Results

In the past two decades, we have revisited and redocumented numerous sites originally reported by Paul Ellenberger [e.g., 29, 31, 43–46] and from this are able to validate that his work from the mid-1900s is a fair representation of the relative stratigraphic placement and ichnological diversity. The original Trisauropodiscus were reported from four localities in Lesotho, and in our investigations, we successfully relocated the field material at Maphutseng (Fig 1), which we describe herein in more detail.

The Maphutseng tracksite records a diverse range of ichnites (Fig 3), initially documented by [47], following its discovery by Paul Ellenberger in 1955. In our current and previous studies, we assessed the sedimentary architecture of the host rocks at the site and in the surrounding areas. Based on the sedimentological analysis and stratigraphic correlations in the region (Fig 3), the tracksite is located in the uppermost part of the lower Elliot Formation (lEF), approximately 20 meters below the lithostratigraphic contact with the overlying upper Elliot Formation (uEF). At Maphutseng, this lithostratigraphic boundary within the Elliot Formation is clearly defined by a mappable pedogenic nodule conglomerate unit, up to 2 meters thick, along with other sedimentary facies specific to the uEF. Compared to the uEF, the lEF is characterized by distinct lithofacies associations (Fig 3B), including differences in external and internal geometries of the sandstone units (Fig 3C), mineralogical composition, grain size, and palaeo-current patterns [48]. Although our litho- and biostratigraphic re-assessments suggest that the Maphutseng tracksite belongs to the uppermost part of the lEF, implying a Late Triassic age, a more precise age was also obtained by us through U-Pb LA-ICPMS age determinations of detrital zircons extracted from sandstones at the study site [29]. These results together with other maximum depositional ages obtained at tracksites across the outcrop area of the Elliot Formation in the main Karoo Basin are fully documented in [29] and references cited therein.

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Fig 3. Sedimentological, stratigraphical and ichnological contexts of the Maphutseng tracksite.

A) Local stratigraphy viewed from the footprint site, facing roughly north. B) Sedimentological log with facies codes of the sandstone-dominated, asymmetrical rock unit that immediately underlies the Maphutseng tracksite (see C for log’s position). C) N–S outcrop view of the stacked sandstone layers and multiple fining-upward successions (see log in B). Note the erosional contact of the sandstone unit and the underlying red, fossiliferous mudstone unit. See D for tracks documented by [47]. D) Maphutseng tracksite ichnodiversity showing at least 10 different individual footprint morphologies on a redrawn map of the original tracks from [47]. Note that cluster H and other magenta tracks mark Trisauropodiscus on the surface in C. The Maphutseng tracksite ichnodiversity map has been modified from [47] under a CC BY license, with permission from the Société Géologique de France, original copyright [1960].

https://doi.org/10.1371/journal.pone.0293021.g003

The Maphutseng footprints are preserved on the upper surface of an asymmetrical sandstone-dominated rock unit (Fig 3C). This unit consists mostly of laterally stacked sandstone layers with multiple fining-upward successions (Fig 3B), which is typical of the laterally accreted, meandering channel deposits common in the lEF [29, 48]. Along a major erosional contact, the sandstone unit is incised into the underlying red mudstone unit (Fig 3C). The same mudstones, which have yielded several hundred fossil bones, are known for having hosted the famous Maphutseng dinosaur cemetery [49], which is one of the laterally most extensive fossil-rich layers ever discovered, and now fully excavated, in upper Triassic of southern Africa.

The track-bearing palaeosurface spans ~80 meters in length and ~5 meters in width, and its extensive documentation was provided by [17]; however, neither the Ellenbergers’ descriptions, including published and unpublished photographs, nor our field investigations revealed any evidence of ancient microbial surfaces or water-saturated substrates in the northern extreme of the surface, where Trisauropodiscus tracks are most common (Fig 3A–3D). Desiccation cracks, on the other hand, are commonly found (S1 Fig) and are preserved both among the tracks and propagating from them in this sector of the palaeosurface. In the latter case, the relative timing of track and crack formation is unequivocal, with the tracks predating the cracks [50]. In the southern sector of the track-bearing palaeosurface, evidence of ancient microbial surfaces is lacking, however water-saturated substrates were likely more common, as indicated by sediment collapse features associated with tracks. The decrease in water saturation levels from south to north aligns with the overall sedimentological evidence at the site, suggesting that the palaeosurface likely represents the surface of a meander point bar that was more emergent in the north relative to the south (Fig 3).

Six discrete, herein unidentified, trace-like markings are exclusively preserved on the northern-most surface in association with Trisauropodiscus on the Maphusteng track-bearing palaeosurface (Fig 4). These markings, preserved as casts, have teardrop-like [51–53], figure of eight-like [51, 52], crescent-to-ring-like [54], curviform-to-sinuous [54], chain-like [42, 54], and bilobate morphologies. Although certain markings may appear similar to feeding traces resulting from bird pecking and probing, their interpretation remains unresolved. This is due not only to their resemblance to invertebrate traces but also because some, such as the teardrop and sinuous shapes, may be partial, overprinted, weathered or eroded track impressions.

