Pyrgoidae

I have referred in the past to there being something of a divide in approaches to the classification of the Foraminifera. This divide arises from disagreements such as the relative significance of various character complexes. One taxon that stands as an example of such disagreements is the subject of this post, the family Pyrgoidae as recognised by Mikhalevich (2005).

Pyrgo williamsoni, copyright Michael.

Pyrgoids are members of the group of forams generally recognised as the Miliolida, the porcelaneous forams. In this group, the wall of the test is composed of calcite but the calcite crystals are not regularly lined up with each other so the wall is not transparent. As a result, the wall of the test resembles porcelain in appearance. Most miliolidans have the chambers of the test coiling in a single plane. The Pyrgoidae were distinguished from other miliolidans by Mikhalevich (2005) by the overall structure of the test which is primarily biloculine (with the whorls of the test composed of two chambers). The family was divided into subfamilies by the nature of the test aperture: single with an inner tooth in Pyrgoinae, single with a flap in Biloculinellinae, and multiple (at least when mature) in Cribropyrgoinae and Idalininae. Idalininae also differed from other subfamilies in that the very last chamber was further enlarged to envelop the entire test. Members of the Pyrgoidae are known from the fossil record going back to the Jurassic period.

In the system of Loeblich & Tappan (1964), however, the pyrgoids were not recognised as a single group. Instead, they were dispersed among separate subfamilies of the family Miliolidae. Part of the reason was simply that Loeblich & Tappan did not divide the miliolidan families as finely as Mikhalevich later would but a bigger difference was one of priority. Loeblich & Tappan regarded the nature as an aperture as a more important feature taxonomically than the arrangement of chambers. Both classifications seem to have been constructed more from a diagnostic viewpoint than necessarily intended to reflect phylogenetic relationships.

Cribropyrgo aspergillum, from the National Museum of Natural History.

As with most other forams, pyrgoids exist in what are called megalosphaeric and microsphaeric forms. These forms represent alternate generations in the foram life cycle: microsphaeric forams are the sexually reproducing generation whereas megalosphaeric forams reproduce asexually. The names refer not to the overall size of the individuals but to the size of the proloculus, the very first embryonic chamber that sits at the center of the test. In megalosphaeric pyrgoids, the developing test is biloculine from the very start. In microsphaeric individuals, the earliest stages of the test are quinqueloculine (with five chambers per whorl) then become triloculine then finally biloculine (with a further progression for the idalinines, of course). The significance of the differences between the two forms has historically been the subject of discussion with some authors arguing that the microsphaeric forms represented a retention and overwriting of ancestral forms, or an expression of the trajectory the lineage might evolve along in the future (Loeblich & Tappan 1964). The most likely explanation, though, seems to me to be the simplest. The size of the proloculus correlates with the amount of cytoplasm in the young foram. Megalosphaeric pyrgoids start with fewer chambers per volution from the start for the simple reason that they don't have the space to pack in more.

REFERENCES

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.

Mikhalevich, V. 2005. The new system of the superfamily Quinqueloculinoidea Cushman, 1917 (Foraminifera). Acta Palaeontologica Romaniae 5: 303–310.