Nemistium Smith, 1928

Fedorowski, Jerzy, Bamber, E. Wayne & Richards, Barry C., 2019, Bashkirian rugose corals from the Carboniferous Mattson Formation in the Liard Basin, northwest Canada - stratigraphic and paleobiogeographic implications, Acta Palaeontologica Polonica 64 (4), pp. 851-870 : 856-859

Genus Nemistium Smith, 1928

Type species: Nemistium edmondsi Smith, 1928 ; Eskett Quarry near Winder Station , West Cumberland, UK; late Viséan, Subzone D 2 (= Brigantian); by original designation .

Emended diagnosis.—After Hill (1981: F383), supplemented, original diagnosis in quotation marks. “Phaceloid, with peripheral increase”; “axial ends of major septa abutting axial structure”, but median lamella derived from counter (?) septum and/or septal lamellae form interrupted axial structure; “arched axial tabulae, each resting on the arch below” form continuous columnotheca; “periaxial tabulae abut axial structure”; dissepiments interseptal; microstructure finely trabecular (?).

Remarks.—Question marks in the proposed diagnosis resulted from: (i) absence of definite data concerning the septum from which pseudocolumella is separated, (ii) lack of data concerning the microstructure of septa in the type species. This feature is suggested on the basis of the Canadian material studied in this paper.

Smith (1928: 114) diagnosed his new genus as “agreeing with Diphyphyllum β in growth habit, manner of gemmation and in most internal structure, but differing from these in having a loosely formed and irregular axial structure”. Thus, he pointed to the most important characters of that genus, indicating its relationships at the same time. He did not describe the microstructure of the septa, the insertion of the septa in offsets, the way the medial lamella appeared or its relationship to the protosepta. Despite the value of those details in terms of the comprehensive characteristic of that genus, nothing has been added concerning those terms since his study. Publications by Hill (1956, 1981) and Nudds (1981) only repeated Smith’s (1928) data. Moreover, Nudds (1981) synonymized Nemistium with Lithostrotion , a suggestion that cannot be followed.

The generic name Nemistium is seldom mentioned in the rugose coral literature (e.g., Poty 1984, 2002; Rodríguez 1984; Semenoff-Tian-Chansky 1985; Fedorowski 2009; Denayer 2014), but some species assigned to Lithostrotion Fleming, 1828 or Diphyphyllum Lonsdale, 1845 may either belong to Nemistium or represent a new, related genus. Poty (1984) suggested the transfer to Nemistium of taxa assigned by earlier authors to other genera. These are: (i) Caryophyllea affinis Fleming, 1828 , listed by Poty (1984: 465) as Nemistium affine ( Fleming, 1828) . That species, redescribed by Kato (1971), is not accepted here as a member of Nemistium (see below). (ii) Three species described by Dobrolyubova (1958) from the Moscow Basin, i.e., Diphyphyllum fasciculatum ( Fleming, 1828) , Diphyphyllum gracile McCoy, 1851 and Lithostrotion scoticum Hill, 1940 . All those species are here considered to belong to a new, as yet unnamed genus. (iii) Lithostrotion (Siphonodendron) dobroljubovae Khoa, 1977 from the Lublin Basin in Poland belonging to the same genus as the Russian species Diphyphyllum fasciculatum , Diphyphyllum gracile , and Lithostrotion scoticum described by Dobroljubova (1958). In addition to those four species, two other European species, not mentioned by Poty (1984), may belong to that new genus. These are Lithostrotion caespitosum ( Martin, 1809) sensu Dobrolyubova (1958) from the Moscow Basin and the British specimens of Diphyphyllum fasciculatum . Poty’s (1984) suggestions published in a short symposium paper cannot be accepted as final for the evolutionary pattern within the family Lithostrotionidae . However, Poty’s (1984) paper is important in drawing attention to the species outside Great Britain that potentially belong to Nemistium or resemble it in morphology. The brief analysis below documents our attitude to those questions.

Caryophyllea fasciculata Fleming, 1828 was assigned by Hill (1938–1941) to Diphyphyllum . Two characters legitimize its relationship with Nemistium (all citations from Hill 1938–1941: 183): “Axial tabulae flattened, domes, their axial parts turning steeply downward either to abut on lower tabulae or to proceed into the outer tabularium”, “the columella is occasionally present, seen as thin lenticle”. Two other characters, i.e., the incompleteness of the columnotheca described above as axial parts of some tabulae extending to the dissepimentarium and the “cardinal septum frequently shorter than others” constitute the most important difference from N. edmondsi ; at least as far as the existing data suggest. The “prevalence of axial increase with 2–4 offsets arising simultaneously” will be an argument against the assignment to Nemistium and the inclusion in the new unnamed genus mentioned above, if the mode of axial increase is confirmed. However, Hill (1938–1941) did not illustrate the lectotype chosen by Smith and Lang (1930) and characterized by Kato (1971: 4) in his redescription of Fleming’s (1828) species as follows: “Axial structure is completely absent. Increase is by ‘fission’”. The latter expression can perhaps be understood as the axial increase mentioned by Hill (1938–1941). Although the existing illustrations do not confirm that increase, two independent opinions allow the exclusion of the peripheral increase typical for Nemistium . The differences prevail over the similarities and therefore Caryophyllea fasciculata is not assigned by us to Nemistium .

