Mongolitubulus squamifer Missarzhevsky, 1977

Wrona, Ryszard, 2004, Cambrian microfossils from glacial erratics of King George Island, Antarctica, Acta Palaeontologica Polonica 49 (1), pp. 13-56 : 43-46

Mongolitubulus squamifer Missarzhevsky, 1977

Figs. 23A–H, 24.

Mongolitubulus squamifer Missarzhevsky ; Missarzhevsky 1977: 13, pl. 1: 1, 2.

Mongolitubulus squamifer Missarzhevsky ; Missarzhevsky and Mambetov 1981: 79, pl. 14: 1, 2.

Mongolitubulus squamifer Missarzhevsky ; Meshkova 1985: 127–128, pl. 46: 1–3.

Mongolitubulus squamifer Missarzhevsky ; Rozanov 1986: 89, fig. 4.

Mongolitubulus squamifer Missarzhevsky ; Peel and Blaker 1988: 56, fig.2.

Mongolitubulus squamifer Missarzhevsky ; Missarzhevsky 1989: 31, figs. 1, 3.

Mongolitubulus squamifer Missarzhevsky ; Wrona 1989: 543, pl. 8: 5.

Mongolitubulus squamifer Missarzhevsky ; Esakova and Zhegallo 1996: 103, pl. 4: 9–13.

Rhombocorniculum aff. insolutum Missarzhevsky and Mambetov ; Brasier 1986: 253, fig. 5j, k.

Rhombocorniculum n. sp.; Landing 1988: 687, fig. 11.6.

Mongolitubulus squamifer Missarzhevsky ; Skovsted and Peel 2001: 137, fig. 2.

Mongolitubulus henrikseni sp. nov.; Skovsted and Peel 2001: 140, fig. 3.? Mongolitubulus ex gr. M. squamifer Missarzhevsky ; Demidenko in Gravestock et al. 2001: 87, pl. 11: 5.

Mongolitubulus squamifer? Missarzhevsky ; Landing et al. 2002: 301, fig. 4: 19.

Diagnosis.—As for the genus.

Material.— Twelve isolated incomplete spines in different states of preservation from erratic boulders Me33 and 66. Figured specimens, ZPAL V. VI/28 S27, 28 ; 29S16; 39S2; 5106 S11, 21; and 22U1, 2.

Description.—Slender phosphatic spines, approximately 1– 2 mm in length and up to 0.3 mm in diameter, straight to gently curved (Fig. 23A–C). The spines taper to a pointed apex; but the basal margin has not been observed (it is broken off and mising in all specimens). The outer surface bears a distinct scaly sculpture (Fig. 23C–F). The rhomboid scales are arranged in spiral rows that cross each other at about 60° to produce a rhomboidal alternating pattern. The density of the scales varies considerably; the more closely spaced scales have sharp, rhombic outlines (Fig. 23D 4, E), whereas the sparsely distributed ones are more irregularly placed and rounded or even ovoid in shape (Fig. 23F 4, D 5). The scales are inclined outwards about 5–10° from their basal ends, the apical raised ends pointing towards the tip of the spine (Fig. 23D 3, D 4). The spines are circular or suboval in cross−section. The large internal cavity extends into the narrow tip. The wall is thin (about 2–3.5 µm), composed of an outer, dense or hyaline, structureless layer which forms the outer surface and scaly ornament, and a thick (about 10–15 µm) inner layer with a distinctly fibrous structure ( Fig. 24 ). The fibres are straight, less than 1 µm thick, and arranged parallel to the long axis of the spine, on its inner wall (Fig. 23G, H). The spines are black in appearance under the light microscope, which may suggest some organic component in their phosphatic matrix.

Remarks.—The spines show distinct morphological variation, especially in their ornamentation, even in this limited assemblage. The spines with sharp, densely packed rhomboid scales have a surface ornament in a clear alternating pattern (Fig. 23D), characteristic also for the type species from Mongolia ( Missarzhevsky 1977: pl. 1: 1, 2), and may represent one end of an intraspecific variability series. Spines with rounded, more randomly packed scales (Fig. 23F) are at the opposite end of the series. Such extreme morphotypes have been recently separated as a new species, M. henrikseni , by Skovsted and Peel (2001) on the basis of their different morphology and the preserved basal area. The Antarctic specimens seem to fit very well, in the morphology and wall structure, within the intraspecific variability of the M. squamifer morphotype as documented by the abundant Greenland collections ( Skovsted and Peel 2001). Spines from Antarctic erratics differ from M. henrikseni type specimens from the Upper Bastion Formation of northeastern Greenland, in laking the flaring base and in having a regular, alternating surface ornament, as well as their thin, hyaline external layer which is not laminated or thicker than the inner layer. The fibrous inner layer, seen as prominent strations on the interior surface (Fig. 23G, H), resembles similar striated structures occuring in coeloscleritophoran sclerites ( Qian and Bengtson 1989: fig. 16A 5, A 6; Bengtson 1992: fig. 8I–K; Conway Morris and Chapman 1996: fig. 8r), but unlike the latter, which presumably represent phosphatic replacement of an originally calcareous wall ( Conway Morris and Chapman 1996), the fibrous structure in the Mongolitubulus spine appears to be a primary phosphatic microstructure.

