A Middle Permian Oasis for Vertebrate and Invertebrate Life in a High-Energy Fluvial Palaeoecosystem of Southern Gondwana (Karoo, Republic of South Africa)

Abstract

The Gansfontein palaeosurface (Fraserburg, Karoo, South Africa), which is correlated with the stratigraphic lowermost part of the continental Middle–Upper Permian Teekloof Formation, is revisited. This treasure trove of peculiar and exquisitely preserved sedimentary structures and invertebrate and vertebrate traces serves as a document of a set of fluvial paleoenvironments ranging from small ponds to marginal lacustrine and muddy riverine outer banks. It represents an isolated and relatively small “oasis” within the dominating sedimentary environments of the Teekloof Formation characterized by fine and medium-grained siliciclastics related to repeated higher-energy flooding events. The vertebrate traces include abundant therapsid trackways and, locally, tetrapod swimming traces. Tetrapod footprints show a very variable preservation in different areas of the palaeosurface, and it also changed based on the time of impression. Fish trails (Undichna) are relatively common. The invertebrate ichnofauna is comprised of abundant arthropod traces and horizontal burrows; however, the palaeosurface was not intersected by intense bioturbation. The occurrence of this scenario of abundant life reflects complex interaction among different tracemakers with the substrate and is evidence of a relatively quiet palaeoenvironment, which was suddenly submerged and sealed during a flooding event. Sedimentological and ichnological insights from such a palaeosurface, therefore, opens a rare window into Middle Permian ecosystems in southernmost Gondwana.

1. Introduction

Studies coupling ichnology and sedimentology of the Middle–Upper Permian continental deposits from Southern Gondwana, and particularly South Africa, are relatively rare [1,2,3,4,5]. Among the most recent papers is the first description of a Cochlichnus-dominated ichnofossil site from the Mid- to Upper Permian Middleton Formation in Eastern Cape (Karoo Supergroup) [6] and a comprehensive review of Mid- to Late Permian tetrapod footprints from South Africa [7].

In 1968, after heavy rains, some vertebrate trace fossils were originally discovered by a landowner between the towns of Fraserburg and Williston. The trace fossils were impressed on a relatively wide surface called the Gansfontein palaeosurface [8]. Ref. [3] was the first to formally study the site, and the author of [4] provided a more detailed sedimentological and ichnological analysis, also comprising other palaeosurface types from the same stratigraphic interval. According to the author of [4], palaeosurfaces represent ancient depositional landforms that are preserved with their original topography and intact surface sculpture.

In the present work, we revisit the Gansfontein palaeosurface (Figure 1A,B,D), making observations from both sedimentological and ichnological viewpoints. A very rich ichnological fossil content is exposed, displaying a combination of both vertebrate (walking and swimming traces) and invertebrate (walking and resting traces, grazing trails, and horizontal burrows) trace fossils. The preserved ecosystem represents a kind of “oasis” within a mostly high-energy fluvial palaeoenvironment. Numerous excellently preserved primary sedimentary structures help to unravel facies and sub-facies of this rare setting in the continental Middle Permian of southern Gondwana, where this biota existed and different faunal elements interacted.

2. Materials and Methods

We surveyed all of the area to identify the best representative stratigraphic section of the palaeosurface. The selected section was then measured in great detail and described for its lithostratigraphy and sedimentology through the exposures of interbedded sandstones, siltstones, and mudstones. Lithologies, color, and every single sedimentary structure were reported and described. Vertebrate and invertebrate traces have been studied first-hand and documented with digital photographs (Canon EOS 70D) in oblique light. Invertebrate traces have been evaluated in terms of invertebrate ichnofacies and invertebrate ichnodisparity, e.g., [9,10].

Three-dimensional models of the tetrapod footprints were obtained through digital photogrammetry with the software Agisoft Photoscan Professional^® (v.1.4.0). Contour lines and color depth maps were obtained by employing the software Cloud Compare^® (v.2.8 beta) and Paraview^® (v.4.1.0). Vertebrate footprints have been evaluated in terms of morphological preservation sensu [11] in relation to the substrate conditions and associated sedimentary structures. Tetrapod trackways from the Gansfontein site were indicated with the acronym GF-TR, as in [7]. Institutional abbreviations: SAM = Iziko South African Museum, Cape Town, South Africa.

