Datheosaurus macrourus Schroeder, 1904

Spindler, Frederik, Falconnet, Jocelyn & Fröbisch, Jörg, 2016, Callibrachion and Datheosaurus, two historical and previously mistaken basal caseasaurian synapsids from Europe, Acta Palaeontologica Polonica 61 (3), pp. 597-616 : 604-609

Datheosaurus macrourus Schroeder, 1904

Figs. 3 , 4 .

1904 Datheosaurus macrourus (gen. nov., sp. nov.); Schroeder 1904: 282–294, pl. 12, 13.

1926 non? Datheosaurus sp. ; Piveteau 1926: 171–172, pl. 17: 2 [MNHN.F.MAP360, holotype of Thadeosaurus colcanapi Carroll, 1981 ].

1940 Haptodus macrourus ( Schroeder, 1904) ; Romer and Price 1940: 308–309.

1979 Haptodus baylei Gaudry, 1886 [sensu novo]; Currie 1979: 138– 160, table 2.

1982 non “ Datheosaurus ”; Currie 1982: 248–259, table 1, figs. 3C, 5, 6 [MNHN.F.MAP360, holotype of Thadeosaurus colcanapi Carroll, 1981 , and referred specimens MNHN.F.MAP318, 331, 334, 349, 350, 613].

Holotype: MB.R. 1015.1–2, an almost complete and fully articulated, but poorly preserved skeleton, split in half and preserved on two blocks, slab and counter slab. MB.R. 1015.1, laying slab, displays ventral elements in internal view ( Schroeder 1904: pl. 12), shown in Figs. 3A and 4A . MB.R. 1015.2, hanging slab, displays dorsal elements in internal view ( Schroeder 1904: pl. 13), shown in Figs. 3B and 4B . The fossil was originally housed in the Königlich-Preussische Geologische Landesanstalt ( Schroeder 1904). It was later considered as having been lost during WWII ( Currie 1979) and documented only by a plaster cast of the specimen in Nowa Ruda ( Haubold 1982), but the original was re-discovered and catalogued at the Museum für Naturkunde Berlin by Johannes Müller in 2008.

Type locality: A quarry 1.5 km east to the town of Nowa Ruda, Lower Silesian Voivodship, Poland ( Dathe 1900). This city was known as Neurode, when the region of Silesia was held by the German Empire until WWII. The fossil was brought to Berlin before or in 1900.

Type horizon: According to Schroeder (1904), the specimen was excavated from the lowermost unit of the Untere Kuseler Schichten (correlation to the Lower Kusel beds from the Saar-Nahe Basin in Germany), Lower Rotliegend (Lower Autunian) from Neurode (today Nowa Ruda), Intra-Sudetic Basin, Poland. The specimen is labeled as of higher Lower Rotliegend. Huene (1956) dated it back to the lowermost Permian. According to modern stratigraphy it corresponds to the Pennsylvanian Ludwikowice Formation ( Niedźwiedzki and Bojanowski 2012: fig. 1). The age can be stated as Stephanian C, corresponding to Gzhelian ( Nowak 1998; Awdankiewicz et al. 2003; Werneburg and Schneider 2006; Schneider and Werneburg 2012; Schneider et al. 2014). The holotype and only specimen of Datheosaurus is preserved in reddish brown sandstone, interbedded with rhyolitic conglomerate. It originates from the so-called Upper Sandstone bank ( Schroeder 1904), a unit of ca. 0.5 m thickness, with high clay content. The holotypic block consists of fine to middle-coarse sandstone with siltic layers and claystone pebbles. The hanging slab MB.R. 1015.2 shows channel fill structures, several decimeters wide ( Fig. 3B ). The lower slab MB.R. 1015.1 is horizontally layered.

Material.— Holotype only.

Diagnosis.—A small, long-tailed caseasaur with indication of a closed ectepicondylar groove and unreduced phalangeal formula. The cranium and manus are proportionally less pronounced than in Callibrachion (see reconstruction; Fig. 5 ).

Description.—The total length of the specimen from the snout to the last preserved vertebra is approximately 0.9 m. The actual bone substance of the specimen is barely preserved, probably due to early diagenetic alteration of the bone tissue. Hence, little can be said about the actual anatomy of the specimen. In addition, some of the original outlines of the bones are obscured by white paint and scratching, younger than 1904, but even Schroeder (1904) identified some elements as bone only based on their greenish-gray appearance.

