Temnocheilina Flower, 1963

Suborder Temnocheilina Flower, 1963

Diagnosis

Suborder of the order Nautilida , in which a ventrolateral shoulder is formed early in ontogeny; advanced species may regress this character. Conch gyroconic, advolute or tightly coiled; general shape usually discoidal, subevolute or evolute. Juvenile whorl profile depressed elliptical or bicarinate in the early species and more circular in the advanced species. Adult whorl profile rounded triangular, but also rounded trapezoidal or circular in the early species, showing minor modifications during evolution. Dorsal whorl zone very small, if present. Juvenile stage with longitudinal ridges or lines. Septa simply domed. Suture line depending on the whorl profile, with shallow lobes and low saddles. Siphuncle in central or subcentral position.

Included superfamilies

Trigonoceratoidea Hyatt, 1884 (Early Carboniferous to Early Permian; 29 genera, 149 species). Koninckioceratoidea Hyatt, 1900 (Early Carboniferous to Early Permian; 2 genera, 17 species).

Remarks

Taxonomy

The suborder Temnocheilina was named with some ambiguity, as Flower (1963) used two slightly different spellings. In the first case Flower (1963: 93) postulated: “It would seem that the Tainoceratidae represent a lineage derived from the Tetragonoceratidae ; however, unpublished material suggests a close relationship between the Devonian Rutoceratidae and the nodose Temnocheilidae and the smoother whorled Koninckioceratidae . The last two families with possible derivatives are a lineage to which the name Temnochelina may be applied.” On the same page, he referred to the publication by Shimansky (1957): “ Shimansky (1957), though using only suborders in a too-comprehensive Nautilida , has suggested the addition of Tainoceratina , Solenochilina and Temnocheilina . It would seem that there is a possible advantage in recognizing Rutoceratina and Tainoceratina , though the writer would place the Koninckioceratidae and Temnocheilidae in the Temnochelina, and add Aipoceratina to replace Solenochilina , a taxonomic recognition of the isolated position of that family.”

These statements were not critically discussed by Shimansky (1967: 38; translated from Russian): “Almost simultaneously, Flower (1963), in his interesting work on the Permian cyrtocones … speaks in some detail about the families Temnocheilidae and Koninckioceratidae , indicates the likelihood of a common origin and suggests the name Temnocheilina for this group.” Later in the article ( Shimansky 1967: 42; translated from Russian) he added: “Flower believes that the group can be considered as a suborder Temnocheilina , including the families Temnocheilidae and Koninckioceratidae .”

The problem here is that Flower (1963) apparently had no intention of defining a new suborder, and mistakenly assumed that Shimansky (1957) had already established the suborder. It is also not clear whether the name “Temnochelina” used twice was a typographical error; in the supposed reference to Shimansky (1957) he used “ Temnocheilina ”. However, it appears to be clear that Flower advocated a suborder Temnocheilina , albeit to a rather limited extent with the families Temnocheilidae and Koninckioceratidae . Furthermore, it may have been Flower’s intention to name the suborder Temnocheilina ; therefore this name is used here.

Shimansky (1957, 1962) included the family Temnocheilidae Mojsisovics, 1902 in the suborder Tainoceratina , the families Triboloceratidae Hyatt, 1884 , Thrincoceratidae Ruzhencev & Shimansky, 1954 and Phacoceratidae Shimansky, 1962 in the suborder Centroceratina and the family Koninckioceratidae Hyatt, 1900 in the suborder Liroceratina . Later, Shimansky (1967, 1979) changed his opinion; the superfamilies Temnocheilaceae Mojsisovics, 1902 , Koninckiocerataceae Hyatt, 1900 and Trigonocerataceae Hyatt, 1884 were placed in the suborder Rutoceratina .

Kummel (1964) placed the taxa of the suborder Temnocheilina as understood here in the superfamilies Tainocerataceae Hyatt, 1883 and Trigonocerataceae Hyatt, 1884 . Dzik (1984) recognised a closer relationship of genera such as Vestinautilus Ryckholt, 1852 and Temnocheilus M‘Coy, 1844 and placed these genera in the family Trigonoceratidae Hyatt, 1884 . From this he directly derived the family Phacoceratidae Shimansky, 1962 .

The concept of Dzik (1984) is followed here for the following reasons: the morphology of the initial whorl is similar in all species of the suborder, with a mostly broadly elliptical whorl profile and a sculpture of coarse longitudinal ridges. In many species a prominent ventrolateral shoulder developed from the lateral margin of the initial whorl. The ventrolateral row of nodes characteristic of Temnocheilus Mojsisovics, 1902 is not considered to be related to the nodes of members of the suborder Tainoceratina (e.g., Metacoceras Hyatt, 1883 ), but is considered to be an independent development. These nodes are not a continuation of the lateral ribs (as in Metacoceras ) and are elongated longitudinally; rather, they may be related to the ventrolateral spiral ridges in genera such as Vestinautilus .

