◄ Carnets Geol. 18 (6) ►
Outline:
[I. Introduction]
[II. Paleontological and systematic study]
[III. Conclusion]
[Bibliographic references] and ...
[Plate]
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Published online in final form (pdf) on May 18, 2018
DOI 10.4267/2042/66955
[Editor: Bruno Granier;
language editor: Stephen Carey]
The species Heteroceras gracile sp. nov. is described; it is a small and slender species, with a small helix and hamuliniform morphology, previously considered as an atypical variant of Heteroceras baylei (Reynès, 1876). In southeast France, it is mainly known in the Morteiron section (Alpes de Haute-Provence, France), but the species is also present in Bulgaria and possibly in Japan.
• Ammonoidea;
• upper Barremian;
• Heteroceras;
• Alpes de Haute-Provence;
• France
Baudouin C., Delanoy G., Bournaud G. & Gonnet R. (2018).- Heteroceras gracile sp. nov., a new species of Heteroceras Orbigny, 1849, from the upper Barremian of Morteiron (Alpes de Haute-Provence, France).- Carnets Geol., Madrid, vol. 18, no. 6, p. 155-165.
Heteroceras gracile sp. nov., une espèce nouvelle d'Heteroceras Orbigny, 1849, du Barrémien supérieur de Morteiron (Alpes de Haute-Provence, France).- L'espèce Heteroceras gracile sp. nov. est créée ; elle est représentée par des formes de petite taille, à enroulement frêle et petit turricône, de morphologie hamuliniforme, précédemment considérées comme des morphes atypiques d'Heteroceras baylei (Reynès, 1876). Dans le sud-est de la France, elle est essentiellement connue dans la coupe de Morteiron (Alpes de Haute-Provence, France), mais l'espèce est aussi présente en Bulgarie et hypothétiquement au Japon.
• Ammonoidea ;
• Barrémien supérieur ;
• Heteroceras ;
• Alpes de Haute-Provence ;
• France
The genus Heteroceras Orbigny, 1849, includes forms of very diverse size, from a few centimeters for the species Heteroceras baylei (Reynès, 1876), H. elegans Rouchadzé, 1933, H. eristavii Kakabadze, 1975, and H. veratiae Frau et al., 2016, to 60 cm or more in Heteroceras coulleti Delanoy, 1995. The small forms of southeast France have been the subject of several recent studies (Delanoy & Bulot, 1990; Delanoy, 1997; Delanoy & Bert, 2006; Frau et al., 2016). The existence in bed no. 3 of the Morteiron section (Alpes de Haute-Provence, France) of a population of small individuals with hamuliniform morphology and bearing a long and slender proversum, was described by Delanoy (1997, p. 98). It was then considered as a variant of Heteroceras baylei (Reynès, 1876), and linked to particular ecological conditions. The collection and study of new specimens showed, however, very marked morphological differences with Heteroceras baylei (Reynès, 1876), including slender coiling, with a long proversum and a relatively short retroversum that is not in contact with the helix, leading us to consider this population as a distinct species.
The biozonation scheme of the Lower Cretaceous employed here (Fig. 1 
)
is
the one developed by the IUGS Lower Cretaceous Ammonite Working Group (Reboulet
et al., 2011, 2014). We retain the
tripartite division of the Imerites
giraudi Zone at the top of the Barremian (Bert et al.,
2008, 2011; Reboulet et al.,
2011).
|
Figure 1:
Biozonation of the Barremian (according to Bert et
al., 2008, 2011, and Reboulet et
al., 2011, 2014). |
All the specimens studied in this work come from the Morteiron section (Fig. 2 ), located in the commune of Saint-Étienne-lès-Orgues (Alpes
de Haute-Provence, France). The important Heteroceratidae fauna of this section
was first reported by Kilian (1889) and described in detail by Delanoy
(1997), Delanoy & Ebbo
(2000) and Delanoy & Bert
(2006). Two levels in particular are remarkable for the diversity and abundance
of the representatives of this family: bed no. 3, with Heteroceras emericianum Orbigny,
1842, and its morphotypes, H.
