◄ Carnets Geol. 15 (1) ►
Contents
[I - Introduction]
[II - Geological setting]
[III - Previous work]
[IV - Sierra de Parras assemblage] [V - Systematic
notes]
[VI - Conclusions] and ... [Bibliographic references]
Calle 5 de febrero #436 -2, Cuauhtémoc, 06880, México, Distrito
Federal (Mexico)
Corresponding author. Tel.: +52 55 5622 4280 ext. 156
Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad
Universitaria, Coyoacán, 04510, México, Distrito Federal (Mexico)
Universidad Autónoma de Nuevo León, Facultad de Ciencias de
la Tierra, Carretera a Cerro Prieto Km. 8, Ex – Hacienda de Guadalupe, C.P. 67700. A.P. 104,
Linares, Nuevo León (Mexico)
Sovereign Pharmaceuticals, 7590 Sand St., Fort Worth, TX
76118, Texas (United States of America)
Published online in final form (pdf) on January 14, 2015
[Scientific
editor:
Michel ; technical editor: Bruno ;
language editor: Stephen ]
We report an interesting Aptian ammonite record from the La Peña Formation in the Sierra de Parras, Coahuila State. This assemblage is analyzed from a paleoecological perspective. It contains the first reported occurrence of a macroconch of Dufrenoyia from Mexico, and the largest known specimen of 'Gargasiceras' adkinsi. Such an assemblage yielding large ammonites is unusual in deposits of this age in Mexico. To properly document this ammonite record, we review 'Gargasiceras' adkinsi, formerly misidentified as Rhytidoplites robertsi, and allied taxa. From our analysis, we conclude that 'Gargasiceras' adkinsi exhibits strong intraspecific variability. We also analyze in detail the differences that exist between 'Gargasiceras' adkinsi and Rhytidoplites robertsi, and emend the concept of the genus Rhytidoplites. These revisions are important from a biostratigraphic point of view since 'Gargasiceras' adkinsi is an index species for the lower Aptian ammonite zonation of Mexico.
Ammonite; Dufrenoyia macroconch; 'Gargasiceras' adkinsi; Rhytidoplites robertsi; Lower Cretaceous; Aptian; Mexico.
J.R., J.A., G. & K.P. (2015).- Lower Aptian ammonites of the Sierra de Parras, Coahuila State, northern Mexico.- Carnets Géol., Madrid, vol. 15, nº 1, p. 1-11.
Ammonites de l'Aptien inférieur de la Sierra de Parras, État de Coahuila, au nord du Mexique.- Ce travail fait état d'une intéressante faune d'ammonites de l'Aptien de la Formation de La Peña, dans la Sierra de Parras, État de Coahuila, analysée ici d'un point de vue paléoécologique. Il permet également de signaler pour la première fois au Mexique la présence d'un macroconque de Dufrenoyia, ainsi que le plus grand spécimen connu de 'Gargasiceras' adkinsi. Une telle association livrant de grandes ammonites est inhabituelle dans les dépôts aptiens du Mexique. Afin de bien caractériser cette ammonitofaune, il est procédé à une révision de l'espèce 'Gargasiceras' adkinsi, autrefois confondue avec Rhytidoplites robertsi, ainsi que des espèces proches. Notre analyse montre que 'Gargasiceras' adkinsi présente une forte variabilité intraspécifique. Les différences entre 'Gargasiceras' adkinsi et Rhytidoplites robertsi sont également analysées en détail, ce qui permet d'émender le concept du genre Rhytidoplites. Ces révisions sont importantes sur le plan biostratigraphique puisque 'Gargasiceras' adkinsi constitue l'une des espèces-index de la zonation de l'Aptien inférieur du Mexique.
Ammonite ; macroconque de Dufrenoyia ; 'Gargasiceras' adkinsi ; Rhytidoplites robertsi ; Crétacé inférieur ; Aptien ; Mexique.
