Geometric morphometric methods achieve type specimen assignment in the cryptic Plagiochila arbuscula complex (Plagiochilaceae: Jungermanniopsida) with the minimum of morphological evidence

Relating type specimens to morphologically cryptic species is particularly vexing. When type specimens are small, fragmentary and sterile, resolution can seem intractable. In this study of the cryptic Plagiochila arbuscula complex, morphological evidence was reconsidered using geometric morphometric methods to gain insight into patterns of variation and differences in leaf shape between four phylogenetic lineages. The study had two aims: to achieve morphological circumscription of species and to attribute challenging type specimens to species. Considerable overlap in leaf shape among the four lineages was observed, but so were high levels of within-individual partitioning of leaf shape variation. Leaf shape exhibited a significant relationship with leaf size, and size was correlated with shoot order. The greatest leaf shape differences among species were manifest in leaves from primary shoots. In linear discriminant analysis, the highest cross-validation success rates for single leaves was achieved using primary shoot leaf shape and so for nine challenging type specimens, the shape of primary shoot leaves was used to assign them to species. Twelve of 15 sampled type leaves were attributed to a single species with high probability, in agreement with subjective assessment. In the Plagiochila arbuscula complex historical type specimens can be related to groups circumscribed by integrated molecular and morphological evidence using the minimum of morphological evidence: the shape of single leaves.