Leptochiton lignatilis, Dell'Angelo, Bertolaso & Sosso, 2015

Leptochiton lignatilis View in CoL Dell’Angelo,Bertolaso & Sosso in Bertolaso,Garilli,Parrinello,Sosso & Dell’Angelo,

2015

( Fig. 18 View FIGURE 18 )

Leptochiton lignatilis View in CoL Dell’Angelo, Bertolaso & Sosso in Bertolaso, Garilli, Parrinello, Sosso & Dell’Angelo, 2015a, p. 6, figs 2–4.

Type material. Holotype, MGPT-PU 108787 (tail valve, width 3.5 mm, Figs 18A–D View FIGURE 18 ). Paratypes from the type locality: MGPT-PU 108788 (head valve, Fig. 18E View FIGURE 18 ); MZB 32033–32034 (2 intermediate valves, Figs 18F–G View FIGURE 18 ); NHMW 2014/0451/0001–0002 (intermediate and tail valves); ZISP 2226–2227 View Materials (2 intermediate valves) ; MSNG 57980 View Materials (1 head and 1 intermediate valves) ; MZPD MAL 2074–2476 (1 head and 2 intermediate valves, Fig. 18H View FIGURE 18 ) .

Paratype from the Langhian of Moncasale di Casina , “Fosso di Moncasale” ( Reggio Emilia): MZPD MAL 2077 (intermediate valve) .

Type locality. Torrente Cinghio, Parma ( Emilia-Romagna, Italy).

Type stage. Miocene, Tortonian ( Termina Formation ), a massive dark grey claystone containing few carbonized wood remains that are concentrated within a small lens-shaped feature.

Material examined. Miocene (Langhian): Italy: Moncasale di Casina : paratype. Miocene (Tortonian) : Italy: Torrente Cinghio : type material plus 100 valves ( BD 382 , LB). Maximum width of the valves: 2.4 / 3.4 / 3.5 mm .

Description. Head valve semioval, posterior margin widely V-shaped, slope concave. Intermediate valve broadly rectangular (W/L = 2.37), carinate in anterior profile, elevated (H/W = 0.5), anterior and posterior margins almost straight, side margins rounded, apex indistinct, lateral areas scarcely differentiated. Tail valve more than semicircular, anterior margin convex, mucro anterior, not prominent, antemucronal slope convex, postmucronal slope a little concave just underneath mucro.

Tegmentum slightly rough, uniformly sculptured with rather irregular roundish, elevated granules (diameter 40–50 μm, up to 70 μm in more elongate granules), well separate from each other, irregularly arranged, except apex portion of HV with concentric ridges, granules becoming more irregularly elongate towards JA of intermediate valves, but always well separated, not coalescing. Each granule with a subcentral megalaesthete, and 2–4 micraesthetes, up to 5–6 in more elongate granules.

Articulamentum without insertion laminae, apophyses small, triangular in intermediate valves, somehow trapezoidal in tail ones, widely separated by a large jugal sinus.

Remarks. The fossil record of Leptochiton lignatilis Dell’Angelo, Bertolaso & Sosso in Bertolaso, Garilli, Parrinello, Sosso & Dell’Angelo, 2015 is limited to the Miocene of N. Italy. No additional material has been found since the original description. In the material studied from Torrente Cinghio there are a few valves complete or sufficiently complete (about a dozen), the others are small fragments (of about 1–1.5 mm or less of width), with clearly visible tegmentum granules, but even difficult to identify as head, intermediate or tail valves.

The small fragment of the intermediate valve from Moncasale di Casina has a sculpturing matching that of the material from Torrente Cinghio and is compatible with an attribution as Leptochiton . Considering the similarities of the sculpture, this valve was provisionally considered conspecific with the material from Torrente Cinghio.

In spite of their considerable presence in recent wood-fall, chitons clearly associated with sunken wood are rare in the fossil records and are only represented by Leptochiton species. A list of these records is reported by Bertolaso et al. (2015), and the description of Leptochiton lignatilis from the Miocene of Italy adds a further record to the scarce number of chitons in fossil wood-fall communities.

Comparisons. A comparison of Leptochiton lignatilis with the other Leptochiton specie known from sunken woods (Sigwart & Sirenko 2012) has been made by Bertolaso et al (2015). The closest species is Leptochiton kurnilatus Kaas, 1985 from the Island of Réunion, carinate and elevated (H/W = 0.57), but differs by the different shape and profile of the tail valve, with the posterior mucro at about three quarters of the valve length.

Distribution. Middle Miocene: Proto-Mediterranean Sea (Langhian): North Italy: Moncasale di Casina ( Bertolaso et al. 2015) . Upper Miocene: Proto-Mediterranean Sea (Tortonian): North Italy: Torrente Cinghio ( Bertolaso et al. 2015).