Mazama tschudii (Wagner, 1855), forgotten by science, re-emerges as a new genetic lineage of Neotropical deer with a proposed neotype (Artiodactyla, Cervidae)

The accurate classification of Neotropical deer is essential for effective conservation strategies; however, many species within the genus <i>Mazama</i> remain taxonomically unresolved due to morphological similarities and historical uncertainties. <i>Mazama tschudii</i>, originally described from the western Andes of Peru, has been debated due to the absence of a designated type specimen and its resemblance to other brocket species. This study integrates morphological, cytogenetic, and molecular data to clarify its taxonomic status, designate a neotype, and highlight its significance for conservation. A recently collected specimen from La Ramada, Lambayeque, Peru, matched the original description of Wagner (1855), exhibiting a smaller size, darker pelage, and distinct cranial features compared to closely related species such as <i>Mazama americana</i>, <i>M. temama</i>, and gray brockets (<i>Subulo gouazoubira</i>, <i>Passalites nemorivagus</i>). Cytogenetic analysis revealed a diploid number of 2n = 42 and a fundamental number (FN) of 68, with extensive chromosomal rearrangements that distinguish it from other <i>Mazama</i> species and suggest reproductive isolation. Mitogenome analysis placed <i>M. tschudii</i> as a sister species to <i>M. temama</i>, yet distinct from <i>M. americana</i>. Despite this close molecular relationship with <i>M. temama</i>, chromosomal divergence and BAC-FISH results demonstrated independent evolutionary trajectories, with ten centric and four tandem fusions differentiating their karyotypes. These results provide robust chromosomal and genomic evidence to validate <i>M. tschudii</i> as a distinct species under the biological species concept. The formal neotype designation from its historical type locality establishes a definitive taxonomic reference, contributing critical insights into the evolutionary complexity of Neotropical deer and reinforcing the importance of integrative taxonomy in shaping conservation priorities.