Wealdenbatrachus sp.

Báez, Ana M., 2021, Barremian anurans of the Iberian Peninsula: new insights into their taxonomic diversity, Comptes Rendus Palevol 20 (6), pp. 75-90 : 78-84

Wealdenbatrachus sp.

MATERIAL. — MUPA-LH 00004. This incomplete specimen consists mainly of an impression of an adult individual in ventral aspect with an estimated 42 mm snout-vent length ( SVL: direct line distance from tip of snout to posterior limit of ischia) and preserved two-dimensionally ( Fig. 2A, B). Photographs of this specimen under incandescent light ( Fig. 2A) and with exposure to UV radiation ( Fig. 2B) were also available for study.

LOCALITY, HORIZON, AND AGE. — Las Hoyas fossil site, 30 km from the city of Cuenca, Cuenca province, Spain; La Huérguina Formation; late Barremian.

REMARKS

The skeleton is almost wholly articulated but the anterior portion of the skull, the left forelimb and corresponding half of the pectoral girdle, and the distal portions of the autopodium of both hindlimbs are missing. A photograph of this specimen was included in a brief note on the amphibians from the Las Hoyas fossil site ( Evans et al. 1995: fig. IV-18, labelled as LH 79900). Previously the specimen was considered referable to the genus Eodiscoglossus ( Evans et al. 1995) but assigned to the genus Eodiscoglossus with doubts and depicted as a line drawing by Roček (2000: fig. 9, labelled as LH 004-R and erroneously scaled). It was also commented on as cf. Eodiscoglossus sp. indicating uncertainty with regard to its generic allocation and depicted partially by Báez (2016: fig. 5, labelled as MUPA-LH 00004). However, herein some of its features are reinterpreted, differing from previous opinions. Examination of the actual specimen and the photographs led to the description below, in which left and right refer to the actual skeletal orientation.

DESCRIPTION

The paired frontoparietals form the boundaries of an anterior pear-shaped fontanelle; posterior to this fontanelle, these bones are medially in contact to one another. The laterally expanded anterior portions of the frontoparietals make the skull roof wider anteriorly than posteriorly. At the level of the posterior margin of the orbits each frontoparietal widens laterally, so it slightly overlapped the medial part of the corresponding otic capsule. A low pars contacta that in life was in contact with the lateral walls of the sphenethmoid is clearly discernable. In the parietal region a rounded scar together with a poorly preserved, similar contralateral scar form a structure with an overall oval outline; this structure is identified as the posterior part of the incrassatio frontoparietalis. The photograph under UV light ( Fig. 2B) reveals a portion of a laminar bone slightly shifted to the left that extends anteriorly as well as laterally to invest the ventral surface of the braincase and the medial region of the otic capsules. This is identified as part of the cultriform process and corpus of the parasphenoid slightly shifted to the left side.

The maxillary arches were complete; they included the missing premaxillae, dentate maxillae, and quadratojugals; on the right side, the last bone is discernable showing that it was relatively long. The three-radiated pterygoids are preserved on both sides of the skull; the anterior ramus articulates with the maxilla at mid-orbit length, whereas the long ventral ramus contacts the pars articularis of the quadrate. On the right side, the ossified pars media plectri of the middle ear is preserved close to the corresponding otic capsule ( Fig. 2B). It has a slender style, distally, and an expanded footplate that was applied to the oval foramen.

The vertebral column is composed of eight presacral vertebrae, the sacrum, and the urostyle ( Fig. 2A, B). The anteroventral margin of the atlas is moderately convex and projects laterally on both sides suggesting the presence of elongated cotyles located ventral to the neural canal; this vertebra lacks transverse processes. The vertebral bodies are poorly preserved although they appear to be longer than wide and slightly waisted at midlength. The type of articulation between successive vertebrae is not discernable. Vertebrae II, III, and IV have well-developed transverse processes associated with stout ribs. The shortest of these ribs are those associated with vertebra II, which are flared posteriorly. Distinct swellings mark the contact between the expanded distal end of the transverse processes and proximal epiphysis of the ribs. The distal epiphyses of the ribs are also expanded. In contrast, the posterior four presacrals have more delicate, shorter transverse processes; these processes are nearly laterally oriented on vertebrae V and VI and moderately anteriorly directed on the last two vertebrae (VII and VIII). The sacrum bears narrow and posterolaterally directed sacral diapophyses that are only slightly expanded distally. The articulation between sacrum and urostyle is seemingly bicondylar. The urostyle is proportionally long, being longer than the length of the presacral column ( Fig. 2A, B). At the anterior end of the urostyle there are indications of a relatively wide coccygeal portion, although transverse processes are not clearly discernable.

