ABSTACT-The reticuloceratid ammonoid Arkanites relictus (Quinn, McCaleb, und Webb, 1962) is represented by hundreds
to thousands of individuals from horizons isolated both stratigraphically und geographically in northern Arkansas.  These
assemblages appear to represent mass mortality events resulting from a semelparous reproductive strategy.  Arkanites
relictus occurs as a dimorphic pair (depressed, widely umbilicate, cadiconic conchs and compressed, narrowly umbilicate,
pachyconic conchs) thought to reflect sexual dimorphism.  Late stage ontogenetic moditications, such as septal crowding
and change in aperture profile, are widely cited evidence of sexual maturity in ammonoids. Septal crowding begins at a
predictable ontogenetic stage in the compressed forms of A. relictus, but specimens with cadiconic conchs do not have
crowded septa even at the largest diameters available.
   
       Depending on the trait examined and the proxy for age of individuals, the dimorphism in Arkanites relictus (using the
depressed form as the reference morph) is the result of acceleration, neoteny, or hypermorphosis plus neoteny.  If size
(diameter) is considered a proxy for age, the dimorphs were the same age at death, und the septa in the compressed
variants devoloped via acceleration relative to the depressed variants.  Regarding conch shape (width vs. diameter),
the compressed morphs devoloped via neoteny relative to the depressed morphs. If septal count is considered a proxy
for age, the dimorphs were not the same age at death, and the compressed forms were produced by a combination of
hypermorphosis plus neoteny, i.e., they grew longer yet slower than the depressed foms. In  A. relictus, the heterochronic
processes of hypermorphosis and neoteny may have been operating simultaneously, which is an interesting possibility
because it is an example of a combination of both peramorphic und paedomorphic processes.