All four species described by Dobrolyubova (1958) and listed above resemble Nemistium in having major septa slightly amplexoid, equal in length (including the protosepta) and the columnotheca either continuous ( D. fasciculatum ) or only slightly broken by rare, sinuous tabulae extended to the dissepimentarium ( Lithostrotion cespitosum , L. scoticum , Diphyphyllum gracile ). Offsetting specimens were not illustrated by Dobrolyubova (1958) in D. gracile , but the offsetting is axial-like in all remaining species, being closely comparable to the offsets in Lithostrotion (Siphonodendron) dobroljubovae Khoa, 1977 . Most Russian species were derived from upper Viséan deposits (Aleksinskiy to Venevskiy horizons). Rare specimens of D. gracile and D. fasciculatum were collected from the Taruskiy and Steshevskiy horizons (lower Serpukhovian, Pendleian). The mode of offsetting and the incomplete columnotheca make the Russian species of D. fasciculatum comparable to the British Caryophyllea fasciculata (= Diphyphyllum fasciculatum ) to an extent which makes their conspecific position acceptable.

Kato (1971: 5, pl. 1: 1, 2) redescribed Caryophyllea affinis Fleming, 1828 , chose its lectotype, illustrated part of that colony and suggested its position within the genus Siphonodendron . He did not illustrate an offsetting corallite, but described the increase as follows: “New buds appear in the dissepimentarium as a vesicular portion for the first time”. Two facts make it possible to contest his conclusion “So the increase is peripheral.”, and to suppose lateral increase in the topotype colony: (i) The parent corallite skeleton disappears from the axial part of peripherally offsetting corallites, whereas offset in C. affine “appear in the dissepimentarium” as Kato (1971: 5) mentioned, i.e., following exactly the lateral increase of dissepimented corals. (ii) Groups of young corallites are invariably observed above the level of offsetting in peripheral increase whereas a single young corallite accompanies a mature one in lateral increase. Such a single young corallite attached to a mature one is illustrated by Kato (1971: pl. 1: 1) in the lectotype of C. affine . Thus, the increase in that species is here accepted as lateral, allowing one to accept its position within the genus Siphonodendron , as suggested by Kato (1971), but not in Nemistium , as suggested by Poty (1984). The lateral increase documented by Khoa (1977: fig. 17) in the Polish specimen assigned by that author to Lithostrotion (Siphonodendron) affine supports that conclusion to some extent.

Khoa (1977: 341, figs. 19–22) introduced the new species Lithostrotion (Siphonodendron) dobroljubovae from the Brigantian (upper Viséan) of Poland and included Lithostrotion scoticum Hill, 1940 of Dobrolyubova (1958) and, questionably, Diphyphyllum lateseptatum McCoy, 1851 of Vassilyuk (1960) in its synonymy. The slightly incomplete columnotheca with the cardinal septum shortened, the axial-like increase and the occurrence of partition but not dividing wall between the offsets in their early growth stage, are the most important characters of that thoroughly investigated species. Those characters combined with the detailed description and illustration of the remaining features, would allow the acceptance of that species as the type for the new, unnamed genus mentioned above.

Semenoff-Tian-Chansky (1985: table 9, pl. 15: 6a, b) listed and illustrated one colony from the upper Serpukhovian or lower Bashkirian strata in Algeria (North Africa) and identified it as Nemistium sp. The specimen illustrated has a continuous columnotheca, the major septa equal in length and an axial structure consisting of median lamella and short septal lamellae. All those characters correspond to the type species of Nemistium . However, the only offsetting specimen illustrated by Semenoff-Tian-Chansky (1985) does not allow us to establish the mode of its increase. It may be either lateral, if the largest corallite is parental or axial-like if that corallite is ontogenetically more advanced than the two remaining offsets. Thus, the taxonomic position of that colony and the occurrence of Nemistium in North Africa, although probable, still remains to be proven.

Poty (2002: pl. 1: 7) illustrated and named Nemistium sp. from the upper Viséan of Nova Scotia ( Canada). He gave no description for this very small, restricted fragment of a colony, represented by a single transverse section. Probable peripheral offsetting, the presence of a median lamella in the mature corallite and its absence from the offset point towards Nemistium , but lack of a longitudinal section and the restriction in size of the specimen preclude a firm identification of that colony. Nevertheless, its position within the genus Nemistium is very probable.

Fan (in Fan et al. 2003) described Diphyphyllum fasciculatum ( Fleming, 1828) and Tschussovskenia gerzeensis Fan, 2003 from the Viséan strata of Tibet, which may either belong or be related to Nemistium . The almost complete columnotheca and the inconsistent median lamella suggest that possibility. Unfortunately, offsetting corallites are not illustrated and the mode of increase is not mentioned in the descriptions. These inadequately studied corals are the only Nemistium- like species found by us in the Chinese literature. They are included in this analysis as potentially important for rugose coral palaeogeography when studied in detail.