Elements of the left anterior limb and pectoral girdle are evident ( Fig. 2C, D). The scapula is medium-sized, waisted at its midlength, and thus probably devoid of a well-developed anterior crest ( Fig. 2D). The partial impression of the clavicle indicates that it was an arched bone, suggesting an arciferal type of girdle; it is not preserved in its natural position probably owing to its partial disarticulation from the scapula. The coracoid of the same side is also disarticulated; its proximal head is expanded, whereas the rest of the bone is directed posteromedially and its impression disappears near the vertebral column. The left humerus and radioulna are clearly discernable ( Fig. 2C). The carpus is incompletely preserved but some carpal elements can be discerned distal to the radioulna; in the proximal row two trapezoidal elements, radiale and ulnare, are evident. Impressions of the four metacarpals are clear although metacarpal V is partially hidden behind metacarpal IV; the longest is metacarpal IV and the shortest is metacarpal II which has a distinctly wide proximal end. The phalangeal formula is 223?. The discernible distal phalanges are distinctly long and narrow; their tips have a bilobed shape owing to the presence of a median groove (Báez 2016: fig. 5B) ( Fig. 2C).

The pelvic girdle includes long ilia, the shafts of which bear well-developed dorsal crests along their lengths. The acetabular regions of these bones are not exposed. The displaced right ischium is exposed laterally near their proximal ends; this displacement indicates that ilium and ischium were not fused to one another. The femur, preserved on both sides, is distinctly sigmoid and its length is greater than that of the tibiofibula (tibiofibula to femur ratio about 1.24). The tibiale and fibulare are in contact with each other at both ends, leaving a long and narrow intertarsal space; the length of this segment of the limb is about 60% that of the zeugopodium. Only the proximal portions of four metatarsals in the left hindlimb are clearly preserved.

Assignment of MUPA-LH 00004 to Wealdenbatrachus Restudy of the original material of the late Barremian Wealdenbatrachus jucarensis Fey ( Báez & Gómez 2019) led to the careful assessment of several features, some of them probably derived, in MUPA-LH 00004 that might be shared with that species and, hence, justify assignment to the same genus. This comparison strongly suggests that the genus Wealdenbatrachus Fey, 1988 previously known from Uña ( Fey 1988) is represented in the coeval site of Las Hoyas, geographically near but with different depositional and taphonomic conditions ( Gomez et al. 2001), confirming the previous opinion of Sanchíz (1998). In particular, the paired frontoparietals of MUPA-LH 00004 form the boundaries of a distinctly long, pear-shaped fontanelle as in W. jucarensis and not a short, V-shaped dorsal opening as it was depicted previously by Báez (2016). As in the species from Uña, the dorsal skull table is wider anteriorly than posteriorly ( Fig. 2A, B) and each frontoparietal bears a distinct narrow pars contacta, whereas these bones are not fused to the sphenethmoid. Also, each frontoparietal extends posterolaterally to overlap a narrow strip of the medial region of the corresponding otic capsule. Most other known features of this specimen from Las Hoyas match those of the original material of W. jucarensis , redescribed and depicted in the revision of Báez & Gómez (2019). As in the latter species, vertebrae II to IV bear transverse processes that are associated with robust ribs, particularly those associated with presacral III. The posterior vertebrae have transverse processes whose degree of development and orientation resemble those of W. jucarensis ; thus, vertebrae V, VI bear transverse processes that are nearly perpendicular to the vertebral column axis whereas the last two presacrals (VII,VIII) have distally blunt, anterolaterally directed transverse processes, although somewhat more anteriorly directed than in W. jucarensis . Transverse processes on the last two presacrals are curved anteriorly in the holotype of Eodiscoglossus santonjae ( Báez & Gómez 2016) . In all these taxa the sacral vertebra has relatively narrow diapophyses that are posterolaterally directed ( Fig. 2A, B). As in W. jucarensis , the scapula is of moderate size and has a distinctly concave anterior margin that suggests that the well-developed anterior lamina that occurs in the short and wide scapula of crown costatans is absent ( Fig. 2D, E). By contrast, in E. santonjae the scapulae appear to have straight anterior margins ( Báez & Gómez 2016). A distinct dorsal crest on the iliac shafts occurs in MUPA-LH 00004 and also in W. jucarensis , whereas in E. santonjae this crest appears to have been less developed. The relative proportions of the hindlimb segments show some resemblance to those of W. jucarensis in having the femur shorter than the tibiofibula, unlike the hindlimbs of E. santonjae , in which these bones are nearly of the same length ( Estes & Reig 1973; Báez & Gómez 2016). Also, the tibiale and fibulare are rather elongate and slender. Although forelimb autopodial bones of W. jucarensis are hitherto unknown, it is interesting to mention that the metacarpals and digits in MUPA-LH 00004 are thin ( Fig. 1C) unlike the more robust elements of most extant costatans and the Barremian basal crown anuran Liaobatrachus from the Jehol Group of China ( Dong et al. 2013). Although the tips of the distal phalanges of the manual digits in “archaeobatrachians” (i.e., non-neobatrachians) have been described as “simple” ( Cannatella 1985), examination of several specimens of extant costatans revealed bilobate or grooved ends in Discoglossu s jeanneae (MNCN 41059, male, pers. obs.). A similar morphology was noted in Discoglossus sardus by Pugener & Maglia (1997) and described as lacking a groove but bifurcated by Kamermans & Vences (2009). The variation and insufficient knowledge on the condition of the tips of the distal phalanges in “archaeobatrachian” anurans prevent me to consider it for justifying taxonomic separation.

