P-1 element morphological variability within the Late Pennsylvanian asymmetrical Idiognathodus clade from the Midcontinent, USA and implications for ontogeny, taxonomy, phylogeny, and function

摘要

Missourian-Virgilian asymmetrical Idiognathodus represent a distinct clade of species with strong dextral-sinistral P-1 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 P-1 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.