New Record of the Ichnogenus Taotieichnus for the Pleistocene of South America

MÉNDEZ, CECILIA R.; LUNA, CARLOS A.; BUATOIS, LUIS; VEZZOSI, RAÚL; ZURITA, ALFREDO E.

Congreso; 5th International Congress on Ichnology; 2024

Bones constitute an important substrate in which various organisms produce modifications as a result of different activities (e.g. feeding, locomotion, attachment). Of the numerous ichnotaxa described to date, only about 8% correspond to bone substrates, the majority of which are attributed to different groups of invertebrates [¹], particularly insects, specifically representatives from various orders such as Dermestidae, Tenebrionidae, Calliphoridae, Tineidae, and Termitidae [²]. Within the fossil record, these modifications are relatively rare but hold significant ecological value, as they enable the characterization of environmental conditions and the behaviour of insects associated with vertebrate remains [³].Except for a few cases, the producer is not preserved in association with its trace fossil, complicating the task of identifying the tracemaker. This is further complicated by the fact that similar traces may be produced by different organisms [4]. Therefore, determining the identity of the producer requires careful comparisons with modern examples [5].Given the diversity of ecological niches occupied by these invertebrates as bioeroding agents, [6] Parkinson (2016) argued that accurately identifying tracemakers in the fossil record is crucial for understanding aspects such as seasonality and microenvironmental/climatic conditions in ancient ecosystems, thereby enhancing taphonomic reconstructions. These aspects have received less attention compared to the applications of ichnology in paleoenvironmental reconstructions [7]. On the other hand, it is equally important to distinguish and correctly interpret post-mortem modification processes and agents of vertebrate remains to obtain a comprehensive understanding of the taphonomic overprints in the fossil record [8]. In the context of vertebrate fossil remains, the analysis of bone modifications allows us to infer specific stages of the decomposition process.This study analyses and compares trace fossils recorded on an undetermined Pleistocene skeletal specimen of a terrestrial vertebrate deposited in the Paleontological Collection of the "Florentino Ameghino" Provincial Museum of Natural Sciences in the city of Santa Fe, Argentina. The specimen, deposited as MFA-G-PV1164, originates from the banks of the Salado del Norte River near the town of Emilia (Santa Fe Province). The material comes from deposits composed of consolidated silt. Deposits are massive to laminated at the base, with an increased concentration of clays towards the upper interval. The upper part of this deposit is structured in the form of horizontal sheets with sub-horizontal carbonate plates and vertical partitions formed by rhizoconcretions, along with crack fillings with secondary carbonates [9]. Absolute ages for these deposits range from 46.5 ±3.2 ka to 25.8 ±2.2 ka BP, a period characterized by humid conditions [10]. The bone material exhibits surface cracking, a result of prolonged exposure to weathering agents. The identified bioerosion trace fossils comprise galleries with a semi-circular to elliptical cross-section, randomly distributed over a large part of the bone surface and locally penetrating inside. Bioglyphs arranged transversely to the gallery axis can be observed on the walls. The average diameter of the galleries is 3.4 mm, reaching a length of 9.72 mm in one case. The morphology of the analyzed trace fossils is like those previously recorded in the Pleistocene Río Bermejo Formation (20 ka) in Formosa Province [11], attributed to Taotieichnus [12], with the exception that they do not exhibit the typical Y-shaped branching described for this ichnogenus and have a higher areal density, covering almost the entire surface. Regarding the average diameter, it is slightly larger than that obtained for the traces fossils from Formosa (3.23 mm) (Figure 1). Concerning the original measurements by [12] Xing et al. (2013) for materials from the Luafeng Formation (Lower Jurassic) in China, unfortunately, a proper comparison is not possible as they only reported minimum and maximum values of the observed diameters. However, the measurements obtained in both Pleistocene materials from Argentina mostly fall within the values reported by the mentioned authors. On the other hand, the diameter obtained by [13] Paz (2022) for similar galleries recorded in the Santa María Supergroup (Upper Triassic) in Rio Grande do Sul (Brazil) is significantly smaller than those mentioned earlier. However, since only one average is mentioned, this could represent a bias.In summary, the morphology, diameter, and arrangement of galleries and perforations on the associated bone substrate from the Pleistocene of the province of Santa Fe are very similar to those assigned to the ichnogenus Taotieichnus in various materials ranging from the Late Triassic to the Late Pleistocene. There are some differences, such as the branching type, which [13] Paz (2022) has used to propose a possible new ichnospecies for the Upper Triassic of Brazil. However, given the scarcity of materials, these features are likely not of taxonomic value but simple variations.The studied trace fossils exhibit a marked morphological similarity to subaerial feeding galleries constructed by subterranean termites. Specifically, this behaviour (possibly seasonal) of utilizing decomposing vertebrate carcasses as a nutrient source and constructing galleries is a known but poorly documented feeding supplement in some species of neotropical xylophagous termites. In this regard, [14] Thorne and Kimsey (1983) argued that the nitrogen demand of termites is not easily satisfied with a cellulose-based diet, and consequently, some species increase their nitrogen supply through physiological or behavioural adjustments. Considering this, the galleries that locally penetrate the spongy bone would indicate that the bones served as a food source, implying that they were not manipulated casually or simply as passageways. Observations indicate that perforations and exploration into the medullary spaces took place within a gallery. Ultimately, if microenvironmental conditions allow, the entire bone covered by galleries is destroyed.