Definition of ammonite in US English:



  • An ammonoid that belongs to the order Ammonitida, typically having elaborately frilled suture lines.

    Typified by ammonoids of the order Ammonitida

    • ‘The egg case secreted by the Cenozoic argonautids was shaped like Cretaceous ammonites, which their ancestors used to occupy and emend.’
    • ‘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.’
    • ‘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.’
    • ‘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.’
    • ‘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.’
    • ‘Argonauta shares other characteristics with some Jurassic and Cretaceous ammonites, namely multicuspid rachidial tooth on the radula.’
    • ‘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.’
    • ‘The children have dug up fossils of ammonites from millions of years ago.’
    • ‘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.’


Mid 18th century: from modern Latin ammonites, from medieval Latin cornu Ammonis ‘horn of Ammon’, from the fossil's resemblance to the ram's horn associated with Jupiter Ammon (see ammoniacal).