During fieldwork in 2013 in the Sierra de Parras, Coahuila State, we found an interesting ammonite assemblage in the La Peña Formation. This study is part of a campaign aimed at locating new Aptian ammonite-bearing sections in Mexico. The data collected from these new sections will be integrated with taxonomic reviews (e.g., & , 2013) to develop a standard zonation for the Central Atlantic province ( et al., 2013; et al., 2014). At one lower Aptian outcrop bed we discovered a macroconch of Dufrenoyia, which constitutes the first record of a macroconch genus from Mexico, and three more ex situ ammonite, including the largest known specimen of 'Gargasiceras' adkinsi. In this study we analyze these discoveries from an environmental point of view. Recently, et al. (2013) employed 'Gargasiceras' adkinsi as an index taxon for the lower Aptian ammonite zonation of the La Peña Formation. 'G.' adkinsi has previously been misidentified as Rhytidoplites robertsi and Penaceras rursiradiatus (e.g., , 1976; , 1977; , 2000, 2001; & , 2008). In the "Systematic notes" (Section V) we address the taxonomic issues associated with 'Gargasiceras' adkinsi and Rhytidoplites robertsi. An improved understanding of 'G.' adkinsi and related forms not only leads to their taxonomic refinement, but is necessary for their proper use in a biostratigraphic framework.
The studied outcrop of
the La Peña Formation is located in Juan Pérez Canyon (at coordinates:
25°18'35"N, 102°13'31"W), 15.5 km SSW of Parras de La Fuente, Coahuila
State, in the core of the Sierra de Parras, which is one of the transverse
ranges of the Mexican Fold Thrust Belt in northern Mexico (Fig.
1 
). The Sierra
de Parras consists of WNW-ESE trending faults and asymmetric folds exposing
Upper Jurassic and Cretaceous sedimentary marine rocks. Deposited in
shallow-water conditions, the La Peña Formation consists of argillaceous
carbonates and shales and grades upwards into deeper-water facies represented by
calcareous lenses with nodules of chert; it is middle to late Aptian in age (
& , 1974; ,
1982; et al.,
1991; et
al., 1999).
In northern Mexico, the La Peña Formation marks a major transgression that drowned the Cupido ramp during the latest Early Aptian ( et al., 1991; , 1999; et al., 1999; et al., 2012). It was deposited over a wide area, and has been correlated with the non-marine Las Uvas Formation in the Coahuila Block, north of the study area, the basinal Otates Formation in the Valles-Golden Lane in East Central Mexico, and the Pearsall Formation of the Texas Gulf Coast ( & , 1993).
Click on thumbnail to enlarge the image.
Figure 1: Geographic location of the State of Coahuila; the star indicates the location of the outcrop studied in Juan Pérez Canyon, Sierra de Parras.
The ammonite record of the La Peña Formation in the Sierra de Parras is poorly known. (1936) was the first author to report Aptian ammonites from this area. He reported but did not illustrate ten species from the Cuesta del Cura, namely, Dufrenoya texana , Dufrenoya aff. dufrenoyi , Dufrenoya ? sp., Parahoplites sp. ind., Parahoplites sp., Douvilleiceras aff. nodosocostatum , Douvilleiceras aff. bigoureti , Douvilleiceras sp., Crioceras trispinosoides and Ammonitoceras sp.
(1940) then described Saynoceras mexicanum from the basal beds of the La Peña Formation of the Rancho El Angel, Sierra de Parras.
Subsequently, (1949) reported eight species from the Cuesta del Cura, Sierra de Parras, and illustrated most of them: Dufrenoya justinae (), Dufrenoya dufrenoyi (), Dufrenoya sp. B, Burckhardtites nazasensis (), Burckhardtites gregoriensis (), Burckhardtites sp. A and B, Uhligella mullerriedi (). The most recent report of Aptian ammonites from the Sierra de Parras was by (1976) who recognized and illustrated "Caseyella reesidei () and Rhytidoplites robertsi ()".
We collected
four ammonites from Juan Pérez Canyon, including Pseudohaploceras
sp., Dufrenoyia sp. and 'Gargasiceras'
adkinsi. These specimens are currently housed in the IGM collection.
The specimen of Dufrenoyia sp. shown
in Figure 2
has a maximum diameter (= D) of about 20 cm, which is the largest specimen of this genus reported in
Mexico. In Europe, (1963) reported several macroconchs that belong
to the genus Dufrenoyia with a maximum
diameter ranging from 7 cm to 38 cm (, 1963: Figs. 135, 138-139;
Pls. 62, 1a-b & 65, 2a-b, 6). More recently, et
al. (2006) reported a macroconch of this genus with a diameter of 16.9 cm,
and et al. (2010:
Electronic annex IX, A) illustrated a macroconch of 33 cm in diameter. Several
authors recognize the presence of macroconchs and microconchs in the Family
Deshayesitidae, which includes the genus Dufrenoyia
(, 1963; et al.,
1996; & ,
1999; & ,
2007; et al.,
2009, 2014; & ,
2010). In the Family Deshayesitidae, the macroconch has an involute
shell with fine ribs and high rib density. In some species, the ornamentation
disappears in the adult stage. In contrast, the microconch is strongly
ornamented, with an evolute shell and robust angular ribs with low costation
density. The Mexican specimen of Figure 2 , with a large, involute shell and high
costation density, can be confidently regarded as a macroconch.
