Tuesday, February 28, 2012

So where did Trilobites come from?

Trilobites are a true Leitfossil for the period of early life on planet Earth called Paleozoic which is Greek for Old Life.

"Trilobites were one of several families of arthropods in the early Cambrian oceans." But when exactly do they first appear in the fossil record and where in the arthropods tree of life did trilobites come from?

Earliest known Trilobites
Sam Gon III writes in his website and I quote (I have added emphasis. For the complete story it is highly recommended to visit the page on trilobite origins.)

The earliest trilobites appear in the lower Cambrian record.

[Note. In another page Sam tells that not in earliest Cambrian but in the so-called Series 2: 
"The first appearance of trilobites defines the start of Series 2 of the Cambrian (521 mya), and they can be found in strata up to the upper Permian (251 mya), after which trilobites (among a large number of marine organisms) went extinct in the great catastrophe that removed over 90% of all species on earth.]

These oldest trilobites include members of
Order Redlichiida, Suborder Olenellina, Superfamily Fallotaspidoidea and
Order Ptychopariida, Suborder Ptychopariina, Superfamily Ellipsocephaloidea.

Even these early representatives bear all of the defining characters of trilobites.

Probably the key distinguishing character, one that also allowed trilobites to be preserved so well (and which accounts for their sudden prominence in the Cambrian), is calcification of the exoskeleton.

It is interesting that some of the most primitive of trilobites lack a few characters that more advanced trilobites bear. Fallotaspidoids lack facial sutures, for example, and their protaspides are apparently uncalcified.

If the ancestors of trilobites in the Precambrian were uncalcified, then their preservation would be restricted to konservat-lagerstätten, which are very rare indeed. Cambrian konservat-lagerstätten such as the Burgess Shale (Canada) and Chengjiang (China) demonstrate the huge diversity of non-calcified arthropods that would have escaped detection if those sites did not exist. Many of these are arachnomorphs closely related to trilobites and relevant in a search for trilobite ancestors. But where did these arachnomorphs come from?

Three comments
1. The general observation Sam makes on the significance of what I call "Divine nature reserves" is of great importance here - the preservation of evidence. As he writes, without the Konservat-lagerstätten researchers of the origins and evolution of trilobites would be left without evidence about life forms that lack the calcified parts which survive better among the fossil record.

2. In a way it is, of course, tautology to say that first recognized trilobites have the defining characters of trilobites.

Of course! Otherwise they would not be defined as trilobites, would they?

Sam goes on describing some typical characteristics missing from the trilobites classed as Fallotaspidoidea but also these have the signs needed to make the fundamental Aristotelean classification into families, subfamilies etc.

The logic is simple
Superfamily A has characteristics ABCD
Superfamily B has characteristics ABCEFG
Specimen X has characteristics ABCD so it is an example of superfamily A.

3. It is quite remarkable that the known record of Cambrian life forms represents such a collection of "ready" organisms that then continue to live and develop from generation to generation. As Sam says, trilobites are a very successful designs and survived about three hundred million years flourishing in thousands of variations to the end of Permian.

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