3. Geological Setting

The Middle Permian (upper Wordian) to lower Middle Triassic (Anisian) Beaufort Group (Figure 1C) is a continental succession dominated by mudstones and sandstones organized in characteristic cyclical fining-upward successions. This group is subdivided into two major units: the Permian Adelaide and Triassic Tarkastad subgroups, which are separated by a regionally mappable contact marked, among others, by a higher abundance of sandstone and red-colored mudstones in the latter unit [12,13]. Throughout the basin, the Beaufort Group conformably and diachronously overlies the Ecca Group and is separated from the overlying Stormberg Group by a major unconformity, which is the largest single stratigraphic gap (up to ~12 Ma) in the Karoo Supergroup. Age estimation of the Beaufort Group is mainly based on pollen, flora, and vertebrate assemblages, and it is believed to be one of the most complete terrestrial successions up to the Permian–Triassic boundary, e.g., [14,15]. Radiometric dating has been applied throughout the Adelaide Subgroup [16,17,18,19,20,21,22]. The stratigraphically meaningful radiometric ages were used to provide absolute dates for the tetrapod assemblage zones (AZ), which are widely employed for correlation within the Beaufort Group, e.g., [7,21,22].

The Beaufort Group sediments have been mainly interpreted as deposits of high sinuosity meandering rivers, e.g., [23,24,25] and less commonly as braided fluvial or lacustrine/delta settings [26]. Using an integrated approach based on vertebrate fossil taphonomy, ichnology, and sedimentology, Smith [4,24,25,27] was the first author to record a detailed account of the formations in the Beaufort Group and reconstruct the finer details of the fluvial habitats in the Permian and earliest Triassic. The Adelaide Subgroup (Middle Permian to earliest Triassic, Figure 1A), about 1000 m-thick, consists of blue-grey to green-grey mudstones, siltstones, and moderate to well-sorted, very fine to medium-grained lithofeldspathic sandstone beds with fining-upward cycles interpreted as overbank and channel deposits of mixed-load meandering rivers [23,24,28]. It is subdivided into the Abrahamskraal and Teekloof Formations (corresponding to Middleton and Balfour (pars) formations, east of 24° E) [29,30]. In the N and NE main Karoo Basin, the Abrahamskraal and Teekloof Formations are equivalents of the Balfour and Emakwezini Formations [30,31].

In particular, the Abrahamskraal Formation can be distinguished from the underlying arenaceous deltaic sediments of the Waterford Formation (top of the Ecca Group) by the first appearance of red/purple mudstones considered to represent the first subaerial fluvial deposition and from the overlying Teekloof Formation based on the sandstone percentage, the presence of chert beds, and the abundance of purple mudstone [32]. The Abrahamskraal Formation is well developed in the southern and southwestern parts of the main Karoo Basin and consists of predominantly grey, bluish to greenish-grey, and olive-green mudstone and siltstone with subordinate mottled, light grey, yellow-brown, greyish orange to greenish grey sandstone. Sandstones (~20–30%) are very fine- to medium-grained, arkosic, mostly massive, and rarely cross-bedded. Upward-fining cycles occur, as do slickensides, rare carbonate nodules, and relatively complete plant fossils [33,34]. The mudstones, which are interpreted as overbank deposits, include incipient calcareous palaeosols, indicating warm to hot semi-arid climatic conditions with strongly seasonal rainfall [35]. The fluvial system was mostly comprised of meandering, locally braided rivers with extensive and vegetated, moist floodplains. The Abrahamskraal Formation was subdivided into six members, named from the base: Combrinkskraal, Leeuvlei, Koornplaats, Wilgerbos, Moordenaars, and Karelskraal [36,37,38]. Later on, ref. [39] changed the name of Wilgerbos Member to Swaerskraal Member and added a seventh one in the lower half of the formation, the Grootfontein Member.

The middle–late Guadalupian age (upper Wordian to upper Capitanian) of the Abrahamskraal Formation was recently confirmed by radiometric and magnetostratigraphic studies [18,22]. According to the former authors, the presence of both normal and reversed polarity zones indicate deposition after the end of the Kiaman Superchron, the top of which is located at 269 Ma by the U–Pb zircon ages of ca. 264–268 Ma. New high-resolution U-Pb zircon ages (CA-ID-TIMS) helped in better constraining the upper two-thirds of the Abrahamskraal Formation [22]. The Abrahamskraal Formation has yielded diverse tetrapod fossils which belong to the Tapinocephalus Assemblage Zone [22,39]. The fauna comprises mostly therapsids (Dinocephalia, Dicynodontia, Therocephalia, Gorgonopsia, Biarmosuchia, basal Anomodontia) with less common reptiles (pareiasaurs and Eunotosaurus), rhinesuchid temnospondyls, and rare fish [32,37,38,40,41,42,43]. Fossil plants are also present in silicified wood fragments, stems, and leaves, some of which are preserved in a growing position [34,44].

