Limnoscelis (/limˈnäsələ̇s/, meaning "marsh footed") was a genus of large diadectomorph tetrapods from the Late Carboniferous to early Permian of western North America.

[5] Limnoscelis had a combination of derived amphibian and primitive reptilian features,[6] and its placement relative to Amniota has significant implications regarding the origins of the first amniotes.

[7][8] The type species Limnoscelis paludis was collected by the fossil hunter David Baldwin between 1877 and 1880[1][9] from the El Cobre Canyon beds[10] of the Cutler Formation, New Mexico.

[1][12] Although Marsh would describe several specimens from Baldwin's collections,[9] many fossils, including Limnoscelis paludis, would be deposited without description at the Peabody Museum of Natural History at Yale College for several decades.

[1] Limnoscelis paludis was finally described in 1911 by the paleontologist Samuel Wendell Williston, who discovered three specimens of the genus in the Yale Peabody Museum collections.

[1][2] Limnoscelis had particularly well-developed incisors,[1][6] peaking in size at the anterior maxilla, similar to the placement of the canine tooth of many derived synapsids.

[5] The vertebrae of Limnoscelis were typically longer than they were wide,[5] but varied in size and shape throughout the vertebral column,[2][5][18] along with neural spine height.

[5] Originally, it was thought that Limnoscelis possessed two proximal tarsals, consisting of the fibulare and a preaxial element comprising a fused tibiale and intermedium.

[1] However, subsequent analyses have cast doubt on this assessment, instead proposing that the two preserved proximal tarsals are the fibulare and intermedium, and that Limnoscelis possessed an unfused tibiale along with these elements.

[2] In its earliest descriptions, Limnoscelis was identified as an early reptile, thought to be closely related to the Captorhinidae or Pareiasauridae based on its flat occiput,[1] as well as its large upper incisors and broad parareptile-like neural arches.

[24] These early descriptions framed Limnoscelis as a member of the paraphyletic group Captorhinomorpha within Cotylosauria, alongside the clades Diadectomorpha and Seymouriamorpha.

[23] This monophyletic grouping of Diadectomorpha is supported by the anterior processes of the atlas and axis intercentra, and the presence of an external iliac shelf,[5][26] features that are shared by all diadectomorphs.

[26] Within the Diadectomorpha, Limnoscelis is often found to be sister to Diadectidae and Tseajaia, with the later clades forming a monophyletic group in many cladistic analyses.

[6] Limnoscelis possessed several reptilian cranial homologies, including the closure of the otic notch and the development of a pterygoid flange on the palatal surface, while retaining a generalized amphibian-like postcranial morphology.

[27] The below cladogram, modified from Laurin and Reisz (1995), showing Limnoscelis and the Diadectomorpha sister to Amniota,[7] agreeing with the original placement from Heaton (1980).

[4] Alfred Sherwood Romer suggested that this might be a retention of an ancestral semiaquatic lifestyle found in amphibians, which might have also been retained in some early pelycosaurs.

[6] However, other studies have suggested a significantly more terrestrial lifestyle for Limnoscelis, based on relatively well-ossified portions of its post-cranial skeleton.

[5] Despite its long conical teeth indicating a carnivorous diet,[1] Williston doubted that Limnoscelis could have been a predator, as he believed its short, robust limbs made it too slow to pursue prey.

[4] This site was originally thought to be early Permian in age,[9] though later studies concluded that the lower beds of the formation were actually from the Late Carboniferous based on biostratigraphy using the brachiopod Anthracospirifer rockymontanus.

[10] However, dating of these lower beds to the late Pennsylvanian was initially found to be dubious based on inconsistencies with the stratigraphic placement of the fossils used for biostratigraphy.

[36] An early Permian age again fell into favor, based on faunal similarities with the Arroyo del Agua beds of the Cutler Formation.

[11] The El Cobre Canyon formation is thought to represent an alluvial plane, with a single-channel meandering river in a semi-arid environment,[3] being one of the earliest representations of a terrestrial fauna.

[11] Also sharing the landscape were several amphibians, including Seymouria sanjuanensis,[38] and the temnospondyls Eryops grandis,[1][9][12] Platyhystrix rugosus,[1][11][12] Aspidosaurus novomexicanus,[1][11][12] and Chenoprosopus milleri.

[13] Limnoscelis dynatis fossils have been found alongside the synapsids Edaphosaurus raymondi,[13] and Xyrospondylus ecordi,[14] the diadectid Desmatodon hesperis,[13] the aïstopod Coloraderpeton brilli,[13] the microsaur Trihecaton howardinus,[14] and labyrinthodont amphibians.