Strandoceras sphynx ( Schmidt, 1858 )

Strandoceras sphynx ( Schmidt, 1858)

Figs 31C–D View Fig , 41A, F View Fig , 42A View Fig

Phragmoceras sphynx Schmidt, 1858: 200 .

Phragmoceras sphynx – Eichwald 1860: 1272–1273. ― Roemer 1861: 61. ― Strand 1934: 93, 107. ― Flower 1946: 434. ― Flower in Flower & Teichert 1957: 57, 114, text-figs 15e, g–h.

Protophragmoceras sphinx (sic) – Teichert 1930: 270, 299–301, pl. 7 fig. 20, pl. 8 fig. 24, text-figs 3–4. ― Balashov 1953a: 212. ― Dzik 1984: 48, 52, 65, text-fig. 14.5, pl. 7 fig. 7.

Diagnosis

Large Strandoceras with mature conch height of more than 70 mm; conch cross section oval compressed with narrower ventral conch margin; mature body chamber slightly endogastrically curved, up to ca 80 mm long, nearly tubular or slightly gibbous with maximum conch height near base; ornamented with fine transverse lirae.

Material examined

ESTONIA • 1 spec.; Vormsi Island; Saxby old quarry ; Moe Formation , Pirgu Regional Stage; TUG 1672-29 • 1 spec.; Raplamaa, Pahkla ; Moe Formation , Pirgu Regional Stage; TUG 1745-195 • 1 spec.; Läänemaa, Niibi hillock; Moe Formation , Pirgu Regional Stage; TAM G432:68 • 2 specs; Sutlepa quarry; Adila Formation , Pirgu Regional Stage ; TAM G149:27 , TAM G149:28 .

Type locality and horizon

Niibi, Estonia; Moe Formation, Pirgu Regional Stage.

Description

Specimen TUG 1672-29 is a nearly complete endogastrically curved conch preserving the outer shell and part of the mature body chamber ( Figs 31D View Fig , 41A View Fig ). The conch surface is ornamented with slightly irregularly spaced, distinct growth bands and / or lirae, which are adapically shifted at the antisiphuncular side of the conch curvature, and which form a hyponomic sinus on the prosiphuncular side. The conch cross section is preserved only at the apical end of the specimen. There, the conch height and width are 17 mm and 15 mm, respectively (CHI = 1.13) and the cross section oval compressed with a narrow margin on the prosiphuncular side. In lateral view, the antisiphuncular conch margin is convex throughout the entire length, the prosiphuncular conch margin is concave in early growth stages and straightens at a position where the conch height is ca 55–60 mm, resulting in an overall inflated conch shape. The conch reaches a maximum height of ca 72 mm at ca 55 mm from the peristome, adorally of which, the conch height decreases to ca 68 mm at the peristome ( Fig. 42A View Fig ; supplementary data 4). The conch is also inflated in width, although, the extent of the inflation cannot be measured in this or other available specimens. The angles of expansion reach a maximum with ca 27° between conch heights of 17 mm and 25 mm.

Where visible, the sutures are straight and directly transverse on the lateral sides and form deep narrow saddles on the prosiphuncular side; they are 7 mm apart where the conch height is 37 mm (RCL = 0.19).

Another nearly complete specimen is TAM G432:68 ( Figs 31C View Fig , 41F View Fig ). This specimen is similarly ornamented to specimen TUG 1672-29 ( Fig. 31D View Fig ). The conch cross section is not preserved. In lateral view, the antisiphuncular conch margin is convex throughout the entire length, the prosiphuncular conch margin is concave in early growth stages and straightens at a position where the conch height is ca 65 mm, resulting in an overall bulged conch shape. The conch reaches a maximum height of ca 73 mm at ca 55 mm from the peristome. Adorad of this, the conch height decreases toward ca 70 mm at the peristome. The conch is also inflated in width. The angles of expansion reach a maximum of ca 37° between conch heights of 31 mm and 43 mm. The sutures are straight and directly transverse on the lateral sides and are 11 mm apart where the conch height is 52 mm (RCL = 0.21). At the base of the body chamber, the conch height is 70 mm. The body chamber has a length of ca 80 mm and is only faintly curved.

Remarks

In the Lyckholm strata, at least three relatively similar species of Strandoceras occur ( S. sphynx , S. kalevipoegi sp. nov., and S. sulevipoegi sp. nov.). Schmidt’s (1858: 200) original description does not allow to specify, which of the three species he referred to. Possibly, Schmidt (1858), more generally referred to the slender morphotype of Strandoceras , which these three species represent, but a lectotype designation is not possible because the Schmidt type-material could not be identified. The slender morphotype is also figured in Teichert (1930) under this species (see list of synonymy). For simplicity, it is suggested here, that this slender morphotype is synonymized with S. sphynx .

Comparison

Strandoceras kalevipoegi sp. nov. is larger (conch height ca 80 mm), and more strongly curved. Strandoceras tyriense ( Strand, 1934) and S. sulevipoegi sp. nov. is smaller (conch heights are ca 55 mm and 65 mm, respectively) and has a lower angle of expansion during late ontogeny. Strandoceras schmidti ( Teichert, 1930) and S. muhvi sp. nov. differ in having a shorter, more rapidly enlarging conch (compare in Fig. 42A–B View Fig ).