For today's post, I'm presenting for your consideration the Osangulariidae, a family within the rotaliid Foraminifera (see here for my post introducing the Rotaliida). These are benthic forams, mostly found in intermediate waters within the top few centimetres of sea floor sediment (Kaiho 1998). The Osangulariidae were first established as a distinct family of Foraminifera by Loeblich & Tappan (1964) to include trochospiral forams with bilamellar walls, with an important distinguishing feature separating osangulariids from related families being their granular rather than radial test wall structure. However, Loeblich & Tappan were criticised by Kaiho (1998) for their utilisation of this character. In developing a more lineage-based classification of the osangulariids and related taxa, Kaiho concluded that "radial-granular texture has no taxonomic significance in the suprageneric classification of calcareous trochospiral benthic foraminifera". Instead, Kaiho defined the Osangulariidae as trochospiral forams with an aperture on the umbilical side of the test, an angular periphery and strongly oblique sutures on the spiral side.
The Osangulariidae first appeared in the Early Cretaceous, during the Aptian epoch. Kaiho (1998) recognised two subfamilies within the Osangulariidae, the Osangulariinae and the Globorotalitinae (not to be confused with the Globorotaliinae), regarding the slightly earlier-appearing Globorotalitinae as probably ancestral to the Osangulariinae. The Globorotalitinae possessed a test with a strongly inflated umbilical side, and can basically be described as looking like a jelly mould. Osangulariids of the globorotalitine type became extinct during the Palaeocene.
The Osangulariinae, on the other hand, have survived to the present day. Their most obvious distinction from the Globorotalitinae is the reduction of the umbilical side of the test, so that osangulariines tend to be more discus-shaped than jelly-mould-shaped. The earliest osangulariine genus, Protosangularia, appeared in the Aptian and survived until the Cenomanian in the early Late Cretaceous. At the end of the Cenomanian, a major anoxic event took place in the ocean followed by a reduction in world ocean temperatures. After this, Protosangularia was replaced by a number of other osangulariine genera appearing from the Turonian to the early Campanian (Kaiho 1998). Two of these genera, Osangularia and Nuttallides, are the family's modern representatives.
Kaiho, K. 1998. Phylogeny of deep-sea calcareous trochospiral benthic Foraminifera: evolution and diversification. Micropaleontology 44 (3): 291-311.
Loeblich, A. R., Jr & H. Tappan. 1964. Treatise on Invertebrate Paleontology pt C. Protista 2. Sarcodina. Chiefly "thecamoebians" and Foraminiferida vol. 2. The Geological Society of America and The University of Kansas Press.