Trocholitidae Chapman 1857

Family Trocholitidae Chapman, 1857 Diagnosis (from Aubrechtová & Korn 2025) Family of the order Tarphyceratida with subglobular to discoidal conchs consisting of three to five or more gradually expanding volutions; body chamber may uncoil from preceding volution. Terminal aperture of body chamber with flares in some taxa, aperture often modified with ventral sinus and lateral projections. Whorl profile usually with dorsal imprint zone; usually depressed except for the adult growth stage that may have an equidimensional or compressed whorl profile; flanks and venter broadly rounded or flattened. Shell ornament varies during ontogeny, usually consisting of imbricating and/or frilled lirae, ribs or annuli, but taxa with smooth shell surface exist; ornament elements form ventral sinus. Conch in some taxa with periodic constrictions. Phragmocone chambers short (on average six phragmocone chambers relative to whorl height). Suture line simple with shallow ventral lobe or rarely saddle. Siphuncle subcentral, subdorsal or marginodorsal in position, ventral to central in initial volution; siphuncle segments tubular, weakly convex or weakly concavo-convex, narrow in diameter (usually less than 0.25 of apertural height); septal necks short (less than 0.20 of segment length), orthochoanitic or loxochoanitic; connecting rings usually thick, layered, but becoming thin and homogeneous in some taxa. Genera included Arkoceras Ulrich, Foerste, Miller & Furnish, 1942; Crenuloceras Flower, 1968; Curtoceras Ulrich, Foerste, Miller & Furnish 1942; Discoceras Barrande, 1867; Hardmanoceras Teichert & Glenister, 1952; Jasperoceras Ulrich, Foerste, Miller & Furnish, 1942; Litoceras Hyatt, 1884; Paradiscoceras Barskov, 1972; Plectolites Flower, 1968; Trocholites Conrad, 1838; Trocholitoceras Hyatt, 1894; Weberoceras Barskov, 1972; Wichitoceras Ulrich, Foerste, Miller & Furnish, 1942; Yushanoceras Chen & Liu, 1976. Remarks Trocholitidae Schröder, 1891 is a junior homonym of Trocholitidae Chapman, 1857 and consequently, Chapman (1857) is the author of the family name. Some authors have previously included Discoceras in the family Discoceratidae Dzik, 1984, which is, however, a junior homonym of Discoceratidae Hyatt, 1894. The latter originally included several early Palaeozoic genera (besides Discoceras also Peismoceras Hyatt, 1894, Systrophoceras Hyatt, 1894, Trochoceras Barrande, 1848, Mitroceras Hyatt, 1894 and Plectoceras Hyatt, 1884), which have later been shown to belong to various, unrelated lineages. For example, the Silurian lechritrochoceratids Peismoceras and Systrophoceras represent basal members of the order Nautilida Agassiz, 1847 (Dzik & Korn 1992; Manda & Turek 2019). When all the unrelated genera are excluded, the family Discoceratidae is left with just Discoceras, the name-bearing taxon. However, the separation of Discoceras at the family level is not justified – the conch geometry, internal conch structures and ornament in Discoceras all perfectly fall within the morphological variability observed among the representatives of the family Trocholitidae. Thus, the family Discoceratidae Hyatt, 1894 is synonymised here with the family Trocholitidae Chapman, 1857.