Drepanoistodus svendi, Rasmussen & Eriksson & Lindskog, 2021

Rasmussen, Jan Audun, Eriksson, Mats E. & Lindskog, Anders, 2021, Middle Ordovician Drepanoistodus (Vertebrata, Conodonta) from Baltica, with description of three new species, European Journal of Taxonomy 774, pp. 106-134 : 122-123

Drepanoistodus svendi sp. nov.

urn:lsid:zoobank.org:act:0921F27A-ECF6-498C-8943-4DEB96CFBB38

Fig. 6I–L

Drepanoistodus aff. basiovalis (Sergeeva) - Rasmussen 2001: 73–74, pl. 5 fig. 17.— Mellgren et al. 2012: fig. 5u.

Diagnosis

A Drepanoistodus species characterised by a geniculate element with a recurved cusp and distinct keels on both the cusp and the upper margin of the base. Weakly curved but distinct carinas are developed on both sides of the cusp, especially well developed on the inner side.

Etymology

Named after the Danish palaeontologist and conodont specialist Svend S. Stouge, Natural History Museum of Denmark, University of Copenhagen.

Material examined

Five geniculate elements including three from the Lynna section. Holotype, geniculate element ( Fig. 6J–K ); LO 12483T . 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 40 cm below the local top of the Volkhov Formation, sample LY 12-14, ca 20 cm above the base of the Lenodus variabilis Zone sensu Lindskog et al. (2020) .

Description

Cusp is recurved (bent weakly downward), with distinct keels developed along the anterior (upper) and posterior (lower) margins. A median, longitudinal, weakly curved carina is developed on both sides of the cusp, most distinct on the inner side of the cusp. Cusp is almost twice as long as the upper margin of the cusp; the mean ratio between the length of the free upper margin and the free cusp ( Fig. 3 ) is ca 0.55 with a standard deviation of 0.13. Basal margin varies from rounded (convex) to almost 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 varies considerable with a mean of 24° and standard deviation of 6.9 ( Fig. 5B ).

Zone. L. Same specimen as I in outer view. M–O. Drepanoistodus viirae sp. nov. M. LO 12484T, holotype, inner view, sample LY12-31, interzone (“uncertain interval”) between the Lenodus variabilis Zone and the Yangtzeplacognathus crassus Zone sensu Lindskog et al. (2020) . N. Same specimen as M in outer view. O. LO 12485t, inner view, sample LY 12-31, interzone (“uncertain interval”) between the Lenodus variabilis Zone and the Yangtzeplacognathus crassus Zone sensu Lindskog et al. (2020) . P–Q. Drepanoistodus aff. basiovalis ( Sergeeva, 1963) , LO 12486t, inner and outer view, respectively, sample LY12-9, L. antivariabilis Zone. R –S. Drepanoistodus stougei Rasmussen, 1991 , LO 12487t, inner and outer view, respectively, sample LY12-13, L. variabilis Zone. T. Drepanoistodus cf. suberectus ( Branson & Mehl, 1933) , LO 12488t, inner view, sample LY14-2, Y. crassus Zone. Scale bar = 200 μm (all specimens illustrated at same scale).

Remarks

Drepanoistodus svendi sp. nov. is distinguished from all the other Drepanoistodus species in the present study by the recurved cusp and the curved carina on each side of the cusp. Like D. iommii sp. nov., it is characterised by a clearly longer upper margin of the base compared to the cusp length than in D. basiovalis . The D. svendi sp. nov. population is located in the lower, right quadrangle of the PCA plot, far from any other species of Drepanoistodus , and the biplot vectors representing the recurved cusp and the curved carina point in this direction ( Fig. 4 ). The PERMANOVA test on the first seven PCA axis shows that the probability that the D. basiovalis and D. svendi sp. nov. populations are the same, is exceedingly low (p (same) = 1.00E- 04).

Occurrence

The lower part of the L. variabilis Zone (samples LY12-13, LY12-14). Moreover, D. svendi sp. nov. has been recorded from Steinsodden, Norway, from the top of the B. norrlandicus – D. stougei Zone and the base of the overlying B. medius – H. holodentata Zone , which correlate with the middle part of the L. variabilis Zone (as D. aff. basiovalis sensu Rasmussen 2001 ), and from the L. pseudoplanus Zone or E. suecicus Zone at the island Osmussaar, Estonia ( Mellgren et al. 2012; reported as D. aff. basiovalis ).

Figures

Fig. 6. Light microscopy photographs of selected geniculate elements of species of Drepanoistodus, as outlined herein, from the Lynna River section, Russia. A–D. Drepanoistodus basiovalis (Sergeeva, 1963). A. LO 12476t, inner view, sample LY12-16, L. variabilis Zone. B. Same specimen as A in outer view. C. LO 12477t, inner view, sample LY12-9,L. antivariabilis Zone.D. LO 12478t, inner view, sample LY12-21b, L. variabilis Zone. E–H. Drepanoistodus iommii sp. nov. E. LO 12479T, holotype, inner view, sample LY12-16, L. variabilis Zone. F. Same specimen as E in outer view. G. LO 12480t, inner view, sample LY12-21b, L. variabilis Zone. H. LO 12481t, inner view, sample LY12-3, L. antivariabilis Zone. I–L. Drepanoistodus svendi sp. nov. I. LO 12482t, inner view, sample LY12-13, L. variabilis Zone. J–K. LO 12483T, holotype, inner and outer view, respectively, sample LY12-14, L. variabilis

Fig. 3. Geniculate Drepanoistodus basiovalis (Sergeeva, 1963) element demonstrating the essential morphological characteristics utilized in the present study. Two characters were measured quantitatively: Angle A between the median line (usually carina) of the cusp and the lower margin of the keel situated on the upper margin of the cusp (A); and the b/c ratio, which is the ratio between the length of the upper margin of the base (b) and the length of the free cusp (c).

Fig. 5. A. Results (probabilities) of the nonparametric PERMANOVA test performed on the first seven components of the PCA plot. Drepanoistodus basiovalis (Sergeeva, 1963) and the three new species, D. iommii sp. nov., D. svendi sp. nov. and D. viirae sp. nov. The three tested null hypotheses are that D. basiovalis is similar to each of the three new species, respectively. Because all the test results are below 0.05, the three null hypotheses are rejected, and there is no statistical evidence from the test that the groups (species) are similar. B. Univariate statistics with regard to the measured angle A and the calculated b/c ratio. Abbreviations: N = number of measured specimens; Min = lowest number; Max = highest number; Mean = average; Stand. dev. = standard deviation. All calculations were performed using the PAST software (Hammer et al. 2001).

Fig. 4. Principal Component Analysis (PCA) biplot based on the correlation coefficient showing the position of conodont specimens (abbreviated names, see Table 1) and morphological characters/variables. The latter is represented by blue vector lines (see Table 2), the direction and length of which mark the relative importance of the various characters/variables. Each coloured field marks the position of a selected species grouping. Whereas 18.4% of the total variance is visible along the PC 1 axis, 16.1% is seen along the PC 2 axis. See main text for further explanation on the analyses performed, and consult Table 1 for data on sample numbers.