Comparison between MUPA-LH 00004 and LH 05429 from Las Hoyas The specimen MUPA-LH 00004 was previously compared with the specimen MUPA-LH 05429 also from Las Hoyas by Báez (2016). The latter specimen consists of the partial, articulated, three-dimensionally preserved skeleton of an adult individual exposed in ventral view and it has been only partially prepared (Báez 2016; pls 1E, F). It should be noted that several parts of this skeleton are still hidden beneath sediment and remain unknown (e.g. anterior palatal region, posterior portions of pelvic girdle and urostyle, femurs, tibiofibulae); therefore, a detailed description, comparisons, and discussion on its taxonomic placement will be presented elsewhere when fully exposed. Despite the evident presence of some features widespread in basal anurans (e.g. eight presacral vertebrae, adults with free ribs associated with vertebrae II-IV), MUPA-LH 00004 and LH 05429 differ from each other in many discernable characters indicating that they represent different taxa, probably different genera. The pterygoid of the latter specimen bears a broad, anteromedially directed flange between the anterior and medial rami of the pterygoid, unlike the same bone in Wealdenbatrachus jucarensis and MUPA-LH 00004. The flange in MUPA-LH 05429 recalls the well-developed orbital flange that extends from the anterior to the medial ramus of the pterygoid described in the extant Asian bombinatorid costatan Bombina microdeladigitora Liu, Hu & Yang, 1960 and apparently is unknown in other anuran species ( Clarke 1988, 2007). In MUPA-LH 00004 the scapula is relatively dorsoventrally long and its shaft is distinctly waisted, indicating the lack of a well-developed anterior crest as in the scapula of W. jucarensis ( Fig. 2D, E). The scapular morphology clearly differs from that in MUPA-LH 05429, in which this bone is relatively shorter and has a straight leading margin despite being slightly broken off ( Fig. 2F). It is also worth mentioning that the mesopodium is partially preserved in LH 05429 ( Fig. 3) with some of the bones hidden beneath sediment; this skeletal part is unknown in W. jucarensis and poorly preserved in MUPA-LH 00004. In MUPA- LH 05429, three nodular proximal mesopodial elements (including a separate small bone between articular facets of a large radiale and a somewhat smaller ulnare) are clearly present ( Fig. 3). Distal to these elements it is possible to identify a large distal carpal 5 on the postaxial side and close to Element Y. The distal carpal 4 is preserved distally between the bases of metacarpals IV and III. The preserved mesopodium also includes a prepollex carpal in contact with and lateral to Element Y ( Fig. 3). Although the metacarpals are partially discernable it is possible to ascertain that they are not relatively long. The presence of a separate small bone in the proximal row of the autopodium was described and identified as the intermedium in the Middle Jurassic Notobatrachus Reig, 1956 “1955” ( Estes & Reig 1973) and in the early salientian Triadobatrachus Kuhn, 1962 ( Ascarrunz et al. 2016). Because developmental data is lacking in these extinct salientians, it is not possible to determine whether this additional bone originates from the ulna or from the ulnare and hence to compare its development with the intermedium of other tetrapods, including salamanders.The impression of this median proximal carpal element is not present in the holotype of Eodiscoglossus santonjae ( Báez & Gómez 2016) nor it occurs in the well-preserved but isolated forelimb from the type locality of E. santonjae ( Vergnaud-Grazzini & Wenz 1975: fig. 5). Even if it has a different origin from that of the intermedium of salamanders, its presence as a distinct free proximal carpal has not been recorded in extant anurans ( Fabrezi et al. 2017).

