PROMETHEUS PRESS/PALAEONTOLOGICAL NETWORK FOUNDATION (TERUEL) Available online at www.journaltaphonomy.com 2006 Fernández-López Journal of Taphonomy VOLUME 4 (ISSUE 3) Taphonomic Alteration and Evolutionary Taphonomy Sixto R. Fernández-López* Departamento de Paleontología, Facultad de Ciencias Geológicas (UCM) and Instituto de Geología Económica (CSIC-UCM), Universidad Complutense de Madrid, E-28040 Madrid, Spain Journal of Taphonomy 4 (3) (2006),111-142. Manuscript received 2 May 2005, revised manuscript accepted 12 March 2006. Every process of taphonomic alteration implies change and modification of the affected taphonomic elements, but it does not necessarily lead to the destruction of taphonomic elements. Taphonomic alteration can be of four types: elementary, populational, taphonic and taphocladal. In order to interpret the differential preservation of fossils and fossilization mechanisms it is necessary to take in mind not only the original architecture of taphonomic elements and the environmental changes, but also the successive changes in architecture of taphonomic elements and the activities carried out by taphonomic elements, as well as the evolutionary modifications of taphons and taphoclades. This systemic and evolutionist procedure allows to explain how the representatives of some taphons or taphoclades have been able to end up being preserved outside of the limits of tolerance of the originally produced taphonomic elements. Keywords: PRESERVATION POTENTIAL, TAPHONOMIC DURABILITY, FOSSILIZATION POTENTIAL, FOSSILIZATION THEORY, AMMONITES Introduction traces of lower physical resistance, chemical stability or durability. The only entities or Most of the current taphonomic units of taphonomic alteration usually investigated interpretations consider that taphonomic are biological remains and traces, as well as alteration has acted during fossilization thanatocoenosis or taphocoenosis (Efremov, processes like a filter or a sieve, eliminating 1940, 1950) and fossil assemblages. From this a large amount of the originally produced point of view, the alteration cannot produce biological remains and traces, which were new elements, which represent new taphonomic non-preservable or of smaller durability due to groups. Taphonomic alteration cannot generate their composition and structure. The destructive new taphonomic entities. It can only role of taphonomic alteration is currently deteriorate or destroy and retain deteriorated accepted in research, and it is considered parts of the initially produced biological that taphonomic alteration can deteriorate remains and traces. Taphonomic alteration is and eliminate the biological remains and a destructive or negative force, which does Article JTa045. All rights reserved. * E-mail: [email protected] 111 Taphonomic alteration not contribute to preservation, being only taphonomic variability, and the regulation responsible for the destruction of biological of such variability by taphonomic alteration. remains and traces. The destructive action The second of these components may be of the taphonomic alteration or the relative regarded as an extrinsic principle of behaviour of the biological remains and regulation, which is able to fix the direction traces cannot serve as causal argument to of evolutionary taphonomic processes. explain the fossilization mechanisms or Representatives of new taphonomic groups processes. Taphonomic alteration can only appear during fossilization processes. eliminate the non-preservable remains and Components of these taphonomic groups traces, and some other cause should play the show different composition and structure positive role of producing or maintaining from those biogenically produced, and they the preservable remains and traces. From increase diversity of the fossil record. In this point of view, the composition and every fossilization stage, the persisting structure of original biological remains and taphonomic groups (or taphons) will be traces, not the external environment or the those whose preserved elements have been differential behaviour of taphonomic stabilized, transformed, and replicated with entities, play a causal role in the changes a higher effectiveness in the prior stages, happened during fossilization. From a but not the most resistant or those preserved different approach, however, an alternative elements less affected by environmental model to that traditionally used in factors. taphonomy can be developed: the model of Taphonomic alteration deteriorates taphonomic modification and differential and eliminates the taphonomic elements of retention vs. the model of palaeobiological smaller durability, but it is also able to modification and selective destruction. cause favourable modifications for the The purpose of the present work is to preservation and to produce new elements show that taphonomic alteration acts at that represent new taphonomic groups, and different levels of organization and, as a it can be considered as a factor responsible consequence, it is convenient to employ a for the fossilization. Some of the taphonomic systemic and evolutionary approach in alteration processes are of destructive taphonomic analyses and interpretations. effect, and some are of conservative effect Several new concepts of evolutionary (cf. Skelton, 1993: 574). Biological remains taphonomy, such as taphon and taphoclade, and traces, fossils or taphonomic elements, allow to give a non-tautological meaning to are not the only entities or units of the concept of preservability or preservation taphonomic alteration. Alteration affects potential. taphonomic entities at different level of organization, and can generate new entities, no longer limited to act as a sieve or filter Evolutionary taphonomy destroying the taphonomic elements of smaller physical resistance, chemical stability Taphonomic preservation is the result of a or durability. Taphonomic alteration causes process, fossilization, where two favourable modifications for the preservation interrelated components are involved: the and produces new taphonomic entities, biogenic and taphogenic production of besides sieving and retaining modified parts 112 Fernández-López of the initially produced taphonomic inhibit or prevent the preservation, but they elements. From this point of view, it is can also condition, channel, facilitate, considered that taphonomic alteration is a intensify, favour, enhance, activate or promote modifier force, not necessarily negative, the alteration processes and the trends or the that can become a generating force paths of fossilization processes. Taphonomic responsible for the appearance of new alteration causes the change and, as a result, taphonomic elements of different chemical taphonomic entities are preserved. composition, representing new taphonomic groups and new taphons. Taphonomic alteration can be considered as a positive Taphonomic alteration force and, therefore, as the primary cause of changes during fossilization. Functionality Differential destruction of taphonomic and evolution of taphonomic entities also entities is just a particular case of have a relative importance as causal taphonomic alteration that can be argument. From this point of view, the distinguished with the name of taphonomic modifier action of the taphonomic sieve. Other cases of taphonomic alteration alteration, the external environment and the correspond to the differential modification relative behaviour of the taphonomic of taphonomic entities, by causal interaction entities can be causal arguments to explain among the components and the external the mechanisms of fossilization. Not only environment, giving rise to the selective the original architecture (composition and preservation. Changes in differential preservation structure) of the taphonomic entities and of two or more taphonomic entities during their external environments, but also their fossilization depends on both intrinsic and modified architectures, the successive changes extrinsic taphonomic factors. Every taphonomic of the external environment and the functional entity is subjected to the action of physical, or evolutionary activities carried out, have a chemical and biological agents from the causal role in the changes happened during external environment. Any component of fossilization. the external environment able to act directly This second approach is evolutionary on taphonomic elements is an extrinsic or and systemic, allowing to develop a theory environmental taphonomic factor. However, of the fossilization denominated theory of the external environment is not the only taphonomic evolution (Fernández-López, source of taphonomic change or selection. 1982, 1984, 1988, 1989, 1991a,b, 1995, Actual properties of taphonomic entities 1999, 2000). Taphonomic changes, selective (such physical or real properties as their preservation during fossilization and the composition, structure and behaviour with resultant differential preservation occur as a respect to environmental changes) also consequence of taphonomic alteration, as intervene during taphonomic alteration. taphonomic entities are fitted to the changes of Biological remains and traces or, in their external environment. The composition general, taphonomic elements are not the only and structure of taphonomic entities, as well entities or units of alteration. Taphonomic as their functional or evolutionary activities alteration also acts at supra-elementary and the environmental changes, can limit, levels, on entities or units of different