Accessing the challenges of descriptive morphology in Barychelidae and Theraphosidae through a morphometric approach (Araneae, Mygalomorphae)

Phylogenetic studies have confirmed that Barychelidae is a sister group of Theraphosidae. Taxonomic instability has led to the transfer of multiple genera between the two families as both families show overlapping diagnostic characteristics, indicating thus a similarity, making species differentiation challenging. In this study, we conducted an evaluation of three morphological characters that are commonly used to distinguish Barychelidae and Theraphosidae. We employed geometric morphometrics to analyze variations in the shapes of the maxilla, labium, and the eye arrangement. Our results revealed that these characters show continuous variation, which can make it problematic for discrete categorization and differentiation between Barychelidae and Theraphosidae. However, our analysis showed that the shape of the eye arrangement is effective in distinguishing the subfamilies within Barychelidae. We found notable distinctions between these characters in the Neotropical Sasoninae genera compared to the Sason Simon, 1887. Therefore, the present study highlights the importance of incorporation of morphometric analyses into taxonomic and cladistics studies to improve the understanding of distinctions between families that are considered as sister groups.