Ilyocypris leptolinea Wang & Zhai, 2022

Wang, Qianwei, Horne, David J., Fan, Jiawei, Wen, Ruilin, Smith, Robin J., Wang, Min & Zhai, Dayou, 2022, Ilyocypris leptolinea Wang & Zhai, sp. nov., an ostracod (Ostracoda, Crustacea) from the late Quaternary of Inner Mongolia, northern China, ZooKeys 1137, pp. 109-132 : 109

Ilyocypris leptolinea Wang & Zhai sp. nov.

Figs 2 , 3 , 4

Type locality.

XJG2 section (43°51'41.9"N, 116°24'57.1"E, Fig. 1 ), lacustrine outcrop cut by intermittent river in central-eastern Inner Mongolia, China.

Type horizon.

Late Pleistocene to Holocene; holotype from 209-210 cm depth in section, c. 70 cm below level of gastropod shells that yielded a calibrated radiocarbon date of 10,108 ± 93 calendar years before present (cal yr BP).

Type material.

Holotype: adult left valve, XJG2-210-1 (XJG2, section code; 210, sample code, corresponding to 209-210 cm depth in section; 1, registration number), length 0.94 mm, height 0.48 mm. Paratypes: seven adult left valves, XJG2-021-1, XJG2-177-1, XJG2-190-1, XJG2-193-1, XJG2-197-2, XJG2-199-2, XJG2-228-2. All type specimens with marginal ripplets on anterior and posterior calcified inner lamellae well-preserved (Figs 2 , 3 ).

Other material examined.

17 adult left valves, 22 adult right valves, six A- 1 juvenile left valves, and one A- 1 juvenile right valve (Table 1 ; Figs 2 - 5 ). Adult right valves and juvenile valves provisionally identified as this species in view of monospecific adult left valves in section.

Etymology.

From the Greek leptos for fine, small, or subtle, and Latin linea for line or thread, referring to fine marginal ripplets on calcified inner lamellae of LV.

Dimensions.

Adult left valves (n = 25, Fig. 5A ) with length 0.78-1.05 mm, height 0.42-0.56 mm, H/L ratio 0.51-0.55. Adult right valves (n = 22, Fig. 5B ) with length 0.79-0.99 mm, height 0.43-0.53 mm, H/L ratio 0.52-0.56. Juvenile valves slightly smaller but with size ranges overlapping with those of adult valves (Fig. 5 ).

Diagnosis.

Intermediate-sized Ilyocypris (length ranging from 0.78 to 1.05 mm, Table 1 ) with shape, sulci, and pits typical of genus. Valve surface without nodes, occasionally with tiny spines along anterior and/or posterior margins. Calcified inner lamellae wide, bearing (in left valve only) two rows of densely arranged, fine marginal ripplets along entire anterior and posterior valve margins; distal row near valve margin, usually well expressed; proximal row in intermediate area less pronounced, absent in poorly preserved specimens. One inner list present between two rows of marginal ripplets in well-preserved specimens.

Description.

Left valves. Intermediate-sized Ilyocypris . Valve sub-reniform in lateral view, with greatest height (antero-dorsal corner) at anterior third. Dorsal margin, i.e., section between antero-dorsal and postero-dorsal corners, nearly straight but with blunt turn immediately behind posterior sulcus due to inflation of postero-dorsal part. Anterior margin broadly rounded, with dorsal part nearly straight and more ventral parts evenly rounded. Posterior margin evenly rounded and less obtrusive. Ventral margin concave. Valve surface carrying two transverse sulci, with anterior sulcus originating from antero-dorsal corner, tapering ventrally, and terminating slightly above mid-height. Posterior sulcus wider and shorter. Adductor muscle scars situated in ovate depression immediately below posterior sulcus. Mandibular scars situated in two small depressions to antero-ventral position of adductor muscle scars. Shell surface densely covered with small pits, with those in front of anterior sulcus, between two sulci and behind posterior sulcus, smaller. Small number of tiny spines present along anterior and posterior margins in some specimens.

