Ecolsonia cutlerensis, an Early Permian dissorophid amphibian from the Cutler Formation of north-central New Mexico

Abstract

Recently discovered specimens of the temnospondyl labyrinthodont amphibian Ecolsonia cutlerensis Vaughn (1969) from the Lower Permian (Wolfcampian) Cutler Formation of north-central New Mexico allow a more complete description of its anatomy and a re-evaluation of its familial position. E. cutlerensis was originally assigned to the Trematopsidae primarily on the basis of its possession of an elongate external naris, a characteristic feature of the family, yet it was also recognized as possessing features characteristic of the Dissorophidae. Although the new materials of Ecolsonia clearly indicate the presence of a trematopsid-like naris, numerous other cranial features, particularly the otic notch, firmly establish its dissorophid affinities. Ecolsonia appears to be the only non-trematopsid labyrinthodont definitely known to possess an external naris identical to that of trematopsids. The postcranial skeleton of Ecolsonia, on the other hand, does not possess features that are diagnostic of either family. The absence in Ecolsonia of the highly specialized dorsal dermal "armor" that characterizes the great majority of dissorophids is unexpected in light of its otherwise highly advanced structural grade of organization. The probable functions of the typical dissorophid dermal "armor," that of providing strength and immobility to the vertebral column and protection from body-moisture loss and from predators, are believed to have been assumed in great part in Ecolsonia by several structures not seen in other dissorophids: (1) greater ossification of presacral intercentra to form ring-like structures; (2) neural spines with a pair of lateral tubercles or with bifid or nodular-like expanded summits that very likely accommodated a greatly increased development of the system of tendinous attachment of the dorsal axial musculature; and (3) well-developed, sculptured osteoderms that formed a dense, armor-like covering over much of the body. The possession by Ecolsonia of numerous unique derived features indicates that it represents a distinct lineage that separated from all other dissorophid lines before the Early Permian. Further, its possession of several primitive features not seen in other dissorophids suggests that the branching may have occurred as early as the Middle Pennsylvanian.