Seminobatrachus boltyschkensis, Skutschas & Gubin, 2012

Skutschas, Pavel P. & Gubin, Yuri M., 2012, A new salamander from the late Paleocene-early Eocene of Ukraine, Acta Palaeontologica Polonica 57 (1), pp. 135-148 : 137-144

Seminobatrachus boltyschkensis sp. nov.

Etymology: From the Boltyshka locality.

Holotype: PIN 3991 /9, part ( PIN 3991 /9a) and counterpart ( PIN 3991 / 9b) skull and anteriormost vertebrae preserved on sapropelite slabs from a drill core, in dorsal and ventral aspects, respectively ( Fig. 1 ).

Type locality: Drill hole near Boltyshka village, Cherkassy Region, central Ukraine.

Type horizon: Lower unit of unnamed sapropelite strata; late Paleocene–early Eocene.

Referred material.—Thirteen incomplete skeletons, all preserved on sapropelite slabs from drill cores: PIN 3991/1a, b, part and counterpart of articulated, incomplete vertebral column (trunk, sacral, and anterior caudal regions) in lateral aspect and limbs ( Fig. 2 ); PIN 3991/3a, b, part and counterpart of skull, anterior part of vertebral column, and right forelimb in, respectively, dorsal and ventral aspects ( Fig. 3); PIN 3991/4a, b, part and counterpart of skull, pre−sacral part of vertebral column, and forelimbs in, respectively, dorsal and ventral aspects ( Fig. 4); PIN 3991/6a, b, part and counterpart of skull, anterior part of vertebral column, and left forelimb in, respectively, dorsal and ventral aspects ( Fig. 5 ); PIN 3991/14, nearly complete skeleton in dorsal aspect ( Fig. 6 ); PIN 3991/2a, b, part and counterpart of posterior part of vertebral column and hindlimbs, both in lateral aspect; PIN 3991/8a,b, part and counterpart of skull and anterior part of vertebral column in, respectively, ventral and dorsal aspects; PIN 3991/10a,b, part and counterpart of skull, anterior part of vertebral column, and forelimbs; PIN 3991/11a, b, part and counterpart of skull, anterior part of vertebral column, and forelimbs in, respectively, ventral and dorsal aspects; PIN 3991/12a, b, part and counterpart of skull, anterior part of vertebral column, and forelimbs in, respectively, ventral and dorsal aspects; PIN 3991/13a, b, part and counterpart of skull, anterior part of vertebral column, and forelimbs in, respectively, dorsal and ventral aspects; PIN 3991/19, of skull and anterior part of vertebral column in dorsal aspect; PIN 3991/20, middle part of vertebral column, limbs, and pelvic girdle.

Diagnosis.—Polarities of character states indicated using the following symbols: primitive (−); derived (+); and uncertain (?). Neotenic crown−group salamander characterized by the following, unique combination of characters: premaxilla with relatively wide, posteriorly elongated dorsal process (+) that overlaps frontal (+); maxilla small, abbreviated (+); nasal nar−

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row and shorter than dorsal process of premaxilla (+); medial contact of nasals absent (+); parietal–squamosal contact absent (+); vomerine tooth row long and parallels maxillary arcade (−); pterygoid with long anterior process (−); ossified quadrate present (−); marginal and palatal teeth pedicellate (?); trunk vertebrae amphicoelous (−), with subcentral keel (+), spinal nerve foramina (+), and anterior basapophysis (+); bicipital ribs (−); carpals and tarsals not ossified (−); phalangeal formulae of 2−2−3−2 (?) and 2−2−3−4−2 (?) for manus and pes, respectively.

Stratigraphic and geographic range.—Late Paleocene–early Eocene, central Ukraine.

Remarks.— Seminobatrachus differs from stem caudates ( Karauridae and Marmorerpeton ) in lacking sculpture on the skull roof bones, in having lightly built vertebrae, and in having spinal nerve foramina in its trunk vertebrae. Differs from Hynobiidae and Cryptobranchidae in having spinal nerve foramina and bicipital transverse processes in trunk vertebrae. Differs from other crown−group salamanders, except Sirenidae , Salamandridae , Ambystomatidae , and Plethodontidae , in having spinal nerve foramina in trunk vertebrae. Seminobatrachus further differs from Salamandridae and Plethodontidae in having amphicoelous trunk vertebrae; from Sirenidae in having pedicellate marginal teeth and nasals lateral to the dorsal process of premaxilla, and from Ambystomatidae in having subcentral keel on trunk vertebrae and abbreviated maxilla.

