Possible Taphonomic Bias in the Preservation of Phosphatic Macroinvertebrates in the Uppermost Maquoketa Formation (Upper Ordovician) of Northeastern Iowa (North-Central USA)
- Autores: Heyo Van Iten, Michael Lichtenwalter, Juliana de Moraes Leme, Marcello Guimarães Simões
- Localización: Journal of taphonomy, ISSN 1696-0815, Vol. 4, Nº. 4, 2006, págs. 207-220
- Idioma: inglés
- Texto completo no disponible (Saber más ...)
-
Resumen
Examination of acid digestion residues can be an indispensable tool in the detection and identification of fragmentary remains of rare and/or fragile, macroinvertebrate fossils preserved in shallow shelf carbonates. We recovered submicroscopic fragments of phosphatic conulariid, Sphenothallus (Cnidaria), and ?Trematis (Linguliformea) skeletons from a slab of highly fossiliferous lime packstone from the uppermost Brainard Shale Member of the Maquoketa Formation (Richmondian, Upper Ordovician) of northeastern Iowa, USA. The bedding planes of this and four similar Brainard Member slabs (total upper bedding surface area approximately 0.38 m2) lack macrofossil specimens of these three taxa, which have never previously been reported from this rock unit. Analysis of the preservation of the abundant calcitic fossils revealed evidence of wave current action, including pervasive disarticulation of the brachiopods, echinoderms, and trilobites, and bimodal alignment of the narrowly conical Cornulites shells and elongate Eoplectodonta valves, almost all of which are also oriented convex up. These results are consistent with the hypothesis that storm wave activity on the Brainard Member sea floor fragmented originally macroscopic conulariid, Sphenothallus, and ?Trematis skeletons, making it difficult to detect these taxa at low magnifications and thus making it appear that they were absent in the original bottom community. Together with results of similar studies of other Paleozoic rock units, our investigation suggests that there is a systematic, taphonomic bias against conulariids, Sphenothallus, and linguliform (phosphatic) brachiopods in storm-influenced shelf deposits.
