RHIZOLITHS: BODY FOSSILS, TRACE FOSSILS OR BOTH?

MELCHOR, RICARDO N.; ESPINOZA, NAHUEL; MANUEL ROJAS MANRÍQUEZ

Florianópolis

Congreso; ICHNIA 2024 - The 5th International Congress on Ichnology; 2024

ICHNIA

Root system development produce bioturbation structures at all scales resulting from thegrowth and development of the plant anchoring system. There is an intrinsic difficulty indistinguishing the resulting structure as a body fossil or a trace fossil, in some cases complicatedby diagenetic modifications, that can be produced during root development or postdate it. Verylittle is known about rhizolith or root trace [1] formation and preservation, which can giveimportant insights in their nature and significance. Plant roots move and compress soil particlesjust as animal producers do thus producing bioturbation [2]. This statement implies that,independent of the preservation type (cast, molds) or diagenetic modifications (concretionary ordiffuse cementation, and/or color change), and the scale of modifications (rootlets or tree root),root generated structures are trace fossils. A particular case can be made for specimens preservingorganic matter including fully carbonized and/or mineralized plant tissues for considering thembody fossils [2], but bioturbation is present, even in this case. We are referring to the samebiogenic sedimentary structure, even if the body of the producer is preserved (e.g., coalified rootcast) or not. It is herein proposed that except those root structures that preserve a substantialamount of any form of preserved organic matter, which are considered body fossils, all other rootgeneratedstructures be considered trace fossils. In consequence, there is no need to demonstratea formation process in several steps (e.g., [2]) to be considered a trace fossil. We also advocatethe use of rhizolith, a general descriptive term for all these structures, deeply entrenched in theichnologic and sedimentologic literature. Diagenetic processes can affect any type of trace fossil,but this is not necessarily considered a factor that change the status as an ichnofossil. Cementationin root traces is commonly linked to interactions among the root, the soil, and microorganisms inthe rhizosphere. This process can even start during the life of the plant [3] or very close to thedeath of the plant (Fig. 1). In nonmarine settings, the presence of cementation is a feature thatdistinguishes root traces from animal burrows (that commonly are devoid of cementation). Inaddition to the architectural pattern, the composition of the root cast or cement of root traces isalso very useful in the paleocological and paleoenvironmental interpretation of these biogenicsedimentary structures, as they commonly result from early diagenetic processes. Another aspectraised by a recent proposal [2], suggest that root systems do not develop a recurrent morphologicalpattern, instead they result in indistinct blurring of existing stratification. We disagree with thisconclusion. As recognized from the botanical literature [4] and observed in fossil examples, rootsystems do have different architectural patterns resulting from soil hydrological constraints andthose typical of the plant types. Examples includes adventitious root traces (Fig. 2A-B), stronglybranched subvertical (Fig. 2D) and tap root traces (Fig. 2C). Fossil root systems can also producea modification of the primary sedimentary structure, thus resulting in an ichnofabric whereidentification of discrete structures can be difficult, as with many other ichnofabrics. To conclude,it is proposed that all root-generated structures be treated as trace fossils except those that containa significant proportion of preserved organic matter, which are considered body fossils. We alsoencourage the use of binomial nomenclature to name morphologically distinctive and repetitiveroot trace fossils.