Callibrachion gaudryi Boule and Glangeaud, 1893b

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 : 599-604

Callibrachion gaudryi Boule and Glangeaud, 1893b

Figs. 1 , 2 .

1893 Callibrachion Gaudryi (gen. nov., sp. nov.); Boule and Glangeau [sic!] 1893b: 646–647.

1893 Callibrachion Gaudryi (gen. nov., sp. nov.); Boule and Glangeaud 1893a: 1–17, text-figs. 1–4, pl. 3.

1893 Callibrachion Gaudryi ; Boule and Glangeaud 1893c: 218. [cop. Boule and Glangeaud 1893a: 1–2, 16–17].

1910 Callibrachion Gaudryi Boule et Glangeaud ; Thévenin 1910: 50, fig. 28. [cop. Boule and Glangeaud 1893a: fig. 1].

1928 [nec] Kallibrachion; Nopcsa 1928: 171.

1940 Haptodus gaudryi (Boule and Glangeaud 1893) ; Romer and Price 1940: 307–308.

1959 Callibrachion Gaudryi ; Mazion 1959: 7–10, figs. 1, 2, 4, 5. [cop. Boule and Glangeaud 1893a: 1–2, 16–17, figs. 1–4].

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

Holotype: MNHN.F.AUT490, single slab with a three-dimensionally preserved partial skull and almost complete skeleton, but missing part of the vertebral column, part of the pectoral and pelvic girdles as well as the left hind limb ( Figs. 1 , 2 ). This specimen was previously cited correctly by Currie (1979: 134) under its entry number MNHN 1891-23 1979: 138) and incorrectly as MNHN 1893-23 .

Two spiral heteropolar coprolites have been added to the plaster jacket, artificially arranged to fit into a wooden frame. This practice was common at the end of the 19th century for specimens intended for exhibition in the new “Galerie de Paléontologie” of the MNHN, including other Autunian tetrapods from the Autun Basin.

Two thin sections have been sampled ( Boule and Glangeaud 1893c: fig. 2), one from the maxillary canines and one containing two of the dorsal vertebrae. The current location of these thin sections is unfortunately unknown.

Type locality: Black shales of Autun, near Margenne, Saône-et-Loire Department, Bourgogne Region, France.

Type horizon: According to modern stratigraphy ( Werneburg and Schneider 2006), the sites that revealed Callibrachion as well as the type and only specimen of Haptodus baylei ( Gaudry 1886) are part of the Upper Millery Formation (Autunian, Rotliegend, Cisuralian), which has recently been shifted from Sakmarian to Artinskian age Schneider et al. 2014). The dark grayish bituminous shale represents lacustrine depositional environment ( Marteau and Feys 1989) where rare amniote skeletons are allochthonous.

Material.— Holotype only.

Diagnosis.—A small to mid-sized caseasaur with slender and straight conical teeth, moderately thickened ribs, possibly an enclosed ectepicondylar foramen, an ilium with the typical caseid high dorsal blade, and robust but not expanded phalanges that show no reduction in phalangeal count.

Description.—The bones in MNHN.F.AUT490 often show questionable outlines, due to the crude original preparation and dispersed pyrite remodeling the shapes and blurring the contrast to the rock matrix. The slab exposes the skeleton on its ventral side, indicated by the visible interclavicular shaft, the inclination of the manual unguals, the rib cage, missing exposure of neural spines, and the relative position of the femur and the ilium with exposed acetabulum.

The type and only specimen of Callibrachion gaudryi is represented by a robustly built skeleton. To estimate its ontogenetic stage, it can be noted that the notochordal canal is well visible, as illustrated by Boule and Glangeaud (1893a: fig. 3). The authors also remark unfused neurocentral sutures, although these were not visible during the current review. As rarely reported in early amniotes, the scapulocoracoid is disarticulated, which underlines the juvenility of the specimen in question. No trace of a scapulocoracoid suture is seen in any known caseasaur shoulder girdle, except for that in the immature holotype of Ruthenosaurus ( Reisz et al. 2011) . In the ilium, the bone vascularization is visible, which together with the disarticulation of this element supports an early ontogenetic stage. The humerus has a broad distal end and a well ossified proximal head. In general, the limb bones show rather undifferentiated heads that correspond to the stages I and III of Brinkman (1988). The metapodials are ossified. Consequently, MNHN.F.AUT490 does not represent a very juvenile but still immature individual.

