Genus Tainoceras Hyatt, 1883

Type species

Nautilus quadrangulus McChesney, 1860; original designation.

Diagnosis

Genus of the family Tainoceratidae with a subinvolute or subevolute conch; whorl profile more or less strongly depressed, ranging from subquadrate and hexagonal to polygonal with a distinct midventral longitudinal groove. Umbilical margin usually pronounced and subangular in the intermediate stage, rounded in the adult stage. Sculpture usually with two rows of conical nodes on the venter and additional rows on the flank. Septa simply domed, suture line strongly depending on the shape of the whorl profile, usually with shallow external lobe and broadly rounded lateral lobe. Siphuncle small with subcentral position ventrad of septum centre.

Included Carboniferous species

North America (McChesney 1860; Miller et al. 1933; Miller & Unklesbay 1942; Lintz 1958; Tucker & Mapes 1978; Sturgeon et al. 1982): Nautilus quadrangulus McChesney, 1860, Gzhelian, Illinois; Nautilus nodocarinatus McChesney, 1860, Gzhelian, Illinois; Tainoceras monilifer Miller, Dunbar & Condra, 1933, Gzhelian, Texas; Tainoceras rotundatum Miller, Dunbar & Condra, 1933, Gzhelian, Texas; Tainoceras murrayi Miller & Unklesbay, 1942, Gzhelian, Nebraska; Metacoceras marylandica Lintz, 1958, Gzhelian, Maryland; Tainoceras sexlineatum Tucker, 1976, Kasimovian, Illinois; Tainoceras collinsi Sturgeon, Windle, Mapes & Hoare, 1982, Gzhelian, Ohio.

Donets Basin (Dernov 2024): Tainoceras luxaeterna Dernov, 2024, Kasimovian.

Western Russia (Waagen 1879): Nautilus Trautscholdi Waagen, 1879, Gzhelian, Moscow Basin.

Included Permian species

North America (Swallow 1860; Hyatt 1891, 1893; Miller et al. 1933; Miller & Thomas 1936; Miller & Unklesbay 1942; Miller & Kemp 1947; Miller & Youngquist 1949): Nautilus occidentalis Swallow, 1860, Artinskian, Kansas; Tainoceras cavatum Hyatt, 1891, Asselian, Texas; Tainoceras Duttoni Hyatt, 1893, Asselian, New Mexico; Tainoceras nebrascense Miller, Dunbar & Condra, 1933, Artinskian, Nebraska; Tainoceras wyomingense Miller & Thomas, 1936, Asselian, Wyoming; Tainoceras schellbachi Miller & Unklesbay, 1942, Kungurian, Arizona; Tainoceras clydense Miller & Kemp, 1947, Kungurian, Texas; Tainoceras unklesbayi Miller & Youngquist, 1949, Kungurian, Texas.

Alps and Southern Europe (Gemmellaro 1889; Simić 1933; Schréter 1974; Prinoth & Posenato 2007): Pleuronautilus Toulai Gemmellaro, 1890, Wordian, Sicily; Tainoceras zmajevatense Simić, 1933, Wuchiapingian, Serbia; Tainoceras bükkense Schréter, 1974, Changhsingian, Bűkk Mountains; Tainoceras crassicostatum Schréter, 1974, Changhsingian, Bűkk Mountains; Tainoceras balestense Prinoth & Posenato, 2007, Changhsingian, Dolomites; Tainoceras malsineri Prinoth & Posenato, 2007, Changhsingian, Dolomites.

NW Iran (this paper): Tainoceras admonens sp. nov., Wuchiapingian; Tainoceras latecostatum sp. nov., Wuchiapingian; Tainoceras unitum sp. nov., Changhsingian.

Central Iran (Korn & Hairapetian in press): new species N to be described by Korn & Hairapetian (in press), Changhsingian, Central Iran.

Pakistan (Reed 1931, 1944): Tainoceras Noetlingi var. subglobosa Reed, 1931, Wuchiapingian, Salt Range; Tainoceras comptum Reed, 1944, Changhsingian, Salt Range; Tainoceras debile Reed, 1944, Changhsingian, Salt Range; Tainoceras trimuense Reed, 1944, Wuchiapingian, Salt Range.

South China (Kayser 1883; Chao 1954; Zheng 1984): Nautilus mingshanensis Kayser, 1883, Wuchiapingian, Jiangxi; Nautilus orientalis Kayser, 1883, Wuchiapingian, Jiangxi; Tainoceras changlingpuense Chao, 1954, Roadian, Hunan; Tainoceras hunanense Chao, 1954, Roadian, Hunan; Tainoceras gibbosum Zheng, 1984, Changhsingian, Guizhou; Tainoceras guizhouense Zheng, 1984, Changhsingian, Guizhou; Tainoceras lateronodosum Zheng, 1984, Changhsingian, Guizhou.

Japan (Hayasaka 1957, 1962; Ehiro & Araki 1997): Tainoceras abukumense Hayasaka, 1957, Capitanian; Tainoceras kitakamiense Hayasaka, 1962, Roadian; Tainoceras carinatum Ehiro & Araki, 1997, Capitanian.