The Abrahamskraal Fm is overlain by the mudstone-dominated Teekloof Formation, which is subdivided into the Poortjie, Hoedemaker, Oukloof, Steenkampsvlakte, and Javanerskop members, e.g., [28,45]. The units comprise an overall upward-fining megacycle showing a gradual gradient decrease of the regional palaeoslope. Mudstones are grey, blue-grey, and green-grey-colored with a few hallmark red- and maroon-colored units. Within the fines are subordinate light grey to grey, fine- to very-fine-grained, and thin (average ~1 m thick) sandstone bodies (litharenites), which are lenticular in shape. A fluvio-lacustrine palaeoenvironment is assigned to this formation. Fluvial style changes from bottom to top: short-lived, low to more permanent, high-sinuosity rivers [28].

A radiometric date from the lowermost Poortje Member provides a CA-ID-TIMS date of 260.259 ± 0.081 Ma, the latest Guadalupian [21]; the base of Poortjie is at 260.16 + 0.10/−0.15 Ma [22]. This is immediately below the Tapinocephalus– Endothiodon AZ transition in the basal part of the same member, which defines the end-Guadalupian extinction event [21,46,47,48].

Different interpretations were carried out to explain the multiple fining-upwards cycles characterizing the mostly fine-grained Abrahamskraal Formation and the overlying Teekloof Formation, even ascribing them to astronomically forced cycles [49]. Most authors agree in considering this sedimentation to be typically fluvial [32,50]. These latter authors ascribed the Abrahamskraal Formation facies associations as related to dominantly high-sinuosity channels and the Teekloof Formation facies as pertaining to floodplain deposits. In the Beaufort West area, ref. [51] identifies different flat-lying, coastal alluvial plains and their deltaic extension facies models in the Abrahamskraal Formation. Since 1977, no subsequent research has confirmed marginal marine environments.

Ref. [52] illustrated the sedimentary structures on some laterally persistent sandy interbeds, commonly interspaced throughout the argillaceous sequences of the Adelaide Subgroup (just below the “Poortjie Sandstone”), and related them to fluctuating hydrodynamic conditions in flood plain deposits. According to the author of [52], the sedimentation of these sandstones took place in a fluvial environment during ephemeral floods along the lower reaches of an arid flat. The author of [53] specified that this environment represented an interior drainage system characterized by a continuously shifting, ephemeral fluvio-lacustrine setting and subdivided the facies associations in fine-to-medium-grained sandstone channel deposits and mudstone, siltstone, and sandstone interchannel deposits. This twofold subdivision of the strata shows fining-upward cyclic sequences with scoured surfaces. On the other hand, the author of [50] subdivided the Abrahamskraal Formation into a lower part, which consists of low-sinuosity channel deposits, and an upper part with high-sinuosity channel deposits.

4. The Gansfontein Palaeosurface

The outcrop is located between the towns of Fraserburg and Williston and can be reached by turning left from Road 353 onto a tarred road to Williston and driving 2.5 km to a gate and track on the left-hand side of the road (Figure 1B). The site is currently protected, and guided tours are arranged by the Fraserburg Museum.

In the area NW of Fraserburg, the so-called Gansfontein palaeosurface (GPS coordinates 31°54′5.40″ S, 21°28′54.60″ E) is a unique site that is very rich in sedimentary structures and invertebrate and vertebrate traces (Figure 1D). The palaeosurface, exposed in a stream bed at the outlet of a dam (about 1.5 km south of the Gansfontein homestead), is about 25 m wide by 60 m long and is preserved within olive-green siltstones and sandstones. Such exceptionally preserved palaeosurfaces are rarely found in the Lower Beaufort. For this reason, the Geological Society of South Africa (GSSA) recognizes this site as a geoheritage locality [8,54].

The palaeosurface was interpreted by [3] as a distal floodplain setting. Later, it was interpreted as either a proximal crevasse splay setting or a proximal floodplain depositional environment at the base of the meanderbelt slope and between prograding crevasse splay lobes [4]. The Gansfontein palaeosurface shows a very well-preserved set of depositional environments related to an alluvial plain area, which was temporarily isolated from destructive high-energy floods and, later, covered by decantation of mud drapes. This kind of environment is rarely preserved in the geological record of the Abrahamskraal Formation, which is mostly represented by fine-grained sandstone to siltstone sheet flows with very frequent upper flow regime (flash floods) bedforms such as parting lineation structures. Moreover, the author of [4] reported other similar palaeosuface occurrences (type 1) in sedimentary sequences of this unit as well in the Teekloof Formation, which he ascribed to proximal floodplain facies.

