Leptolepidae, Pictet, 1854

Konwert, Martin & Stumpf, Sebastian, 2017, Exceptionally preserved Leptolepidae (Actinopterygii, Teleostei) from the late Early Jurassic Fossil-Lagerstätten of Grimmen and Dobbertin (Mecklenburg-Western Pomerania, Germany), Zootaxa 4243 (2), pp. 249-296 : 254-256

Leptolepidae sp. 1

Figure 2 ; Plate 1A, B

Material. GG 431/5a, b isolated head missing postcranial bones, preserved in part and counterpart. Many of the bones are missing but molds of the bones are preserved on both slaps, showing their outlines and ornamentation.

Geographical distribution. Former clay pit of Dobbertin, Mecklenburg-Western Pomerania, Germany.

Stratigraphical distribution. Lower Jurassic, lower Toarcian “Green Series”, Harpoceras falciferum Zone.

Description.

Cranial bones ( Fig. 2 ; Pl. 1B): The parietal is incompletely preserved, but the impressions of the bone on GG 431/5a and b show that the anterior portion of the bone was thin and elongate. The posterior part is broad with an undulating posterior margin. The surface of the bone is ornamented with several fine grooves. The postparietal is not preserved. The pterotic is somewhat L-shaped with a broad ventral and a thin dorsal limb, the latter is mainly formed by a heavily ossified bony ridge. Some short vertical grooves are present on the surface of the bone, the most posterior one, that is deeper and broader than the previous ones, seems to be the middle pit line. A short section of the parasphenoid is preserved in the specimen, it is thin and heavily ossified. The posterior margin of the extrascapula is damaged, but the impression of this bone on GG 431/5b shows that it was semicircular, with a straight anterior margin.

Upper and lower jaws ( Fig. 2 ; Pl. 1B): The jaws are only known from impressions on the rock. The maxilla is long and slightly convex, at least its anterior part must had been ornamented with longitudinal grooves and ridges. The impressions on GG 431/5b indicate the presence of teeth at least in the posterior part of the maxilla, their size remains unknown. Two supramaxillae are present. They are placed dorsal to the maxilla. The posterior supramaxilla bears a well marked, spine-like anterodorsal process. According to the impressions, both must had been covered with a strong ornamentation of longitudinal grooves and ridges. The molds of the dentary and angular are uninformative.

Circumorbital bones ( Fig. 2 ; Pl. 1A): The bones of the circumorbital series are known from impressions of the supraorbital and the left and right infraorbital 1. The dermosphenotic ( Fig. 2 ; Pl. 4B, E) is preserved in GG 431/5a, as well as its impression in GG 431/5b. The supraorbital is elongate with a sharp anterior tip. Its medial margin is sharply angled along its entire length. The possible presence of a posterior supraorbital is unknown. The infraorbital bone 1 was large and broad, with its anterior portion deeper than the posterior. The dermosphenotic is very large for basal teleosts (about 10% of head length), it is somewhat triangular and expands in anterodorsal direction.

Opercular bones and branchiostegal rays ( Fig. 2 ; Pl. 1B): The opercular series consists of opercle, subopercle, preopercle; the interopercle was not observed. The opercle is the largest bone of the series, it bears a strongly ossified ridge at its anterior margin. The articulation with the subopercle is oblique. Some irregular shaped grooves are present on the surface of the bone. The subopercle is about as half as deep as the opercle and slightly broader. Its ventral and posteroventral margins are convex. The preopercle consists of a long dorsal and a ventral limb, both forming an angle of 110°. The dorsalmost part of the preopercle is only formed by the bony tube of the preopercular sensory canal. There is a well-marked notch in the posterior margin, and a small process in the anterior margin. Three incompletely preserved, plate-like branchiostegal rays are present ventral to the opercular bones.

Sensory canal system ( Fig. 2 ; Pl. 1A, B): All preserved sensory canals are bone enclosed. The preserved part of the supraorbital canal runs along the parietal and forms a slight curve posterior to the orbit. The supraorbital canal ends at the posterior margin of the parietal. The canal probably continues on the postparietal. The canal does not give off any tubules, but four pores are observed. The infraorbital canal is positioned near the dorsal margin of infraorbital 1, at least five tubules are present in this bone. These are long and thin, nearly reaching the ventral margin of infraorbital 1. The infraorbital canal ends at the anterior margin of the dermosphenotic. On the dermosphenotic, the infraorbital canal gives off four tubules, all are directed dorsally, ending close to the dorsal margin of the bone. The infraorbital sensory canal is posteriorly continuous with the otic canal. The otic canal runs along the dorsolateral margin of the pterotic. This canal does not have tubules but two pores are present. The supratemporal canal runs near the anterior margin of the extrascapula, it gives off at least five long tubules that probably reached the posterior margin of the extrascapula. A short part of the middle pit line is probably present in the posterior part of the pterotic. The preopercular canal runs near the anterodorsal margin of the ventral limb of the preopercle, and along the center of the dorsal limb. At least eight tubules are present in the ventral limb of the preopercle, their length increases in posterior direction. They end near the ventral respectively posterior margin of the preopercle. One long tubule was observed in the ventral portion of the dorsal limb of the preopercle. A single pore is present in the dorsal portion of the preopercular canal.

Identification: Although poorly preserved and lacking the postcranial skeleton, GG 431/5a, b can be assigned to Leptolepidae based on the presence of two relatively large supramaxillae that are placed at the dorsal margin of the maxilla and the L-shaped preopercle. The specimen cannot be assigned to any existing species of Leptolepidae , since the undulating posterior margin of the parietal, the L-shaped pterotic, and the number of tubules in the infraorbital canal in the dermosphenotic seem to be unique among leptolepids.

Figures

FIGURE 2. Leptolepidae sp. 1 (GG 431 / 5 a) from Dobbertin. Line drawing of cranial bones and molds. Dashed lines indicate molds. Abbreviations: a. smx, anterior supramaxilla; br, branchiostegal rays; de, dentary; dsp, dermosphenotic; exs, extrascapula; io 1, infraorbital 1; op, opercle; pa, parietal bone; par, parasphenoid; pt, pterotic; pop, preopercle; pop. c, preopercular sensory canal; p. smx, posterior supramaxilla; sop, subopercle; so. c, supraorbital sensory canal;?, unknown bone.