The Phanerozoic four revolutions and evolution of paleosol ichnofacies

GENISE J, BEDATOU E, BELLOSI E, SARZETTI L, SANCHEZ M, KRAUSE M

The Trace Fossil Record of Major Evolutionary Events

Springer. Topics in Geobiology

Lugar: Dordrecht; Año: 2016; p. 301 - 370

Our analysis of a database composed of 166 cases of invertebrate and plant tracefossils in paleosols resulted in the recognition of four major evolutionary steps fromthe fi rst paleosols-bearing rhizoliths preserved in Early Devonian rocks to those ofthe Holocene. Each step constitutes a revolution for paleosol ichnofaunas, whichtriggered the appearance of a new ichnofacies. These events refl ect some of themajor changes in the history of life. Other historical events, such as the Cretaceous?Paleocene event, have no refl ection in paleosol ichnofaunas or alternatively the datais too scarce for detection.The first revolution is associated with the appearance and expansion of vascularplants, and took place in the Early Devonian with the fi rst paleosols showing onlyichnoassemblages composed of rhizoliths, which constitute half the cases of thePaleozoic and are recorded up to the Holocene. The appearance of vascular plantsby the late Silurian resulted in the Early Devonian appearance of several types ofpaleosols (i.e., Histosols, Spodosols, Alfi sols, and Ultisols) linked to the developmentof new ecosystems. These rhizolith assemblages compose an archetypal ichnofacies,the Rhizolith Ichnofacies, which would be indicative of subaerial exposure and the presence of different types of vegetation. Depending on needed and pendingstudies on rooting patterns, in the future this ichnofacies can yield more precise andsignifi cative paleoenvironmental data. Also, the absence of invertebrate trace fossilsmay be a clue to interpret better the paleoenvironment. Other Paleozoic ichnofaunas,composed of scarce invertebrate trace fossils, some of them meniscate, areconsidered herein as representative of the Scoyenia Ichnofacies , reflecting the periodicalsubaerial exposure of the deposits.These ichnoassemblages are the only recorded for the rest of the Paleozoic untilthe second revolution, after the end-Permian mass extinction occured, with theappearance of trace-fossil assemblages dominated by earthworm ( Edaphichnium ,Castrichnus , diffuse boxworks) and crayfi sh ( Camborygma , Loloichnus ,Dagnichnus , Cellicalichnus , Katbergia ) trace fossils. These ichnoassemblages,indicative of paleosols with high fl uctuating water tables and representative of localmarshes, bogs, swamps, and wetlands, may be grouped into a new archetypal ichnofacies:the Camborygma Ichnofacies, which should be defi ned more completelyin the future, when better-documented cases are available.The third revolution occurred in the Late Cretaceous. By that time, the diversification of fl owering plants triggered the diversifi cation of certain soil-inhabitinginsects, such as ants, termites, bees, wasps, and some beetles, which were capableof constructing linings and free-standing walls for their chambers and nests thatconsequently acquired a high preservation potential. By the Late Cretaceous, thefi rst recognizable insect trace fossils in paleosols occurred, either as isolated examples,as part of the Camborygma Ichnofacies, or in a new one: the CelliformaIchnofacies. The end Cretaceous mass extinction shows no global changes inpaleosol ichnofaunas, although the Paleocene record is scarce for a thorough evaluation.In southern South America, the Camborygma Ichnofacies, which was dominantsince the Late Jurassic, dissapeared. In North America the CamborygmaIchnofacies remains dominant in Paleocene?Early Eocene deposits, bearing recognizableinsect trace fossils, resembling those assemblages of the South AmericanLate Cretaceous.The fourth and most important revolution for ichnodiversity took place in themiddle Eocene, infl uenced by the advent and expansion of grass-dominated habitats.Other favorable conditions that promoted this ichnofacies were the early EoceneClimatic Optimum, and the establishment of modern insect groups. These conditionsalso lead to the appearance of a new soil type, the Mollisols, displaying granular orcrumb peds. Trace fossils of cicadas ( Feoichnus , Monesichnus , Naktodemasis ), dungbeetles ( Coprinisphaera , Eatonichnus , Chubutolithes), bees ( Palmiraichnus ,Elipsoideichnus , Uruguay , Corimbatichnus , Rosellichnus ), sphinx moths ( Teisseirei ),ants ( Attaichnus ), termites (Krausichnus ), and cleptoparasites and detritivores(Tombownichnus , Lazaichnus ) appeared or diversifi ed during this revolution that ledto the establishment of the Coprinisphaera Ichnofacies. By the Oligocene, inclosed-forest environments, also appears the fi rst assemblage dominated by termiteand ant trace fossils ( Termitichnus , Fleaglellius , Vondrichnus , Krausichnus ), comprisingthe Termitichnus Ichnofacies. The Coprinisphaera revolution is followedduring the Neogene by stasis that was interrupted by the occasional appearance ofnew trace fossils of the same, previously mentioned groups of insects. In contrastwith older ichnofacies ( Scoyenia and Camborygma ), which are recorded in fl oodedor high water table paleosols, the younger ichnofacies ( Celliforma , Coprinisphaera ,and Termitichnus ) refl ect paleosols with lower water tables. It is as if the terrestrialichnofaunas would have shift to drier environments, favored by morphological andbehavioral adaptations of their producers.The ichnofacies model for paleosols is an ongoing approach that still has weakpoints, all of which arise in the present incompleteness of the trace-fossil record.For developing a stronger foundation, additional documentation will be required,and surely it will be a future task of ichnologists to complete and strengthen themodels. There are about 166 case studies (25 for the Paleozoic, 39 for the Mesozoic,and 102 for the Cenozoic), many of which are understudied or involve few tracefossils that presently are insuffi cient to resolve the entire scenario. Despite this, themodel is a very promising one, and currently allows us to recognize within the fogof incompleteness, the silhouettes of fi ve paleosol ichnofacies, each one including anumber of ichnoassemblages that show sharply or broadly defi ned paleoenvironmentalsignifi cance. After the novel approach presented in this chapter, the paleosolichnofacies model also provides an evolutionary scenario to track the major changesin the history of terrestrial life.