This paper examines the uppermost Sinemurian and lowermost Pliensbachian ammonite fauna recorded in NW Iberia with a special focus on Echioceratidae from the Aplanatum Subzone. In total, 852 specimens have been collected bed-by-bed from five reference sections from the Asturian, Basque-Cantabrian, and Lusitanian basins. Echioceratidae, namely Paltechioceras genus, represent 86.2% of the sample, Polymorphitidae (Leptonotoceras genus) the 10%, and less than 4% of the specimens are Eoderoceratidae, Phricodoceratidae, or Oxynoticeratidae. This study describes five species of Paltechioceras, namely P. tardecrescens, P. oosteri, P. recticostatum, P. elicitum, and P. romanicum. They allow a biochronostratigraphic division for the studied basins and its worldwide correlation. One of the species, P. romanicum, is the latest one registered in all the studied sections, occurring directly below the Sinemurian–Pliensbachian boundary. The disappearance of this widely distributed species marks the echioceratids extinction. This work reviews the global events described around the Sinemurian–Pliensbachian transition (the Sinemurian–Pliensbachian boundary event). Finally, we present a palaeobiogeographical analysis of the Paltechioceras species described, giving more information to discuss the opening of the Hispanic Corridor during the late Sinemurian.