According to references [3,4], this outcrop lies in the uppermost part of the Abrahamskraal Formation (Karelskraal Member) and about 500 m above the base of the Beaufort Group. However, this area lacks vertebrate remains and is placed north of the Great Escarpment, where the succession is very difficult to measure and correlate because of the low relief, low angle of dip, large geographic distance, and lack of continuous exposure [39] (p. 230). Moreover, from the cores, it is evident how the succession is reduced in thickness, and some units recognized in the basin epocenter south of the Great Escarpment are missing [39], so younger strata may appear lower in the succession [55]. Also, in the vicinity of Fraserburg, the Abrahamskraal Formation is attenuated and lacks the uppermost Karelskraal Member. Therefore, the Moordenaars Member of the Abrahamskraal Formation is in contact with the Poortje Member of the overlying Teekloof Formation [39], and because of the lithofacies similarity and analog depositional palaeoenvironments, this boundary lacks other elements for a correlation [4,39], and it is possible that this locality was incorrectly mapped and it is actually within the Poortje Member of the Teekloof Formation. The tetrapod ichnoassociation, recently thoroughly revised by the authors of [7], suggests the dominance of large gorgonopsian footprints (Karoopes gansfonteinensis), with the occurrence of small therocephalian footprints (cf. Capitosauroides isp.) and rare dicynodont footprints (Dolomitipes icelsi). This ichnoassociation has been assigned to the Footprint Assemblage II and is also part of the Dicynodontipus sub-biochron of the Erpetopus footprint biochron, dated as, at the latest, Capitanian to early Wuchiapingian [7,56]. This tetrapod ichnoassociation is in agreement with a post-Tapinocephalus AZ fauna, therefore supporting an attribution of the Gansfontein palaeosurface to the Poortje Member of the Teekloof Formation. The lack of large gorgonopsian forms in the Poortje Member is not a limiting factor because recent studies have evidenced how gorgonopsians reached a large size since the Guadalupian [57]. The possible occurrence of dinocephalian remains in the vicinity of Gansfontein (R.M.H. Smith comm. pers.) is also not limiting because dinocephalians have been found in the Poortje Member [58]. A small sandstone cast, with the possible shape of a vertebrate burrow, was found near the palaeosurface (see Figure 10D). Small tetrapod burrows are common in the Poortje Member of the Teekloof Formation, e.g., [59]. For the reasons explained above and because of its trace fossil content, we consider the Gansfontein palaeosurface as pertaining to the Poortje Member of the Teekloof Formation (also see [7]).

5. Results

5.1. Lithostratigraphy

The Gansfontein palaeosurface is situated in a flat area that hampers a thick profile description. Reference [3] provided a 3-meter-thick stratigraphic section, probably measuring and also correlating with some outcrops in the surroundings (composite section). Here, we focus on a detailed stratigraphic section (Figure 2) of the Gansfontein site measured at the centimeter scale containing the palaeosurface horizon. The package containing the palaeosurface is analyzed at the millimeter scale (Figure 2).

The stratigraphic log measures about 1.70 m, and its description is from base to top as follows:

