Pleuronautiloidea Hyatt, 1900

Superfamily Pleuronautiloidea Hyatt, 1900

Diagnosis

Superfamily of the suborder Tainoceratina with a discoidal, subinvolute to subevolute conch. Whorl profile in early species subquadrate with distinct ventrolateral shoulder and distinct umbilical margin. Derived species show a variation of modifications including trapezoidal, inverted trapezoidal or hexagonal whorl profiles with a less angular ventrolateral shoulder and umbilical margin. Whorl overlap is always very small. Sculpture in early species with transverse ribs and ventrolateral nodes, in derived species often with ribs and several rows of nodes. Septa simply domed, in derived species with a dorsal inflexion that produces an annular process. Suture line with broadly rounded lateral lobe and shallow lobe or low saddle on the venter.

Included families

Pleuronautilidae Hyatt, 1900 (Middle Permian to Late Triassic; 1 Permian genus, 10 Permian species).

Gzheloceratidae Ruzhencev & Shimansky, 1954 (Early Carboniferous to Early Permian; 5 genera, 37 species).

Mosquoceratidae Ruzhencev & Shimansky, 1954 (Late Carboniferous to Early Permian; 3 genera, 11 species).

Aktubonautilidae Ruzhencev & Shimansky, 1954 (Early Permian; 2 genera, 2 species).

Rhiphaeoceratidae Ruzhencev & Shimansky, 1954 (Early to Late Permian; 6 genera, 15 species).

Metacoceratidae fam. nov. (Late Carboniferous to Late Permian; 12 genera, 101 species).

Foordiceratidae fam. nov. (Middle to Late Permian; 3 genera, 14 species).

Remarks

Based on the phylogenetic reconstruction that was proposed by Dzik (1984), six families within the superfamily Pleuronautiloidea are distinguished here and briefly characterised as follows:

Metacoceratidae fam. nov. – Ancestral taxa with a commonly subquadrate or weakly depressed whorl profile; sculpture with conical ventrolateral nodes and sometimes with dorsolateral nodes and ribs on the flank ( Fig. 24 View Fig ).

Gzheloceratidae Ruzhencev & Shimansky, 1954 . – Ancestral taxa with a small conch and an elliptical or reniform whorl profile; sculpture with short ribs or transversely elongated tubercles in the middle of the flank ( Fig. 25A View Fig ).

Aktubonautilidae Ruzhencev & Shimansky, 1954 . – Taxa with a semicircular or reniform whorl profile and a broadly rounded venter; sculpture with elongate nodes on the flank ( Fig. 25B View Fig ).

Mosquoceratidae Ruzhencev & Shimansky, 1954 . – Taxa with a trapezoidal whorl profile and a convex venter; sculpture with longitudinally elongated tubercles on the outer flank ( Fig. 26 View Fig ).

Rhiphaeoceratidae Ruzhencev & Shimansky, 1954 . – Taxa with an oval, reniform or trapezoidal whorl profile and a flattened venter; sculpture with short ribs on the flank ( Fig. 27 View Fig ).

Foordiceratidae fam. nov. – Derived taxa with a trapezoidal whorl profile and a flattened venter; sculpture with coarse ribs or coarse conical nodes on the outer flank ( Fig. 28 View Fig ).

Pleuronautilidae Hyatt, 1900 . – Derived taxa with parallel or convergent flanks and rounded ventrolateral shoulder; sculpture with coarse ribs and sometimes with multiple rows of nodes ( Fig. 29 View Fig ).

The absence of the midventral groove separates the Pleuronautiloidea from the Tainoceratoidea . In addition, the species of the Pleuronautiloidea usually have a rectangular, trapezoidal or inverted trapezoidal whorl profile, whereas the Tainoceratoidea have a polygonal whorl profile. The sculpture of the Pleuronautiloidea does not have the characteristic rows of nodes typical for the Tainoceratoidea .

Family Metacoceratidae fam. nov.

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Fig. 24 View Fig

Type genus

Metacoceras Hyatt, 1883 .

Diagnosis

Family of the superfamily Pleuronautiloidea with an equidimensional or more commonly weakly depressed, trapezoidal to inverted trapezoidal whorl profile. Venter usually flattened, but ranging from slightly convex to slightly concave. Ventrolateral shoulder often prominent, ranging from broadly rounded to subangular. Flanks weakly convergent, parallel or weakly divergent, usually flattened and ranging from weakly convex to weakly concave. Umbilical margin usually pronounced, usually subangular in the intermediate growth stage. Sculpture with ventrolateral conical nodes, often with dorsolateral nodes and low ribs on the flank. Suture line with shallow lobes and low saddles. Internal lobe very shallow, without annular process.

