''Acroplous'' is an extinct

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

dvinosauria Dvinosaurs are one of several new clades of Temnospondyl amphibians named in the phylogenetic review of the group by Yates and Warren 2000. They represent a group of primitive semi-aquatic to completely aquatic amphibians, and are known from t ...

Temnospondyli Temnospondyli (from Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') is a diverse order of small to giant tetrapods—often considered primitive amphibians—that flourished worldwide during the Carb ...

within the

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

Eobrachyopidae Eobrachyopidae is a family of dvinosaurian temnospondyl Temnospondyli (from Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') is a diverse order of small to giant tetrapods—often considered primi ...

History of study

''Acroplous'' was described by Nicholas Hotton in 1959 for the type species, ''A. vorax''. The type locality is in Riley County,

Kansas Kansas () is a state in the Midwestern United States. Its capital is Topeka, and its largest city is Wichita. Kansas is a landlocked state bordered by Nebraska to the north; Missouri to the east; Oklahoma to the south; and Colorado to th ...

within the

Speiser Shale The Speiser Shale or Speiser Formation is a geologic formation in Kansas, Oklahoma, and Nebraska dating to the early Permian period. See also * List of fossiliferous stratigraphic units in Kansas * Paleontology in Kansas Paleontology in Kansas ...

( early Permian). The original description only described the

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of sever ...

(KUVP 9822), a partially disarticulated skull with isolated, associated postcrania. The genus name comes from the Greek word for 'swimming at the top,' based on Hotton's inference of the animal as being a surface cruising animal. The species name comes from the inferred voracity of the taxon. Hotton suggested that some material from the Dunkard Group of

Pennsylvania Pennsylvania (; ( Pennsylvania Dutch: )), officially the Commonwealth of Pennsylvania, is a state spanning the Mid-Atlantic, Northeastern, Appalachian, and Great Lakes regions of the United States. It borders Delaware to its southeast, ...

that had been previously described by Romer (1952) as Saurerpeton obtusum''' might represent an Appalachian occurrence of ''Acroplous''. However, this material is not considered to belong to either ''Acroplous'' or to ''

Isodectes ''Isodectes'' is an extinct genus of dvinosaurian temnospondyl within the family Eobrachyopidae. The genus ''Saurerpeton'', named in 1909, is considered to be a junior synonym The Botanical and Zoological Codes of nomenclature treat the concep ...

'', the senior synonym of ''Saurerpeton,'' and instead has been listed as merely Dvinosauroidea incertae sedis by Schoch & Milner (2014). The type locality was reopened in 1971 by Orville Bonner and Larry Martin at the University of Kansas, which led to the discovery of additional vertebrate material, including a new specimen of ''Acroplous'' that was described by Coldiron (1978). Like the holotype, this specimen also consisted of a partially disarticulated skull with isolated, associated postcrania. Coldiron also presented one of the earliest phylogenies of Dvinosauria (see also for discussion of relationships), but these early analyses in general were compromised by the perception of the time that brachyopoids were closely related to dvinosaurs, whereas this has not been supported by more recent cladistic analyses. Nonetheless, these workers were correct in identifying many similarities with ''Isodectes,'' which is currently the only other eobrachyopid. Additional material of ''Acroplous'' was discovered from another locality in the Speiser Shale about 40 miles from the type locality in Wabaunsee County, Kansas in 1976. This material was described by Foreman (1990), who also noted additional, previously undescribed material from the type locality. The specimens described by Foreman were the most completely known skulls, which permitted a complete cranial reconstruction. Foreman continued the framework in which ''Acroplous'' was placed in the same family as ''Isodectes'' (Saurerpetontidae, now a junior synonym of Eobrachyopidae following synonymy of ''Saurerpeton'' with ''Isodectes''). Most recently, Englehorn et al. (2008) described new material of ''Acroplous'' from several localities in Eskridge Formation exposures in Richardson County, Nebraska as well as another specimen from the type locality that had been collected by Hotton in the 1960s. The Nebraska exposures (the "Humboldt localities") are thought to be

Asselian In the geologic timescale, the Asselian is the earliest geochronologic age or lowermost chronostratigraphic stage of the Permian. It is a subdivision of the Cisuralian Epoch or Series. The Asselian lasted between and million years ago (Ma) ...

in age and thus older than the horizons from the Speiser Shale. This description provided additional information on the skull based on the most completely known specimens and the most detailed information on the lower jaw. By 2008, the notion that dvinosaurs and brachyopoids were closely related had fallen out of favor, and thus Englehorn et al.'s phylogenetic analysis only sampled other Paleozoic taxa, in which they recovered ''Acroplous'' as closely related to ''Isodectes''.

Anatomy

''Acroplous'' is diagnosed by several

autapomorphies In phylogenetics, an autapomorphy is a distinctive feature, known as a derived trait, that is unique to a given taxon. That is, it is found only in one taxon, but not found in any others or outgroup taxa, not even those most closely related to t ...

(among dvinosaurs), including an internarial fenestra between the premaxillae; anteroposteriorly shortened postparietals; and interlocking mandibular symphyses formed by medially projecting prongs. Compared to ''Isodectes'', it has a proportionately broader skull (wider than long), lacks an intertemporal, and has less developed lateral line grooves. Notably, the most complete specimens of ''Acroplous'' are smaller than those represented by more fragmentary or isolated cranial remains, which suggests that previous descriptions have been based on sub-adult or juvenile material.

Phylogeny

Historically, ''Acroplous'' has been placed in the same family as ''Isodectes'' (and associated junior synonyms with ''Saurerpeton''). However, this family has sometimes been recovered as paraphyletic within the broader Dvinosauroidea. Below is the phylogeny from Marsicano et al. (2021):

References

{{Taxonbar, from=Q4676400 Dvinosaurs Permian temnospondyls Permian temnospondyls of North America Fossils of the United States