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

FUNDING: South African National Research Foundation (grant no. 113394] and DST-NRF Centre of Excellence in Palaeosciences (grant no. CoE PAL 2020) 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.


Introduction
Mounting evidence for body size changes in theropods 1-3 , 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. Late Triassic to Early Jurassic dinosaur footprints, for which southern Gondwana, especially southern Africa, is an excellent archive, are particularly useful proxies for such biogeological investigations. In particular, the Lower Jurassic of southern Africa (i.e. the upper Stormberg Group; Figure 1) 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 (uppermost Stormberg Group; Figure 1) of South Africa and shows that these tracks are also, 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 also 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 thus 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 thus 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 .

Geological background
The dinosaur footprints investigated in this study were discovered north of Maclear in the Eastern Cape of South Africa ( Figure 1) by a local resident, Mrs Adele Moore, and her field party in 2014 along a footpath leading to the Storm Shelter archaeological site. 13 Found at the same altitude as the rock art site, the very fine-grained sandstone slab, containing the single trackway of five well-impressed dinosaur footprints, is part of the lowermost Clarens Formation (Figure 1). Here, at Storm Shelter, and throughout the main Karoo Basin (South Africa and Lesotho), the basal Clarens Formation, which is likely Sinemurian 4,14 in age, is characterised by tabular, fine-to medium-grained sandstone beds and subordinate mudstones. Suites of sedimentary structures (e.g. horizontal lamination, cross-bedding, ripple cross-lamination, desiccation cracks; Supplementary figure 1) in these rocks point to deposition in decelerating powerful traction currents and stagnant water. 15,16 Overall, these rocks indicate that the ancient wet aeolian ecosystem was prone to intermittent flash flooding and drying episodes. 15,16 While being less fossiliferous than the conformably underlying Hettangian to Sinemurian upper Elliot Formation, the palaeontological record of the Clarens Formation is diverse, both in vertebrate skeletal remains and trace fossils 4,6,14,17,18 , particularly in footprints, and most notably in dinosaur tracks, as recently summarised by Mukaddam et al. 19

Methods
The footprints investigated in this study were subjected to morphometric measurements as shown in Figure 2. Moreover, for comparative purposes, four additional footprints collected from Lesotho, two Grallator-like (LES 283 and 288) and two Anomoepus-like (LES 111 and 112) tracks from the Clarens and upper Elliot Formations, respectively, were also briefly analysed for this study. These four Lesotho tracks were collected by Dr Paul Ellenberger pre-1970, and are now housed in the Ellenberger Collection at the University of Montpellier (France).
The ichnological morphometric measurements and the standard proxies derived from them, which estimate the trackmaker's hip height, gait, body length and speed, are summarised in Table 1 and detailed in Supplementary tables 1-5 (with references). The relevant modus operandi is explained elsewhere 5,8,9,10,19 and in references therein. The track measurements were taken physically in the field using a calliper. After photographing each track with a Nikon D5100 digital camera (focal length 50 mm), individual photogrammetric 3D models and falsecolour depth maps were generated with Agisoft Metashape Professional (version 1.6.4) and CloudCompare (v.2.11.0) software packages, respectively. Following the standard protocol of Falkingham et al. 20 , the photogrammetric 3D models and relevant raw data are publicly available online via this open access data repository link: https://doi.org/10.6084/ m9.figshare.13007240