• Unit 1: 4 cm thick dark grey, greenish to silver-colored, massive siltstone layer with straight to sinuous wave ripples;
• Unit 2: 2 cm thick grey-colored, medium coarse sandstones with sparse mud pellets;
• Unit 3: 30 cm thick dark olive-green-colored, finely laminated, sub-covered mudstones;
• Unit 4: 7 cm thick dark gray-greenish-colored, laminated mudstones;
• Unit 5: 17 cm thick, well-cemented, FU (fining upwards) grey-colored, medium-grained sandstones with erosive base and parallel to low angle ripple cross-lamination; mud pellet layer in the upper part and mud-cracked veneer at the top; raindrop-like imprints also recorded;
• Unit 6: about 30 cm thick sub-covered, dark grey-greenish-colored, fissile mudstones;
• Unit 7: about 18 cm thick, massive to finely laminated, grey-colored, very fine sandstones/siltstones with localized dark grey-colored mud veneers. In the first 7 cm from the base, a fine repetition of siltstones/mudstones occurs. Above, a thin mud veneer (about 3 cm) belonging to the middle part of this unit represents the main palaeosurface layer: it allows the preservation of vertebrate and invertebrate traces, several types of sedimentary structures like different kinds of ripples, mud-cracks, algal mats, rill marks, scour marks; in the last 9 cm thick dark gray-colored fine sandstones interspaced with fine siltstones and mm-scale mud surfaces occur again; a wave ripples layer and very large mud cracks at the top; abundant green-colored mud chips layers occur laterally;
• Unit 8: about 6 cm thick, fissile and cracked, dark grey-colored siltstones/mudstones with mud cracks; sparse calcareous nodules;
• Unit 9: about 10 cm thick grey to light brown-colored, massive, sandy siltstones with mud-chip conglomerate at the base, ripples, mud cracks, and invertebrate traces;
• Unit 10: about 37 cm thick yellowish to light green-colored, medium-grained, well-cemented, and sorted feldspathic sandstones with ripple cross-lamination; repeated occurrence of shallow scour surfaces with mud chips, particularly occurring at the base of the unit under even lamination. Such channel sandstones frequently show a lag represented by a mud pellet conglomerate. These intraformational lenticular and thin conglomerates characteristically line channel bottoms on the basal scoured surfaces of cyclothems [52].

5.2. Sedimentary Structures

In most cases, the original surface topography displays a suite of sedimentary bedding planes, which are commonly overprinted with run-off and desiccation structures, as well as a variety of trace fossils. A thin mudstone veneer that is non-eroded or overprinted by subsequent deposition/sediment re-working has allowed for the preservation of many of the sedimentary structures and high-detail invertebrate and vertebrate traces. This kind of preservation is generally rare, e.g., [4]. The palaeosurface is preserved on the gently undulating upper bedding plane of sheet-like, tabular silty sandstone. The sedimentary structures on the palaeosurface record the fluxes in energy during sediment deposition, ranging from high-energy flows depositing sand to the low-energy, quiet settling of mud from suspension [3,4]. During flooding stages, a series of pools and interpool ridges formed, some of which preserve the vertebrate trackways.

The scour pools are shallow depressions (50–75 cm deep, 2–5 m wide), and their margins contain sedimentary structures denoting falling water levels [4]. Water-level marks on the sandstone bedding planes (Figure 3B,E) are indicative of falling water levels (under very shallow water conditions) and the eventual emergence of the sandy sediments with sudden changes in flow directions and current velocities [52,60]. Sedimentary features related to falling water levels range from ‘emergence’ structures on the lip of the pool margin (consisting of adhesion ripples delimiting the water’s edge (Figure 3E)) to concentric zones of adhesion ‘warts’, which decrease in size downslope and are bounded on the lower margin by run-off rills, indicating the dropping water levels in the pools [3,4,61]. The adhesion ‘warts’ suggest wind shear on the sand with interstitial water.

The pools are often linked by late-stage, narrow, low-sinuosity run-off channels, which appear to have scoured the surface secondarily due to their steep (10 cm high) walls (Figure 3A,F). Pictures of modern run-off channels and pool margins in fine-grained deposits are shown as a comparison (Figure 3C,D). The bases of the narrow run-off channels are covered in linguoid ripples or small antidunes and indicate a higher flow regime (Figure 3A,F and Figure 4A). Drying of the palaeosurface is expressly shown on the interpool ridges as sandstone-filled casts of desiccation cracks.

The bottom of the palaeo-pools preserves small symmetrical oscillation ripples (Figure 4B), while an association of various kinds of wave ripples and current ripples, interference or “ladderback” ripples, and flat-topped ripples occur widely in various sectors of the palaeosurface. Ladderback ripples have been considered diagnostic of late-stage emergence run-off in intertidal environments but are not confined to tide-dominated environments [62]. Asymmetrical wave ripple surfaces are also observed (Figure 4C–E,H): this type of ripple shows well-developed bifurcation of crests, and crests are rather regular [63]. Flat-topped ripple marks (Figure 4F) indicate beveling by wavelets approximately two inches (50 mm) or less deep in water [64].

