Directed evolution of life through millions of years, such as increasing
adult body size, is one of the most intriguing patterns displayed by
fossil lineages. Processes and causes of such evolutionary trends are
still poorly understood. Ammonoids (externally shelled marine
cephalopods) are well known to have experienced repetitive morphological
evolutionary trends of their adult size, shell geometry and
ornamentation. This study analyses the evolutionary trends of the family
Acrochordiceratidae Arthaber, 1911 from the Early to Middle Triassic
(251228 Ma). Exceptionally large and bed-rock-controlled collections of
this ammonoid family were obtained from strata of Anisian age (Middle
Triassic) in north-west Nevada and north-east British Columbia. They
enable quantitative and statistical analyses of its morphological
evolutionary trends. This study demonstrates that the monophyletic clade
Acrochordiceratidae underwent the classical evolute to involute
evolutionary trend (i.e. increasing coiling of the shell), an increase
in its shell adult size (conch diameter) and an increase in the
indentation of its shell suture shape. These evolutionary trends are
statistically robust and seem more or less gradual. Furthermore, they
are nonrandom with the sustained shift in the mean, the minimum and the
maximum of studied shell characters. These results can be classically
interpreted as being constrained by the persistence and common selection
pressure on this mostly anagenetic lineage characterized by relatively
moderate evolutionary rates. Increasing involution of ammonites is
traditionally interpreted by increasing adaptation mostly in terms of
improved hydrodynamics. However, this trend in ammonoid geometry can
also be explained as a case of Copes rule (increasing adult body size)
instead of functional explanation of coiling, because both shell
diameter and shell involution are two possible paths for ammonoids to
accommodate size increase.