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Fig 4. Photographs, interpretive outlines and false-colour depth maps of distinct traces associated with Trisauropodiscus at Maphutseng.

False colour depth maps are applied to 3D photogrammetric models, where blue and red reflect negative and positive relief, respectively, relative to the average relief. See Methods for details. A, A’, A”) Markings resembling either pecking and probing activity, partial track (digit) impressions or invertebrate traces. B, B’, B”) Markings resembling either a continuous pecking trace or invertebrate trace. C, C’, C”) Unequivocal invertebrate trace and markings resembling either pecking and probing activities, or partial track (digit) impressions or invertebrate traces. Abbreviations: b–bilobate trace, c–chain-like trace, cr–crescent ring-like trace, cs–curviform-to-sinuous trace, f–figure of eight-like trace, i–invertebrate trace, t–teardrop-like trace.

https://doi.org/10.1371/journal.pone.0293021.g004

Seven ichnospecies, based on varying morphological features, were originally assigned to the Trisauropodiscus ichnogenus [15–20]. Based on size, track length-to-width ratios, and digit morphologies of the holotype material, we assign them to two distinct morphotypes (Fig 5; S1 Fig; S1 and S2 Tables): Morphotype I, comprising T. galliforma, T. levis, T. popompoi, and T. superaviforma, is marginally longer than wide with robust digits that have a smaller total digit divarication, while Morphotype II, comprising T. aviforma, T. phasianiforma, and T. superavipes, is wider than long with slender digits and a larger total digit divarication.

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Fig 5. Morphological comparison of southern African Trisauropodiscus morphotypes with established ichnogenera attributed to avian, ornithischian and theropod trackmakers.

Morphotypes I and II are largely indistinguishable when considering their track length-to-width ratio and total digit divarication, though Morphotype I generally has lower total digit divarication angles. When considering additional features using landmark-based principal component analysis, the two morphotypes form distinct fields. Although Morphotype I resembles Anomoepus, when plotted, the two ichnogenera fields do not overlap. Morphotype II morphospace consistently corresponds to established avian ichnogenera. Notably, both morphotypes are discrete from the classical, contemporaneous theropod ichnogenera Kayentapus, Grallator, Anchisauripus and Eubrontes. A) Photograph and false colour depth map of a typical Morphotype I and Morphotype II track. Key differences include larger track size, more robust digits and smaller total digit divarication of Morphotype I relative to Morphotype II. False-colour depth maps are applied to 3D photogrammetric models, where blue reflects negative relief and red reflects positive relief relative to the average relief. See Methods for details. B) Length-to-width ratio and total digit divarication angle plot. Measurements for the southern African morphotypes were performed by a single operator on > 80 tracks comprising field material (27 tracks), cast material (13 tracks), historical photographs (15 tracks), and published sketches (34 tracks, Table 1; S1 and S2 Figs, S1 Table). See Methods for details and S2 Fig for the track numbers corresponding to the data points. C) Landmark-based principal component analysis. Eight landmarks were placed on orthophotographs of the Trisauropodiscus morphotypes and comparative ichnotaxa by a single operator to account for additional morphological features such as digit thickness and anterior projection of the central digit. The morphospaces are most strongly controlled by landmarks, which are influenced by track width, total digit divarication and digit thickness. See Methods for details and S2 Fig for the track numbers corresponding to the data points. Abbreviations: TL–Track length, TW–Track width, II-IV–Digit numbers, II^IV–digit divarication angle.

https://doi.org/10.1371/journal.pone.0293021.g005

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Table 1. Track measurements for field material, cast material, historical photographs and illustrated sketches of Trisauropodiscus morphotypes I and II.

See S1 Fig and S1 and S2 Tables for details.

https://doi.org/10.1371/journal.pone.0293021.t001

On average, Morphotype I tracks are larger than Morphotype II (6.59±5.19 cm vs 3.83±1.80 cm) but their distinct morphological characteristics are unlikely attributed to allometric changes as morphologies typifying Morphotype II are also preserved in larger tracks with track lengths typical of Morphotype I and vice versa (e.g., tracks 10 and 39; S1 Fig; S2 Table). Both morphotypes are known from both Late Triassic and Early Jurassic ichnosites and commonly co-occur on these track-bearing surfaces (Figs 3 and 4; S1 Fig; S1 Table). While the morphological differences between Morphotype I and II could justify their classification into separate ichnogenera, this study does not include any ichnotaxonomic assessment or revision of the diagnosis of Trisauropodiscus.

