PHYLOGENETIC IMPLICATIONS OF THE MORPHOLOGY OF THE EMBRYONIC SHELLS OF DEVONIAN AMMONOIDS

The structure of the embryonic shell (ammonitella) of the Ammonoidea is well known. Most of our information comes from studies on well preserved Mesozoic taxa. Some data do exist for the earliest ammonoids from the Devonian. Recent studies on very well preserved Devonian ammonoids from the Mimagoniatitidae, the Agoniatitidae and the Anarcestidae, have increased our knowledge of the morphology of the early ammonoid ammonitellas and raised questions as to how this morphology relates and evolved into the better known morphology of post-Devonian ammonitellas. The specimens in this study are from Morocco and New York State.

There are similarities in the morphology of the ammonitellas of Devonian and Mesozoic taxa. Both have a prosiphon, caecum and primary constriction. But the ornament is different. The shell is smooth or tuberculate in post-Devonian ammonitellas, but covered with transverse lirae in Devonian taxa. At the aperture of the ammonitella the ornament changes becoming finer and closely spaced. Devonian taxa have no shell thickening (primary varix) but it is present in all post-Devonian ammonoids. In the specimens studied the postembryonic shell can be seen emerging from beneath the embryonic shell.

A study of the lirae spacing across the ammonitellas in the three above families yields several conclusions. The lirae do not seem to be produced by a slow regular progression at an apertural margin. There is variability in the distance between the lirae, and the ratio of the distance between the ventral and dorsal margins of the shell is not consistant. The patterns of the increase and decrease in the distance between lirae are different for the three families with most of the differences taking place in the initial chamber. The number of lirae produced in the three families is also different: In the Mimagoniatitidae there are 70-77 on the ammonitella, with 25-30 on the initial chamber. In the Anarcestidae there are 95 on the ammonitella, with 40-45 on the initial chamber. In the Agoniatitidae there are 65 on the ammonitella with 30-35 on the initial chamber. These differences in both the spacing and number of lirae are taxonomically significant. The initial chamber, the earliest development point, also appears to be key in affecting the shape and size of the ammonitellas. These early developmental changes may have helped fuel the rapid radiation of the ammonoids.