gracile sp. nov. (= H. baylei (Reynès,
1876) in Delanoy, 1997), Rouchadzeites
mascarellii (Delanoy & Ebbo,
2000), Egoianites
mikhaili Delanoy & Bert,
2006, Paraimerites ? sp., Argvethites
sp., and bed no. β, with Heteroceras emericianum Orbigny,
1842, H. moulladei Delanoy
& Bert, 2006, H. gracile
sp. nov., H. gonneti Delanoy, 1997, H. baylei ? (Reynès,
1876) and several forms of Heteroceras
sp. which will be the subject of a future study.
|
Figure
2:
Geographical location of the studied section (source: www.geoportail.gouv.fr). |
The terminology of the different parts of the shell is the one used by Frau et al. (2016, Fig. 4).
The measurements (Fig. 3
and Tab. 1) were carried out according to the
protocol employed by Delanoy (1997, p. 18, fig. 2 and p. 26) for the
measurements N (ribbing density), and Cp
and Cc (growth rate). On incomplete specimens, measurement H
corresponds to the height of the preserved part of the shell. The h2
measurement was performed at the last suture if it was visible. If the last
suture is not visible, h2 was measured at the top of the proversum, at
the level where the boundary between the phragmocone and the body chamber is
usually observed. In order to allow the calculation of the growth rates Cp
and Cc on the maximum number of specimens, where h1 or h3 are not
measurable (incomplete specimens), additional measurements h1* or h3* (as well as the
lp* and lc* lengths of the corresponding shell segments) were carried out,
corresponding respectively to the lowest measurable whorl height on the
phragmocone and to the highest measurable whorl height on the body chamber.
|
Figure
3:
Explanatory diagram of the measurements made on the specimens
studied. |
Table 1: Abbreviations used in the text.
| H | Total length |
| L | Total width |
| Dt | Turricone diameter |
| h1 | Whorl height at the end of the turricone |
| h1* | Whorl height at the base of the preserved part of the proversum (when h1 is not measurable) |
| h2 | Whorl height at the last suture |
| h3 | Whorl height at the end of the body chamber |
| h3* | Whorl height at the end of the preserved part of the body chamber (when h3 is not measurable) |
| lp | Length of the phragmocone (without the turricone) |
| lp* | Length of the preserved part of the phragmocone (when lp is not measurable) |
| lc | Length of the body chamber |
| lc* | Length of the preserved part of the body chamber (when lc is not measurable) |
| Cp | Relative growth in whorl height of the phragmocone: Cp = (h2 – h1)÷lp (or of the preserved part of the phragmocone when it is incomplete: Cp = (h2 – h1*)÷lp*) |
| Cc | Relative growth in whorl height of the body chamber: Cp = (h3 – h1)÷lc (or of the preserved part of the body chamber when it is incomplete: Cp = (h3* – h1)÷lc*) |
| N | Number of ventral ribs belonging to a longitudinal interval of length h, where h is the whorl height in the middle of this interval |
Suborder Ancyloceratina Wiedmann, 1966
Superfamily Ancyloceratoidea Gill, 1871
Family Heteroceratidae Spath, 1922
Genus Heteroceras Orbigny, 1849
(= Lindigia Karsten, 1858; Atopoceras Jaubert in Kilian, 1889, nomen nudum; Hemibaculites Hyatt, 1900, nomen dubium; ? Dirrymoceras Hyatt, 1900, nomen dubium; Colchidites Djanélidzé, 1926; ? Santandericeras Royo y Gomez, 1945)
Type species: Turrilites emericianus Orbigny, 1842 (by subsequent designation of Meek, 1876, p. 477).
Heteroceras gracile sp. nov.
