Isorthoceras suave Kröger 2013, comb. nov.
Creators: name: Kröger, Björn
Publisher: name: Zenodo
Year: 2013
Subjects: Biodiversity
Taxonomy
Animalia
Mollusca
Cephalopoda
Orthocerida
Proteoceratidae
Isorthoceras
Isorthoceras suave

description: Isorthoceras suave (Angelin in Angelin & Lindström, 1880) comb. nov.
Figs 17I, 23B, 28B
Orthoceras suave Angelin in Angelin & Lindström, 1880: 3, pl. 7, figs 13-17.
Diagnosis
Isorthoceras with weakly curved conch and circular or slightly compressed cross section; angle of expansion ca. 9° in premature growth stages; ornamented with fine longitudinal raised lirae and faint
transverse irregularly spaced growth lines; mature size more than 20 mm; siphuncle eccentric to subcentral at convex side of conch curvature.
Type material
Lectotype
NRM-Mo 151187 (Angelin & Lindström 1880: pl. 7, figs 16-17) (designated herein).
Paralectotype
NRM-Mo 151186 (Angelin & Lindström 1880: pl. 7, fig. 14).
Type locality and horizon
Ostbjörka, Siljan district, Dalarna, Sweden; Boda Limestone, Boda Core Member, late Katian, Ordovician.
Other material examined
Eight specimens: three specimens (PMU 26835–26837), Osmundberget, Dalarna; four specimens (PMU 26838– 26841), Kallholn, Dalarna; all from the Boda Limestone, Boda Core Member, late Katian.
Decription
The lectotype is a fragment of a very slightly curved phragmocone, 42 mm long, with a diameter that increases from 8 to 14 mm (angle of expansion 8.1°). It is ornamented with fine longitudinal raised striae.
Specimen PMU 26835 is a nearly straight portion of phragmocone 43 mm long, increasing in diameter from 10 to 17 mm (angle of expansion 9.2°) (Fig. 23B) and a circular conch cross section. The siphuncle is nearly central, 5.5 mm from the conch margin at a corresponding conch diameter of 12.5 mm (SPR 0.44). The chamber height is 3 mm at a diameter of 14.5 (0.21 of conch diameter). The diameter of the septal perforation is 1 mm at a conch diameter of 12.5 mm (0.08 of conch diameter). The specimen is ornamented with fine raised lirae, ca. five per millimeter, and very faint, irregularly spaced growth lines ca. 1 mm apart (Fig. 17I). Approximately 34 ca. 0.7 mm wide dark bands run longitudinally around the entire circumference (Fig. 28B).
In another specimen (PMU 26836) the siphuncle is more eccentrically positioned. At a conch diameter of 21 mm, the siphuncle is positioned 8 mm from conch margin (SPR 0.38).
The angle of expansion of the six available specimens varies between 5.7 and 9.9°, with no size trend. The chamber height varies between 0.2 and 0.3 of the corresponding conch height.
Remarks
The adult size of this species must be clearly more than 20 mm. The largest specimen (PMU 26836) in the collection is a fragment of a phragmocone with a diameter of 21 mm and the septal spacing shows no signs of septal crowding. The longitudinal ornamentation is crossed only by faint, irregularly spaced growth lines. A total of 34 longitudinal color bands are preserved around the entire circumference of the specimen PMU 26835. This occurrence of longitudinal color bands is known from other species of Isorthoceras (see Kröger et al. 2009; Fig. 28), but their significance for taxonomy is not known, because of the rarity of this type of preservation.
Comparison
This species is unique amongst species considered to belong to Isorthoceras in having exclusively longitudinal lirae.
Stratigraphic and geographic range
Boda Limestone, late Katian, Dalarna, Sweden.
description: Published as part of Kröger, Björn, 2013, The cephalopods of the Boda Limestone, Late Ordovician, of Dalarna, Sweden, pp. 1-110 in European Journal of Taxonomy 41 on pages 64-66, DOI: 10.5852/ejt.2013.41, http://zenodo.org/record/3814221