Eduardo S. Bellosi

An approach to the description and interpretation of ichnofabrics in palaeosols

Abstract Studies on ichnofabrics have focused mainly on marine environments. Attempts to apply the ichnofabric methodology and theoretical framework to continental deposits bearing palaeosols are few and poorly developed. Methodologies analysed in this contribution include the applicability of current ichnofabric indexes and diagrams, the assessment of the destruction of the original bedding by ichnofabrics and by other soil characters separately, and the relationships between different stages of palaeosol and ichnofabric development. Many soil features may be formed without the intervention of bioturbation, or may be the result of interactions of physical, chemical and biological processes, in which traces of organisms may have only a subsidiary role. Ichnofabrics can be well developed in palaeosols devoid of other soil characters and, conversely, palaeosols showing a well-developed soil structure can bear almost no trace fossils. This fact adds a third component to classical methods that normally consider only original bedding and ichnofabrics. Theoretical analysis includes the possibility of recording composite ichnofabrics in palaeosols, and the value of individual ichnotaxa as possible indicators of subaerial conditions and environmental changes. The ecological preferences and requirements of trace-makers provide the key to understanding composite ichnofabrics; however, only complex traces can be certainly attributed to particular modern taxa. Insect nests, pupal chambers and earthworm burrows are the most reliable indicators of subaerial exposure and, in many cases, very particular environmental conditions.

Fossil bee nests, coleopteran pupal chambers and tuffaceous paleosols from the Late Cretaceous Laguna Palacios Formation, Central Patagonia (Argentina)

Abstract The Late Cretaceous Laguna Palacios Formation in Central Patagonia (San Jorge Basin), southern South America, is composed of tuffaceous deposits supplied by periodical volcanic ash falls partly reworked by rivers, on broad plains. Variations in ash-fall rates allowed the formation of stacked, mature paleosols, which are one of the most characteristic features of this formation. The mature paleosols show well-developed horizons, ped structure and bear an intricate network of trace fossils mostly produced by small roots and invertebrates. Two different insect trace fossils could be recognized in this formation: sweat bee nests and coleopteran pupal chambers. Fossil bee nests are composed of inclined tunnels with cells attached to them by means of short necks, a typical construction of bees of the subfamily Halictinae. Similar halictine constructions were reported from the Cretaceous of the USA. Coleopteran pupal chambers are discrete, ovoid structures, having an internal cavity with a smooth surface, and an outer wall of lumpy appearance composed of different layers of soil material. They are commonly constructed by the larvae of different families of Coleoptera. Similar trace fossils were previously reported from the Asencio Formation (Late Cretaceous–Early Tertiary) of Uruguay and from the Djadokhta Formation of Mongolia (Late Cretaceous). These trace fossils constitute some of the only paleontological data from the Laguna Palacios Formation, allowing inferences about its paleoecology, paleoclimatology and paleogeography. Ecological preferences of Halictinae, as well as some features of the nests, suggest a temperate, seasonal climate and an environment dominated by low vegetation for the Laguna Palacios Formation, which is also compatible with sedimentologic and pedogenic evidence. The morphology of the nests, typical of North American halictinae, adds more evidence to the hypothesis of the existence of faunal interchange between North and South America by the Late Cretaceous. The fossil nests constitute some of the oldest evidence of bees in the fossil record, the third known record of bees of Cretaceous age and the first for the Southern Hemisphere. The two traces described are, together with those of Dakota and the Gobi Desert, the only trace fossils from paleosols of Cretaceous age that can be certainly attributed to insects.

Comment—Advanced Early Jurassic Termite (Insecta: Isoptera) Nests: Evidence from the Clarens Formation in the Tuli Basin, Southern Africa (Bordy et al., 2004)

“The characteristics of termite nests are better understood if the main requirements of these insects are kept in mind (Noirot, 1970, p. 102). More than the gross architecture, some details may be significant…A very precise description of the chambers and galleries would be necessary…” (Noirot, 1977, p. 179) Insect paleoichnology, being a young discipline, needs to gain acceptance and recognition as a sister counterpart of paleoentomology and entomology. Accordingly, the analyses and discussions to ascertain the attribution of continental trace fossils to insects should be carried out very carefully. Such analysis (e.g., Machado, 1983; Sands, 1987) for fossil termite nests, which is lacking in the recent contribution by Bordy et al. (2004) on supposed Jurassic termite nests, is critical, because their results are at odds with our previous knowledge of the evolutionary history of termites and their relationship with coevolving groups of plants and fungi. Many invertebrate trace fossils are more preservable than their constructors are. For example, fossil bee nests predate the oldest known bees by about 25 My (Elliot and Nations, 1998; Genise, 2000; Engel, 2000), which is an expected gap. In contrast, the gap between the oldest termites, which come from the Lower Cretaceous (Jarzembowski, 1981; Martinez-Delclos and Martinell, 1995), and the supposed termite nests described by Bordy et al. (2004) would be about 60 My. However, it is neither the time involved nor the difficulties of imagining fungus-growing termites in an early Jurassic environment deprived of Basidiomycotina and grasses that promoted this comment. Instead, it is the understanding that the description and affinities of the Tuli structures are not treated with the necessary detail and their termitic origin was not demonstrated. Termite nests comprise closed and dynamic systems, largely isolated from the external environment, within which the microclimate can be …

