Fragmentary Fossils

Mr. Elsberry;
I came across the talk origins site on accident as I was doing some browsing about the subject of origins (particularly intermediate or transitional forms). As a complete neophyte to paleontology I had a couple of questions and your answers seemed to be some of the most cohesive that I found. How are partial fossil remains (I am supposing that most fossil finds are at least partially incomplete) classified and dated? Is a date deduced first then a classification or the other way around? And is most of the dating done by strata location or radioisotope dating of surrounding material? Sorry for my ignorance I am a professor of languages and this is new but interesting material for me.

I think that the example of “conodonts” may help illuminate all your questions.

First, check the Wikipedia page on conodonts.

And second, take a look here.

Conodonts were first described in 1856 on the basis of phosphatic tooth-like fossils. This sort of fossil was the only type of fossil remains known until 1983, when the same tooth-like fossils were found with soft-tissue preservation showing the organism that bore them. So for over a century, this group was solely known from highly fragmentary fossil evidence. There was plenty of conjecture, but the actual affinity of conodonts as members of Phylum Chordata was only worked out relatively recently.

The easily discovered conodont fossils are in the Cambrian or pre-Cambrian, and are commonly found in strata dating to the late Triassic period, so conodonts were around for somewhere near 350 MY. As fossils go, conodonts are relatively common. The group was diverse, too, and those two things meant that they were nearly ideal as index fossils. Index fossils are those fossils that are reliably found only in a limited range of strata. Because of this, they are used for assigning relative dates to those strata based on standard geological heuristics like superposition (a rock layer above another rock layer is generally a younger rock layer).

Conodont fossils are extensively used in petroleum geology for locating oil deposits. Besides their use there as index fossils, they also give evidence of paleoclimates due to temperature-dependent changes in their phosphatic composition.

Now, recall that all of this utility came well before anyone had any good idea of what a conodont might actually look like. In the absence of complete evidence, conodonts were assigned their own phylum. Classification within Conodonta proceeded entirely based upon morphology and chemistry of the tooth-like fossils. Despite knowing next to nothing about the organisms themselves, geologists and paleontologists found conodont fossils highly useful for assigning relative dates to other fossils found in association with them.

In this case, most of the dating associated with conodonts was relative, not absolute, and classification played a very minor role, as it was based on known-fragmentary information.

I can’t really say what the typical approach might be toward fragmentary fossil evidence (since I’m not a paleontologist), but the conodont story indicates that paleontology has successfully and fruitfully dealt with that situation before.

I think that you should have a look at the TalkOrgins “Fossil Hominids” sub-page.

There’s quite a bit of discussion there about the partial skeletons of various hominids and what we can learn from them.

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Wesley R. Elsberry

Falconer. Interdisciplinary researcher: biology and computer science. Photographer. Husband. Christian. Activist.

8 thoughts on “Fragmentary Fossils

  • 2009/01/04 at 8:59 pm
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    “This sort of fossil was the only type of fossil remains known until 1983, ”

    Did you really mean to say this ?

  • 2009/01/05 at 6:38 am
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    Given that the discussion was begun with the qualification that the group under discussion was conodonts and that the statement was followed by another sentence making it clear that this was about that group in particular, I don’t see a problem.

    Of course, fossils of other sorts for other groups existed prior to 1983, but I clearly was not talking about those.

  • 2009/01/05 at 7:50 am
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    D McCallum –

    I had to do a double-take too, but reading carefully, its clear Wes is referring specifically to conodonts and not to fossils in general. Maybe “fossil remains of this group known” would be a tad more clear than “fossil remains known”, but the original is fine. But that’s just me. You could also argue that that’s being redundant.

  • 2009/01/05 at 4:51 pm
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    OK, your question has two parts.

    Firstly. you are correct, most fossils that are found are incompete, but this is generally enough to classify the fossil to a specific group, or genus. Once enough diagnostic material has been collected to reconstruct the organism, or a complete specimen is found, a “type” specimen is described and published. Future fragmentary finds can then be compared with the “type” specimen. Often it is only necessary to find a fragment of the organism, provided that the fragment contains diagnostic features that identify the fragment as belonging to a particular species.

    For example, here is the “type” specimen for Anomalocaris briggsi from the Lower Cambrian of South Australia. Anomalocarids have similar bodies, and the diagnistic features of each species are the frontal appendages. So therefor it is only necessary to find the appendages to identify the species. The body is not necessary. Indeed the “type”: specimen of A. briggsi is an appendage, as that is the diagnostic feature which identifies the species. Should the body be found, it will add to the suite of diagnostic characters for A. briggsi.

    Fragmentary finds often provide more information that a complete specimen. Once we have a complete secimen, finding another one does not provide any new information. However, fragmentary remains can provide information on the organism, for example did the material tear, snap or fracture.

    As to dating, if the fossils are found in close association with rocks that can be dated directly, then an absolute age cam be given (in millions of years). If not, then the deposits can be relatively dated (e.g. Permian age, Cretaceous age) by any associated fossils that have been identified and dated elsewhere, and/or correlated stratigraphically and palaeontologically with rocks elswhere that have absolute ages.

  • 2009/01/05 at 5:33 pm
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    Chris, thanks for the additional information on handling fragmentary fossil evidence.

  • 2009/01/06 at 8:36 am
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    Chris Nedin writes:

    Once we have a complete secimen, finding another one does not provide any new information.

    It might. For example it might provide information about the geographic distribution of the species (depending on where it was found) or the temporal distribution (if it was found in a different strata). Also, even complete specimens may have been deformed by geolgic processes or may not be entirely clear because of the way they were lain when fossilized, and a separate complete specimen may provide that information.

    Maybe I’m being overly pedantic here.

  • 2009/01/06 at 4:12 pm
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    Dave S,

    What I meant to say was that finding a second complete specimen would provide no new morphological information for classification purposes, but you are correct, new specimens can provide new stratigraphic or geographic information.

  • 2009/01/06 at 5:21 pm
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    Chris,

    I’ve added your blog to my blogroll.

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