Calcareous foraminiferans have been featured on this site before: planktic floaters, living stars, microscopic jelly moulds and gigantic reef-formers. All these forms have belonged to the group of calcareous forams known as the rotaliids. Today's subject is another group of forams, the Rhapydionininae, belonging to a different calcareous group, the Miliolida. Miliolids may have shell walls made of calcite like the rotaliids, but differ in the wall structure: while the walls of rotaliids are glass-like and porous, those of miliolids are structured like porcelain. Phylogenetic studies of forams have not placed the miliolids close to the rotaliids, and the two groups seem to have evolved their secreted shells independently (Sen Gupta 2002).
The Rhapydionininae were defined by Loeblich & Tappan (1964) as a group of miliolids with a conical test composed of broad chambers stacked one on top of another (the overall shape being kind of like a fan or an ice-cream cone), with each of these chambers subdivided by internal septa into multiple chamberlets (the difference between a 'chamber' and a 'chamberlet' being that the latter are not completely divided from each other by the walls). The opening of the test took the form of a sieve-like array of pores at the top end. However, subsequent researchers have discovered that Loeblich & Tappan's definition was inadequate. Rhapydioninines start life growing as a flat spiral, with growth becoming linearised at maturity. However, it turns out that not all Rhapydionininae become linear; some retain their juvenile coiling into maturity (Vicedo et al. 2011). At least some species are believed to have both a linear megalospheric form and a coiled microspheric form. To explain, forams can be divided between microspheric forms, in which the first chambers of a new test are much smaller, and megalospheric forms with larger initial chambers. In those relatively few forams whose life cycles have been studied in detail, these two forms correspond to an alternation of generations, with a mostly microspheric asexually-reproducing generation giving rise to the generally megalospheric sexually-reproducing phase. Loeblich & Tappan's (1964) concept of rhapydionines, therefore, would have potentially placed members of a single species into separate families.
Rhapydionines are best known as fossils, with a definite range from the Upper Cretaceous to the mid-Eocene (Loeblich & Tappan 1984). Believe it or not, whether there are still rhapydioninines in the world is something of an open question. Loeblich & Tappan (1964) listed two Recent genera in the Rhapydionininae, each represented by only a single known specimen. Ripacubana conica was originally described from sand deposits in Cuba; however, Loeblich & Tappan (1964) suggested that Ripacubana may actually represent what has been referred to as a 'zombie taxon'. Some of you may be familiar with the palaeontological concept of a 'Lazarus taxon', where a species disappears from the fossil record only to reappear at a later date. What has actually happened in these cases is that the species had only become locally extinct, but survived in some other locality that has not been preserved, subsequently recolonising its old range. A 'zombie taxon', however, is one that has genuinely become extinct at the earlier date, but its fossilised remains have since been transported into a younger sediment deposit, giving the impression that it survived later than it did*. In the case of Ripacubana, it is difficult to know just how long a foram shell buried in sand has been lying there.
*Identifications of Lazarus taxa also have to be on the look-out for 'Elvis taxa': where the more recent population does not in fact represent the same species, but a different species that has convergently evolved similar features.
Loeblich & Tappan (1964) did not express the same reservations about Craterites rectus, described from a beach on Lord Howe Island east of Australia. Craterites was later separated as its own subfamily by Loeblich & Tappan (1984) on the basis of its being attached to the substrate, and so differing from other free-living Rhapydionininae. Nevertheless, they kept the two subfamilies together as the family Rhapydioninidae, so Craterites may still be the only known survivor of the rhapydioninine lineage. However, with only one known specimen, the details of the internal structure of Craterites remain unknown.
Loeblich, A. R., Jr & H. Tappan. 1964. Treatise on Invertebrate Paleontology pt C. Protista 2. Sarcodina, chiefly "thecamoebians" and Foraminiferida, vol. 1. The Geological Society of America and The University of Kansas Press.
Loeblich, A. R., Jr & H. Tappan. 1984. Suprageneric classification of the Foraminiferida (Protozoa). Micropaleontology 30 (1): 1-70.
Sen Gupta, B. K. 2002. Modern Foraminifera. Springer.
Vicedo, V., G. Frijia, M. Parente & E. Caus. 2011. The Late Cretaceous genera Cuvillierinella, Cyclopseudedomia, and Rhapydionina (Rhapydioninidae, Foraminiferida) in shallow-water carbonates of Pylos (Peloponnese, Greece). Journal of Foraminiferal Research 41 (2): 167-181.