Interior view, anterior and posterior calcified inner lamellae comparatively wide but with anterior one slightly wider. Three inner lists present on anterior calcified inner lamella (Figs 2D, J , 3E ). First one running in intermediate zone, usually weakly expressed, sometimes not preserved. Second and third ones running close to inner margin. Inner lists also present on posterior calcified inner lamella but with first one usually very faint or absent (Figs 2C, I , 3B, G ). Two rows of fine, densely arranged marginal ripplets present on both anterior and posterior calcified inner lamellae (Figs 2 , 3 ): distal row (Figs 2C, D, I, J , 3B, E, G ) with each ripplet extending from exterior margin of selvage near valve margin to almost first inner list, distributed throughout entire anterior and posterior calcified inner lamellae, usually well expressed; proximal zone (Figs 2C, D, J , 3B, E, G ) present between first and second inner lists, better observed at antero-ventral and postero-ventral areas, sometimes not preserved (Fig. 2I ). Adductor muscle scars (Fig. 3F ), consisting of six scars of different sizes and shapes, arranged in two rows (four anterior scars and two posterior ones). Mandibular scars (Fig. 3F ) consisting of two sub-oval elements.

Dorsal view (Fig. 2E ) semi-elliptical, with part behind posterior sulcus wider than anterior part. Anterior end pointed. Posterior end bluntly pointed. Antero-dorsal corner with one small blunt expansion representing tooth-structure of hinge. Middle part of ventral calcified inner lamella (corresponding to highest part of ventral margin in lateral view; observed from oblique-dorsal view) with gentle, inward, i.e., adaxial, expansion. Two proximal inner lists, i.e., second and third ones, showing complex crossing patterns on this part (Fig. 2F, G ).

Right valves. Shape similar to that of left valve, but dorsal margin straighter (Fig. 4A, B ) with posterior section not inflated dorsally (Fig. 4A, B ). Valve margin also with three inner lists but first one very faint on anterior calcified inner lamella (Fig. 4C ) and almost absent on posterior calcified inner lamella (Fig. 4D ).

Valves of A - 1 juveniles. Shape similar to that of adults but with dorsal margin more inclined (Fig. 4E, F ). Pits on valve surface smaller and shallower. Calcified inner lamellae of left valve narrow, with only one row of densely arranged marginal ripplets distally: extending to median area in anterior calcified inner lamella (Fig. 4H ), almost to proximal area in postero-ventral area (Fig. 4G ). Only one inner list present, running close to inner margin (Fig. 4G, H ).

Differential diagnosis.

The new species can be easily distinguished from congeners by the fineness, number, and distribution of the two rows of marginal ripplets as well as the presence of the outer-most (first) inner list in the intermediate zone that separates the two rows of marginal ripplets. Ilyocypris bradyi , for example, has fewer, thicker, and more widely spaced ripplets confined to the postero-ventral area, typically four or five in the outer row (see e.g., Mazzini et al. 2014: fig. 11). Considering potential taphonomic loss of finer morphologies in fossil material (e.g., the abrasion of the proximal row of marginal ripplets and the inner lists), several species with morphologies that may be confused with the poorly preserved specimens of the new species are compared here. Ilyocypris lacustris Kaufmann, 1900 seems to have two rows of fine marginal ripplets separated by one inner list on the anterior valve margin, which resembles the new species ( Fuhrmann 2012: Tafel (= pl.) 76 1b), but the ripplets on the posterior part are confined to the postero-ventral margin and consist of only one row ( Fuhrmann 2012: Tafel (= pl.) 76 1f and 2d). The highest point of the carapace of I. lacustris is situated at approximately the anterior quarter, more anterior than that of the new species. The marginal ripplets on the posterior part of I. salebrosa are also fine and consist of two rows ( Mazzini et al. 2014: fig. 11, panel 8), but are confined to the postero-ventral part too. And the prominent postero-dorsal node on the exterior valve surface offers easy distinction from the new species. Ilyocypris hanguk Karanovic & Lee, 2013 described from South Korea, has a valve shape somewhat similar to the new species, and its H/L ratio (0.55 for LV and 0.54 for RV as measured from the holotype in Karanovic and Lee 2013: fig. 6A, B) comes close to that of the new species. However, no marginal ripplets were observed on the left valve of I. hanguk ( Karanovic and Lee 2013: fig. 6A, D). Furthermore, small lateral projections are present on the postero-central part of the valve of I. hanguk ( Karanovic and Lee 2013: fig. 6B), which are not observed in I. leptolinea Wang & Zhai, sp. nov. (Figs 2 - 4 ). Ilyocypris glabella Fuhrmann & Goth, 2011, Ilyocypris sebeiensis Yang & Sun, 2004 (in Yang et al. 2004), I. tibeta ( Peng et al. 2021), and Qinghaicypris crassa Huang, 1979 ( Yang et al. 2004) possess marginal ripplets distributed along most parts of the anterior and posterior calcified inner lamellae. However, in all these species, there is only one row of ripplets distributed near the valve margins, which are thicker and more sparsely arranged compared with the new species. As a result, even if the proximal row of marginal ripplets is taphonomically lost in the new species, it would not be confused with these species. Besides, the valves of I. glabella , I. sebeiensis , and Q. crassa are significantly stouter than those of the new species. (Note that Shen et al. (1993) and Hou et al. (2002) considered Qinghaicypris a junior synonym of Ilyocypris and moved all the species in this genus to Ilyocypris ).

