The material on which the present study is based was collected over a period of almost 200 years (Jaeger 1828; Weber 2013; Schoch and Seegis 2016; Schoch et al. 2022). In the 1820s to 1860s, the Gaildorf locality yielded five skulls of large to giant size (Meyer in Meyer and Plieninger 1844; Meyer 1855). One of them was associated with an articulated postcranial skeleton that was partially figured by Plieninger in Meyer and Plieninger (1844). The surviving material is housed in the collections at Tübingen (GPIT) and Stuttgart (SMNS), and the now-lost giant snout was fortunately described and figured by Meyer (1855).

The skull from Bedheim in Thuringia collected by Rühle von Lilienstern (1935) is housed in the Natural History Museum Berlin (MB), whereas the more recent finds from Arnstadt are housed in the Natural History Museum Schleusingen (NHMS).

Further material accumulated from deposits in Baden-Württemberg are now housed at the SMNS, as is the entire collection from the Kupferzell excavation of 1977 (Schoch et al. 2022). In the 1980s, finds from Michelbach an der Bilz and Wolptershausen came into the MHI collection at the Muschelkalkmuseum Ingelfingen, as did the bulk of the material collected by private collector Werner Kugler in Vellberg-Eschenau (Schoch and Seegis 2016). Since 1998, much additional material was collected by the first author and colleagues at Vellberg that is now housed at the SMNS.

Analyzing ontogenetic changes in extinct taxa faces a range of challenges, such as incomplete specimens, poor preservation of small stages, taxonomic identification of all growth stages and variation of the studied samples in space and time. Even if restricted to samples collected at the same locality and within one horizon, time averaging is unavoidable in most cases (Boy 2003; Schoch 2009).

The studied material of Mastodonsaurus giganteus was collected at numerous localities, nine of which yielded diagnostic skull material (Gaildorf, Markgröningen, Hoheneck, Kupferzell, Vellberg, Michelbach an der Bilz, Wolpertshausen, Bedheim and Arnstadt). Among the diagnostic finds, only Vellberg and Kupferzell produced samples that are rich enough to study ontogenetic changes, whereas the other localities gave only adult or relatively large juvenile specimens. Larval specimens are known from Kupferzell K3 and Vellberg E6, whereas juvenile and subadult specimens are known from Vellberg E6 and E7.

Our focus on the ontogenetic changes in the skeleton of M. giganteus is therefore entirely based on the sample from Kupferzell K3, K4 and Vellberg E6 and E7. These two localities represent coeval and regionally neighbouring lake deposits, separated by 25 km distance. They both fall within the topmost units of the Untere Graue Mergel, and the lacustrine facies have been studied in detail (Schoch and Seegis 2016; Schoch et al. 2022).

All the studied specimens referred to Mastodonsaurus giganteus were identified on the basis of autapomorphies (see diagnosis). The samples from Vellberg (E6, E7) and Kupferzell (K3, K4) were thoroughly examined for differences between all preserved stages of ontogeny, but we identified only individual variation rather than geographically consistent clusters. We therefore conclude that the samples from Kupferzell and Vellberg, on which our ontogenetic study rests, likely represent the same species and are coeval within the limits of time averaging.

Anatomical

ap, anterior process; apv, anterior palatal vacuity; ch, choana; d, dentary; eo, exoccipital; f, frontal; HHL, posterior skull length; HHW, posterior skull width; IOW, interorbital width; ipv, interpterygoid vacuity; ju, jugal; la, lacrimal; m, maxilla; n, nasal; nar, naris; ORL, orbit length; p, parietal; pap, parapophysis; pf, postfrontal; pl, palatine; pm, premaxilla; pmf, premaxillar fenestra; po, postorbital; pop, posterior process; pp, postparietal; prf, prefrontal; ps, parasphenoid; pt, pterygoid; q, quadrate; qj, quadratojugal; SL, skull length; sq, squamosal; st, supratemporal; stf, subtemporal fenestra; ta, tabular; vo, vomer.

Institutional

GPIT, Institut für Geologie Tübingen; MB, Museum für Naturkunde Berlin, Leibniz-Institut für Evolutions- und Biodiversitätsforschung; MHI, Muschelkalkmuseum Hagdorn Ingelfingen, NHMS, Naturhistorisches Museum Schloss Bertholdsburg, Schleusingen, SMNS, Staatliches Museum für Naturkunde Stuttgart.

