Meandering Forams

Specimen of Meandropsina vidali, showing the patterning on the external surface, from Loeblich & Tappan (1964).

There are some taxonomic names that just instantly bring up a mental image of the sort of organism to which they refer. For my part, I've always felt that Meandropsina is one of those names. The Meandropsinidae are another family of relatively large and complex foraminifera (growing up to a number of millimetres across) that are known only from the Upper Cretaceous. The several genera of the family are predominantly European, with only the genus Fallotia also known from the West Indies.

Cross-section of Meandropsina vidali, from Loeblich & Tappan (1964).

Meandropsinids are (as far as I know) more or less lenticular in shape with chambers enrolled in a flat spiral. The name of the type genus Meandropsina refers to the way that the outer margins of the chambers tend to meander irregularly around the test, giving it something of an ornate appearance. Both molecular and structural evidence indicate that multi-chambered forams arose from ancestors with undivided tests on more than one occasion, and the majority of multi-chambered forams can be assigned to two major lineages (Pawlowski et al. 2013). In one lineage, the Globothalamea (which includes, for instance, the rotaliids), the basic chamber shape is globular with successive chambers in the test being wider than long. In the other lineage, the Tubothalamea (including the miliolids and spirillinids), the basic chamber shape is tubular, and the test may grow through a number of spirals before it even starts to be divided into chambers (if at all). Members of the two lineages with calcareous tests may also be distinguished by their test structure: in calcareous globothalameans, the crystals making up the test are arranged regularly so the overall appearance of the test is hyaline (glass-like). In contrast, tubothalameans have the crystals of the test arranged irregularly so the appearance of the test is porcelaneous (like porcelain). Meandropsinids are unmistakeably tubothalameans in both regards.

Like other large forams of the Mesozoic, meandropsinids did not make it past the end of the Cretaceous. Early Palaeocene taxa that have been included in the families represent distinct lineages that evolved to take their place, occupying the ecological spaces opened up by the mass extinction ending the era.

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.

Pawlowski, J., M. Holzmann & J. Tyszka. 2013. New supraordinal classification of Foraminifera: molecules meet morphology. Marine Micropalaeontology 100: 1–10.