Phenotypic plasticity and taxonomy of Schloenbachia varians (J. Sowerby, 1817) (Cretaceous Ammonoidea)

Abstract

Taxonomic concepts of Early–Middle Cenomanian representatives of Schloenbachia Neumayr, 1875 are evaluated based on well-preserved, abundant faunas from NE Iran (Atamir Formation, Koppeh Dagh) and Germany (Essen Greensand and Baddeckenstedt formations at several localities in northern Germany; Meisen Formation of Saxony). From a single Lower Cenomanian horizon in the Atamir Formation, a complete suite of specimens of Schloenbachia varians (J. Sowerby, 1817), ranging from moderately involute, compressed and finely ornamented (“forma subplana”) to more evolute, depressed and strongly spinose forms (“forma ventriosa”) with all transitional morphologies, is documented. Also in the shallow-water faunas of the “Klippenfazies” of the Essen Greensand Formation at Mulheim-Broich and of the Meisen Formation, all morphotypes co-occur. Thus, these forms cannot be regarded as geographic subspecies or successive chrono-subspecies, but reflect a high degree of variability in shell form and ornament (phenotypic plasticity) in S. varians. Interestingly, strongly tuberculate forms are rare to absent in contemporaneous offshore settings as documented by the Baddeckenstedt Formation. The phenotypic plasticity in Early Cenomanian S. varians populations is explained by ecophenotypic variation along a depth (proximal–distal) gradient: strongly tuberculate, depressed forms reflect comparatively shallow, nearshore environments with higher water energy and predation pressure, whereas compressed, weakly ornamented morphs are forms of open (and deeper) marine waters. The complete range of different forms is also observed in micro- and macroconchs, suggesting that the morphological variability is not controlled by sexual dimorphism. Based on its similarity to S. varians, also the Middle Cenomanian Ammonites coupei Brongniart, 1822 may be placed in the synonymy of the former.