however, with a structure called a lophophore, comprising
a number of tentacles used to filter food from the water in a
net-like arrangement, which is often held within the shell, with
water being pumped in to be strained by the lophophore.
Modern
lophophorate animals are all bilaterally symmetrical, whereas
Namacalathus appears
to have a hexaradial symmetry, which presents a promlem for the
interpretation of Namacalathus
as a Lophophorate. However Namacalathus appears
to have reproduced by budding, with new daughter cups growing from
the parent on either side in a symmetrical arrangement. This suggests
that the soft tissue of the living animal may have been arranged in a
bilaterally symmetrical fashion, possibly with sex organs (or asexual
reproductive organs that could potentially have later evolved into
sex organs) arranged on either side.
This
suggests that Namacalathus may
have been a bilaterally symmetrical Lophophorate Animal living within
a hexaradiate shell. This shell would have offered little protection
against predators (it had six openings around its side as well as the
central opening at its top) but with a triple-layered shell
presumably supported by tougher organic material, would have been
highly resilient to the battering associated with a high-energy
environment on a reef top, while the openings in the shell allowed
water to pass through and be filtered by the lophophore. The
organisms formed bilaterally branching colonies with daughter cups
probably sharing a common gut with the parents, a system still found
in modern colonial Lophophorates, and which has been suggested as
possibly linked to the origin of bilateral symmetry in animals.
Reconstruction of the
living Namacalathus. (1) stem; (2) parental cup; (3) daughter
cups; (4) hollow ciliated tentacles; (5) spines; (6) lateral lumen;
(7) central opening; (8) inner skeletal layer—foliated with
columnar microlamellar inflections; (9) internal (middle) skeletal
later—organic rich; (10) external outer skeletal layer—foliated
with columnar skeletal inflections. John Sibbick in
Zhurayev et al. (2015).
See
also
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to any...
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Annelida, Mollusca, Bryozoa, Cycliophora, Brachiopoda, Entoprocta and
Phoronida. Within this group several groups are united by the presence
of a crown of tentacles (the Lophophore) surrounding the mouth...