The concept of the suborder Temnocheilina presented here differs significantly from previously published concepts of comparable taxonomic groups. It contains the majority of the Tournaisian and Viséan coiled nautiloids, which were previously distributed in the suborders Tainoceratina , Centroceratina and Liroceratina ( Shimansky 1957, 1962), the suborder Rutoceratina ( Shimansky 1967) , the superfamilies Tainocerataceae Hyatt, 1883 and Trigonocerataceae Hyatt, 1884 ( Kummel 1964) or the family Trigonoceratidae Hyatt, 1884 ( Dzik 1984).

Morphology and subdivision

The vast majority of species in the suborder Temnocheilina have a discoidal conch with a widely opened umbilicus. Especially in the stratigraphically older Tournaisian and Viséan species, there is a morphocline ranging from gyroconic, advolute to more or less tightly coiled conchs. The umbilical foramen is quite large in most of the species; it is usually more than 5 mm in diameter, but can be considerably larger. A common character of most early representatives is the triangular adult whorl profile, which is preserved in various variations throughout the phylogeny of the group. This shape evolved from an early ontogenetic depressed oval or bicarinate shape of the whorl profile, in which the initial lateral margin developed into the ventrolateral shoulder ( Fig. 3A, F View Fig ). In later species, the whorl profile of the early ontogenetic stage is usually circular. In the juvenile stage of the early species, the shell is decorated with coarse spiral ridges. These lines evolved in the different evolutionary lineages into either fine spiral ornament (e.g., Rineceras Hyatt, 1893 ) or very coarse ridges (e.g., Vestinautilus Ryckholt, 1852 ), which gave the whorl profile a polygonal shape. More advanced species usually had a pattern of finer spiral lines. Longitudinally elongated ventrolateral nodes are present in the family Temnocheilidae Mojsisovics, 1902 .

In this paper, the suborder Temnocheilina is redefined on the basis of a phylogenetic hypothesis based on the shape of the initial whorl and its modification during ontogeny. The suborder Temnocheilina contains the species with the most plesiomorphic characters of the Carboniferous and is the dominant suborder of the Early Carboniferous. The suborder Temnocheilina is divided into two superfamilies, which are characterised as follows:

Trigonoceratoidea Hyatt, 1884 . – The whorl profile in the ancestral species is often triangular or trapezoidal with a flat venter; derived species show various modifications leading to circular shapes. During ontogeny, the geometry of the conch changes slightly. The shell is usually ornamented with spiral ridges or spiral lines.

Koninckioceratoidea Hyatt, 1900 . – The whorl profile is depressed oval and does not change during ontogeny. The shell surface is largely smooth.

Origin

There are several concepts to explain the evolutionary pathways of curved and coiled nautiloids during the Devonian–Carboniferous transition and thus the origin of the taxa defined here as Temnocheilina . This group was partly named suborder Centroceratina and was interpreted by Shimansky (1957) to start with the Devonian family Centroceratidae Hyatt, 1900 ; according to this it had been derived from the family Rutoceratidae Hyatt, 1884 . At the end of the Triassic, the suborder Centroceratina gave rise to the suborder Nautilina , which includes all Jurassic to Recent coiled nautiloids.

According to the phylogenetic scheme outlined by Dzik (1984), all post-Devonian coiled nautiloids, with the exception of the aipoceratids, form a monophyletic unit with roots in the earliest Carboniferous.

Dzik & Korn (1992) presented Dasbergoceras Dzik & Korn, 1992 as a possible ancestor of the Trigonoceratidae and thus the suborder Temnocheilina and with this the majority of post-Devonian coiled nautiloids. Dasbergoceras alternans (Tietze, 1871) has an advolute conch with a large umbilical foramen ( 27 mm) and a trapezoidal whorl profile. It possesses coarse radial ribs terminating in prominent conical nodes on the ventrolateral shoulder. The siphuncle is located close to the venter. Although this species shows some superficial resemblance to species of Temnocheilus , it can hardly be considered a direct ancestor; Temnocheilus was probably derived from Subvestinautilus Turner, 1954 , as suggested by Dzik & Korn (1992: 88), and the ventrolateral row of nodes is considered a secondary character.

The ancestry of the Temnocheilina , with or without Dasbergoceras included, is an unsolved problem because of several unanswered questions:

(1) What was the morphological inventory of a possible ancestor of the Temnocheilina ? – The characters of the almost central siphuncle and the ornamentation with spiral ridges in the early juvenile stage, which are important for the Temnocheilina , are not present in the Devonian nautilids, including Dasbergoceras .

(2) Are the Temnocheilina derived from fully coiled or loosely coiled conchs? For example, does Trigonoceras M‘Coy, 1844 represent a plesiomorphic form or did the cyrtoconic conch arise by secondary uncoiling? – Intuition, based on knowledge of some other evolutionary lineages in cephalopods, would favour an evolutionary lineage from loosely coiled to fully coiled conchs. However, this observation in some of the groups may not apply in every individual case; some other cases of probable secondary uncoiling are known, for example in Maccoyoceras Miller, Dunbar & Condra, 1933 where the terminal whorl separates slightly from the preceding one ( Foord 1900; Histon 1999).

