Introduction

The Saar-Nahe Basin of southwest Germany is among the largest and best-studied of late Palaeozoic sedimentary basins, and has yielded the largest quantities of vertebrate fossils for that time interval in Europe (Boy 2007). Several thousand specimens of fishes and amphibians have been collected from numerous successive lake deposits at various localities in the last decades (Boy 1987; Schindler and Heidtke 2007; Schoch 2009). These often exquisitely preserved discoveries have provided profound insights into the palaeobiology of aquatic taxa, and the palaeoecology and taphonomy of these richly-documented faunas (Boy 2003). However, despite intensive search, not a single tetrapod other than the abundant aquatic amphibians was found in the more than 165 years of collecting in this basin. This differs from the record of other basins (Lodève, Autun, Thuringian Forest, Döhlen), which have all yielded a range of fully terrestrial taxa (Boy 1977; Werneburg and Schneider 2006).

The absence of amniote skeletons in the Saar-Nahe Basin is also in a stark contrast to the rich amniote track record reported from the same area (Haubold 1973; Fichter 1983; Voigt 2007). According to this evidence, diadectids, parareptiles, diapsids, and synapsids are likely to have been present in the Saar-Nahe Basin. One particularly obvious group of fully terrestrial tetrapods that diversified rapidly and that represents the dominant element in Late Palaeozoic terrestrial vertebrate ecosystems are synapsid amniotes, which ultimately led to the evolution of mammals (e.g., Olson 1966). The synapsid tracks of the Saar-Nahe Basin agree with skeletal finds from other, roughly coeval basins, such as Autun (Gaudry 1886; Currie 1979), Thuringian Forest (Berman et al. 2001, 2004), Döhlen in Saxony (Credner 1888; Huene 1925; Currie 1979), and Upper Silesia (Schroeder 1905).

Synapsids are traditionally divided into two major groups, the paraphyletic grade of “pelycosaurs” and the monophyletic Therapsida, which includes mammals (e.g., Romer and Price 1940; Kemp 1982, 2005; Reisz 1986). As “pelycosaurs” do not represent a natural group, they are commonly referred to as non-therapsid or pelycosaur-grade synapsids. This assemblage includes a number of characteristic and well-defined clades, whose inter- and intrarelationships are comparably well resolved and stable (e.g., Reisz 1986; Reisz et al. 1992a, 2009; Laurin 1993; Berman et al. 1995; Modesto 1995; Sidor and Hopson 1998; Maddin et al. 2008; Botha-Brink and Modesto 2009). These clades include Caseasauria (Caseidae and Eothyrididae) as the sister clade of all other synapsids, the Eupelycosauria, comprising Varanopidae, Ophiacodontidae, Edaphosauridae, and Sphenacodontia. Sphenacodontia consist of a number of mainly European stem taxa such as Haptodus, as well as Spenacodontoidea, including Sphenacodontidae and Therapsida.

Fig. 1.

Geological setting of the locality. A. Stratigraphic framework of the Rotliegend in the Saar Nahe Basin. B. Location of Saar-Nahe Basin (B₁), geological map of the basin (B₂), and location of the Remigiusberg quarry (B₃). Lithostratigraphy follows Schindler (2007); isotopic ages after Burger et al. (1997), Königer et al. (2002), and Lippolt and Hess (1989); position of the Permo-Carboniferous boundary (299.0 ±0.8 Ma) is based on Ogg et al. (2008).

As sister taxon of Therapsida, sphenacodontids are central to our understanding of therapsid origins and the acquisition of mammalian characters within synapsids. Moreover, the moderately to large-sized sphenacodontids from the Late Carboniferous and Early Permian represented the dominant terrestrial predators of their time and are thus particularly important for understanding the evolution of terrestrial ecosystems. A number of genera are currently recognised within Sphenacodontidae. The four best-known genera, Dimetrodon, Sphenacodon, Secodontosaurus, and Ctenospondylus, are based on several well-preserved skulls and skeletons and are mainly known from North America; the only exceptions being Dimetrodon teutonis from Thuringia, Germany (Berman et al. 2001, 2004), and a possible occurrence of Sphenacodon, previously “Oxyodon” brittanicus (see Huene 1908; Paton 1974), from the English midlands (but see Eberth 1985). In addition, the poorly known Bathygnathus from Prince Edward Island, Canada, Macromerion from Kounová, Czech Republic, and Neosaurus from Moissey, France, are based mainly on isolated maxillae (e.g., Leidy 1854; Fritsch 1889; Nopcsa 1923).

