Decoritheca cyrene (Walcott, 1905)

Sun, Haijing, Malinky, John M., Zhu, Maoyan & Huang, Diying, 2018, Palaeobiology of orthothecide hyoliths from the Cambrian Manto Formation of Hebei Province, North China, Acta Palaeontologica Polonica 63 (1), pp. 87-101 : 90-92

Decoritheca cyrene (Walcott, 1905)

Fig. 3 .

1905 Orthotheca cyrene sp. nov.; Walcott 1905: 19. 1905 Orthotheca cyrene dryas sp. nov.; Walcott 1905: 19. 1913 Orthotheca cyrene Walcott, 1905 ; Walcott 1913: 94, pl. 5: 21, 21a. 1913 Orthotheca cyrene dryas Walcott, 1905 ; Walcott 1913: 93, pl. 5:

22, 22a–c. 1946 Hyolithes (Orthotheca) cyrene (Walcott), 1905 ; Sinclair 1946: 75. 1946 Hyolithes (Orthotheca) dryas (Walcott), 1905 ; Sinclair 1946: 75. 1987 Decoritheca cyrene (Walcott, 1905) ; Malinky 1987: 948–951,

figs. 2.4, 6.1–6.6, 6.14.

Material. — Thirty-seven specimens (NIGPAS 166329–35, 166337, 166338, 166341, 166344, 166349–74) preserved as internal moulds of conchs.

Diagnosis. —See Malinky (1987: 948).

Description. —Conch straight with a planar (orthogonal) aperture, length of conchs ranges from 7.1–20.1 mm, and apertural width ranges from 2.9–5.5 mm. The dorsum is broadly rounded and covered with rows of tubercles ( Fig. 3C ), with diameters of 6–10 μm. Venter developed with a prominent longitudinal median concavity, becoming shallower from aperture to apex. Sub-rounded ridges adjacent to the concavity grade into narrowly rounded lateral margins ( Fig. 3B 1 , B 3 , D, F, H) and lateral margins are straight in dorsal view. Ventral external surface is covered with closely spaced transverse lines ( Fig. 3B 2 ). The morphology of dorsum and venter gives rise to a reniform cross section, but only one specimen displays the full image of transverse section ( Fig. 3E ), the apertural height is 0.7 mm, apertural width is 1.4 mm, and apertural width/height ratio is 2. Conchs terminate in pointed apex, and angle of divergence at apex ranges from 15–18°. Operculum and helens unknown.

G

Remarks. —This species bears resemblance to D. lineatula Holm, 1893 ) from Sweden, but the presence of only longitudinal sculpture on the latter serves to distinguish it from the former. D. cyrene also bears limited resemblance to the Swedish D. excavata ( Holm, 1893) , but the two species can be differentiated by the absence of rounded ridges at the ventral lateral margins in the latter.

One specimen of this species ( Fig. 3C 1 ) appears to preserve a conch with an unattached operculum near the aperture. Given that the operculum is nearly circular but the cross section of D. cyrene is reniform, it is unlikely that the conch and operculum represent the same taxon. It is more likely that these specimens are coincidentally located next to each other as result of movement by currents or perhaps bioturbation. Circular opercula such as these are usually found in clusters of C. convexa sp. nov. shells, and therefore we suggest that these circular or oval opercula more reasonably belong to C. convexa sp. nov.

Stratigraphic and geographic range.—Cambrian Series 3 Stage 5 to Furongian Stage 10. The Manto Formation of Tangshan, Hebei Province (herein), the Chaomitien and Changhsia formations of Yantai, Shandong Province (Walcott 1905), and the Changhsia Formation in Anhui Province, China ( Qian and Xiao 1995); Snowy Range Formation of Gallatin, Montana; Wilberns Formation of Burnet County, Texas ( Malinky 1987; see also SOM 2).

Figures

Fig. 3. Novitatid hyolith Decoritheca cyrene (Walcott, 1905) from the Manto Formation (Cambrian Stage 4/5) in the Zuojiawu section of Tangshan, China. A. NIGPAS 166338 in dorsal view. B. NIGPAS 166335 in ventral (B1) and ventro-lateral (B3) views; detail showing transverse lines on the external surface of venter (B2). C. NIGPAS 166337 in dorsal view, conch of Decoritheca cyrene with operculum of Cupitheca convexa sp. nov. in apertural region (C1); rows of tubercles (C2), inset shows enlarged individual tubercle with nearly flat top. D. NIGPAS 166329 in ventral view. E. NIGPAS 166341b, aperture with reniform cross section. F. NIGPAS 166331 in ventral view; F1, incomplete shell showing well developed median concavity and adjacent ridges; F2, nearly complete shell with broken aperture and adapically shallowing median concavity. G. NIGPAS 166330, both shells display partial venter and partial dorsum from dorsal view. H. NIGPAS 166333 in ventral view. I. NIGPAS 166332 in dorsal view, white arrows indicate algae. Scale bars: A, D, B3, F–I, 2 mm; B1, C1, 1 mm; B2, 100 μm; C2, 20 μm; E, 200 μm.

Fig. 1. Aggregations of orthothecide hyoliths preserved in purplish-red shales from the Manto Formation (Cambrian Stage 4/5) in the Zuojiawu section of Tangshan, China. A. NIGPAS 166336. B. NIGPAS 166345. C. NIGPAS 166334. D. NIGPAS 166341b. E. NIGPAS 166346. Scale bars 5 mm, except E, 2 mm.

Fig. 2. Scanning electron micrographs of shell and shale fabrics as well as mineral elemental analyses of matrix from the Manto Formation (Cambrian Stage 4/5) in the Zuojiawu section of Tangshan, China. A. NIGPAS 166334, fine grained clay minerals and hematite crystals in the matrix (A1). Tabular crystals of hematite forming rosette-shaped aggregations in the matrix (A2, A3); the cross in A2 shows the location of elemental analysis (A4). B. NIGPAS 166339a, skeleton coated with hematite. C. NIGPAS 166337, halite pseudomorphs distributed on the surface of undetermined fossil (C1), calcite imprints in the matrix (C2). Scale bars: A1, A2, C2, 2 μm; A3, 1 μm; B, 10 μm; C1, 50 μm.