Mapes and Doguzhaeva—New Pennsylvanian coleoids from Nebraska and Iowa
mosaic combination of “old” and “new” traits in which a body chamber is an ancestral (bactritoid) trait, a pro-ostracum-like structure is a novelty, and an ink sac is a coleoid trait (Doguzhaeva et al., 2003). The Pennsylvanian genus Mutveico- nites Doguzhaeva, 2002 from Texas combines an ancestral (bactritoid) trait that is a long body chamber and a coleoid trait that is a rostrum (Doguzhaeva et al., 2006). The Mississippian genus Hematites Flower and Gordon, 1959 from Arkansas and Gordoniconus Mapes, Weller, and Doguzhaeva, 2010b from Montana, as well as the Pennsylvanian age Shimanskya Doguzhaeva, Mapes, and Mutvei, 1999 from Texas and Oklahoma, Donovaniconus, Mutveiconites Doguzhaeva, 2002, and Saundersites Doguzhaeva, Mapes, and Mutvei, 2007 from Illinois reveal the appearance of a novelty at different evolu- tionary stages together with a retention of ancestral traits for a long period after the appearance of a novelty in another form (Doguzhaeva et al., 1999, 2002a, 2006, 2010). For example, the evolutionary loss of the body chamber occurred in Hematites, while the retention of the body chamber was maintained in younger genera like Donovaniconus. The Late Mississippian Hematites and Gordoniconus and the Pennsylvanian Mutveiconites and Donovaniconus demonstrate the parallel development of a new morphological trait, namely, a rostrum, that is a long well mineralized well-shaped structure in Hematites, a long spine-shaped weakly mineralized structure inGordoniconus, a short loosely mineralized cap-like structure in Mutveiconites, and a sheath-like weakly mineralized structure in Donovaniconus. The Pennsylvanian age Shimanskya shows the ultrastructural coleoid novelty that is a shell wall formed by two loosely mineralized distinctly separated prismatic layers. A similar shell wall is secreted in the modern Spirula-Sepia branch of coleoids, but it is unknown in externally shelled cephalopods that have a shell wall formed by a nacreous layer located between two prismatic layers. The nacreous layer in the shell wall was retained in the Pennsylvanian coleoid Donovaniconus (Doguzhaeva et al., 2003). Donovaniconus shows another ultrastructural coleoid novelty in having lamello-fibrillar nacre in its septum. This ultrastructure is also known fromsepta inmodern Spirula Lamarck, 1799 (Mutvei, 1964), the Cretaceous coleoid Naefia Wetzel, 1930 (Doguzhaeva, 1996), and late Eocene cut- tlefish Belosaepia Voltz, 1830 and Mississaepia Doguzhaeva, Weaver, and Ciampaglio, 2014 (Doguzhaeva et al., 2014). Two additional evolutionary innovations in coleoid mor-
phology also deserve mention. One is the complete loss of an internal shell or internal support structure. This innovation was reported in Pohlsepia mazonensis Kluessendorf and Doyle, 2000 from Desmoinesian (=Moscovian) age sediments of the Mazon Creek Lagerstätte by Kluessendorf and Doyle (2000). The second is replacement of the calcareous internal shell with an internal organic (chitin?) skeletal support structure. This innovation is seen in Glochinomorpha Gordon, 1971 from the lower Permian of Utah (see Doguzhaeva and Mapes, 2015 for a complete discussion). To date, no transitional coleoid forms from a bactritoid-like ancestor have been recovered and descri- bed. We suspect, given the extreme shell change in chemical composition in Glochinomorpha and the complete loss of an internal shell in Pohlsepia, that such an evolutionary change must require significant time to occur and that this must have been at least as early as the Middle or Early Devonian.
147 As can be seen above, a number of different Carboniferous
coleoids with different new and old morphological combina- tions have been reported from North America in the past 20 years. These discoveries point to an evolutionary radiation in the upper Paleozoic (i.e., the upper part of the Mississippian into the Permian), as reported by Doguzhaeva et al. (2010). This variety of morphological combinations has increased the diffi- culty in determining the ancestor-descendant relationships in early coleoid taxa with those known in the Mesozoic and Cenozoic where coleoids are much better represented in the fossil record. Additionally, the earliest origins of the coleoid lineage still remain to be discovered in the lower part of the Lower Mississippian or the Devonian. By describing the Nebraska and Iowa coleoids from the Stark Shale, additional insights into upper Paleozoic coleoid morphologic features are added to the features that have been previously reported. This additional information provides greater insight into coleoid diversity and into the coleoid evolutionary radiation.
Material, geological setting, and generalized depositional environment
The examined specimens were collected from quarries in Pennsylvanian (Missourian = Kasimovian) limestone in southeast Nebraska near Bellevue, Nebraska and southwest central Iowa at the Howe Quarry in Adair County (Pope et al., 2002) (Fig. 1). In that region, the Winterset Limestone, which overlies the Stark Shale, and the underlying Canville Limestone are mined and the thin (~1m thick) Stark Shale, which is con- sidered waste rock, is removed as irregularly shaped blocks and piled in abandoned parts of the quarries. After weathering, these shale blocks can be split along bedding planes. The Stark Shale in this region is rich in phosphate, which
occurs as discrete spheroidal concretions and as thin plates between the bedding planes. The Stark Shale is currently inter- preted as having been deposited in an offshore, outer shelf environment in relatively deep water with the bottom of the water column at the water sediment interface being anoxic (Heckel, 1977). Such shales in the Carboniferous are typically conodont-rich (Heckel and Baesemann, 1975), and several conodonts are seen on the bedding planes of the coleoid-bearing shale specimens. In addition to the coleoids listed above, other recovered fauna from the Stark Shale at the coleoid-bearing localities include iniopterygians (Zangerl and Case, 1973), articulated sharks and other fish tentatively assigned to the pla- tysomoids and paleoniscoids, rare, small-sized, tightly umbili- cate ammonoids (tentatively identified as Neodimorphoceras [Schmidt, 1925], Schistoceras Hyatt, 1884, Gonioloboceras Hyatt, 1900), and arthropods including concavidcarids, tyrannophonitids, eocarids, and eurypterids (Schram, 1984; R. Pabian personal communication, 2009). The preservation of shell material from the Stark Shale is
distinctive in that all the aragonite of the coleoids (and other cephalopods) has been replaced by phosphate. This replacement has altered the ultrastructure of the shell and septa, and as such, the replacement has placed limits on utilization of an important morphological tool in the recognition of coleoids. It is not
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