Figures

Figure 1. A digital Elevation Model showing the position of the study area (red rectangle) in central-eastern Inner Mongolia, China B digital Elevation Model showing the location of the XJG 2 section C field photo of the section D lithology of the section. Red triangle indicates the result of 14 C dating of gastropods from depth 139 - 140 cm. See text for details.

Figure 2. Adult left valves of the ostracod Ilyocypris leptolinea Wang & Zhai, sp. nov. from the late Quaternary Xiaojinggou section of Inner Mongolia, China A-D XJG 2 - 210 - 1 (holotype, 0.94 x 0.48 mm) A exterior view B interior view C interior view of the posterior part, showing the marginal ripplets D interior view of the anterior part, showing the marginal ripplets. Arrows and numerals indicate three inner lists on the calcified inner lamella E, F XJG 2 - 206 - 4 E oblique-dorsal view F enlarged view of the rectangle in (E), showing the crossing pattern of the inner lists at the median part of ventral margin. Arrows and numerals indicate the three inner lists G XJG 2 - 206 - 1, enlarged oblique-dorsal view of the median part of calcified inner lamella, showing the crossing pattern of inner lists H-J XJG 2 - 199 - 2 (paratype) H interior view I interior view of the postero-ventral part, showing the marginal ripplets J Interior view of the antero-ventral part, showing the marginal ripplets. Arrows and numerals indicate the three inner lists. Scale bars: 200 μm (A, B, E, H); 100 μm (C, D, I, J); 50 μm (F, G).

Figure 3. Adult left valves of the ostracod Ilyocypris leptolinea Wang & Zhai, sp. nov. from the late Quaternary Xiaojinggou section of Inner Mongolia, China A-C XJG 2 - 21 - 1 (paratype) A exterior view B interior view of the postero-ventral part, showing the marginal ripplets C interior view D-G XJG 2 - 197 - 2 (paratype) D interior view E interior view of the antero-ventral part, showing the marginal ripplets and three inner lists (arrowed) F adductor muscle scars (AMS) and mandibular scars (MS) G interior view of the posterior part, showing the marginal ripplets. Scale bars: 200 μm (A, C, D); 100 μm (B, E, G); 50 μm (F).

Figure 4. Ostracod valves tentatively identified as Ilyocypris leptolinea Wang & Zhai, sp. nov. from the late Quaternary Xiaojinggou section of Inner Mongolia, China A-D XJG 2 - 203 - 1, adult right valve A exterior view B interior view C interior view of the antero-ventral part, showing three inner lists (arrowed) D interior view of the postero-ventral part E XJG 2 - 193 - 4, right valve of A- 1 juvenile, exterior view F-H XJG 2 - 199 - 1, left valve of A- 1 juvenile F interior view G interior view of the postero-ventral part, showing the marginal ripplets H interior view of the antero-ventral part, showing the marginal ripplets. Scale bars: 200 μm (A, B, E, F); 100 μm (C, D, G, H).

Figure 5. Height-length biplots of the left valves of Ilyocypris leptolinea Wang & Zhai, sp. nov. (A) and the right valves tentatively identified as this species (B) from the late Quaternary Xiaojinggou section of Inner Mongolia, China.