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Australia: The Land Where Time Began |
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Cambrian Explosion - Branches that Link
Lobopoda and Crown Arthropoda There is a wide array of fossils that appear to
have affinities with arthropods between the lobopods of
Cambrian age and
crown-group arthropods; that have what appear to be segmented
exoskeletons and some sort of jointed appendages in most, the crown
group being called the Euarthropoda to distinguish it from such stems.
Some of the synapomorphies of crown groups are missing from more basal
groups, though they have synapomorphies of their own. There are 2 forms
near the base of this assemblage, and close to the lobopodians,
Kerygmachela
and
Pambdelurion, that have
been interpreted as having flexible, as opposed to jointed, sclerotised
appendages that can be seen as having derived from lobopods (see Budd,
1999). These forms have gills, and
Pambdelurion has a radial
mouth which is surrounded by sclerotised plates resembling the mouths of
anomalocaridids. These forms differ from canonical lobopodians in these,
as well as other respects. These “gilled lobopods” have been commonly
cited as evidence that the lobopodians gave rise to the arthropods. Another step towards arthropodisation may be
represented by
Opabinia. This form,
about 5 cm long, has an anterior appendage on a stalk, multiple eyes and
odd posterior blades or flaps.
Opabinia is believed to
have been mostly a swimming form, and it is suggested by the authors1
that it is probably allied with the anomalocaridids, known from several
genera that have been grouped into the clade Radiodonta. The
anomalocaridids were fairly large animals, some possibly reaching up to
1 m in length. Anomalocaridids have a radial mouth, as does
Pambdelurion that has
been preserved as a circlet of sclerotised plates, some forms having 32
plates, around the mouth opening. It had paired claws flanking the mouth
anteriorly. Anomalocaridids persisted into the
Ordovician, at least, and
it was specimens, that the authors1 describe as ‘beautiful
(and gigantic) specimens’ from the Ordovician that have recently been
recovered (Van Roy & Briggs, 2011). It has been that the jointed claws
could presumably be flexed to capture prey and transfer it to the mouth circlet,
which the authors1 suggest was probably specialised for
crushing exoskeletons. Swimming is suggested to probably have been
accomplished by the movement of the lateral flaps, to which gills were
attached, that were along the sides of the body. Both radiodontids and
Opabinia belong to the
paraphyletic stem group assemblage between the lobopodians and
Euarthropoda. To understand the evolution of arthropod limbs, and
therefore the origin and early diversification of arthropods it is
critical to resolve the phylogenetic relations among these groups (Budd
& Telford, 2009; Edgecombe, 2010). There are a number of problems facing
researchers in this area. An example is that lobopodians all have
relatively simple, unspecialised legs, though in
Opabinia and
anomalocaridids there are paired lateral flaps instead of legs and these
flaps have gills along the upper aspect, particularly in
Opabinia (X.L. Zhang &
Briggs, 2007). Jointed
appendages that are well sclerotised reappear beyond the Radiodonta.
Some of the complexities involved in understanding the evolutionary
pathways among these groups are illustrated by the ongoing debate
regarding a number of questions, such as:
1.
Are the appendages of arthropods
homologous to those of lobopods, as argued by Budd?
2.
Are they homologous to the lateral flaps
of the Radiodonta?
3.
Or are they completely novel structures?
According to the authors1 this debate is
not close to settlement (Budd, 1996; Budd & Daley, 2012; Waloszek et
al., 2007; X.L. Zhang &
Briggs, 2007), illustrating some of the complexities of understanding the
evolutionary pathways among these groups. There are still other stem
arthropods that are almost certainly closer to crown arthropods than are
the radiodontids, which have been recovered from assemblages of
Burgess
Shale type. There are 2 such groups, that could possibly be closely
related, the Canadaspids, bivalves that have sometimes been assigned to
crown Crustacea, and
Fuxianhuia
and related
taxa. They differ from crown groups mainly in the details of their limbs
(see Edgecombe, 2010). The Megacheira, the “short great appendage
arthropods” such as
Fortiforceps, are another
stem group that are well represented among the Lower Cambrian
lagerstätten. Relatively large, paired appendages arising on the
anterior portion of the head, characterise this group, tough it is often
difficult to be sure precisely where they insert as the heads are
partially hidden beneath a shield and it is not evident where the
boundaries of the head segments are. Anterior head appendages are
innervated from the brain, which has 3 lobes, in living arthropods: in
anterior-posterior order they are the procerebrum (bearing eyes), the
deuterocerebrum, and the tritocerebrum. Arthropods have their eyes on
their most anterior head segment, the ocular segment, and they are
innervated from the most anterior lobe. The antennae in crustaceans and
the chelicerae in chelicerates, are on the second segment only in both
groups, and are innervated from the middle lobe. Appendages arising on
the ocular segment are not present on any extant arthropods, though
extant onychophorans have 2 lobes to their brains and their antennae
arise on their ocular segment, the eyes being situated at the base of
the antennae and both are innervated from the anterior brain lobe.
