Mongolitubulus

Mongolitubulus is a form genus encapsulating a range of ornamented conical small shelly fossils of the Cambrian period. It is potentially synonymous with Rushtonites, Tubuterium and certain species of Rhombocorniculum, and owing to the similarity of the genera,[1][4][5] they are all dealt with herein. Organisms that bore Mongolitubulus-like projections include trilobites, bradoriid arthropods and hallucigeniid lobopodians.[6]

Mongolitubulus
Temporal range: Lower Cambrian–Early Middle Cambrian[1]
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Stem group: Crustacea (?)
Order: †Bradoriida
Genus: †Mongolitubulus
Missarzhevsky, 1977[2]
Species
  • M. decensus Betts et al., 2017
  • M. henrikseni Skovsted & Peel, 2001
  • M. squamifer Missarzhevsky, 1977 (type)
  • M. spinosus Hinz, 1987
  • M. unispinosa Topper et al., 2007[3]
  • M. reticulatus Kouchinsky et al., 2010[4]
Synonyms
  • Rushtonites Hinz, 1987
  • Tubuterium Melnikova, 2000

Morphology

The fossils consist of round, slender, pointed, spines with a slight curvature,[1] and are covered with short rhomboid processes that spiral around the spine surface, forming a regular mosaic with a 60° angle of intersection.[5] Spines vary from sub-millimetric up to two centimetres in length, but do not show any growth lines, suggesting that they were moulted and replaced.[7] Species are defined on the basis of the ornamentation, which may of course be convergent.

Spines of Rhombocorniculum cancellatum have a similar surface ornamentation and are also curved, sometimes in two dimensions to form a 'screw'; they had an inner and outer organic layer that surrounded a layer of pillar-like apatite crystals; these enclosed a honeycomb-like structure of narrow edge-parallel chambers.[8] This genus is a useful biostratigraphic marker of the Lower Cambrian. The rhomboid ornament uniformly covers all the spine, with the exception (in some cases) of the smooth-surfaced tip.[9]

Mongolitubulus has a comparable structure; phosphatic fossils show that there was a smooth outer layer about 2–3.5 µm thick, a 10–15 µm-thick inner layer comprising axis-parallel fibres that are each ~1 µm wide, and a large cavity in the centre of the spine.[5]

Species
Species Distinguishing features Probable affinity
M. henrikseni Skovsted & Peel, 2001 Ornament of rounded conical protrusions, sometimes associated with faint ridges, at high angle to spine, irregularly arranged at low density. Spines circular in cross-section, straight to gently curved, more parallel-sided than M. squamifer. Tip of spine may be truncated or bifurcate slightly. Original microstructure unknown, but apparently formed two layers. Flaring of base, which is attached to bradoriid carapace.[1][10] Bradoriid
M. squamifer Missarzhevsky, 1977 (type) Regular ornament of rhomboidal to rounded projections arranged in spirals at 60° angle to form regular pattern; these lift slightly from the surface of the spine, directed towards its tip; their intensity fades at the tip, which is generally smooth-walled. Fibrous internal layer with high organic content.[1] Further, very thin layer within this, no structure apparent.[11] Spines are straight or gently curved; the pointed tip is always straight. Circular cross-section. Reaching 2.3 mm in length.[3] Base does not flare.[12] Trilobite?[13]
M. spinosus (Hinz, 1987)[14] Described as Rushtonites and since synonymized. Incomplete fragments reach 3 mm in length. Apical angle 5°. Base unknown. Gentle curvature. Spine ornament of rhombi with rounded tips, arrangement slightly deviates from regular pattern, with adjacent rhombi in places so close as to seem to merge. Wall in two layers, microstructure unknown. Unknown
"Rushtonites" astiaca [15] Spinose processes, not scales.[1] Unknown
M. unispinosa Topper et al.. 2007[3] (=Spinella unialata)[12] Carapaces with a single central spine. Ornament irregularly distributed, comprising short cones or pustules at steep angle to spine. Spine straight or slightly curved, with pointed tip. Cross-section circular. Base flares to join with carapace. Bradoriid
M. reticulatus Kouchinsky et al., 2010 Flaring base. Parallel edged near base, becoming highly curved in final quarter of length near tip. Pointed tip and circular cross-section. Two mineralogical layers (both calcium phosphate); inner layer bears transverse lineations, resembling growth lines, not entirely straight. Outer layer bears a polygonal network, grading into a more linear, axis-parallel fabric near the base. Up to 2.5 mm long. Observed attached to carapaces. Bradoriid

Affinity

M. henrikseni has been shown to be part of the carapace of a bivalved bradoriid arthropod.[10] However, the affinity of M. squamifer is still unresolved; the genus may transpire to be a form taxon, which would require M. henrikseni to be re-classified into a new genus.[10] Unlike the spines of M. henrikseni, which flare out at the base where they attach to the cuticle, the spines of M. squamifer are more parallel-sided, with the fossil material becoming thinner towards the base: consistent with their attachment to non-mineralized cuticle.[7] M. squamifer spines appear to have formed in pairs, owing to their symmetry; on this basis they have been likened to the spines of certain armoured lobopods known from Burgess shale-type deposits.[7] This speculative claim has been substantiated for some material attributed to Mongolitubulus, based on similarities with the spines of the hallucigeniid lobopodians.[6]

The trilobite Hupeidiscus orinentalis has spinose projections with a rhomboidal ornamentation that resembles that seen in Mongolitubulus, so some Mongolitubulus material may represent trilobites.[13]

Preservation

The spines often comprise layers of phosphate, with a central void often infilled with diagenetic phosphate.[7] Similar spines have been recovered from acid macerations, where they are preserved as films of organic carbon.[16]