Results
The Storm Shelter ichnosite preserves a single trackway of five, very small, narrow, elongated and strongly mesaxonic footprints ( Figure 3, Table 1, Supplementary table 1) with the following main parameters (average values): length 7.5 cm, width 3.6 cm, length/width ratio >2.1. The ratios of the anterior triangle 21 length to width and the anterior triangle length to track width are 1 and 0.9, respectively. Manus tracks were not observed.
These elongate, digitigrade tracks preserve evidence for claw marks at the tips of the three well-impressed digits and one lightly impressed digit, indicative of a hindfoot that was functionally tridactyl and featured a welldeveloped hallux (digit I). The slender digit impressions only sporadically show discrete, oval digital pads, most likely due to extensive sediment collapse within the tracks, which would also suggest that these are likely penetrative tracks 22,23 (i.e. the foot penetrated the substrate much more deeply than is apparent from the track outlines on the tracking surface). The penetrative nature of the tracks is further supported by the very narrow distal portion of digit III. Relative to digits II and IV, digit III is better impressed and extends beyond digits II and IV, which are subequal. Both the apex of the anterior triangle and the divarication between digits II and IV form acute angles of 54° and 29°, respectively. On average, digit I is ~1 cm long and its long axis encloses an acute angle of 54° with the long axis of digit III. The variability of the position of the digit I impression relative to the other digits ( Figure 3, Table 1) indicates that the hallux did not have any functional use for locomotion or stability.
The single trackway that these five pes tracks form is narrow (~6.1 cm), and relative to the footprint length of 7.5 cm, preserves long strides of nearly 1 m ( Figure 4, Table 1, Supplementary table 2). The long axis of digit III is essentially parallel to the trackway axis. The average pace is 47.6 cm with an angulation of 172°, and the track width to trackway width ratio is 59%, which define a narrow-gauge trackway. Along the trackway, the morphological quality of the tracks is uniform, although well-developed wrinkle structures, a type of microbially induced sedimentary structure, are present both inside and outside the tracks on the tracking surface ( Figure 4 and Supplementary figure 1), and interfere with the outlines of some of the tracks (e.g. see the posterior of track #1 in Figure 3). The microbially induced sedimentary structures and the digit I impression demonstrate that the tracks are not underprints, but rather primary, penetrative footprints impressed directly and deeply on the tracking surface.
Because of the incomplete/missing digital pad impressions, some illdefined imprint walls, and superimposition of microbial structures, all five Storm Shelter tracks rank low, around 1.5 on the recently Using standard ichnological formulae 26 The relatively rapid locomotion of the trackmaker is also suggested by the narrow trackway, as during fast movement, running bipedal animals place their feet closer to the midline (cf. 29 ). The stride/footprint length ratio for the Storm Shelter trackway is 12.9 and thus is only slightly greater than the median value 30 for trackways in this size category (i.e. foot length of 5-10 cm) from the early Mesozoic and attributed to theropods. This suggests that the trackmaker was moving in a typical manner for a small theropod.

