Abstract:

         A small egg-shaped initial chamber is a common feature of the Ammonoidea.  This plus the shell up to the primary constriction defines the embryonic shell, called the ammonitella.  All ammonitellas possess the same general characteristics, but their expression is different. This has both phylogenetic and ontogenetic implications. Most available data are from Mesozoic ammonoids, due to preservational biases.  As a result, most models of embryonic development are based on the characteristics of the better preserved Mesozoic ammonoids and hypothesize a non-accretionary mode of growth.  These models include the Paleozoic ammonoids by default or exclude them outright, postulating that they formed differently, namely by accretionary growth.
        In this study we have examined the ammonitellas of some Lower-Middle Devonian representatives of the most primitive ammonoid groups: primarily the Order Agoniatitida(Archanarcestes, Mimagoniatites, and Agoniatites) from North Africa and North America.The ammonitellas of these primitive ammonoids are covered by a transverse ornamentconsisting of lirae. The apex of the initial chamber is free of omament and is encircled by the lirae.  This "bald spot" may correlate to the thin apical portion of the initial chamber of Mesozoic ammonoids.  Adoral of the primary constriction (on the postembryonic shell), growth lines appear between the lirae.  No such growth lines occur on the ammonitella. The embryonic lirae are different from the postembryonic lirae, both in course and form,and probably formed differently.  We conclude from the study the embryonic ornament that the formation of these ammonitellas was the same as that of all other Ammonoidea, i.e., a non-accretionary mode.
           Data from the internal morphology of the ammonitella are also examined.  Like more advanced Paleozoic and Mesozoic ammonoids, a prosiphon and caecum are present.  The siphuncle, however, lies far adapical in the initial chamber.  In addition, a flange and primary varix are probably absent in these primitive ammonoids.
          In conclusion, embryonic shell characters support the monophyly of the Ammonoidea.  The actual character states are variable, allowing for further phylogenetic analysis.