Mickwitzia cf. occidens Walcott, 1908

Skovsted, Christian B. & Holmer, Lars E., 2003, The Early Cambrian (Botomian) stem group brachiopod Mickwitzia from Northeast Greenland, Acta Palaeontologica Polonica 48 (1), pp. 1-20 : 3-4

Mickwitzia cf. occidens Walcott, 1908

Figs. 3–5, 7–14 .

cf. Mickwitzia occidens ; Walcott 1908: 54, pl. 7: 1.

cf. Mickwitzia occidens Walcott ; Walcott 1912: 331, pl 4: 1.? Mickwitzia sp. ; Cowie and Adams 1957: 26.

cf. Mickwitzia occidens Walcott ; Rowell 1977: 79, pl. 1: 12.? Mickwitzia multipunctata sp. nov; McMenamin 1992: 180, figs. 1.1, 1.7, 4.3–4.6, 5.1–5.3.

?Perforate plate; Landing and Bartowski 1996: 759, figs. 9.16, 9.17.

Material.—Total of 410 ventral and 4 dorsal valves in addition to an undetermined number of fragments.

Taxonomic remarks.— M. occidens Walcott, 1908 from the Lower Cambrian of Nevada and California remains a very poorly understood species ( Rowell 1977; see also McMenamin 1992). Rowell (1977) re−described the type material which, as noted by him, comprises a single poorly preserved indeterminate fragment of a valve and four smaller fragments. Additional specimens referable to the species occur in the acid resistant residue of a limestone sample collected (by C.B.S) from the middle member of the Poleta Formation in Indian Springs Canyon of the northern Montezuma Range, Esmeralda county, Nevada ( Fig. 2 ). The sample (CBS 990918−2) was collected about 5 m above the proposed base of the Dyeran Stage at a proposed boundary stratotype section ( Palmer 1999: 38). These new specimens are preserved in a similar manner to our material of M. cf. occidens from Northeast Greenland and appear to have similar ornamentation and shell structure. The shell appears to include acrotretoid−type columns as well as thicker tubes that are open to the exterior surface of the valves ( Fig. 2B ). The external ornament of fine radiating ridges ( Fig. 2A ) is identical to that described by Walcott (1912) and Rowell (1977) for M. occidens , but differs from the ornamentation of M. cf. occidens from Northeast Greenland in having more pronounced radiating ridges and in lacking the distinctive pustulose sculpture, with high pustules arranged in offset radiating rows. A full description of M. occidens and the associated fauna are outside the scope of the present paper, as more and better material is in the process of being collected from this and other levels in Nevada (L.E. Babcock personal communication 2000). The Greenland specimens are referred conditionally to the American species awaiting further revision of topotypic material.

Mickwitzia multipunctata described by McMenamin (1992) from the Puerto Blanco Formation of Sonora, Mexico, is very similar to both M. occidens and M. cf. occidens . According to McMenamin (1992: 180), M. occidens differs from M. multipunctata only “by being three times as large and by having proportionally larger and less densely spaced punctae”. The density of the “punctae” (corresponding to the larger cylindroids, here termed tubes) varies strongly within M. cf. occidens and depends on preservational factors, and the size−range of the new specimens of M. occidens includes individuals as small as those referred to M. multipunctata . The dorsal valves of M. multipunctata ( McMenamin 1992: fig. 5: 3) and M. cf. occidens are almost identical in all observable characters.

Figures

Fig. 7. Mickwitzia cf. occidens Walcott. Ventral valves; MGUH 26300 (A), + MGUH 26301 (B), MGUH 26302 (C), MGUH 26303 (D), MGUH 26304 (E). A1. Oblique lateral view of early mature valve; scale bar 200 µm. A2. De− tail of juvenile shell of A1; scale bar 100 µm. A3. Detail of filar ornamentation with nick−points of A2; scale bar 50 µm. B1. Oblique lateral view of mature valve; scale bar 500 µm. B2. Detail of apex with juvenile shell of B1; scale bar 200 µm. C. Mature valve with exfoliated apex and radiating trails of nick−points; scale bar 500 µm. D. Mature valve showing transition to pustulose mature ornamentation; scale bar 500 µm. E. Detail of ornamentation with radiating trails of nick−points on early mature shell; scale bar 100 µm. All specimens from the Bastion Formation. A, sample GGU 314835; B, sample GGU 314807; C, E, sample GGU 314910; D, sample GGU 314804.

Fig. 8. Mickwitzia cf. occidens Walcott.A, B. Ventral valves; MGUH 26305 (A), MGUH 26306 (B). A1. Early mature valve showing beginning pustulose ornamentation; scale bar 500 µm. A2. Detail of pustule of A1; scale bar 20 µm. B1. Mature valve with exfoliated apex showing pustulose ornamentation with offset radiating rows; scale bar 500 µm. B2. Detail of pustulose ornamentation B1; scale bar 200 µm. B3. Oblique lateral view of B1; scale bar 500 µm. C. Dorsal valve MGUH 26307; scale bar 200 µm. All specimens from the Bastion Formation. A, sample GGU 314804; B, sample GGU 314814; C, sample GGU 314905.

Fig. 9. Mickwitzia cf. occidens Walcott. Fragmentary mature valves (indeterminate) MGUH 26311 (A), MGUH 26312 (B), MGUH 26313 (C). A1, A2. Pustulose mature ornamentation with low cone−shaped pustules, perforated by the radiating rows of large tubular openings; scale bar 200 µm. A3. Detail of cone−shaped pustules of A, also showing three large tubular openings; scale bar 50 µm. A4. Detail of pyramidal pustules of A1, showing also scattered large tubular openings; scale bar 50 µm. A5. Detail of large tubular opening of A1. Note lack of depressions around the aperture; scale bar 10 µm. A6. Detail of granular surface structure of A1 and pyramidal pustules; scale bar 50 µm. B. Detail of exfoliated surface of mature valve showing numerous openings to acrotretoid−type columns and single large tubular opening; scale bar 20 µm. C1. Section through re−crystallized primary layer with pyramidal pustules; scale bar 20 µm. C2. Detail of primary layer of C1; scale bar 10 µm. All specimens from the Ella Island Formation, sample GGU 314816.

Fig. 2. Mickwitzia occidens Walcott. A. Ventral valve MGUH 26294, lateral view of exterior; scale bar 500 µm. B. Ventral valve MGUH 26295, detail of exfoliated exterior with large tube and cocci; scale bar 20 µm. All specimens from the middle member of the Poleta Formation in Indian Springs Canyon, northern Montezuma Range, Esmeralda County, Nevada.