Experimental Effects of Water Abrasion on Bone Fragments.
Keywords: TRANSPORT, SEDIMENT, WATER STREAM, EXPERIMENT, ABRASION, ROUNDING
[+info] VOLUME 1. ISSUE 3. 2003 (1 issue)
Water transport is a frequent taphonomic agent in continental environments that may affect and disturb original bone associations. Fossil allochthony occurs as a result of resedimentation (before burial) and/or secondary deposition (after initial burial) altering palaeoenvironmental and palaeoecological indications provided by fossils. Skeletal element sorting or preferred orientations of fossils are evidence of fluvial transport as studied by several authors. Bone surface abrasion is another trait recorded on fossils that may provide evidence of water transport in a fossil association. Results of a preliminary experiment on the effects of abrasion have shown characteristic differences relating to the type of sediment (coarse to fine) and the type of bone involved (fresh, dry, weathered or fossil). This indicates that the effects and consequences of water transport on bone associations can be identified from traits of abrasion. This paper also considers other experiments involving abrasion on large and small mammal bones and owl pellets.
Conulariids provide good examples of how biometric and other characters that have been used to diagnose fossil species can be affected by taphonomic processes, possibly leading to the erection of taphonomic taxa, or taphonomic artifacts. Based on analysis of the taphonomy of Conularia quichua Ulrich from the Devonian Ponta Grossa of southern Brazil, we argue that caution must be exercised when using biometric and other characters to diagnose conulariid species. For example, measurements of the spacing of the transverse ribs must be corrected for compaction of the theca parallel to its long axis. C. quichua oriented at high angles to bedding almost always exhibit this kind of deformation, which if not corrected for results in substantial additional measurement error. Similarly, the value of the apical angle of C. quichua differs between compressed and uncompressed specimens, making it difficult to measure this character with a high degree of consistency and reproducibility. Other characters (geometry of the transverse ribs, presence or absence of interspace ridges and nodes) used to diagnose conulariids are susceptible to modification and information loss through weathering. In reviewing published descriptions of other conulariids, we have found that certain species may be taphotaxa. Future descriptions of new species should be based on collections encompassing the known spectrum of preservational patterns. Also, the erection of new conulariid taxa should be based as much as possible on complete or nearly complete specimens, and morphometric comparisons should be made using specimens showing similar patterns of preservation.
Phosphatic concretions, containing remains of the lobster Palaeonephrops browni (Whitfield), are described from the Upper Cretaceous Bearpaw Formation of southern Alberta. Two modes of burial are interpreted to have enhanced the preservation potential of the lobsters: (1) burial of remains by volcanic ash, and (2) burial of remains within burrows due to sediment injection during storms. The latter mode of burial is indicated for the majority of the specimens studied, suggesting that the exceptional preservation of Palaeonephrops, and probably some other fossil decapod taxa may have been more strongly influenced by their life-habits than previously assumed. Within-burrow preservation of lobster specimens also demonstrates that obrution is not only important for the preservation of faunal elements residing on the sea-floor, but can also bias the preservation of some deep infaunal taxa. Following rapid burial, the preservation potential of the lobster remains was further enhanced by virtue of its phosphate-bearing cuticle, upon which, early diagenetic phosphate cements were preferentially precipitated. Further phosphate precipitation resulted in the entombment of the remains within phosphatic concretions, further protecting them from destructive taphonomic processes.
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