(Pl. 1 ,
figs. 1-12)
| ? | 1964 | Anahamulina subcylindrica (Orbigny, 1850). - Nikolov, p. 126, Pl. 4, fig. 6; non Pl. 5, figs. 1-2 (= Heteroceras sp.) |
| ? | 1976 | Acrioceras (?) aff. julivertii Etayo-Serna, 1968. - Obata & Ogawa, Pl. 4, fig. 6 |
| 1989 | Heteroceras cf. elegans Rouchadzé, 1933. - Conte, p. 39, fig. 9 ?, 10 ?, 11 | |
| pars | 1997 | Heteroceras baylei (Reynès, 1876). - Delanoy, p. 95, Pl. 13, fig. 4; Pl. 16, fig. 3; Pl. 20, figs. 3-4; Pl. 50, fig. 3; non Pl. 9, fig. 3; Pl. 13, fig. 5; Pl. 14, figs. 4-7; Pl. 20, fig. 2; Pl. 50, figs. 4, 8; Pl. 51, figs. 2-8 [=Heteroceras baylei (Reynès, 1876)] |
| 2013 | Heteroceras baylei (Reynès, 1876). - Ivanov & Idakieva, fig. 6.E ?, F-G |
Holotype: Specimen no. FSL13388, coll. Claude Bernard University, Lyon, France. Examination of the matrix indicates that the holotype comes from bed no. 3 of the Morteiron section (Alpes-de-Haute-Provence, France).
Derivatio nominis: In reference to the slender and hamuliniform character of this new species.
Locus-typicus: Morteiron, Saint-Étienne-lès-Orgues (Alpes-de-Haute-Provence, France).
Stratum-typicum: Upper Barremian, Imerites giraudi Zone, Imerites giraudi Subzone, Heteroceras emericianum Horizon.
Geographical and stratigraphical distribution: H. gracile sp. nov. is mainly known in the Heteroceras emericianum Horizon of the Morteiron section (Alpes-de-Haute-Provence, France), more precisely in beds no. 3 and β (Delanoy, 1997, p. 255).
H. gracile sp. nov. also appears to be present in Bulgaria in the Heteroceras astierianum Zone of Nikolov (1964, Pl. 4, fig. 6). Its presence seems certain at the base of the Martelites sarasini Subzone of the Opaka section (Ivanov & Idakieva, 2013, p. 55 and Fig. 6).
Its presence in Japan is still hypothetical and is based on the morphological affinities between Acrioceras (?) aff. julivertii Etayo-Serna, 1968, figured by Obata & Ogawa (1976, Pl. 4, fig. 6) and H. gracile sp. nov.
Material and dimensions (N=29): Specimens no. FSL13388, coll. Claude Bernard University (Lyon, France), mot02, mot03, mot04, mot05, mot06, mot07a, mot07b, mot09, mot10, mot11, mot12, mot13, mot14, mot16, mot17, mot18, mot19, mot20, coll. Baudouin, 28824, 28838, 28839, 28840, coll. Delanoy, MOT GBD 01, MOT GBD 02, coll. Bournaud, bed no. 3, upper Barremian, Imerites giraudi Zone, Imerites giraudi Subzone, Heteroceras emericianum Horizon, Morteiron, Saint-Étienne-lès-Orgues (Alpes-de-Haute-Provence, France); no. RG419a, RG419b, coll. Baudouin leg. Gonnet, bed no. β, upper Barremian, Imerites giraudi Zone, Imerites giraudi Subzone, Heteroceras emericianum Horizon, Morteiron, Saint-Étienne-lès-Orgues (Alpes-de-Haute-Provence, France); no. MOT GBD 03, MOT GBD 06, coll. Bournaud, bed no. β ?, upper Barremian, Imerites giraudi Zone, Imerites giraudi Subzone, Heteroceras emericianum Horizon, Morteiron, Saint-Étienne-lès-Orgues (Alpes-de-Haute-Provence, France). See Tables 2 and 3 for dimensions.
