ABSTRACT: Two landmark-based morphometric procedures were used to analyze asymmetry in P1 elements of Idiognathodus magnificus Stauffer and Plummer 1932 from the Upper Pennsylvanian Beeman Formation of the Sacramento Mountains in NewMexico. The first procedure, named the PLA(platform landmark analysis), uses landmarks on platforms and adcarinal ridges of Idiognathodus P1 elements to analyze morphological variation in platform shape. PLA allows comparison between most species of Idiognathodus and Streptognathodus, whether lobes or grooves are present or absent, and is the ‘base protocol’formorphometric analysis of these genera. The second procedure, named BLA(bi-lobed landmark analysis), supplements PLAby the addition of landmark points that are defined by lobe shape and position. BLA better describes morphological variation in two-lobed P1 elements, and as the name implies, it is only used on two-lobed elements. Thesemorphometric procedures show that I. magnificus has the same pattern of P1 element asymmetry as seen in the I. simulator group, where the rostral margin is wider on the ventral end of the dextral element than it is on the sinistral element. Landmark analyses of I.magnificus, together with specimen abundances, demonstrate that there are two morphotypes of I.magnificuswith asymmetrical P1 element pairs that can be diagnosed on the basis of lobe variation and platform shape. One morphotype pair has more elongate rostral lobes on both P1 elements than the other morphotype pair. PLA distinguished specimens of I. magnificus from specimens of the I. simulator group, the former having longer adcarinal ridges and more symmetrical platform margins. The robust applicability of PLA serves as a ‘base protocol’ for studying morphological variability between different species of Idiognathodus.