Orthothecida Marek, 1966

Malinky, John M. & Skovsted, Christian B., 2004, Hyoliths and small shelly fossils from the Lower Cambrian of North-East Greenland, Acta Palaeontologica Polonica 49 (4), pp. 551-578 : 564-565

Order Orthothecida Marek, 1966

Remarks.—This order of hyoliths was defined to encompass individuals that have either a planar aperture or one with a broad indentation usually on the ventral side. Within this concept, a ligula would therefore be lacking. In addition, the orthothecid operculum lacks not only clavicles but also a furrow and rooflets.

Orthothecids fall into two broad groups based on crosssectional shape: the first with a poor− to well−developed kidney or heart shape, and a second with either a circular, sub−circular, lenticular or elliptical cross−section. The former are readily identifiable as orthothecids because opercula that match in outline those cross−sections have cardinal processes typically seen among hyoliths. The second group is more problematic because opercula are unknown for most such tubular fossils, and the tubular morphology is a trait seen in other groups of calcareous organisms, such as Coleolus and Coleoides ( Fisher 1962). Circotheca Syssoiev, 1958 is a tubular hyolith with an operculum that clearly possesses cardinal processes ( Berg−Madsen and Malinky 1999), thus its affinity to the Hyolitha is beyond question. However, many tubular fossils particularly from the Lower Cambrian of Siberia and South China, such as Turcutheca Missarzhevsky, 1989 and Paracircotheca Qian, 1989 have been referred to Orthothecida , and many others to Circotheca itself ( Syssoiev 1960; Meshkova 1974; Kerber 1988) even though the operculum is unknown. Relatively few calcareous tubular fossils are known from Greenland, although some do occur ( Figs. 10G , 11–13 ) but without an operculum their affinity to the Hyolitha cannot be demonstrated.

It is also worth noting that apical curvature has not been documented among any undoubted adult orthothecid hyoliths with a heart− or kidney−shaped cross−section except for Contitheca ? sp. and “ Orthotheca bayonet var. groenlandica ( Poulsen, 1932) ” below. In contrast, many small, strongly curved tubular fossils of circular or sub−circular cross−section are assigned to Circotheca ( Kerber 1988: pl. 1: 10, 11, 14), even though such curvature has not been seen among larger and presumably adult individuals of Circotheca ( Berg−Madsen and Malinky 1999) .

Stratigraphic range.—Lower Cambrian–Middle Devonian.

Figures

Fig. 10. A–C. “Orthotheca bayonet var. groenlandica (Poulsen, 1932)”. A. MGUH 3521; A1, venter; A2, right lateral edge; A3, dorsum. B. MGUH 3523, venter. C. MGUH 3525; C1, dorsum; C2, venter; C3, cross−section. D. “Orthotheca communis (Billings, 1872)”, MGUH 3524; D1, cross−section; D2, dorsum; D3, right lateral edge with venter to right; D4, venter. E. “Orthotheca bayonet var. longa (Poulsen, 1932)”, MGUH 3526, venter. F. “Orthotheca billingsi (Walcott, 1886)”, MGUH 3528, venter. G. Coleolus sp.? MGUH 3517. All specimens from the collections of C. Poulsen. Scale bars 2 mm.

Fig. 11. Cupitheca hemicyclata (Bengtson, 1990b). A. MGUH 27112. B. MGUH 27113. Both specimens from GGU sample 314835. Scale bars 0.2 mm.

Fig. 12. Cupitheca holocyclata (Bengtson, 1990b). A. MGUH 27114; A1, apical view showing septum; A2, lateral view showing shell with ornamentation. B. MGUH 27115; B1, B2, oblique views of apical septum with structure of shell wall visible in B2. C. MGUH 27116. From GGU sample 314809. Lateral view showing apical septum. D. MGUH 27117; D1, lateral view showing apical septum; note oblique angle of constriction; D2, enlarged view of apical septum and constriction. E. MGUH 27118, lateral view showing apical septum. All specimens except C from GGU sample 314807. Scale bars 0.2 mm except B2 which is 0.1 mm.

Fig. 13. Coleolus sp. A. MGUH 27119, from GGU sample 314933, lateral view. B. MGUH 27120, from GGU sample 314835; B1–B3, lateral views; B4, cross−section. C. MGUH 3527 from the collections of C. Poulsen, lateral view. Scale bars 1 mm.