The key phrase I used was "mineralised skeletons". Many crustaceans lack
mineralised skeletons, but get by very well with chitinous ones. My
understanding of the trilobite skeleton was that it consisted of chitin
mineralised by calcite. Trilos that whose skeleton was not mineralised would
still function, but would not leave a fossil record.
I don't see why "To conclude that the trails that look like the trails made by
trilobites were made by trilobites without hard parts because no skeletal
remains were found in the same sediments" is an example of "wishful thinking on
someone's part" when dealing with the early Cambrian. The other examples you
cite (and I could add to them) come from eras when mineralised trilos were
common. Tommotian trilo body fossils are absent, but not their trace fossils.
Without looking it up, I understand this is a world wide phenomon in those
places which have a complete or relatively complete Neoproterozoic-Early
Cambrian succession.
Arthur V. Chadwick wrote:
> Maybe you can explain how a trilobite without a hardened exoskeleton can
> make Cruzina-type trace fossils, since these are made by the hardened
> exoskeleton of the telopodites displacing sediment. It appears to me that
> the case is somewhat different from that which you suggest. For example,
> in Grand Canyon we find cruzina below the first trilobites in the Tapeats
> Sandstone, but there are no body fossils until the overlying Bright Angel
> Shale. However, this is upper Lower Cambrian to lower Middle Cambrian, and
> nobody has suggested the trace fossils were made by other than the same
> kind of bugs we find in the shale. To conclude that the trails that look
> like the trails made by trilobites were made by trilobites without hard
> parts because no skeletal remains were found in the same sediments is
> wishful thinking on someone's part.
> Art
> http://biology.swau.edu