Euestheria jingyuanensis Chen

TENG, XIAO, 2022, A morphological re-study of three spinicaudatan species from the Middle-Upper Jurassic of Gansu Province, northwestern China, Palaeoentomology 5 (4), pp. 327-339 : 332-336

Euestheria jingyuanensis Chen in Zhang et al., 1976

( Figs 5 , 6 )

1976 Euestheria jingyuanensis Chen in Zhang et al., p. 131, pl. 22, figs 14–17

2002 Euestheria jingyuanensis Chen in Shen et al., pl. 3 figs 1–2

Material. NIGP 29784 , holotype, left valve . NIGP 29785 , paratype, left valve . NIGP 29783 , paratype, external mould of holotype. Upper Jurassic oil shale member of the upper parts of the Wangjiashan Formation at Wangjiashan of Jingyuan County, Baiyin City , Jingyuan, Gansu Province, northwestern China .

Emended diagnosis. Carapace of small to medium size, shape elliptical or oval; dorsal margin short; larval valve small; umbo in anterior and marginal position; growth lines thin, 26–30 in number; early-growth-stage growth lines form a large postero-dorsal angle with the dorsal margin; late-growth-stage growth lines meet at a point at the end of the dorsal margin; growth bands narrow and flat; in the dorsal part of the carapace, growth bands ornamented with fine reticulation, mesh wall very thick and bulge, makes reticulation seems very sparse; on growth bands in the ventral part of the carapace, mesh wall strongly bulge and change to irregularly stretched tubercles; on growth bands near the antero-ventral and ventral margin of the carapace, tubercles with linear arrangements to form radial lirae.

Description. Carapace of small to medium size (L = 4.7–5.7 mm, H = 3.2–3.8 mm, H/L = 0.67–0.68), shape elliptical or oval; dorsal margin short; larval valve small; umbo in anterior and marginal position; growth lines thin, 26–30 in number; early-growth-stage growth lines form a large postero-dorsal angle with the dorsal margin( Fig. 5A–C ); late-growth-stage growth lines meet at a point at the end of the dorsal margin( Fig. 5A–C ); growth bands narrow and flat; in the dorsal part of the carapace, growth bands ornamented with very fine reticulation, mesh is 2–4 μm in diameter, mesh wall very thick and bulge, makes reticulation seems very sparse ( Fig. 6B–E ); on growth bands in the ventral part of the carapace, mesh wall strongly bulge and change to irregularly stretched tubercles, mesh is 11–18 μm in diameter ( Figs 5E , 6F– H ); on growth bands near the anterior, antero-ventral and ventral margin of the carapace, tubercles stretched vertically and with linear arrangements to form radial lirae ( Figs 5E , 6F–H ).

Measurements.

Locality and horizon. Upper Jurassic , the oil shale member of the upper parts of the Wangjiashan Formation. Wangjiashan of Jingyuan County, Baiyin City , Jingyuan, Gansu Province, China; Upper Jurassic Honggou Formation at Tanshanling of Tianzhu, Gansu Province, P.R. China.

Remarks. In the original description of this species, only fine reticulation was recorded ( Zhang et al., 1976). Considering the early study relied on an optical microscope, the very fine reticulation mentioned by Chen in Zhang et al., 1976 is the fine reticulation formed by tubercles. Liao et al., 2014 have re-examined the type specimens of this species by scanning electron microscope, and recorded the radial lirae on the ventral part of the carapace, but failed to revealed the very fine reticulation (2–4 μm in mesh diameter) on growth bands on the upper part of the carapace.

It is interesting that Sinokontikia also has the same ornamentation: tubercular ornamentation which is changed from bulge thick mesh wall on the lower part of the carapace, and fine reticulation on the upper part of the carapace ( Wang, 1985). It suggests that this species and Sinokontikia species probably share the same ancestor.

Figures

FIGURE 5. Euestheria jingyuanensis Chen in Zhang et al., 1976 emend. A–C, Focus-stacked images. D, E, SEM images. A, The holotype (NIGP 29784), left valve. B, The paratype (NIGP 29783), external mould of holotype. C, The paratype (NIGP 29785), left valve. D, SEM images of the same individual in Fig. 5C (NIGP 29785). E, Growth bands on the antero-ventral part of the carapace, ornamented by irregular tubercles, tubercle stretched irregularly to form reticulation, mesh is 11–18 μm in diameter; and tubercles on the marginal region of the carapace stretched vertically to form radial lirae. Scale bars = 1 mm in A–D, 100 μm in E.

FIGURE 6. Euestheria jingyuanensis Chen in Zhang et al., 1976 emend. SEM Images of the holotype specimens in Fig. 5A (NIGP 29784). A, Left valve. B–E, growth bands in the upper and middle part of the carapace, ornamented with very fine reticulation, mesh is 2–4 μm in diameter, mesh wall very thick and bulge, makes reticulation seems very sparse. F, growth bands near the anterior, antero-ventral, and postero-ventral margin of the carapace, ornamented with irregular tubercles, tubercles stretched vertically and with linear arrangements to form radial lirae. Scale bars = 1 mm in A, 10 μm in B and C, 50 μm in D–G, 100 μm in H.