Discussion.— Mongolitubulus spines certainly do not represent an entire organism, but single elements of a disarticulated scleritome or fragments of exoskeletal elements armouring the body of an enigmatic animal. Their external morphology superficially resembles scaly ornaments widespread among fossils in the Lower Palaeozoic, particularly trilobite, eurypterid or bradoriid carapaces, remains of the vertebrate Anatolepis ( Bockelie and Fortey 1976) , and problematic ornamented tubes or conodont−like fossils. In particular, enrolled integument fragments from the Upper Cambrian (Trempealeauan– Dreshbachian) Broom Point section in western Newfoundland, show rhomboid surface ornament, and have been assigned to Anatolepis sp. by Landing (in Fortey et al. 1982: 115: fig. 9x), but they are clearly similar to Mongolitubulus tube fragments. Brasier (1986: fig. 5k) described one specimen from the Serrodiscus bellimarginatus Limestone Bed (Ac 3) at Comley, Shropshire, and referred it tentatively to Rhombocorniculum aff. insolutum Missarzhevsky and Mambetov, 1981 . However, the Comley specimen differs from typical forms and from the rest of Brasier’s specimens in having stronger, scale−like sculpture and a trumpet−shaped basal cavity. The same remark concerns Landing’s specimens of Rhombocorniculum sp. nov. from the Lower Cambrian of eastern Massachusetts, which are straight and slender tubes with wide internal cavity, nearly circular cross−section and characteristic Mongolitubulus − type surface ornament ( Landing 1988: 687: fig. 11.6). These characters indicate that both the English and American specimens represent a species of Mongolitubulus .

Mongolitubulus spines also show some similarities with ornamented tubes, form “B”, from the Parara Limestone and form “C” from the Ajax Limestone in the Mount Scott Range, South Australia ( Bengtson et al. 1990: figs. 102and 103) and with apical cones of Lapworthella ( Bengtson et al. 1990: figs. 78F and 80A, B), as well as with Rushtonites spinosus Hinz (1987) from the Lower Cambrian of Comley. The latter form was also compared with scale−sculptured organic−walled spines or setae from the Middle Cambrian of the Mackenzie Mountains, Canada ( Butterfield and Nicholas 1996). Some similarities can be recognized between the Mongolitubulus scale ornament and the external tuberculate ornamentation on sclerites of the coleoscleritophoran Halkieria (Fig. 7F; also Conway Morris and Chapman 1996: fig. 6: 10 and fig. 8: 2, 3). The most important differences with ornamented tubes are in the composition of the wall and the interior surface of the tube, which shows pits corresponding to the external scale−like ornament (e.g., Bengtson et al. 1990: fig. 101H). Thus, histological investigations of all conodont−like forms and ornamented tubes are crucial. Observations made on the scale ornament of such coniform tubes and lapworthellid apical cones suggests that scales have rather mechanical significance in construction of the spine wall than any functional meaning. The strongly abraded spine fragments from the Lower Cambrian of Ville Guay, Québec very likely represent specimens of M. squamifer ( Landing et al. 2002) . The recently discovered M. henrikseni with flaring spine base and co−occurring plate fragments with broken spines from the Lower Cambrian Bastion Formation of northeastern Greenland may have been an arthropod carapace bearing spines, although possibly this interpretation cannot be extended to the type species ( Skovsted and Peel 2001). The alternative interpretation of the Mongotubulus spines as sclerites armouring the body of a lobopodian animal, similar to Xenusion , has been suggested by Dzik (2003) on the basis of a low−diversity fossil assemblage dominated by Mongolitubulus spines and Microdictyon sclerites. Both these interpretations are founded on different incomplete skeletal elements and cannot be extended to all Mongolitubulus forms, in particular the type material, until new complete scleritomes or articulated spine arrays of Mongolitubulus are discovered.

Occurrence.— Mongolia, Sanashhtykgol Horizon, Lower Cambrian (Botomian); Maly Karatau, Koksu, and Ushbas, from Geress Member of the Shabakty Formation, Lower Cambrian (Botomian or latest Atdabanian, see Missarzhevsky and Mambetov 1981). Turkestan, Middle Cambrian. Centralnorthern Greenland, Henson Gletscher Formation, Lower Cambrian (Bonnia–Olenellus Zone); Lower Cambrian of eastern Massachusetts and Ville Guay, Québec, Norh America; Lower Cambrian Comley Limestone (Ac 3) at Comley, Shropshire, England; allochthonous Early Cambrian (Botomian) boulders (Me32), King George Island, Antarctica.

Hyolithyelminths

Phylum and class uncertain

Order Hyolithelminthida Fisher, 1962

Family Hyolithellidae Walcott, 1886

Genus Hyolithellus Billings, 1871

Type species: Hyolithes micans Billings, 1871 .

Figures

Fig. 24. A. Diagrammatic cross−section of the wall of Mongolitubulus squamifer Missarzhevsky. B. Energy dispersive spectrum (EDS) for the M.squamifer sclerite. C. Broken end of the specimen figured in Fig. 23D, showing layered wall structure including outer hyaline layer and fibrous inner layer.