Drepanoistodus viirae Rasmussen & Eriksson & Lindskog 2021, sp. nov.

Drepanoistodus viirae sp. nov.

urn:lsid:zoobank.org:act: AD10D9B3-9802-4DAC-97C0-B44EB8DE195D

Fig. 6M–O

Drepanoistodus basiovalis (Sergeeva, 1963) – Löfgren 2000b: fig. 4p; 2003: fig. 7aa. — Lindskog et al. 2020: fig. 7v–w.

partim Drepanoistodus cf. basiovalis – Rasmussen 2001: 73, pl. 5 fig. 16 (only).

Drepanoistodus cf. stougei Rasmussen, 1991 – Rasmussen 2001: 76, pl. 6 fig. 12.

Drepanoistodus aff. suberectus (Branson & Mehl, 1933) – Mellgren & Eriksson 2010: fig. 7f.

aff. Drepanoistodus basiovalis – Feltes & Albanesi 2013: fig. 3.12.

? partim Drepanoistodus basiovalis – Zhen 2020: 18–19, fig. 7b (only).

Diagnosis

A Drepanoistodus species characterised by a geniculate element with a wide, straight, compressed cusp and a very short base, where the free cusp typically is ca 4 times longer than the upper margin of the base.

Etymology

Named after the Estonian palaeontologist and conodont specialist Viive Viira, Tallinn University of Technology, Estonia.

Material examined

Nine geniculate elements including five from the Lynna section. Holotype, geniculate element (Fig. 6M–N); LO 12484T.

Type locality

River bank near the mouth of Lynna River, village of Kolchanovo, St. Petersburg region, Russia (60°00′39″ N, 32°33′49″ E).

Type stratum

Approximately 15 cm above the local base of the Sillaoru Formation, sample LY 12-31. Lower part of the 90 cm thick interzone (“uncertain interval”) between the Lenodus variabilis Zone and the Yangtzeplacognathus crassus Zone sensu Lindskog et al. (2020).

Description

Cusp is reclined, wide (from upper to lower margin) and straight, with keels developed along the anterior (upper) and posterior (lower) margins. A weak, median, longitudinal carina is developed on the inner side of the cusp. Occasionally, the carina may be distinct. Basal margin is weakly rounded or straight. A distinct keel is developed on the upper margin. Anterior margin is rounded or weakly rounded (convex). Angle A (Fig. 3) between the cusp and upper margin of the base is ca 30° (mean) with a standard deviation of 4.3 (Fig. 5B), and mean ratio between length of the free upper margin and the free cusp is ca 0.25 with a standard deviation of 0.05.

Remarks

Drepanoistodus viirae sp. nov. is situated in the lower left quadrangle of the PCA plot (Fig. 4). Like D. basiovalis, it is clearly separated from D. iommii sp. nov. and D. svendi sp. nov., whereas it partly overlaps with the D. basiovalis population, when only the PC 1 (x) and PC 2 (y) axis is plotted. The vectors in the biplot reinforce that D. viirae sp. nov. is characterised by a convex basal margin, a weakly developed carina and a short upper margin on the base (= low b/c value), the latter because it is situated in the opposite direction of the b/c vector, as seen in Fig. 4. The partial overlap with D. basiovalis occurs because the two species share some characters. A significant difference, however, is that D. viirae sp. nov. has a relatively shorter upper margin of the base, where the mean b/c ratio is 0.40 in D. basiovalis but only 0.25 in D. viirae sp. nov. (Fig. 5B). Moreover, D. viirae sp. nov. is characterised by a wider cusp when viewed from the side and, typically, a less developed carina on the cusp. The hypothesis that the D. viirae sp. nov. population is morphologically separate from the D. basiovalis population is supported by the PERMANOVA test (Fig. 5A), which shows that the probability that the two populations are the same is low (p (same) = 8.00E- 03). Drepanoistodus viirae sp. nov. is distinguished from the stratigraphically older Drepanoistodus contractus on the relatively wider and more compressed cusp and the usually less distinct longitudinal carina, and from D. cf. suberectus on the markedly smaller angle between the cusp and the upper margin of the base (mean angle = 46° in D. cf. suberectus, 30° in D. viirae sp. nov.).

Occurrence

The lower part of the L. variabilis Zone (sample LY12-13) to the lower part of the interzone (“uncertain interval”) between the L. variabilis Zone and the Y. crassus Zone (sample LY12-31) sensu Lindskog et al. (2020). In addition, D. viirae sp. nov. has been recorded from the B. norrlandicus and basal Y. crassus zones at Gillberga, Sweden (Löfgren 2000b, 2003); the uppermost part of the P. rectus – M. parva Zone at Steinsodden, Norway, which correlates with the uppermost P. originalis Zone (as D. cf. stougei sensu Rasmussen 2001); the lower part of the B. medius – H. holodentata Zone at Andersön, Sweden, correlating with the uppermost part of the L. variabilis Zone (as D. cf. basiovalis sensu Rasmussen 2001), and the L. variabilis Zone at Hällekis, Sweden (as D. aff. suberectus sensu Mellgren & Eriksson 2010). Moreover, it shares some characteristics with the geniculate element from strata correlated with the L. pseudoplanus Zone of the Canning Basin, Australia, which was included in D. basiovalis (Zhen 2020: fig. 7b), but this identification is questionable.