Re-evaluation of an early sphenacodontian synapsid from the Lower Permian of England

Abstract

The holotypic isolated maxilla of the early sphenacodontian Haptodus grandis from the Cisuralian of England is revisited. A unique character combination includes haptodontine-grade traits like less specialised teeth and a high number of precanines, but at the same time a shortened lacrimal that is separated from the naris, which is strongly diagnostic of Sphenacodontoidea. As the specimen cannot be assigned to any known taxon, the new taxon Hypselohaptodus grandis gen. nov. is proposed. Comparison with other sphenacodontians reveals a mosaic distribution of maxillary features, most significantly regarding the precanine region. Preliminary character histories preclude Hypselohaptodus from Sphenacodontidae, but suggest a haptodontine-grade or basal therapsid position. The latter hypothesis is substantiated by an ecological model of episodic wet phases in an overall trend of aridification throughout the Permian, to explain the rareness of non-sphenacodontid sphenacodontians in the fossil record. Also from the early Permian of England, an isolated dentary has previously been assigned to Ophiacodon, but can be shown to be either a sphenacodontian, possibly affiliated with Hypselohaptodus, or a robust, Stereorhachis-like ophiacodontid. The absence of Ophiacodon in the intramontaneous Permian basis of Europe is explained by a narrow environmental tolerance range requiring limnic connection with lowland basins.

Synapsid tracks with skin impressions illuminate the terrestrial tetrapod diversity in the earliest Permian of equatorial Pangea

“…The stratigraphic ranges of particular taxa and locality ages are based on Refs. 25 , 26 , 99 , 101 – 103 , 109 , 110 . …”

“…The members of non-therapsid Sphenacodontia, with their characteristic proximodistally-elongated ulnare, reached the highest diversity during the wetter phases of the early Permian and mainly inhabited equatorial lowland basins [99][100][101] . While their occurrence in the Boskovice Basin would not be unexpected, given the semihumid intervals recorded in several horizons (Fig.…”

“…The stratigraphic ranges of particular taxa and locality ages are based on Refs. 25,26,99,[101][102][103]109,110 A complete edaphosaurid autopodium has not been described yet; however, the isolated phalanges of Edaphosaurus indicate that the foot was rather broad and short, with strong claws 57 . Edaphosaurids are best known from coal deposits of the Pennsylvanian and their diversity decreased gradually with the continual aridification during the early Permian 26 .…”

The age of North America’s youngest Paleozoic continental vertebrates: a review of data from the Middle Permian Pease River (Texas) and El Reno (Oklahoma) Groups

“…Olson, 1962, which appears to be the most recent representative of this clade (Didier and Laurin, 2021). The oldest known sphenacodontids are Cryptovenator hirschbergeri Fröbisch et al, 2011 and Sphenacodon ferox Marsh 1878, both of which date from the latest Carboniferous or the earliest Permian (Spielmann et al, 2010;Fröbisch et al, 2011); the enigmatic "Macromerion" schwarzenbergii Fritsch 1879 may be of comparable age (Spindler 2020). Other than D. angelensis, the most recent sphenacodontids include Dimetrodon giganhomogenes Case 1907 andD.…”

A faunivorous early sphenacodontian synapsid with a diastema

“…Distinct from "Haptodus" garnettensis on basis of taller facial blades of the lacrimal and maxilla, less postcanine and precanine teeth, the convex precanine margin, presence of diastema, and shorter contribution of maxilla to naris. The diagnosis of Sphenacodontia based on node apomorphies is subject to an ongoing debate (Spindler, 2019), however, the assignment of ROM 43608 to Sphenacodontia is confirmed by the presence of stout marginal teeth, a convex ventral margin of the maxilla, and tall lacrimal and maxillary dorsal lobe. ROM 43608 is excluded from Sphenacodontoidea due to the narial contribution of the lacrimal.…”