Nemistium liardense, Fedorowski & Bamber & Richards, 2019
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publication ID |
https://doi.org/ 10.4202/app.00636.2019 |
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persistent identifier |
https://treatment.plazi.org/id/3B6D8103-F362-FFF5-7C97-1FC8DC9722C4 |
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treatment provided by |
Felipe |
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scientific name |
Nemistium liardense |
| status |
sp. nov. |
Nemistium liardense sp. nov.
Figs. 4–7 View Fig View Fig View Fig .
ZooBank LSID: urn:lsid:zoobank.org:act:8AA4EF27-438D-4449-8B20-5EF589E63209
Etymology: Named for its occurrence in the Liard Basin.
Holotype: GSC 140520 View Materials , fragments of four colonies.
Type locality: GSC locality C-442050, Mattson Gap section 20RAH9, Northwestern Territories, Canada.
Type horizon: Upper member Mattson Fm.between 1056.7 and 1057.1 m above base (1021.64 to 1022.04 m above base Mattson Fm.; 401.03 to 401.43 m above base upper member); Foraminiferal Biozone 20 of Mamet and Ross in Bamber and Waterhouse (1971), Mamet (1976).
Material.— Ten thin sections and five peels studied from Liard Basin type locality ( GSC 140520–140523 View Materials ); also, several thin sections ( GSC 140524 View Materials , 140525 View Materials ) from Stikine Accreted Terrane ( GSC locality C-442050, Foraminiferal Biozone 20 of Mamet 1976, lower Bashkirian ) examined and some illustrated in part for comparison. Colonies ranging in approximate size from 7 × 9 × 13 cm to 1 × 14 × 20 cm. Corallites silicified in part. Preservation inadequate for serial sectioning with acetate peels clear enough for blastogenetic study. Slightly altered septal microstructure locally preserved in parts of some corallites. Some or many corallites in each colony crushed by compaction .
Diagnosis.— Nemistium with n:d value of 23–25 × 2 (rarely 27 × 2), septa at 6.2–7.5 mm (rarely 10 mm) corallite diameter; median lamella thin, interrupted, absent from early growth stage of offsets; 1–3 short, thin septal lamellae may rarely occur; minor septa ranging in length from short, thin plates at periphery to penetrating tabularium; 1–3 rows of rectangular, interseptal dissepiments.
Description.—Corallites slender, densely packed in sections cut just above level of offsetting; distance between corallites increases during colony growth; very large corallites rare ( Figs. 4A View Fig 1 View Fig , B 2, D 2, C 2 View Fig , 7A View Fig 2 View Fig ). Major septa slightly amplexoid, radially arranged, slightly thickened at tabularium border, generally equal in length. Cardinal fossula - like structure in very few corallites, marked by reduction of dissepiments (Fig. 6C). Minor septa differentiated in length both within individual transverse sections of a given corallite and between corallites; may penetrate tabularium or be restricted to peripheral parts of corallites within sections of a single colony. Median lamella thin, mostly short, interrupted, in many corallites reduced to few short plates either attached to upper surfaces of tabulae or intersecting two to several tabulae, long lasting median lamellae rare, present mainly in colonies from Stikine Terrane ( Fig. 4A View Fig 2 View Fig , B 1 View Fig vs. 4C 1, D 1), extension of median lamella above tabulae (i.e., calice floor), allows it to be called a pseudocolumella; in rare instances it may protrude so as to be present in middle of offsetting specimen, from which the polyp’s body is absent at that level ( Fig. 5C View Fig ); derivation of pseudocolumella from septa observed in rare young corallites (Fig. 6B 1), but its relationship to protosepta not established. Very weak axial structure, consisting of thin pseudocolumella, one to three septal lamellae and axial tabellae exceptional (Fig. 6B 2, B 3). Dissepiments interseptal, regular in 1–3 rows; peripheral row largest, vertical in longitudinal section.
