Tuesday, March 6, 2012

Trilobite evolution - Origins

From Parvancorina to Trilobite in Four Easy Steps

The series of ontogeny diagrams is highly illustrative and gives fundamental insight on the evolution of a significant family among Cambrian life forms upon Earth.

How do these specimens fit into the chronology of fossil discovery? For the pattern in the schematic chart could suggest that all these branches are variations of a single basic form and they could at some point exist all at the same period of time.

However, Sam tells us that these are called different species and that there is a chronological sequence:
"Taken in sequence, and with legs added to accentuate their underlying shared arthropod heritage, the links between Parvancorina, Primicaris, Naraoia, Kuamaia (a helmetid), and trilobites seem easier to visualize in the sequence below:"

Parvancorina Primicaris  Naraoia
Kuamaia Redlichia

The oldest Trilobite is the Redlichia that appears in the lower Cambrian (Series 2).

I understand from Sam that Parvancorina and the fellows including the helmetid Kuamaia are known from the Ediacaran period of enigmatic life forms and these do not appear in the known Cambrian record.

In my untrained eyes the sequence looks like the evolution according to the species (microevolution, in Biblical Hebrew leminehu) rather than the birth of trilobites from some very different species (macroevolution). In other words, that the DNA determining the major characteristics of the species is already locked in the Parvancorina and allows the kind of variations that the main chart is so nicely demonstrating.

Of course, it is a matter of taxonomy and what arguments we use to define a separate species, family or subordinates and so on. But the impression is given that we have something that resembles some  mathematical factor defining the evolution of these things.

An un-named "soft-bodied trilobite" from the Flinders site in Australia also might seem a reasonable Precambrian candidate antecedent to true trilobites ( Gehling, J.G. 1991. The case for Ediacaran fossil roots to the metazoan tree. Memoirs Geological Society of India. 20: 181-223. ).

At first glance, it resembles the many-segmented Redlichiida, but it also resembles small specimens of ArchaeaspinusDickinsonia, or Vendia, which most workers do not consider to be of arthropod affinity.

Nonetheless, this taxon, as well as Parvancorina minchami (Glaessner 1980) both suggest that arthropods did not miraculously appear in the Lower Cambrian, but took form during the Precambrian. The specimen of Parvancorina to the right even bears fine lines that some workers have interpreted as evidence of paired limbs.

(Glaessner, M.F. 1980. Parvancorina - an arthropod from the Late Precambrian fauna of the Ediacara Fossil Reserve. Records of the South Australia Museum 13:83-90).
Sam Gon III

Okay! So Sam suggests actually the same thing, a species starting in Ediacaran and continuing in Cambrian and living all the way to the end of Permian.

This pushes the knowledge of trilobites origins forward and raises the question how then did the Parvanorina begin their life upon planet Earth.

Or to paraphrase Sam Gon III "Where did the Parvancorina come from?"

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