A skull fragment of a Devonian tetrapod with a unique lateral line morphology in the collection of the Museum für Naturkunde Berlin
Abstract. A dermal bone from the late Famennian of Ketleri in Latvia, identified as a probable tetrapod postorbital by Oleg Lebedev and stored in the Museum für Naturkunde Berlin, is described in detail. Its tetrapod status is confirmed based on the dermal sculpture consisting of polygonal pits and radially aligned ridges and furrows. The sculpture resembles that of Devonian tetrapods but is nevertheless not as regular as in most post-Devonian forms. The bone can be designated as a postorbital because of its concave anterior margin, the lateral line that shows no bifurcation, and the ossification centre that is located in the dorsal portion of the bone. The lateral line (i.e., the postorbital branch of the infraorbital line) forms an open, deep sulcus in the ventral portion of the postorbital but is enclosed within the bone in the dorsal portion and opens through pores to the surface. This morphology of the postorbital branch is unique among basal tetrapods. The bone might belong to the tetrapod Ventastega curonica Ahlberg et al., 1994 that is known from the same strata at Ketleri, however, since the postorbital is unknown in this taxon, this cannot be stated with certainty.