The Fossil Record and Vertebrate Taphonomy

John Merck

Taphonomy:

The realm of paleontology dedicated to the study of the processes by which an organism becomes part of the fossil record. Yields an understanding of the filters and biases of the fossil record.

Becoming a fossil:

From the birth of the organism to discovery by a paleontologist, fossils go through four general stages.

The Biotic Stage


Apatite (Ca5(PO4)3) from Wikipedia
Hard tissues

The organism develops tissues and substances with a range of preservation potential. The greatest potential exists for hard tissues including:

As a mineral, apatite is relatively durable, so vertebrate skeletal elements that make it into the rock record have a decent shot at long-term preservation. The problem is that phosphate ion is relatively rare and important for metabolism. Thus, vertebrate bodies hoard it, and bony tissue is continually remodeled to provide metabolic access. Not surprisingly, vertebrates secrete skeletal tissue sparingly in comparison to creatures like mollusks with their massive (CaCO3) skeletons. Hydroxyapatite occurs in:


Tough tissues


Fossil bird from University of Bristol Paleobiology Research Group
Complex organic molecules forming the framework for robust soft tissues: While not exactly "hard parts" some soft tissues form tough parts having reasonably good fossilization potentials. Although susceptible to decomposition, these are tough enough to withstand much of the physical abuse of the interment stage, and can be preserved if they are quickly removed from agents of decomposition. Among vertebrates these include:
Soft tissues


Haikouichthys from SVT A L'AFFICHE – LeWebPédagogique
Genuine soft tissues, susceptible to decomposition and mechanical forces, are only rarely preserved as fossils as a result of rapid burial in quiet, anoxic environments. When they are, we make a big fuss, as in the case of Haikouichthys (right) and other soft-bodied creatures from the Cambrian Chengjiang locality.
Behavior and habitat preference


Mountain goat Oreamnos americanus
Of course, organismal behavior also influences fossilization potential. By exclusively inhabiting an erosional environment, the mountain goat (right) has almost no chance of becoming a fossil. In contrast, creatures the burrow in sediments in depositional environments are preferentially preserved.

Response to predation: The hardest, most robust, and least digestible parts of an organism have enhanced preservation potential simply because so many vertebrate remains are processed by predators and scavengers. Thus, teeth are significantly more likely to enter the fossil record than fragile bones.

The Interment Stage


Red salmon shuffles off mortal coil
Necrolysis:

  • Decay: The removal of soft tissues from the carcass is accomplished by everything from bacteria to large carnivores, but microbial action predominates. The speed of decay is a function of: Low values of any of these can retard or halt decay.

  • Posture: Aspects of decay including: Can cause the carcass to assume postures that it would not assume in life.

    Transport:


    Beach gravel with basalt and marine iguana bones - Fernandina, Galápagos Islands.
    Fossils (and sedimentary particles generally) are either: In the latter case, we are concerned with the effects of transport. Vertebrate remains are subject to the same modes of transport as other clasts of similar size:

    Generally, each transport mode imposes its own filter on the final survival, distribution, and orientation of each element of the body.



    Beach gravel with basalt and marine iguana bones - Fernandina, Galápagos Islands.
    Disarticulation:

    In discussing transport, we have treated the skeleton merely as a pile of bones, and considered their relative transportability. In fact, the vertebrate skeleton consists of elements whose articulation - geometric relationship one to another - conveys considerable information. What is the fate of that information upon death?

    In the absence of transport agents, disarticulation progresses as scavengers and agents of decay attack the carcass. Thus:

    Identification: Disarticulation is the randomization of the location and orientation of skeletal elements, with disturbance concentrated at the top. If the agents of disarticulation are predators or scavengers, bones will be abraded and broken, also.


    Wildebeast carcasses in Mara River from Wild Eye.
    Bloated carcasses: Where transport meets disarticulation. Carcasses commonly are floated down rivers and out to sea, where they can remain intact for up to weeks. As the carcass decays, pieces fall off starting with jaws and appendages and working inward. This can have the effect of transporting disarticuated remains of terrestrial organisms into shallow marine depositional environments.

    Bone modification:


    Eusthenopteron foordi a cylindrical fish in two dimensions

    Although modification of bone shape and surface texture can happen at any taphonomic stage, it is concentrated in the interment stage. Sources of modification include:


    Unlike other clasts, vertebrate elements must put up with other organisms as agents of transport, including:

    Diagenesis:

    The mineral component of bone: Whereas the mineral component of calcareous (CaCO3) skeletons is highly susceptible to chemical alteration after deposition, hydroxyapatite is relatively stable, although:

    Pore Space within Bone: Most vertebate bone has pore space in the form of:



    Fossil bone microstructure from Illuminating Microstructure
    Modes of fossilization:


    Fossil fish in concretion
    Diagenesis of surrounding sediment: Of course, being porous, chemical changes in vertebrate bone are coupled with those in the surrounding sediment. Ironically, buried decomposing vertebrate carcasses alter pore-fluid chemistry in their immediate vicinity, often promoting precipitation of ions in solution and prompting the formation of concretions. Because concretions tend to be more resistant to weathering than surrounding matrix, the effect is often to protect vertebrate fossils. Indeed, in lagerstätten like Mazon Creek, fossils are primarily preserved inside concretions.

    Taphonomic Information



    Haikouichthys from Chengjiang from leWebPedagogiques

    Some Famous Lagerstätten