Chapter 7 Invertebrate and Vertebrate Trace Fossils from Continental Carbonates

This chapter presents a review of trace-fossil assemblages from carbonate sediments deposited in continental settings, including carbonate-rich paleosols, lacustrine and eolian carbonates, and examples from travertine and tufas. Carbonate-rich paleosols from the Paleozoic are scarce and trace fossils are mostly only briefly described; Mesozoic examples are more abundant and better studied, although associations of trace fossils cannot now be included in the ichnofacies model because of a lack of recurrence. The Cenozoic shows the largest and best-studied cases of trace fossils in carbonate-rich paleosols. Two Seilacherian ichnofacies can be recognized: the Coprinisphaera ichnofacies, which occurs in paleosols, probably with a relatively lower carbonate content, and the Celliforma ichnofacies, which occurs in deposits with a higher carbonate content. The latter, developed in calcretes and mostly in palustrine carbonates, displays a recurrent association of insect trace fossils dominated by bee and wasp ichnofossils, particularly Celliforma ichnospecies and Rebuffoichnus sciuttoi, associated with freshwater and terrestrial gastropods and hackberry endocarps. Seven cases and three additional possible examples of these assemblages are reviewed in detail. They range in age from Late Cretaceous to Miocene and are known from South and North America, Europe, and Africa. These examples are used to further support an incipient Celliforma ichnofacies. When occurring in calcretes, the Celliforma ichnofacies would be indicative of drier paleoenvironments, such as scrubs to woodlands, than those represented by the Coprinisphaera ichnofacies. When occurring in palustrine carbonates, it would reflect the presence of bare soils exposed after periodic waterlogging under subhumid to sub-arid climates. The examples of trace-fossil assemblages from lacustrine carbonates are more numerous (39 cases cited in this chapter) as well as more varied. These examples can be subdivided into assemblages from carbonate-dominated, mixed carbonate/siliciclastic, and evaporite lacustrine basins. The Mermia ichnofacies is represented by a few cases of subaqueous trace-fossil assemblages. Carbonate microbial/caddisfly mounds constitute a distinctive biogenic structure from carbonate-dominated and wave-agitated littoral lacustrine facies, although no ichnofacies assignment is possible to date. Some cases of intermittently exposed and submerged shallow-lacustrine deposits, where the subaerial exposure was not enough to allow insects to nest, can be ascribed to the Scoyenia ichnofacies. Many trace-fossil assemblages contain tetrapod tracks and invertebrate traces in shallow-lacustrine facies that were frequently exposed and submerged. Some of the Mesozoic examples have been attributed to the Brontopodus ichnofacies, whereas the Cenozoic examples (containing shorebird and mammal footprints) have been regarded as representing the “shorebird ichnofacies” or Grallator ichnofacies. A case assignable to the later ichnofacies, from the Triassic Ischichuca Formation (Argentina), is discussed in more detail. In addition, a few examples of trace-fossil assemblages from Late Cenozoic carbonate eolianites are summarized. One of these examples was tentatively considered as representative of the Psilonichnus ichnofacies.

The Phanerozoic Four Revolutions and Evolution of Paleosol Ichnofacies

The analysis of a database composed of 166 cases of invertebrate and root trace fossils in paleosols allowed us to recognize four major evolutionary steps in the Phanerozoic evolution of paleosol ichnofaunas. Each step constitutes a revolution that is reflected in the appearance of a new ichnofacies. The emergence of the first vascular plants during the late Silurian–Early Devonian produced the most significant change in soil evolution, the appearance of rooted Histosols, Spodosols, Alfisols, Ultisols, and forest Oxisols through the Devonian and Carboniferous. The first revolution, then, occurred in the Early Devonian with these first paleosols that exhibit ichnoassemblages composed of rhizoliths. Paleosols bearing only rhizoliths constitute half of the cases in the Paleozoic and are recorded through the whole Phanerozoic. These cases may compose an archetypal ichnofacies termed the Rhizolith Ichnofacies. The Rhizolith Ichnofacies would be indicative of subaerial exposure and depending on needed studies on root morphology, probably would yield more precise and significative data on paleoenvironment and vegetation in the near future. Other cases of Paleozoic ichnoassemblages can be included in the Scoyenia Ichnofacies. The second revolution took place after the end-Permian mass extinction. It is characterized by the appearance of trace fossil assemblages that include or are dominated by earthworm and crayfish trace fossils. These ichnoassemblages are grouped in a new archetypal ichnofacies, the Camborygma Ichnofacies. The second revolution was followed by stasis that ended during the Cretaceous, when the third revolution occurred. By that time, the appearance and diversification of flowering plants triggered the diversification of groups of soil-inhabiting insects, such as ants, termites, bees, wasps, and certain beetles, which were capable of constructing linings and free-standing walls for their chambers and nests that consequently acquired a high potential of preservation. By the Late Cretaceous, the first recognizable insect trace fossils in paleosols appear as isolated examples, integrating the Camborygma Ichnofacies, or composing a new one: the Celliforma Ichnofacies. The fourth and most diverse revolution took place in the Middle Eocene, with the advent of grass-dominated habitats during the long-term cooling-drying period after the EECO, and the full establishment of all groups of modern insects, including the ball-making dung beetles. The spread of grasses produced the last step in soil evolution, that is, the appearance of soils with fine granular peds that derived in Mollisols. Traces of cicadas, dung beetles, bees, sphinx moths, ants, termites, cleptoparasites, and detritivores appear or diversify during this revolution composing the Coprinisphaera Ichnofacies. By the Oligocene, in closed-forest environments, the first assemblage dominated by termite trace fossils is recorded, composing the Termitichnus Ichnofacies. This last revolution is followed during the Neogene by a stasis only interrupted by the occasional appearance of new trace fossils from the same groups of insects.

