Palaeocurrent directions deduced from cross bedding, erosional structures and channel shape, indicate a dominant transport of the sediments from west to east [14]. Lenses of red or grey sandstones can be several metres thick. Their lateral extent varies from a few metres to more than a hundred metres. The lower, erosive surface, of the sandstones exhibits an assemblage of flute casts, groove casts, prod and bounce marks. The upper surface is covered with ripple-marks. The sandstone is built up of a succession of laminae with well-developed parting lineation and crescent-shaped scour marks. The vertical distribution of the sedimentary structures indicates a decrease in strength of the current with time. Sandstones resulted from sediments deposited at times of flood [14] and [17]. They contain plant debris derived from equisetales, ferns and conifers and amphibian bones (stegocephalids). The fossils which are disarticulated and broken, usually are intersecting the bedding plane and are jumbled together. Obviously, they have undergone prolonged transport and reworking before accumulating downstream in the watercourses. Thus these assemblages provide valuable data about the communities that lived upstream from where the fossils are found. In particular, amphibians are never found in the more distal area of the ‘Grès à Voltzia’ delta.

Lenses of green or red silty clay represent overbank deposits laid down by the overspilling of flood water from river channels onto adjacent lowlands. In the resulting temporary ponds and pools, a rich aquatic fauna flourished and their margins were colonised by a variety of plants that were themselves inhabited by many arthropods. The occurrence of Lingula in some shale lenses indicates that some pools were supplied with brackish water. It is likely that the delta platform was scattered with a set of water bodies from more fresh upstream to brackish downstream. The fossil assemblages represent biocoenoses, as indicated by the preservation of animals in life position, such as the brachiopod Lingula, and by the fossilisation of in situ root systems as well as by the occurrence of trails of limulids and clutches of insect and fish eggs.

The water bodies were harsh environments. Indeed, the fossiliferous lenses of shale correspond to pools of small lateral extent, perhaps on the sites of abandoned channels, which dried out relatively quickly, resulting in mass mortality among the aquatic fauna. As one goes upwards investigating a single shale lens, a diverse aquatic fauna is gradually replaced by terrestrial vegetation, sometimes with the root systems preserved in situ, and desiccation cracks appear at the top. Similarly, trace element content of the clay minerals, especially boron, increases and indicates that the salinity of the pool rose over time. The occurrence of pyrite testifies that the water body became oxygen depleted. All these observations confirm that the pools decreased in size and dried out. Concerning the gymnosperms for instance, a 60-cm-thick shale lens exhibits only one cycle from the immature reproductive organs to the dispersed seeds, showing that it corresponds to a short period of time [14]. It is evident that the organisms must have suffered great variations in temperature, oxygenation and salinity as time passed. Indeed, some palaeoecological characters can be considered as responses of the fauna to harsh environmental conditions, such as the sheaths of mucilage that protected the insect eggs from desiccation [18], and the very short life cycle, a couple of weeks, of the estheriids that allows these crustaceans to survive long enough to reproduce in ephemeral ponds [12] and [34].

As far as the systematic attributions are reliable, Table 2 shows that the marine fauna is composed of a greater proportion of holdover taxa than the freshwater/brackish fauna, which in turn is composed of more holdover taxa than the terrestrial fauna. This may imply that extinction intensity was greatest amongst terrestrial faunal communities and lowest in the marine ones. Indeed, the terrestrial fauna from the ‘Grès à Voltzia’ is represented by new genera and new families, whereas the marine organisms still resemble the Palaeozoic ones. The land vegetation partly resembles the Late Permian flora, from which it is considered to be descended [9]. Moreover, the ‘Grès à Voltzia’ fossil record confirms the statement that the replacement of the Palaeozoic taxa by the Mesozoic ones was not an abrupt or a complete event. For instance, a comparison of the faunas from the ‘Grès à Voltzia’ and the middle Pennsylvanian Mazon Creek (Illinois, USA) shows that they have 2 genera and 12 families in common [7].

The crustaceans, which are the dominant group among the aquatic fauna, provide good examples of this statement. They include the family Euthycarcinidae, which has representatives in both Carboniferous and Triassic faunas, and the superfamily Cycloidea, which comprises the Carboniferous genus Cyclus and the Triassic genus Halicyne (Fig. 4). The latter survived until the end of the Triassic [50]. In the ‘Grès à Voltzia’, they coexist with more evolved Malacostraca, such as the isopods Palaega and Anhelkocephalon and the decapods assigned to the recent families Penaeidae (Antrimpos) (Fig. 3) and Erymidae (Clytiopsis). It was recently suggested that the crustaceans were amongst the final groups of organisms to reappear in the marine realm [58]. From this point of view, environments such as the ‘Grès à Voltzia’ delta may have acted as a refugium for these arthropods during the Permian ocean anoxia, unless they invaded later from some other unknown refugia.