NEW GENUS AND SPECIES?

MATERIAL

MUPA-LH 11392. This specimen is represented by the latex-rubber cast of a partial skeleton exposed in dorsal view preserved in one slab ( Fig. 4 A-C), lacking the skull and probably the anterior two vertebrae, as well as by a photograph of possible counterpart which was obtained by the scanning of a slide ( Fig. 5C).

LOCALITY, HORIZON, AND AGE Las Hoyas fossil site , 30 km from the city of Cuenca, Cuenca province , Spain. La Huérguina Formation, late Barremian.

REMARKS

The latex cast (deposited in the Museo de Paleontología de Castilla- La Mancha) and photograph (Repositorio fotográfico de Las Hoyas LHSa-001, MUPA-LH 11392,

Museo de Paleontología de Castilla-La Mancha) document the existence of an actual single specimen probably preserved as part and counterpart because of their incongruent positive and negative reliefs, but whose whereabouts are unknown at present. According to the International Code of Zoological Nomenclature, recommendation 73.1.4 designation of the illustration of a single specimen as a holotype is to be treated as designation of the specimen illustrated; the fact that the specimen no longer exist or cannot be traced does not of itself invalidate the designation. However, because of the insufficient anatomical data coupling with the impossibility of examining the actual specimen, I refrain from formally recognizing a new taxon herein. This specimen possesses a distinctive set of features that set it aside from other recognized taxa, as discussed below, and probably represents a new taxon.

The specimen MUPA-LH 11392, now seemingly lost but from which a latex cast was prepared in the early 1990s ( Fig. 4 A-C), consisted of the impressions and scraps of bone of one adult individual preserving the six most posterior presacral vertebrae, sacrum, urostyle, pelvis, most of the right forelimb and scapula, and right hindlimb lacking the distal portion. Photograph and interpretive drawing of this cast, labelled as H-5 of the private Diaz-Romeral collection, appeared in a brief preliminary summary on the first fossil anurans discovered from Las Hoyas fossil site presented during a conference in Cuenca ( Sanchíz 1991). A photograph of a slide showing the same individual H-5, but that it seems to belong to a counterpart (also lost) is available for study ( Fig. 5C). Subsequently, this specimen was mentioned as LH 11392 ( Evans et al. 1995). It is evident that the lost fossil has a distinctive combination of features that differs from those of other known specimens not only from the same beds but also from other Barremian sites, indicating that it likely pertains to an additional taxon among the anurans of Las Hoyas as suggested previously ( Evans et al. 1995). However, the specimen(s) remained undescribed to date.

DESCRIPTION

Examination of both the photograph ( Fig. 5C) and the latex cast of the putative counterpart (MUPA-LH 11392; Fig. 4 A-C) revealed the features described below.

The neural arch laminae of the four most anterior preserved vertebrae are longer than wide, distinctly dorsally protuberant at the level of the postzygapophyses, and fully imbricated so that the spinal cord was completely concealed between successive vertebrae. On these vertebrae large neural spines project from the neural arches, each overlapping the posteriorly adjacent vertebra; the tips of these spines are missing or remained cartilaginous and thus are not preserved ( Fig. 5A). By contrast, the neural arches of the last two presacral vertebrae are slightly wider than long and somewhat flatter than those of the more anterior vertebrae. These vertebrae are less imbricate, with posterior margins excavated to the left and right of the small spinous processes where the neural canal might have been slightly exposed dorsally. The two most anteriorly preserved vertebrae are associated with long processes indicating that these are probably vertebrae III and IV; these processes are delicate, sigmoid, slightly posteriorly directed, and distally expanded according to the photograph ( Fig. 5B, C). The presence of a distinct swelling in the process borne by vertebra III might be an indication that this element consists of a relatively long rib articulated to a proximal short transverse process; this is not so clearly obvious in the processes of vertebra IV suggesting a more complete ankylosis of these parts, if present. Both pairs of processes are of nearly the same length and reach as far laterally as the sacral diapophyses. The transverse processes of the four following vertebrae (presumably vertebrae V-VIII) are short and nearly of the same length. Those of vertebra V are laterally oriented; instead, the fine, distally acuminating processes of the last three presacral vertebrae (VI-VIII) are oriented at an acute angle with respect to the longitudinal axis of the column, especially those of the two most posterior vertebrae ( Figs 4A; 5B, C). Mid-dorsal ridges on the neural arches of the vertebrae V-VII are clearly discernable in the latex cast ( Fig. 4A, C).