It
is remarkable that the specimen of 'Gargasiceras'
adkinsi studied here (Fig. 3.A ) with D = 8.9 cm, is the largest
specimen known from Mexico. Despite the scarcity of ammonites collected, we
found unusually large specimens of Gargasiceras
and Pseudohaploceras (Fig. 3 )
and
the macroconch of Dufrenoyia described
above (Fig. 2 ). This assemblage is quite different from the one studied by
et al. (2013), which contains mostly
small forms (microconchs, juveniles, etc.), an assemblage interpreted as being
deposited along the most distal part of the outer continental shelf. Other
authors have reported European assemblages similar to the one detailed in this
work, with an abundance of macroconchs and larger shells (e.g., ,
1963; et al., 2006;
et al., 2010). This kind of assemblage
has been found to be associated with more proximal marine external platform
environments. Such Aptian ammonite assemblages are poorly known in Mexico, so
that a return trip will be made to Juan Pérez Canyon to conduct a systematic
bed-by-bed sampling of the section.
Click on thumbnail to enlarge the image.
Figure 2: Dufrenoyia sp. Lateral view of the specimen IGM 9838, Sierra de Parras [Scale bar 1 cm].
Click on thumbnail to enlarge the image.
Figure 3: A: 'Gargasiceras' adkinsi. Lateral view of the specimen IGM 9839, Sierra de Parras. B: Pseudohaploceras sp. IGM 9840, Sierra de Parras. C: Pseudohaploceras sp. Lateral view of the specimen IGM 9841, Sierra de Parras [Scale bar 1 cm].
IGM = Museo María del Carmen , Colección Nacional de Paleontología, Instituto de Geología, UNAM, Mexico D.F.
BEG = Bureau of Economic Geology. The specimen is in the Types Collection at the University of Texas.
UMMP = University of Michigan, Museum of Paleontology.
Superfamily Acanthohoplitoidea , 1949
Family Acanthohoplitidae , 1949
Subfamily Colombiceratinae , 1979
Genus Gargasiceras , 1954
'Gargasiceras' adkinsi (, 1949)
(Figs. 3.A
- 4.A-D & I-N
- 5.A-E )
| v. | 1925 | Parahoplites sp. , p. 23, Pl. 4, figs. 19-23. |
| v. | 1949 | Acanthoplites ? adkinsi, , p. 139, Pl. 13, figs. 2-3. |
| v. | 1949 | Acanthoplites ? sandidgei, , p. 140, Pl. 13, figs. 1, 4. |
| 1976 | Rhytidohoplites robertsi, , p. 15, Pl. 1, figs. 4, 4a, 6, 6a, 7. | |
| 1977 | Rhytidohoplites robertsi, , p. 14, Pl. 6, figs. 2-3. | |
| v. | 1989 | Acanthoplites ? adkinsi, et al., p. 182, Fig. 61h. |
| v. | 1989 | Acanthoplites ? sandidgei, et al., p. 182, Fig. 61j. |
| v. | 1992 | Acanthohoplites ? adkinsi, et al., p. w.n. (= without number), Fig. w.n. |
| v. | 1992 | Acanthohoplites ? sandidgei, et al., p. w.n., Fig. w.n. |
| 1992 | Rhytidohoplites robertsi, et al., p. w.n., Fig. w.n. | |
| 1996 | Rhytidoplites robertsi, et al., p. 275, Fig. 215, 3a-3b. | |
| ? v. | 2000 | Acanthohoplites acutecosta, , p. 110, Pl. 56, figs. 6 ?, 7-9. |
| ? v. | 2000 | Acanthohoplites protreritensis, , p. 115, Pl. 57, figs. 1, 2 ?. |
| v. | 2000 | Penaceras rursiradiatus, , p. 122, Pl. 58, figs. 1-3. |
| v. | 2000 | Rhytidoplites robertsi, , p. 125, Pl. 58, figs. 4-13. |
| v. | 2001 | Rhytidoplites robertsi, , p. 192, Fig. 4, 3-4. |
| v. | 2003 | Acanthohoplites acutecosta, , p. 72, Pl. 7, figs. 4-6. |
| ? v. | 2003 | Rhytidoplites robertsi, , p. 80, Pl. 8, figs. 1-2, 3 ?. |
| v. | 2005 | Acanthohoplites aschiltaensis, , p. 38, Pl. 2, figs. 5A, 5B. |
| v. | 2005 | Rhytidoplites sp., , p. 40, Pl. 2, fig. 6. |
| v. | 2005 | Penaceras rursiradiatus, , p. 42, Pl. 2, fig. 7. |
| v. | 2008 | Penaceras rursiradiatus, & , p. 152, Fig. 3g. |
| v. | 2013 | Gargasiceras ? adkinsi, et al., p. 154, Figs. 4, C-G, I. |