The arachnomorphs of the ‘Grès à Voltzia’ also show a similar mixture of Mesozoic and Palaeozoic representatives. The scorpions comprise two genera, one of which belongs to the recent super-family Buthoidea (Protobuthus), whereas the other (Gallioscorpio) exhibits several features inherited from Palaeozoic lineages [38]. In contrast, the only spider described from the Buntsandstein belongs to the living mygalomorph Hexathelidae [54].

The rich entomofauna of the ‘Grès à Voltzia’ needs more extensive investigations. Fossil insects systematic of family level remains unsatisfactory [5]. Considering the ordinal rank, among the 15 orders which are present, 13 were present in the Palaeozoic. The order Blattodea, for instance, comprises genera having Palaeozoic features (Transitoblatta) and genera with features of modern cockroaches (Voltziablatta, Scleroblatta) [39]. The Odonatoptera and the Ephemeroptera reveal the same story (Figs. 1 and 2). Concerning the Odonatoptera, one taxon (Voltzialestes), which belongs to an undetermined family of the Protozygoptera, announces the modern Odonata, whereas another taxon (Triadotypus guillaumei), which belongs to the family Triadotypidae, is close to some Palaeozoic representatives [39]. As to the Ephemeroptera, one of the isolated wings (Triassodotes) shows affinities with those of the Permian family Misthodotidae, whereas a larva, assigned to the genus Triassonurus, is similar to those of the living family Siphlonuridae (Marchal-Papier, oral com.)

With regard to the flora from the ‘Grès à Voltzia’, it shows many similarities with the Permian flora and therefore it is considered to have been descended from it [9]. Indeed most of its taxa seem to be inherited from the Palaeozoic, such as the horsetail Schizoneura paradoxa, the fern Anomopteris mougeoti [22], the lycopsid Pleuromeia [21], a descendant from the widespread Carboniferous taxon Chaloneria [24], and the conifer Yuccites, a possible survivor of the Palaeozoic cordaites [19].The fern genus Neuropteridium, the foliage of which resembles that of Neuropteris, similarly may be a descendant of this Carboniferous genus. Likewise the conifer genus Voltzia, which is the most characteristic fossil of the ‘Grès à Voltzia’, already existed in the Middle Permian of Germany [53]. The regular branching pattern shown by the species V. walchiaeformis from the ‘Grès à Voltzia’ quite resembles that of the Permian Walchia. Moreover, it is worth noting that the pollen cones from the ‘Grès à Voltzia’ (Darneya and Willsiostrobus) show two kinds of organisation, of which only the type represented by Willsiostrobus (Fig. 7), with a structure similar to cones of the extant Araucariaceae [20] and [23], survived beyond the Middle Triassic. In contrast, those assigned to Darneya show archaic features and do not seem to have survived the Anisian. Indeed, Darneya (Fig. 6) resembles the pollen cones of some Permian cordaites and some Permian conifers, particularly Voltzia hexagona [23], [32] and [53]. Moreover, it is likely that the conifer foliage of the ‘Grès à Voltzia’ attributed to Albertia existed in the Permian, where it is called Archaeopodocarpus germanicus, Ullmannia bronni or Culmitszia florini ([20] and unpublished observations). These observations show that Albertia already existed in the Palaeozoic. A Permian refugium where arboreous forests from the Carboniferous still persisted has already been described in China [65].

According to this analysis of the Triassic floras and their interpretation in the light of the generalized model of biotic recovery proposed by Kauffmann and Erwin [30], the ‘Grès à Voltzia’ flora would represent the first stage of the recovery interval. The Early Triassic Pleuromeia flora would correspond to the preceding survival interval [24].

In brief, the end-Permian extinction corresponds to a step-wise, graded mass extinction rather than to a catastrophic extinction, which implies that the crisis was interrupted by more or less long intervals during which new species can appear [25].

Jablonski [27] introduced the concept ‘Lazarus taxa' to refer to organisms that vanish from the stratigraphic record during the biotic crisis and reappear later. Schubert and Bottjer [51] used the term ‘disaster forms' to identify opportunistic taxa that bloom during the aftermath of mass extinction events. ‘Progenitor taxa' were defined by Kauffman and Harries [31] as species that evolved during the crisis and seeded the post-extinction recoveries, thus helping to rebuild the new ecosystems.

