Cretaceous Nerineacean gastropods.
The Nerineacea form a distinctive and significant component of shallow marine
Mesozoic Tethyan macrofaunas. They occur in shelf sediments deposited in
tropical regions during the Jurassic and Cretaceous. The group first appears in the
early Jurassic and high diversity levels had developed by late Jurassic and early
Cretaceous times. A major extinction episode occurred during the latter part of the
Cenomanian and the Turonian, with the Nerineacea becoming finally extinct in the
The heterostrophic nature of the ncrineacean protoconch (described here from
Nerineidae and Nerinellidae species) indicates opisthobranch affinities. The
Nerineacea is placed in the Entomotacniata, an independent order within the
Opisthobranchia. The Entomotaeniata is considered to show greatest affinities with
some members of the Acteonacea. The Pyramidcllacea are not included in the
Entomotaeniata. The order contains the Ceritellidae, Nerineidae, Nerinellidae and
ltieriidae. Early phylogenetic separation of the first three families is indicated.
Within each family a limited number of "stable" internal fold patterns developed
independently which reflected anatomical features of the abapical portion of the
The Nerineacea typically inhabited warm, clear, carbonate-dominated shallow
marine environments on and around off-shore "highs", however, some genera lived
in prelittoral or lagoonal situations. Only Ptygmatis shows any evidence of
tolerance of abnormal salinities. Carbon and oxygen isotope analyses demonstrate
that Nerinella and Simploptyxis specimens from Austrian Gosau deposits lived in
water of normal marine salinity. Most Cretaceous nerineid genera were probably
mobile epifaunal herbivores, although Nerinellidae species may have been infaunal.Cretaceous Nerineacea are taxonomically reviewed; the diagnoses of the subfamily
Umboneinae; Diozoptyxis, Adiozoptyxis; Julesia and Phaneroptyxis are revised.
Stratigraphic and palaeogeographic ranges indicate that certain species could
potentially be used for correlation. Quantitative measurements of various
parameters are included in specific descriptions; the value of these in specific
resolution is tested in appropriate cases. An analysis of morphological range in one
species (Diozoptyxis cochleaeformis) demonstrates wide continuous variation in
overall shell shape and whorl concavity. However, external ornament and internal
fold pattern do not show significant intraspecific variation; these features therefore
provide reliable characters for species definition.