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Journal of Paleontology 92(3):412–431
diversification and dispersion event of tarphycerids, terminated by the mid-Homerian extinction. A single tarphycerid genus, Ophioceras Barrande, 1865, which includes two long-ranging species, survived this extinction. The geographic distribution of Discoceras in the Silurian is
restricted to proximal areas of low-latitude platforms, and occasionally more distal areas of black shale sedimentation. The occurrence of Discoceras in off-shore shales and the dispersion of D. graftonense in distant continents indicate its migration potential and swimming ability in surface currents across an open sea.
Figure 14. Schematic drawing of changing biological orientation in D. lindstroemi n. sp. throughout ontogeny.
the contact of the animal with the bottom (Stridsberg and Turek, 1997; Turek and Manda, 2016). Any advantage to this first decoiling is somewhat problematic. Decoiling of the shell could significantly alter the drag coefficient. Such a decoiled shell would have had a higher relative drag in comparison with a tightly coiled shell. The highly elaborate sculpture in D. lindstroemi n. sp. probably also negatively influenced the hydrodynamic properties of the shell (Chamberlain, 1976, 1981). The appearance of a heteromorph shell in D. lindstroemi n. sp. was probably an unsuccessful random evolutionary event among stratigraphically younger tarphycerids.
Conclusions
The first tarphycerids appeared in the early Tremadocian, then their diversity suddenly increased, reaching a maximum in the early Floian. Their generic diversity slowly declined in the late Upper Ordovician (Kröger and Zhang, 2009). Starting at the Late Ordovician extinction, diversity of tarphycerids was low throughout the Silurian, prior to their extinction just below Silurian-Devonian boundary. Three genera of tarphycerids are known from the Silurian, two of which survived the Late Ordovician extinction: Discoceras Barrande, 1867 and Trocholites Conrad, 1838, from which Ophioceras Barrande, 1865 probably diverged. Including the four species described here and evaluating previously published data, Discoceras comprises six Silurian species occurring in Llandovery (peri- Gondwanan Perunica) and Wenlock strata (Baltica, Laurentia, NE Gondwana, Northeast China Plate, peri-Gondwanan Per- unica). The widespread geographic distribution of D. graftonense and the origin of four endemic species of Discoceras in the middle Wenlock represented the last weak
spacing in the first whorl of Ordovician tarphycerids with their hatching phase. In this concept, hatching in tarphycerids is manifested by a sudden decrease in phragmocone chamber volume in phragmocone chambers six through eight. As a consequence, the shell of early-hatched tarphycerids would have reached slightly more than one whorl, and a shell diameter of 10–20mm. Nevertheless, there is as yet no clear evidence indicating hatching time in Discoceras. A repaired injury in a Silurian Discoceras that took place after hatching indicates that the shell of the hatched individual did not reach three-quarters of a whorl, and thus hatching preceded the change in septal spacing. Moreover, this change in spacing is not pre- sent in every specimen studied. Growth lines appear in Silurian Discoceras at about three-eighths to one-half of the first whorl. Their appearance on the shell surface coincides with a slightly changed course of the shell spire, and a slight shell expansion. More likely, the hatchling possessed only a curved shell with two or three phragmocone chambers. Early hatched specimens thus differed in habit from adults (demersal swimmers). The large volume of the first phragmocone chamber and the undeveloped hyponomic sinus in juveniles suggest a macro- planktic habit, which is consistent with early development of the evolutionarily youngest tarphycerid Ophioceras (Turek and Manda, 2016). Silurian Discoceras retained the morphology and habitats
of its Ordovician ancestors. Nevertheless, the early shell of D. lindstroemi n. sp. is tightly coiled, but subsequently, during growth, the second whorl becomes loosely coiled and then reverts again to tight coiling; furthermore, a slight indication of a second decoiling may once again appear. A heteromorphic planispiral shell with coiling that changed during ontogeny resulted in changing aperture orientation and maneuverability in life. The appearance of a heteromorphic shell in D. lindstroemi n. sp. was probably an unsuccessful random evolutionary event among Silurian tarphycerids. Simultaneously, it was the strati- graphically first appearance of a well-elaborated heteromorph shell in externally shelled cephalopods in the fossil record.
Acknowledgments
Strategic Research Plan of the Czech Geological Survey project no. 339900, Czech Grant Agency 14124S (ŠM), and Ministry of Culture project 2018/06, National Museum, 00023272 (VT) supported the research. The authors thank J. Bergström, C. Franzen, and J. Hagström (Rickmuseet Stockholm, Sweden) and S. Stridsberg (University Lund, Sweden) for access to col- lections and their kind assistance during our stay in their
Schindewolf (1934) was the first to link a change in septal
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