A new small-sized stem salamander from the Middle Jurassic of Western Siberia, Russia

Salamanders (Caudata) are one of the three modern groups of amphibians known from the Middle Jurassic. The early stages of evolution of these amphibians are still poorly known, especially for stem taxa of Jurassic age. A new small-sized stem salamander, Egoria malashichevi gen. et sp. nov., from the Middle Jurassic (Bathonian) Itat Formation of the Berezovsk Quarry locality in Western Siberia, Russia, is described on the basis of isolated vertebrae, including an atlas centrum and a fragmentary trunk vertebra centrum previously referred to an undescribed salamander taxon ("Berezovsk salamander A"). The new taxon is diagnosed by the following unique combination of vertebral characters: atlantal anterior cotyles with elliptical anterior outline, located at an angle of approximately 135-137 degrees to each other; wide posterior portion of the atlantal centrum; ossified portion of the intercotylar tubercle represented by dorsal and ventral lips; absence of a deep depression on the ventral surface of the atlantal centrum; absence of pronounced ventrolateral ridges on the atlas; absence of spinal nerve foramina; presence of a pitted texture on the ventral and lateral surfaces of the centra and lateral surfaces neural arch pedicels; presence of a short atlantal neural arch with its anterior border situated behind the level of the anterior cotyles; short trunk vertebrae; and upper transverse process (= diapophysis) larger than lower transverse process (= parapophysis) on the trunk vertebrae; notochordal canal opens in the upper half of the cotyle (= the lower portion of the centrum is more massive and less compact than the upper portion). The microanatomical organization of the atlas and trunk vertebrae is characterized by the presence of inner cancellous endochondral bone. The small body size (about 180-215 mm) of Egoria malashichevi gen. et sp. nov. indicates that that not all stem salamanders were large neotenic forms (up to 550-600 mm in Urupia and Marmorerpeton) and hints at a broader ecological role for stem salamanders.