The earliest discoveries of Lentiarenium date to the early 19th century, with a mandible, ribs, vertebrae and molars being found in sandpits of the city of Linz.

Following communication between several researchers across multiple Austrian institutions, the remains were correctly identified as belonging to an extinct species of seacow by paleontologist Leopold Fitzinger, who would go on to describe the material in 1842.

Fitzinger believed the bones to belong to the sirenian Halitherium, described by Johann Jakob Kaup just four years prior.

Additionally, the stability of Halitherium itself is questionable, as its type species, H. schinzii, was based on a single isolated tooth found to have no diagnostic value, thus rendering the genus a nomen dubium.

[8] As part of this spike in research; Voss, Berning and Reiter published their examination of the upper Austrian seacow material in 2016, with the express goal to test Domning's synonymisation of H. christolli, H. abeli and H. pergense.

As the type species was described on a series of syntypes, Voss and colleagues chose OLL 2012/1, a complete mandible, as the genus' lectotype.

Like the frontal, the parietal is flat between the crests with a marked constriction that reaches its strongest point just behind the center of the skull roof.

This differs from species previously combined under Halitherium schinzii, which only have a weakly concave mandibular symphysis prior to the downturn.

[9] Phylogenetic analysis have repeatedly shown that Lentiarenium was a derived sirenian compared to other Oligocene and Eocene taxa, but basal to the clade that contains modern dugongs, hydrodamalines, the various Metaxytherium species and Caribosiren.

Although Voss and colleagues deem this less likely given the fossil record (as both taxa are from the West Atlantic), it still shows that the genus is neither close to Metaxytherium nor does it nest with other species and genera previously lumped together in Halitherium.