The ontogenetic stage of MB.R. 1015 is difficult to estimate. Romer and Price (1940) considered it to be a halfgrown individual, based on unfinished long bone epiphyses, a separate anterior coracoid (procoracoid), and a presumably unossified posterior coracoid. However, the present re-study of the specimen indicates a much older ontogenetic stage, based on the differentiated coracoid, the complete closure of the thyroid fenestra in the pelvis, the blunt posterior tip of the ischium, and the polygonal shape of the articulating proximal tarsals. In fact, the long bone ossification is not in a young juvenile stage, but rather that of a still growing subadult according to the growth stages established by Brinkman (1988; confirmed by Shelton et al. 2013).

Cranial skeleton: The skull of Datheosaurus measures 56 mm in width. In dorsal view it is short and sub-triangular. In general, the skull of Datheosaurus resembles that of Eothyris in all observable details, including the overall proportions and the size and position of the pineal foramen. Schroeder (1904) reported potential palatal elements on the hanging slab, but it seems instead that this slab only reveals the broad exposure of the skull roof in ventral view. As a result of the poor preservation, no sutures can be identified with certainty. The snout and left antorbital region are preserved on the lower slab.

Due to the complete preservation in full articulation the jaws are in a natural anatomical position. The upper jaw projects beyond the anterior margin of the lower jaw, forming a distinct overbite and possibly resulting in protruding nostrils. Although close to the block margin, the anterior outline of the premaxilla is completely preserved. The only definite statement that can be made about the premaxillary morphology is that it was short and probably tall below the naris. The exact location of the suture between the premaxilla-maxilla is uncertain.

In the laying slab ( Fig. 4A ), the skull remains are preserved in a depression of the matrix rock. Therefore, the left maxillary is lingually exposed. It is the only skull bone that seems to preserve its actual and complete outline. Although as badly preserved as the remainder of the specimen, it provides key information for the identification of MB.R. 1015. The body of the maxilla has a low, rounded dorsal process on its anterior half, as in Eothyris ( Reisz et al. 2009: fig. 2). This process may indicate the position of a caniniform region in Datheosaurus , as already suggested by Romer and Price (1940), even if their opinion was possibly influenced by their interpretation of this taxon as sphenacodontian. However, given that such a dorsal process is present in all adequately known caseids (e.g., Olson 1968; Sigogneau-Russell and Russell 1974; Reisz 2005), it cannot be considered as evidence for the presence of caniniforms. The maxilla of Datheosaurus seems to have been taller than that of Eothyris and especially than that of Oedaleops ( Reisz et al. 2009) , but lower and more rounded than that of caseids ( Olson 1968; Sigogneau-Russell and Russell 1974; Reisz 2005). The presence of a broad, rounded embayment between the dorsal and anterior maxillary processes show also that Datheosaurus had probably large, rounded external nares. Posteriorly, the maxilla forms a stout ramus with a slightly concave dorsal margin which most likely marks the ventral rim of the orbit, resembling more Eothyris than Oedaleops ( Reisz et al. 2009) . The maxilla contributes widely to the orbital margin in Eothyris but not in Oedaleops and in caseids in which it is excluded from it by a short lacrimal-jugal contact ( Sigogneau-Russell and Russell 1974; Laurin and Reisz 1995: fig. 6C; Reisz 2005; Maddin et al. 2008; Reisz et al. 2009). For Datheosaurus , this is supported when both slabs are graphically superimposed over each other to compare the preserved portions of the skull. When superimposed, the supposed position of the orbit relative to the maxilla matches that on the counter slab. The skull table is a broad, flat surface that is widely emarginated by the orbits. Overhung by a lateral expansion of the prefrontals and postfrontals, the orbits were therefore facing dorsolaterally at mid-length of the skull of Datheosaurus .

Most conspicuous in the skull table, the parietals are wide and enclose a large pineal foramen, as already recognized by Schroeder (1904). It is located along the middle to anterior part of the interparietal suture, at least a certain distance from their posterior edge. This condition matches the pattern seen in all members of Caseasauria, in contrast to the smaller pineal foramen of Edaphosauridae ( Modesto 1995) and a more posterior position seen in basal Sphenacodontia ( Laurin 1993). The posterior edge of the parietals is well visible, outlining the dorsal margin of the occiput.

