In 1969 paleobotanist Rafael Herbst discovered the partial skeleton of a small reptile in sandstone of the Upper Jurassic Vaca Muerta Formation in Mendoza Province, Argentina. Rodolfo Casamiquela described the skeleton in 1974, naming it Herbstosaurus pigmaeus and considering it a compsognathid theropod. In a 1981 anatomical review of Compsognathus John Ostrom pointed out that Herbstosaurus is not a theropod, but clearly a pterosaur.
The specimen consists of the sacrum, most of the left ilium, part of the right ilium, the prepubis, and most of the femora. The bones are articulated and lie on the surface of a sandstone block about 10 cm square. The sacrum is made up of at least five vertebrae and are preserved in dorsal view. The ilia are long and narrow with a rectangular preacetabular process, a feature found in pterosaurs and not theropods. The prepubis has a narrow and rod-like proximal end that widens into a broad fan. This element was initially identified by Casamiquela as an ischium. The femora are both about 9 cm long and have distinctive bowing.
Because Herbstosaurus is so fragmentary, it is difficult to determine what pterosaur lineage it belongs to. Five or more sacral vertebrae are present in short-tailed pterodactyloids rather than their long-tailed ancestors, so it is likely Herbstosaurus is some kind of pterodactyloid. In 1996 David Unwin, pointing to features of the femur suggested that it may represent an early dsungaripteroid similar to Germanodactylus. If it was indeed similar to Germanodactylus, its wingspan would be about 1.2 meters (4 feet). Without any skull material it’s impossible to say anything about its diet.
The Vaca Muerta Formation was formed roughly 145 million years ago and represents a shallow marine environment off the west coast of ancient South America. The only known individual of Herbstosaurus may have died while flying or fishing over the ocean, or could have been washed out to sea after dying near a river.