On an unknown lamprey holotype (Petromyzon macrostomus Burmeister 1868) and the genus Exomegas Gill 1883 from South America

Abstract

The taxonomy of the South American genus Exomegas Gill 1883 has been unstable owing to the unknown status of the type specimen of its type species, Petromyzon macrostomus Burmeister 1868. Here the authors announce the finding of the holotype in the Museo Argentino de Ciencias Naturales "Bernardino Rivadavia" ichthyology collection. Whereas Exomegas type species is recognized based on its holotype, Exomegas gallegensis (Smitt 1901) new status is proposed. These preliminary results resolve a century-long taxonomic uncertainty and foster a nomenclatural arrangement for a Southern Hemisphere lamprey genus.

Identification of long‐preserved specimens reveals the historical geographic range of the Patagonian lamprey Geotria macrostoma (Burmeister, 1868) in southern South America

“…The results of this study align with the recent taxonomic reclassification of G. macrostoma, reverting it to its earliest valid designation in RivaRossi et al (2020). The study provides strong evidence to reject the taxonomic separation proposed by FirpoLacoste et al (2021) between lampreys from de la Plata River and those collected in Patagonian drainages. Additionally, this study highlights the value of often undervalued ichthyological collections, particularly those containing holotypes, in resolving taxonomic disputes and improving our knowledge of biodiversity.…”

“…Following Riva Rossi et al (2020) and Baker et al (2021), Firpo Lacoste et al (2021) proposed a new taxonomy for the Argentinian lamprey based on the examination of century‐old specimens from the ichthyological collection of the Museo Argentino de Ciencias Naturales “Bernardino Rivadavia” (MACN, Buenos Aires). They resurrected the genus Exomegas Gill, 1883 to reclassify Argentinian lampreys under the assumption that this genus was distinct from Geotria Gray, 1851, since the former had three TLL cusps rather than two as in the latter.…”

“…They resurrected the genus Exomegas Gill, 1883 to reclassify Argentinian lampreys under the assumption that this genus was distinct from Geotria Gray, 1851, since the former had three TLL cusps rather than two as in the latter. This led Firpo Lacoste et al (2021) to name specimens collected during the 19th century from the de la Plata River (Berg, 1895; Burmeister, 1868) as Exomegas macrostomus Gill, 1883 and those from Patagonian drainages as E. gallegensis (Smitt, 1901). It thus elevated Geotria macrostoma f. gallegensis Smitt, 1901, to the species rank without providing distinguishing diagnostic characters.…”

“…It thus elevated Geotria macrostoma f. gallegensis Smitt, 1901, to the species rank without providing distinguishing diagnostic characters. The proposal of Firpo Lacoste et al (2021) to allocate lampreys from Argentina under Exomegas was subsequently refuted by Riva Rossi et al (2022), who clearly demonstrated that the three TLL cups were reduced to two during adult life, which is a key a diagnostic character of Geotria .…”

“…It is important to recall that the collection in 1867 of the holotype of P. macrostomus Burmeister, 1868 predates by 28 years the creation of the Ichthyology Division of MACN in 1895(Chiaramonte et al, 2015) and that Hermann Burmeister passed away in 1892, 3 years earlier. MACN-Ict 12711 is definitely a century-old specimen as it was found in a methanol-filled paraffinsealed glass flask(Firpo Lacoste et al, 2021) and according to the…”