The sacral vertebra (presumably vertebra IX) bears moderately distally expanded, dorsoventrally flat sacral diapophyses with slightly convex lateral margins. These diapophyses are represented by their impressions and fragments ( Fig. 5C). These diapophyses are anteriorly and posteriorly symmetrically developed with respect to the line of maximum width perpendicular to the sagittal plane of the column. Although fusion of the sacral vertebra with the urostyle can be ruled out based on the slightly more dorsal portion of the sacral laminae with respect to the coccygeal region of the urostyle, poor preservation of this area prevents unambiguous determination of the nature of the sacrourostylar articulation. The urostyle is slightly rotated to the right; examination of the cast shows a broad portion resembling a neural arch with a fine, acuminate, lateroposteriorly directed transverse process on the anterior part of the urostyle ( Fig. 4C). A low ridge extends on the dorsal surface of the urostyle along its anterior half. According to the length of the preserved portion of the vertebral column, the urostyle was somewhat longer than the presacral column.

Elements of the right half of the pectoral girdle in dorsal view are discernible ( Figs 4A, B; 5A, D). The scapula is medium-sized; the leading edge of the bone is concave, thus lacking a well-developed anterior crest, and the shaft is markedly waisted ( Fig. 4B). Proximally (ventrally) a notch between the partes acromialis and glenoidalis is not clearly discernable whereas the anteromedial corner of the bone is damaged. The clavicle is not well exposed owing to the overlapping transverse process of vertebra III; a poorly preserved, acuminate piece of bone lying anterior to the scapula is barely discernable in the photograph ( Fig. 5C); this piece of bone might be the proximal portion of the clavicle. The coracoid is partially obscured by the transverse process of vertebra V. The photograph clearly shows a triangular laminar element lying on the articulation of the right humerus and radioulna ( Fig. 5C); this element might be the displaced cleithrum of this side. The leading margin of this element is straight, forming a right angle with the margin that probably extended along the entire scapula-suprascapular cartilage articulation. The proximal and distal ends of the humerus are hidden by the overlying scapula and cleithrum respectively. Likewise, the proximal end of the radioulna is obscured by the putative isolated cleithrum but a clear distinction between the radial and ulnar components appears evident along most of the bone length. In the cast, the silhouettes of at least three metacarpals are discernable distal to unidentifiable carpal elements hidden beneath a rounded mass. Their proximal ends are partially overlapped by the latter mass, but it is evident that they are not particularly long and have slightly expanded distal ends ( Fig. 4A). The proximal two phalanges of digits V, IV, and III are also discernable in the cast; they are robust and expanded at both ends. Other phalanges are barely evident.

The ilia have distinctly long shafts ( Fig. 5C, D). The presence of dorsal crests is difficult to assess; these structures appear to be present along the posterior half of the shafts although they were partially scraped off as shown in the latex cast. These bones are not well exposed in lateral aspect and, consequently, it is not possible to describe the acetabular region. Impressions of the ischia in situ are barely discernable.

The femur is sigmoid ( Fig. 4A), whereas a proximal femoral crest is not clearly evident. The tibiofibula is also entirely preserved; its length is slightly greater than that of the femur (tibiofibula to femur ratio of about 1.109); deep grooves occur along its anterior and posterior portions revealing its dual nature. The tibiale and fibulare are in contact with one another at both ends, leaving an oval intertarsal space; the length of this hindlimb segment represents about 50% of that of the tibiofibula. The compound structure is slightly wider distally than it is proximally.

Impression of the body outline is evident in the photograph ( Fig. 5C), suggesting that MUPA-LH 11392 was an individual with highly muscular hindlimbs. Whitish masses surrounding yellowish corpuscles fill most of the body cavity; they might be interpreted as eggs, if so, the specimen belongs to an adult female.