The egg case secreted by the Cenozoic argonautids was shaped like Cretaceous ammonites , which their ancestors used to occupy and emend.
When the tests for dimorphism are now applied to these adults, it is clear that they are in fact dimorphic in just the same manner as other Jurassic ammonites .
The children have dug up fossils of ammonites from millions of years ago.
A feature of especial interest is that, while the youngest Triassic ammonites are quite rare, the oldest Jurassic ammonites are extremely abundant, entering the succession in a veritable flood.
In eastern Mongolia late Permian marine sediments with brachiopods, crinoids and Bryozoa and Triassic marine sediments with ammonites occur in the suture zone, indicating that marine conditions existed during these times.
Argonauta shares other characteristics with some Jurassic and Cretaceous ammonites , namely multicuspid rachidial tooth on the radula.
We love it there because rather than having just a sandy beach, there are loads of rocks and rock pools to explore and as the program showed last night, the place is abundant with fossils, especially ammonites .
The fossil content consists of ammonites , belemnites, brachiopods, echinoderms, bivalves, crinoids, gastropods, ostracodes and benthic foraminifers.
Among the Middle Jurassic ammonites , they may be one of the more polymorphic groups: a feature which has made them a particularly useful subject for quantitative and biostatistical approaches.
This increase in disparity also corresponded to a phyletic radiation, as all the earliest Jurassic ammonites derived from scarce ammonites of rather homogeneous and medium-sized species.