The posterolateral margin of the skull table, which was probably made up by part of the squamosal, supratemporal and parietal wings, projects far posteriorly. Even considering diagenetic deformations, the jaw joint seems to have been set far behind the posterior margin of the skull roof, around 20% of the total skull length. On the hanging slab, the anatomically left cheek exhibits a longitudinal step that tentatively marks the position of the temporal opening ( Fig. 4B ).

The lower jaws are preserved in articulation on the lower slab and only slightly flattened due to compaction. Regarding the straightness of each jaw ramus, the mandible is acutely triangular with a pointed tip, as typical for caseasaurs. The symphyseal region appears to be higher than in Eothyris . Both hemi-mandibles are exposing their lingual sides. The left jaw ramus is mostly complete and measures 60 mm in length. It shows a slight dorsal eminence in the posterior third. There is no convincing indication of the adductoral fossa or sutural contacts.

A singular element, distinguished by its smooth surface, might represent the only preserved tooth of MB.R. 1015. It is located left to the mandibular symphysis, possibly arising from the upper jaw. If correctly identified, it reveals a slen- der, rod-like morphology, but lacks information about the tip of the tooth.

Axial skeleton: The holotype of Datheosaurus displays an almost complete and fully articulated vertebral column, but its preservation is extremely poor. Only few anatomical details of the vertebrae themselves can be observed. The widths of the vertebrae appear to be subequal to their lengths from the cervical to the proximal caudal region. The cervical and posterior dorsal vertebrae are slightly longer than the mid-dorsals, whereas a certain count of a cervical or possible ‘lumbar’ region is hampered by indistinct preservation. The exceptionally complete articulation of the specimen provides a presacral vertebral count of 26 including an unpreserved atlas, which has been reconstructed to represent the plesiomorphic condition for synapsids and all amniotes ( Müller et al. 2010). Except for Cotylorhynchus with a presacral count of 26, caseids have a reduced number of presacrals (24–25) ( Olson 1968). Among caseasaurs, only the recently described Eocasea martini with a count of 27 has a higher number of presacrals ( Reisz and Fröbisch 2014).

The size of the pelvic girdle suggests a number of two or three sacral vertebrae, but a certain assessment is not possible as the iliac blade is not preserved. Based on the position of the first caudal rib, the sacrum most likely consisted of two segments only, which is consistent with other basal synapsids, including Eocasea ( Reisz and Fröbisch 2014) , but contrasts a higher number of three to four sacrals in more derived caseids.

The caudal vertebrae are more slender than the presacrals and continuously decrease in size posteriorly. It is likely that most of the tail is preserved, with a number of at least 75 caudals. In parts of the series, a segmented midline structure can be seen ( Fig. 4A ), which resembles the notochordal canal, whereas a section of the neural canal would scarcely produce a trail this constant in thinness. The posteriormost portion of the tail shows preservation of actual vertebrae, contradicting Schroeder (1904) who assumed this part to only represent a tug-trace. Nonetheless, the surrounding area of the tail does indeed have a modified texture compared to the matrix rock, thus potentially supporting an interpretation of this structure as traces from the last movements of the suddenly buried and possibly suffocating animal. The reconstructed biotratonomy implies that the animal might bave been surprised by a mass flow event, embedded while resting or walking. Its exaggerated walking posture suggests that it tried to excape until sudden death. The specimen in question may represent one of the rare cases where an ichnofossil is preserved with its producer.

The presacral ribs of Datheosaurus are rather broad and interestingly show a constant thickness. The majority bends along their complete length, whereas more posteriorly the ribs display a bending zone just proximal to their mid-length. The posterior cervical ribs and anterior dorsal ribs are the shortest, while from around the 10 th to the 21 st presacral rib they are of approximately the same length. Towards the posterior dorsal series, the ribs are incomplete, but retain a broad shape, as typical for Caseidae and especially contrasting the slender posterior dorsal ribs in sphenacodontians. Since Eocasea recently revealed slender ribs ( Reisz and Fröbisch 2014), Datheosaurus is either more derived or independently developed broad ribs.

Five proximal caudal rib pairs are preserved. They strongly decrease in size posteriorly, but share the same strong, hook-like shape. However, the two body sides are slightly asymmetrical, which can be explained by tectonic shear in the clayish embedding and largely altered bone tissue, distorting the sacral region.