The holotype no. FSL13388 and specimens no. 28839 and no. 28840, as well as casts of specimens no. mot09 and no. mot10 (registered as UCBL-FSL 391837 and UCBL-FSL 391838) are deposited in the collections of the Claude Bernard University, Lyon 1 of Villeurbanne (France).
Table 2: Morphometric measurements of Heteroceras gracile sp. nov.
| no. | "H" | L | h1 | h1* | h2 | h3 | h3* | Dt | lp | lp* | lc | lc* | Cp | Cc |
| FSL13388 | 65.9 | 24.5 | 3.8 | - | 8.2 | 13.7 | - | 4.8 | 58.3 | - | 48.4 | - | 7.5 | 11.4 |
| mot02 | 50.9 | - | - | 3.9 | 8.2 | - | - | - | - | 39.6 | - | - | 10.9 | - |
| mot03 | 59.6 | 24.3 | - | 4.3 | 8 | 12.3 | - | - | - | 47.8 | 30.2 | - | 7.7 | 14.2 |
| mot04 | 57.3 | 23.2 | - | 2.9 | 7.8 | 13.9 | - | - | - | 48.3 | 49.7 | - | 10.1 | 12.3 |
| mot05 | 51.4 | 20.7 | - | 4.6 | 7.2 | - | 11.6 | - | - | 39.6 | - | 29.5 | 6.6 | 14.9 |
| mot06 | 55.5 | - | - | 4.7 | 7.7 | - | - | - | - | 39.6 | - | - | 7.6 | - |
| mot07a | 80 | 29.5 | - | 5.2 | 9.7 | 16.2 | - | - | - | 60.3 | 34.9 | - | 7.5 | 18.6 |
| mot07b | 69.3 | - | - | - | - | - | - | - | - | - | - | - | - | - |
| mot09 | 83.3 | 28.2 | 3.5 | - | 10.7 | 15.5 | - | - | 75.2 | - | 51.5 | - | 9.6 | 9.3 |
| mot10 | 59 | - | 4.1 | - | c9.8 | - | - | c8.1 | 52 | - | - | - | 11.0 | - |
| mot11 | 56.9 | 25.1 | - | 4.1 | 7.8 | 16.2 | - | - | - | 42.8 | 44.3 | - | 8.6 | 19.0 |
| mot12 | 72.6 | 36.4 | - | 6.4 | 10.2 | 18.4 | - | - | - | 58.3 | 48.2 | - | 6.5 | 17.0 |
| mot13 | 45.3 | 27.9 | - | 5.2 | 7.4 | 15.4 | - | - | - | 38.4 | 50.9 | - | 5.7 | 15.7 |
| mot14 | 67.9 | 23.4 | - | 5.3 | 9.1 | - | - | - | - | 45 | - | - | 8.4 | - |
| mot16 | 55.3 | 24.5 | - | 6.6 | 8.5 | 13.8 | - | - | - | 37.3 | 63 | - | 5.1 | 8.4 |
| mot17 | 56.7 | 28.3 | - | 6 | 9.1 | 13.7 | - | - | - | 42.3 | 44.9 | - | 7.3 | 10.2 |
| mot18 | 34.4 | 26 | - | - | 7.9 | 12.8 | - | - | - | - | 42.1 | - | - | 11.6 |
| mot19 | 49.6 | 21.6 | - | 4.8 | 7.6 | 12.3 | - | - | - | 39.6 | 47.5 | - | 7.1 | 9.9 |
| mot20 | 50.9 | 27.6 | - | 3.9 | 6.7 | 13.4 | - | - | - | 42.6 | 49 | - | 6.6 | 13.7 |
| RG419a | 62.9 | 23.9 | 4.2 | - | 7.6 | 11.6 | - | - | 51.2 | - | 50.3 | - | 6.6 | 8.0 |
| RG419b | 55.8 | 20 | - | - | 6.7 | - | - | - | - | - | - | - | - | - |
| MOT GDB 01 | 58.4 | 23.5 | - | 7 | 9.3 | - | 11.8 | - | - | 42.2 | - | 29.8 | 5.5 | 8.4 |
| MOT GDB 02 | 65.6 | 25.3 | 3.4 | - | 8 | c13 | - | 9.4 | 53.5 | - | 50.1 | - | 8.6 | 10.0 |
| MOT GDB 03 | 46.7 | 25.2 | - | 5.5 | 8.4 | 14.9 | - | - | - | 34.1 | 57 | - | 8.5 | 11.4 |
