Ariekanerpeton sigalovi

Introduction

Ariekanerpeton sigalovi was found in Lower Permian strata of Tadzhikistan. It is represented by over nine hundred compressed but otherwise well-preserved specimens. These specimens constitute one of the best known growth series of seymouriamorphs that includes skulls ranging from 6 mm to 54 mm in length (Ivakhnenko, 1981). However, the poor ossification of the skeleton of the largest known specimens suggests that even these are not fully mature.

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Figure 1. One of the largest known specimens of Ariekanerpeton sigalovi. The poor ossification of the long bones and the absence of the quadrate suggest that this specimen is immature. Anatomical abbreviations: An, angular; At. r., atlantal rib; Ax. r., axial rib; Bc, braincase; Cl, clavicle; D, dentary; Ec, ectopterygoid; H, humerus; Icl, interclavicle; Ps, parasphenoid; Psp, postsplenial; Pt, pterygoid; Qj, quadratojugal; Ra, radius; Sa, surangular; Sc, scapula; Sp, splenial; Sq, squamosal; U, ulna.

Characteristics

Ariekanerpeton sigalovi, like other small seymouriamorphs, has been reconstructed with a broad, triangular skull, but recent study of this taxon suggests that the skull was slightly more narrow (Laurin, 1996). There is usually no contact between the postorbital and the supratemporal. This suggests that Ariekanerpeton is more closely related to Seymouria and Discosauriscus than to Utegenia, who consistently retained this contact in postmetamorphic specimens (the contact is occasionally present in Discosauriscus and Ariekanerpeton, but only rarely).

Ariekanerpeton had twenty-four presacral vertebrae. The neural arches are paired and disarticulated from the pleurocentra even in the largest known specimen, but this is probably a juvenile character.

Larval specimens of Ariekanerpeton had circular scales with concentric rings similar to those found in Discosauriscus, but postmetamorphic specimens appear to have lost them. Ariekanerpeton lacked the rectangular ventral scales (gastralia) found in most other early terrestrial choanates. The loss of gastralia suggests that Ariekanerpeton was more closely related to Discosauriscus than to Utegenia, who still had gastralia. In most fossil taxa, such characteristics of the integument are not known, but the great number of known specimens and their excellent preservation (scales are often preserved in the larval specimens) allow changes in the integument of Ariekanerpeton to be followed through its growth.

References

Ivakhnenko, M. F. 1981. Discosauriscidae from the Permian of Tadzhikistan. Paleontological Journal 1981: 90-102.

Laurin, M. 1996. A reevaluation of Ariekanerpeton, a Lower Permian seymouriamorph (Tetrapods: Batrachosauria) from Tadzhikistan. Journal of Vertebrate Paleontology 16 (4): 653-665.