Fine-ripple cross-lamination or micro-cross-bedding, ubiquitous in the slightly calcareous siltstones and very fine-grained sandstones, gives rise to characteristic rib-and-furrow structures on bedding planes. Desiccation cracks repeatedly occur over fine-grained layers (Figure 5A,D,F), often associated with structureless greyish mudstone. Occasionally, large desiccation cracks form low, polygonal ridges on bedding surfaces, showing algal mats preserved in the mud portions (Figure 5C). In some cases, unconsolidated mud cracks are deformed by the impression of vertebrate footprints (Figure 5C). These dark grey-green mudstones often contain invertebrate traces and sometimes display wrinkled laminae (Figure 5I), formed during subaerial exposure when the wind blew across a sandy surface covered by a thin film of water. Dendritic rill marks occur very often on the rims of the pools or on rippled surfaces (Figure 5B,G). Rarely can raindrop-like imprints (Figure 5E), poorly preserved in whitish silty deposits, be observed.

5.3. Vertebrate Trace Fossils

5.3.1. Tetrapod Footprints

Several trackways are impressed and excellently preserved on the palaeosurface [3,4,7] (Figure 6). The total number of trackways is estimated to be around 20, and the number of tracks around 200. The degree of preservation of the trackways can be seen as largely controlled by the rheology of the substrate (degree of water saturation) and the animal’s body mass and pedal mechanism. It varies considerably, from 0 (worse preservation) to 3 (best preservation). In the first case, the footprints are simple sub-circular holes or indistinct depressions and are not assignable to any ichnotaxon/producer. In the second case, the footprints are wonderfully preserved, showing all details of digit and palm/sole impressions. This is the case of the type of material of Karoopes gansfonteinensis [7].

Modified true tracks preserved on the palaeosurface are from several therapsids, including abundant gorgonopsians, common therocephalians, and rare dicynodonts. Many tracks preferentially preserve expulsion rims due to the visco-elastic rheology of the sediment.

The most common trackway types were assigned to Karoopes gansfonteinensis by the authors of [7] and have previously been referred to as trackway 1 in [3,4]. These are trackways of large quadrupedal tetrapods with pentadactyl manus and pes impressions up to 26 cm long and with a high pace angulation (110–170°) (Figure 6A,C). The distinct gait of the trackmaker is characterized by the inward curvature of the digits towards the trackway-midline. The trackways have formerly been interpreted as being left by dinocephalian therapsids [4]. Ref. [7] comprehensively revised this material and instead attributed it to gorgonopsian therapsids.

These tracks are abundant (more than 100 footprints arranged in several trackways) and show a complete range of preservation, including indistinct broad footprints, digitigrade incomplete footprints, and completely impressed and well-preserved footprints. Often, transitions between different types of preservation are visible along the same trackway. Footprints commonly overprint ripples and rills and are instead cut by mud cracks.

The second most common trackways, assigned to cf. Capitosauroides isp. [7] are preserved adjacent and within a shallow pool and associated with swimming traces (trackway 3) in [4]. The swimming traces are constituted by three parallel elongated traces up to 4 cm long (Figure 6D). They can be attributed to digitigrade, quadrupedal animals walking on saturated sediment and show toe-drag marks. The five-toed manus and pes tracks leave distinct, small, sub-circular pad impressions (Figure 6E–G). The author of [4] attributed these tracks to the small dicynodont Diictodon. The author of [4] considered these trackways as being made subaqueously, based on the claw scrapes and the sedimentary structures, which suggest the margin of a pool. Also, swimming traces are observed, constituted by three parallel elongated traces up to 4 cm long. This material has been comprehensively revised by the authors of [7], who attributed it to therocephalian therapsid producers.

A third track type, of which only one trackway was identified at the Gansfontein site, shows relatively large paw-like impressions (Figure 6H,I). It was assigned to Dolomitipes icelsi and has been attributed to dicynodont therapsids by the authors of [7]. The large sub-circular and indistinct impressions occurring in some parts of the palaeosurface (non-digitiform footprints) of [4] may have been deformed Dolomitipes icelsi tracks; however, a clear assignment is not possible because of their poor preservation.

5.3.2. Fish Trails

The first to report detailed fish trails referred to the ichnogenus Undichna from the Early Permian of the Dwyka and Ecca Series was the author of [1]. This author classified the trails as Undichna simplicitas, U. bina, and U. insolentia.

Ref. [6] also described Undichna traces from the Middleton Formation. The traces display the characteristic morphology of Undichna britannica [65], which has been interpreted as a pair of intertwined waves of different amplitudes generated in soft sediment by the ventrally protruding fins of swimming fishes in freshwater settings [65,66,67]. Combined ichnological and sedimentological evidence suggests there was a low-energy, freshwater lacustrine depositional environment, where occasional higher energy currents brought in nutrients [6].