The smallest specimens comprise symphyses that were identified in screen-washed samples from Kupferzell (K3) and small elements of the pectoral girdle from Vellberg (E6). As the two localities are coeval, both yield adult specimens of M. giganteus that do not show consistent morphological differences, and the two lake environments were very probably interconnected, we consider the Kupferzell (K3) and Vellberg (E6) samples as part of the same species. We did not identify small growth stages of M. giganteus in older or younger deposits.

The symphyses are well-preserved and measure 1 mm and 2 mm in length, respectively (Fig. 6A, B). They rank among the smallest identified stereospondyl specimens worldwide, third only to Rewanobatrachus aliciae (Warren and Hutchinson 1988) and Trematolestes hagdorni (Schoch and Mujal 2022).

The smallest symphysis stems from a specimen with approximately 12–15 mm skull length (SMNS 97035, Fig. 6A). The larger symphysis (SMNS 97036, Fig. 6B) is about double the size but has proportionately similar-sized fangs. The size of the marginal teeth decreased gradually in proportion from the smallest specimen over the next larger to the juvenile and adult symphyses (Fig. 6A–D).

Both symphyses can be easily distinguished from all other temnospondyls: they differ from Tatrasuchus (Fig. 6E, F) by the absence of a transverse row of teeth posterior to the fangs and the larger size of these, from Callistomordax (Fig. 6G) in the presence of a continuous dentary tooth row anterior to the fangs and from Trematolestes (Fig. 6H) in the different outline of the bone, the absence of a sagittally extended symphysis and the different arrangement of the fangs with the medial one being well anterior. The symphyses of plagiosaurids are still more different, especially in the absence of tusks and the outline of the dentary (Hellrung 2003; Damiani et al. 2009; Witzmann and Schoch 2024).

Likewise, we identified a range of small and tiny interclavicles in the Vellberg (E6) sample. These are consistent with juveniles and adults of M. giganteus in the slender posterior process and the overall proportions; they differ from small interclavicles of Tatrasuchus in the slenderer posterior process and from Callistomordax and Trematolestes in the greater width of the lateral process.

Despite their fragmentary nature, the small specimens here attributed to M. giganteus are highly informative in their great consistency with the juvenile and adult morphologies. At least regarding the interclavicle, this agrees with the ontogenetically rather conservative Palaeozoic eryopiforms (Boy 1988; Witzmann 2006).

The smallest juveniles are known from partial skulls, mandibles, and interclavicles (Fig. 6). Juvenile bones from other regions are also present but unambiguous referral to M. giganteus remains impossible because the postcranium of the closely related Tatrasuchus wildi remains largely unknown.

Skull roof

The best small juvenile specimen of M. giganteus is a partial postorbital skull (SMNS 97037) from Vellberg (E6) that preserves the left half of the posterior skull table and squamosal (Fig. 3I, J). This skull shares all features of the ornament of adults, and in comparison to other temnospondyl taxa of similar size, the bones are much thicker. This is especially obvious along the occipital margin. The small juvenile differs from all larger specimens in having a shorter squamosal and a longer supratemporal. The orbit is somewhat larger with respect to the postorbital skull compared with adult specimens; this is interesting, because larger juveniles have relatively small orbits (Fig. 8). Larger juvenile skulls, which are especially well preserved at Vellberg (E6, E7), are remarkably consistent with adults in proportions and suture topologies (Fig. 4). A few trends are apparent from morphometric analysis, in which juveniles have among the widest interorbital distances, the smallest orbit lengths and the narrowest postorbital skulls (Fig. 8). This said, there is substantial variation, especially in Vellberg E6.

Palate

The palate is well preserved in MHI 1992/31 (30 cm SL; Fig. 5A, E) and MHI 1991/11 (38 cm SL). The proportions of the palate elements are similar to those of adults, and the pattern and size of dentition is consistent with later stages. The basal plate of the parasphenoid is slightly shorter and the deltoid area in its anterior part is not yet established. However, its sutures with the pterygoid and exoccipital are consistent with the adult stages, as is the relative size of the exoccipital and its condyle.

Mandible

The Kupferzell and Vellberg localities yielded a wide range of juvenile mandibles that are readily distinguished from those of Tatrasuchus wildi by the following features in those of M. giganteus: (1) no transverse row of small teeth posterior to the symphyseal fangs, (2) larger size of the symphyseal fangs, (3) Meckelian window elongate and low, reaching 25% the length of the mandible, and (4) long and tall postglenoid area. The juvenile mandibles differ hardly from those of adults, with the postglenoid area only slightly shorter and the hamate process only gently lower in juveniles.

