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Australia: The Land Where Time Began |
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Cambrian Explosion - Lobopodians and
Onychophora Extant Onychophora are morphologically similar,
having paired antennae and lobopodal walking legs, (legs that are
stubby, rounded and annulated), and all extant forms are terrestrial and
the extinct forms are marine. The Cambrian fossils with lobopodial
appendages are much more diverse and have generated considerable
controversy concerning their relationship to crown panarthropods (Hou &
Bergström, 1995; Liu et al.,
2008; Ma, Hou & Bergström, 2009; Ramskold & Chen, 1998). The first
onychophoran to be recognised from the Burgess Shale was
Aysheaia. Many
lobopodians from the Cambrian Explosion were “armoured”, with a diverse
array of plates and spines. The earliest confirmed body fossils of
lobopods were recovered from deposits of Cambrian Stage 2 age, and it
has been argued (J. Liu et al.,
2008) that
Zhijinites, a small
shelly fossil from Cambrian Stage 1 is likely to be a lobopodian
sclerite, and there have been reports from rocks of similar age in
Siberia of
Microdictyon plates
(Kouchinsky et al., 2012).
All forms from the Cambrian have elongated bodies and have lobopod-limbs
that are circled by putative haemal channels, though the only exception
is that the location of the mouth is different from extant
onychophorans, the variable nature of the morphology of the limbs, the
presence of terminal claws on limbs, the variety of spines and plaques
on the trunk, and the general absence of prominent paired, muscular
antennae, though small antennae have been reconstructed on some species
from the Cambrian, which differ markedly from the muscular antennae of
extant onychophorans.
Antennacanthopodia gracilis
was an exceptional form from the Cambrian
Chengjiang Fauna, with 2 pairs
of antennae (Ou et al.,
2011), the larger, more anterior of which, resemble the antennae of
crown onychophorans. There are also possible paired eye spots on
A. gracilis, a cuticle
that are lightly sclerotised, and on the limbs there are terminal
walking pads instead of claws, though these are other features that are
more similar to extant onychophorans than to those of the lobopods.
A. gracilis had body
spaces that are relatively large, which are presumed to be haemocoels,
that are arranged in a similar way to those of onychophorans. It has
reasonably been speculated that (Ou et
al., 2011)
A. gracilis may be
related to a branch of the lobopodians that gave rise to crown
Onychophora. Among those arguing for a lobopodian origin for
arthropods Graham Budd, Uppsala University, has been prominent, citing
anatomical comparisons between crown groups and fossil morphologies
interpreted by him as having features that link onychophorans with stem
plus crown arthropods (Budd, 1996, 1999). The description of
Diania cactiformis, a
spectacular new form from the Chengjiang Fauna (J. Liu et
al., 2011), had added a new
dimension to the question of the origins of arthropods. Diania is
heavily armoured, which includes spiked elements that encase its legs,
which suggests jointing and at least a degree of incipient segmentation.
The armour on the trunk, the elements of which are quite narrow,
suggests a different type of flexibility. It is easy to see how the
jointing characterising the carapace of arthropods could have evolved
from such integumental armour, while integrating within the jointed
skeletal architecture internal segmental features to produce the body
plan of arthropods (J. Liu et al.,
2011), Though there has been a considerable amount of debate over the
phylogenetic placement of
Diania.
Diania was proposed to be
truly intermediate between Cambrian age lobopods and Euarthropods. The
authors1 suggest that this hypothesis seems to be unlikely
because of the discovery by several new phylogenetic analyses that have
placed
Diania well within other
lobopods of the Cambrian (Mounce et
al., 2011). This emphasises
both the difficulty of resolving phylogenetic relationships among the
Panarthropoda and the importance of phylogenies that are rigorously
developed. Many of the known fossils from the Cambrian are
poorly preserved and incomplete, with key morphological features being
difficult to discern. Therefore advances in knowledge of morphological
details and their interrelationships is dependent on the continued
discovery of new fossils revealing more features, though the general
impression of the variety of body shapes and ornamentation of lobopods
from the Cambrian is correct. There have already been challenges by
subsequent study to the early phylogenetic hypothesis. A panarthropod
phylogeny has been presented with a series of paraphyletic lineages in
which the tardigrades and the onychophorans are extant representatives
of a once more disparate assemblage of forms. The authors1
suspect that phylogenetic studies in the future may alter the picture,
possibly significantly, of the topology shown in Fig. 6.27 in their book1.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |