Abstract
The Karoo igneous rocks represent one of the largest continental flood basalt events (by volume) on Earth, and are not normally associated with fossils remains. However, these Pliensbachian–Toarcian lava flows contain sandstone interbeds that are particularly common in the lower part of the volcanic succession and are occasionally fossiliferous. On a sandstone interbed in the northern main Karoo Basin, we discovered twenty-five tridactyl and tetradactyl vertebrate tracks comprising five trackways. The tracks are preserved among desiccation cracks and low-amplitude, asymmetrical ripple marks, implying deposition in low energy, shallow, ephemeral water currents. Based on footprint lengths of 2–14 cm and trackway patterns, the trackmakers were both bipedal and quadrupedal animals of assorted sizes with walking and running gaits. We describe the larger tridactyl tracks as “grallatorid” and attribute them to bipedal theropod dinosaurs, like Coelophysis, a genus common in the Early Jurassic of southern Africa. The smallest tracks are tentatively interpreted as Brasilichnium-like tracks, which are linked to synapsid trackmakers, a common attribution of similar tracks from the Lower to Middle Jurassic record of southern and southwestern Gondwana. The trackway of an intermediate-sized quadruped reveals strong similarities in morphometric parameters to a post-Karoo Zimbabwean trackway from Chewore. These trackways are classified here as a new ichnogenus attributable to small ornithischian dinosaurs as yet without a body fossil record in southern Africa. These tracks not only suggest that dinosaurs and therapsids survived the onset of the Drakensberg volcanism, but also that theropods, ornithischians and synapsids were among the last vertebrates that inhabited the main Karoo Basin some 183 Ma ago. Although these vertebrates survived the first Karoo volcanic eruptions, their rapidly dwindling habitat was turned into a land of fire as it was covered by the outpouring lavas during one of the most dramatic geological episodes in southern Africa.
Highlights
The main Karoo Basin of southern Africa (Fig 1) is an excellent study area for land-based manifestations of several Palaeozoic and Mesozoic mass extinction events
From left to right (Trackway): each trackway is labelled with a letter (e.g., B, D, E). (Footprint No.): number of pes mark in the trackway. (FL): pes length. (FW): pes width; (FL/FW) foot length to width ratio; (PP): pes pace; (PS): pes stride; (P ANG): Pace Angulation for pes; (PR): pes rotation, which is the orientation of the longitudinal axis of the pes respect to the trackway midline; rotation is expressed in qualitative terms: 0 = no rotation, 1 = inward rotation, -1 = outward rotation of pes; (Digits Y/N): presence or absence of digit marks on the footprint; (No of P digits): number of visible digit marks on the foot mark; (PTW): pes trackway width. : incomplete track measurement
Our study specially demonstrates that the: 1. Track-bearing Pliensbachian–Toarcian sandstone interbeds of the Karoo continental flood basalts at Highlands were deposited in a moderately wet, seasonally dry palaeo-climate that was relatively more humid compared to that of the underlying Clarens Formation, a mostly aeolian succession
Summary
The main Karoo Basin of southern Africa (Fig 1) is an excellent study area for land-based manifestations of several Palaeozoic and Mesozoic mass extinction events. Being one of the largest igneous provinces on Earth, the Karoo-Ferrar Large Igneous Province extended from the Karoo, across Antarctica to South Australia with a total length of > 5000 km in the Early Jurassic, and its rock record is best exposed in the main Karoo Basin Associated with these large igneous events, major possible environmental perturbations for life were the increase in atmospheric CO2 and other gases from volcanic degassing and wildfires causing warming, volcanic dust causing dimming of daylight, volcanic winters, food chain collapse, sea level fluctuations, methane hydrate release and perturbations in the global biogeochemical cycles leading to oceanic anoxia (e.g., [11, 13–31]). The uncertainty in determining the controlling factors and their relative contributions to the Toarcian biotic turnover on land is associated with: (a) the low number of complete and well-dated Lower Jurassic continental sections, (b) difficulties in correlating the results of the different stratigraphic studies (e.g., biostratigraphy, isotope stratigraphy, magnetostratigraphy) on globally dispersed sections, and (c) lack of modern equivalents for massive continental scale volcanic events (e.g., [31, 32])
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