A Late Carboniferous Spirulid Coleoid from ahe Southern Mid-Continent (USA)

TL;DR: Ultrastructural study of a longiconic cephalopod, formerly described as B. postremus, reveals that the shell wall, consisting of two prismatic layers and lacking a nacreous layer, differs strongly from that of bactritoids, and is considered to be the earliest known member of the order Spirulida.

Abstract: Ultrastructural study of a longiconic cephalopod, formerly described as Bactrites postremus (Miller, 1930; Mapes, 1979), reveals that the shell wall, consisting of two prismatic layers and lacking a nacreous layer, differs strongly from that of bactritoids In the bactritoids, the shell wall after hatching is composed of the principal nacreous and thin outer prismatic layers, and only the embryonic shell, or bactritella, has a prismatic ultrastructure (Doguzhaeva, 1996a-c) Phragmocones possessing a marginal siphuncle and a prismatic shell wall are known in Recent Spirula and Cretaceous members of Spirulida: Groenlandibelus, Naefia and Adygeya (Doguzhaeva, 1996d) Ultrastructural comparison of the shell wall in so-called B postremus and the four spirulid genera listed above supports the conclusion that the two distinctly separated prismatic layers of the shell wall represent the outer and inner plates sensu Appellof (1893), which were secreted as an internal shell Thus, the form described as B postremus cannot be assigned to Bactritoidea It is redescribed as Shimanskya gen nov postremus (Miller, 1930), and is considered to be the earliest known member of the order Spirulida On the basis of this form the family Shimanskyidae fam nov is established