Foordiceratidae, Korn, 2025

Family Foordiceratidae fam. nov.

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Fig. 28 View Fig

Type genus

Foordiceras Hyatt, 1893 .

Diagnosis

Family of the superfamily Tainoceratoidea with a trapezoidal whorl profile; ventrolateral shoulder rounded, flanks strongly divergent. Sculpture with ventrolateral conical nodes, sometimes with 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

Foordiceras Hyatt, 1893 (Wuchiapingian to Changhsingian; 8 species).

Foordoceras Girty, 1908 [nomen nullum].

Araxonautilus Shimansky, 1979 (Wordian to Wuchiapingian; 3 species). New genus E to be described by Korn & Ghaderi (in press) (Wuchiapingian to Changhsingian; 3 species).

Remarks

The repeated stratigraphic occurrence of nautiloids with an open umbilicus, a trapezoidal whorl profile and a sculpture with ventrolateral ribs or nodes in the Late Carboniferous (e.g., Latitemnocheilus ), Early Permian (e.g., Pseudotemnocheilus , Articheilus ) and Late Permian (e.g., Foordiceras ) is a phenomenon that is not easy to explain. In order to approach this problem, four hypotheses can be discussed:

Hypothesis 1: the species of interest are a monophyletic unit descended from an ancestor with similar conch geometry and sculpture, such as the Early Carboniferous genus Temnocheilus . This may be the most parsimonious explanation in terms of morphological evolution, but it would imply a very long and simple evolutionary lineage starting in the Late Viséan and ending in the Changhsingian. It should be noted, however, that this hypothesis is mainly based on adult morphology; the juvenile conch, which is unknown in many species, plays only a minor role. For example, it is not clear whether the genera discussed also possess the characteristic bicarinate juvenile whorl profile, the longitudinal ornamentation and the deep and V-shaped inner lobe of Temnocheilus .

Sturgeon et al. (1982: 1461; 1997: 48) proposed an origin of Latitemnocheilus from Temnocheilus , because they found the longitudinal ornament characteristic for Temnocheilus also in similar development in two species of Latitemnocheilus , but not in Metacoceras . As a consequence, they concluded that “… temnocheilids were not the ancestors of, or closely related to, Metacoceras as suggested by Miller et al. (1933: p. 160), Miller & Owen (1934, p. 221) and Miller & Youngquist (1949, p. 94)”.

Hypothesis 2: the species compose a monophyletic unit, descended from a Late Carboniferous genus such as Metacoceras by a transformation of the inverted trapezoidal or almost rectangular whorl profile to a trapezoidal shape. This was accompanied by a regression of the angular umbilical margin. After this initial evolution, the lineage continued with only minor morphological changes throughout the Late Permian.

An origin of Latitemnocheilus from Metacoceras may be explained by the rather close resemblance of the adult morphology of these two genera, which differ mainly in the presence or absence of a pronounced umbilical margin.

Hypothesis 3: the species do not share the same phylogenetic origin; the Late Carboniferous and Permian nautiloids with trapezoidal whorl profile represent unrelated clades. In this hypothesis, the Early Permian genera such as Pseudotemnocheilus independently originated from Metacoceras or a similar genus by deflation of the umbilical margin. This scenario was proposed by Ruzhencev & Shimansky (1954: 45). Dzik (1984: 160) suggested to amalgamate the Early Permian species attributed to Metacoceras and Pseudotemnocheilus by Ruzhencev & Shimansky (1954) in one genus because of the minor morphological differences between the species.

Hypothesis 4: the species repeatedly descended from ancestors with a pronounced umbilical margin in the Late Carboniferous, Early Permian and Late Permian, respectively. This may be the morphologically least parsimonious solution. At the same time, it turns out that in all three cases there are morphoclines in each of the time intervals, which can be used as a good supporting argument for this hypothesis.

The ontogeny of the juvenile conch can provide solid information on the phylogenetic relationships between these species. Although the juvenile conch of the Late Permian genera is poorly known, it appears that it is more similar to the Early Permian genus Pseudotemnocheilus , with its more circular whorl profile, than to the Early Carboniferous genus Temnocheilus , which has a bicarinate juvenile conch. This may argue for an evolutionary lineage connecting the Permian groups, i.e., a descent of the Foordiceratidae from the metacoceratids.

