Fusulinid Foraminifera are important for biostratigraphic correlation of strata in the Upper Paleozoic of the central and southwestern United States due to their high abundance and diversity in shallow marine carbonates. These correlations rely on consistent species-level identifications as well as geographically consistent morphological variation within species. However, many taxonomically important characters are described qualitatively, which can lead to conflicting identifications among taxonomists and cause biostratigraphic disagreements. Quantitative morphometric comparisons among species can increase the consistency of identifications among workers and capture temporal and geographic morphological gradients. Here, we used 14 linear measurements to describe the morphology of 18 biostratigraphically important species within the genus Triticites. Canonical variates analysis (CVA) showed that whereas specimens of a given species occupy similar areas of morphospace, species overlap considerably and, thus, these linear measurements poorly differentiate among species. However, species morphology covaried with geological age and multivariate analyses focused on age differences reveal an increase in the size of the initial chamber (proloculus) and a relative decrease in test expansion along the long axes over time. These morphological changes are consistent with an adaptation to shallow water conditions following the fall in relative sea-level across the Virgilian-Newwellian/Bursumian transition. Given the consistency of the morphological shift among North American basins, these morphological changes may be more useful for the recognition of the Virgilian-Newwellian/Bursumian transition than species-level biozones because they do not depend on consistent species identification.

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