Palaeoecology and environmental interpretation

A reconstruction of the probable life appearance of ?Opsieobuthus tungeri sp. nov. is shown in Fig. 7. Applying the criteria of McCoy and Brandt [31] for recognizing fossil scorpion mortalities (cf. moulted exoskeletons) we can interpret the Chemnitz fossils as more likely to be mortalities. The pedipalps are preserved drawn in, rather than extended out, and the body is largely straight, without a curved mesosoma. We also have evidence for additional fragments of (?moulted) cuticle immediately behind TA 1116 (Fig. 8a) and the isolated accumulations of cuticle fragments in specimens such as TA1177 and TA1187 (Fig. 8b). At least TA1126, the holotype, was discovered in what was likely to have been its original life position within its natural habitat (Fig. 4). It was found in a compacted depression, positioned beneath a woody root of almost 6 cm width. This depression is located 2 to 6 cm below the flattened root and about 8 cm beneath the palaeosol surface. From the entrance to the distal range of this putative burrow there is recognizably a gradually decreasing amount of red-coloured clay (clearly visible under polarised light; see Fig. 9). Because this kind of a pure clay layer has not been observed anywhere in the soil horizon, it is interpreted as clay illuviation into the (compacted) scorpion burrow. In support of this illuviation interpretation there is also a difference in grain size between the burrow infill – the red clay layer – and the surrounding palaeosol.

Fig. 8 Evidence for ecdysis. a Alternative view of the paratype TA1116 under polarised light showing additional (?metasomal) cuticle fragments immediately behind the main body. b Additional specimen (TA1187) showing isolated remains of scorpion body segments; again perhaps from a moulted exuvium. Scale bars 10 mm Full size image

Fig. 9 Evidence for being preserved in situ in a burrow. Alternative view of the holotype TA1126 showing its wider context in the matrix. The sediments surrounding the scorpion outline and define a putative burrow around this scorpion (delimitated by a white dotted line), which was discovered immediately below a woody gymnosperm root (cf. Fig. 4); whereby red-coloured clay appears to have been washed into the burrow entrance (arrow indicates direction of clay illuviation). Image taken under polarised light. Scale bar 10 mm Full size image

The width of this clay layer corresponds exactly to the body width of the scorpion TA1126 which supports our interpretation of the animal being buried in its burrow. The putative burrow extends for several cm behind the scorpion body while maintaining this same width, i.e. it does not form a halo around the body as would be expected if this effect was caused by decay. Furthermore, we know of no decay-related (bio)chemical reaction which would yield illicit clay; while at the same time it is common knowledge that hollows in soils caused by burrowing organism, or by decayed roots or other plant remains, are framed (or more or less completely filled) by clay illuviations as in our material. Modern burrowing scorpions often moult inside their burrows [32]. The additional presence of isolated body remains (also preserved as imprints and here interpreted as further possible moulted exuvia remains) inside the burrow range also points to the in situ situation of TA1126.

In relation to the preservation of the scorpions, the volcanic ash fall effectively acted as a ‘coffin lid’ to cover the material. The palaeosol itself was too coarse-grained to reveal fine anatomical details, and at the same time was apparently too aerated/oxidised to preserve any organic polymers from the original cuticle. According to the general litho- and biofacies characteristics of the sedimentary unit containing the horizon yielding the scorpions, this layer is interpreted as an alluvial palaeosol [15]. The most conspicuous feature is the common presence of roots in different forms of preservation, intensive colour mottling, and the occurrence of carbonate glaebules of different sizes. The rooting of plants and other processes involved in soil formation, such as swelling, shrinkage, pedoturbation or various animal activity, have altered or completely destroyed most pre-existing sedimentary structures. Both the red and purple mottling of the muddy sediment and the loss of organic matter indicate periods of soil oxidation that are usually observed in well-drained surface soils. The lack of carbonaceous root preservation and rubefaction on one hand, and evidence of periods of more sustained plant growth, waterlogging, and the lack of visible endogenous ichnia of the Scoyenia-ichnofacies on the other, point to a polygenetic palaeosol that formed at a relatively low accommodation rate. The last phase of palaeosol formation took place during seasonally high groundwater levels [15].