Discussion: In 2013, et al. began employing the name Gargasiceras ? adkinsi, previously identified by several authors mostly as Rhytidoplites robertsi or Penaceras rursiradiatus (e.g., , 1976; , 1977; , 2000, 2001; & , 2008), as an ammonite biozone for the uppermost lower Aptian of the La Peña Formation. They neither explained the reason for using the name Gargasiceras ? adkinsi nor defined this species, and we will now address these issues in detail, comparing the holotypes and other specimens of the relevant species housed in several collections.
According to
(1940: p. 1035) and our own observations, the lower Albian species Rhytidoplites
robertsi (holotype, BEG 34825, herein illustrated in Fig.
4.F-H ) is
characterized by a shell with a sub-rectangular whorl section, and flattened
venter. The ornamentation consisting of primary and secondary ribs is well
developed on the flanks, especially on the lower third where they are quite
prominent. The secondary ribs are weaker than the primaries and appear
approximately at midflank. On the venter, the primary and secondary ribs are
indistinguishable. Three or four secondary ribs are intercalated between the
primary ribs. Though Rhytidoplites
robertsi and the Aptian ammonites have quite similar ornamentation, the
morphology of the shells is clearly different, as was previously mentioned by
(1949: p. 139). Whereas Rhytidoplites
robertsi has a more involute shell with a sub-rectangular whorl section, the
Aptian forms are more evolute and have an elliptical whorl section, and are
clearly distinguishable.
(1949) described two Aptian species with ornamentation quite similar to Rhytidoplites
robertsi: Acanthoplites ? adkinsi
and Acanthoplites ? sandidgei.
According to (1949: p. 139) and our own observations, Acanthoplites
? adkinsi (holotype UMMP 22681, herein illustrated in Fig.
4.A-D ) is
characterized by an evolute shell with an elliptical whorl section. The
ornamentation consists of primary and secondary ribs. One or two secondary ribs
are regularly intercalated between the primary ribs. The primary ribs are well
developed in the flanks, especially in the lower third where they are quite
prominent. The secondary ribs appear in the last third of the flank. Along the
ventral region, the primary and secondary ribs are undifferentiated. According
to (1949: p. 140) and our own observations, Acanthoplites
? sandidgei (holotype, UMMP 24297; herein illustrated in Fig.
4.J-K )
has the same form and whorl section as Acanthoplites
? adkinsi. The pattern of
ornamentation consists of primary and secondary ribs. One or two secondary ribs
are regularly intercalated between the primary ribs. The primary ribs are well
developed on the flank, especially in the lower third where they are quite
prominent. The secondary ribs usually appear on the last third of the flank, but
some of them appear in the middle of the flank. On the venter, primary and
secondary ribs are identical. The differences that used to
separate the two species are related to the scarce and indistinct bifurcations
of secondary ribs in Acanthoplites ? sandidgei.
He also observed that the secondary rib pattern of Acanthoplites
? sandidgei is more irregular than
that of Acanthoplites ? adkinsi.
In our opinion the uncommon and indistinct bifurcation pattern is not only
present in Acanthoplites ? sandidgei
but can also be observed in Acanthoplites
? adkinsi. We agree that the secondary
rib pattern of Acanthoplites ? sandidgei
is more irregular than Acanthoplites ?
adkinsi, but we attribute this difference to intraspecific
variation. We therefore consider 'G.'
sandidgei a junior synonym of 'G.'
adkinsi, with the name 'Gargasiceras'
adkinsi taking priority for this
species, since it was first described by (1949: p. 139-140).
