160


Journal of Paleontology 92(2):157–169


Fig. 3. Oklaconus okmulgeensis n. gen. n. sp., AMNH 82600, Moscovian, Oklahoma: (1) position of well-preserved fragment of proostracum-like structure and muscular mantle shown on Figures 2.2 and 3 (detail of 1.1); (2) proostracum-like structure with a central concentric pattern surrounded by longitudinally folded muscular mantle; (3) proostracum-like structure and muscular mantle shown in (2), positioned with an angle of ~ 180º to show that the dorsal side of the proostracum-like structure is covered by folds of the muscular mantle; (4) ink sac surrounded by shell-wall debris partially fractured by compaction of body chamber. bch=body chamber; ch=chamber of the phragmocone; is=ink sac; ls=last septum; m=matrix; mm=muscular mantle; ms=mural part of septum; pro=proostracum-like structure; pro+mm=proostracum-like structure plus muscular mantle; s=septum; sf=septal foramen; su=suture.


exposed, which enabled observation of these structures on the dorsal side of the conch (Figs. 2.6–2.7, 3.1–3.3). The specimen was then examined using scanning electron microscopy (SEM). It was etched with a 1–3% solution of acidic acid for 6–10 sec and coated with gold for further ultrastructural analyses of shell wall, sheath-like rostrum, siphuncle, ink sac, and mantle (see Figs. 4, 5, 6). Stronger etching chemicals could have been used to better expose the shell wall and sheath-like rostrum ultra- structures, but these were not applied to avoid destruction of nonbiomineralized structures.


Repositories and institutional abbreviations.—AMNH, American Museum of Natural History, New York City, USA; SUI, University of Iowa, Paleontological Collection, Iowa City, Iowa, USA; UNSM, University of Nebraska State Museum, Lincoln, Nebraska, USA.


Ink sac and muscular mantle associated with the analyzed conch


Ink sac.—A large black ‘body’ situated deep in the body chamber (Figs. 1.2, 3.4) is here shown to be an ink sac; the


three-dimensional shape of this black ‘body’—if it is flask-like with a rounded posterior reservoir and a tubular, broad gradually narrowing forward duct, like that of well-shaped ink sacs of the Carboniferous age (see Doguzhaeva et al., 2004a, fig. 4A–B; Mapes et al., 2010a, fig. 2A–C)—is unknown. The ‘pockets’ in the black ‘body’ around the chips of the fractured shell wall reveal the elastic wall of this structure (Fig. 3.4). The ink sac wall is fibrous and multilayered; three layers are distinguished due to different orientations of fibers in each layer (Fig. 6.1, 6.4). Like the ink sac of Donovaniconus (see Doguzhaeva et al., 2003, figs. 14, 15), the black ‘body’ is subdivided into com- partments by internal partitions (Fig. 6.2). The compartments are filled with a structureless material that, when evaluated under high magnification, shows that this is a massive aggregate of globular, 0.1–0.4 µm diameter, microparticles consisting of smaller particles (Fig. 6.2–6.4). A similar microglobular ultra- structure was observed in the dried ink extracted from the ink sacs of the Recent squid Loligo Schneider, 1784 (Doguzhaeva et al., 2004a, fig. 2A–B), as well as in fossil ink of the Jurassic Loligosepia Quenstedt, 1839 and Teudopsis Deslongchamps, 1835 (Doguzhaeva et al., 2004a, fig. 1C, E, F), the Late Triassic Phragmoteuthis vonMojsisovics, 1882 (Doguzhaeva et al., 2007c,


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