Vertebrae

The morphological consistency of the trunk intercentra is remarkable (Figs 6N–Q, 7K–N). In contrast to the close relative M. cappelensis, M. giganteus had closed disc-shaped intercentra already in juvenile stages. Intercentra of this type range in transverse width between 4 cm and 12 cm, representing sizes between 30 and 100 cm skull length. Smaller intercentra were found but cannot be distinguished from those of Tatrasuchus wildi.

Pectoral girdle

A wide range of clavicles and interclavicles were collected in Vellberg E6, spanning a full range from the smallest to various juvenile stages. These are consistent with adult interclavicles of M. giganteus in the slender posterior process, the much-elongated anterior process, and the shape of the clavicular facet (Fig. 7E–G). Independent of size, these features are distinct from all other temnospondyls of the Lower Keuper and especially Tatrasuchus. The clavicle of M. giganteus has a large convex dorsolateral flange anterior to the ascending process (Fig. 7H, I) that is not present in Tatrasuchus. In contrast to the dermal pectoral girdle, diagnostic and undisputed juvenile humeri or other limb elements have not been identified in the studied material.

Pelvic girdle

Small ilia consistent with the morphology of the adult ilium of M. giganteus were collected in Kupferzell K3 and figured in Schoch (1999, fig. 46), but their distinction from the yet unknown ilium of Tatrasuchus is unknown; the ilia of all other temnospondyls of the Lower Keuper are very different (Schoch 2006, 2008).

Here we define an arbitrary adult stage beginning with a skull length of 50 cm onwards. As depicted in Fig. 2, adult specimens ranging from 50–60 cm form the bulk of the samples Kupferzell (K3, K4) and Vellberg (E5, E6). Their morphology has been described and figured in detail by Schoch (1999).

Skull roof

The large specimens from Gaildorf (G2) were described by Meyer and Plieninger (1844), Fraas (1889) and Moser and Schoch (2007). The three complete skulls range from 60 to 68 cm skull length (Fig. 4; Moser and Schoch 2007). Fraas (1889) further described and figured a large skull without cheeks from Markgröningen (SMNS 4974), found in a dark mudstone that was roughly equivalent to the Untere Graue Mergel of Vellberg and Kupferzell.

The grey mudstones of Vellberg E6 yielded numerous isolated bones of giant specimens. These include intercentra, humeri, mandibles, and fragments of skulls in the 100–120 cm size range. It is unknown from how many specimens these bones stem, but a minimum assessment identifies at least three giant individuals collected over 30 years (Werner Kugler, pers. comm. 2016). The Kupferzell fossillagerstätte (K3) gave only a single very large skeleton, referred to as giant specimen (“Riesenexemplar”, SMNS 81310). It encompasses a palatine, tabular, complete mandible and 34 vertebrae plus a series of associated ribs (Schoch 1999), representing a specimen with 88.5 cm skull length.

At a highway construction site near Arnstadt in central Thuringia (Erfurter Kreuz, see Werneburg in Hagdorn et al. 2015), a large partial skull was collected (NHMS-WT3323-3368, see Werneburg and Witter 2005) in the upper part of the Lower Keuper. The complete skull measured around 101 cm in length and preserves a good portion of the skull roof (restored in Fig. 4I). This includes portions of the preorbital, interorbital and cheek regions. It is consistent with the large Kupferzell and Vellberg material in that ornament is coarser than in 40–70 cm skulls, meaning that the size of the polygons is proportionately larger in these specimens. In turn, the ridges are low and thinner than in smaller skulls, whereas the lateral line sulci are proportionately of similar width as in smaller specimens.

The orbit of the largest skull is proportionately somewhat smaller, albeit retaining its characteristic outline with the pointed anterior end. In the other material, the length of the orbit varies broadly, with the Gaildorf and Markgröningen specimens ranking among those with the longest orbits. Such variation was also reported by Sulej (2007) on Metoposaurus krasiejowensis. The large size of the orbits in M. giganteus has been hypothesized to correlate with large adductor musculature (Schoch et al. 2023), which might explain that the size did not essentially change between juveniles and adults.

A rather clear trend was found in the length/width ratio of the postorbital skull table, which is smaller in larger specimens, except for the Markgröningen skull. Conversely, the ratio length of the postorbital skull through skull length increases slightly from juveniles to large adults (Fig. 8).

The area of the postorbital increases markedly between juveniles (30 cm) and small adults (40–50 cm) but remains constant throughout later stages. Likewise, the width of the squamosal increases between juveniles and adults. Both features are typical of adult M. giganteus, whereas juveniles retain the plesiomorphic condition seen in adult M. cappelensis (Schoch et al. 2023).