Family Pleuronautilidae Hyatt, 1900

Fig. 29 View Fig

Diagnosis

Family of the superfamily Pleuronautiloidea with a commonly subquadrate or weakly depressed whorl profile; venter ranging from convex to weakly concave, ventrolateral shoulder and umbilical margin often pronounced, flanks usually weakly convergent. Sculpture with numerous ribs on the flank, sometimes with conical tubercles and more rarely with spiral ridges. An annular process is present in the advanced species.

Included genera

New genus C to be described by Korn & Hairapetian (in press) (Wordian to Changhsingian; 10 species). Pleuronautilus Mojsisovics, 1882 (Triassic).

Phloioceras Hyatt, 1884 (Triassic).

Anoploceras Hyatt, 1900 (Triassic).

Encoiloceras Hyatt, 1900 (Triassic).

Enoploceras Hyatt, 1900 (Triassic).

Holconautilus Mojsisovics, 1902 (Triassic).

Trachynautilus Mojsisovics, 1902 (Triassic).

Sibyllonautilus Diener, 1915 (Triassic).

Phaedrysmocheilus Shimansky & Erlanger, 1955 (Triassic).

Arctonautilus Sobolev, 1989 (Triassic).

Grumantoceras Sobolev, 1989 (Triassic).

Remarks

Pleuronautilus is a genus that has been the subject of very different opinions in the literature over the last few decades. The genus was established by Mojsisovics (1902) for the very distinctive Triassic species Pleuronautilus trinodosus . von Arthaber (1900: 215) also used the genus name for the Late Permian forms similar to the species “ Nautilus dorso armatus ” that was described by Abich (1878) and his newly established species “ Pleuronautilus Verae ”; he considered both to be closely related.

While Miller & Youngquist (1949) did not use the genus name Pleuronautilus for Permian nautilids, Kummel (1953: 34) placed 24 Permian species in this genus, together with 34 Triassic species. The reason for this high number is that Kummel also included a number of species in Pleuronautilus that had previously been placed in other genera ( Metacoceras , Foordiceras , Huanghoceras ).

Ruzhencev & Shimansky (1954) reduced the species composition of Pleuronautilus by accepting the genera Huanghoceras and Shansinautilus and by establishing the new genus Pseudofoordiceras for some species from the Leonard Formation of Texas, which were previously placed in the genus Metacoceras by Miller (1945) and in Foordiceras by Miller & Youngquist (1949). However, Shimansky (1967) considered these three genera as being synonyms of Pleuronautilus ; he listed 19 species belonging to this genus. Kummel (1964), in the Treatise on Invertebrate Paleontology, had already before synonymised the genera Huanghoceras , Shansinautilus , Tungkuanoceras , Basleonautilus and Pseudofoordiceras with Pleuronautilus .

It has already been suggested by previous authors that it is difficult to distinguish clearly between genera such as Metacoceras and Pleuronautilus in the Permian nautiloid assemblages as interpreted at that time (e.g., Kummel 1953: 34). To ensure a monophyletic definition of Pleuronautilus and related genera, it is necessary to investigate the possible phylogenetic origin of these genera and their relationships. It also needs to be clarified whether species with Pleuronautilus -like conch morphology and sculpture, such as the recently described Late Permian Serometacoceras and Lutonautilus, could have evolved independently during the Permian.

Earlier authors had already considered that, starting from the putative ancestral genus Metacoceras , lateral branches with strengthened sculpture gave rise to several genera with coarse sculpture, such as Huanghoceras from the Taiyuan Series of North China ( Yin 1933) and Pseudofoordiceras from the Leonard Formation of Texas ( Ruzhencev & Shimansky 1954). Apparently, such considerations have not yet been made for the Late Permian species of the Transcaucasus. Both Shimansky (1965) and Teichert & Kummel (1973) assigned the species of this group of species to the two genera Metacoceras and Pleuronautilus . Such a practice would imply that the latter genus actually has a Late Permian origin. However, the empirical data is hardly sufficient for such a statement.

Here, the family Pleuronautilidae is reduced in its content to the Triassic species, which share some morphological characteristics, such as the rather dense transverse ribbing. Furthermore, they could be united by the presence of an annular process of the suture line, which means that they have a dorsal inflexion of the septum.

There are some Late Permian species that have a very similar shell and sculpture to Pleuronautilus , but the apparent lack the annular process. These will shortly be described as new genus C by Korn & Hairapetian (in press) and may be the ancestors of the Triassic pleuronautilids.