Since approximately the 1800s there have been single observations (d’Orbigny, … ),
since the 1960s (Makowski 1962, Callomon 1963),it was scientifically recognized that
there are certain ammonite shells, found in the same beds, that may be showing
signs of sexual dimorphism – usually with these specimen, the most inner whorls are
the same for the dimorphs, after that growth of the partners diverge, with one partner,
today assumed to be the female (= macroconch) showing continued growth and/or
changes in the sculpture of the shell, while the other, assumed to be the
male (= microconch) , remains relatively small with little sculptural changes.
One of the most obvious potential (we shall never know for sure…) dimorphic pairs is
Promicroceras / Xipheroceras.
For about the first 15-20 mm, the whorls of Promicroceras and Xipheroceras are
indistinguishable, after that, Xipheroceras as the macroconch develops broader whorls,
strong spines and a change in ribbing, attaining sizes up to more than 30 cm
while Promicroceras as the microconch does not grow much beyond 30-40 mm,
never changing its sculpture.
The following species have been found at Robin Hoods Bay:
On the first 15-20 mm the ammonite develops just like a Promicroceras, after
which the shell shows strong spines at about every 30-40 degrees and an otherwise
weakening of the ribs inbetween.
Spines stop at about 50-70 mm, only the weak ribbing continues.
Xipheroceras dudressieri (D´ORBIGNY, 1844)
In contrast to X. ziphus, the ribbing of X. dudressieri continues well beyond the
“Promicroceras” stage, and the spines are nowhere near as strong, but on every rib .
EDMUNDS et al suggested a potential synonymy with X. planicosta as a
junior synonym, see literature.
Promicroceras planicosta (SOWERBY, 1814)
Promicroceras is relatively common and can be found associated with Asteroceras and
of course Xipheroceras.
Promicroceras planicosta has about 24 ribs/whorl at 3 cm, and a characteristic
broadening/flattening of the ribs on the venter.
Promicroceras capricornoides (QUENSTEDT, 1884)
Marston Magna specimen
Xipheroceras and Promicroceras have been described in great detail from an excavation
in the Marston Magna area recently by
EDMUNDS,WHICHER, LANGHAM, CHANDLER (see literature).
the later discovery of a well spined Xipheroceras was an added bonus !
preservation make up the appeal of these ammonites, and there is certainly much still
to be learned about their evolution.
SSSI – only responsible collecting is allowed.
Cope J. (1994), Preservation, sexual dimorphism and mode of life of some Sinemurian
ammonites // Palaeopelagos Special Publication I, Rome