Arganaceras and Sphodrosaurus

Arganaceras vacanti Jalil and Janvier (2005) described this new Late Permian pareiasaur with twin nasal horns. They nested it as a “much derived form ” close to Elginia, a pareiasaur with atypical bull-like horns developing from enlarged and pointed supratemporals.

Here Arganaceras nests between Sclerosaurus at the base of the softshell turtle clade that includes Odontochelys and Trionyx.

Distinct from Sclerosaurus, the skull of Arganaceras was probably losing (reducing) its nasal horn, supratemporal horns and descending quadratojugal cheeks because more derived taxa do not have them. The naris was reclined. Some sort of upper temporal fenestra seems to be developing. That would become confluent with the posttemporal fenestra in Trionyx. The body remains unknown at present, but was probably low, wide and flat with short limbs and a robust tail.

Sphodrosaurus pennsylvanicus (Colbert 1960; North Museum No. 2321; Newark Supergroup, Late Triassic) was originally described as a procolophonid like Hypsognathus. Here Sphodrosaurus nests between Sclerosaurus and Arganceras as a basal soft-shell turtle with teeth. Distinct from Sclerosaurus, the femur is longer, the coracoid is smaller. As in Trionyx, pedal digit 5 is gracile. Below these taxa are shown to scale along with Odontochelys.

Sclerosaurus, Sphodrosaurus and Odontochelys to scale

Colbert EH 1960. A new Triassic procolophonid from Pennsylvania. American Museum Novitates 2022:1–19.

Jalil N-E and Janvier P 2005. Les pareiasaures (Amniota, Parareptilia) du Permien supérieur du Bassin d’Argana, Maroc. Geodiversitas 27(1):35-132.

Rice R, Kallonen A, Cebra-Thomas J and Gilbert SF 2016. Development of the turtle plastron, the order-defining skeletal structure. PNAS 113 (19):5317–5322.

Sues H-D, Baird D and Olsen PE 1993. Redescription of Sphodrosaurus pennsylvanicus Colbert, 1960 (Reptilia) and a Reassessment of its Affinities. Annals of Carnegie Museum 62(3):245-253.

Ezcurra MD andSues H-D 2022. A re-assessment of the osteology and phylogenetic relationships of the enigmatic, large-headed reptile Sphodrosaurus pennsylvanicus
(Late Triassic, Pennsylvania, USA) indicates archosauriform affinities. Journal of Systematic Palaeontology. 0 (0): 1–35. doi:10.1080/14772019.2022.2057820

wiki/Arganaceras
wiki/Sphodrosaurus

Pareiasaurs

Stephanospondylus

Deltavjatia

Anthodon

Scutosaurus • Bradysaurus

Softshell Turtles & Kin

Scoloparia 83.1

Sclerosaurus

Arganaceras

Odontochelys

Trionyx