Jaws of ammonites which inhabited the Panboreal Superrealm during the Jurassic and Cretaceous are poorly known in comparison to those of Tethyan ammonoid faunas. This paucity may be explained by limited thickness, or even absence of an outer calcitic layer, in lower jaw elements (aptychi) of Boreal ammonites. Here we describe, for the first time, the jaws (both lower and upper) of ammonites of the Boreal family Polyptychitidae, of Early Cretaceous (Valanginian) age. Polyptychitid lower jaws are of the aptychus type, but have an unusual pointed and convex shape. However, lower jaws of Late Jurassic ancestors of polyptychitids (Craspeditidae) as well as Middle Jurassic cardioceratids (Pseudocadoceras) have a near-identical shape, as do previously described aptychi of the Late Cretaceous genera Neogastroplites and Placenticeras (Hoplitoidea). The close resemblance of lower jaws of evolutionarily distant ammonites may be linked to a similar lifestyle, but more data are needed to substantiate this. Upper jaws of polyptychitid are closely similar to previously described upper jaws of Jurassic ammonites, which indicates the conservatism of this part of the jaw apparatus. Together with shells and jaws of the Valanginian ammonites described herein, jaws of coleoids (likely belemnites) as well as arm hooks (onychites) have been found.
Aptychi - mirror-symmetrical halves of the lower jaws of the Jurassic and Cretaceous ammonites have been studied very unevenly. For more than a century, paleontologists have collected a great deal of information on ammonite aptychi from the Tethyan realm, whereas aptychi of various Boreal ammonites have remained largely unexplored. In this article, we filled in one of these "blank spots" and for the first time described the aptychi of ammonites of the family Polyptychitidae from the Valanginian of Russia. These aptychi turned out to be surprisingly similar to those of some other ammonites, including those not related to Polyptychitids. This indicates that the unusual shape of such aptychi is related with environmental factors, and not the origin of their hosts from a common ancestor. By the way, it is worth noting that the aptychi of Polyptychitids did not have a calcitic layer.
Jaw apparatus, Ammonites, Polyptychitidae, Aptychi, Lower Cretaceous, Russia