Cranial skeleton: The skull is possibly not only represented by the region originally interpreted as such, but also by a number of very uncertain elements between the jaw fragments and the left manus. This is supported by the mostly articulated postcranium of which all elements are known or at least identifiable, and thus can be excluded as a source for these additional elements.

Several certain skull bones have been identified in the original description ( Boule and Glangeaud 1893a, c). Among these, the pterygoid and a dorsal coronoid process could not be confirmed in the current study, and the supposed quadrate is tentatively accepted.

The maxilla (and probable continuation to the premaxilla) is the only unquestionably identifiable element in the skull, characterized by its marginal tooth row. The ventral rim is slightly convex, with a canine region at the anterior end indicated by the broad tooth bases in contrast to the smaller ones preserved posteriorly to the latter. It is not possible to provide a definite tooth count or a length estimate for the maxilla, since the outline of the element is somewhat questionable. However, the maxilla seems to bear a steep dorsal process, as it has been reported for Varanopidae (e.g., Langston and Reisz 1981: fig. 5) and Caseasauria (e.g., Sigogneau-Russell and Russell 1974: fig. 2), even though both groups show differing sutural patterns. If not partly belonging to the lacrimal, the element designated as the maxilla is relatively high compared to eothyridids (see Reisz et al. 2009). According to its proportions, a contribution of the maxilla to the rim of the orbit is possible, but not certain. The most diagnostic and striking feature of the maxilla of Callibrachion is in fact the marginal dentition. The preserved teeth have a slender, conical crown with a smooth surface, lacking any significant curvature except in the anterior part of the preserved tooth row. Some teeth are partly broken and expose the internal pulp cavity. Among basal synapsids, a similar tooth type is found only in ophiacodontids, the basal sphenacodontian Pantelosaurus ( FS personal observation), eothyridids, and the undescribed basal caseid from the German Bromacker locality ( Reisz and Fröbisch 2014; Martens et al. 2014; Thomas Martens, personal communication 2014). Reconstructing the maxillary tooth row of Callibrachion with the canines in an anterior position, a second region of enlarged teeth posterior to the caniniform positions is reflecting the eothyridid pattern. Considering the combined information regarding the overall skull dimensions, the maxillary outline and the dentition, the limited data available for Callibrachion only matches the condition in Caseasauria.

With the same preservation as in the posterior part of the maxilla, some bluish-white fractures of tooth bases indicate that part of the palatal dentition may have been preserved in the region ventral to the mandible. They seem large compared to the smallest marginal teeth, strongly resembling the coarse denticles in the anterior part of the palate of Cotylorhynchus romeri ( OMNH 04327; FS personal observation). Unfortunately, the poor preservation precludes a certain identification of the dentigerous element, but considering its size, it possibly includes a pterygoid.

The mandible is represented by a straight, slender, and rather shallow right jaw ramus. Its preserved length of about 70 mm could be close to its original length, since large and slightly curved teeth are found in what is most likely the dentary tip, lacking a deepened symphysis. The identification of individual elements is difficult, given its preservation, but the configuration suggests nevertheless that the ventral element is in fact a downturned splenial. Another element contacting the dorsal margin of the mandible was tentatively described by Boule and Glangeaud (1893a) as a “coronoid process” in a lepidosaur fashion. Such a structure would not resemble the more confluent process present in caseids, such as in Euromycter ( Sigogneau-Russell and Russell 1974) . As for now, there is no evidence for the presence of a coronoid process in Callibrachion , given the poor definition of this area in the holotype.