| MOT GDB 06 | 37.5 | 16.8 | - | 3.4 | 5.3 | 9.6 | - | - | - | 31 | 43.2 | - | 6.1 | 10.0 |
| 22839 | 55 | 21.9 | - | 5 | 8.6 | - | 10.8 | - | - | 37.7 | - | 22.8 | 9.5 | 9.6 |
| 28840 | 61 | - | - | 6.6 | 10.4 | 17.1 | - | - | - | 45.8 | 46 | - | 8.3 | 14.6 |
| Heteroceras baylei, holotype no. MHNM 1989-44 |
65.7 | - | - | 8.6 | 11.7 | 16.7 | - | - | - | 34.3 | 62 | - | 9.0 | 8.1 |
| Heteroceras elegans, holotype no. GIT 350/1016 |
53.1 | 28 | 8.1 | - | 9.4 | 14.8 | - | 13.6 | 29.8 | - | 39.7 | - | 4.4 | 13.6 |
| Heteroceras veratiae, holotype no. FCC.8A |
33 | - | 5.7 | - | 7.4 | 9 | - | 11 | 16.2 | - | 25.1 | 10.5 | 6.4 |
Table 3: Ribbing density measurements of Heteroceras gracile sp. nov. (Bold italic type indicates that these values were measured on photographs).
| no. | |||||
| FSL13388 | h | 4.2 | 6.3 | 8.6 | 13.7 |
| N | 4 | 5 | 6 | 8 | |
| mot02 | h | 6.4 | |||
| N | 6 | ||||
| mot03 | h | 4.6 | 6.5 | 8.5 | 11.8 |
| N | 4 | 5 | 6 | 6 | |
| mot04 | h | 3.7 | 8.4 | 11.5 | |
| N | 5 | 11 | 9 | ||
| mot05 | h | 4 | 7.2 | 11.1 | |
| N | 4 | 7 | 7 | ||
| mot06 | h | 5.1 | 7.6 | ||
| N | 5 | 6 | |||
| mot07a | h | 4.9 | 8.2 | 10.7 | 14.9 |
| N | 5 | 7 | 10 | 9 | |
| mot07b | h | 3.2 | 6.6 | ||
| N | 3 | 6 | |||
| mot09 | h | 4 | 6.1 | 10.7 | 13.2 |
| N | 5 | 5 | 9 | 8 | |
| mot10 | h | 4.4 | 5.5 | 8.8 | |
| N | 5 | 5 | 6 | ||
| mot11 | h | 5.2 | 7.2 | 14.4 | |
| N | 5 | 5 | 7 | ||
| mot12 | h | 7.5 | 10.9 | 17.3 | |
| N | 7 | 9 | 10 | ||
| mot13 | h | 8 | 14.2 | ||
| N | 6 | 8 | |||
| mot14 | h | 5.5 | 9.4 | 11.6 | |
| N | 5 | 6 | 6 | ||
| mot16 | h | 7.3 | 8.7 | 13.8 | |
| N | 6 | 6 | 6 | ||
| mot17 | h | 6.5 | 9.3 | 13.5 | |
| N | 7 | 8 | 8 | ||
| mot18 | h | 7.1 | 12.4 | ||
| N | 5 | 6 | |||
| mot19 | h | 5.1 | 7.2 | 11.7 | |
| N | 5 | 6 | 7 | ||
| mot20 | h | 5.4 | 13 | ||
| N | 5 | 8 | |||
| RG419a | h | 4.6 | 7.4 | 12.6 | |
| N | 6 | 8 | 7 | ||
| RG419b | h | 11.2 | 10.6 | ||
| N | 5 | 6 | |||
| MOT GDB 01 | h | 6.8 | 9.2 | ||
| N | 6 | 6 | |||
| MOT GDB 02 | h | 3.4 | 7.6 | 12 | |
| N | 4 | 6 | 8 | ||
| MOT GDB 03 | h | 7.2 | 8.8 | 13.8 | |
| N | 6 | 7 | 8 | ||
| MOT GDB 06 | h | 3.6 | 5.3 | 9.2 | |
| N | 4 | 5 | 7 | ||
| 28839 | h | 6.5 | 8 | 11 | |
| N | 6 | 6 | 6 | ||
| 28840 | h | 7 | 10 | 16 | |
| N | 6 | 7 | 9 | ||
| Heteroceras baylei, holotype no. MHNM 1989-44 |
h | 9.3 | 16.7 | ||
| N | 5 | 5 | |||
| Heteroceras elegans, holotype no. GIT 350/1016 |
h | 8.5 | 9.3 | 14.9 | |
| N | 5 | 6 | 6 | ||
| Heteroceras veratiae, holotype no. FCC.8A |
h | 5.7 | 8.6 | ||
| N | 5 | 6 |
