Craspedochiton mutinocrassus ( Sacco, 1897 )

Craspedochiton mutinocrassus ( Sacco, 1897)

Fig. 148 View FIGURE 148

Acantochiton (sic) costatus (Rov.) Sacco, 1897, p. 91 , pl. 7, figs 33–35 ( non figs 36–37 = Craspedochiton View in CoL ? sp., fide Dell’Angelo et al. 2016: 91).

Acanthochiton costatus ; Van Belle 1981, p. 32; Ferrero Mortara et al. 1984, p. 300, pl. 55, figs 4–5; Zanaroli 1985, tab. 2; Dell’Angelo et al. 1999, p. 283.

Acanthochiton costatus var. mutinocrassa Sacco, 1897, p. 91 , pl. 7, fig. 38; Laghi 1977, p. 112; Dell’Angelo et al. 1999, p. 283.

Craspedochiton costatus ; Laghi 1977, p. 112, pl. 4, figs 1–3; Bałuk 1984, p. 293. Craspedochiton mutinocrassus ; Dell’Angelo et al. 2016, p. 90, pl. 6, figs 12–15; Dell’Angelo et al. 2020b, p. 44, tab. 9.

non Acanthochiton costatus ? var. astensis ; Sacco 1897, p. 91, pl. 7, figs 39–47 (= Craspedochiton altavillensis View in CoL ).

Coronula bifida? Bronn, 1831 ; Doderlein 1864, p. 12; Laghi 1977, p. 112; Van Belle 1981, p. 24 [= Craspedochiton costatus (Sacco) , fide Laghi 1977].

Type material. Syntype MPM 6090, head valve, coll. Doderlein, figured by Sacco (1897: pl. 7, fig. 38) and Laghi (1977: pl. 4, fig. 2), Fig. 148A View FIGURE 148 .

Type locality. Montegibbio ( Emilia-Romagna, Italy) .

Type stage. Miocene (Tortonian).

Material examined. Lower Miocene: Italy: Sciolze: 1 head valve, BS.105.04.001, width 10 mm, figured by Sacco (1897: pl. 7, fig. 33) and Laghi (1977: pl. 4, fig. 1). Maximum width of the head valves: 29 mm.

Description. Intermediate and tail valves unknown. Head valve semicircular, large and strong, flat, with five well evident radial ribs which continue on the wide insertion lamina.

Tegmentum covered with large granules of irregular oval shape.

Articulamentum with insertion lamina particularly expanded, greater than length of tegmentum, with 5 deep incisions that divide insertion plate into 6 strong insertion teeth.

Remarks. Sacco (1897) described the new species Acantochiton (sic) costatus based on some head and tail valves from the Torino Hill ( Sacco 1897: pl. 7, figs 33-37), although he considered doubtful conspecificity among those head plates. The head valves are characterized by the presence of large radial ribs, and especially by the insertion lamina particularly expanded. Only one of the three valves figured by Sacco is still present in the Bellardi-Sacco collection (Sacco: pl. 7, fig. 33, width 10 mm), whereas the other two valves from the Collection Rovasenda are missing. Sacco also described another head valve from Montegibbio as Acantochiton (sic) costatus var. mutinocrassa (Sacco: pl. 7, fig. 38); this specimen is identical to the head valves from the Torino Hill, but larger (width 22 mm).

Craspedochiton costatus ( Sacco, 1897) was considered a valid species by some authors ( Laghi 1977; Bałuk 1984), or a synonym of other related species by others, e.g., C. profascicularis (Boettger, 1907) ( Šulc 1934; Zilch 1934; Studencka & Dulai 2010) or C. altavillensis (Seguenza, 1876) (Dell’Angelo et al. 1999).

Dell’Angelo et al. (2016) considered the remarkable expansion of the insertion lamina as an important character to differentiate these valves from C. altavillensis , where the insertion lamina is reduced to a rather narrow band that does not appear to vary with the valve’s size. The expansion of the insertion lamina in each of the two head valves from Montegibbio figured by Laghi (1977), the valve described by Sacco as A. costatus var. mutinocrassa (Laghi, pl. 4, fig. 2) and a still larger one (Laghi, pl. 4, fig. 3, width 29 mm) is greater than the length of the tegmentum ( Figs 148A–B View FIGURE 148 ).

The taxon Acanthochiton costatus Adams & Angas, 1864 has priority over the fossil Acantochiton costatus Sacco, 1897 , for which Dell’Angelo et al. (2016) proposed the name Craspedochiton mutinocrassus ( Sacco, 1897) .

Therefore, only three head valves can be reliably attributed to Craspedochiton mutinocrassus : the valve from Sciolze ( Sacco 1897, pl. 7, fig. 33; Laghi 1977, pl. 4, fig. 1; Ferrero Mortara et al. 1984, pl. 55, fig. 4; Figs 148C–D View FIGURE 148 ) and the two valves from Montegibbio ( Sacco 1897, pl. 7, fig. 38; Laghi 1977, pl. 4, figs 2–3; Figs 148A–B View FIGURE 148 ). The tegmentum is generally abraded, only in one valve (MPM 6090, Fig. 148A View FIGURE 148 ) is possible to see, in some area, an ornamentation with ellipsoidal granules ( Laghi 1977, pl. 4, fig. 2B).

It is not possible to identify with certainty the tail valve figured by Sacco (1897, pl. 7, figs 36–37) as A. costatus , whose conspecificity with head valves was already considered doubtful by the same author. The shape of the tegmentum of the valves is different from that of C. altavillensis , and e ven the attribution to the genus Craspedochiton is uncertain. This valve is discussed and figured in the section dedicated to the “Species of unclear taxonomic position”, and it is left in open nomenclature as Craspedochiton ? sp.

Doderlein reported from Montegibbio one species of Polyplacophora [ Chiton zibinicus ( Doderlein, 1864) = Rhyssoplax olivacea ( Spengler, 1797) ], and also a species of barnacle ( Coronula bifida Bronn, 1831 ), common within the Pliocene record of the Mediterranean ( Dominici et al. 2011). Laghi (1977) made a provisional assignment of this finding to C. costatus , and we include here this taxon, that Laghi probably has seen during his work at Modena, studying the Doderlein collection.

Comparisons. See Tab. 23 for a comparison with the Craspedochiton spp . considered in the present study.

Distribution. Lower Miocene: Proto-Mediterranean Sea (Burdigalian): N. Italy: Sciolze ( Sacco 1897; Laghi 1977; Dell’Angelo et al. 2016). Upper Miocene: Proto-Mediterranean Sea (Tortonian): Po Basin, N Italy: Montegibbio ( Sacco 1897; Laghi 1977; Dell’Angelo et al. 2016).