The family Acrochordiceratidae Arthaber, 1911 ranges in age from latest
Spathian to the middle/late Anisian boundary, and it represents a major
component of ammonoid faunas during that time. The middle Anisian genus
Acrochordiceras Hyatt, 1877 is the most widespread taxon of the family
and occurs abundantly worldwide within the low paleolatitude belt.
However, there is a profusion of species names available for
Acrochordiceras. This excessive diversity at the species level
essentially results from the fact that sufficiently large samples were
not available, thus leading to a typological approach to its taxonomy.
Based on new extensive collections obtained from the Anisian (Middle
Triassic) Fossil Hill Member (Star Peak Group, north-west Nevada) for
which a high resolution biostratigraphic frame is available, the
taxonomy and biostratigraphy of the genus Acrochordiceras Hyatt, 1877 is
herein revised with respect to its intra-specific variation.
Morphological and biometric studies (c. 550 bedrock-controlled specimens
were measured) show that only one species occurs in each stratigraphic
level. Continuous ranges of intra-specific variation of studied
specimens enable us to synonymize Haydenites Diener, 1907,
Silesiacrochordiceras Diener, 1916 and Epacrochordiceras Spath, 1934
with Acrochordiceras Hyatt, 1877. Three stratigraphically successive
species are herein recognized in the low paleolatitude middle Anisian
faunas from Nevada: A. hatschekii (Diener, 1907), A. hyatti Meek, 1877
and A. carolinae Mojsisovics, 1882. Moreover, an assessment of
intra-specific variation of the adult size range does not support
recognition of a dimorphic pair (Acrochordiceras and Epacrochordiceras)
as previously suggested by other workers (Epacrochordiceras is the
compressed and weakly ornamented end-member variant of Acrochordiceras).
The successive middle Anisian species of Acrochordiceras form an
anagenetic lineage characterized by increasing involution, adult size
and intra-specific variation. This taxonomic revision based on new
bedrock-controlled collections is thus an important prerequisite before
studying the evolution of the group.