Living fossils refer to taxa with low evolutionary rates. They often possess wide environmental tolerance, e.g. to fluctuations of salinity and/or oxygen content, or survive in areas where predators and competitors are rare. The absence of those taxa during long stratigraphical intervals implies that unsampled refugia existed. In the ‘Grès à Voltzia’ Formation, living fossils are represented by the brachiopoda Lingula tenuissima Bronn, which is omnipresent in the different facies, as well as by the limulids and the coelacanth. But the most astonishing example of morphological stasis belongs certainly to the crustacea Triops (Apus) cancriformis (Schaeffer), a species fully identical to the living one (Fig. 5). The same taxon was described from the Permian of the French Massif Central [19]. Its re-appearance in the Middle Triassic, after apparently vanishing from the fossil record, is typical for a ‘Lazarus taxon’.

The branchiopod crustacea estheriids (Isaura, Palaeolimnadia, Palaeolimnadiopsis, Praeleaia) are the most common animals in the ‘Grès à Voltzia'. Thanks to their small body size, their short life history of only a few weeks and their rapid population growth, they may be considered as typical ecological opportunists adapted to colonise temporary water bodies. Their numerical dominance in the ‘Grès à Voltzia’ emphasizes the persistence of stressful environmental conditions during the Early Triassic.

It is well known that the body size of organisms depends on the availability of resources and space. That is why animals from islands are often smaller than their representatives from nearby continents. The stressed, often brackish and fragmented habitats of the ‘Grès à Voltzia’ environment certainly affected the body size of the organisms. Indeed, small body size is widespread among the ‘Grès à Voltzia’ living communities. For instance, the body length of the spider Rosamygale does not exceed 6 mm [54]. The isopod crustacea Palaega pumila measures less than 7 mm. The insect fauna is mostly represented by isolated wings, the size of which varies from 2 to 20 mm, with an average of 5–6 mm. However, it is worth noting that it also contains a big dragonfly Triadotypus guillaumei, the span (27 cm) of which is half the span (about 60 cm) of the very large Carboniferous Meganeura, from which it is supposed to be descended. Likewise, the size of the bivalve Myophoria vulgaris (Schlot.) is half the size of the same species in the marine Middle Triassic (Muschelkalk Formation).

A similar tendency to body size reduction was documented by Urbanek [61] in his study on the recovery of graptoloids after the Silurian biotic crisis. He termed this the ‘Lilliput' effect, and it is a phenotypic response to the unfavourable conditions that prevailed during the immediate aftermath of this biotic crisis [29] and [45]. A different interpretation of the body size reduction has been suggested by Twitchett [57]. According to him, the drop of primary productivity during the crisis involves a decrease in the biomass, which leads to a reduction either in abundance of the populations or in body size of the taxa. The latter have more chances to be preserved in the fossil record.

Size reduction is also documented in the flora, particularly by the conifer Aethophyllum stipulare. Indeed, whereas fossil and recent conifers are typically shrubby or arborescent, Aethophyllum is an herbaceous plant of only one or two metres high. Moreover, in very poor environmental conditions, its height can be even more reduced. Indeed, one of the entire specimens with root and ovulate cones is only 30-cm tall. This conifer is interpreted as a fast growing ruderal species rapidly invading vacant ecospaces thanks to a prolific production of small seeds [48].

It is commonly observed that after a disastrous event like a forest fire, the recovery of the devastated environments starts with fast growing herbaceous plant communities, such as weeds and grasses, and trees arrive later. Lycopsids and ferns are often pioneer species that take over disturbed sites. Indeed, spore-bearing vascular plants are superior to seed plants for preparing recolonisation sites [43]. The ‘Grès à Voltzia’ Formation contains some common ferns such as Anomopteris, proving that they found propitious environmental conditions to settle and grow. Fuchs et al. [13] showed that Anomopteris colonized the borders of ephemeral water bodies in the Middle Buntsandstein of the Eifel (Germany), together with the lycopsid Pleuromeia. This latter genus was an opportunistic pioneering plant in the Early Triassic, which took advantage of the vacuity of the environment to colonise the empty niches after the end-Permian life crisis. Indeed, it often is the only fossil plant in the Lower Triassic plant record, being represented by dense monospecific populations preserved in life position [21]. The same observation has been made in the Lower Triassic of northern China, showing that it used the same ecologic strategies [64]. The conifer Aethophyllum also very likely used its fast growth to proliferate and occupy quickly wide areas. This also may explain why its pollen grains, Illinites kosankei Klaus, are widespread in many palynological assemblages from the Triassic of Europe and Asia [20].

Concerning the colonization of the ephemeral water bodies, the omnipresence of estherids crustacea in all fossiliferous localities of the ‘Grès à Voltzia’ may be interpreted as one of the first stages of the biological recovery of these biotopes, preceding the arrival of bigger predators.