In the Thuringian and Gaildorf specimens, the postorbital is still proportionately large. The interorbital distance varies more broadly in the largest skulls, with the Markgröningen and two of the Gaildorf specimens ranking among the narrower skulls, together with K4 specimens from Kupferzell (Fig. 8). The occipital condyles are more massive than in smaller stages and the occipital margin of the postparietals is more strongly concave, especially in the Gaildorf and Markgröningen specimens.

Palate

Meyer (1855) figured the now-lost snout of a giant specimen from Gaildorf (G2) in original size, which measured approximately 107 cm in total skull length (Meyer and Plieninger 1844). Based on Meyer’s drawing (modified in Fig. 5D), the marginal and vomerine teeth are throughout proportionately smaller than in 50–60 cm long skulls, whereas the fangs especially of the symphyseal region are enlarged (SMNS 81310). The choana was drawn rounded by Meyer (1855) but reported to be rather oval by Fraas (1889); we suppose that Meyer referred to the outline of the opening within the main plane of the vomer, whereas Fraas described the outline of the opening, which is more deeply emplaced in the dorsal portion of the narial capsule. At any rate, the choana is distinctly shorter and smaller in the giant Gaildorf specimen than in smaller adult stages (Fig. 5C, D).

Vertebrae

The largest vertebrae are mostly heavily crushed, sometimes even more flattened than intercentra of adults in the 50–60 cm skull range. This probably results from a higher porosity of the bones. This is particularly obvious at the top of layer K3 in Kupferzell, which contained both the giant specimen (SMNS 81310) and several smaller disarticulated vertebral columns; in the smaller specimens, the intercentra are hardly crushed at all and retained most of their 3D structure. Apart from the higher porosity, the parapophyses are proportionately larger by area in the giant specimen. The caudal vertebrae are low and wedge-shaped but very massively ossified, not as porose as the trunk vertebrae and consequently less affected by compaction. The neural arches are peculiar in giant specimens by having disproportionately large transverse processes; their length was subject to strong positive allometry from adult to giant size classes (Fig. 6L–M).

Pectoral girdle

Several huge clavicles and interclavicles are housed in the MHI and SMNS collections, among which the giant specimen (SMNS 81310) from Kupferzell (K3) permits the calibration of mandible size and interclavicle length. The proportions appear not to have changed, whereas the ornament underwent a similar coarsening as reported from the skull roof. The ontogenetic changes of the cleithrum involved a broadening of the dorsal head region and an area increase of the scapular facet, well preserved in the K3 sample (Schoch 1999, fig. 41). The humerus of large specimens differs between the Gaildorf and other samples. Large humeri in the 20–24 cm range are known from Vellberg, where they are poorly differentiated without a clear supinator and poorly defined, mostly cartilage-capped condyles. In contrast, the giant Gaildorf humerus (SMNS 4706) figured by Meyer and Plieninger (1844) has a prominent supinator process and a large fully ossified radial condyle. (Schoch 1999, fig. 43). As preserved in one Gaildorf specimen (SMNS 4706), the carpal region contained at least a few ossified carpals (Schoch 1999, p. 17 and fig. 45).

Pelvic girdle. This region is again best represented by the small Gaildorf sample, which preserves three divergent size classes. Interestingly, the smallest of these includes an ilium with attached ischium and pubis (Schoch 1999, fig. 46), being much more fully ossified than all other specimens from Vellberg and Kupferzell.

In Fig. 2, we mapped the frequency of M. giganteus in six different localities and horizons: Gaildorf (G2), Kupferzell (K3, K4) and Vellberg-Eschenau (E5, E6, E7). The basis of these distributions form registered finds of skulls, articulated skeletons, and isolated elements that together give a minimum number of individuals. To avoid counting isolated bones from the same disarticulated individual twice, we constrained the sample of single elements to the most robust and common bones, the exoccipitals. Especially at Kupferzell the number of interclavicles and mandibles, the other two common elements, matched the distribution of exoccipitals.

Lake Gaildorf G2

The Gaildorf sample is the smallest and most difficult to interpret because the locality is currently poorly accessible. It includes three complete large skulls (60–68 cm), two skull fragments (61–65 cm), one lost giant specimen (107 cm), and articulated as well as isolated postcranial bones. The minimal number of specimens is difficult to assess, because the belonging of cranial and postcranial material remains unclear. As in most other tetrapod fossillagerstätten in the Erfurt Formation, the fossiliferous horizon was restricted to a small area, and in currently accessible neighbouring outcrops it is absent.

