Doguzhaeva and Mapes—A new Carboniferous bactritoid-like coleoid
that the different shell morphologies of the Hematitida and Donovaniconida, discussed briefly above, illustrate principle differences between these two orders that do not allow combining them into a single evolutionary lineage. As is shown above, in hematitids, a pronounced rostrum is associated with the loss of the body chamber and a proostracum-like structure, whereas in donovaniconids, there is a long body chamber, a proostracum-like structure, and a sheath-like rostrum. These two different morphological combinations of conch traits support the hypothesis that there are two very different evolutionary strategies involved in the evolutionary histories of the Hematitida and Donovaniconida. The assump- tion that belemnoids and modern coleoids originated from the order Donovaniconida via the order Phragmoteuthida (Kröger et al., 2011; Fuchs et al., 2013) in our opinion erroneously postulates that the proostracum-like structure of the donovaniconids gave rise to the broad three-part proostra- cum of the phragmoteuthids by means of elimination of the ventrolateral wall of the body chamber in donovaniconids. This still popular, but doubtful, hypothesis on formation of the proostracum by means of elimination of the ventrolateral wall of the body chamber in precursors (see Naef, 1922) has not yet been supported by the available data on the early–late Carboniferous orthocone cephalopods. Moreover, recent exam- inations of the ultrastructure of proostraca in different belemnoids and fossil gladii suggests that proostracum (Doguzhaeva, 2012; Doguzhaeva and Summesberger, 2012; Doguzhaeva et al., 2006a, 2007c) as well as gladii (Gordon, 1971; Doguzhaeva and Mutvei, 2006; Doguzhaeva and Mapes, 2015) can hardly have been derived from a conch wall. Ultrastructural and chemical examinations of the well-preserved proostraca in Late Triassic (Carnian) Phragmoteuthis and Lunzoteuthis Doguzhaeva, Summesberger, and Mutvei, 2006c from the Austrian Alps (Doguzhaeva et al., 2006c, 2007c; Doguzhaeva and Summesberger, 2012), Early Jurassic (Sinemurian) Nannobelus Pavlow, 1914 from Belgium (Doguzhaeva, 2012), and Middle Jurassic Belemnoteuthis from England (Doguzhaeva et al., 2006a) favor the interpretation of proostracum as a novelty of the skeleton in coleoids rather than as a dorsal projection of the phragmocone wall. Additionally, the small Late Triassic coleoid Lunzoteuthis, which coexisted with Phragmoteuthis in the northern Tethys, has an innovative proostracum structure by lateral fields with thin converging striations that are absent in Phragmoteuthis (compare Doguzhaeva et al., 2006c, fig. 1A–C; Doguzhaeva and Summesberger, 2012, figs. 1, 2A–B). The Lunzoteuthis proostracum type seems to have further evolved in the Early Jurassic belemnitids, in which lateral fields show overlapping, longitudinal, forward tapering, chevron-like bends that supposedly provided proper fixation of the mantle to the proostracum, which could have allowed for increased maneuverability in belemnites (Doguzhaeva, 2012, figs. 1A– D, 2A–D, 3A–F). The ancestor/descendent evolutionary rela- tionship between donovaniconids and phragmoteuthids sug- gested by Kröger et al. (2011) and Fuchs et al. (2013) is not supported by currently available data on the early Carbonifer- ous–Late Triassic coleoids (Doguzhaeva et al., 2006a, c, 2007c, 2010a; Doguzhaeva, 2012; Doguzhaeva and Mapes, 2015; Brayard et al., 2017).
Conclusions
(1) Recognition of high biodiversity among the Carboniferous coleoid cephalopods is reinforced by the bactritoid-like coleoid Oklaconus okmulgeensis n. gen n. sp. in Oklaconidae n. fam. described herein.
(2) Records of the late Carboniferous coleoids having an ink sac is expanded by the described Oklaconus okmulgeensis n. gen. n. sp.
(3) Amuscular mantle in Oklaconus okmulgeensis n. gen. n. sp. is reported; it is fossilized as a dense sheet-like structure between the conch and the matrix of the concretion and is distinguished by a crisscross pattern, fine longitudinal folds, and globular-lamellar ultrastructure.
(4) Thinning of a weakly mineralized external portion of the shell wall in the adapical direction observed in Oklaconus okmulgeensis n. gen. n. sp. supports the interpretation that this skeletal part acted as a sheath-like rostrum rather than as an outer shell wall layer.
(5) Different shell morphologies of members of the orders Hematitida and Donovaniconida refute their consideration as a single evolutionary lineage.
(6) Carboniferous evolutionary development in shelled coleoid cephalopods was perhaps principally driven by the capacity for variation among bactritoid-like coleoids, as can be seen by the diverse combinations of ‘bactritoid’ and ‘coleoid’ structures (orders Hematitida, Donovaniconida, Aulacocerida, Spirulida).
(7) Recent knowledge of the ultrastructural differences between the shell wall and proostracum or gladius, as well as an Early Triassic record of a slender gladius similar to that of extant squids, refute the hypothesis that extant gladius-bearing coleoids evolved fromDonovaniconida via Phragmoteuthida.
Acknowledgments
This study was support by the Royal Swedish Academy of Sciences and personally by Professors S. Bengtson, E. Friis, and V. Vajda, Department of Palaeobiology of the Swedish Museum of Natural History, in 2013–2016. We are grateful to C. Klug (Palaeontological Institute and Museum, University of Zurich, Zurich, Switzerland), D. Fuchs (Freie Universität, Berlin, Germany), B. Hunda (Department of Invertebrate Paleontology, Cincinnati Museum Center, Cincinnati, Ohio, USA), P. Weaver (North Carolina Museum of Natural Sciences, Raleigh, North Carolina, USA), and M. Yacobucci (Department of Geology, Bowling Green State University, Bowling Green, Ohio, USA), all of whom reviewed the manuscript.
References
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