Description

Skull overview.—Skulls and skull fragments are present in several specimens ( Figs. 1 , 3–6). The skull is relatively short (width to length ratio is about 1.1), with the widest part at the level of the jaw–skull articulation, and an anteriorly narrowing rostrum. The orbit is large and the cheek is widely emarginated from posterior abbreviation of the maxilla. The skull roof bones have no dorsal sculpture. The presence of lacrimals or septomaxillae cannot be confirmed in the available specimens.

Maxillary arcade and suspensorium.—The premaxilla (Figs. The quadrate ( Figs. 1 , 4) is well ossified and has a typical

1, 5) has an elongate maxillary process (= posterior process) salamander morphology, with expanded distal and narrow and a narrow, relatively long and pointed dorsal process (= proximal portions.

alary process); the latter arises from the medial part of the Skull roof.—The nasal ( Fig. 1 ) is small, with a narrow, trianbone and posteriorly overlaps the anterior part of the frontals. gular posterior portion that posteriorly overlaps the anterior The length of the dorsal process is nearly equal to the width part of the frontal and lies parallel and lateral to the dorsal of the dental margin. The dorsal processes on the paired process of the premaxilla. The nasal does not extend as far premaxillae contact medially along their anterior halves. posteriorly as the dorsal process of the premaxilla. The struc− The maxilla ( Figs. 1 , 5 ) is posteriorly abbreviated, but ture of the anterior portion of the nasal is unknown.

retains a short dorsal process and a slender posterior pro− The prefrontal ( Figs. 1 , 6 ) is larger than the nasal. The cess. There is a large foramen in the anterior part of the dor− posterior portion of the prefrontal tapers posteriorly. The sal process. structure of the anterior portion of the prefrontal is unknown. +

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The frontal ( Figs. 1 , 3A, 4–6) is long, slender, and slightly tapered anteriorly, with nearly parallel lateral edges and a small anterolateral extension. The anterior portion is overlapped by the dorsal process of the premaxilla and the posterior portion of the nasal. The posterior portion is nearly triangular and overlaps the anterior portion of the parietal. The frontals contact one another only anteriorly, whereas more posteriorly they are separated by the median fontanelle. This fontanelle is elongate and rhomboidal in shape (with its widest part level with the anterior edge of the posterolateral extension of the parietal), and also separates the parietals posteriorly. The frontals contribute less than 50% of the orbital margin.

The parietal ( Figs. 1 , 3–6) is the longest bone of the skull roof (ratio of maximum lengths of frontal versus parietal is about 0.8), with a narrow and long posterolateral extension. The parietals appear to be slightly wider than the frontals. The posterior part of the parietal is curved and tapers posteriorly.

The squamosal ( Figs. 1 , 4, 6 ) is “T”−shaped bone, with a broad dorsal portion and a tapered ventral portion. It contacts the braincase dorsally, but has no contact with the posterolateral extension of the parietal. The ventral portion of the squamosal contacts the quadrate posteroventrally along most of its length.

Palate.—The vomer ( Figs. 1 , 4) is large, with a broad choanal notch and well−developed postchoanal flange. It clearly contacts the premaxilla anteriorly, but the detailed structure of the medial part of the vomer is unknown. The vomer has one tooth row that is long and extends close to and parallel with the maxillary arcade.

The triradiate pterygoid ( Figs. 1 , 4) has a wide and short medial process (= medial ramus or basipterygoid ramus). The structure of the pterygoid–parasphenoid contact (and basicranial articulation) is unknown but it seems to be loose. The anterior process (= palatine ramus) is long, strongly arcuate, and tapers anteriorly. The anterior portion of the anterior process is anteromedially oriented.

The parasphenoid ( Figs. 1 , 3, 5 ) is the largest bone of the palate. It has a long cultriform process that is relatively narrow, nearly parallel−sided in its medial part, and slightly expanded anteriorly. The cultriform process is overlapped anteriorly by the vomers (this feature is visible in PIN 3991/11, which is not figured in this paper). The lateral processes (= lateral ala) are not preserved.

Braincase.—The orbitosphenoid ( Fig. 1 ) is long (about onehalf the length of the cultriform process of the parasphenoid). The optic foramen is situated nearly at the anteroposterior midpoint of the orbitosphenoid. The otic capsule is well ossified, and there is no indication of a suture between the left and right otic capsules. The posterior end of the braincase bears a pair of large occipital condyles ( Fig. 5B ).