Here we report the first amniote from the Saar-Nahe Basin, consisting of a diagnostic anterior portion of a right mandible with 11 teeth and one empty alveolus. It can be safely referred to Sphenacodontidae, whose presence in that basin is otherwise indicated only by the ichnotaxon Dimetropus, which occurs in several horizons of the Lower Rotliegend (Voigt 2007). The new mandible originates from the very base of the Rotliegend, which falls into the latest Carboniferous (see Fig. 1 and citations therein). As such it is the oldest amniote record from Germany, and among European sphenacodontids it is the second oldest in stratigraphic age to Macromerion schwarzenbergii from the Stephanian B (Gzhelian) of Kounová, Czech Republic (Fritsch 1889; Romer 1945; see Supplementary Online Material at  http://app.pan.pl/SOM/app56-Frobisch_etal_SOM.pdf).

Fig. 2.

Lithological column of the Remigiusberg quarry, arrow marks the horizon (layer 26) of the holotype of Cryptovenator hirschbergeri.

Institutional abbreviation.—LFN, Landessammlung für Naturkunde Rheinland-Pfalz, Mainz, Germany.

Geological setting

The locality of the new find, an anterior portion of a sphenacodontid mandible (LFN-PW 2008/5599-LS), is at the western rim of the Remigiusberg quarry (49°54′88″N, 7°26′05″E) on the summit of the Remigiusberg mountain range (Fig. 1), about 1 km northeast of Haschbach in Rhineland Palatinate, Germany. LFN-PW 2008/5599-LS originates from a dark, fine grained sandstone of the middle Remigiusberg Formation in layer 26 of the quarry profile (Fig. 2), which forms part of a lac ustrine, mixed carbonate-siliciclastic sequence, representing the lower part of Lake Theisbergstegen (Boy and Schindler 2000). The fauna of this lake deposit falls within the climax of the “Remigiusberg invasion”, an ecostratigraphic bio-event described by Boy and Schindler (2000).

Large blocks of these beds (1–3 m in length) were deposited by quarry workers alongside a dirt road paralleling the western cliff of the Remigiusberg quarry (Fig. 1B). The sphenacodontid mandible was found in one of these blocks of calcareous, fine-grained sandstone of gray-blue-green colour with coarse silty lamina. Examination of the same blocks in 2008 yielded fragments of acanthodians and palaeonisciforms. The bones, scales, and teeth have a bright white coloration as a result of contact metamorphosis of the sediments with subvolcanic intrusions.

The age of the fine grained sandstone of the middle Remigiusberg Formation is basal Rotliegend, late Gzhelian, latest Carboniferous (Fig. 1). The available radiometric date for this formation is 300.0 Ma ±2.4 Ma (Lippolt and Hess 1989; Burger et al. 1997; Menning et al. 2000; Königer et al. 2002), which agrees with ecostratigraphic evidence for a late Stephanian age (Boy and Schindler 2000). The new find is therefore older than all other sphenacodont material reported from Europe, except for Macromerion schwarzenbergii from the Stephanian B Czech locality of Kounová (Romer 1945; see SOM).

Systematic palaeontology

Fig. 3.

The holotype of Cryptovenator hirschbergeri gen. et sp. nov. (LFN-PW 2008/5599-LS), Remigiusberg Formation (Upper Carboniferous), Rhineland Palatinate, Germany. A. Photograph. B. interpretative drawing. C, D. Diagrammatic illustrations of skull of Dimetrodon grandis (C) and Cryptovenator hirschbergeri (D). E, F. Diagrammatic illustrations of isolated mandibles of Sphenacodon ferocior (E) and Dimetrodon grandis (F). C, E, F after Romer and Price (1940).

Fig. 4.

Strict consensus cladograms of the eight most parsimonious trees including (A) and excluding (B) Cryptovenator hirschbergeri. Italic numbers indicate bootstrap values above 50% and bold numbers indicate Bremer decay values. Bremer decay values for nodes that collapse at one extra step are not shown.

Phylogenetic relationships of Cryptovenator

Analysis.—The phylogenetic relationships of Cryptovenator hirschbergeri are tested in the most comprehensive cladistic analysis of non-therapsid sphenacodontians undertaken to date. The analysis comprises 15 synapsid taxa and 122 morphological characters. The higher-level taxa Varanopidae, Ophiacodontidae, and Edaphosauridae are used for outgroup comparison to determine character polarities. The ingroup consists of five haptodontine-grade sphenacodontians (Haptodus, Palaeohatteria, Pantelosaurus, Ianthodon, and Cutleria), the four best-known members of Sphenacodontidae (Sphenacodon, Ctenospondylus, Dimetrodon, and Secodontosaurus), two therapsids (Biarmosuchus and a composite coding for Dinocephalia), and the new taxon (Cryptovenator hirschbergeri). The morphological characters used in this study include 50 cranial, 13 mandibular, 16 dental, and 43 postcranial characters. The dataset represents a combination of characters used in previous analyses of basal synapsid relationships, specifically those of Laurin (1993), with the addition of those used by Reisz et al. (1992a) and selected character used by Sidor (2003). All parsimony uninformative characters were excluded from the analysis. Codings of previous authors were scrutinised and in most cases could be corroborated via personal observations of relevant specimens. Data on taxa that could not be studied in person were obtained from the literature. A list of all characters and the data matrix are provided as part of the Supplementary Online Material.

