- © 2008 Journal of Foraminiferal Research
Scanning electron microscopy of foraminifera from the lowermost Barremian (Lower Cretaceous) deposits at Speeton, eastern England, reveals that about 4% of the agglutinated tests are constructed almost exclusively of coccoliths of the species Watznaueria barnesae. The majority of the foraminifera that agglutinate coccoliths are trochospiral forms resembling Trochammina depressa, but planispiral, unilocular, uniserial, biserial and triserial forms have also been observed. The tests are small, rarely exceeding 125 μm in length. Specific identification is difficult due to the small size of the tests, the unusual building material and compression.
In the simplest state, the walls of the trochamminids consist of a single layer of coccoliths. The foraminifera were highly efficient in selecting and handling the coccoliths, which were always placed with their distal surfaces facing outward. The outline of the coccoliths was modified from oval to hexagonal, probably by a dissolution process, apparently in order to fit the coccoliths closer together and to diminish the amount of empty spaces between them.
On larger tests, a secondary layer of cement coated the layer of coccoliths. Today the cement consists of silica, but it was probably originally organic. The silica contains a few grains of mixed sedimentary and skeletal origin. Most of the grains in the cement-rich layer are coccoliths, but in contrast to the inner coccolith layer, which was constructed exclusively of W. barnesae, the outer layer contains a large number of forms apparently picked at random among the species available on the sea bottom. A second dissolution process affected 10–20% of the tests by attacking all the crystals of the coccoliths, not just their edges as in the primary layer. The dissolution resulted sometimes in the almost total destruction of the coccoliths. In most tests, the dissolution occurred post mortem. However, in a few cases the dissolution took place while the protists were still alive. In many of the serial species, the coccoliths interlock as in coccospheres, producing similar globular structures. In a triserial species, the tests show a distinct ontogenetic shift from an early part constructed mainly of coccoliths to a later part constructed of sedimentary grains and silica cement.