Diagnosis: Small Heteroceras Orbigny, 1849, of hamuliniform morphology with a relatively slow growth rate, having a small helix, a long and thin proversum and a retroversum approximately half the length of the shell. The ribbing is very regular and consists of simple ribs on most of the shell, with bifurcated ribs present only on or near the flexus.
Description: Shell of small size and hamuliniform morphology. The whorl section,
rarely visible in its entirety, appears to be subcircular to suboval, higher
than wide. The helix, rarely preserved, is small, with whorls in contact and
with both dextral (specimen no. mot10, Pl. 1 , fig. 4) and sinistral coiling
(specimen no. FSL13388, Pl. 1 , fig.
6). The proversum
is long, straight or slightly curved, with a relatively moderate whorl height
growth (Cp between 5.7% and 11%). On
specimens with visible sutures, the body chamber begins at the top of the proversum,
at the junction with the flexus. The retroversum is straight, half to two thirds of the total length of
the shell. Growth in whorl height is increasing, particularly on the flexus
and the start of the retroversum, with
a Cc growth rate of between 8% and 18%. The gap between the proversum
and the retroversum is generally narrow, especially in specimens with
straight proversum.
On the helix, the ribs are simple or sometimes bifurcated from the umbilical shoulder.
For most of the proversum, the
ornamentation consists solely of straight, simple and regular, slightly
prorsiradiate, more or less fine and dense ribs, N varying between 5 and 11 at whorl height h between 6 and 10 mm
(Fig. 4 ). The ribs appear in the dorsal region
where they can be relatively attenuated. During growth, the ribbing gradually
strengthens, and, in the upper third of the proversum, bifurcated ribs appear, with the bifurcation point
generally lying between the lower third and the middle of the flanks. The number
of bifurcated ribs increases gradually.
|
Figure
4:
Ribbing density N in
function of h in Heteroceras gracile sp. nov. (see Table 2). |
On the flexus, the proportions of bifurcated and single ribs are highly variable; however, bifurcated ribs are most often predominant, with single ribs isolated and irregularly arranged, or even absent.
On the retroversum, the ribbing
becomes even stronger and the ribs are then slightly rursiradiate, sometimes
rectiradiate. It again consists mostly of simple ribs; bifurcated ribs, however,
are present in the lower half of the retroversum where they may be dominant (specimen no. mot17,
Pl. 1 ,
fig. 7).
