Kulicki,C., Neil H. Landman, Michael J. Heaney, Royal H. Mapes & Kazushige Tanabe (2002):
Abstract:
     We investigated the morphology and microstructure of the early whorls of goniatites with aragonitic preservation
from the Upper Carboni ferous Buckhorn Asphalt (Oklahoma, USA).  These specimens probably all belong to the same
species or to several closely related species. The ammonitella diameter is approximately 0.8 mm and the ammonitella
angle is approximately 360°.  The initial chamber is ellipsoidal and is surrounded by the first whorl of the ammonitella.  
The outer surface of the ammonitella is smooth without any trace of ornamentation or   growth lines.  In contrast,
growth lines and lirae are present on the postembryonic shell.  The dorsal end of the initial charnber terminates in a
thick flange.  There is an eloflgate muscie scar on the inside surface of the initial chamber above the flange.  
The wall of the initial chamber consists of three layers, the outermost of which is the dorsal wall of the next whorl.  
The wall of the first whorl of the ammonitella consists of four layers:
   1 ) Inner prismatic layer (= the mural part of the proseptum and subsequent septa).
   2) Middle granular/subprismatic layer (=thewall proper of the ammonitella).
   3) Very thin outer prismatic layer .
   4) Dorsal wall of the next (postembryonic) whorl.
     The wrinkle layer first appears on the ventral surface of the initial chamber near the ammonitella edge.  This layer
is the outer component of the dorsal wall and disappears on the postembryonic shell.  In median cross-section the
proseptum and second septum are closely spaced on the dorsal side. The proseptum is prismatic and the second
septum is nacreous. There are many similarities between the ammonitellas of  these goniatites and those of Mesozoic
ammonoids, implying a similar mode of embryonic development. However, there are also several important differences
relating to the shape of the ammonitella and its ornamentation.  This variation in the morphology of the ammonitella
can be used for phylogenetic analysis.