Discovering the first tetrapod footprints from the Early Triassic of Antarctica: a multidisciplinary approach

Rationale
Ichnology is the branch of life sciences that study all the traces left by both extant (neoichnology) and extinct organism (paleoichnology). Within vertebrate ichnology, this discipline includes study of footprints and trackways, but also of burrows, coprolites (fossil dung), eggs and nesting sites. Among these, footprints represent a unique ‘living image’ of the organisms that left them, with opportunities to make inferences about behavior and many other elements of physiology and biology in general of putative trackmakers. In recent decades a new multidisciplinary approach to paleoichnology has shown the enormous inferential power of fossil traces, throwing light on biological and behavioral aspects, not obtainable by simply studying skeletons in the collections. In particular, the study of fossil footprints in numerous vertebrate groups (from basal tetrapods to dinosaurs and synapsids sensu lato) provided novel inferences on the type of locomotion, behavior, and biomechanics, and in various fields of their biology, such as physiology, type of metabolism, and ecological requirements (e.g., Farlow, 1981; Lockley, 1994; Avanzini, 1998; Gierliński et al., 2009; Sacchi et al., 2014; Citton et al., 2015, 2017a; Romano et al., 2016; Romano & Citton, 2017). In the context of behavior, one of the most important inferences drawn from trace fossils relates to the possibility of gregariousness and complex social interrelations in some groups of extinct animals. Parallel trackways, especially of herbivorous dinosaurs (e.g., Matsukawa et al., 1997; Castanera et al., 2012; Lockley et al., 2012), often referable to individuals of different sizes (and thus, presumably, ages), indicate that, as observed in many extant large-sized continental mammals, some clades of dinosaurs lived in groups and moved together as large herds. In in the field of vertebrate ichnology, several analyses permit generalizations about average speed and type of gait in the various studied groups (e.g. Alexander 1976; Farlow 1981; Lockley et al. 1994; Mossman et al. 2003). The attitude of the tail in dinosaurs is confirmed as straight and aligned with the remainder of the spine, since traces of the tail are relatively uncommon in the context of the entire ichnological record (in many classical representations, the tail – especially in sauropods – is shown as resting on the ground and being dragged during locomotion). The study of theropod and ornithopod footprints with substantial metatarsal impression has shed light on particular behaviours in these dinosaurs, on the modalities of resting and crouching and on the peculiar pubic impressions when an animal adopts this posture (e.g. Gierliński et al. 2009; Romano and Citton, 2017). Recently, the application of multidisciplinary analyses (geometric morphometry, differential depth analysis, use of high definition 3D photogrammetric models) to a unique ichnological cave site in Northern Italy (Grotta della Basura, Toriano, Savona), led some of the participants to the here presented project (Marco Romano, Marco Avanzini) to shed new crucial light on the possible social structure of restricted human groups from the Upper Paleolithic (Citton et al., 2017b). The multidisciplinary study of the unique site showed how the integrated study of human
footprints, and track association, preserved in a cavity frequented around 12ka BP, provide highly confidential information about the anatomy and instant behavior of human subjects, on their social inter-relationships and on the mode of exploration of the hypogeum environment. In addition the study confirmed that children were ubiquitous in the daily lives of the Pleistocene people following adults also in non-utilitarian, challenging and risky activities. These few examples show the great potential of a multidisciplinary study of the fossil ichnological record, which, in some respects, represents a young field in vertebrate studies, with a lot of research yet to be done. While Permo-Triassic fossil footprints are well known from Laurasia and part of the Gondwana supercontintents, with several publications dedicated to such ichnological material, tetrapod tracks of that age have been reported once from Antarctica (Macdonald et al. 1991). Very recently, the sensational discovery of the first Triassic tetrapod footprints from the Lower Triassic of Antarctica was made by the multidisciplinary team headed by Prof. Christian Sidor, in the framework of the Antarctica project “Understanding the evolution of high-latitude Permo-Triassic paleoenvironments and their vertebrate communities”. Several trampled levels with numerous footprints were identified and collected from outcrops at Halfmoon Bluff, Collinson Ridge, and McIntyre Promontory, in the field work conducted in December 2017–January 2018. All collected material come from the lower Fremouw Formation in the Shackleton Glacier region. Based on biostratigraphic correlation, these rocks are inferred to be Early Triassic in age, coeval with the Lystrosaurus Assemblage Zone of South Africa’s Karoo Basin. Several stratigraphic sections of high detail were analyzed, measured, and described directly in the field by one of the components of the present project (Prof. Roger
Smith), in order to contextualize the recovered material. Triassic and Jurassic sedimentary rocks crop out on the margin of the polar plateau from northern Victoria Land to the Amundsen Glacier in the central Transantarctic Mountains. Triassic rocks were deposited in two separate and related basins, in terrestrial environments consisting of lakes, rivers and flood plains. Transantarctic Mountains deposits result from the accumulation in a foreland basin setting, connected to thrusting
and folding along the subducting Panthalassan Ocean margin. Victoria Land rocks were deposited in a relatively narrow basin or basins further inland of the Panthalassan margin. The break-up of the Gondwanan supercontinent in the Jurassic rifting led to the accumulation of widespread volcanic deposits and emplacement of intrusive dolerite sills and dikes within the Permian-Triassic strata of the region. The Triassic Fremouw Formation has been divided into lower, middle and upper informal members. The lower Fremouw member returned several vertebrate fossils in many localities, and is represented by channel-formed sandstones and interbedded siltstone and mudstone. From the middle member, dominated by fine-grained components, just bone fragments were recovered. Several localities at the base of the upper member in the
Beardmore Glacier region returned Middle Triassic vertebrate remains. The stratigraphic sequence characterizing the Shackleton Glacier region preserves a rich fossil record of Triassic vertebrates and a unique record of high-latitude environments. The main goal of the project headed by Prof. Christian Sidor is to characterize the Permo-Triassic boundary within Shackleton area strata and correlate it to other stratigraphic successions in the region. The research project is designed to throw new light on Antarctic life during the Permian and Triassic through an interdisciplinary approach; main objectives are to address relationships between environmental change, faunal composition, and biogeographic patterns in the context of the high-latitude strata preserved in the Buckley and Fremouw formations in the Shackleton Glacier region.
In addition to the already ongoing study of body fossil material, the here proposed muldisciplinary analyses of the first fossil footprints from the Triassic of Antarctica, will represent another important step forward in the understanding of a unique highest paleolatitude tetrapod fauna of the entire Triassic, and in general on polar life evolution during the early Mesozoic.

