The ammonoids from the Gattendorfia Limestone of Oberrödinghausen (Early Carboniferous; Rhenish Mountains, Germany)
Author
Korn, Dieter
286CA4F3-7EBC-4AEF-A66A-B2508D001367
Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Invalidenstrasse 43, 10115 Berlin, Germany.
dieter.korn@mfn.berlin
Author
Weyer, Dieter
A09A1738-C70E-4F22-A069-8B7DB4A1753D
Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Invalidenstrasse 43, 10115 Berlin, Germany.
dieter.weyer@t-online.de
text
European Journal of Taxonomy
2023
2023-07-19
882
1
230
http://dx.doi.org/10.5852/ejt.2023.882.2177
journal article
58033
10.5852/ejt.2023.882.2177
57d1d191-2bf5-4229-a0f5-9ab472a114a6
2118-9773
8177581
67C909E4-C700-4F8D-B8CE-5FD9B2C5D549
Imitoceras initium
sp. nov.
urn:lsid:zoobank.org:act:
F4100C55-64E5-48AF-8EDC-87CDA35580F1
Fig. 65
;
Table 63
Diagnosis
Species of
Imitoceras
with a conch reaching
60 mm
diameter. Conch at
50 mm
dm discoidal, involute (ww/dm ~0.45; uw/dm ~0.05); whorl profile at
30 mm
dm weakly compressed (ww/wh ~0.75); coiling rate very high (WER ~2.30). Venter rounded, umbilical margin rounded, flanks strongly convergent. Growth lines very fine, narrow-standing, with slightly biconvex course. Weak constrictions on the shell surface of the inner flank. Suture line with a weakly pouched external lobe and a V-shaped adventive lobe.
Etymology
After Latin ‘
initium
’ = ‘the beginning’; because of the position of the species at the base of a long-ranging evolutionary lineage.
Material examined
Holotype
GERMANY
•
Rhenish Mountains
, Oberrödinghausen, west of railway cutting; Hangenberg Limestone, loose material; Korn 1977 Coll.; illustrated in
Fig. 65
; MB.C.31144.
Description
Holotype
MB.C.31144 is well preserved and has a conch diameter of
52 mm
(
Fig. 65A
). It is thinly discoid with a slightly opened umbilicus (ww/dm = 0.43; uw/dm = 0.05) and has a compressed whorl profile that is widest at the rounded umbilical margin; it has fairly strongly converging flanks and a rounded venter. The coiling rate is very high (WER = 2.30). The shell bears barely visible, extremely faint growth lines with a biconvex course and also a shell constriction limited to the inner flank. The suture line has a weakly pouched external lobe that is about two-thirds the depth of the nearly symmetric V-shaped adventive lobe (
Fig. 65B
).
Fig. 65.
Imitoceras initium
sp. nov.
, holotype MB.C.31144 (Korn 1977 Coll.) from the forestry road cutting west of the Oberrödinghausen railway cutting.
A
. Lateral and dorsal view.
B
. Suture line, at ww =14.0 mm, wh =15.4 mm. Scale bar units=1 mm.
Table 63.
Conch measurements, ratios and rates of
Imitoceras initium
sp. nov.
from Oberrödinghausen and Oese.
|
specimen
|
dm
|
ww
|
wh
|
uw
|
ah
|
ww/dm
|
ww/wh
|
uw/dm
|
WER
|
IZR
|
| MB.C.31144 |
52.0 |
22.2 |
29.6 |
2.6 |
17.7 |
0.43 |
0.75 |
0.05 |
2.30 |
0.40 |
| MB.C.31144 |
34.2 |
16.6 |
19.0 |
2.0 |
– |
0.49 |
0.87 |
0.06 |
– |
– |
Remarks
The occurrence of the genus
Imitoceras
in the
Gattendorfia
Limestone
near Oberrödinghausen was previously unknown, but is actually not a great surprise. Representatives of
Imitoceras
have already been found together with early Tournaisian ammonoids in
Guizhou
(
Sun & Shen 1965
;
Ruan 1981
), originally described as “
Imitoceras planolobatum
” and “
Imitoceras
(
Imitoceras
)
crassum
”, and the Anti-Atlas (
Bockwinkel & Ebbighausen 2006
).
The only specimen available is unique in the ammonoid assemblage from Oberrödinghausen because of its high coiling rate of 2.30. Such a value is rarely reached by other members of the prionoceratid ammonoids. In the subfamily
Acutimitoceratinae
, the whorl expansion rate is normally between 1.70 and
1.90 in
Stockumites
and between 1.90 and
2.05 in
Nicimitoceras
.
Of these genera, only
Nicimitoceras
has a short external lobe like in
Imitoceras initium
sp. nov.
However,
I. initium
is stouter than the size-equivalent
holotypes
of other
Nicimitoceras
species
from Oberrödinghausen: its ww/dm ratio is 0.43 at
52 mm
dm in
I. initium
, while the value in
N. heterolobatum
, for example, is 0.36 at
54 mm
dm. In addition, the whorl profile shows rapidly converging flanks in
I. initium
, which tend to be subparallel in species of
Nicimitoceras
.
A superficially similar species to
Imitoceras initium
sp. nov.
is
Acutimitoceras procedens
from the Stockum limestone equivalent of the Müssenberg (
Korn 1981
). This also shows a high coiling rate, rapidly convergent flanks, and a shell ornament with weak growth lines and constrictions. The
holotype
of this species is much stouter (ww/dm = 0.54 at
33 mm
dm) than
holotype
MB.C.31144 of
A. initium
(ww/dm = 0.43 at
52 mm
dm). This difference cannot be explained by the advanced ontogenetic evolution towards a more slender conch in specimen MB.C.31144, as this has half a whorl in front of the largest diameter, a ww/dm ratio of only 0.47 at
36 mm
conch diameter.