Middle Triassic (Ladinian) amphibian tracks from the Lower Keuper succession of southern Germany: Implications for temnospondyl locomotion and track preservation
Introduction
Tetrapod tracks are abundant fossil components of Triassic terrestrial and coastal deposits worldwide (Klein and Lucas, 2010a). Such ichnofossils are useful for the reconstruction of past ecosystems, as they provide direct evidence of the habitat of the potential trackmakers (Melchor, 2015). In this regard, tetrapod ichnofaunas are often considered a reflection of the tetrapod faunas themselves. However, this is not the case in the Triassic temnospondyl amphibian record because a direct correlation between skeletal and footprint data is problematic. The bone record shows that temnospondyls were common in Triassic ecosystems, where they acquired gigantic sizes (Schoch, 1999) and occupied a wide range of (often aquatic) environments (Schoch and Milner, 2000; Warren, 2000; Fortuny et al., 2016). By contrast, temnospondyl tracks are extremely scarce within the Triassic fossil record (Klein and Lucas, 2010a, Klein and Lucas, 2010b; Marsicano et al., 2014). Until recently, the Triassic ichnogenus Capitosauroides was attributed to amphibian trackmakers (e.g., Haubold, 1971a, Haubold, 1971b). However, the revisions of Capitosauroides by Marchetti et al., 2019a, Marchetti et al., 2019b now suggest therapsids as potential trackmakers. Other evidences of Triassic amphibian tracks rely on sparse and sometimes poorly preserved ichnites that deserve further analyses (Peabody, 1948; Klein and Lucas, 2010b; Marsicano et al., 2014). So far, only one ichnotaxon from the Upper Triassic of Lesotho has been unambiguously attributed to a temnospondyl producer (Marsicano et al., 2014). Therefore, perhaps due to a preservational and/or environmental bias, the osteological fossil richness of temnospondyl amphibians is not reflected in the ichnological record.
Herein we report tetrapod ichnites recovered from the Middle Triassic Lower Keuper succession of the Vellberg Fossil-Lagerstätte, southern Germany. The shape of the tracks, which consist in tetradactyl manus impressions, suggests temnospondyl amphibians as the most likely trackmakers. These ichnofossils widen the palaeontological record of the already rich Middle Triassic Lower Keuper of Germany (Schoch and Seegis, 2016). The different modes of impression and preservation of the ichnites reveal palaeoenvironmental and palaeoecological affinities of the potential trackmakers, and shed light on the notable bias between osteological and ichnological remains of temnospondyls. Furthermore, these ichnites widen the picture of the Triassic tetrapod track record worldwide, as they represent the earliest unambiguous evidence of temnospondyl tracks from the Triassic.
Section snippets
Geological setting
During Middle Triassic time, the Central European Basin formed a vast but shallow depression with a very subdued relief, which was readily and frequently affected even by minor sea level changes (Beutler et al., 1999). After the regression of the Muschelkalk epicontinental sea (mostly represented by carbonate deposits), the Lower Keuper was deposited in a deltaic setting (Nitsch, 2015). In the study area, which is located in the southern sub-basin, lakes and lagoons of various sizes alternated
Material and methods
The track-bearing surface was found in spring of 2019 during palaeontological prospecting at the Eschenau Quarry (Vellberg municipality, Baden-Württemberg, Germany) (Fig. 1). Footprints were discovered in the lowermost layer of a fallen block from the lower half of the Anthrakonitbank bed (block dimensions: ~90 cm each side, ~35–40 cm thick) (Figs. 1B, 2B). The layer was manually detached from the fallen block. It is composed of fragmented but well-connected pieces and preserves the footprints
Sedimentology of the track-bearing layer
The deposits below the track-bearing layer, corresponding to the Sandige Pflanzenschiefer, consist of greyish and laminated marly siltstones and fine-grained sandstones. The deposits overlying the track-bearing layer, the Anthrakonitbank unit, correspond to layered dolomites (Figs. 1B, 2). Layers of the Anthrakonitbank unit are evidenced as an alternation of greyish (in fresh fracture) micrite and medium- to coarse-grained deposits building up cyclic sequences (Fig. 2A, B). Such layering or
Temnospondyl amphibian tracks
Fig. 3, Fig. 4, Fig. 5, Fig. 6; Table 1
Palaeoenvironment, preservation and taphonomy
The sedimentological analyses of the track-bearing layer, together with the preservation state of the ichnites and the whole stratigraphic framework, from the Sandige Pflanzenschiefer to the Anthrakonitbank (Fig. 1B), allow to decipher the palaeoenvironmental setting and the taphonomic processes of the uncovered tetrapod footprints. The carbonate sandstone intervals with abundant quartz and bone fragments at the base of each cycle of the track-bearing layer (Fig. 2C, D, E, G) are interpreted as
Conclusions
Temnospondyl amphibian tracks are rare components of the Triassic fossil record. This forms a strong contrast to both the rich record of other tetrapod footprints and the relatively abundant temnospondyl bone remains of this period. Here we report an exceptional record of diagnostic amphibian tracks from the upper Middle Triassic (Ladinian) of Vellberg (southern Germany). They were recovered from the base of the Anthrakonitbank, within the Lower Keuper succession, being the first tetrapod
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We thank Norbert Adorf and Isabell Rosin (SMNS) for invaluable help during fieldwork, and for the preparation of the track-bearing layer. Christoph Wimmer-Pfeil (SMNS) is thanked for the skillful preparation of thin sections. We thank Erin Maxwell (SMNS) for fruitful discussions and logistics, and Michael Rasser (SMNS) to help in scanning the thin sections. We thank the Schumann Quarry owners for allowing access to the outcrops. We acknowledge the reviewers Lorenzo Marchetti and Hendrik Klein,
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2022, Palaeogeography, Palaeoclimatology, PalaeoecologyCitation Excerpt :In this regard, the mixed terrestrial-shallow marine Lower Keuper facies from southwestern Germany are particularly informative for the understanding of Middle Triassic continental ecosystems and the identification of potential biases in the preservation of vertebrates. In this region, a range of fossil lagerstätten are known for their abundance of fossils and quality of preservation (Wild, 1980; Schoch, 2002; Hagdorn et al., 2015; Schoch and Seegis, 2016; Schoch et al., 2018; Mujal and Schoch, 2020; Mujal et al., 2022). Among these, the Kupferzell locality was the first to be discovered and also yielded the largest quantity of vertebrate fossils collected in a single period.
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2021, Palaeogeography, Palaeoclimatology, PalaeoecologyCitation Excerpt :The potential trackmaker of Chelonipus is traditionally referred to turtles (Lichtig et al., 2018). Nevertheless, other potential producers, such as temnospondyl amphibians (e.g., Mujal and Schoch, 2020) or other unknown producers cannot be discarded. The most abundant morphotype of the middle Muschelkalk is Rhynchosauroides Maidwell, 1911 (Figs. 3G, H, 5D, E, F, S4, S5).