Mandible.—The mandibles are present in several specimens ( Figs. 1 , 3–6). Two bones are easily recognizable in all available specimens: dentary and prearticular. The dentary is elongate and deep. The medial surface has a wide Meckelian groove that narrows anteriorly and is bordered dorsally by a wide subdental shelf.

The prearticular ( Figs. 1 , 3–5) is long, has a narrow and tapering anterior part and an expanded posterior part. The posterior part has a sharp and high dorsomedial edge, which gradually decreases in size anteriorly, and a dorsomediallyoriented coronoid process. The posterior (the highest) part of the dorsomedial edge is nearly the same height as the coronoid process. The lateral edge of the prearticular is thickened, and there is a shallow groove between its lateral and dorsomedial edges.

Hyobranchial apparatus.— The only preserved element of the hyobranchial skeleton is basibranchial 2 (present in PIN 3991 /6). Additionally, branchial denticles are present in the holotype PIN 3991 /9 and three specimens ( PIN 3991 /4, 9, 6) have imprints of the external gills .

Basibranchial 2 ( Fig. 5 ) is triradiate (inverted “Y”−shaped), with all three processes equal in size (as in the Early Cretaceous Valdotriton , ambystomatids, and some modern salamandrids; see Evans and Milner 1996; Rose 2003).

There are six rows of branchial denticles ( Fig. 1 ) on the four branchial arches, having a configuration of 1.2.2.1. This linear arrangement of the branchial denticles allows us to reconstruct the presence of the four ceratobranchials: ceratobranchials 1 and 4 each support one row, whereas ceratobranchials 2 and 3 each support two rows of branchial denticles. Each denticle is conical, with an expanded base and a relatively thin, curved crown.

Only two pairs of external gills are present in any of the available specimens ( Figs. 1 , 4, 6 ). The first pair of external gills was supported by ceratobranchial 2 and the second by ceratobranchial 3. All external gills are nearly equal in size. If not a preservation artefact, the presence of only two pairs of external gills in Seminobatrachus is a unique feature among salamanders. Normally three pairs of external gills are present in larvae and in adults of those neotenic taxa, such as Necturus , that have external gills.

Dentition.—Marginal dentition is present on the premaxillae, maxillae, and dentaries. The number of teeth on the maxilla and dentary is unclear; the estimated premaxillary tooth count is 18–20. All marginal teeth are pedicellate. The tooth crowns of the marginal teeth are sharp but their detailed structure (e.g., number of cuspids) is unknown. Palatal dentition is present on the vomers; the estimated vomerine tooth count is more than 35–40. The vomerine teeth are smaller than the marginal teeth. The vomerine teeth are pedicellate as well, but the structure of their crowns is unknown.

Axial skeleton.—The vertebral column consists of 14–16 presacral vertebrae (15 or 16 in PIN 3991/14; 14 in PIN 3991/4) and more than 25 caudal vertebrae (the end of the tail is lacking in PIN 3991/14 and the estimated number of caudal vertebrae based on that specimen is 30–33). All trunk vertebrae were articulated with bicipital (= twoheaded) ribs.

The detailed structure of the atlas ( Fig. 4) is unknown. It is wider than the anterior trunk vertebrae in dorsal view and has no transverse processes. The length of the atlas is nearly equal to that of the following anterior trunk vertebrae.

The anterior trunk vertebrae ( Figs. 2 , 4) are relatively elongate, narrow, low, and consistently lack sculpture. The centrum is amphicoelous. In lateral view, the centrum is longer than wide, hourglass−shaped, and its ventral surface bears a prominent subcentral keel. Anterior basapophyses are present as anteriorly elongate knobs along the ventrolateral sides of the anterior cotylar rim. Posterior basapophyses are consistently absent. The transverse processes (= rib−bearers) are elongate, bipartite (inferred from the presence of bicipital ribs), and extend posterolaterally. The base of the transverse

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process is perforated by a vertebrarterial canal. Two alar processes (= laminae) are associated with the transverse process. The anterior alar process is a relatively long and wide flange that extends anteroventrally from the base of the transverse process and it almost reaches the anterior basapophysis. The base of the posterior alar process is nearly equal in length with that of the anterior alar process and extends posteroventrally from the base of the transverse process. The neural arches are poorly preserved and their structure is unknown. The spinal nerve foramen is visible on the inner surface of the base of the neural arch ( Fig. 2C ) just behind the anteroposterior midpoint of the vertebra.

