Natantesprifmata, Hussain & Rogers & Blackburn, 2022

Hussain, Rowshi, Rogers, Steven L. & Blackburn, Joel A., 2022, Ovummuridae (calcareous microfossils) from the Much Wenlock Limestone Formation, Shropshire, UK, Palaeontologia Electronica (a 27) 25 (2), pp. 1-24 : 12

Genus NATANTESPRIFMATA gen. nov.

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Derivation of name. Latin, natantes, floating; prifmata, prisms; genus; neutral

Type Species. Natantesprifmata rogersi sp. nov., the only species known.

Diagnosis. Ovummuridae with a prismatic test divided into two chambers by a pseudo-septum. The larger dorsal chamber is prismatic in shape. The smaller ventral chamber is conical or pyramidal. Apertures may be present within the central axial plane of the dorsal chamber. The wall is characteristic of Ovummuridae , exhibiting the lamellar brick-like arrangement of calcite tablets.

Remarks. These Ovummuridae specimens do not conform to the specifications of the genera: Minourella , Ovummurus , Arouxina or Samtlebenella because: (1) their chambers are not globular like Minourella or Ovummurus , (2) their chambers are not symmetrical like Ovummurus , (3) they do not exhibit hemispherical chambers around the initial chambers like Arouxina and (4) they do not contain chambers within their walls. On this basis, a new genus has been established to account for the prismatic nature of these variants of Ovummuridae . The ventral chamber is difficult to determine the test shape based on 2D cross sections, hence it is undetermined whether the ventral chamber is of conical or pyramidal nature. It is most likely pyramidal based on the terminations of the wall surrounding the secondary chamber and the pseudo-septum.

There are no known calcareous fossils of a prismatic nature, and neither do Palaeozoic foraminifera show such test geometries. The genus can be compared to Radiolaria and Dinoflagellates for their similarities in polygonal, polyhedron or prismatic test morphologies, however, these microfossil groups are siliceous rather than calcareous. Dinoflagellates such as: Protoperidinium communis and Protoperidinium oceanicum Biffi and Grignani 1983 , are polygonal and exhibit the same anterior sharp apex as seen in Natantesprifmata . The pyramidal morphology is seen due to the apical horn in Protoperidinium communis and Protoperidinium oceanicum Biffe and Grignani 1983 , rather than a pyramidal-shaped dorsal chamber as seen in these Ovummuridae . However, Protoperidinium are characteristic of having apical and antapical horns that are not seen in Ovummuridae .

The Family Pterocorythidae , described by Haeckel (1881) such as Stichopilium bicorne Haeckel (1887) , show similarities to Natantesprifmata as their thorax and external shells are pyramidal when observed proximally due to their polyhedron nature. Similarly, Podobursa mersinensis sp.and Pterocanium prismatium ( Lazarus, 1986) can appear prismatic in 2D cross-sections. However, these Radiolaria do not comprise chambers but consist of porate cephalis and commonly more than one postcephalic chamber is present ( Haeckel, 1862). Radiolarians do not have the same wall structure that is characteristic of Ovummuridae as their shells are latticed (Haeckel, 1882; Minoura and Chitoku, 1979). They express long spines that emerge from their cephalis, which are not observed within Ovummuridae ( Haeckel, 1862). The nassellarian skeletons are usually preserved as microcrystalline calcite.

Stratigraphical Range. Lower Silurian (Homerian).