Ammonoids – cephalopod molluscs with external shells that existed from the Early Devonian up to the end of the Cretaceous – had well-developed jaws. During ammonoid evolution, several different types of their jaw apparatus arose, the study of which is of undoubted interest since it allows researchers to draw conclusions about the feeding strategies of ammonoids and their position in trophic chains. However, there is a lack of findings relating to the evolution of ammonoid jaws during the Permian. Here we describe a collection of almost thirty of cephalopod jaws from the Divjinskian Formation (Artinskian Stage, Cisuralian, Lower Permian), from the Sverdlovsk region of Russia. Most likely, these jaws belong to goniatitid ammonoid Uraloceras, the most abundant cephalopod mollusc in the Divjinskian (Divya) Formation. Uraloceras lower jaws are typical ammonoid anaptychi which have a rounded, wide and convex shape with smooth or slightly ribbed surface. They have a large inner lamella with a trapezoidal platform in the central part. One of the jaws bears a possible bite trace of a predator or scavenger. The upper jaws, described here for the first time, are slightly smaller than the lower jaws, their shape is narrow and pointed. Originally, both jaws were completely organic without calcareous elements. The absence of sculpture, consisting of frequent ribs and growth lines, characteristic of the more ancient Carboniferous goniatitid jaws, makes the jaws of the Uraloceras closer to the structure of the jaw apparatus of Triassic ammonoids. Judging by the pointed shape of the tips of both jaws, Uraloceras were active predators.
In this publication, we describe for the first time a large collection of Permian ammonoid jaws, which were collected by S.V. Naugolnykh over the course of a decade in the vicinity of Krasnoufimsk, Russia. The collection includes both lower and upper jaws of the ammonoid genus Uraloceras. Until now, only the single isolated specimens of the lower jaws of Permian ammonoids have been described, and the upper jaws have not yet been found. The lower jaws of Permian ammonoids belong to the anaptychus-type and are more similar to Triassic than to Carboniferous ammonoid jaws. One of the previously described Permian ammonoid jaws had been identified as an aptychus, but we show that this definition is erroneous. Nevertheless, we discuss the existence of the aptychi in the Paleozoic and show that discussion on this topic is still open for debate.
Permian, Cephalopoda, Ammonoidea, jaw apparatus, anaptychi, Artinskian