Leptochiton maguntiacus (de Rochebrune, 1882 )

Leptochiton maguntiacus (de Rochebrune, 1882) View in CoL

Fig. 34 View FIGURE 34

Lepidopleurus maguntiacus de Rochebrune 1882, p. 58 View in CoL ; Janssen 1978, p. 219, pl. 14, figs 11–15, pl. 15, figs 16–17; Gürs 1983, p. 57, pl. 1, figs 2a–c; Hocht 1986, p. 209; Dell’Angelo & Palazzi 1989, p. 64; Gürs 1995, p. 20, pl. 1, figs 1–4.

Chiton virgifer ? juv. [non Lepidopleurus virgifer (Sandberger, 1859) View in CoL ]; Boettger 1869, p. 9, pl. 1, figs 11a–g; Boettger 1870, p. 39, pl. 9, figs 11a–g; Van Belle 1981, p. 80 [= Leptochiton maguntiacus (de Rochebrune, 1882) View in CoL partim and L. poirieri (de Rochebrune, 1882) View in CoL partim, fide Janssen 1978, p. 219, 221].

Chiton virgifer ? juv. var. agnata Boettger MS ; Janssen 1978, p. 220; Van Belle 1981, p. 19 (= Leptochiton maguntiacus View in CoL , fide Janssen 1978, p. 220).

Lepidopleurus ( Leptochiton) algesirensis View in CoL [non Leptochiton algesirensis ( Capellini, 1859) View in CoL ]; Dell’Angelo & Palazzi 1989, p. 61, pl. 8, fig. 2, pl. 9, figs 1–2, pl. 10, fig. 2, pl. 12, fig. 2.

Type material. SMF, Lectotype (intermediate valve) and paralectotype (head valve) designated by Janssen (1978: 219).

Type locality. Gienberg ( Germany) .

Type stage. Oligocene (Rupelian).

Material examined. Oligocene (Rupelian): Germany:Steigerberg: 10 valves ( BD 405, Figs 34A–L View FIGURE 34 ). Maximum width of the valves: 4.8 / 8 / 7.5 mm.

Description. Head valve less than semicircular, posterior margin widely V-shaped, notched in middle. Intermediate valves broadly rectangular, elongate (W/L = 2.8), semicarinate in anterior profile, moderately elevated (H/W = 0.35–0.41), anterior margin straight, side margins rounded, posterior margin straight, apex not indicated, lateral areas hardly raised, though clearly indicated. Tail valve semicircular (W/L = 1.86), front margin straight, mucro subcentral, antemucronal slope slightly convex, postmucronal slope concave.

Tegmentum rough, growth lines are often present in variable numbers. HV, LA, PMA sculptured with dense, minute, roundish granules arranged in radial series ( 70 in HV, 60–70 in PMA), initially fine, becoming coarser towards the margins. CA, AMA sculptured with roundish granules (diameter 43–57 µm) arranged in longitudinal series (CA 60–65, AMA 60–70 and joined together, space between striae of granules large. Each granule with a central megalaesthete and up to 8 micraesthetes around edge.

Articulamentum lacking insertion laminae to the girdle, apophyses small, widely spaced, triangular but tending to become rounded in tail valve.

Remarks. Leptochiton maguntiacus (de Rochebrune 1882) was always classified in the old collections as Lepidopleurus virgifer . A thorough discussion given by past authors (including Boettger) is reported in Janssen (1978: 220). Boettger was the first to recognize that in addition to Lepidopleurus virgifer even finer sculpted forms occur, and called this material Chiton virgifer ? juv. var. agnata , but without publishing this taxon.

Dell’Angelo & Palazzi (1989: 64) considered Leptochiton maguntiacus as a synonym of L. algesirensis ( Capellini, 1859) . The two species undoubtedly have remarkable similarities, well highlighted by Dell’Angelo & Palazzi (1989), but the rougher sculpture of L. maguntiacus , especially in HV, LA and PMA (minute, roundish granules arranged in radial series, initially fine, becoming coarser towards the margins) is different from that of L. algesirensis (more regular radial series), and this alone is sufficient to separate the two species. However, the two species differ in other features (e.g., the larger space between striae of granules in CA and AMA, the anterior profile of the intermediate valves more rounded and a lower dorsal elevation in L. algesirensis ) and in the stratigraphic distribution [ L. algesirensis does not exceed the Miocene (Tortonian)].

Comparisons. The characteristics of the sculpture of Leptochiton maguntiacus agree with the belonging of the species to the Leptochiton cancellatus group.

Distribution. Lower Oligocene: North Europe, Germany: Eckelsheim, Gienberg, Heimberg, Trift, Zeilstück, Welschberg, Würzmühle ( Janssen 1978; Gürs 1983, 1995; Hocht 1986).