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Australia: The Land Where Time Began |
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Acute Vision in
Anomalocaris – Origin of compound eyes Intimate details of the optical design of
non-biomineralised arthropod eyes remained elusive until recently in
deposits of Cambrian Burgess Shale type, though there had been
exceptional preservation of soft-part anatomy in such
Konservat-Lageretätten. According to the authors1 a single
origin some time prior to the latest common ancestor of crown-group
arthropods is supported by the development and structure of ommatidia of
arthropod compound eyes, though the earliest appearance of compound eyes
has been poorly constrained due to the lack of adequate fossils. In this
paper the authors1 report the discovery of 2-3-cm paired eyes
from the Emu Bay Shale, Kangaroo Island, South Australia that dates to
the Early Cambrian, about 515 Ma, that has been assigned to
Anomalocaris, the apex
predator of the Cambrian. There
are at least 16,000 ommatidia lenses (in a single eye) that are
hexagonally packed, which the authors1 suggest rivals the
most acute compound eyes in extant arthropods. There are 2 distinct
taphonomic modes shown by the specimens, preserved as iron oxide (after
pyrite) and calcium phosphate, which demonstrates that the same type of
extracellular tissue (i.e. cuticle) can be replicated by disparate
styles of early diagenetic mineralisation within a single deposit of
Burgess Shale-type. Compelling evidence of the arthropod affinity of
anomalocaridids is provided by these fossils, which moves the origin of
compound eyes deeper down the stem lineage of arthropods, the authors1
suggesting that compound eyes developed earlier than such features as a
hardened skeleton. The inferred acuity of the eyes of anomalocaridids is
consistent with other evidence that suggests these animals were visual
predators in the water column that were highly mobile. The escalatory
‘arms race’ that began more than 500 Ma was probably helped to
accelerate by the presence of large macrophagous nektonic predators with
sharp vision, such as
Anomalocaris within the
ecosystem of the Early Cambrian.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |