''Hapsidopareion'' is an extinct

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

microsaur Microsauria is an extinct, possibly polyphyletic order of tetrapods from the late Carboniferous and early Permian periods. It is the most diverse and species-rich group of lepospondyls. Recently, Microsauria has been considered paraphyletic, as s ...

belonging to the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Hapsidopareiidae Hapsidopareiidae is an extinct family of microsaurs. Hapsidopareiids are known from the Early Permian of the United States and possibly Germany and the Czech Republic. Hapsidopareiids are characterized by a large temporal embayment near the chee ...

. Fossils have been found in the early Permian of Oklahoma.

History of study

''Hapsidopareion'' was named in 1973 by American paleontologist Eleanor Daly based on material collected from the early Permian South Grandfield locality in southwestern Oklahoma. The genus name is given for the Greek ''hapsido''- ('arch') and -''pareion'' ('cheek'). The species name, ''H. lepton'', is given for the slightness of the animal. The taxon is known from several partial to complete skulls and possibly by some isolated postcranial material.

Anatomy

''Hapsidopareion'' was originally differentiated from other microsaurs by the large temporal emargination, which produced other variable morphology of the circum-emargination bones (e.g., postorbital). It is similar to the more recently described '' Llistrofus pricei'' in this regard, but can be differentiated from ''L. pricei'' by features such as the absence of a quadratojugal and a frontal excluded from the orbit. Because all specimens of ''H. lepton'' are notably smaller than those of ''L. pricei'', and a number of anatomical differences are likely ontogenetically variable (e.g., contact between neural arch and centrum), some researchers considered that the former maybe represent a juvenile of the latter, but recent workers have maintained the separation of these taxa. Using high-resolution

micro-CT In radiography, X-ray microtomography uses X-rays to create cross-sections of a physical object that can be used to recreate a virtual model (3D model) without destroying the original object. It is similar to tomography and X-ray computed tomog ...

scans of three skulls of the coeval ''Hapsidopareion'', Jenkins et al. (2025) identified that many of the characters used to distinguish ''Llistrofus'' from it are shared by both taxa, documenting the presence of a quadratojugal in ''Hapsidopareion'' and demonstrating that the supposed exclusion of the frontal from the orbit was due to crushing in the holotype specimen. Due to its larger size, they identified ''Llistrofus'' as representing a more mature

ontogenetic Ontogeny (also ontogenesis) is the origination and development of an organism (both physical and psychological, e.g., moral development), usually from the time of fertilization of the egg to adult. The term can also be used to refer to the stu ...

stage of ''Hapsidopareion'', and argued that the former should be regarded as a

junior synonym In taxonomy, the scientific classification of living organisms, a synonym is an alternative scientific name for the accepted scientific name of a taxon. The botanical and zoological codes of nomenclature treat the concept of synonymy differently. ...

of the latter.

Relationships

''Hapsidopareion'' is the

sister taxon In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

to ''Llistrofus'' forms the family Hapsidopareiidae (traditionally called the 'Hapsidopareiontidae') within the

Recumbirostra Recumbirostra is a clade of tetrapods which lived during the Carboniferous and Permian periods. They are thought to have had a fossorial (burrowing) lifestyle and the group includes both short-bodied and long-bodied snake-like forms. At least one ...

. The family is recognized by the greatly enlarged temporal emargination. In their 2025 publication, Jenkins et al. discussed the possible synonymy of ''Hapsidopareion'' and ''Llistrofus''. The results of their phylogenetic analyses are displayed in the

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

below, where they found recumbirostrans to be a clade along the

amniote Amniotes are tetrapod vertebrate animals belonging to the clade Amniota, a large group that comprises the vast majority of living terrestrial animal, terrestrial and semiaquatic vertebrates. Amniotes evolution, evolved from amphibious Stem tet ...

stem Stem or STEM most commonly refers to: * Plant stem, a structural axis of a vascular plant * Stem group * Science, technology, engineering, and mathematics Stem or STEM can also refer to: Language and writing * Word stem, part of a word respon ...

References

{{Taxonbar, from=Q5653247 Microsauria Cisuralian tetrapods of North America