Axial skeleton: Although mostly articulated, the vertebrae provide only limited information. The vertebral column of Callibrachion is only represented by about twelve centra of the dorsal region, as well as uncertain remains of the cervical series and neural arches. Two barely outlined cervical centra appear to be located between the skull and the pectoral girdle. Some of the vertebrae have been damaged by preparation, while some others are overlapped by the pectoral girdle. Nonetheless, as a result of the articulation of the skeleton, it is possible to provide an estimate of the presacral count. In particular the uninterrupted series of about 21 dorsal ribs indicates a minimal presacral count of 23 vertebrae, in agreement with previous interpretations ( Huene 1908). As this number is close to the plesiomorphic dorsal count reconstructed for synapsids ( Müller et al. 2010), it is likely that only a few vertebrae are missing or still in the matrix.

As shown by a thin section, the dorsal centra are amphicoelous, notochordal, and rather elongated ( Boule and Glangeaud 1893a: fig. 3). In average, the dorsal centra measure about 11 mm in width, and 15 mm in length. Given this ratio of about three-quarters, Callibrachion differs from basal sphenacodontians ( Currie 1979: table 4). Although their articular facets are rounded, the lateral surfaces of the dorsal centra are slightly compressed. The resulting subtriangular cross-section ends in a weak ventral ridge.

The dorsal neural arches mentioned by Boule and Glangeaud (1893a) could not be identified with confidence. According to them, only two are well preserved enough for description, which they considered to be separated from the centra, showing a strong prezygapophysis, a smaller postzygapophysis, no transverse process, and a well-developed neural spine with a slight transverse dilatation.

The dorsal ribs are strongly built. The heads are difficult to reconstruct. Some appear to be elongated, but do not compare to the enormous length related to the expanded transverse processes in the neural arches of derived caseids. In the posterior dorsal region the ribs decrease in length, but not in width. The ribs have a distinct inflexion zone in their proximal region, which contrasts with all known members of Caseasauria including Datheosaurus (see below), but this morphology is very similar to a pattern seen in other pelycosaur-grade synapsids (e.g., Archaeothyris , Ianthodon , Ianthasaurus , and basal Sphenacodontia).

Pectoral girdle: The pectoral girdle of Callibrachion is well preserved and nearly complete, retaining its articulation on the anatomical right side of the skeleton. Boule and Glangeaud (1893a) already recognized the unfused condition of the scapulocoracoid. The clavicle has a relatively straight, rod-like shaft and overlaps the scapula with its wide ventral plate. Medially it meets the interclavicle, which displays a long, straight, parallel-sided and relatively slender shaft, whereas the head shape and proportions are not known with certainty. This is very much unlike the condition in Oedaleops ( Sumida et al. 2014: fig. 2.5a), which has an interclavicular shaft that is broad proximally and narrows continuously distally and its head is very short but projects far laterally on either side.

With a height of about 50 mm, the right scapula of Callibrachion is much shorter than its humerus. It has a flaring dorsal edge and a strongly developed supraglenoid buttress. No diagnostic feature can be identified, but it does not seem to resemble the stout and broadly rounded shape of the scapula of Oedaleops ( Sumida et al. 2014: fig. 2.5b), which may be the result of a low level of ossification in Callibrachion . The coracoids show little of their anatomy. Their outlines indicate nevertheless that the larger anterior coracoid has a strongly convex anterior margin and that the posterior coracoid has a weak triceps tubercle. Neither a supraglenoid foramen in the scapula nor a coracoid foramen are observable, but this may also be an artifact of poor preservation.

Forelimb: Both humeri are well exposed, each measuring about 70 mm in length and 40 mm in width distally. The proximal end is rather well ossified, with a short but robust deltopectoral crest and a well-developed process for the Musculus latissimus dorsi. Currie (1977, 1979) interpreted this shape to be less ossified than that of “ Haptodus ” garnettensis (holotype RM 14,156). Personal examination of the latter ( Spindler 2015) in fact indicates a progressed ossification in Callibrachion , with the two distal foramina being enclosed within the epiphysis. In contrast to the description by Romer and Price (1940), who reported a ventrally pointing supinator process in Callibrachion , the distal epiphysis is almost flat and shows diffuse outlines of both an entepicondylar and ectepicondylar foramen. Though not fully ossified in distal outline, the ectepicondylar groove is clearly closed, which represents a condition seen in Edaphosaurus (Reisz 1986) , Heleosaurus ( Botha-Brink and Modesto 2009) , and Caseidae ( Olson 1968: fig. 16).

