P1 element morphological variability within the Late Pennsylvanian asymmetrical Idiognathodus clade from the Midcontinent, U.S.A. and implications for ontogeny, taxonomy, phylogeny, and function
Abstract Missourian-Virgilian asymmetrical Idiognathodus represent a distinct clade of species with strong dextral-sinistral P1 element asymmetry that exhibit a series of morphological events before the Late Pennsylvanian extinction of Idiognathodus. Previous taxonomic work and morphometric analyses were completed separately for the three major groups of the clade. From oldest to youngest, these are the I. magnificus, I. eudoraensis, and I. simulator groups. The dextral-sinistral P1 element asymmetry is more apparent in larger specimens and the asymmetry is caused by ontogenetic distortion of the dextral element, which widens in the ventral part of the rostral side. At smaller sizes, both sinistral and dextral elements grow predominately dorsal-ventral, and at larger sizes growth is predominately rostral-caudal. Lobe groups correspond with different platform shapes, suggesting that lobe groups are useful taxonomic identification criterion for classifying morphologically distinct species. There are four major morphological events that occur within the asymmetrical Idiognathodus clade: 1) the development of simulator-style P1 element asymmetry in the I. magnificus group; 2) the replacement of flat ungrooved platforms in the I. magnificus group with the grooved platforms of the I. eudoraensis group; 3) the shortening of the adcarinal ridges and development of stronger dextral-sinistral element P1 asymmetry in the I. simulator group; and 4) the replacement of the asymmetric Idiognathodus clade with the I. tersus group which have grooveless narrow platforms, long adcarinal ridges and no apparent dextral-sinistral P1 element asymmetry. These morphological events likely reflect significant biological and paleoecological changes that ultimately culminate with the extinction of Idiognathodus.