A total of 163 Trisauropodiscus tracks from field and historical photographs were evaluated on 11 individual track-bearing slabs from the Maphutseng and Thejane track sites to determine the track orientation and density (see Methods; S4 and S5 Tables). The average track number per slab is 10, when disregarding the lowest and highest track numbers, which are 4 and 64, respectively. On each slab, the tracks are directed at random, and this lack of orientation is well illustrated by the vector statistics. Track directional data has a standard deviation range of ~ 18°–80° and a consistency ratio of 0.01–0.95. When disregarding the lowest and highest values, the average standard deviation and consistency ratio of the directional data is ~ 66° and ~ 0.49, respectively. The vector statistics of the slab with the highest track number (n = 64) shows the highest standard deviation and lowest consistency ratio at 80.42° and 0.01, respectively.

Morphotype I

Morphotype I tracks are documented from the Maphutseng field ichnosite, cast material, and interpretive sketches (Figs 1 and 3; Table 1; S1 Fig; S1–S3 Tables). At Maphutseng, the isolated, asymmetrical tracks are preserved as slightly longer than wide (mean TL/TW of 1.07±0.09) shallow hyporeliefs. The digit impressions (LII<LIII<LIV) are relatively robust, with rounded tips and pad impressions preserved in tracks 9 and 10, and claw marks preserved in track 10 (S1 Fig). Where digital pads are present, the proximal digit IV pad is distinctly aligned with the central axis of digit III. Cast materials preserve the same morphology, with a digital pad formula of 2-3-4 clear in track 37 (repository A#BM056; Fig 5). Comparable moderate average total digit divarication angles are preserved for the field and cast material (84° vs 81°; Table 1; S2 Table). A single, bipedal Morphotype I trackway is documented in repository A#G041 (tracks 33 to 35; S1 Fig) with the tracks having a slight rotation relative to the midline (track 33–6°, track 34 4°), a large pace angulation of 151°, and a stride 6 times the track length (S3 Table). Interpretive sketches of Morphotype I have measured parameters within a single standard of deviation overlap with the measured parameters of the field and cast material, but on average, the illustrated material is wider and has larger digit splays (Fig 5; Table 1; S1 Fig; S1 Table). The sketched trackways are also bipedal, with average track rotations of -13°, pace angulations of 174° and stride 8 times the track length (S3 Table).

The wide and round digit morphology of Morphotype I, with the digit IV proximal pad aligned with digit III, resembles Anomoepus [33, 34], a contemporaneous tridactyl track. The pes impression of Anomoepus is small (track length < 20 cm), asymmetrical, and distinctly preserves digit IV’s metatarsophalangeal pad in line with digit III. Anomoepus digit impressions are cigar-shaped with LII < LIII < LIV and the outer digits have moderate total digit divarication (~ 68°). Furthermore, Morphotype I trackways preserve pace angulations and inward track rotations consistent with Anomoepus [55].

When considering measured parameters (TL/TW vs II^IV), Morphotype I is indistinguishable from Morphotype II, but generally preserves lower total digit divarication, which is most clearly seen when considering field and cast material (Fig 5). Morphotype I field and cast material TL/TW and II^IV coincides with Anomoepus, while the historical photographs and published illustrations depict forms that are wider with more splayed digits than Anomoepus. When considering landmark-based principal component analysis which accounts for more of the track morphology, Morphotype I predominantly plots in the same quadrant as Anomoepus and Kayentapus (Fig 5). This quadrant is most strongly influenced by landmarks 1, 3 and 4, directly related to the digit II and IV ends i.e., track width, “heel” and total digit divarication. Although in the same quadrant, Morphotype I morphospace does not coincide with Anomoepus or Kayentapus i.e., Morphotype I is distinct from Anomoepus and Kayentapus. The holotype P. pilulatus [9] plots within the Morphotype I morphospace; however, Morphotype I never preserves hallux impressions nor a pronounced digit III metatarsophalangeal pad and does preserve defined digital pad impressions which is dissimilar from P. pilulatus.

Based on Ellenberger’s works, some authors consider Trisauropodiscus to be a junior synonym of Anomoepus [e.g., 21], while others [25], several of whom have recently documented new field specimens [e.g., 12, 23, 26], have argued Trisauropodiscus is a distinct ichnotaxa. It is worth noting that in Ellenberger’s original report on the Maphutseng tracksite [47], he noted that the small tridactyl tracks resembled Anomoepus but still considered them as distinct. Our principal component analysis supports Ellenberger and these more recent authors who posit Trisauropodiscus is distinct from Anomoepus.