The dataset was analysed using the branch-and-bound parsimony algorithm implemented in PAUP* 4.0b10 (Swofford 2001). All characters were treated as unordered and were given equal weight. Multiple character states in taxa were treated as polymorphisms; missing data and inapplicable characters were coded as “?”. The support for each node was measured by calculating the Bremer decay (Bremer 1988) and bootstrap (Felsenstein 1985) values with 100 repetitions for 1000 bootstrap replicates.

Results.—The phylogenetic analysis yielded eight most parsimonious trees (MPTs) with a tree length of 265 steps, a consistency index (CI) of 0.743, a retention index (RI) of 0.767, and a rescaled consistency index (RC) of 0.570. The tree topology of the strict consensus cladogram (Fig. 4) is in large part con sistent with those of the previous analyses (Reisz et al. 1992a; Laurin 1993; Kissel and Reisz 2004). Haptodus (based on Haptodus garnettensis and not the type species H. baylei) represents the most basal sphenacodontian and Cutleria represents the sister taxon of Sphenacodontoidea. In contrast to previous analysis, the relationships of the haptodontine-grade sphenacodontians Palaeohatteria, Pantelosaurus and Ianthodon are not resolved; instead they form a polytomy with Cutleria + Sphenacodontoidea. The analysis recovers a monophyletic Sphenacodontidae as the sister taxon of therapsids, exemplified by Biarmosuchus and Dinocephalia. However, the position of Secodontosaurus at the base of Sphenacodontidae is novel. The new taxon from the Saar-Nahe Basin, Cryptovenator hirschbergeri, represents the progressively more derived taxon with a sister-taxon relationship to the well-supported but unresolved monophylum that includes Sphenacodon, Ctenospondylus, and Dimetrodon.

Discussion and conclusions

Cryptovenator hirschbergeri can be unequivocally referred to the synapsid family Sphenacodontidae Williston, 1912, on the basis of its enlarged canines, characteristic teardrop outline of the marginal teeth in lateral view, possession of a deep symphyseal region, and the pronounced upward curvature of dorsal margin of the dentary (Fig. 3). These features are shared only with the advanced sphenacodontid genera Dimetrodon, Sphenacodon, and Ctenospondylus, but notably not Secodontosaurus or the more basal haptodontine-grade sphenacodontians. Moreover, Cryptovenator is autapomorphic in the extreme shortness and robustness of the lower jaw and the development of moderate heterodonty only. This includes the absence of a greatly reduced first tooth and only a slight caniniform development of the second and third teeth. The heterodonty is much more pronounced in Sphenacodon, Ctenospondylus, and Dimetrodon, whereas it is absent in Secodontosaurus. In addition, the phylogenetic analysis of sphenacodontian synapsids (Fig. 4) indicates that Cryptovenator represents a basal member of the Sphenacodontidae, being less derived than most primarily Permian taxa.

