Maccoyoceras pentagonum, Korn & Miao & Bockwinkel, 2022

Korn, Dieter, Miao, Luyi & Bockwinkel, Jürgen, 2022, The nautiloids from the Early Carboniferous Dalle à Merocanites of Timimoun, western Algeria, European Journal of Taxonomy 789, pp. 104-129 : 107-110

Maccoyoceras pentagonum sp. nov.

urn:lsid:zoobank.org:act:5C272A6C-5B23-4D36-B28B-8650C8A6D38B

Figs 2–3 ; Table 1

Diagnosis

Species of Maccoyoceras reaching about 80 mm conch diameter. Conch with weakly compressed to circular whorl profile (ww/wh = 0.80–1.00); venter flattened or completely applanate, ventrolateral shoulder narrowly rounded or subangular. Whorls very weakly embracing or just touching. Ornament in the juvenile stage with spiral lines on the flank and outer area of the venter, in the adult stage without spiral lines. Growth lines with weakly biconvex course with a shallow lateral sinus and a moderately high ventrolateral projection.

Etymology

From the Latin ‘ pentagon ’; referring to the shape of the whorl profile.

Type material

Holotype ALGERIA • Gourara, Sebkha de Timimoun 14.5 km west-southwest of Timimoun; “ Dalle à Merocanites ” (Tournaisian-Viséan boundary interval); illustrated in Fig. 2A ; MB.C.30378.1.

Paratypes ALGERIA • 33 specimens; Gourara , Sebkha de Timimoun 14.5 km west-southwest of Timimoun; “ Dalle à Merocanites ” (Tournaisian-Viséan boundary interval); MB.C.30378.2 – MB.C.30378.34 .

Description

Holotype MB.C.30378.1 is the largest specimen available; it has a conch almost 74 mm in diameter ( Fig. 2A ). It is extremely discoidal and subevolute (ww/dm = 0.31; uw/dm = 0.37) with compressed whorls (ww/wh = 0.80) and a very high coiling rate (WER = 2.65). The inner whorls are loosely coiled with a 6 mm wide umbilical window; the dorsum touches the venter of the previous whorl after one and a quarter whorl. The first whorl and a half are circular in profile. Thereafter, the venter becomes flattened and bordered to the flanks by a subangular shoulder.At 25 mm in diameter, the umbilical margin becomes narrowly rounded. During the last half whorl, the ventrolateral shoulder becomes rounded and the venter is broadly convex in profile ( Fig. 3A ).

The ornament shows a succession of four developmental stages in the present specimens:

(1) On slightly more than the first volution, up to a conch diameter of 15 mm, coarse spiral lines and growth lines form a granular surface on the flanks. The growth lines extend backwards on the flank.

(2) For almost three quarters of a whorl between 15 and 28 mm in diameter, the flanks are ornamented with spiral lines that become strikingly finer during this growth interval. At the beginning of this stage, they are much wider than their interstices, but at the end they are visible as very fine lines separated by interstices ten times wider. Some of the spiral lines granulate out to a diameter of the conch of 23 mm.

(3) A short segment of a volution (between 28 and 32 mm dm) shows hardly visible spiral lines; biconvex, rather coarse growth lines with a weakly biconvex course are the dominant type of ornament.

(4) Beyond 32 mm diameter of the conch, the ornament consists only of coarse growth lines. They are weakly biconvex with a very low dorsolateral projection, a shallow lateral sinus, a low ventrolateral projection; the venter is almost smooth.

The other specimens illustrated here largely confirm the ornament development of specimen MB.C.30378.1 or provide additional information. Specimen MB.C.30378.4 ( Fig. 2D ) has a very wellpreserved shell ornament; it clearly shows the sudden change that terminates the second ontogenetic ornament stage at 26 mm diameter. The third stage with clearly weaker spiral lines is only 45 degrees long.

Specimen MB.C.30378.8 ( Fig. 2H ) shows, up to at least 40 mm conch diameter, spiral lines on the venter. Specimen MB.C.30378.5 ( Fig. 2E ) possesses, between 15 and 20 mm diameter, rather coarse spiral lines.

Between 20 and 70 mm shell diameter, there are ontogenetic changes in the shell proportions ( Fig. 3B– E ). These specimens show that the conch becomes more slender (ww/dm decreases from 0.40 to 0.30) and slightly more involute (uw/dm decreases from 0.45 to 0.40). Additionally, the whorl profile becomes more compressed (ww/wh decreases from 1.10 to 0.80). For the first three parameters of the conch, the

intraspecific variation is small ( Fig. 3B–D ). In contrast, the plasticity of the coiling rate is very striking; the whorl expansion rate ranges from 2.10 to 2.70 between 30 and 40 mm conch diameters ( Fig. 3E ).

Remarks

Maccoyoceras pentagonum sp. nov. has a similar conch shape and ornament to the other species of the genus already known from Belgium and Ireland. However, the new species differs from M. discors ( M‘Coy 1844) in the compressed whorl profile, which is approximately quadrate at 80 mm conch diameter in M. discors . Maccoyoceras leveilleanum has a convex venter ( de Koninck 1844) and M. wrightii ( Foord, 1900) has a convex venter and an even more compressed whorl profile (ww/wh ~ 0.70 at 50 mm dm) ( Foord 1900).

Figures

Fig. 2. Maccoyoceras pentagonum sp. nov. A. Holotype MB.C.30378.1. B. Paratype MB.C.30378.2. C. Paratype MB.C.30378.3. D. Paratype MB.C.30378.4. E. Paratype MB.C.30378.5. F. Paratype MB.C.30378.6. G. Paratype MB.C.30378.7. H. Specimen MB.C.30378.8. Scale bar units = 1 mm.

Fig. 3. Maccoyoceras pentagonum sp. nov.A. Reconstructed cross section of the holotype MB.C.30378.1. B –E. Ontogenetic trajectories of the cardinal conch parameters of selected specimens. Scale bar units = 1 mm.