(3) Did the oldest representatives of the Temnocheilina undergo a rapid middle and late Tournaisian evolution after the Hangenberg Event at the Devonian–Carboniferous boundary, or was it a slow, long evolutionary process that started already in the Late Devonian, but was hidden without a fossil record? – The ‘nautiloid gap’ spanning most of the Late Devonian and the earliest Carboniferous with the only one known genus Dasbergoceras in the latest Devonian is the cardinal obstacle to answering this question. This is related to the general question of possible very different evolutionary rates in nautiloids.

Phylogeny

Gaps in our knowledge mean that the evolutionary pathways within the suborder Temnocheilina are, at least in part, unknown. Several questions concern the degree of involution of the conch, the shape of the whorl profile and the ornamentation:

(1) Do the gyroconic conchs (e.g., Trigonoceras M‘Coy, 1844 ) represent the plesiomorphic state or are they the result of secondary uncoiling? – This is a difficult question to answer as there are only very fragmentary records of the stratigraphically oldest representatives. Moreover, openly coiled conchs are known from both the oldest known assemblages such as Chouteauoceras Miller & Garner, 1953 in early Late Tournaisian ( Miller & Furnish 1939; Miller & Garner 1953) and from much younger assemblages such as Trigonoceras in the Viséan ( Maillieux 1925). It is possible that both evolutionary paths, including increased coiling and also secondary uncoiling, were realised in these Early Carboniferous nautiloids. However, if a form like Dasbergoceras was the ancestor, then repeated cases of secondary uncoiling must be assumed.

(2) Does the triangular (e.g., Trigonoceras M‘Coy, 1844 , Triboloceras Hyatt, 1884 ), compressed oval ( Chouteauoceras ), or depressed ovate (e.g., Rineceras Hyatt, 1893 ) whorl profile represent the plesiomorphic condition? – Similar to the general conch shape, the evolutionary pathways of the whorl profile are also not fully understood. For example, in the proposed evolutionary lineage Triboloceras - Vestinautilus - Subvestinautilus , an evolutionary trend can be observed from angular triangular to more rounded triangular and trapezoidal whorl profiles.

(3) Are coarse longitudinal ridges or more delicate spiral lines of the juvenile conch the plesiomorphic state? – Longitudinal ridges in the stratigraphically oldest known species have been reported from several Middle and early Late Tournaisian formations, including the Calcaire de Vaulx and Calonne in Belgium ( de Koninck 1878), the Chouteau Formation in Missouri ( Miller & Furnish 1939), the Marshall Sandstone in Michigan ( Miller & Garner 1953) and the Argiles de Teguentour in central Algeria ( Korn & Bockwinkel 2022). At the same time, the coarse spiral ornamentation of the earliest juvenile stage has been reported to have regressed in several species and genera. These records provide good evidence for the hypothesis that the longitudinal ridges represented a plesiomorphic state. In addition, the stratigraphic position of the material also suggests that in one derived evolutionary lineage (family Subclymeniidae Shimansky, 1962 of the suborder Domatoceratina subordo nov.) there was a phylogenetic change in the juvenile ornamentation from coarse spiral ridges (early Late Tournaisian) to fine spiral lines (latest Tournaisian) and finally to the loss of spiral ornamentation (Viséan). In others, the spiral ridges were reduced in number but became much coarser (family Epistroboceratidae fam. nov. of the suborder Domatoceratina ) or were gradually reduced (e.g., Vestinautilinae subfam. nov. and Temnocheilidae Mojsisovics, 1902 ).

Despite these partly unresolved problems, at least some possible phylogenetic lineages can be identified, which are also supported by stratigraphic data. A morphological transformation of the different conch characters can be observed:

(1) General coiling. – Within the genera Rineceras and Vestinautilus there is an evolutionary trend towards increasingly dense coiling, which could be seen as a general trend among the early representatives of the Temnocheilina .

(2) Whorl profile. – There is a trend from triangular to rounded trapezoidal shapes. This is related to the tighter enclosure of the preceding whorls. However, there is also a trend from an originally concave or at least strongly flattened venter to an increasingly convex rounded venter.

(3) Sculpture. – A general trend can be seen in the decreasing strength of the spiral ridges. Such a trend affects, for example, the number and strength of spiral ridges, as can be seen in Vestinautilus and Subvestinautilus .

Descendants

According to the current state of knowledge, three further suborders can be derived from the Temnocheilina ; these are the Domatoceratina subordo nov., the Tainoceratina and the Liroceratina . The first two are distinguished from the Temnocheilina by the more or less simultaneous early ontogenetic formation of an umbilical margin and a ventrolateral shoulder; the latter developed only an umbilical margin but no prominent ventrolateral shoulder. The demarcation of some of the genera (e.g., Catastroboceras Turner, 1965 and Epidomatoceras Turner, 1954 ) placed here in the Domatoceratina is sometimes difficult because it is not always clear whether they already possess an umbilical margin. However, this is formed by the reduction of the spiral ridges present in the early growth stage. The early ontogenetic stage of Tainoceratina , at least those from the Early Permian, apparently does not show a spiral ornament ( Ruzhencev & Shimansky 1954).