''Mongolitubulus'' is a

form genus Form classification is the classification of organisms based on their morphology, which does not necessarily reflect their biological relationships. Form classification, generally restricted to palaeontology, reflects uncertainty; the goal of s ...

encapsulating a range of ornamented conical

small shelly fossil The small shelly fauna, small shelly fossils (SSF), or early skeletal fossils (ESF) are mineralized fossils, many only a few millimetres long, with a nearly continuous record from the latest stages of the Ediacaran to the end of the Early Cambri ...

s of the

Cambrian period The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago (m ...

. It is potentially synonymous with ''Rushtonites'', ''Tubuterium'' and certain species of ''

Rhombocorniculum ''Rhombocorniculum'' is a species of small shelly fossil comprising twisted ornamented cones. It has been described from the Comely limestone and elsewhere. ''R. cancellatum'' straddles the Atdabanian/Botomian boundary. The structure of its in ...

'', and owing to the similarity of the genera, they are all dealt with herein. Organisms that bore ''Mongolitubulus''-like projections include trilobites, bradoriid arthropods and hallucigeniid lobopodians.

Morphology

The fossils consist of round, slender, pointed, spines with a slight curvature, and are covered with short rhomboid processes that spiral around the spine surface, forming a regular mosaic with a 60° angle of intersection. 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. Species are defined on the basis of the ornamentation, which may of course be

convergent Convergent is an adjective for things that converge. It is commonly used in mathematics and may refer to: *Convergent boundary, a type of plate tectonic boundary * Convergent (continued fraction) * Convergent evolution * Convergent series Converg ...

. 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. 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. ''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.

Species

Affinity

''M. henrikseni'' has been shown to be part of the carapace of a bivalved bradoriid arthropod. However, the affinity of ''M. squamifer'' is still unresolved; the genus may transpire to be a

form taxon Form classification is the classification of organisms based on their morphology, which does not necessarily reflect their biological relationships. Form classification, generally restricted to palaeontology, reflects uncertainty; the goal of sc ...

, which would require ''M. henrikseni'' to be re-classified into a new genus. 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. ''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 The lobopodians, members of the informal group Lobopodia (from the Greek, meaning "blunt feet"), or the formally erected phylum Lobopoda Cavalier-Smith (1998), are panarthropods with stubby legs called lobopods, a term which may also be used as ...

known from

Burgess shale-type deposit The Burgess Shale of British Columbia is famous for its exceptional preservation of mid-Cambrian organisms. Around 69 other sites have been discovered of a similar age, with soft tissues preserved in a similar, though not identical, fashion. Additi ...

s. This speculative claim has been substantiated for some material attributed to ''Mongolitubulus'', based on similarities with the spines of the hallucigeniid lobopodians. The trilobite ''Hupeidiscus orinentalis'' has spinose projections with a rhomboidal ornamentation that resembles that seen in ''Mongolitubulus'', so some ''Mongolitubulus'' material may represent trilobites.

Preservation

The spines often comprise layers of phosphate, with a central void often infilled with diagenetic phosphate. Similar spines have been recovered from

acid maceration Fossil preparation is the act of preparing fossil specimens for use in paleontological research or for exhibition, and involves removing the surrounding rocky matrix and cleaning the fossil. Techniques Acid maceration Acid maceration is a tech ...

s, where they are preserved as films of organic carbon.

Distribution

''Mongolitubulus'' is known from the Botomian to the lower strata of the Middle Cambrian, and have a worldwide distribution, being found on every continent including Antarctica. ''Rhombocorniculum'' is known from a variety of localities, including England and Massachusetts.

References

{{Taxonbar, from=Q6899766 Cambrian arthropods Arthropod enigmatic taxa Prehistoric crustacean genera Controversial taxa Paleozoic life of Nova Scotia Paleozoic life of Quebec