Eventually, this palaeosol supported a dense vegetation dominated by hygrophilous elements, but did not develop any organic deposits such as peat, pointing to a nearly complete recycling rate of the plant litter within this forested habitat. As remnants of the primary sediment composition and structures in both the soil horizon and the sediments beneath the palaeosol indicate, soil formation and growth of the forest took place in a special local sedimentary environment of the typical Leukersdorf Formation wet red beds. Deposition was dominated by suspension, in places also with a minor bedload of sandy-pebbly braided river channels, and caused a multistacked, fine-grained deposit to form in a distal floodplain environment. Recent investigations based on specific geochemical proxies and anatomical characteristics of the plant fossils provided evidence of strong seasonality of this environment and revealed a mean annual precipitation of 800 to 1100 mm [15].

Terrestrial predatory arachnids

The mode of preservation and depositional environment documented here in the Petrified Forest of Chemnitz contrasts markedly with the environment interpreted for the majority of the Paleozoic scorpions. There remains a long dispute regarding the original habitats of Paleozoic scorpions with some authors, especially Kjellesvig-Waering [22], interpreting almost all Paleozoic scorpions as aquatic. By contrast, comparative anatomy [33] has been used to argue that the arachnid book-lung has a single origin, which would imply that all scorpions were terrestrial throughout their geological history. Anatomical characters which could support a marine/aquatic or terrestrial mode of life have been proposed [34]. However, recently, even taxa whose mode of life appeared to be well substantiated have come under discussion again. For example, the Devonian Palaeoscorpius devonicus Lehmann, 1944 was, for many years, regarded as the epitome of a marine form, in part based on the presence of unequivocally marine animals in the Hunsrück palaeoenvironment, but restudy has challenged this interpretation by finding evidence for possible lungs in this scorpion [35]. At the same time, another Devonian genus, Waeringoscorpio Størmer, 1970 bizarrely seems to show features consistent with having external gills similar to those of certain modern insects which have become secondarily adapted for benthic aquatic life [36]. It is possible that scorpions occupied a wider range of habitats in the past than they do today. Even clearly terrestrial forms, such as those from the Viséan of Scotland [26], or representatives of modern families from the Lower Cretaceous Crato Formation of Brazil [37], were obviously washed into their lacustrine depositional areas alongside the many associated plant remains.

The Chemnitz scorpions come from an unequivocally terrestrial palaeoenvironment, significantly without any indication of large standing water bodies, but with multiple lines of evidence (calcic to ferric palaeosols, frequent growth rings in woody plants) for at least seasonally dry conditions. A further reconstruction of ?Opsieobuthus tungeri sp. nov. in its original environmental setting is shown in Fig. 10. Scorpions today inhabit a wide range of environments from hot, dry deserts to warm, humid rainforests. Extant scorpions commonly live in burrows (see e.g. [38] and references therein), either excavating them themselves or using existing burrows made by other animals. They may spend the best parts of their lives within these retreats, emerging only infrequently and often at night in order to hunt [39]. Burrowing scorpions of around 12 cm body length (or larger) are very common in the tropics, such as Heterometrus spp. in Asia (Scorpionidae: burrows of up to 40 cm in wet soils), Pandinus spp. in Africa (Scorpionidae), or Brotheas spp. in South America (Chactidae). They all make burrows in the organic soil, fine mineral soil, and inside rotten logs. Presence of exuvia (moult skin) next to one fossil specimen at Chemnitz (Fig. 8a) suggests an ecdysis hideout during moulting.

