It is known from the holotype UFRGS PV 1027 T, a partial left dentary (lower jaw bone) found in what is now Faxinal do Soturno, Rio Grande do Sul, southern Brazil, in the geopark Paleorrota.

Cargninia was named by José Fernando Bonaparte, César Leandro Schultz, Marina Bento Soares and Agustín G. Martinelli in 2010.

[1] The preserved dentary has small, peg-like teeth, with the underlying jaw bone about four times deeper than the height of each tooth crown.

However, during preparation the front half of the fossil was destroyed, leaving only the rear part of the jaw with five preserved teeth and one or two empty spaces.

[2] Several additional pleuroacrodont jaw fragments are known from Faxinal do Soturno, though a lack of overlap prevents an unambiguous referral to Cargninia.