The Gaildorf specimens of M. giganteus stem from a hard coaly siltstone rich in pyrite, sphalerite, and alum (Kurr 1852). This layer occurs immediately above a light grey mudstone (G1) containing a bonebed and below a 25–30 cm thick coal seam (G3) that was rich in plants, a coaly mudstone poor in fossils (G4), and a dark grey sandstone that yielded occasional plant debris and vertebrate bones (G5). This succession might have formed in a lake that provided good conditions for life, harbouring a fish fauna that is preserved in the 25 cm-thick bonebed-bearing mudstone (G1).

The 30 cm thick pyrite-rich type horizon G2 yielded the skulls and articulated skeletons of M. giganteus, along with a skeleton of Plagiosuchus pustuliferus, a skull fragment of the small capitosaur Tatrasuchus wildi (Schoch 1997) and mass accumulations of bivalves (Unionites letticus). A jaw and vertebra of the enigmatic archosauriform Zanclodon laevis further documents the presence of terrestrial predators (Schoch 2011), whereas isolated bones of the marine reptile Nothosaurus sp. (Meyer and Plieninger 1844) suggest the presence of lagoonal environments in the vicinity. The articulated state of the aquatic tetrapods in G2 indicates a calmer environment than in the preceding unit G1, and the abundant pyrite must have formed under euxinic conditions.

According to Nitsch (2015), many coal deposits of the Lower Keuper are associated with channel sandstones and probably formed in oxbow lakes. At Gaildorf, the reported sequence of mudstones, pyrite-bearing siltstones, coal, and dark grey sandstones is consistent with this interpretation. Weber (2013) cited historical sources that described the setting of the coal seam as lenticular with 1200 m longitudinal extension and 40 m width.

The lateral variation of the horizons is well documented in the three different sections that were published over a range of 36 years (Plieninger in Meyer and Plieninger 1844; Kurr 1852; Quenstedt 1880). These were probably measured in successively exposed sections of the mine.

Plieninger (1844) noted the presence of a second, thinner coal seam at the top immediately below the first sandstone unit, and he highlighted the occurrence of estherians in G1 and coaly clasts in G2 and teeth in G4. Quenstedt (1880) reported the horizons G2, G3 and G5, but noted the absence of G4 in the section available to him, he also mentioned estherians and reddish ironstone nodules in G2. In sum, these sections provide evidence of a freshwater lake environment that developed into a peat lake with calm, euxinic conditions, relatively similar to layers K1 and K2 from Kupferzell (see Schoch et al. 2022). The abundance of freshwater bivalves indicates good living conditions in G2, and the presence of four temnospondyl predators provides evidence of a rich ecosystem (Mastodonsaurus, Tatrasuchus, Plagiosuchus, Trematolestes).

Plagiosuchus probably was a bottom-dwelling sit-and-wait predator that, together with the abundance of Unionites, indicates a well-aerated lake floor of G2. At the same time, the abundance of pyrite and the dark colour of the sediment probably formed after seasonal poisoning of lake water, most probably by algal blooms, whereas the occurrence of coaly clasts suggest either the presence of already existing peat deposits in the vicinity or, alternatively, the input of larger quantities of plant material from the lake shore. Plieninger (Meyer and Plieninger 1844) reported that the skulls and skeletons of M. giganteus were found in the top of G2, with the bones reaching into the coal seam. This might indicate a drying up of Lake G2 (somewhat similar to Lake K3 in Kupferzell, see Schoch et al. 2022), followed by a peat lake preserved in the coal seam of G3 which contains Taeniopteris, Calamites and Equisetites (Kurr 1852).

The large size of most specimens of M. giganteus in G2 is remarkable; the complete skulls all fall into the 60–68 cm size range, as do most of the postcranial elements, with few bones stemming from individuals with a skull length of 50–60 cm, and several finds (humerus, ilium, fang, giant snout reported by Meyer and Plieninger 1844) of very large individuals in the 100–120 cm size range. The sample also includes three ilia of very different size, probably matching the three size classes (50, 60, and 110 cm skull length). Among all occurrences of M. giganteus, these finds are the most fully articulated, consistent with the find of a Plagiosuchus skeleton in G2 (Huene 1922).

Within this relatively small sample, the dominance of large and giant specimens is unparalleled among Lower Keuper deposits (Fig. 2). This may point at differences in life histories: either the Gaildorf population of M. giganteus had a longer life span or specimens reached large sizes more often than the later populations due to more favourable conditions. Two anatomical features are further noteworthy: the largest humerus is more heavily ossified than that of other samples and bears a prominent supinator process that is not developed in any other specimen, and the pelvis of some specimens includes an ossified pubis, which is unique among capitosauroid taxa except for Sclerothorax (Schoch et al. 2007). Ongoing analysis of bone microanatomy might shed more light on this interesting difference to the Vellberg and Kupferzell samples.