Undichnia quina and Undichna britannica from the Gansfontein palaeosurface has been described by the authors of [68]. Undichna quina is diagnosed by at least two pairs of intertwined out-of-phase sinusoidal waves, with one pair showing higher amplitudes [68].

Additionally, U. unisulca, the most simple Undichna trace characterized by a single sinuous trail, can be observed (Figure 7A). Undichna trails are very common in the central-western portion of the Gansfontein palaeosurface (Figure 7A–F). It is likely that the tracemakers were trapped in isolated channel bar ponds or shallow pools (precise dimensions unknown) in the waning phase of a flood.

5.3.3. Invertebrate Trace Fossils

Various invertebrate trace fossils also occur on the Gansfontain palaeosurface. Different kinds of bioturbation can be linked to the life activities of shallow infaunal and epifaunal invertebrates, possibly annelids, aquatic oligochaetes, nematodes, insect larvae, and arthropods.

Anyway, the invertebrate traces did not intensively re-work the sediment, thus allowing the preservation of the palaeosurface. In fact, few or no non-horizontal burrows are observed.

Ref. [3] (Figure 4.4) reported an arthropod trackway with a central furrow (?Umfolozia isp.), and the author of [4] (Figure 10) documented further indeterminate arthropod trackways. This material and further trackways (Figure 8A,E) with two parallel rows of evenly spaced impressions are assignable to Diplichnites gouldi (type A) sensu [69]. Further arthropod trackways include two parallel rows of comma-like closely-spaced impressions with a central furrow assignable to Dendroidichnites isp. (Figure 8C) and two parallel sinuous furrows assignable to Diplopodichnus isp. (Figure 8G and Figure 9F). Possibly, arthropod body imprints, first reported by (3, Figure 4.7), can also be found in grey-colored fine sediments on the palaeosurface or in its surroundings (Figure 9D,E).

Horizontal burrows and grazing trails are common on the Gansfontein palaeosurface. The author of [4] (Figure 8) described beaded trails arranged in loops (potentially made by insect larvae). Ref. [8] interpreted these beaded traces as belonging to Fodinichnia, the ethological category of traces that includes deposit feeders. These traces (Figure 9A–C) are assignable to Gordia isp. Further horizontal burrows include Planolites (Figure 10A), first reported from the locality by the author of [3], meniscate burrows (?Scoyenia Figure 10B), and the slightly sinusoidal grazing trace Helminthopsis (Figure 8I). Undetermined mud-loving beetle traces are also recorded (Figure 10C).

6. Discussion

In the Middle Permian, paleosurfaces preserving vertebrate and invertebrate tracks are very rare. Exceptions, but not all specifically ascribed to the Middle Permian, are those reported by the authors of [70] at the transition from the Ecca Group to the Beaufort Group, where a paleosurface preserves a non-marine aquatic setting with numerous ichnofossils, including tetrapod footprints and fish swim-trails, as well as body impressions attributed to rhinesuchid temnospondyl. The Wordian Onder Karoo lagerstatte [71] in the southwestern Karoo Basin also records a very rich lake-shore palaeoecosystem but with mainly insects and plants preserved. Other Permian sites with Undichna, invertebrate traces, and tetrapod footprints are the tidal flat units of the Early Permian Robledo Mountains Formation of New Mexico [72], the long-lived ephemeral water bodies, rich invertebrate–vertebrate tracks, of the Clear Fork Group at Castle Peak in Texas, USA [69], and the fluvial and marginal marine units of the Late Permian Val Gardena Sandstone Formation of Italy [68]. Some Middle Permian basins in Southern France, e.g., [73,74], also show tracks and trails of vertebrates and insects.

Therefore, on the basis of several sedimentological and ichnological aspects, we can hypothesize that this palaeosurface represented an extremely rare depositional setting that was preserved for a likely short time interval (possibly months) into a surrounding much more energetic hydrodynamic depositional environment. The traces were produced during such a time span of shallow water conditions in ponds, which alternated with emerging small and low reliefs. Preservation is most probably attributable to flooding of the floodplain with silt- and mud-rich water but under low energy conditions where the main channels are distant, and the water entered the floodplain lakes via crevasse splay events. The “sudden” but no-erosive covering of the palaeosurface, necessary for the preservation of the trace fossils, could be simply the result of mud settling from the sediment-laden water and subsequent subsidence in the basin, preventing erosion of the palaeosurface during the next major high water stand in the river systems. In particular, the laminated mudstones occurring in the Gansfontein section indicate the slow settling of mud-size particles from suspension.