Discussion
The morphological characteristics describing the Storm Shelter footprint proportions (Table 1) closely resemble those of Grallator-like tracks; however, due to the moderate to low quality of these tracks, their ichnotaxonomic treatment is not warranted here. Grallator is an ichnogenus that refers to globally occurring tracks of bipedal theropod dinosaurs of early Mesozoic age. Grallator tracks are small (<15 cm long), elongate with a footprint length/width ratio of ≥2, narrow digit divarication angles (10-30°) of the slender, pointy toes and a great anterior projection of digit III (i.e. mesaxonic tracks; e.g. 21,26,31,32 ). Moreover, Grallator tracks are digitigrade, often preserve claw traces and occasionally also the impression of digit I 28 , and thus the ichnogenus refers to functionally tridactyl tracks.
In the early Mesozoic ichnological record of the main Karoo Basin of southern Africa, tracks referable to Grallator are not rare. [6][7][8][9][10][33][34][35][36][37] Being only ~1 cm longer than the smallest Grallator tracks in southern Africa (described 10 5,10 ). Compared to these older, likely pre-Sinemurian 4 , tridactyl tracks (Figure 6), the Storm Shelter tracks are typically more elongated with narrow digit divarication angles (10-30°) and a greater anterior projection of digit III (i.e. more mesaxonic; see Supplementary table 1). All in all, these comparisons show that, thus far, the Storm Shelter tracks are among the most elongated (i.e. high length/width ratio) Grallator-like tracks reported from the Lower Jurassic of southern Africa, and the smallest in the Clarens Formation.  Although Grallator tracks rarely preserve 28 impressions of digit I, remarkably, all Grallator-like tracks at Storm Shelter are associated with hallux (digit I) traces ( Figure 3). Moreover, unlike in typical Grallator tracks 26,31,39 , the impression of the hallux is not directed medially or to the rear, but forward. Figure 3 shows how these ~1-cm-long impressions point in the forward direction, at ~54° (average) away from the long axis of digit III. The hallux impressions are not only important for identifying the trackmaker (see below) but also for determining that the trackmaker's feet sunk hallux-deep into the wet, soft sediment while the animal ran across the surface. A pliable, squidgy tracking surface also explains why digits II, III and IV in these tracks lack well-defined digital pad impressions, imprint walls, etc. (Figure 3), and thus limit the tracks' ichnotaxonomic assessment and conclusive attribution to any specific trackmaker. The wrinkle structures (Figure 4, Supplementary  figure 1), which are linked to fossilised microbial mats and are abundant on this tracking surface, were possibly present already at the time of trackmaking and thus enhanced the footprint generation and overall preservation. Microbially induced sedimentary structures are known to be associated with dinosaur tracks in the southern African 10,34,35 and global [40][41][42] ichnofossil record. Likely, these sediment-binding biofilms developed as localised algal blooms flourished in shallow, stagnant pools of water that were generated in ephemeral downpours and that evaporated over time (Figure 7). Although the host Clarens Formation at Storm Shelter does preserve desiccation cracks (Supplementary figure 1), there are none preserved on the tracking surface itself, which possibly remained wet, or at least moist, until it was buried by the next layer of flash flood generated sediment (Figure 7).
Grallator is commonly attributed to early theropod dinosaurs that were obligatory bipeds with long, thin toes, including well-developed halluces (e.g. 21,26,31,32 ). Based on the morphometric parameters of the Storm Shelter tracks ( Figure 3, Table 1, Supplementary table 1) and the regional Early Jurassic bone fossil record, a likely trackmaker candidate is Megapnosaurus rhodesiensis (formerly Syntarsus rhodesiensis and Coelophysis rhodesiensis 39,40 ). This early theropod is a coelophysoid dinosaur (Figure 7) that is common in the same stratigraphic interval (i.e. Lower Jurassic) of southern Africa, especially in Zimbabwe (e.g. [43][44][45]. A similar coelophysoid dinosaur, Coelophysis bauri, is often assumed to have made the early Mesozoic Grallator tracks in North America (e.g. 26,31,32,[46][47][48]. Both of these early theropods were smallto-medium-sized, lightly built, agile, carnivorous bipeds and are often found in richly fossiliferous strata where entire animal populations are jumbled together as partially intact or complete skeletons that also include articulated foot bones. 43 Foot reconstructions show that these early theropods had four pedal digits and a pes phalangeal formula (inclusive of unguals) of 2:3:4:5:0. 43 Reconstructions also show that digit I (the hallux; Figure 7b) did not touch the ground during the steeply digitigrade locomotion of the animal (Figure 7a; e.g. 43,48,49 ). Moreover, the well-formed hallux and associated foot bones of Megapnosaurus rhodesiensis preserve anatomical features that indicate that the hallux was forwards directed 26,43 , alongside digit II (Figure 7b). Raath 43(p.93) remarked that the hallux 'retained a specialised function in life. Its use as a grooming accessory seems quite feasible'. The congruence in the morphological features of this foot skeleton and the tracks (including the non-reversed hallux configuration) is used here to suggest that the Storm Shelter footprints likely belong to Megapnosaurus rhodesiensis.
It is noteworthy that in other early Mesozoic Grallator tracks 26,31,39,48 , the orientation of the hallux impression is reversed (posterior directed) in contrast to the original anatomical configuration. This backwarddirected hallux mark resulted from the way the forward-directed hallux interacted with the sediment during the track-generating process as demonstrated by Gatesy et al. 48 In the Storm Shelter tracks, however, the hallux impression closely corresponds to the typical anatomical (forward) orientation of the digit I in coelophysoid dinosaurs 43,48 , and this makes the Storm Shelter tracks unique among Grallator-like tracks. It is probable that this anterior-directed hallux impression resulted from the fortuitous combination of the microbially influenced substrate consistency that allowed the penetrative tracks to form, the trackmaker's tiny body proportions and the fast speed of motion.
Theropods were the main terrestrial carnivores in the post-Triassic part of the Mesozoic. Their smaller bodied varieties, like the trackmaker of the Storm Shelter tracks, most probably took on the dual role of predator and prey, and thus had good reason to leave behind tracks indicative of  a running gait. The ability to run and occasionally sprint at speeds up to 12.5 km/h, even in the smallest individuals, must have been a great advantage for these agile, highly successful predators 43 that had to adapt to an increasingly harsh, desert ecosystem, prone to flash flooding and dry spells in the Early Jurassic of southern Africa 4 (Figure 7).
The minute, elongated Storm Shelter tracks together with the region's smallest 10 and two largest 6-9 tridactyl tracks are taken as evidence, not only of the abundance but also the size diversity of the Early Jurassic theropod dinosaurs in southern Africa. Given the regional abundance and diversity of the footprint record, the dinosaur tracks of southern Africa, in all shapes and sizes, remain an important proxy for meaningfully assessing concepts on macroevolutionary changes in dinosaur body size during the early Mesozoic. [1][2][3]50 The true potential of this rich ichnological record is only achievable if the collected, but largely undescribed materials in various museum collections, as well as new discoveries like this one at the Storm Shelter, are quantified and integrated with the global early Mesozoic ichnological and osteological fossil records.