Morphotype II

Morphotype II tracks are documented from the Maphutseng field ichnosite, cast material, historical photographs, and interpretive sketches (Figs 1, 4 and 5; Table 1; S1 Fig; S1 and S2 Tables). At Maphutseng, the high density, (45 tracks preserved on a 90 cm by 65 cm slab), asymmetrical tracks are preserved as epireliefs. Although some of the tracks may be penetrative, it is unlikely the case for the measured tracks as they do not appear to have exaggerated depth differences (all tracks are shallow) and have consistent morphological parameters within a single site and across sites of variable age (Table 1; S1 Fig; S2 Table). Morphotype II tracks preserved at the Maphutseng field ichnosite are small (track length < 5.5 cm), wider than longer (TL/TW of 0.88±0.18), with slender digits which taper to sharp clawed tips. Digital pad impressions are rarely preserved, and when present, they are indistinct (e.g., track 17 digit III; S1 Fig). The tracks are preserved either as discrete digit impressions (where LII<LIV<LIII) or with a merged, V-shaped proximal “heel” where proximal digit IV is offset from the central axis of digit III. The outer digits are widely splayed, with an average total digit divarication of 109°, reaching a maximum of 160°. A single step (tracks 20 and 21) is preserved at Maphutseng, with the tracks oriented parallel to the step midline (S1 Fig). Morphotype II cast materials have comparable morphologies to the field material (average TL/TW of 0.83±0.09 vs 0.88±0.18, DP/TW of 0.41±0.02 vs 0.49±0.16, and II^IV 96° vs 109°; Table 1), though a single larger track is documented (track 39, repository A# 48,2, with TL of 7.2 cm; S1 Fig). Tracks from historical photographs and the interpretive sketches have morphologies and measured parameters comparable to the field and cast material: average TL/TW of 0.86±0.12 vs 0.87±0.16, DP/TW of 0.47±0.09 vs 0.47±0.14 and average II^IV of 105°±14° vs 106°±21°, though a single larger track is documented in the photographs (TL of 11. 74 cm; Table 1; S2 Table). Historical photographs from Maphutseng were matched to tracks preserved in the field (S1 Fig); however active local brick making has destroyed much of the track-bearing surface. It is important to note that tetradactyl Morphotype II tracks are depicted in the interpretive sketches (e.g., track 62) but are not observed in the field material or cast material. Ellenberger [17] noted apparent hallux impressions for T. aviforma at Maphutseng, and we have matched one of the tracks (his plate 15) to our field material (S1 Fig)–it is our interpretation that in this example the feature close to the heel is not a hallux impression. Morphotype II trackways are exclusively known from the interpretive sketches: most of the tracks have an inward rotation (average -15°), with high pace angulations (average 158°) and stride length up to 10 times the track length (S1 Fig; S3 Table).

The small track lengths, slender digits and wide digit divarication of Morphotype II resembles Gruipeda [22], known in the stratigraphic record since the Late Cretaceous. Gruipeda is defined as having three forward facing digits (with a fourth backwards facing hallux), interdigit angles less than 70° (II^III and III^IV), with an absence of webbing and, where present, a digital pad formula of 2:2:3:4. G. dominguensis [22] is small (track length < 5 cm) and wide (TL/TW of 0.7–0.9), and can be preserved as tridactyl or tetradactyl (three forward facing digits with a fourth backward facing hallux). Digit II is the shortest and digit III is the longest (LII < LIV < LIII), with a large total interdigit divarication angle of 90–135° (II^IV). Bipedal G. dominguensis trackways comprise tracks that are parallel or inwardly rotated relative to the midline, with high pace angulations (150° to 182°) and stride lengths up to 5 times the track length. Morphotype II is rarely preserved with a hallux impression but the forward-facing track morphologies are consistent with Gruipeda which is attributed to an avian trackmaker.

When considering conventional TL/TW vs II^IV biplots, Morphotype II is highly variable and coincide with Morphotype II (Fig 5B). The TL/TW and II^IV ranges of Morphotype II are indistinguishable from recently reported Jurassic Trisauropodiscus and Cretaceous avian ichnogenera, with minor Morphotype II tracks overlapping with Anomoepus. When considering landmark-based principal component analysis which accounts for more of the track morphology, Morphotype II occupies the same morphospace as younger Middle Jurassic Trisauropodiscus reported from North Africa and various Cretaceous ichnotaxa attributed to avians (Fig 5C). These morphospace quadrants are most strongly influenced by landmarks 3–8, which capture digit IV ends and digit width; these ichnotaxa are typified by their narrow digits and wide total digit splay. The Morphotype II morphospace only partially overlaps with recent reports of Trisauropodiscus from Early Jurassic strata in Lesotho.

The strong resemblance of Morphotype II to established avian ichnogenera and their clear distinction from classical ornithischian and theropod tracks, which is now reinforced by geometric morphometric analyses, supports the original avian affinity of Trisauropodiscus originally proposed by [15, 16, 17, 18, 19, 20].