Etymology

The family name refers to the type genus.

Included genera

Metacoceras Hyatt, 1883 (Moscovian to Roadian; 45 species).

? Shansinautilus Yabe & Mabuti, 1935 (Roadian; 1 species).

Cooperoceras Miller, 1945 (Kungurian; 1 species).

Epimetacoceras Librovitch, 1946 (Carboniferous) (nomen nudum).

Pseudofoordiceras Ruzhencev & Shimansky, 1954 (Artinskian to Kungurian; 7 species). Pseudotemnocheilus Ruzhencev & Shimansky, 1954 (Artinskian to? Changhsingian; 11 species). Tanchiashanites Zhao, 1954 (Roadian; 1 species).

Mahoningoceras Murphy, 1974 (Moscovian; 3 species).

Lichuanoceras Xu, 1977 (Wuchiapingian; 1 species).

Sinotitanoceras Pan, 1983 (Kungurian; 1 species).

Anthodiscoceras Qin, 1986 (Wuchiapingian; 1 species)?

New genus C to be described by Korn & Ghaderi (in press) (Wuchiapingian to Changhsingian; 22 species).

Mojsvaroceras Hyatt, 1883 (Triassic).

Huanghoceras Yin, 1933 (Asselian to Wuchiapingian; 8 species).

Remarks

Miller et al. (1933: 166) placed six Pennsylvanian genera from the American Midcontinent in the family Tainoceratidae , namely Tainoceras , Temnocheilus , Metacoceras , Endolobus , Titanoceras and Coelogasteroceras Hyatt, 1893 . This list contains a very heterogeneous collection of genera, but the authors were aware of a serious problem: “The classification of the nautiloid cephalopods is not in a satisfactory condition as is that of most of the other major groups of fossil invertebrates.” ( Miller et al. 1933: 38). They discussed an earlier idea expressed by Girty (1915) to subdivide the genus Metacoceras into subgenera or genera, but concluded that “… such subdivision, however justifiable it may be from a phylogenic point of view, is so difficult, if not impossible to carry through in practice, that it would lead to endless confusion.” Consequently, they listed about 25 Carboniferous taxa at the species level.

Miller & Youngquist (1949: 105) discussed the genus Metacoceras including the Permian material and repeated the problems posed by the wide range of variation in conch shape and ornament: “Normally, we would be inclined to regard many of the forms under consideration as varieties of established species, but such a procedure does not seem to be practicable in this case because of the extreme amount of variation in all of the characters involved. Therefore, more or less as a matter of expediency, we are recognizing most of the variants as distinct species.” Miller & Youngquist (1949) restricted Metacoceras to those species with conical nodes only on the outer flank or the ventrolateral shoulder; species with ventrolateral nodes extending as ribs onto the flank were assigned to Foordiceras . Because of this interpretation, they stated that many of the species formerly referred to Metacoceras should be better placed in Foordiceras .

Kummel (1953: 19) proposed a different concept for the genus Metacoceras and understood it with a much wider morphological range. He separated the two subgenera M. ( Metacoceras ) and M. ( Mojsvaroceras ), the former with nearly 50 species occurring in the Carboniferous and Permian and the latter with 17 species in the Triassic. Kummel did not accept the placement of species with umbilical nodes in Foordiceras as done by Miller & Youngquist (1949), instead he stated: “The basic pattern of ornamentation of Metacoceras is that of ventrolateral and umbilical nodes.” However, at the same time he stated: “The species can be separated into two groups, the first including those that have only ventrolateral nodes, and the second those that have both ventrolateral and umbilical nodes.”

Contrary to Miller & Youngquist (1949), Kummel (1953) saw close relationships between the genera Metacoceras and Pleuronautilus and transferred a number of Permian species that were previously assigned to Metacoceras to Pleuronautilus , although they show close affinities to Metacoceras . This reduced the number of species of Metacoceras but made Pleuronautilus a very large genus, spanning the Early Permian to the Late Triassic. Kummel estimated that there may be 58 species in the subgenus Pleuronautilus ( Pleuronautilus) , 24 of which are from the Permian. It is worth noting that Kummel argued that many Permian tainoceratid species previously assigned to Metacoceras or Foordiceras should be placed in Pleuronautilus based on the presence of radial ribs: “Many of these species appear to be transitional between Metacoceras and the Triassic Pleuronautilus and should be placed in the latter genus.” ( Kummel 1953: 34). He listed the species, including “ Nautilus dorso armatus ” from Dzhulfa, under Pleuronautilus ( Pleuronautilus) .