The phylogenetic position of Secodontosaurus at the base of Sphenacodotidae, as suggested by the current phylogenetic analysis, warrants further discussion. Previously, material of Secodontosaurus has variously been identified as belonging to an ophiacodontid and, more recently, to a sphenacodontian synapsid (Cope 1880; Case 1907; Williston 1916; Romer 1936; Romer and Price 1940; Reisz et al. 1992a, b). Romer and Price (1940) further discussed the phylogenetic relationships of Secodontosaurus within Sphenacodontia, offering two alternative hypotheses: (i) Secodontosaurus as an advanced sphenacodontid; or (ii) Secodontosaurus as a basal haptodontine-grade sphenacodontian. They favoured the second hypothesis, whereas Reisz et al.'s (1992a) cladistic analysis of sphenacodontids supported the first hypothesis and a sister-taxon relationship of Secodontosaurus with Dimetrodon, mainly supported by characters of the neural spines. All of those characters are also included in the present phylogenetic analysis. However, whereas the resulting phylogenetic position of Secodontosaurus at the base of sphenacodontids represents a novel hypothesis of sphenacodontid relationships, the decay and bootstrap values indicated that it is robust (Fig. 4). The topology within Sphenacodontidae is particularly well supported when Cryptovenator is excluded from the analysis (Fig. 4B). A monophyletic Sphenacodontinae (sensu Romer and Price 1940; contra Reisz et al. 1992a) to the exclusion of Secodontosaurus (and possibly Cryptovenator) is supported by the following characters: (1) orbital process of frontal extends far laterally; (2) extensive postorbital-squamosal contact; (3) supratemporal-postorbital contact; (4) small contribution of lacrimal to orbit; (5) narrow, blade-like paroccipital process; (6) one to three maxillary precaniniform teeth; (7) strongly convex maxilla ventral margin; (8) dorsoventrally deepened symphyseal region of the dentary (also present in Cryptovenator); (9) teardrop shaped marginal teeth (also present in Cryptovenator); (10) greatly enlarged second dentary tooth. Nonetheless, Secodontosaurus clearly represents a sphenacodontid synapsid and Cryptovenator seems to be intermediate between the former taxon and a monophyletic Sphenacodontinae that includes Sphenacodon, Ctenospondylus, and Dimetrodon.

The discovery of a sphenacodontid in the Remigiusberg Formation of the Saar-Nahe Basin is unexpected because this formation has so far not yielded any remains of larger tetrapods or their tracks, whereas large quantities of tetrapods are found only in much higher stratigraphic levels, predominantly in the Lauterecken and Meisenheim formations (Boy 1987; Schindler 2007). However, tracks of synapsids are known from beds above the Remigiusberg Formation (Voigt 2007), and judging from ichnological evidence, synapsids must have been much more widespread and long-ranging than suggested by this single skeletal find. Synapsid tracks have a long stratigraphic range in the Saar-Nahe Basin, occurring throughout the Altenglan and Standenbühl formations and spanning a time interval of at least 10 Ma (Voigt 2007).

Cryptovenator hirschbergeri is not only one of the oldest synapsids, but also represents the oldest skeletal remain of an amniote from Germany. With the exception of Macromerion schwarzenbergii from the Stephanian B (Gzhelian) of Kounová (a maxilla very similar to Dimetrodon, see Romer 1945), all other European sphenacodontids are substantially younger than Cryptovenator, occurring in the Early Permian (see SOM). They either date to the Asselian—Sakmarian (Autun, Döhlen, and Silesian material) or Artinskian (Tambach Formation) (e.g., German Stratigraphic Commission 2002; Schneider and Werneburg 2006). Even the more primitive haptodontine-grade sphenacodontians, such as Haptodus, Palaeohatteria, and Pantelosaurus, make their first occurrence only slightly above the stratigraphic levels of the Kounová and Remigiusberg finds (Kissel and Reisz 2004).

Sphenacodontians were the largest terrestrial predators of their time, ranging in body length between 60 cm (Haptodus garnettensis, Dimetrodon teutonis) and 320 cm (Dimetrodon grandis). The Remigiusberg specimen (Cryptovenator) pertains to a relatively small animal (estimated 100 cm), which is consistent with the size of some Dimetropus tracks, although other tracks of up to 20 cm pedal length indicate that much larger sphenacodontians must have been present in the Saar-Nahe Basin (see Voigt 2007). Nonetheless, Cryptovenator predates the record of sphenacodontid tracks in the Saar-Nahe Basin, as the oldest occurrence of Dimetropus tracks is in the slightly younger Wahnwegen Formation (Voigt 2007).

The palaeoenvironment of Lake Theisbergstegen was analysed by Boy and Schindler (2000). The biofacies of this deposit includes root horizons and mud cracks, and is characterized by a rapid lateral change of beds, which suggests shallow water conditions. The discovery of a terrestrial amniote is therefore not surprising, although certainly a rare event. Vertebrates from the same beds include only aquatic taxa, among them acanthodians and small, smooth-scaled palaeonisciforms. In general, the lake deposits of the Remigiusberg time interval were small and shallow, but rich in species (Boy and Schindler 2000; Schindler 2007). It is probable that such small lake deposits are more likely to bear terrestrial tetrapods, which is one type of deposit to be focused on in the further search for amniote remains.

The Late Carboniferous occurrence of sphenacodontids, the most derived pelycosaur-grade synapsids, once more emphasises that the major phylogenetic diversification of early amniotes took place well before the Permo-Carboniferous transition. However, as a result of the comparatively poor fossil record of Carboniferous amniotes, it remains unclear if this diversification event proceeded relatively fast within only few million years, or if it occurred over a longer period of time, thereby extending well into the earlier Carboniferous.