Spiral, then Spread

Vertical (1-3, 7) and transverse (4-6) sections of Miogypsinoides dehaarti. Figure from Cole (1939).

Miogypsinoides dehaarti was a large calcareous foraminiferan (up to a few millimetres in length) that lived during the Aquitanian period (lower Miocene). The test of M. dehaarti and other species of Miogypsinoides was divided vertically into three distinct layers: the chambers were laid in a plane along the midline of the test with a thick unchambered wall above and below. The ventral wall contained a lattice system of canals that in life may have contained cytoplasm running between the chambers and the outside world (de Bock 1976); the upper wall was unperforated. The first part of the test to grow is visible in the lower point of the transverse sections above: from the embryonic chambers, the juvenile test initially grew in a spiral (with the size and proportions of the spiral varying between species). After the juvenile test reached maturity*, new chambers were added in a fan from one side of the juvenile test, so the eventual form of Miogypsinoides was not dissimilar to a piece of candy corn (though we do not know whether it, too, tasted of sweetness and death). The juvenile test of Miogypsinoides is similar to the adult form of other rotaliid forams, and it probably evolved from such forms through a process of hypermorphosis (the addition of new adult stages to development).

*Developmental maturity, that is. Obviously, we have no idea when the foram reached reproductive maturity.


Sectional diagram of the apical end of Miogypsinoides, showing the chambers in the equatorial plane and with some sections of the canal system visible below. Figure from Bock (1976).

The genus Miogypsinoides may have been ancestral to other genera in the family Miogypsinidae (Hanzawa 1964), from which it differs by the presence in the latter of accessory lateral chambers in the dorsal and ventral walls (and hence the loss of the canal system in the ventral wall). In the genera Miolepidocyclina and Heterosteginoides, the juvenile test moved from the apical position of Miogypsinoides to a a central position, and the adult test became conical (Hanzawa 1962). Some authors treat some or all of these genera as subgenera of Miogypsina (which, in the sense of Hanzawa 1962, had lateral chambers like Miolepidocyclina but an apical juvenile test like Miogypsinoides). Hanzawa (1964) treated them as separate but nevertheless derived both Miogypsina and Heterosteginoides from Miogypsinoides polyphyletically, evidently basing his classification on the overall shape of the adult test but his phylogeny on stratigraphy and the juvenile test. The Miogypsinidae as a whole became extinct in the middle Miocene.

REFERENCES

Bock, J. F. de. 1976. Studies on some Miogypsinoides-Miogypsina s.s. associations with special reference to morphological features. Scripta Geologica 36: 1-135.

Cole, W. S. 1939. Large Foraminifera from Guam. Journal of Paleontology 13 (2): 183-189.

Hanzawa, S. 1962. Upper Cretaceous and Tertiary three-layered larger Foraminifera and their allied forms. Micropaleontology 8 (2): 129-186.

Hanzawa, S. 1964. The phylomorphogeneses of the Tertiary foraminiferal families, Lepidocyclinidae and Miogypsinidae. Science Reports of the Tohoku University, second series, Geology 35 (3): 295-313.