New age constraints for early Paleogene strata of central Patagonia, Argentina: Implications for the timing of South American Land Mammal Ages

The Rio Chico Group in the San Jorge Basin of central Patagonia (Argentina) preserves some of South America’s most significant Paleogene records of biotic and climatic change. Three of its constituent formations, the Penas Coloradas, Las Flores, and Koluel-Kaike, host vertebrate faunas referred to the “ Carodnia faunal zone,” the Itaboraian South American Land Mammal Age (SALMA), and the Riochican SALMA. However, the precise absolute ages of these units, and thus their associated faunas and paleoclimate records, are poorly resolved. Herein, we report new paleomagnetic and geochronologic results from these formations in south-central Chubut Province, Argentina. U-Pb dating of four volcanic ashes, using both laser ablation−multicollector−inductively coupled plasma−mass spectrometry and high-resolution chemical abrasion−isotope dilution−thermal ionization mass spectrometry, indicates ages of igneous crystallization of 51.403 ± 0.037 (0.045) [0.071] Ma for a level within the middle Las Flores Formation and 46.714 ± 0.012 (0.026) [0.057] Ma, 44.579 ± 0.013 (0.026) [0.054] Ma, and 42.856 ± 0.017 (0.027) [0.054] Ma for levels in the lower, middle, and upper Koluel-Kaike Formation, respectively. Combining these with previous isotopic ages in our new magnetostratigraphic framework, we correlate the Penas Coloradas Formation to chrons C27n-26r (ca. 62.5 to ca. 61.6 Ma; late Danian) and the section from the middle Las Flores to the uppermost Koluel-Kaike to chrons C23n to C19r (ca. 51.4−42.2 Ma; mid Ypresian−late Lutetian). We combine these data with other recently published chronostratigraphic results from Paleogene units in Patagonia to better constrain the ages of noteworthy Paleogene plant and mammal fossil sites in Patagonia and to develop a revised temporal calibration of the Las Flores, Vacan, and “Sapoan” faunas.

Paleogene laterites bearing the highest insect ichnodiversity in paleosols

Research on laterites deals mostly with geomorphological and geochemical characterization of extant tropical systems, with few examples of sedimentologic and paleosol analyses of mid-latitude detrital records. Extra-tropical, Lower Eocene laterites from Uruguay include Fe-rich indurated horizons (duricrusts) that preserve paleosol features, and nodular beds. Discrete trace fossils are generally absent in laterites, despite that they are formed in tropical ecosystems with high biodiversity. Uruguayan laterites bear abundant, highly diverse, and superbly preserved insect trace fossils. Association, distribution, orientation, and abundance of trace fossils preserved in strongly developed paleosols (Ultisols), are key for reconstructing cyclic depositional and weathering processes governed by hydrology and tectonics. Each cycle lasted ∼420 ka and included iterative stages of fluvial sedimentation in a pericratonic scenario, pedogenesis and insect nesting and pupation in a seasonal tropical savanna, duricrust formation by desiccation and interruption of insect activities, and duricrust disaggregation (nodular beds) when rainfall increased. The proposed cycle can be used as a heuristic tool in the study of laterites.

LOWER PALEOGENE COMPLEX ANT NESTS FROM ARGENTINA: EVIDENCE FOR EARLY POLYDOMY IN ANTS?

Abstract: Complex ant nests from the lower Paleogene Maíz Gordo Formation of Salta (northwestern Argentina), included in the new ichnospecies Krausichnus sisi (Krausichnidae), are composed of columns of up to 1 m high and 160 flat, stacked chambers which are connected by vertical, straight, and parallel shafts. Columns are interconnected by lateral extensions of chambers, which are recognizable by their dark linings. Columns of densely packed flat chambers are similar to extant nests of Monomorium (Myrmicinae), which are postulated as the potential producers. Independent entrances and strong enlargements of columns by addition of chambers suggest that these functioned as individual, yet interconnected, nests. Such systems are compatible with polydomy, which is defined as the arrangement of an ant colony in at least two spatially separated nests. Krausichnus sisi traces polydomy in ants to the early Paleogene of South America in warm and semiarid fluvial valleys with calcareous sandy soils and dry woodland vegetation. These nests provide the oldest record of this behavior in ants and are a rare case for study of the underground architecture of polydomous nests.