Liroceratina Flower, 1955

Suborder Liroceratina Flower, 1955

Fig. 31 View Fig

Diagnosis

Suborder of the order Nautilida , in which an umbilical margin is formed early in ontogeny; advanced species may regress this character. Conch usually pachyconic and rarely discoidal or globular, subinvolute to involute. Juvenile whorl profile circular. Adult whorl profile usually circular or depressed oval without distinct ventrolateral shoulder in the early species, showing modifications during evolution (inverted trapezoidal with convergent flanks and flattened venter). Dorsal whorl zone always present, small to moderately deep. Juvenile sculpture with spiral lines that may be restricted to the umbilical area in the early species; adult sculpture usually lacking except for spiral lines in some species. Septa simply domed in the early species, with dorsal inflexion in advanced species and with corrugated septa in two derived clades. Suture line depending on the whorl profile, usually with shallow lobes and low saddles, with distinct lobes in two clades. Siphuncle in central or subcentral position.

Included superfamilies

Liroceratoidea Miller & Youngquist, 1949 (Early Carboniferous to Late Triassic; 23 Palaeozoic genera, 142 Palaeozoic species).

Ephippioceratoidea Miller & Youngquist, 1949 (Early Carboniferous to Early Permian; 3 genera, 26 species).

Clydonautiloidea Hyatt, 1900 (Middle to Late Triassic).

Remarks

Taxonomy

The suborder Liroceratina was interpreted by Shimansky (1957) as consisting of the superfamily Lirocerataceae Miller & Youngquist, 1949 with the mostly Palaeozoic families Koninckioceratidae , Liroceratidae Miller & Youngquist, 1949 , Ephippioceratidae Miller & Youngquist, 1949 and Paranautilidae Kummel in Flower & Kummel, 1950 and the fully Mesozoic superfamily Clydonautiloidea Hyatt, 1900 . Kummel (1964) included the families Liroceratidae , Ephippioceratidae , Clydonautilidae , Gonionautilidae Kummel in Flower & Kummel, 1950 and Siberionautilidae Popov, 1951 in his superfamily Clydonautilaceae .

Morphology and subdivision

Almost all Carboniferous and Permian genera of the suborder Liroceratina , which all belong to the superfamilies Liroceratoidea and Ephippioceratoidea , are easily recognised by their stout, narrowly umbilicate conch. All have either a very simple, almost straight suture line ( Liroceratoidea ) or a suture line with a high ventral saddle ( Ephippioceratoidea ). Only the Triassic descendants (superfamily Clydonautiloidea ) show a modification of the conch by lateral compression and complete closure of the umbilicus. These species show a complication of the suture line with the formation of lateral lobes; they are the nautiloids with the most complex suture lines.

The suborder Liroceratina represents a large group of species that differ in their conch morphology from the other nautiloid suborders of the Late Palaeozoic and Triassic. The liroceratids are predominantly pachyconic or globular with a narrow or completely closed umbilicus ( Fig. 31 View Fig ). The whorl profile is usually reniform, the flanks and venter often merging into a more or less uniformly convex arch. In stratigraphically older genera (e.g., Bistrialites Turner, 1954 , Liroceras ) the umbilical margin is usually uniformly rounded and the umbilical wall convex; in derived species the umbilical margin may be subangular and the umbilical wall flattened. Some derived genera ( Coelogasteroceras Hyatt, 1893 , Permonautilus Kruglov, 1933 and members of the Clydonautiloidea ) show a longitudinal depression on the venter or a galeate form ( Callaionautilus Kieslinger, 1924 ).

The suborder Liroceratina consists of three superfamilies, which differ mainly in the morphological evolution of the septal shape and the suture lines:

Liroceratoidea . – Forms with a simple suture line extending almost straight across the umbilical wall, the flanks and the venter. The dorsal suture is also usually almost straight. Some genera (e.g., Paranautilus Mojsisovics, 1902 ) have an annular process, but this may occur only intermittently during ontogeny ( Sobolev 1989: 18).

Ephippioceratoidea . – Forms with a highly elevated ventral saddle created by a striking division of the septal surface by a prominent ventrodorsal ridge into two broadly arched domes.