Fig. 10 Ecological reconstruction. ?Opsieobuthus tungeri sp. nov. placed in its suggested original environment at the mouth of a burrow on the forest floor. Drawing by Frederik Spindler Full size image

Scorpions today are invariably non-specialized predators. Modern species mostly feed on other arthropods, but will also take gastropods and even small vertebrates. Among fossil animals in the Chemnitz Fossil-Lagerstätte, these large (~12 cm) scorpions were probably among the top invertebrate predators. Their prey could include smaller animals: insects (fossils not found so far), myriapods and gastropods. We do not know if they were predated upon (in comparable modern habitats, large scorpions are often the top predators) but different vertebrates are also known from this fauna. Scorpion density at Chemnitz can be formally estimated as not less than one per 200 m2 (i.e. at least two specimens in a c. 400 m2 plot: Fig. 4). Such a range is not uncommon in modern forests. Among extant scorpions, densities can be higher (deserts >1 m2; littoral c. 10 m2), i.e. they are most common predators in these environments.

Morphological adaptations and dimorphism

Pectines are unique to scorpions and commonly preserved in Carboniferous forms. Their presence may indicate a terrestrial (litter) adaptation for prey/mate localization and general olfactory orientation. The number of pectinal plates (teeth) on these organs is generally species-specific; with slight variation and some indication the number increases slightly with age. Furthermore, the number of pectinal teeth in extant scorpions is usually sexually dimorphic (Fig. 11); males always have a higher number of teeth, which also are markedly longer than those of females. A larger size (length) of male pectinal teeth allows them to house a considerably higher number of chemosensory sensilla within much larger sensory fields. Although only two complete specimens are known from Chemnitz, their similar size and similar habitus suggest that these fossils are probably conspecific (see also above). Nevertheless, there are a few subtle differences between the holo- and paratype and, in the light of what is known about modern scorpion pectines, it is interesting to speculate whether we have a male and a female, expressing sexually dimorphic characters. In particular the pectines of the paratype appear to be longer and thinner than those of the holotype (Fig. 6b), and to express the male character (see above) of slightly longer pectinal teeth (Fig. 6d). If this interpretation is correct, with a female holotype and male paratype, then the informal names Birgit and Jogi were assigned to the correct genders.

Fig. 11 Pectines of living scorpions. Hadrurus anzaborrego Soleglad et al., 2011 [49] (Scorpiones: Caraboctonidae) from California, USA; above, the female, below, the male. Like the paratype of the new fossil species, the male here has more pectinal teeth and the individual teeth are slightly longer. After ([49]: figs. 45–46; courtesy of Michael E. Soleglad) Full size image

In a wider context, the number of pectinal teeth appears to be very high (50–70 per comb [21]) in at least some of the extinct mesoscorpions; a group which can be defined [26] on anteriorly located median eyes, 30–60 ocelli in each lateral-eye group (when present), and a preanal segment which is equal to or shorter than, the preceeding postabdominal segment. Our new material supports this basic assertion in documenting ca. 40–50 teeth borne on noticeably large pectines (see Description). The number of teeth in Orthosterni of comparable size is lower; maximal pectinal teeth numbers in extant males reach over 40. The exceptional, highest number of 45–58, in extant scorpions has been recorded in males of the South American species Brachistosternus multidentatus Maury, 1984 (Bothriuridae) [40].

The presence of two, probably conspecific, specimens assignable to different sexes implies a resident, reproducing population. They were found within ca. 2 m from one another (Fig. 4) and may thus even be a mating couple; many extant scorpions, especially of this large size, would normally maintain a larger distance from one another (R. Teruel, pers. comm.). Their preservation in situ and in close proximity – one male and another female, the female perhaps even being freshly moulted if the cuticle fragments are not so old and originate from the same animal – could even be the earliest tentative evidence of mate-guarding behaviour, a phenomenon first reported in scorpions by Benton [41]. Mate-guarding involves an adult male detecting a subadult female and staying with her in the same shelter to prevent other males from entering until she moults to maturity. Rolando Teruel (pers. comm.) observed mate-guarding (always by the male) in essentially all Caribbean species of Buthidae and Scorpionidae.

