This genus and species is represented by MML-PV-42, a well-preserved right maxillary bone (including 10 teeth of varying completeness).

This bone was found at the Cerro Tortuga site near Lamarque, Argentina, which preserves fossils from the Cretaceous Allen Formation.

These wear facets indicate that Lamarquesaurus was an advanced sphenodont capable of propalinal (front-to-back) jaw movement.

Although they do not closely resemble the wide, rectangular teeth of eilenodontine opisthodonts, there are no qualities which preclude a relationship between Lamarquesaurus and basal opisthodonts such as Opisthias, or alternatively members of the family Sphenodontidae such as the modern tuatara (Sphenodon).

Lamarquesaurus's discovery bolsters the theory that eupropalinal sphenodonts continued to dominate the "small reptile" niche of South American ecosystems during the Cretaceous and early Cenozoic, while other sphenodonts went extinct and were replaced by lizards in the Northern Hemisphere.