Involutinina, Hohenegger and Piller, 1977

Rigaud, Sylvain, Blau, Joachim, Martini, Rossana & Rettori, Roberto, 2015, Taxonomy, phylogeny, and functional morphology of the foraminiferal genus Involutina, Acta Palaeontologica Polonica 60 (1), pp. 235-244 : 236

The Involutinina classification: its origin and limitations

The suborder Involutinina Hohenegger and Piller, 1977 unites tubular foraminifers with an aragonitic wall structure. Piller (1978) and di Bari and Laghi (1994) have defined two major models for the Involutinina mode of test construction. In the “ Triadodiscus model”, the laminar deposits or first order lamellae (L1 lamellae sensu Piller 1978) are discontinuous but would form by stacking one continuous laminar extension or second order lamella (L2 lamella sensu Piller 1978) per whorl. In the “ Aulotortus model”, the L1 lamellae are continuous but are laterally tapered such that they form two distinct L2 lamellae per whorl (or one L2 lamella per half whorl), which are successively interfingered in the umbilical region. A third model was proposed by Piller (1978, 1983) for Involutina and Trocholina . This model is close to that proposed by di Bari and Laghi (1994) for Triadodiscus and can be considered as a variant of the “ Triadodiscus model”, like the “ Lamelliconus ” and “ Prorakusia ” submodels of di Bari and Laghi (1994).

The aim of this paper is not to discuss these two major models. However, it has to be noted that the reliability of each model has not been irrefutably proved. The distinction between the two modes of test construction results from the observation of a small number of very well-preserved specimens. As inadequately oriented sections can be misleading for the observer (see Piller 1983: fig. 4), a model established only on a few specimens is unreliable. In addition, the correspondence between the “ Triadodiscus model” ( di Bari and Laghi 1994: pl. 4: 2) and the associated high-quality illustrations of Triadodiscus ( di Bari and Laghi 1994: pl. 3: 4; pl. 4:. 1) is questionable. Finally, in specimens known to represent the “ Aulotortus model” (e.g., Aulotortus , Coronipora , Frentzenella ), the diagenesis often obliterates the lamellae, giving the impression that only one L2 lamella is formed per whorl. This diagenetic alteration particularly affects the L1 lamellae, which are especially minute and discontinuous in Triadodiscus .

The present high rank Involutinina systematic subdivision is partly based on these two major models ( Zaninetti 1984; Zaninetti et al. 1987; Loeblich and Tappan 1987). Nevertheless, because of preservation problems, architectural models have been identified in only few forms. The suborder is typified by the genus Involutina in which the lamellae arrangement remains uncertain, entailing confusion in the lineage classification.Actually, the systematic position of the involutinins is for the most part hypothetic and phylogenetic links between different lineages are speculative. In spite of that, in the latest proposed phylogenetic tree ( di Bari and Laghi 1994: fig. 7), the majority of Involutinina taxa, Involutina included, have been postulated to have originated from Triadodiscus (“ Triadodiscus model”).