Pectoral girdle: The holotype of Datheosaurus only exposes the ventral elements of the pectoral girdle, all articulated, indicating that the vertical elements such as the scapula have not been affected by compaction. The outlines of the ventral plate of the clavicles, the anterior half of the interclavicle, and the anterior coracoids are visible on MB.R. 1015.1. This is not the case for the posterior extension of the interclavicle, which is still covered by vertebral remains in the laying slab (contra Williston 1912, who interpreted the matrix gap as the space for the interclavicular shaft). The clavicles have a rather broad, triangular ventral plate, which is only half the size of the anterior coracoid. From what can be seen, the interclavicle has a rhomboidal head. It shows otherwise no apparent features. The anterior coracoids are subcircular in outline. A shallow dorsal concavity marks the sutural contact for the scapula, meaning that the anterior coracoids were nevertheless largely ossified. On both sides the location of the glenoid is indicated by a diagonal bar, the coracoid ridge, pointing towards the proximal head of the humerus. A posterior coracoid is not exposed (contra Huene 1910, who suggested both coracoids were co-ossified). The position of the anterior-most ribs suggests that the actual posterior coracoids may still be located below the first dorsal vertebrae and ribs, as is the posterior portion of the interclavicle. As a whole, the pectoral girdle is massively built, even though not co-ossified. It does not provide any diagnostic features beyond representing a typical ‘pelycosaur’ condition. The strengthened anterior rim supported by the clavicles resembles that of large skeletons, e.g., in derived caseids ( FS personal observation).

Forelimb: Both humeri are exposed, but their proximal ends are poorly preserved and a distinct deltopectoral crest is not visible. On the hanging slab, the right humerus appears as a robust bone with a straight shaft ( Fig. 4C ). It has a minimal length of 54 mm and a distal width of 35 mm. Exposing the actual ventral surface, the anatomically right humerus of the hanging slab is the better preserved of the two and serves therefore as a basis for the following description. The entepicondylar foramen is located rather proximally and close to the posterior margin. A shallow trough in the anterior portion is definitely anatomical, separating a very large supinator process from the shallow swelling that represents the capitulum. Thus the radius articulated close to the middle of the distal head of the humerus. The trough most certainly represents the closed ectepicondylar groove, producing a unified semicircular contour in the distal humerus.

Currie (1977: 938) interpreted the humerus of Datheosaurus as representing a younger ontogenetic stage than that of the holotype of “ Haptodus ” garnettensis ( RM 14,156; see Spindler 2015).Later, Currie(1979:160;based on Schroeder 1904) mentioned the same features of the humerus stated here. He interpreted the humerus of Datheosaurus as indicating a juvenile, at the beginning of the ossification of the supinator and epicondyles, but ontogenetically older than in the juvenile series of Palaeohatteria longicaudata ( Haptodus baylei sensu Currie 1979 ). However, in contrast to the condition in Datheosaurus , the entepicondylar foramen is placed more distally in juvenile individuals of Palaeohatteria , whereas the supinator process is already separated from the capitulum by a step, even in some very small individuals (e.g., SNSD SaP 342, former “D 2223” in Currie 1979, see Spindler 2015: fig. 5.17). Therefore, the absence of a stepped supinator process in Datheosaurus does not indicate an early ontogenetic stage under a supposed sphenacodontian classification. The holotype of Datheosaurus is here considered subadult with respect to its ossification, with the closed ectepicondylar foramen representing a phylogenetic signal comparing to adult caseids, rather than representing an early juvenile stage. The outline of the foramen itself is blurred by the poor preservation, as it is also the case for the entepicondylar foramen.

In the forelimb only one partial zeugopodial element is preserved, which most likely represents a radius, judging from its overall shape and the concave proximal articulation surface for the humerus.

In the laying slab, a partial right manus is preserved in ventral view. Given the size of the skeleton, it documents a heavily built forefoot. Only the distal ends of metacarpals II to V are exposed and nothing can be said about their relative lengths. The respective digits show the phalangeal formula? -3-3+-5-1+, with digits II and IV including their terminal phalanges. This indicates the likely presence of an unreduced, plesiomorphic phalangeal formula of 2-3-4-5-3. All non-terminal phalanges are moderately expanded, with only the middle phalanx of digit IV being significantly shortened.

