Spinochiton gaasi ( Cherns & Schwabe, 2019 )

Spinochiton gaasi ( Cherns & Schwabe, 2019) View in CoL

Fig. 98 View FIGURE 98

Chiton sp. 2 Varone, 2008, p. 157, figs 15–16.

Chaetopleura gaasi Cherns & Schwabe, 2019, p. 8 , figs 6A–I.

Spinochiton gaasi View in CoL ; Dell’Angelo et al. 2018a: p. 34, figs 8A–U, 9A–L; Dell’Angelo et al. 2019c: p. 106; Dell’Angelo et al. 2020b: tab. 7, 9.

Type material. Holotype ZSM Mol 20060779A, tail valve. Paratype ZSM Mol 20060779D, head valve.

Type locality. Gaas ( France) .

Type stage. Oligocene (Rupelian).

Material examined. Lower Oligocene: France: Gaas : 60 valves ( AC, BD 836 , Figs 98K–M View FIGURE 98 , DA, JVC, MHNBx 2017.7.3, Figs 98A–E View FIGURE 98 , MHNBx 2017.10.2, Figs 98F–J View FIGURE 98 , MHNBx 2017.11.1, Figs 98N–P View FIGURE 98 ). Maximum width of the valves: 6.2 / 8.6 / 5.7 mm .

Description. Head valve semicircular, elevated, apex slightly recurved, front slope concave, posterior margin V-shaped, notched in middle. Intermediate valve broadly rectangular (W/L = 2.41–2.53), carinate in anterior profile, elevated (H/W = 0.45–0.54), anterior margin convex with jugal part forwardly produced, side margins oblique, posterior margin straight, apex inconspicuous, lateral areas sligthly raised. Tail valve almost semicircular (W/L = 1.53–1.70) anterior margin about straight with jugal part forwardly produced, mucro not prominent, in central position, antemucronal slope slightly convex, postmucronal slope concave just underneath mucro.

HV sculptured with 13–14 radial rows of conical spinous processes rounded at top (without any type of sculpture), rows starting from a small smooth area around semicircular notch with spinous processes initially small and growing and getting more elevate towards anterior margin, 6–7 spinous processes well developed for each row, and 2–3 obsolete ones near notch. Two rows of spinous processes are on two sides of posterior margin, surface between striae rough, with evidence of growth lines but without a well-defined sculpture, everywhere perforated by micraesthetes, but not on spinous processes. LA sculptured with 2 radial rows of conical spinous processes rounded at top, like in HV, obsolete near apex and growing towards side margins, one of them on posterior margin. PMA sculptured with conical spinous processes that seem irregularly arranged, not in radial rows as in HV. CA and AMA sculptured with longitudinal coarse ribs with from 2 to 6 very irregular oval pustules, getting larger and fused together towards JA, latter formed by a single longitudinal rib with larger pustules fused together to reach two adjacent ribs, interspaces with growth lines vaguely indicated, intersected by very small and fine striae of longitudinal granules, giving a reticulate appearance.

Articulamentum strongly developed with apophyses expanded, rounded, connected by a jugal lamina with external slits in intermediate and tail valves; head valve with a large, elevated apical area, extending obliquely to articulamentum, traversed by a series of more or less parallel and columnar thickenings branching towards apex; intermediate valve with a smaller but similar apical area; teeth solid, finely striated on upper side, in head valve corresponding to radial rows of spines, except for two near posterior margin for each side, slit rays vaguely evidenced, eaves solid, slit formula 9–13 / 1 / 11.

Remarks. Cherns & Schwabe published, firstly online ( 1 Nov 2017) the description of the new species Chaetopleura gaasi , based on 4 valves (1 head and 3 tail) from Gaas. The printed version appeared later ( Cherns & Schwabe 2019). The same species was studied simultaneously by Dell’Angelo et al. (2018a), on a greater number of valves (60), also including intermediate valves (not present in the Cherns & Schwabe material). Their description strongly completemented what reported by Cherns & Schwabe (2019), highlighting some important morphologically characters that induced Dell’Angelo et al. (2018a) to define the new genus Spinochiton .

The large apical area extending obliquely to the articulamentum ( Figs 98E, 98I, 98L–M View FIGURE 98 ) is a weird character, not present in chitons from the Cenozoic, where the apical area is normally little evident in head valve, and more pronounced but always narrow in intermediate ones.

Comparisons To our knowledge, no living or fossil chitons share, the same characters of Spinochiton gaasi . A few species show the head valve character of the apex slightly recurved and front slope strongly concave, e.g., living species of the genus Nuttallochiton Plate, 1899 (Kaas & Van Belle 1987), and the fossil Oochiton halli Ashby, 1929 from the Miocene of Australia ( Ashby 1929; Smith 1960).

Distribution. Lower Oligocene: northeastern Atlantic (Rupelian): Aquitaine Basin, France: Gaas ( Cherns &

Schwabe 2019; Dell’Angelo et al. 2018a, 2020b).