Denayer (2014) identified as Nemistium cf. affine ( Fleming, 1828) a single, poorly preserved colony derived from a loose block in the Kisla section ( Turkey) and modified Hill’s (1981: F383) diagnosis of the genus. Some characters introduced by him in that new diagnosis are absent from the type species. These are: “major septa long, reaching the axis or not”. The septa in N. edmondsi are slightly amplexoid, but they do not reach the corallite axis. Dissepiments “V-shaped and herringbone”. Rectangular dissepiments are prevalent in N. edmondsi whereas the two kinds noted by Denayer (2014) are absent from the illustrations of Smith (1928: pl. 5: 1, 2). “Increase axial, parricidal, bipartite, tripartite or quadri-partite”, if followed, would eliminate at least one paratype colony of N. edmondsi from the genus Nemistium . That colony produced five offsets ( Smith 1928: pl. 5: 2). Also, the term “axial”, introduced by Denayer (2014) is incorrect for N. edmondsi . Its increase is peripheral as correctly indicated by Hill (1981) in her diagnosis of Nemistium . The substantial difference between those two modes of offsetting was previously diagnosed by Fedorowski and Jull (1976: 40, 41). “Minor septa long” is one of the correct items introduced by Denayer (2014) in his emended diagnosis. However, minor septa are commonly variable within the framework of particular lithostrotionid genera and cannot be considered as diagnostic at the genus level. Their length bears some diagnostic value for species, but only when compared to the length of the major septa or the width of the dissepimentarium. Denayer’s (2014) diagnosis is rendered doubtful by the above statements and by the absence of a character so important as the occurrence of the continuous columnotheca of Fedorowski (2009). Also, the latter feature was described by Hill (1981: F383) as “strongly arched axial tabulae each resting on the arch below”. Thus, we follow Hill’s (1981) diagnosis, supplemented by the data from both the type species and Nemistium liardense sp. nov.

Denayer (2014: fig. 16: Ca–c) appears to be correct in considering the increase as “axial” in his poorly preserved colony, although that increase should rather be referred to as axial-like when compared to that in Stauria (see Smith and Ryder 1927: pl. 9). The walls of the offset in Denayer’s (2014) colony, which are most probably partitions, are directly joined at the parent corallite‘s axis. The mode of increase, the occurrence of a cardinal fossula with the cardinal septum shortened, and the lack of an unquestionably continuous columnotheca in longitudinal section makes the Turkish species closely comparable with Lithostrotion (Siphonodendron) dobroljubovae Khoa, 1977 . It is also comparable in most characters to the Russian species described by Dobroljubova (1958), except for the shortened cardinal septum, but those characters exclude it from Nemistium . Thus, the phrase “All the generic characters of Nemistium Smith, 1928 are present in that [i.e., Turkish] specimen.” ( Denayer 2014: 262) does not find support in the morphology of that specimen.

The following should be pointed out to sum up this discussion: (i) Nemistium sp. of Semenoff-Tian-Chansky (1985) and Nemistium sp. of Poty (2002) may belong to that genus if the peripheral offsetting in the former and the presence of a columnotheca in the latter species are confirmed. Both are included here in the paleogeographical considerations below. (ii) The following taxa should be grouped together in a new genus closely related to Nemistium : Lithostrotion (Siphonodendron) dobroljubovae Khoa, 1977 [with Lithostrotion caespitosum of Dobrolyubova (1958) and D. latesept atum of Vassilyuk (1960) as synonyms], Diphyphyllum fasciculatum of Dobrolyubova (1958), Nemistium cf. affine of Denayer (2014) and Diphyphyllum fasciculatum of Hill (1938–1941), all of which have an incomplete columnotheca and axial-like increase. The main differentiating features with this group are the presence or absence of shortening of the cardinal septum and the presence or absence of a cardinal fossula. Those differences may be sufficient to subdivide them into two subgenera, but this question is beyond the scope of the present paper. The occurrence of a partition in the very early stage ( Khoa 1977), but the lack of a dividing wall ( Fedorowski and Jull 1976) is of special significance in that group of corals. (iii) Caryophyllea affinis Fleming, 1828 , transferred to Nemistium by Poty (1984) is here accepted within Siphonodendron as suggested by Kato (1971).

Colonies described here from both the Liard Basin and the Stikine Accreted Terrane bear all of the main diagnostic characters of Nemistium , except in their axial structures. The main axial features differentiating them from N. edmonsi are the presence of a simple, interrupted median lamella, very seldom accompanied by short septal lamellae, and a cardinal fossulae (?) developed temporarily in rare corallites. We do not consider those differences important enough to assign the North American specimens to a separate genus or subgenus. However, the trabecular septal microstructure established in our specimens (see description) may constitute a substantial generic difference if that character is absent from N. edmondsi . However, we conditionally included that character in the emended diagnosis of Nemistium .