Orthonychia yutaroi Ebbestad, 2023

Nuetzel, Alexander, Ebbestad, Jan Ove, Seuss, Barbara, Munnecke, Axel, Mapes, Royal H. & Cook, Alex G., 2023, On Paleozoic platycerate gastropods, Zitteliana 97, pp. 29-51 : 29

Orthonychia yutaroi Ebbestad sp. nov.

Fig. 2

Material.

Only the holotype ( PMU 37146 ) is known, from the Late Ordovician (Katian) Boda Limestone at Jutjärn quarry, Siljan district, Dalarna .

Derivation of name.

After Dr Yutaro Suzuki, Shizuoka University, Japan, who found the specimen and who has made extensive studies of fossils and the mound ecology of the Boda Limestone.

Diagnosis.

Shell tall, teleoconch forming a weakly cyrtoconic tube, open coiled with apex coiled through half a whorl; supra-apical surface formed by raised and convex median section; teleoconch ornamentation consist of densely spaced, slightly undulating, strong co-marginal ribs on initial parts, widening later in ontogeny to distinct growth increments that end in a flared lamellae; growth lines corrugated, laterally forming two or three spaced flutes, across the median part only weakly undulating; last part of protoconch without visible ornament and abrupt transition to teleoconch; early protoconch whorls poorly preserved as imprint, possibly tightly coiled consisting of ca. two whorls.

Description.

Shell tall, weakly cyrtoconic, open coiled with apex coiled through at least half a whorl before shell coiling ends and a straight tube forms, widening gradually with length at base about 45% of total height (= 20 mm). Shell nearly planispiral but with weak translation down the axis if viewed with aperture in normal right-handed position. Apex overhang sub-apical margin. Supra-apical surface of shell forms a raised and convex median section, with concave transition to lateral surfaces. Ornamentation on initial conch consist of densely spaced, slightly undulating, strong co-marginal ribs. A couple of millimeters from the apex, the space between the ribs widens gradually and distinct growth increments (here taken as the growth accumulated in the space between subsequent lamellae) become apparent. Increments are spaced no more than 1 mm apart, consist of fine co-marginal growth lines and end with a lamella that extends prominently and nearly horizontal from the shell (arrows in Fig. 2A4, A5 ). Near the aperture the space between the increments again becomes narrower. The growth lines are corrugated, laterally forming two or three spaced flutes that have a pointed bell-shaped, whereas the growth lines across the median part only are weakly undulating. Ornamentation on inner margin obscured by matrix. Medially the growth-lines form a broad low-relief U-shape.

Last part of protoconch without visible ornament and abrupt transition to teleoconch; width of last protoconch whorl at termination about 200 µm; early protoconch whorls poorly preserved as imprint, possibly tightly coiled consisting of ca. two whorls.

Remarks.

The conch of Orthonychia yutaroi resembles an orthonychian shell morphology in the tall elongated tube-like shell and has a similar crenulated teleoconch ornamentation as the Silurian O. enorme (see below). Compared with O. enorme it differs markedly in attaining an open coiled shell earlier, as the initial shell of O. enorme coils more tightly. However, the teleochonch in O. yutaroi expands more slowly in late ontogeny than that of O. enorme and the shell of the Dalarna species is proportionally taller and straighter (less pronounced supra-apical curvature). The periodic flaring lamellae in Orthonychia yutaroi are much more extensive and more pronounced, and ribbing is much denser, especially in the early teleoconch, than in O. enorme . Furthermore, O. yutaroi has stronger crenulations forming flutes laterally, whereas the longitudinal flutes are weak or absent in O. enorme . But both species share the development of periodic growth increments that end with a crenulated lamella.

The protoconch of Orthonychia yutaroi is incompletely preserved but it is clear that it lacks visible ornament and has an abrupt transition to the teleoconch. It seems to be possible that the protoconch is tightly coiled and consists of ca. 2.5 whorls, similar to what we have found in O. enorme (see below).

Figures

Figure 2. Orthonychia yutaroi Ebbestad, sp. nov., holotype PMU 37146. (1, 2) Right lateral and anterior views. (3) Detail of initial whorls. (4) Detail of left periphery with arrow pointing to lamella. (5) Detail of left periphery with arrow pointing to lamella.