Journal of Paleontology, 92(3), 2018, p. 412–431 Copyright © 2018, The Paleontological Society 0022-3360/18/0088-0906 doi: 10.1017/jpa.2017.122
Silurian tarphycerid Discoceras (Cephalopoda, Nautiloidea): systematics, embryonic development and paleoecology
Štěpán Manda,1 and Vojtěch Turek2
1Czech Geological Survey, Klárov 3, 118 21 Praha 1, Czech Republic, ⟨
stepan.manda@
geology.cz⟩ 2National Museum, Department of Palaeontology, Václavské náměstí 68, 115 79 Praha 1, Czech Republic, ⟨
vojtech.turek@nm.cz⟩
Abstract.—Tarphycerids were diverse and abundant in Ordovician marine faunas. Beginning at the Late Ordovician extinction, the diversity of tarphycerids declined throughout the Silurian, until their extinction in the latest Silurian. Two genera survived the Late Ordovician extinction: Trocholites Conrad, 1838 (from which Ophioceras Barrande, 1865 probably diverged) and Discoceras Barrande, 1867 (= Graftonoceras Foerste, 1925). Discoceras graftonense (Meek and Worthen, 1870), so far known from the US, China, and Australia, is recorded from the Silurian of Bohemia and Gotland. Discoceras stridsbergi n. sp., D. lindstroemi n. sp., and D. sp. indet. from the Wenlock of Gotland and D. amissus (Barrande, 1865) from the Llandovery of Bohemia are all endemic species probably derived from D. graftonense. The distribution of D. graftonense and the origin of four species of Discoceras in the latest Sheinwoodian and early Homerian represent the last diversification and dispersion of the Tarphycerida. No tarphycerid species originated after the mid-Homerian extinction (Mulde and Lundgreni events). Silurian Discoceras retained the morphology and habitats of their Ordovician ancestors. The hatching time and autecology of juveniles has remained unclear. Evidence from the material studied suggests that juveniles were planktonic in habit, possessing a minute curved shell with few phragmocone chambers. Discoceras lindstroemi n. sp. is exceptional owing to its heteromorphic planispiral shell with coiling that changed during ontogeny, resulting in a changing aperture orientation and decreased maneuverability.
Introduction
Order Tarphycerida (Early Ordovician–latest Silurian) includes nautiloids with a tightly coiled planispiral shell, a wide siphuncle that changed its position during ontogeny, layered and thick connecting rings, and an aperture possessing a deep, narrow hyponomic sinus (Furnish and Glenister, 1964; Dzik, 1984). Tarphycerids inhabited shallow tropical and subtropical epeiric seas (Flower, 1957; Westermann, 1998), but occasion- ally spread pole-ward, into open sea or deeper water settings (Babin and Gutierréz-Marco, 1992; Manda, 2008a; Kröger et al., 2009). The planispiral shell and deep hyponomic sinus indicate that tarphycerids were capable of active swimming and were, like Nautilus, most probably demersal animals (Dzik, 1984; Westermann, 1998). In some species, the last whorl becomes uncoiled, which indicates a changed mode of life from nektonic towards a demersal habit, with limited swimming ability (Flower, 1955). The first tarphycerids appeared in the early Tremadocian, and their diversity suddenly increased, reaching a maximum in early Floian time. Their generic diver- sity fluctuated only slightly, until a slow diversity decline in the middle Katian and Hirnantian (Kröger and Zhang, 2009). The tarphycerida included the vast majority of Ordovician cephalopods possessing coiled shells. The diversity of tarphycerids after the Late Ordovician
extinction was very low throughout the Silurian. Although some species were locally abundant, only three genera/families are
known. Ophioceras Barrande, 1865 is the single known repre- sentative of the family Ophioceratidae Hyatt, 1894, and it is the only tarphycerid genus to have originated in the Silurian. It comprises only two long-ranging species, occurring in middle and upper Silurian rocks of the US, England, Bohemia, Poland, Sweden, Estonia (Stridsberg and Turek, 1997), and China (Chen et al., 1981). Trocholites Conrad, 1838 (Trocholitidae Chapman, 1857) includes more than 20 Ordovician species, but is represented in the early Silurian by only a single species known from the Anglo-Welsh Basin (Holland, 2010). Discoceras Barrande, 1867 (= Graftonoceras Foerste, 1925 and Baterboceras Zou, 1983; Discoceratidae Dzik, 1984) is a genus with more than 30 Middle and Upper Ordovician species. Including the four species described here and evaluating previously published data, Discoceras includes six Silurian species occurring in Llandovery strata in Bohemia, andWenlock strata in North America (Foerste, 1925), Australia (Etheridge, 1904), InnerMongolia (Zou, 1983), Sweden, and Bohemia. We describe here three new Silurian species of Discoceras
from Gotland (Sweden) and Central Bohemia (Czech Republic). Silurian Discoceras retained the tarphycerid shell morphology and habitat of its Ordovician ancestors, but one new species— D. lindstroemi n. sp.—exhibits an unusual coiled shell, resemb- ling that of some heteromorph ammonoids. Well-preserved specimens of Silurian Discoceras from
Gotland have made possible the study of the early growth stages of the shell as well as the reconstruction of the early ontogeny of
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