Crossognathus danubiensis, Cavin & Grigorescu, 2005

Cavin, Lionel & Grigorescu, Dan, 2005, A new Crossognathus (Actinopterygii, Teleostei) from the Lower Cretaceous of Romania with comments on Crossognathidae relationships, Geodiversitas 27 (1), pp. 5-16 : 8-12

Crossognathus danubiensis n. sp.

HOLOTYPE AND ONLY KNOWN SPECIMEN. — FGGUB.V.210 .

TYPE LOCALITY. — On the bank of the Danube , vicinity of Giurgiu, Romania.

TYPE LEVEL. — Middle Albian.

ETYMOLOGY. — From the Latin Danubius, the name of the Danube River.

DIAGNOSIS. — Crossognathus with elongated posterior infraorbitals covering the vertical limb of the preopercle and part of the opercle, opercular bones with fine posterior radiating ridges, small teeth on the ectopterygoid and palatine, numerous and well developed branchiostegal rays, scales small, numerous, with radii on the posterior exposed area.

DESCRIPTION ( FIGS 2-4 )

The single-known specimen is incomplete. The head is preserved, but the anterior extremity of the skull roof, the ethmoid region and the tips of mandibles are missing. The ventral part of the squamation is preserved back to the level of the pelvic fins, whilst dorsally the squamation is preserved up to the level of the midline between the pectoral and the pelvic fins. The specimen is slightly dorso-ventrally flattened. The head is tapered ventrally at the level of the mandibles, but broadens posteriorly at the levels of the opercular apparatus.

Skull roof

Only the posterior part of the skull roof is preserved. The anterior tips of both frontals (Fr) are missing. The preserved posterior parts show that the frontal is broader posteriorly than anteriorly, giving a roughly triangular outline in dorsal view. The surface of both frontals is not well preserved, but ridges and weak reticulated networks of ridges radiate from each centre of ossification. A stronger ridge runs posteromedially from each centre of ossification and meets its counterpart in the midline, defining the posterior margin of a median shallow depression (md). Such a median depression is present in crossognathids (Taverne 1989) and pachyrhizodontids (Forey 1977) among other teleosts. The suture between frontals is smooth and forms a sigmoid line in the half of the length of preserved bones. Most of the posterior part of the skull roof is covered by the hypertrophied extrascapulars (Ext) and only reduced surfaces of both parietals (Pa) are visible. They contact the frontals with interdigitate sutures. Because of the size of extrascapulars, we cannot determine if both parietals are in contact in the midline (the medioparietal condition versus the lateroparietal condition). Posterolaterally to the frontals and laterally to the parietals is a pair of badly preserved ossifications, regarded as dermopterotics (Dpt). The general shape of these ossifications show that they protrude posterolaterally and extend the lateral margins of the wide frontals posteriorly. The posterior lateral development of the dermopterotics corresponds to the maximum width of the skull roof. Such a shape of the posterior skull roof in dorsal view is reminiscent of crossognathids (Wenz 1965; TellerMarshall & Bardack 1978; Taverne 1989). A piece of the anterior part of the parasphenoid (Psp), exposed in dorsal view because the anterior part of the skull roof is lacking, and a piece of the right lateral ethmoid, are visible. Pieces of thin curved bones inside the orbit indicate the presence of sclerotic rings (sr). Other elements of the skull roof and of the braincase, as well as the path of sensory canals, are either not preserved, or not visible on the specimen.

Circumorbital bones

Only the posterior half of the ring formed by the circumorbital bones is preserved, and we cannot determine if the circumorbital ring was complete. All the bones forming the circumorbital ring are very thin and fractured, making difficult the observation of limits between the ossifications. The dermosphenotic (Dsp) is located in the posterodorsal edge of the orbit and shows a sinusoidal contact with the last infraorbital. The posterior margin of the orbit is formed by two large infraorbitals (Io) that extend posteriorly over the vertical limb of the preopercle and the anterior part of the opercle. Ventrally, the margin of the orbit is formed by an elongated infraorbital that covers part of the dorsal border of the horizontal limb of the preopercle. The three visible infraorbitals are ornamented on their posterior half with fine radiating shallow grooves. From the infraorbital sensory canal, portions of canals are visible parallel to the posteroventral margin of the orbit, as well as some posteriorly orientated short bone-enclosed branches that seem to radiate from two points located dorsally and ventrally along the posterior section of the canal. A relatively similar pattern is visible on the reconstruction of the cheek of Crossognathus sabaudianus Pictet, 1858 proposed by Taverne (1989: fig. 2).