Ruzhencev & Shimansky (1954) proposed an alternative approach, which was very different from those previously outlined; their approach was based on proposed phylogenetic relationships that should be expressed in the classification of Metacoceras and its relatives. These authors had excellently preserved material for study and were therefore able to include the size, shape and ornamentation of early juvenile conch in their phylogenetic analysis. Ruzhencev & Shimansky (1954: 45) postulated that there are two separate evolutionary lineages within Metacoceras ; the European species (including those from the South Urals) are characterised by a single row of tubercles on the ventrolateral shoulder, whereas the American species possess one row of tubercles on the ventrolateral shoulder and another on the umbilical margin. They reduced the extend of the genus Metacoceras by separating the North American Permian species with ribs on the flank as Pseudofoordiceras . Furthermore, they accepted the Asian genera Huanghoceras and Shansinautilus .

In the Treatise of Invertebrate Paleontology, Kummel (1964) expressed a much more restrictive view on the tainoceratids. He did not accept the families Gzheloceratidae and Mosquoceratidae that were previously established by Ruzhencev & Shimansky (1954) and included them in the Tainoceratidae . Furthermore, ignoring the family Mosquoceratidae , he synonymised Mosquoceras Ruzhencev & Shimansky, 1954 with Metacoceras and Articheilus and placed Leonardocheilus Ruzhencev & Shimansky, 1954 in synonymy with Temnocheilus . He also synonymised a number of other genera such as Huanghoceras and Shansinautilus . In summary, Kummel’s attempt suggested the existence of some long-ranging and geographically widespread genera ( Metacoceras , Pleuronautilus ) with a very large number of species.

Shimansky (1965), when describing the Late Permian nautiloids from Dzhulfa, noted the transitional morphology of “ Metacoceras dorsoarmatum ” and “ M. dorashamense ” with “ Pleuronautilus dzhulfensis ”. It seems that Shimansky avoided to name a clear character to separate the two genera.

Metacoceras remained a species-rich genus. Shimansky (1967) listed 34 species, about half of which were from the Late Carboniferous and half from the Permian. He also listed 19 Permian species of Pleuronautilus , including some that had been assigned to the genera Huanghoceras and Pseudofoordiceras in an earlier paper ( Ruzhencev & Shimansky 1954). These two genera as well as Shansinautilus Yabe & Mabuti, 1935 and Tungkuanoceras Hajasaka, 1947 were synonymised with Pleuronautilus .

Teichert & Kummel (1973), when describing the nautiloids from the Iranian side of the Aras Valley, accepted the separation of Metacoceras (with “ Metacoceras dorsoarmatum ” and “ M. dorashamense ”) and Pleuronautilus (with “ Pleuronautilus sp. indet. 1”) as previously outlined in a similar way by Shimansky (1965). Like Shimansky, they did not provide a clear reason for this choice of separation.

Sturgeon et al. (1997: 31) discussed in detail the morphological spectrum of Metacoceras and its relationships with other tainoceratid genera. They included Late Carboniferous species with lateral ribs and umbilical nodes in Metacoceras .

An alternative division of families within the superfamily Pleuronautiloidea is proposed here. The family Metacoceratidae includes all genera that have a trapezoidal to inverted trapezoidal whorl profile and whose sculpture consists largely of ventrolateral nodes.

The family Metacoceratidae is distinguished from the other families of the superfamily Pleuronautiloidea by the following criteria:

The main difference with the partly rather similar species of the Pleuronautilidae is the sculpture, which in the Pleuronautilidae consists mainly of sharp ribs on the flanks, whereas in the Metacoceratidae it consists mainly of conical nodes and a few low and mostly rounded ribs.

The families Mosquoceratidae and Aktubonautilidae differ from the Metacoceratidae in the very large juvenile whorl; the families Gzheloceratidae and Rhiphaeoceratidae differ from the Metacoceratidae in the more elliptical whorl cross section and, at least partly, in the presence of coarse transverse ribs. The family Foordiceratidae is easily distinguished from the Metacoceratidae by the highly divergent flanks and the absence of an umbilical margin.