In view of their propositional content (i.e. they can be right or wrong), character statements (i.e. statements that predicate characters of organisms) are treated as low-level hypotheses. The thesis of the present study is that such character statements, as do more complex scientific theories, come with variable scope. The scope of a hypothesis, or theory, is the domain of discourse over which the hypothesis, or theory, ranges. A character statement is initially introduced within the context of a certain domain of discourse that is defined by the scale of the initial phylogenetic analysis. The doctrine of ‘total evidence’ requires the inclusion of previously introduced characters in subsequent studies. As a consequence, the initial scope of character statements is widened to the extent that the scale of subsequent analyses is broadened. Scope expansion for character statements may result in incomplete characters, in the subdivision of characters, or in ambiguity of reference (indeterminacy of the extension of anatomical terms). Character statements with a wide scope are desirable because they refer to characters with the potential to resolve deep nodes in phylogenetic analyses. Care must be taken to preserve referential unambiguity of anatomical terms if the originally restricted scope of a character statement is expanded to match a broad-scale phylogenetic analysis.

The Hapsidopareiontidae are microsaurian amphibians known only from the Lower Permian of the U.S. The hapsidopareiontid skull has a large temporal embayment that occupies most of the area of the cheek, in which the quadratojugal is either absent (Hapsidopareion) or present and greatly reduced (Llistrofus). the unique holotype skull of Llistrofus pricei is well preserved in soft clay. It was originally prepared and described in dorsal view only, but has now been prepared completely free of the matrix. This permitted the first description of the palate, occiput and mandible along with a redescription of the rest of the skull, resulting in substantial emendation of previously published descriptions. Comparison with the eight known complete and partial skulls of Hapsidopareion lepton demonstrates that the two genera are much more similar than had been recognized earlier. Their continued separation is advocated, however, based on size difference, a small number of osteological differences, and the difficulty of obtaining comparably detailed morphological information for Hapsidopareion, which is less well preserved than the Llistrofus specimen. the new morphological information, combined with results of recently published cladistic analyses, suggests that Hapsidopareion and Llistrofus are the only microsaurs that can at present be assigned to the Hapsidopareiontidae. the large temporal embayment of hapsidopareiontids might be thought to have significant functional implications, possibly including “escape” of jaw adductor musculature onto the skull roof. Detailed consideration of this possibility reveals morphological evidence to the contrary, however, and the functional implications of the embayment remain uncertain.