Suspensorium

Only a small part of the suspensorium is visible. The quadrate (Q) is antero-posteriorly elongated, its posterior margin being almost horizontal in lateral view. The posteroventral process of the quadrate is well developed and forms a thickened body of bone by comparison with thin plate-like remaining part of the quadrate. The anterior extremity of the posteroventral process forms a broad concave articular facet that receives the postarticular process of the mandible as described in Apsopelix (Teller-Marshall & Bardack 1978) . Ventrally, the posteroventral process of the quadrate bears a groove in which rested the symplectic. Only a piece of the latter bone is still visible on the left side of the specimen. Suturing with the anterior margin of the quadrate is the ectopterygoid (Ecpt), which shows a concave ventral edentulous margin. The rest of the suspensorium is not visible, except the dorsal face of the entopterygoid (Enpt) and of the elongated palatines (Pal) that extend horizontally from the lateral margins of the suspensorium to the parasphenoid. Two patches of minute alveoli are visible medially on the right ectopterygoid and on the ventral side of the right palatine, the latter one still bearing a few small rounded teeth.

Upper jaw

From the upper jaw, only fragments of the right maxilla (Mx) and parts of both supramaxillae (Smx) are preserved; no teeth are visible. The posterior supramaxilla bears on the posterior half of the bone a pattern of radiating grooves extending from a longitudinal axis and the anterior supramaxilla bears trace of a similar pattern on its dorsalmost margin. Such ornamentation has been described in Apsopelix anglicus (Teller-M a r s h a l l & B a r d a c k 1 9 7 8) a n d f i g u r e d i n Crossognathus sabaudianus (Taverne 1989) . In “ Syllaemus albiensis ”, regarded as a synonym of Apsopelix anglicus by Patterson & Rosen (1977) and Teller-Marshall & Bardack (1978), and as a s y n o n y m o f C r o s s o g n a t h u s s a b a u d i a n u s b y Taverne (1989: fig. 2), Wenz (1965) described a shallow longitudinal groove running along both ossifications.

Lower jaw Both lower jaws are preserved, but only their posterior part show any details. The general shape of the hemi-mandibles is apparently proportionally longer and shallower than in Apsopelix anglicus and Crossognathus sabaudianus , in which it is described as leptolepid-like (Taverne 1989). We should, however, point out that the posterior oral margin of the mandible is reconstructed in C r o s s o g n a t h u s s a b a u d i a n u s (T a v e r n e 1 9 8 1: fig. 20; 1989: fig. 5) and in “ Syllaemus albiensis ” (Wenz 1965: fig. 5). No teeth are visible on the holotype, but the oral margin of the dentary is not well preserved. The dentary (D) has a concave ventral outline in lateral view and forms ventrally a sharp edge extending medially as a horizontal shelf, as in Apsopelix anglicus (Teller-Marshall & Bardack 1978). The medial margins of these shelves are ornamented with fringes. The angular (An) extends anteroventral into a thin process on the edge of the mandible. The ventral part of the retroarticular process and of the facet bears a reticulate ornamentation. As the hemimandibles are visible in lateral view only, we cannot distinguish the sutures between the bones of the posterior extremity of the mandible (angular, articular, retroarticular).