These data coincide with the occurrence of the smaller capitosauroid Tatrasuchus. The unusually large size of M. giganteus might be a result of character displacement, with Mastodonsaurus forming the apex predator and Tatrasuchus representing the guild of smaller ambush predators at this locality. At Gaildorf, M. giganteus appears to have invaded the lake at later ontogenetic stages and larger sizes than in other lakes, and its huge size might be a result of either plasticity or separate microevolution. Since Tatrasuchus probably occupied the same ecological niche as small adults of M. giganteus, the latter could have undergone an ontogenetic niche shift between juveniles and small adults. M. giganteus might have targeted larger prey and thus developed a stronger bite by proportionately increasing the area of the postorbital and the size of the squamosal (differing from M. cappelensis) in order to accommodate larger adductor musculature.

Lake Kupferzell K3

At Kupferzell-Bauersbach, a thick greenish mudstone (K3) has yielded a rich sample of tiny, juvenile, and adult specimens of M. giganteus. By the sheer quantity and density of bones, this deposit forms the richest tetrapod fossillagerstätte in the Lower Keuper and well beyond. Especially considering the excavated area, the temnospondyl specimens collected in that horizon by far outnumber those collected at the much larger excavated areas of Vellberg (E5, E6, E7). K3 yielded a loosely articulated giant specimen but otherwise contained mostly smaller adults, juveniles, and tiny specimens (Fig. 2).

The unit K3 has been interpreted as littoral facies of a lacustrine system (Schoch et al. 2022). This is also in line with the high number of bones with bite marks inflicted by the pseudosuchian archosaur Batrachotomus kupferzellensis, a more terrestrial predator feeding on carcasses of M. giganteus (Mujal et al. 2022), probably stuck in drying-up lakes (Schoch et al. 2022). Characeans and ostracods indicate well-aerated water in a basin less than 10 m deep. This water body spanned probably several km, but bone accumulations are restricted to only 500 m. The lake was subject to pulses of higher energy that produced currents, disarticulated skeletons, and transported the bones (Schoch et al. 2022). The absence of dark components and the light green colour of the sediment suggest that organic components were not as abundant as in other lakes, probably indicating a somewhat less nutrient-rich environment. Salinity fluctuations are indicated by euryhaline ostracods (Urlichs 1982) and histological evidence in bones of Gerrothorax point at an environment with fluctuating conditions (Sanchez and Schoch 2013). These fluctuations may have been caused by the occasional contact to a lagoonal water body in the vicinity (Schoch et al. 2022). Conversely, the poor carbonate content of the sediment was probably caused by inflowing streams that also produced channel-like erosions (Urlichs 1982). Only in the last phase of Lake K3, a diverse fish fauna (14 taxa) similar to, but not identical with E5 and E6 at Vellberg was established, and the accumulation of tetrapod bones is confined to that horizon (Schoch et al. 2022).

Despite the diverse fish assemblage, only aquatic tetrapod predators were present, M. giganteus and Gerrothorax pulcherrimus. The latter was distinctly smaller than Plagiosuchus pustuliferus of Gaildorf G2 and Vellberg E5. However, the abundance of this taxon in K3 concurs with the above-cited evidence of clear and well-aerated bottom water.

Within K3, the frequency distribution of M. giganteus is bimodal with specimens clustering in the 18–32 as well as 40–65 cm range (skull length; Fig. 2). Apart from that, tiny specimens were identified in the screen-washed fraction, and a single very large specimen (88.5 cm skull length) was found, but the giants in the range of Vellberg and Gaildorf specimens are absent. In Lake K3, M. giganteus must have primarily fed on Gerrothorax and large bony fishes such as dipnoans, actinistians and scanilepiforms.

Occasionally, M. giganteus may also have predated the smaller capitosaur Tatrasuchus wildi whose remains are always completely disarticulated and less common; this taxon probably dwelled in neighbouring habitats, and the disarticulated bones were washed in from there. The mass accumulation of bones and skulls of M. giganteus and G. pulcherrimus was probably caused by the final drying-up of Lake K3, which is documented by mud cracks; of note, bones are often found in the interface between layers K3 and K4, i.e., remaining half buried and thus exposed for a relatively prolonged time after the lake dried up (Schoch et al. 2022). At the same time, carcasses of M. giganteus, including that of the giant specimen, were scavenged by Batrachotomus kupferzellensis, which occasionally probably also actively hunted M. giganteus individuals (Mujal et al. 2022).

