Aulacoseira helianthus Mohan & Stone, 2016

Mohan, Joseph & Stone, Jeffery R., 2016, Three novel species of Bacillariophyta (Diatoms) belonging to Aulacoseira and Lindavia from the Pliocene Hadar Formation, Afar Depression of Ethiopia, Phytotaxa 272 (4), pp. 235-247 : 238-240

Aulacoseira helianthus Mohan & Stone , spec. nov. ( Figs 3 , 4 )

Frustules form filamentous chains and exhibit heterovalvy. Separation valves and linking valves are observed. Valves are cylindrical, 20– 43 μm in diameter and 6–12 μm height. The valve face is flat or concentrically undulates to a small degree. Areolae are punctate and large (1.0–1.5 μm in diameter). Areolae commonly blanket the entire valve face, rarely it is largely hyaline and areolae distribution is sporadic. A row of spines projected outward and perpendicular to the valve face encompasses the margin. In girdle view, the spines conjoin from two costae that run the length of the mantel in a straight line. Between each set of costae, striae with 7–9 areolae in 10 μm, run the length of the mantle. With regards to separation valves, every 5 th –7 th spine there exists an elongated separation spine. Correspondingly, every 5 th –7 th striae is replaced with a trench that runs the length of the mantle, herein designated as the separation furrow. Separation spines co-occur with the sister-cell’s separation furrow. A collum is present at the base of the mantle, the valve however lacks a sulcus. The ringleiste is approximately 1/6 the diameter of the valve diameter, raised toward the center and chamfered. Rimoportulae are not observed.

Type: — ETHIOPIA. Afar Regional State: Osi Isi site, 11.31518° N, 40.73689° E, 514 m a.s.l., late Pliocene lake sediment core ( HSPDP-NAO 14-1B) material, sediment depth ~ 19 m below surface, HSPDP Northern Awash drilling team, February 2014 (holotype ANSP! GC 17186, circled specimen on slide HSPDP-NAO 14-1B-9Q-1 (91–93 cm), here illustrated as Fig. 3 : B).

Etymology: —Named after the plant genus Helianthus (sunflower) as A. helianthus in valve view exhibits a superficial likeness to the central area of the flower with spines radiating in a way like petals on a flower.

Remarks:— Aulacoseira helianthus differs from A. muzzanensis ( Meister 1912: 41) Krammer (1991: 98) in its full-bodied costae structure, whereas A. muzzanensis retains more refined lacy mantle costae. Aulacoseira muzzanensis can exhibit slightly twisted striae on the mantle and A. helianthus always exhibits straight striae on the mantle. Aulacoseira helianthus differs from A. agassizii var. malayensis ( Hustedt 1942: 10) Simonsen (1979: 56) most notably by separation spine concentration. Aulacoseira helianthus have separation spines every 3–5 costae while A. agassizii var. malayensis contains separation spines on every 2–3 costae ( Fig. 9 ). The linking valves of A. helianthus have tapered spines whereas A. agassizii var. malayensis are noted to exhibit cruciform spines on linking valves ( Tremarin et al. 2012), (Figs 82–83).

Figures

FIGURE 3. Size diminution series of A. helianthus (A–L). Holotype shown (B). Ringleiste view (A, L). Scale bars are 10 μm throughout.

FIGURE 4. Scanning electron micrographs of A. helianthus. (A)Valve view of a separation valve. Note the separation furrow (arrowed). A linking valve (B) in valve view lacks separation spines and furrows. Internal view of two separation valves (C) two thickened costae are the internal structure of the separation furrow (arrowed). Two linking valves lacking separation spines (D). A filament of A. helianthus with separation furrow arrowed (E). Fimbria attach the two valves of the frustule (E), arrowed.

FIGURE 9. Scanning electron micrographs of Aulacoseira helianthus (left) and A. agassizii var. malayensis (right). Note the spine morphology originating from two mantle costae, location of the separation furrow in relation to the separation spine, and spine distribution.