Opercular series, branchiostegal rays, and gular

The preopercle (Pop) is crescent in shape with a rounded and regular posterior angle. Its dorsal extremity is hidden by the infraorbitals. The posterior and posteroventral margins of the bone are ornamented with very fine grooves, reminiscent of the ornamentation observed on the infraorbitals. At least seven bone-enclosed canals extend radially from the main sensory canal in the posteroventral and ventral parts of the ossification. The anterior tip of the preopercle is tapered. The opercle (Op) is a well developed bone. Its dorsal and posterior margins are rounded, and its ventral margin is obliquely orientated and slightly concave. The subopercle (Sop) is also well developed, with a regular rounded posterior margin and a straight ventral margin. Only a small part of the interopercle is visible on the right side of the skull, the main body of the bone being hidden under the horizontal limb of the preopercle. This condition is common in crossognathids, since Wenz (1965) regarded the interopercle in “ Syllaemus albiensis ” as absent, because the ossification was hidden under the preopercle. The dorsalmost branchiostegal ray differs from the other ones by its stronger ornamentation and by its anterior extremity that disappears below the horizontal limb of the preopercle with a different angle that the mean angle formed by the bases of the more anterior branchiostegal rays. This ossification is specific enough to deserve the designation of branchiopercle (Bop), as described in Amia calva (Grande & Bemis 1998) . Apart from the branchiopercle, 21 left, elongated and platelike branchiostegal rays (br) are visible. The bases of the anteriormost rays are covered by the thin gular plate. The left series of rays covers the right series (a sinistral individual). The gular plate is an elongated bone that extends anteriorly from the symphysis to a level situated at the anterior extremities of the preopercles posteriorly. The ossification is tapered and very thin at the symphysis, then broadens in the middle length of the mandible and reaches a constant width posteriorly. A few minute teeth similar to those borne by the palate are visible of the floor of the buccal cavity. They are probably borne by the basihyal and/or basibranchial, although the latter bones are not visible on the specimen.

Pectoral girdle and fins

The extrascapulars are very large, meet together in the middline, and cover most of the parietals. The canal for the extrascapular commissure runs parallel and close to the straight anterior margin of the extrascapular. The posterior margin is rounded and the surface of the bone is ornamented with fine radiating ridges. The postemporal is also very large, but badly preserved on the specimen. The cleithrum has a well developed posteroventral edge, that expands above the pectoral fin as in Apsopelix anglicus (Teller-Marshall & Bardack 1978). An elongated axillary process is visible on the left side of the specimen, the surface of which is marked by pittings as in Apsopelix anglicus (Woodward 1903). The number of postcleithra cannot be determined. The bases of fin rays and their traces on the squamation visible on both sides of the specimen indicate that the pectoral fins were elongated and contained at least 12 rays. Scales

The scales broadly overlapped each other. They are small, cycloid, with numerous fine circuli and radii in the exposed posterior area. We estimate that 30 horizontal scale rows are present on each side of the body just behind the pectoral girdle. The general scale morphology corresponds to the d e s c r i p t i o n m a d e b y T a v e r n e (1 9 8 9) f o r Crossognathus sabaudianus , but differ from the scales observed in Apsopelix anglicus from the English Gault and Chalk (pers. obs.) and from North America ( Dunkle 1958). Moreover, the relative size of the scales is much smaller in C. danubiensis n. sp., since 10-12 horizontal rows only are present on each side at midbody in Apsopelix (Woodward 1903; Teller-Marshall & Bardack 1978).

Figures

FIG. 2. — Crossognathus danubiensis n. sp., photographs of the specimen in dorsal (top) and ventral (bottom) views. Scale bar: 50 mm.

FIG. 3. — Crossognathus danubiensis n. sp., holotype (FGGUB.V.210), drawings of the head in dorsal (A), ventral (B), left lateral (C) and right ventrolateral (D) views. Abbreviations: An, angular; Bop, branchiopercle; br, branchiostegal ray; Cl, cleithrum; D, dentary; Dpt, dermopterotic; Dsp, dermosphe- notic; Ecpt, ectopterygoid; Enpt, entopterygoid; Ext, extrascapular; ext.com, extrascapular commissure; Fr, frontal; G, gular; Io, infraorbital bone; md, median depression; Op, opercle; Pa, parietal; Pal, palatine; Pop, preopercle; Psp, parasphenoid; Q, quadrate; Scl, supracleithrum; Smx, supramaxilla; Sop, subopercle; Sy, symplectic. Scale bar: 50 mm.

FIG. 4. — Reconstruction of the head of Crossognathus danubiensis n. sp. in dorsal (A) and lateral (B) views. Abbreviations: Ao, antorbital; Axp, axillary process; Bop, branchiopercle; br, branchiostegal ray; Cl, cleithrum; Dpt, dermopterotic; Dsp, dermosphenotic; Ext, extrascapular; Fr, frontal; Io, infraorbital bone; Mes, mesethmoid; Mx, maxilla; Na, nasal; Op, opercle; Pa, parietal; Pmx, premaxilla; Pop, preopercle; Ptt, posttemporal; Q, quadrate; Scl, supracleithrum; Smx, supramaxilla; Sob, supraorbital; Sop, subopercle; sr, sclerotic ring.