The peristome is not preserved on any of the specimens studied.
At the approach of the peristome, the rib pattern seems to become slightly prorsiradiate again
(specimens no. mot04, Pl. 1 , fig. 1; no. mot09,
Pl. 1 , fig. 3; no. RG419a,
Pl. 1 , fig. 9).
Suture lines are difficult to observe in detail. They show, however, a
relatively deep and feebly incised, trifid lateral lobe (specimen no. MOT GBD
02; Fig. 5 ).
|
Figure
5:
Detail of the suture line (lateral lobe) of Heteroceras gracile sp. nov. (specimen no. MOT GBD 02). |
Discussion and comparisons:
Heteroceras gracile sp. nov. is close to Heteroceras
baylei (Reynès, 1876), of which it was considered a variant,
and linked
to particular ecological conditions by Delanoy (1997, p. 98). Heteroceras
gracile sp. nov. is distinguished by its generally more hamuliniform and
slender form, with a much longer proversum
and a relatively short retroversum
that never comes into contact with the helix, as well as by its much denser
ribbing (Fig. 4 ).
Heteroceras gracile sp. nov. is also close to Heteroceras
elegans Rouchadzé, 1933, whose type-specimens come from the
upper Barremian ("Colchidites securiformis
Zone", Kakabadze, 1975, p. 89) of Goresha (Georgia). Heteroceras
gracile sp. nov. differs from it by its larger size and more slender
morphology, with a significantly smaller helix and a much longer and thinner proversum,
and by its denser ribbing (Fig. 4 ).
Heteroceras helicoceroides (Karsten, 1858) is a small form from the upper Barremian of Colombia, known only from the original picture of Karsten. This makes it difficult to establish the specific characteristics of this taxon, which appears to differ from Heteroceras gracile sp. nov. by its less slender morphology and the chevron-like ribbing on its ventral region.
Heteroceras vermiforme Rouchadzé, 1933, of the upper Barremian ("Colchidites securiformis Zone", Kakabadze, 1975, p. 90) of Georgia, is known only from its holotype, which has many similarities to Heteroceras gracile sp. nov., in particular, a rather long proversum and quite similar ribbing. The very incomplete state of this holotype, figured by Rouchadzé (1933, Pl. 13, fig. 5) and Kakabadze (1975, Pl. 3, fig. 6), makes it difficult to compare the two taxa; the general morphology of the Georgian species seems less slender than that of H. gracile sp. nov.
Heteroceras veratiae Frau et al., 2016, a species known only in the Martelites sarasini Subzone of Cape Croisette (Marseille, France), differs from Heteroceras gracile sp. nov. primarily in its much smaller size which does not exceed 38 mm, and its less slender morphology. Its proversum and retroversum are clearly divergent instead of subparallel.
The particular morphological characteristics of the specimens studied (slender and almost hamuliniform shell, long proversum and relatively short retroversum) justify the creation of the taxon Heteroceras gracile sp. nov. This new species makes it possible to complete the inventory of the genus Heteroceras Orbigny, 1849, particularly for small taxa. These still poorly known species are often difficult to characterize on the basis of incomplete and/or poorly stratigraphically located specimens, which preclude us from appreciating their great morphological diversity and establishing clear specific distinctions.
We would like to thank Emmanuel Robert (Codirector of Geological Collections, University Lyon 1, France) for his hospitality during our investigations of the collections of the Claude Bernard University, and both Keith P. Minor (University of Texas at Austin, USA) and Stephen Carey (Federation University, University of Ballarat, Australia) for the improvement of the English text. We also warmly thank our reviewers Miguel Company (Granada University, Spain) and Zdenĕk Vašiček (Academy of Sciences, Czech Republic) for their constructive remarks.
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Plate 1: All
specimens are from Morteiron, Saint-Étienne-lès-Orgues
(Alpes de Haute-Provence, France). |
|
