Ischnochiton zbyi, Dell'Angelo & da Silva, 2003

Ischnochiton zbyi View in CoL Dell’Angelo & Silva, 2003

Fig. 69 View FIGURE 69

Ischnochiton zbyi View in CoL Dell’Angelo & Silva, 2003, p. 10, figs 5–11; Schwabe 2005, p. 105; Dell’Angelo et al. 2012, p. 60; Cherns & Schwabe 2019, p. 5; Dell’Angelo et al. 2019a, p. 48; Dell’Angelo et al. 2022, p. 7, figs 5, 6.

Type material. Holotype: MNHN /UL.II.406 (intermediate valve) . Paratypes: MNHN /UL.II.407-408 (head and tail valves); MZB 12692 (3 valves); BD FW49 (3 valves); GeoFCUL VFX.03.312-332 (21 valves) .

Type locality. Vale de Freixo site, near the village of Carnide, Pombal municipality (central-west Portugal) .

Type stage. Pliocene ( upper Zanclean to lower Piacenzian), Carnide Sandstone Formation .

Material examined. Pliocene: Portugal: Vale de Freixo : type material plus 505 valves ( GeoFCUL VFX.03.334– 336, GeoFCUL VFX.03.351–354, RGM.1364002–1364006, MNHN.F. A81983 View Materials – A81985 View Materials , BD 240, Figs 69A–L View FIGURE 69 ). Maximum width of the valves: 5.8 / 6.5 / 4.3 mm .

Description. Head valve semi-circular, posterior margin widely V-shaped, notched in middle, slope straight to slightly concave. Intermediate valves broadly rectangular, three times wider than long or more (W/L = 3.00–3.50), moderately elevated (H/W = 0.26–0.36), carinate in anterior profile, anterior margin straight, slightly concave in jugum, side margins straight to slightly rounded, posterior margin straight, with apex not evident, lateral areas moderately elevated, jugal area triangular. Tail valve semicircular, depressed (W/L = 1.90–2.10), anterior margin straight or slightly concave in jugal tract, mucro subcentral, slightly raised, antemucronal slope straight, postmucronal slope slightly concave directly behind mucro.

Tegmentum rough. HV, LA and PMA sculptured with microgranulose sculpture of roundish/squarish granules up to 30–40 μm, with 3–5 aesthetes of equal size, and granulose radiating ribs (HV 19–24, not starting from apex, but from mid-valve, concentrically crossed by numerous growth lines, some ribs splitting towards anterior margin; LA 2–8, starting from apex, with microgranules always present between ribs; PMA 24–30), granules larger towards margins (up to 100 μm, with 10–15 or more aesthetes of equal size), with microgranules present in all rib interspaces. CA and AMA sculptured with longitudinal granulose ribs (10–26 on either side), with granules coalescent forming uninterrupted ribs, interstices narrower than ribs with subobsolete microgranulose sculpture, JA triangular with longitudinal striae converging towards center on both sides in wider valves, forming larger, irregular granules; not arranged in longitudinal striae in smaller valves.

Articulamentum whitish, apophyses wide, triangular in intermediate valves, trapezoidal in tail valve, jugal sinus narrow, slit formula 10–11 / 1 / 7–11, slits inequidistant.

Remarks. The fossil record of Ischnochiton zbyi Dell’Angelo & Silva, 2003 seems limited to the Pliocene of Portugal.The abundant material studied, representing the complete growth series from juvenile to adult, has permitted a better understanding of intraspecific variability ( Dell’Angelo et al. 2022), e.g., the sculpture of intermediate valves, with the granulose radial ribs in the lateral areas ranging from 5–8 in the larger valves ( Fig. 69D View FIGURE 69 ) to 2–3 in smaller valves ( Fig. 69J View FIGURE 69 ), with microgranules always present in the rib interspaces, and the number of longitudinal ribs in the pleural areas varying from 22–26 on either side in the larger valves ( Fig. 69D View FIGURE 69 ) to 10–15 in juveniles ( Fig. 69J View FIGURE 69 ).

Comparisons. This species is similar to Ischnochiton crovatoi Dell’Angelo, Sosso & Bonfitto, 2019, from which it differs by the shape of the tail valve, more squared in I. crovatoi (W/L = 1.71) respect to I. zbyi (W/L = 1.90–2.10), the greater number of radial riblets on the postmucronal area ( 42 in I. crovatoi vs. 24–26 in I. zbyi ) and, mainly, by the absence in I. crovatoi of any concentric grooves on HV, LA and PMA, concentric grooves instead well present in I. zbyi .

Distribution. Pliocene: northeastern Atlantic, Mondego Basin, Portugal: Vale de Freixo ( Dell’Angelo & Silva 2003; Dell’Angelo et al. 2022).