Objectives
The main objectives of the proposed project are a multidisciplinary study and understanding of the first Early Triassic tetrapod footprints form
Antarctica, and the enhancing, education, conservation and dissemination of such unique material to both specialists and to a wider audience.
To fully reach the first objective, the following intermediate goals are planned:
-A detailed sedimentological and taphonomical analysis of the trampled substrates, in order to better contextualize and understand the impressions processes.
-Detailed stratigraphic description of the section that returned the material under study, in order to contextualize the fossil footprints within the geological framework characterizing the lower Fremouw Formation, and, in on a broader scale, the Shackleton Glacier region in Antarctica.
-Classical ichnological study of all the preserved material, in order to have complete picture of the possible trackmakers, including the application of classical formulae to infer the possible size and locomotion speed of the producers.
-Production of high resolution 3D photogrammetric models of all the recognized tracks, which allow, through the study of the differential depth of impression, to infer the possible biomechanics of the trackmaker and complex locomotor cycles in the footprints producers.
-Recognition, through the integrated and comparative study of classical ichnology, 3D photogrammetric models, different depths of impression, and comparison with osteological material preserved in museum collections (also through the use of Principal Component Analyses), of the possible zoological trackmakers who left the footprints.
-Once studied in detail, use of the footprints to better understand faunal diversity in a unique highest paleolatitude tetrapod fauna, and as a biochronological tool for correlations with comparable material from low latitudes.
-Realization, in collaboration with paleoartists, of paleo-environmental reconstructions of the unique high latitudes Early Triassic Antarctic environment, and realization of 3D hyperrealistic in-vivo models of the putative trackmakers; realization of HD digital video illustrating the possible complex cycle of locomotion in footprints producers.

To reach the education, dissemination and storytelling objective, the following goals will be achieved:
-The project will be a great opportunity in the framework of training and mentoring, giving the opportunity to various young students directly involved in the study, to learn the basic techniques ichonology, and the use of new cutting edge multidisciplinary methodologies, including also the new technologies of 3D models acquisition and processing.
-Realization of video and photographic documentation of the various phases of the research, to be used later as a basis for the storytelling of the discovery and scientific research.
-Participation to several national and international congresses to present both the first preliminary data and the final results of the research.
-Organization of press releases to announce the discovery, and dissemination of results through various international media coverage. Among these, the productive and long collaboration of some participants with the National Geographic preferential channel will surely be carried forward (see Education and outreach program section below).