Distribution

Mongolitubulus is known from the Botomian to the lower strata of the Middle Cambrian,[1] and have a worldwide distribution,[7] being found on every continent including Antarctica.[3]

Rhombocorniculum is known from a variety of localities, including England[17] and Massachusetts.[18]

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References
  1. Skovsted, C. E.; Peel, J. S. (2001). "The problematic fossil Mongolitubulus from the Lower Cambrian of Greenland" (PDF). Bulletin of the Geological Society of Denmark. 48: 135–147.
  2. Missarzhevsky, V. V. (1977). "Konodonty (?) i fosfatnye problematiki kembriya Mongolii i Sibiri". In Tatarinov, L. P (ed.). Bespozvonochnye Paleozoya Mongolii [Paleozoic invertebrates of Mongolia] [Conodonts(?) and phosphatic problematica from the Cambrian of Mongolia and Siberia]. Trudy Sovmestanya Sovetsko-Mongolskaya Paleontologicheskaya Ekspeditsiya [Transactions of the Joint Soviet-Mongolian Paleontological Expedition] (in Russian). 5. pp. 10–19. ISSN 0320-2305.
  3. Topper, T. P., Skovsted, C. B., Brock, G. A. & Paterson, J. R. (2007). "New bradoriids from the lower Cambrian Mernmerna Formation, South Australia: systematics, biostratigraphy and biogeography". Memoirs of the Association of Australasian Palaeontologists. 33: 67–100. hdl:1959.14/29153. ISSN 0810-8889.CS1 maint: multiple names: authors list (link)
  4. Kouchinsky, A.; Bengtson, S.; Clausen, S. B.; Gubanov, A.; Malinky, J. M.; Peel, J. S. (2011). "A middle Cambrian fauna of skeletal fossils from the Kuonamka Formation, northern Siberia". Alcheringa: An Australasian Journal of Palaeontology. 35: 123–189. doi:10.1080/03115518.2010.496529.
  5. Ryszard Wrona (2004). "Cambrian microfossils from glacial erratics of King George Island, Antarctica". Acta Palaeontologica Polonica. 49 (1): 13–56.
  6. Jean-Bernard Caron, Martin R. Smith & Thomas H. P. Harvey (2013). "Beyond the Burgess Shale: Cambrian microfossils track the rise and fall of hallucigeniid lobopodians". Proceedings of the Royal Society B: Biological Sciences. 280 (1767): 20131613. doi:10.1098/rspb.2013.1613. PMC 3735267. PMID 23902914.
  7. Dzik, J. (2003). "Early Cambrian lobopodian sclerites and associated fossils from Kazakhstan" (PDF). Palaeontology. 46: 93–112. doi:10.1111/1475-4983.00289.
  8. Li, G.; Zhu, M.; Steiner, M. (2003). "Microstructure and functional morphology of the Early Cambrian problematical fossil Rhombocorniculum *". Progress in Natural Science. 13 (11): 831. doi:10.1080/10020070312331344510.
  9. Otto H. Walliser (1958). "Rhombocorniculum comleyense n. gen., n. sp". Paläontologische Zeitschrift. 32 (3–4): 176–180. doi:10.1007/BF02989029.
  10. Skovsted, C. (2005). "A carapace of the bradoriid arthropod Mongolitubulus from the Early Cambrian of Greenland". GFF. 127 (3): 217–220. doi:10.1080/11035890501273217.
  11. Wrona, R. (2009). "Early Cambrian bradoriide and phosphatocopide arthropods from King George Island, West Antarctica: biogeographic implications". Polish Polar Research. 30 (4): 347–377. doi:10.4202/ppres.2009.19.
  12. Topper, T. P.; Skovsted, C. B.; Brock, G. A.; Paterson, J. R. (2011). "The oldest bivalved arthropods from the early Cambrian of East Gondwana: Systematics, biostratigraphy and biogeography". Gondwana Research. 19 (1): 310–326. Bibcode:2011GondR..19..310T. doi:10.1016/j.gr.2010.05.012.
  13. Li, G.; Steiner, M.; Zhu, Maoyan; Xin, Zhao (2012). "Early Cambrian eodiscoid trilobite Hupeidiscus orientalis from South China: ontogeny and implications for affinities of Mongolitubulus-like sclerites" (PDF). Bulletin of Geosciences. 87 (1): 159–169. doi:10.3140/bull.geosci.1224.
  14. Hinz, I. (1987). "The Lower Cambrian microfauna of Comley and Rushton, Shropshire/England". Palaeontographica Abteilung A. 198: 41–100.
  15. Landing, E. (1991). "Upper Precambrian through Lower Cambrian of Cape Breton Island: Faunas, Paleoenvironments, and Stratigraphic Revision" [2017-01-01]. Journal of Paleontology. 65 (4): 570–595. doi:10.1017/s0022336000030675. JSTOR 1305670.
  16. Butterfield, N. J.; Nicholas, C. J. (1996). "Burgess Shale-Type Preservation of Both Non-Mineralizing and 'Shelly' Cambrian Organisms from the Mackenzie Mountains, Northwestern Canada" [2017-01-01]. Journal of Paleontology. 70 (6): 893–899. doi:10.1017/s0022336000038579. JSTOR 1306492.
  17. Brasier, M. D. (1986). "The succession of small shelly fossils (especially conoidal microfossils) from English Precambrian–Cambrian boundary beds". Geological Magazine. 123 (3): 237. Bibcode:1986GeoM..123..237B. doi:10.1017/S0016756800034737.
  18. Landing, E (1988). "Lower Cambrian of Eastern Massachusetts: Stratigraphy and Small Shelly Fossils" [2017-01-01]. Journal of Paleontology. 62 (5): 661–695. doi:10.1017/s0022336000018965. JSTOR 1305390.
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