Shaped by evolution: geometric morphometrics and phenotypic divergence in Thrichomys inermis (Pictet, 1841) and Thrichomys aff. laurentius (Thomas, 1904) (Rodentia: Echimyidae) from northeastern Brazil

Variations in skull, mandible, scapula, and pelvis shape of Thrichomys inermis and Thrichomys aff. laurentius were explored using geometric morphometrics techniques. The shape variations of these bony structures of both species were compared in specimens from several localities in two states of northeastern Brazil. The results obtained revealed significant morphological differences between these species, evidence of a possible process of adaptive divergence. The main shape differences in the skull were observed in the parietal, maxilla, and nasal bones. In the mandibles, variation was concentrated in the posterior condylar region, the anterodorsal alveolar area, and the horizontal ramus. These differences suggest adaptations to different dietary regimes, possibly related to the availability of resources in their respective habitats. In the scapula, shape deformations were found in the coracoid process, infraglenoid tubercle, and the acromion and meta-acromion regions, indicating possible differences in locomotion and activity patterns, which could be linked to the exploitation of different ecological niches. Additionally, sexual dimorphism was detected in pelvis. In T. inermis, notable differences between males and females were observed in the iliac crest region, gluteal fossa, and pubic symphysis. In T. aff. laurentius, the main differences were found in the iliac crest and pubic symphysis region. These sexual differences suggesting morphological adaptations related to reproduction and locomotor function. The results obtained suggest that selective pressures imposed by ecological and evolutionary factors have shaped the morphological characteristics of these species, leading to a phenotypic differentiation that reflects their adaptation to different ecological niches.