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Journal of Paleontology 92(2):157–169


can be explained as a result of a chemical reaction in a slightly acid or neutral (as opposed to alkaline) bottom water medium (Fox, 1966) causing melanin (the main ink component providing the black color) to precipitate into a solid phase while still in the coleoid body. In an alkaline environment, melanin would be dispersed colloidally (Fox, 1966). To preserve the ink, the burial environment must have also been anoxic or strongly dysoxic, which is the case for the Bear Gulch Limestone in Montana, the Wewoka Formation in Oklahoma, and the Stark Shale in Nebraska. By contrast, the Illinois occurrence is thought to have been a shallow-water deposit in a delta, however, rapid concretion formation acted as an oxygen shield, which preserved the ink in the coleoids.


Recent knowledge on the early to late Carboniferous shelled coleoids.—The first recognized Carboniferous coleoids— Hematites, Paleoconus, and Bactritimimus—were distinguished by the presence of a well-developed rostrum covering a brevi- conic phragmocone (Flower and Gordon, 1959). An ultra- structural approach for recognition of Carboniferous coleoids was introduced at the end of the twentieth century. This method is based on the evolutionary stability of shell ultrastructure in cephalopods (for more information, see Doguzhaeva, 1994, 1996, 2002a, 2008, 2012; Doguzhaeva et al., 1996, 1999, 2002a, 2006b, 2010a, 2017). Ultrastructural methods were used to help recognize the late Carboniferous coleoid Shimanskya Doguzhaeva, Mapes, and Mutvei, 1999 in which the shell wall, as in extant Spirula Lamarck, 1799 and the Early Cretaceous spirulid Adygeya Dogushaeva, 1996, lacks a nacreous layer (Doguzhaeva et al., 1999). Shimanskya shell-wall type was also identified in several taxa that were earlier described as bactritoids (Mapes, 1979). The taxa ‘Bactrites’ quadrilineatus Mapes, 1979, ‘Ctenobactrites’ lesliensis Mapes, 1979, and ‘Rugobactrites’ jacksboroensis Mapes, 1979 from North America are now considered to be coleoids (Mutvei et al., 2012). Based on shell-wall ultrastructure, ‘Bactrites’ carbonarius Smith, 1903 is referred to Coleoidea as well (Doguzhaeva and Mapes, 2017). Using ultrastructural exam- ination of the concretion matrix in front of conch apertures, the arm hooks were revealed and defined as a coleoid affiliation of Saundersites from the Mazon Creek Lagerstätte in Illinois (Doguzhaeva et al., 2007a) and Gordoniconus from Bear Gulch Bes in Montana (Mapes et al., 2010a). This approach also helped identify the arm hooks, a Saundersites-type radula, and the cartilaginous capsule of an unnamed coleoid from Eudora Shale, Oklahoma preserved without any traces of a mineralized conch (Doguzhaeva et al., 2010b). Therefore, we now know that Carboniferous coleoids


possessed the following innovative structures: (1) a rostrum with a free postalveolar portion (Hematites, Gordoniconus, Saunder- sites), (2) a primordial rostrum (Mutveiconites Doguzhaeva, 2002a), (3) a thin sheath-like rostrum(Donovaniconus, Oklaconus n. gen.), (4) loss of the body chamber (Hematites), (5) a shell wall without nacre (Shimanskya,


‘Bactrites’ quadrilineatus,


B. carbonarius, ‘Ctenobactrites’ lesliensis, ‘Rugobactrites’ jacksboroensis, Oklaconus n. gen.), (6) a proostracum-like structure (Donovaniconus,


Saundersites, Gordoniconus,


Oklaconus n. gen.), (7) an ink sac (Gordoniconus, Saundersites, Donovaniconus, Flowerites Mapes et al., 2010a, Oklaconus n.


gen.), (8) arm hooks (Gordoniconus; Saundersites; unnamed coleoid from Eudora Shale, Oklahoma, Doguzhaeva et al., 2010b), (9) lamellar-fibrillar nacre in the septa (Shimanskya, Donovaniconus), (10) a radula with two marginal plates on each side, which is atypical for nautiloids (Saundersites; unnamed coleoid fromEudora Shale, Oklahoma, Doguzhaeva et al., 2010b), and (11) a muscular mantle on the conch surface (assumed in all; observed in Saundersites and Oklaconus n. gen.). These novelties, together with morphological structures derived from the ancestral bactritoid stock (i.e., a spherical protoconch, a straight phragmo- cone, a small ventral siphuncle, thin nonbiomineralized connecting rings, a long body chamber, a nacreous layer in the shell wall, and columnar nacre in the septa) provide the diverse morphological combinations considered to be high-level taxonomic traits. These diverse combinations of both ancestral and innovative structures show that Carboniferous coleoids possessed high morphological plasticity with a capacity for being altered (Doguzhaeva et al., 2010a). Novelty appeared in Carboniferous coleoids at an early evolutionary stage in one taxon, but the ‘old’ traits existed for a long time after the novelty appeared in other taxa. An example of this phenomenon is the lack of a body chamber in the early Carboniferous Hematites (Doguzhaeva et al., 2002a). Hematites and Gordoniconus are the earliest recorded Carboniferous coleoids (dated 318–333 Ma), however, Gordoniconus has a long body chamber (Mapes et al., 2010b). Many of the younger coleoids, e.g., the late Carboniferous Oklaconus n. gen. and Donovaniconus in the Donovaniconida, and juvenile Mutveiconites in the Aulacocerida, have body chambers (Doguzhaeva, 2002a; Doguzhaeva et al., 2002d, 2003, 2006b; herein). In Hematites, loss of the body chamber did not lead to formation of a proostracum or proostracum-like structure. This illustrates forms at an early stage of coleoid evolution that lacked skeletal protection in the form of a body chamber, yet did not have dorsal support for the body in the shape of a proostracum (Doguzhaeva, 2012). Another character of the Carboniferous stage of coleoid


evolution is the independent appearance of new morphological traits and their further convergent evolution. This is exemplified by rostrum development. The early Carboniferous Hematites has a comparatively massive rostrum with a unique ultrastruc- ture (Flower and Gordon, 1959; Doguzhaeva et al., 2002a), whereas the early to late Carboniferous donovaniconids Gordoniconus, Saundersites, Donovaniconus,and Oklaconus n. gen. have thin sheath-like rostra, and the late Carboniferous aulacocerid Mutveiconites has a primordial rostrum continuing as a sheath-like rostrum along the phragmocone (Doguzhaeva, 2002a; Doguzhaeva et al., 2006b). These observations suggest that the long-term (~60 Myr) Carboniferous evolution of coleoid cephalopods included many experimental attempts before a comparatively stable coleoid model, similar to that of modern gladius-bearing taxa, appeared in the Early Triassic (Olenekian) (Brayard et al., 2017). Kröger et al. (2011), in our opinion, erroneously suggested that the orders Hematitida and Donova- niconida could represent the evolutionary lineage that gave rise to Middle–Late Triassic phragmoteuthids and Permian–Cretac- eous belemnitids. Fuchs et al. (2013, fig. 12) also erroneously assumed that the Carboniferous Donovaniconida and the Middle Triassic–Early Jurassic Phragmoteuthida gave rise to all of the Mesozoic coleoids. It is worth noting in this context


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