Remarks

Tainoceras is the typical representative of the family, superfamily and suborder named after it and is considered one of the cardinal genera within the order Nautilida. About 40 species of Tainoceras have been described so far and the genus has a long stratigraphic range extending from the latest Carboniferous to the latest Permian. However, only a few efforts have been made to clearly define the genus in its morphological range. While Miller et al. (1933: 147) and Miller & Youngquist (1949: 80) gave a detailed characterisation of the genus, Shimansky (1962b: 121) and Kummel (1964: K413) characterised the genus Tainoceras with just one sentence: “Like Metacoceras but with a double row of nodes on the venter.” Sturgeon et al. (1997: 29) were more precise: “Similar to Metacoceras but possessing two ventral rows of nodes or ribs separated by a median sulcus.” They gave a more detailed outline of the characters typically present in Tainoceras.

It is apparent that a simple definition is not adequate to define the rather complex genus, especially in respecting the fact that other tainoceratid genera have been established in recent decades. A precise morphological delineation and taxonomic interpretation of Tainoceras requires a discussion of several questions:

(1) What are the morphological characters that can be used to clearly distinguish Tainoceras from other genera?

(2) Are the tainoceratids (Tainoceras and derived genera of the Late Permian) really a monophyletic unit?

(3) Did long-ranging evolutionary lineages with stable morphology exist within Tainoceras, or did similar conch shapes and sculptures emerge iteratively and independently?

The first question is not easy to answer. The previously used character of a double row of ventral nodes cannot be used universally as a cardinal separating character because some species have only one row of ventral nodes or no ventral nodes at all in the adult stage (e.g., the Late Carboniferous T. collinsi and T. marylandicum and the Late Permian T. balestense). Instead, other supporting characters, such as the presence of the midventral groove, must be used. It should be made clear that in Tainoceras this groove incurves a broadly rounded venter. This is in contrast to genera such as Metacoceras, in which a concave venter, if present at all, always occurs as a shallow depression of the entire venter.

The second question is easier to answer. Although the morphological spectrum of Tainoceras is rather broad and somewhat variable, the combination of several morphological characters, such as the presence of the midventral groove, the rows of nodes on the venter and ventrolateral shoulder and the pronounced umbilical margin, suggests a monophyletic series of forms. Due to the complexity of the morphology, a polyphyletic origin of Tainoceras can be excluded.

To answer the third question, it is necessary to evaluate the characters of conch geometry and sculpture with regard to their variation within the genus Tainoceras. The following characters have proven to be particularly variable (with some representative examples):

- General shape of the whorl profile: it can range from rectangular (T. nebrascense) to octagonal or polygonal (T. clydense, T. admonens sp. nov.); the ww/wh ratio can range from approximately equidimensional (T. cavatum) to weakly depressed (T. schellbachi, T. admonens sp. nov.) and moderately depressed (T. duttoni).

- General shape of the venter: in all of the species, the venter is more or less tripartite, but the degree of tripartition varies from weak with nearly convex venter (T. cavatum) to very strong with clearly defined ventrolateral applanation forming a tectiform venter (T. clydense, T. duttoni).

- Arrangement and shape of the flanks: the flanks can be divergent (T. quadrangulum, T. admonens sp. nov.), parallel (T. quadrangulum) or convergent (T. duttoni). They can be flattened (T. cavatum) or weakly concave (T. duttoni, T. admonens).

- Shape of the umbilical margin and umbilical wall: while the umbilical margin is usually narrowly rounded (T. nebrascense) or subangular (T. duttoni), the umbilical wall ranges from oblique (T. cavatum, T. clydense) to steep (T. duttoni) and from weakly convex (T. nebrascense) to flattened (T. duttoni, T. admonens sp. nov.).

- Width and depth of the midventral groove: the groove can vary from narrow (T. nebrascense, T. clydense) to wide (T. schellbachi, T. admonens sp. nov.) and from shallow (T. collinsi, T. cavatum, T. admonens) to deep (T. duttoni, T. wyomingense).

- Formation of the ventral nodes: ventral sculptural elements may appear as short plications (T. collinsi), small tubercles (T. nebrascense, T. monilifer, T. nodocarinatum, T. admonens sp. nov.), or also large conical, blunt nodes (T. clydense) or pointed nodes or spines (T. quadrangulum, T. schellbachi). Some species possess coarse ventral transverse ribs (T. unklesbayi). The ventral tubercles may be arranged symmetrically (T. nebrascense) or alternating on the right and left sides of the midventral groove (T. monilifer).

- Formation of lateral ribs: lateral ribs do not occur in many of the North American species, but are present in species from other regions (T. debile, T. admonens sp. nov.).

- Formation of umbilical nodes: umbilical sculptural elements are present in some species; they range from being small tubercles (T. clydense) to large conical nodes (T. schellbachi).

The list of mostly bipolar character pairs shows numerous theoretically possible combinations; however, covariation is very common and some of the characters often appear simultaneously.