This is different from the massive mudstones, which are common all throughout the Abrahamskraal Fm. and may be interpreted as a result of a quick deposition (mass movement) or bioturbation disrupting any primary sedimentary structures. In particular, as also stated by the author of [4], the distribution of palaeosurfaces in the Beaufort strata was ultimately controlled by flash floods, which transported sand out of the large mid-Permian meandering river channels into the floodplains. In the case of the Gansfontein palaeosurface, preservation of the vertebrate and invertebrate traces was enhanced by a silty clay veneer that accumulated on the sand surfaces during a waning flood. Also, in the case of the vertebrate footprints, it is very likely that the sandy base favored their preservation by providing stable support, and the silty clay veneer enhanced the preservation of fine details.

It was likely the rapidly fluctuating hydrodynamic regime of the topmost Abrahamskraal Formation and basal Teekloof Formation rivers [52,75] that promoted the formation and preservation of the different kinds of floodplain palaeosurfaces analyzed by Smith [4]. Anyway, the difficulty of preservation of such palaeosurfaces is probably due to both the originally limited area of sedimentary environments in which they were formed and the mechanical weathering that rapidly destroyed the fragile surfaces soon after they were exposed [4].

The tetrapod footprints are generally superimposing ripples and rill structures, suggesting a subsequent passage of the trackmaker, i.e., after the dynamic phase of the flow. The very variable preservation suggests that the rheologic conditions of the sediment varied through time (different preservation in the same spot) and area (different preservation along the same trackway). In any case, the exposure time was sufficient for the sediment to pass from completely water-saturated (indistinct footprints) to plastic (good footprint preservation) to dry (digitigrade footprints). Also, the tracks are commonly cut by mud cracks, testifying to a drying process before burial. Some tracks were clearly associated with pools because they preserved sliding and swimming traces together with locomotion traces.

According to the authors of [76], who studied the response of modern macroinvertebrate communities after river floods, over the short term, their diversity decreased when compared to pre-flood levels, whereas total macroinvertebrate density remained unchanged. Over the long term (1 and 10 years after the floods), macroinvertebrate diversity and community structure at the studied site returned to pre-flood levels without increasing reference conditions. This is consistent with the invertebrate ichnoassociation observed at the Gansfontein palaeosurface, which is not very diverse and also shows a low disparity, sensu [10], with a prevalence of surface trackways, grazing trails, and resting traces. This represents a typical depaupered Scoyenia ichnofacies, typical of floodplain depositional environments [11].

The lack of infaunal traces and the simultaneous abundance of surface traces are in agreement with a relatively short time interval of deposition and rapid burial. The occurrence of subaqueous grazing traces, as well as swimming traces of fish and tetrapods and the clear presence of pools and rills, testify to the occurrence of localized but interconnected small water bodies.

Undoubtedly, the Gansfontein palaeosurface is a true substrate, sensu [77], i.e., a preserved surface that demonstrably existed at the sediment–water or sediment–air interface at the time of deposition. These surfaces possess, e.g., [71], high value as repositories of palaeoenvironmental information, revealing fossilized snapshots of microscale topography from deep time. Again, according to the above-cited authors, “true substrates are fundamentally related to a state of stasis in ancient sedimentation systems, and distinguishable from other types of bedding surfaces that formed from a dominance of states of deposition or erosion. Stasis is shown to play a key role in both their formation and preservation, rendering them faithful and valuable archives of palaeoenvironmental and temporal information” [77] (p. 1).

On the basis of the many and different sedimentary structures and sedimentologic inferences, we can argue that the Gansfontein palaeosurface formed in a medium-to-distal crevasse splay (Figure 11), which represented a topographic “high” with respect to the alluvial plain areas, and thus could permit a longer preservation of the surface before its definitive sealing by a flood or alternatively by flooding of the entire floodplain area during the next flood cycle of the main river system.

7. Conclusions

Palaeosurfaces like the one described in this discussion are very important as repositories of palaeoenvironmental information because they reveal a kind of fossilized snapshot of micro- to mesoscale topography from deep time. They are notable for their sedimentary structures and for the numerous preserved vertebrate and invertebrate traces, providing insight into the environment of a key interval of Earth’s history. The Gansfontein “oasis” represents a unique window to a distinct animal community and habitat, permitting the acquisition of detailed data on Middle Permian life in continental areas of Southern Gondwana.