Llistrofus is an extinct genus of early Permian microsaur within the family Hapsidopareiidae that is known from Oklahoma.
Llistrofus was described by Canadian paleontologists Robert Carroll and Pamela Gaskill in 1978.[1] The genus name is an anagram of Fort Sill, the historic name of the Richards Spur locality from which material of Llistrofus was collected. The species name, L. pricei, is given for the Brazilian paleontologist Llewellyn Price. The holotype of this taxon is currently reposited at the Field Museum of Natural History. The skull of the holotype was reappraised by Bolt & Rieppel in 2009.[2] New material from Richards Spur was described by Gee et al. (2019).[3]
Llistrofus is readily identified by the presence of a large temporal emargination, a feature shared with Hapsidopareion lepton that unites the Hapsidopareiidae. Carroll & Gaskill (1978) differentiated Llistrofus from Hapsidopareion by four features: (1) postorbital contacting the tabular; (2) postparietal contacting the squamosal; (3) frontals entering the orbit; and (4) presence of a quadratojugal. Bolt & Rieppel (2009) revised the diagnosis following additional preparation of the holotype's skull and list three features (only one of which is shared with the previous work): (1) skull length approximately twice that of Hapsidopareion; (2) frontals entering the orbit; and (3) cultriform process of the parasphenoid offset from the basal plate by a narrow base. Gee et al. (2019) further revised the diagnosis based on additional material from Richards Spur and listed 11 features: (1) frontals entering the orbit; (2) prefrontal entering the naris; (3) premaxilla contributing to ventral narial margin; (4) postfrontal excluded from the temporal emargination; (5) postorbital contacting the tabular; (6) denticles on the vomer; (7) teeth on the palatine smaller than the marginal teeth; (8) absence of a pterygoid-premaxilla contact; (9) a splenial that contributes to the symphysis; (10) presence of a Meckelian foramen; and (11) presence of a retroarticular process.
Llistrofus is recovered as the sister taxon to Hapsidopareion in phylogenetic analyses, producing the Hapsidopareiidae. A 2022 phylogenetic analysis recovered it within Recumbirostra.[4]