Lake Kupferzell K4

The deposit K4 encompasses yellow to brown, dolomitic marlstones. The yellow mud also fills desiccation cracks on top of layer K3, preserving equal amounts of characeans but much larger quantities of ostracods and microvertebrates than K3 (Schoch et al. 2022). Lake K4 represents a shallow, carbonate-rich environment that harboured a rich benthos. It probably formed in a more protected sub-basin that was not subject to frequent currents and therefore less siliciclastic input than K3. The oligohaline ostracod Darwinula was accompanied by the 14 fish taxa that also occur in K3 (Urlichs 1982; Schoch et al. 2022). The high abundance of small actinistian bones has been interpreted as evidence of a nursery ground of these fishes (Schoch et al. 2022). This lake probably existed within the same basin as K3, probably on a carbonate mudflat. Like K3, it is rich in ostracods and characeans, thus indicating the persistence of shallow water conditions.

In contrast to K3, the deposits of Lake K4 preserve skeletons rather than accumulations of single bones, indicating lower energy conditions and the preservation of skeletons after short-term droughts which are preserved as minor desiccation cracks. Gerrothorax was distinctly less abundant and Tatrasuchus more common than in K3 (Schoch 1997). In contrast to K3, Tatrasuchus is represented with several well-preserved skulls, suggesting that this sub-basin was inhabited by that taxon. M. giganteus was present with specimens in the 50–70 cm range (skull length), whereas giant specimens are entirely absent. The absence of juveniles and the slightly larger mean size compared with K3 coincide with the occurrence of Tatrasuchus (size range 25–40 cm skull length). Like at Gaildorf (see above), character displacement may explain this distribution (Schluter and McPhail 1992).

In K4, M. giganteus primarily fed on Gerrothorax and lungfishes such as the 2 m long Ptychoceratodus which is present both with skeletons and large teeth with traces of gastric acids. As in K3, Batrachotomus kupferzellensis primarily fed on carcasses of M. giganteus (Mujal et al. 2022). Littoral or marine influence in Lake K4 is indicated by the presence of a partial skeleton of Nothosaurus sp. (Schoch et al. 2022).

Lake Vellberg E5

In the Schumann quarry of Vellberg-Eschenau, the brown silty mudstones of E5 have been excavated by private collectors in concert with the more fossiliferous horizon E6. The horizon E5 formed in a small lake basin that developed after the retreat of a large brackish lagoon (Schoch and Seegis 2016). It is associated with autochthonous coal and roots at least in the northwestern area of its occurrence (Ummenhofen and northern Eschenau quarries). Its fauna includes a diverse fish assemblage (15 taxa) in which juvenile lungfishes and small polzbergiid actinopterygians form the bulk of the sample. Juvenile actinistians are present but much less abundant than in K4. Together with the occurrence of larval and juvenile temnospondyls (Callistomordax, Trematolestes, Plagiosuchus), these finds suggest the prevalence of well-structured, protected areas of a lake. The dominance of the ostracod Speluncella over Darwinula points to slightly brackish water (Schoch and Seegis 2016). The large predators in this habitat were the bottom dwelling Plagiosuchus and the medium-sized predator Tatrasuchus, both present with numerous well-preserved specimens. In contrast, M. giganteus is represented by relatively few skulls in the narrow range of 50–60 cm skull length (Fig. 9). This setting formed in a small lake in which M. giganteus was probably only occasionally present due to the dominance of competing Tatrasuchus. Adult Trematolestes and Callistomordax probably visited the lake only during the breeding season. In contrast to the other deposits studied by us, Lake Vellberg E5 probably could not support a population of adult M. giganteus.

Lake Vellberg E6

The grey clayey mudstones of E6 have been excavated over a period of 35 years and during all field seasons yielded large quantities of vertebrate remains (Schoch and Seegis 2016). They contain thin bonebeds as well as articulated skeletons and probably formed in a stratified lake (>5 km) that was at least seasonally subject to euxinic conditions.

Like in E5, baby lungfishes and two polzbergiids dominate the fish fauna, which also encompasses redfieldiids and semionotids, as well as medium-sized predators (Saurichthys, scanilepiforms). The high-bodied polzbergiids indicate that the lake floor was differentiated and offered protected areas, consistent with the presence of small juvenile temnospondyls (Trematolestes, Callistomordax, Mastodonsaurus). Layer E6 is rich in remains of the temnospondyl Callistomordax, an elongate eel-like form with large, keeled fangs that is only known from Vellberg. This taxon might have dwelled in protected places because it is the only aquatic tetrapod in this deposit to be preserved with fully articulated skeletons.