Therefore, if crown onychophorans and arthropods are both derived from
among the lobopodians, then at some point there was significant
evolution of the brain and ocular appendages to produce their different
derived features. An hypothesis has been proposed to overcome this
evolutionary problem (Budd, 2002) according to which the “great
appendages” could be interpreted as arising from the ocular segment, as
is the case of extant Onychophora. He suggested that if this
interpretation is correct the appendages on the ocular segment on those
forms from the Cambrian then the appendages would be innervated from the
procerebrum, which differs from the condition in the crown arthropods,
i.e., the switch from anterior innervation of anterior appendages took
place after the Megacheira, or other arthropods with similar appendage
origins, had branched from the stem arthropods. It has been found by
study of more recent fossil finds that in at least some forms the great
appendages arise from just behind the eyes, so they may not have been on
the ocular segment. The simplest explanation of evolution of these
features is that the condition present in crown arthropods arose prior
to the branching that led to the Megacheira. The question then remains
as to when the brain style of crown arthropods and anterior innervation
arose. The authors1pose the question of could it simply have
been after the LCA of Onychophora and Arthropoda, within the lineage,
though not known as fossils, that gave rise to the basal arthropods?
Also, in what form was the brain in lobopods? The austhors1
suggest that the answer may never be known for certain. Trilobites were the best studied and most widely
known arthropods of the Cambrian prior to the redescription of the
Burgess Shale Fauna that began in the mid-1960s. They are among the
best-studied taxa from the
Palaeozoic and remain the most numerous by
far, outnumbering all other groups in terms of the number of species and
genera known from the Cambrian. The base of Cambrian Stage 3 is defined
by the earliest-known trilobites. The carbonate exoskeletons of
trilobites are more readily preserved than those of most arthropods from
the Cambrian that have organic integuments that are tough but not
mineralised, and they are commonly found in the many fossil faunas where
only skeletons that are mineralised are preserved. The obvious
correlation between the ability to preserve well and variability leads
to speculation that the groups with unmineralised integuments that have
been recovered from the Burgess Shale,
Chengjiang,
Emu Bay, and other
similar faunas may have been far more diverse than it is possible to
document. The Naraoiidae are a group with unmineralised integuments that
are a closely related sister group to trilobites that have been found
only in the Chengjiang and Burgess Shale assemblages (Hou & Bergström,
1997). It is uncertain whether trilobites branched before or after the
LCA of crown arthropods. Among the larger, better-known fossils that have
been described from the assemblages of the Chengjiang and Burgess Shale
crustaceans don’t appear to be represented, though the group may be
represented by fragmentary mouth parts in drill cores of Stage 3 or
Stage 4 from the Northwest Territories, Canada (T.H. Harvey &
Butterfield, 2008). These fragments indicate a mandibular feeding
apparatus that is more similar to the apparatuses that have been found
in crown-group crustaceans than to those of known stem-group taxa. An
animal that may have been as large as 5 cm that is believed to have
likely fed by scraping particles from benthic surfaces, that were then
filtered and eventually comminuting them on mandibular molars in a
sophisticated feeding
system. Also, in the Stage 4 rocks from south China a crustacean larva
has been identified (X.G. Zhang et
al., 2010). Therefore, crown
crustaceans, and therefore crown arthropods, are likely to be
represented among fossils from the Early Cambrian. In summary, among these spectacular though
problematic ecdysozoans groups from the Early Cambrian, there appears to
have been a radiation of scalidophorans, lobopods and arthropods in the
latest Neoproterozoic and Early Cambrian. The scalidophorans of the
Cambrian appear to be stem lineages displaying more morphological
diversity than their crown groups. Lobopodians from the Cambrian are a
very disparate, paraphyletic stem clades including ancestral groups of
the arthropods, tardigrades and onychophorans. In the Cambrian the
burgeoning lobopod and arthropod faunas, almost all clades of which are
extinct, were clearly major elements in the economy of Cambrian
communities, and according to the authors1 their ecological
effects must have been felt in
most corners of the marine biosphere, judging from their great
variety of morphology. Erwin, Douglas H., & Valentine, James W., 2013, The Cambrian Explosion: The Construction of Animal Biodiversity, Roberts & Co., Greenwood Village, Colorado |
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |