426


Journal of Paleontology 92(3):412–431


Figure 12. Early shell of Discoceras lindstroemi n. sp.; arrows indicate appearance of growth lines, cc indicates a change in shell coiling. (1) RM Mo 59442, lateral view, counterpart, Färö, Slite beds, late Sheinwoodian. Growth ridges regularly spaced, visible from first half of whorl. Minor shell damage seen dorsolaterally at one and one-half whorls; (2, 3) RM Mo 59420, lateral view, Othem, Samsugun, Slite beds, late Sheinwoodian; (4) RM Mo 59422, lateral view, specimen with well-developed lateral furrow, Othem, Samsugun, Slite beds, late Sheinwoodian. Specimens in Figure 12.1, 12.3, and 12.4 coated with ammonium chloride before photographing. Scale bars 5mm.


clear evidence indicating hatching time in Discoceras. More likely, the hatchling possessed a curved shell (one-half or greater than three-quarters of a whorl) with two/three phragmo- cone chambers. Early hatched specimens thus differed in life from adults; the large volume of the first phragmocone chamber and the undeveloped hyponomic sinus suggest a macroplanktic habit. This is consistent with the early development of the evolutionarily youngest tarphycerid, Ophioceras (Turek and Manda, 2016). Longer chambers in the first whorl may be linked with accelerated growth before the shell reached one whorl, resulting in a stable shell orientation and higher rigidity of the shell.


Heteromorph shell in Discoceras


Silurian Discoceras shows shell morphology and habitat derived directly from Ordovician species. However, D. lindstroemi n. sp. (Figs. 4, 5) possesses a heteromorphic shell and elaborate sculpture. 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 being


tightly coiled (a slight indication of a second decoiling may once again appear; Fig. 13.3). In adult specimens, the final growth stage of the shell is straight. Changes in shell coiling resulted in a change in both the aperture orientation of the animal and the hydrodynamic properties of the shell (Chamberlain, 1976, 1981; Naglik et al., 2015). Growth stages with tightly coiled shells were nektonic, with the aperture oriented forward as in other tarphycerids (e.g., Flower, 1957; Westermann, 1998). In the growth stage with a loosely coiled shell, and in the adult stage possessing a protruding adapertural part of the shell, the aperture was oriented obliquely downward. Shells of the openly coiled stage exhibit longer phragmocone chambers, while whorl height slightly decreases. Apertures of fully grown specimens are constricted, with deepened ocular sinuses, suggesting well- developed eyes. Similarly, the deep hyponomic sinus suggests a well-developed hyponome. A premature part of the shell, either loosely coiled or


straight, is known in coiled Tarphycerida (Furnish and Glenis- ter, 1964), Nautilida (Hyatt, 1894; Turek, 1975), and Oncocer- ida (Manda and Turek, 2011). A similar shell shape is unknown in post-Triassic nautilids, probably due to increasing


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