Clydonautiloidea . – Forms with a highly elevated ventral saddle, which is flat ( Styrionautilus Mojsisovics, 1902 ) or divided by an external lobe ( Proclydonautilus Mojsisovics, 1902 ). There may be several secondary external lobes ( Siberionautilus Popov, 1951 ) or extensive separation of the entire suture line ( Yakutionautilus Arkhipov & Barskov, 1970 ) ( Arkhipov & Barskov 1970; Sobolev 1989). The internal lobe is shallow; an annular process may be present.

Origin

The liroceratids were considered by Flower & Kummel (1950) to be the basal representatives of the order Nautilida (corresponding to the suborder Nautilina in current understanding), which, like the three other nautiloid orders of the Carboniferous and Permian accepted by them, should have originated in the Devonian family Barrandeoceratidae Foerste, 1925 . Shimansky (1957) derived the suborder Liroceratina (as he used it) from the Devonian family Litogyroceratidae Shimansky, 1957 (suborder Rutoceratina ). Kummel (1964) also saw the origin of his superfamily Clydonautilaceae (which corresponds to the Liroceratina of Shimansky) in the Devonian; he assumed that the group originated from the order Oncocerida Flower.

The assumption that the liroceratids originated in the Devonian is probably based on the problematic genus Potoceras Hyatt, 1894 . The only species, P. dubium Hyatt, 1894 , was based on a single specimen that was poorly illustrated by Hyatt. Kummel (1963) gave a detailed description and a good photographic illustration of the holotype. Hyatt (1894: 538) did not know the provenance or stratigraphic position of the specimen and believed it to be Devonian. This was confirmed by Charles Schuchert, who suggested the Iberg in the Harz Mountains as the locality, based on a spiriferid brachiopod attached to the nautilid. Kummel (1963: 356) had this information checked by G.A. Cooper, who concluded that it was probably a Viséan brachiopod. If the specimen indeed comes from the Iberg, which is by no means certain, it could actually be a specimen from the well-known Early Carboniferous Neptunian dykes, in which cephalopods and spiriferids have been identified (e.g., Schindewolf 1951).

The conch morphology of the holotype of Potoceras dubium is very similar to the typical Early Carboniferous representatives of Liroceras or Bistrialites , such as those described by Foord (1891) and Turner (1954) from northern England, by Trenkner (1868) and Schmidt (1951) from the Harz Mountains, and by Korn & Klug (2023) from the Anti-Atlas of Morocco. For this reason, too, it is reasonable to assume that the stratigraphic age of Potoceras dubium is Early Carboniferous. Potoceras may even be a senior synonym of Bistrialites or Liroceras , but this problem cannot be solved at present because of the limited data available.

Dzik (1984: 168) proposed a fundamentally different hypothesis in which the family Liroceratidae (which he defined more broadly than the other authors) was derived from an Early Carboniferous group of nautilids, that is the family Trigonoceratidae . He based this hypothesis on the juvenile ornament with spiral ridges present in both Liroceras and Vestinautilus and postulated that both genera were related through Bistrialites Turner, 1954 . There are several reasons for accepting this suggestion. An evolution from Vestinautilus to Liroceras would mainly involve a narrowing of the umbilicus, although this would be mainly due to an expansion of the ventral zone ( Fig. 3E, J View Fig ). Therefore, the spiral ridges are still located in the area of the (topographic) ventrolateral shoulder in Vestinautilus , in the middle of the flank in Bistrialites and in the (topographic) umbilical margin in Liroceras .

Phylogeny

Several scenarios have been developed to clarify the phylogeny within the liroceratids. Shimansky (1957, 1962) proposed three independent evolutionary lineages, the first of which ( Koninckioceratidae ) is placed here in the suborder Temnocheilina . The superfamily Clydonautilaceae was derived from the Lirocerataceae by Shimansky. This phylogenetic scheme was supported by Kummel (1964).

Descendants

Flower & Kummel (1950) linked, albeit with a question mark, the families that are now considered part of the suborder Nautilina Agassiz, 1847 (e.g., Nautilidae de Blainville, 1825 , Aturiidae Chapman, 1857 ) to the family Paranautilidae , which they accepted as valid. This view was not shared by later authors ( Shimansky 1962; Kummel 1964; Dzik 1984).