The middle and posterior trunk vertebrae differ from the anterior trunk vertebrae in being more elongate and in having a shallower subcentral keel. Spinal nerve foramina are consistently present.

The sacral vertebrae are poorly preserved. According to specimen PIN 3991/4 ( Fig. 4) the sacral vertebra does not differ in shape or size from the adjacent posterior trunk and anterior caudal vertebrae.

The anteriormost caudal vertebra ( Figs. 2 , 4) is as elongate as the posterior trunk vertebrae and it lacks haemapophyses. In successively more posterior caudal vertebrae, the centrum length gradually decreases and haemapophyses are consistently present. The latter processes are relatively narrow (in comparison with neural arches on the same vertebrae), rod−like, and extend posteroventrally. The depths of the haemapophyses decrease posteriorly along the caudal series.

All ribs are bicipital. The largest ribs are associated with the second and third trunk vertebrae. These robust ribs have expanded distal ends. The ribs become weaker and shorter towards the sacrum.

Pectoral girdle and forelimb.—The scapula and coracoid form a single ossification. The scapulocoracoid ( Figs. 4, 6 ) has an expanded coracoid portion and an elongate scapular portion that is slightly constricted at its base. The humerus ( Figs. 2–4 , 6 ) is straight, with expanded and flattened proximal and distal ends. The ulna ( Figs. 2–4 ) is slightly longer than the radius and roughly half the length of the humerus. The carpals are not ossified. Four digits ( Fig. 4B 2 ) are present in the manus, with digit IV the longest. The phalangeal formula of the manus is 2−2−3−2.

Pelvic girdle and hindlimb.—The ilium ( Fig. 4) has a relatively long and narrow proximal part. The ischia are preserved in only one specimen (PIN 3991/20); they are kidney−shaped and contact one another medially along most of their lengths. The femur ( Figs. 2 , 6 ) is of similar length to the humerus, and the proximal and distal ends are expanded and compressed. The trochanter and crista trochanterica are well developed. The tibia and the fibula ( Figs. 2 , 6 ) are nearly similar in length. The tibia is more robust than the fibula and has a more expanded distal end. The fourth digit is the longest of the five ( Fig. 2 ), and the phalangeal formula of the pes is 2−2−3−4−2.

Figures

Fig. 1. A salamander Seminobatrachus boltischki gen. et sp. nov. from the lower unit of the Boltyshka sapropelite strata (late Paleocene–early Eocene, Ukraine), skull and anteriormost vertebrae with traces of soft tissue, holotype, PIN 3991/9. A. PIN 3991/9a, part in dorsal aspect. B. PIN 3991/9b, counterpart in ventral aspect. Photographs as exposed (A1, B1) and interpretative drawings (A2, B2). Grey areas in interpretive drawings represent soft tissues.

Fig. 2. A salamander Seminobatrachus boltischki gen. et sp. nov. from the lower unit of Boltyshka sapropelite strata (late Paleocene–early Eocene, Ukraine), articulated, incomplete vertebral column (trunk, sacral, and anterior caudal regions) and limbs, PIN 3991/1a, part in lateral aspect. A. Photograph as exposed. B, C. Digital reconstructions based on high−resolution computed tomography of entire specimen (B) and close up of pectoral region rotated 90° counterclockwise (C).

Fig. 5. A salamander Seminobatrachus boltischki gen. et sp. nov. from the lower unit of Boltyshka sapropelite strata (late Paleocene–early Eocene, Ukraine), close up of part and counterpart of skull, anterior part of vertebral column with traces of soft tissue, PIN 3991/6. A. PIN 3991/6a, part in dorsal aspect. B. PIN 3991/6b, counterpart in ventral aspect. Photographs as exposed (A1, B1) and interpretative drawings (A2, B2). Grey areas in interpretive drawings represent soft tissues.

Fig. 6. A salamander Seminobatrachus boltischki gen. et sp. nov. from the lower unit of Boltyshka sapropelite strata (late Paleocene–early Eocene, Ukraine), nearly complete skeleton with traces of soft tissue, PIN 3991/14, in dorsal aspect. Photograph as exposed (A) and interpretative drawing (B). Grey areas in interpretive drawings represent soft tissues.