M. giganteus is present with specimens of all size classes, and the largest quantities of specimens fall in the 20–50 cm size range (Fig. 2). Notably, there are also remains of giant specimens up to 120 cm skull length, whereas the large adults (60–70 cm range) are much rarer than at Gaildforf G2. In contrast to Callistomordax, the remains of M. giganteus are nearly always disarticulated and often heavily damaged by predation.

In contrast, Tatrasuchus and Trematolestes are much less frequent, and only two remains of Plagiosuchus were found. Traces on adult remains of all taxa suggest predation by M. giganteus (regurgitates, bite marks, lungfish teeth and vertebral centra of temnospondyls corroded by gastric acids). Lake E6 also forms the only deposit so far to evidence cannibalism in M. giganteus (Schoch and Seegis 2016; Mujal et al. 2022), including a 60 cm skull plus mandibles with bite marks of a much larger conspecific. At the same time, a reduced number of bones of M. giganteus display bite marks produced by Batrachotomus kupferzellensis, indicating scavenging as in the Kupferzell lakes, but in lesser frequency (Mujal et al. 2022), possibly because the lakes in Vellberg persisted for longer periods, so that the large pseudosuchian archosaurs could not frequently reach the large carcasses of M. giganteus.

These data indicate that Lake E6 formed a protected and nutrient-rich nursery ground for lungfishes and temnospondyls, with adult Ptychoceratodus, Trematolestes and Tatrasuchus appearing only during the mating season to lay eggs. In contrast, Callistomordax and M. giganteus were autochthonous throughout their life cycles and sufficiently different to avoid direct competition as adults. The numerous regurgitates preserved in E6 fall into two different size classes: 5–20 cm long skeletons of juvenile lungfishes and small archosauromorphs (potentially produced by Callistomordax) and 1–2 m long skeletons of archosauriforms (especially the amphibious Jaxtasuchus salomoni, Schoch and Sues 2013) and pseudosuchians (likely produced by Mastodonsaurus). Nevertheless, it cannot be discounted that some of the regurgitates were produced by Batrachotomus kupferzellensis (Mujal et al. 2022).

Lake Vellberg E7

The pale yellow micritic dolostones of E7 (Anoplophora Dolomite) form a 70–90 cm thick unit that is subdivided by thin horizons of greyish-brown mudstones. The basal unit has yielded skeletons of the terrestrial apex predator Batrachotomus kupferzellensis (thus the unit was coined Batrachotomus dolomite) skulls and disarticulated skeletons of M. giganteus, occasional finds of large adult Plagiosuchus and two juvenile skulls of Tatrasuchus. Fish remains are confined to single scales and teeth, with dominance of large polzbergiids (Serrolepis).

Like the Kupferzell facies, E7 was probably a clear water environment, although characeans have not been found. The dolostones formed under very shallow water conditions (Schoch and Seegis 2016). Like most of the aforementioned lakes, E7 developed in a sabkha-like setting, emplaced on a tidal flat that underwent frequent periods of desiccation. This is evidenced by the relative abundance of well-preserved skulls and skeletons of M. giganteus as well as Batrachotomus.

Lake Arnstadt A42

During highway construction near Arnstadt (north of the bridge at the Autobahn exit “Arnstadt Nord”), a large exposure that was open in 2003 yielded tetrapods in several horizons of the Lower Keuper (Werneburg and Witter 2005). The stratigraphically higher one yielded M. giganteus, among them the giant specimen restored in Fig. 4I. This horizon (Oberes Dunkles Band, Bed A42 within the upper sandstone S2o) forms a succession of dark coaly siltstones grading into dark grey mudstones. The disarticulated, large skull of M. giganteus was found below a bed rich in fossil wood, in a coaly and pyrite-rich layer. The preservation of bones closely resembles that of the Gaildorf specimens (Werneburg in Hagdorn et al. 2015), as does the stratigraphic association of the coaly siltstones with channel sandstones. Bed A42 also contained remains of the marine-lagoonal tetrapods Plagiosternum and Nothosaurus, as well as vertebrae of large pseudosuchians (Werneburg and Witter 2005). This setting might have formed in a lagoonally-influenced, stagnant water body with terrestrial input (reworked pseudosuchian bones). The occurrence of very large specimens of M. giganteus in similar facies at Gaildorf and Arnstadt is noteworthy and might either indicate a preference of swampy environments or simply a wider distribution of such giants.