Scorpion evolution

A major unresolved question for fossil scorpion research is whether the observed peak of Carboniferous diversity reflects a genuine period of scorpion diversity, or whether it is an artifact of sampling bias resulting from coal mining and the huge mine dumps and/or unreliable taxonomy. In favour of the latter hypothesis is the fact that many fossil species stem from the work of Kjellesvig-Waering [22] and Petrunkevitch [28] on the Mississippian of Scotland and the late Carboniferous Coal Measures of Europe and North America in particular. Both authors tended to recognise multiple species, often diagnosed on dubious characters. Revision has invariably reduced the observed diversity, such as in the case study of Compsoscorpius buthiformis (Pocock, 1911) from the British Middle Coal Measures for which nine junior synonyms could be recognised [4].

At the same time, the Carboniferous does appear to have been a crucial time of transition for these animals. As noted above, Kjellesvig-Waering [22] regarded almost all Paleozoic scorpions as aquatic and developed a typological scheme of families and superfamilies. Translating these, commonly monotypic, groups into a meaningful phylogenetic classification is still a work in progress. Three major lineages of fossil scorpions were recognised by Jeram [26] termed palaeo-, meso- and neoscorpions, and defined by accumulations of apomorphic characters to yield increasingly modern-looking species. A modification of these groups, recognising a series of increasingly derived clades at a finer level of resolution, can be found in the thesis of Legg [24]. This (unpublished) model includes higher taxa successively defined by the presence of two pairs of coxapophyses, the presence of a pentagonal sternum, and finally the crown-group orthosterns which are defined by book lungs opening in the middle of the sternites. Fossils assignable to of all of these grades can be found coexisting during the late Carboniferous. Thus the elevated number of fossil scorpion species could reflect a genuinely wider range of scorpion body plans at this time. Essentially, current data suggests that both stem- and crown-group species lived alongside one another in the Coal Measures. With this in mind, our new Permian finds are of particular interest in revealing whether one of the more basal scorpion lineages outlasted the Coal Measures, or whether the decline of the coal forests witnessed – or perhaps even induced – radiations of scorpions much closer to the living clades.

Our results suggest that the Chemnitz scorpions are probably best placed in a genus previously known from the Coal Measures: Opsieobuthus (see Systematics). In other words, these new Permian fossils express a mixture of plesiomorphic and derived characters including a number of typical features for Coal Measures scorpions, such as large and anteriorly positioned median eyes, spatulate coxapophyses on the first pair of legs, and the absence of an elongate preanal segment prior to the telson. Unfortunately, the position of the book lung spiracles (either marginal on the sternites or within the sternites, as in modern (orthostern) species is not resolvable in the Chemnitz fossils. Nevertheless, the implication is that at least one stem-group lineage did outlast the Carboniferous coal swamps, and was still present in the early Permian (ca. 291 Ma) of Germany.

This condition is also reflected in the Triassic Mesophonus Wills, 1910 [42] scorpions from the Lower Keuper (ca. 230 Ma) of England [42, 43], and the slightly older (Anisian: ca. 247 Ma) Gallioscorpio voltzi Lourenço and Gall, 2004 [11] from the Buntsandstein of the Vosges in France. These fossils, both mesoscorpions in traditional terminology, also retain evidently plesiomorphic character states, such as anteriorly placed median eyes and compound lateral eyes. In this context, one could argue that stem-group scorpions maintained a presence until at least the middle part of the Triassic period. At the same time, the early Triassic also yields the oldest putative member of a modern scorpion superfamily: Protobuthus elegans Lourenço and Gall, 2004 [11], also from the Vosges. However, the exact position of this fossil, assigned to an extinct family, Protobuthidae within the superfamily Buthoidea, has not been tested cladistically and its buthoid affinities have been questioned [44]. Jurassic records of scorpions are extremely sparse and have either been shown to be misidentifications, or are poorly preserved and not referable to any particular higher taxon [45]. Demonstrably modern scorpion families first appear as Chactidae and Hemiscorpiidae from the ca. 115 Ma Early Cretaceous Crato Formation of Brazil [37], with Chaerilidae reported from the ca. 99 Ma Late Cretaceous Burmese amber from Myanmar [46]. Note that Burmese amber also yields a number of putatively extinct family groups (reviewed by [47]). Modern families such as Buthidae begin to appear in subsequent Cenozoic deposits like the Baltic and Dominican Republic amber.