Darting towards Storm Shelter: A minute dinosaur trackway from southern Africa

Emese M. Bordy

doi:10.17159/sajs.2021/9145

Abstract

Theropod dinosaurs are considered the main terrestrial carnivores in the Jurassic and Cretaceous. Their rise to dominance has been linked to, among others, body size changes in their early history, especially across the Triassic–Jurassic boundary. However, to qualitatively assess such temporal trends, robust skeletal and trace fossil data sets are needed globally. The richly fossiliferous southern African continental rock record in the main Karoo Basin offers an unparalleled perspective for such investigations. Herein, by documenting a newly discovered Early Jurassic trackway of very small, functionally tridactyl tracks near Storm Shelter (Eastern Cape) in South Africa, the track record can be expanded. Based on ichnological measurements at the ichnosite and digital 3D models, the footprint dimensions (length, width, splay), locomotor parameters (step length, stride, speed), and body size estimates of the trackmaker are presented. In comparison to other similar tracks, these footprints are not only the smallest Grallator-like tracks in the Clarens Formation, but also the most elongated dinosaur footprints in southern Africa to date. The tracks also show that the small-bodied bipedal trackmaker dashed across the wet sediment surface at an estimated running speed of ~12.5 km/h. During the dash, either as a predator or as a prey, the trackmaker’s small feet sunk hallux-deep into the sediment. The tracking surface is overgrown by fossilised microbial mats, which likely enhanced the footprint preservation. Based on track morphometrics and the regional dinosaur skeletal record, the trackmakers are attributed to Megapnosaurus rhodesiensis (formerly Syntarsus rhodesiensis), a small-to-medium-sized, early theropod common in southern Africa.Significance:  A newly discovered Early Jurassic theropod trackway in South Africa contains not only the smallest tracks in the Clarens Formation, but also the most elongated dinosaur footprints in southern Africa to date. The tracks show that the small bipedal trackmaker dashed across the wet sediment surface at an estimated running speed of ~12.5 km per hour. During the run, the trackmaker’s feet sunk so deeply into the sediment that even the forwards-directed halluces were impressed.  Open data set: https://doi.org/10.6084/m9.figshare.13007240.v1

Highlights

Mounting evidence for body size changes in theropods[1,2,3], a class of carnivorous dinosaurs, during the dawn of the dinosaurs is increasingly placing the southern African fossil record into the focus of global palaeontological studies
The Lower Jurassic of southern Africa is key in this regard as: (1) it was deposited in a semi-arid continental ecosystem of rivers, lakes and deserts; (2) it is richly fossiliferous, and (3) it provides clues on how life recovered after the global biodiversity crisis event that occurred ~200 million years ago.[4,5]
This study reports on the smallest dinosaur footprints, forming a single trackway, in the Clarens Formation of South Africa and shows that these tracks are to date, the most elongated dinosaur footprints from the Early Jurassic of southern Africa

Summary

Introduction

Mounting evidence for body size changes in theropods[1,2,3], a class of carnivorous dinosaurs, during the dawn of the dinosaurs is increasingly placing the southern African fossil record into the focus of global palaeontological studies. This study reports on the smallest dinosaur footprints, forming a single trackway, in the Clarens Formation (uppermost Stormberg Group; Figure 1) of South Africa and shows that these tracks are to date, the most elongated dinosaur footprints from the Early Jurassic of southern Africa These tiny, slender tracks in the lower Clarens Formation are part of the Lower Jurassic footprint assemblage, which in southern Africa is contained in the upper Elliot Formation (Hettangian–Sinemurian) and in the comformably overlying Clarens Formation (Sinemurian– Pliensbachian).[4] These two rock units contain the two largest (footprint lengths: 55–57 cm)[6,7,8,9] and the smallest (footprint length: ~6.5 cm)[10] tridactyl tracks on record far, all reported from Lesotho. Because the current tracks are part of the ichnofauna of the lower Clarens Formation, which is likely Sinemurian[4], they enrich the Early Jurassic track record locally and globally, and can contribute to the evaluation of the temporal patterns in the footprint record, which, in turn, can add to the ongoing debate on dinosaur body size changes across the Triassic– Jurassic boundary[1,2,3,11]

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