The right ulna is well preserved and exposed on the right side of the specimen, measuring 58 mm in length. Its concave proximal articulation lacks a distinct olecranon. The radius is only partially exposed on both sides and little can be said about its anatomy except that it was certainly a more slender element compared to the ulna.

Both manus are well preserved and mostly articulated. Although present, the proximal carpals cannot be identified with certainty, including the ulnare mentioned by Boule and Glangeaud (1893a). The individual elements of the manus are robust and well developed. In digit IV, which is the longest, the metacarpal measures 22 mm. The metacarpals and phalanges are of plesiomorphic amniote morphology, contrasting the stout and broadened shape of all known caseids except Eocasea martini , Casea broilii , and the intermediate “ Casea ” nicholsi . Due to its poor preservation, some of the interphalangeal joints are not easy to identify, but the specimen seems to display the plesiomorphic phalangeal formula (2-3-4-5-3) of amniotes ( Figs. 1A , 2 ). The middle phalanx of digit IV is shortened, measuring about 70% the lengths of the neighboring elements. With this feature Callibrachion resembles Datheosaurus (see below). The terminal phalanges are strongly curved, with a strong flexor tubercle and a broadened flat dorsal roof.

Pelvic girdle: In the pelvic girdle, only the right ilium is exposed measuring 33 mm in length and 30 mm in height. The iliac blade has a simple plate-like morphology, showing no feature except a distinct anterodorsal expansion, resembling the ilium of Sphenacomorpha as well as Caseidae (and even Oedaleops , see Langston 1965: fig. 4; contradicting Sumida et al. 2014: figs. 2.5, 2.9). This is the only region of the entire postcranial skeleton preserving the original bone texture. Radiating from the neck of the ilium, the surface of the blade is unfinished, displaying long osteons. None of the surrounding fragments can be identified as a pubis or ischium with certainty.

Hind limb: Despite its poor preservation and partial disarticulation, the hind limb appears to have equivalent dimensions to the forelimb, but exact measurements of the individual elements are not available as they overlap in part with other bones. Only the proximal half of the right femur is exposed, clearly showing a relatively shallow intertrochanteric fossa delimited by the posterior ridge and a deep, distinct internal trochanter. As a side note, the presence of a fracture parallel to the femoral long axis shows that the orientation of this trochanter is artifactual. Now resting along the shaft of the femur, the internal trochanter was likely projecting ventrally originally. The femoral shaft is straight. The proximal articulation of the femur is convex, but its simplicity suggests it is not fully ossified.

The zeugopodials are disarticulated. The element that most likely represents the tibia is positioned across the distal end of the femur. It is robust and slightly curved with a broad proximal end. Another, more slender element is only partially exposed and possibly represents the fibula.

The pedal elements are mostly disarticulated. It is not possible to make any definite statement about the phalangeal formula or potential phalanx shortening, but the individual elements seem to be equally robust and strongly developed as those of the manus.

Scales: Several small, shiny scales with a rhomboidal outline and fine surface fluting have been associated with Callibrachion ( Boule and Glangeaud 1893a, who remarked that these scales have been extracted from the slab). As already pointed out by Romer and Price (1940), this description indicates they actually belong to a palaeoniscoid actinopterygian, very common in the Millery Formation.

Stratigraphic and geographic range.— Known only from type locality and horizon; Margenne ( France), Artinskian (Cisuralian) .

Figures

Fig. 1. Holotype of caseasaurian synapsid Callibrachion gaudryi Boule and Glangeaud, 1893 from Cisuralian of Autun, France (MNHN.F.AUT490). A. General view. B. Close-up of right manus. C. Right ilium, dorsally covered by proximal femur. D. Isolated shark coprolithes from the holotypic slab (Jörg W. Schneider, personal communication 2015).

Fig. 2. Explanatory drawing of the holotype of caseasaurian syn- apsid Callibrachion gaudryi Boule and Glangeaud, 1893 from Cisuralian of Autun, France (MNHN.F.AUT490).