Offsetting peripheral; formation and separation of particular offsets sequential in their individual, slightly differentiated timing ( Fig. 5A, B View Fig ); beginning of offsetting results in changes difficult to recognize in transverse sections ( Fig. 5A View Fig , arrow), but well documented in longitudinal sections ( Figs. 4D View Fig 1 View Fig , 6A 3, 7A 1). Each offset inherits peripheral part of parent’s skeleton including peripheral tabularium, dissepimentarium and septa, i.e., “atavotissue” (this and following terms in inverted comas after Smith and Ryder [1927], followed in part by Hill [1935]). Thus, all septa on peripheral margins of offsets are “atavosepta”. Walls between offsets absent in early stage of offsetting ( Fig. 5B View Fig 1 View Fig , B 2 View Fig ). Uppermost basal skeleton of parent corallite becomes
Fig. 6. Transverse thin sections (except A 3, which is longitudinal section) of lithostrotionid rugose coral Nemistium liardense sp. nov. from Liard Basin, Northwestern Territories , Canada, Mattson Formation, perhaps lower Bashkirian. A. GSC 140520 View Materials , holotype. Microstructure in transverse (A 1) and oblique A 2) sections; in A 1: black arrows, tabula apparently continued sclerenchymal cover of septum; white arrow, sclerenchymal cover isolated from primary septum; in A 2: black arrow, tabula joins sclerenchymal cover directly; white arrow, lack of border between sclerenchymal cover of septum and tabula; A 3, longitudinal thin section, beginning of offsetting; two lower white arrows, last parent’s tabulae; black arrow, first offset’s tabula; neotheca indicated →
modified ( Figs. 4D View Fig 1 View Fig , 6A 3, 7A 1); particularly the axial part of columnotheca. Those modifications, slightly differentiated in terms of arrangement of tabellae in parent/offset intermediate area of individual offsets, lead to formation of offset’s columnotheca. At very early growth stage (Figs. 6A 3, 7A 1), last parential tabulae indicated by black arrows and offset’s “neotheca” marked by white arrow; peripheral tabularium absent from “neotheca” in offsets ( Fig. 7A View Fig 1 View Fig ). It appears as “neotissue” during further growth of offsets. All septa inserted in “neotheca” are “neosepta”. Median lamella or pseudocolumella almost invariably absent from offsets during their early growth stage ( Figs. 4E View Fig , 5B View Fig 3, 6B 4); it appears at various growth levels of offset, but not until complete isolation of offset in overwhelming number of instances observed, except for possibly one ( Fig. 5C View Fig , lower offset); one major septum in that offset, which is lengthened to reach the axis, can be interpreted as forming median lamella. Median lamella arose from inner margin of major septum in another, ontogenetically more advanced offset (Fig. 6B 1); formula of that septum (cardinal or counter) not established. Connection of median lamella to septum rarely observed; thus, it may last a very short time; median lamella free from all septa in almost all corallites studied. Formation of dividing wall of Fedorowski and Jull (1976) begins from depressions in “neotheca” on both sides of partition ( Fig. 5C, B View Fig 3). Those depressions separate middle line of dividing wall, step by step up to its complete division into two external walls of young corallites. Trabeculae present in partition are replaced by cryptocrystalline peripheral line of external offset’s wall, which is typical for every basal skeletal element in corals, as documented independently by Wells (1969) and Sorauf (1970). Remnants of parent’s skeleton and tissue may persist in some corallites until they accompany separated offsets (Fig. 6B 4, arrow). Offsetting in Nemistium liardense sp. nov. resembles that in Tizraia ? sp. from Bashkirian strata of Canadian Arctic Archipelago, described in detail by Fedorowski et al. (2012). Unfortunately, sequence of septal insertion in offsets of N. liardense sp. nov. could not be established. Microstructure of septa is trabecular; trabeculae approximately 0.02 mm thick, long (as documented in oblique sections of septa), and rather loosely arranged (Fig. 6A 1, A 2). One peculiar character in the relationship between the septa, dissepiments or tabulae, and the distribution of sclerenchymal sheets should be pointed out. Some basal skeletal structures join the sclerenchymal covering of septa directly (Fig. 6A 2, black arrow), or appear to be a continuation of those coverings (Fig. 6A 1, black arrows). Sclerenchymal coverings in other instances may be locally isolated from a primary septum (Fig. 6A 1, white arrow). Also, sclerenchymal coverings of septa and basal skeletal by upper white arrow. B. GSC 140523, median lamella united with major septum; slight break caused diagenetically (B 1), axial structure consisting of median lamella and irregular tabellae (B 2, B 3), offsets separated from each other completely, but rudiment of parent’s skeleton continued to occur (arrow) (B 4). C. GSC 140521, lack of dissepiments in one loculum (arrow) suggests cardinal fossula.
structures may meet without a clear border (Fig. 6A 2, white arrow). Reasons for apparent differences in distribution of sclerenchyme unknown.
Remarks.—There are two main characters of Nemistium liardense sp. nov. that distinguish it from the type species: (i) the minor septa are strongly differentiated in length, with some restricted to the most peripheral parts of corallites and (ii) the axial structure is reduced to a single, thin pseudocolumella, which appears only sporadically in most corallites. The rare incompleteness of the columnotheca ( Fig. 4C View Fig 1 View Fig , left corallite) may have resulted either from the obliqueness of the longitudinal section or from changes leading to the formation of offsets. This especially concerns the colonies from the Mattson Fm. In colonies from the Stikine Terrane both those characters are better developed, forming the main difference between those two groups of colonies of N. liardense sp. nov. We do not consider those features adequate for either distinguishing between those two groups as different species or distinguishing them from N. edmondsi as a different genus or subgenus. Specimens derived from the Stikine Terrane are more numerous and much more diversified than those from the Mattson Fm. (JF and EWB preliminary identifications, unpublished material), but their description is beyond the scope of the present paper.
Stratigraphic and geographic range. — Type locality and horizon only.
| GSC |
Geological Survey of Canada |
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