the presence of strong tubercles on flank costae, but they differ
from it in having an area surface barely differentiated from the
flank. While Steinmanella is more similar to Myophorella s.s. in
shell outline (Fig. 1(J)), Quadratotrigonia is more quadrate, being
truncated at both anterior and posterior ends.
This general evolutionary scheme was followed later on by
many authors (Levy, 1966; Pe´ rez and Reyes, 1977; Fleming, 1987;
H. Leanza and Garate-Zubillaga, 1987; Cooper, 1991; H. Leanza,
1993; Francis and Hallam, 2003) although sometimes the taxa
were grouped under different generic names and some species
were variably referred to different genera. As a result, the
Myophorellidae remains as one of the unresolved clades within
Trigoniida (Schneider and Kelly, 2014).