Pelvic girdle: Of the pelvic girdle of Datheosaurus , only the ventral plates are clearly identifiable. A long fragment located on the left lateral side of the caudal ribs in the hanging slab corresponds to the left ischium on the lower slab, and thus does not represent an ilium ( Fig. 4 ). The ventral elements are largely ossified, with a fully developed shape, including a complete symphyseal rim. Hence, the only immature features are the lack of a co-ossified pelvic symphysis and a visible puboischiadic suture. The pubis is shorter than the ischium, with a strong anterior ridge running toward the acetabulum. Likewise, the ischium has a thickened dorsal edge posterior to its acetabular contribution. The ventral plates of the ischia are sutured to each other along their anterior half but are separate and rounded in their posterior ends. No diagnostic features are observable in the pelvis. The length ratio of pubis and ischium, with the pubis being shorter than the ischium, seems to be plesiomorphic in synapsids ( Romer and Price 1940: figs. 25–28) and contrasts the inverse condition in some caseids ( Olson 1968: fig. 19).

Hind limb: The two hind limbs are in a flexed position, with the femora perpendicular to the pelvic girdle and the zeugopodia and pedes directed posteriorly. Both femora are present but poorly preserved, with a minimum length of 59 mm. The right femur exhibits a distinct sigmoid curvature, with the proximal head weakly bent dorsally and the distal head strongly bent ventrally. There is also a distinct strong fourth trochanter, at about one-third of the femur, which continues apparently in a high adductor ridge. Although this might be purported by the plane of splitting between the two blocks, there is some resemblance with caseid femora ( Olson 1968: fig. 20). The femoral curvature was erroneously interpreted as typically sphenacodontid by Currie (1979). In fact, this feature is not consistently present within Sphenacodontidae (see Romer and Price 1940: figs. 36, 37).

All posterior zeugopodials are preserved, with both tibiae measuring 41 mm in length. They are only slightly bent and have well-developed terminal articulations. The fibulae are slightly more slender and more strongly bent. The left fibula from the hanging slab reveals a finished ossification of the distal articulation facets, indicating a mature state of ossification in this area.

On the lower slab, only traces of the right pes are preserved, as a barely visible altered bone fan, including the astragalus, calcaneum and possibly three digits pointing towards the base of the tail. The left pes is better and slightly more completely preserved on the hanging slab and includes the proximal tarsals that are large and approximately of equal size. Their polygonal shape and straight midline contact indicate full ossification. The foramen for the perforating artery is placed directly distal to the center of the articulation between the two elements. The low neck of the astragalus in Datheosaurus is a pattern seen also in most varanopids and caseids.Additional tarsals cannot be identified with certainty, but a single distal tarsal is preserved at the base of the third metatarsal in the same pes. This foot also displays the complete metatarsus, of which the fifth metatarsus is only faintly preserved. The metatarsals differ only slightly in length.

Stratigraphic and geographic range.— Known only from type locality and horizon; Upper Pennsylvanian of Nowa Ruda , Poland .

Figures

Fig. 3. Holotype of caseasaurian synapsid Datheosaurus macrourus Schroeder, 1904 from Upper Pennsylvanian of Nowa Ruda, Poland. A. MB.R. 1015.1 laying slab), exposing an internal view onto ventral elements (A1), with close-up of distal-most tail portion (A2). B. MB.R. 1015.2 (hanging slab), exposing an internal view onto dorsal elements (B1), detail (B2), trough stratification of the area indicated by the arrowhead in B1.

Fig. 4. Explanatory drawing of the holotype of caseasaurian synapsid Datheosaurus macrourus Schroeder, 1904 from Upper Pennsylvanian of Nowa Ruda, Poland. MB.R. 1015.1 (A) and MB.R. 1015.2 (B). C. Right humerus of MB.R. 1015.2 in ventral aspect, photograph (C1), drawing (C2).

Fig. 5. Reconstructions of caseasaurian synapsid skeletons in dorsal views (A2, B2) and lateral aspects of skulls (A1, B1). A. Datheosaurus macrourus Schroeder, 1904 (Upper Pennsylvanian, Poland). B. Callibrachion gaudryi Boule and Glangeaud, 1893 (Cisuralian, France).