DEGRADATION, DIAGENESIS AND DISPARITY: THE IMPACT OF TAPHONOMY ON MORPHOLOGICAL DATA IN CAMBRIAN LAGERSTÄTTEN

Burgess Shale-type faunas from the Cambrian of North America and elsewhere embrace a variety of modes of preservation. In one variant the tissues of the organism, notably the cuticle of arthropods, acted as a template for the precipitation of authigenic clay minerals. It has been suggested that these clay minerals precipitated either on the external surface of the cuticle, or internally, in voids created by decay; the latter is supported by elemental mapping, imaging of the fossils using ESE or BSE detectors, and computer simulations of the analytical methods. This also occurs in other, similarly-preserved, non-biomineralized arthropods.

The ecology and affinities of many Cambrian arthropods from the Burgess Shale and other exceptional faunas are controversial. The relative impact of the different modes of preservation on such issues has yet to be fully determined, but is potentially significant. In similarly preserved fossil arthropods known to be deposit/detritus feeders the mud-filled guts are preserved in three-dimensions; i.e. not compacted to the same extent as the surrounding sediment. This is attributed to early diagenetic mineralization inside the gut, possibly encouraged by the high abundance of microbes or organic material. This is an alternative to the interpretation of the guts of Cambrian arthropods preserved in clay minerals having originated by weathering of phosphatized biological tissues.

Decay experiments indicate that the preservation potential of different appendages, or parts of an appendage, vary strongly within an individual arthropod. Complete decay of certain appendages can precede appreciable deterioration in the morphology of others. This variation in the recalcitrance of non-biomineralized cuticle reflects factors such as its thickness and degree of sclerotization. It is independent of the mode of mineralization and can be shown to occur in arthropods preserved in a similar manner to those in the Burgess Shale. Thus some of the disparity in morphology exhibited by exceptionally preserved arthropods may be taphonomic rather than biological in origin.