The generic reference of 'Gargasiceras' adkinsi remains unclear. (1949) used the genus Acanthoplites with a question mark, and we agree that this species should not be assigned to this genus, according to the current concept of the genus Acanthohoplites , 1907. The early whorls of the genus Acanthohoplites have a very conspicuous tuberculation that is not present in the Mexican specimens. In addition, the subadult and adult stages of Acanthohoplites do not show the characteristic prominent primary ribs on the lower third of the flank of the Mexican species. et al. (2013) also used a question mark with the genus Gargasiceras because of some differences between 'G.' adkinsi and the more typical members of the genus. The generic assignment will be clarified in a forthcoming work comparing these Mexican species with other similar Aptian ammonites. For the present, we use the generic name Gargasiceras with quotation marks.
In our opinion,
'Gargasiceras' adkinsi exhibits strong intraspecific variability, especially in the
secondary rib pattern. In the Sierra de Parras specimen IGM 9839 (Fig. 3.A ),
we can see an irregular secondary rib pattern, with some of them appearing
approximately in the middle of the flank and others appearing in the outer third
of the flank. In this work, we also illustrate two additional specimens of this
species from the University of Michigan collection, UMMP 21845 and UMMP 41880 (Fig.
4.E, I & L-N ), with UMMP 21845
(Fig. 4.E & I ) showing a unique
secondary rib pattern. In this case, the secondary ribs are less prominent than
the primaries, whereas usually they are equally prominent. In addition, the
maximum number of secondary ribs between primary ribs (up to six) is uncommonly
high, and one secondary rib appears to be bifurcated.
(1925)
illustrated two specimens (, 1925: Pl. 4, figs. 19-23) from
east of Rancho el Mulato, Rio Nazas, Durango State. These specimens are
currently housed in the IGM collection (IGM 1881 and 1882) and herein
illustrated in Figure 5A-E . gave a detailed description of
both specimens, commenting that they have an evolute shell with a rib pattern of
primary and secondary ribs. He wrote that one or two secondary ribs are
intercalated or sometimes bifurcated between the primary ribs, and added that
the secondary ribs can appear on the last third of the flank and less commonly
in the middle of the flank. In the ventral region, primary and secondary ribs
are undifferentiated, but in IGM 1881, the primary ribs are stronger than the
secondary ribs. (1925) identified these specimens as a Parahoplites
sp., but remarked on their similarity to Gargasiceras
gargasense (, 1841), the type species of the genus. In our
opinion, these two specimens should be assigned to the species 'Gargasiceras' adkinsi.
(1925) failed to describe an important feature that can be seen in both
specimens: the primary ribs are well developed on the flank, especially in the
lower third where they are quite prominent. This feature is apparent in 's
original illustrations and Figure 5
of this work. It is important to note that
the primary ribs in ventral region of IGM 1881 (Fig. 5.C ) are stronger than the
secondary ribs. We attribute this to intraspecific variation.
Superfamily Deshayesitaceae , 1949
Family Parahoplitidae , 1922
Subfamily Acanthohoplitinae , 1949
Genus Rhytidoplites , 1940
Rhytidoplites robertsi (, 1940)
(Fig. 4.F-H ,
holotype)
| v. | 1940 | Rhytidoplites robertsi, , p. 1035, Pl. 61, fig. 11; Pl. 63, fig. 7; Fig 159 (holotype). |
| v. | 1974 | Rhytidoplites robertsi, , p. 213, Pl. 12, fig. 3; Pl. 13, figs. 10, 15; Fig. 5d (holotype). |
| non | 1976 | Rhytidohoplites robertsi, , p. 15, Pl. 1, figs. 4, 4a, 6, 6a, 7. |
| non | 1977 | Rhytidohoplites robertsi, , p. 14, Pl. 6, figs. 2-3. |
| v. | 1992 | Rhytidohoplites robertsi, et al., p. w.n., Fig. w.n. (holotype) |
| non | 1992 | Rhytidohoplites robertsi, et al., p. w.n., Fig. w.n. |
| non | 1996 | Rhytidoplites robertsi, et al., p. 275, Fig. 215, 3a-3b. |
| non v. | 2000 | Rhytidoplites robertsi, , p. 125, Pl. 58, figs. 4-13. |
| non v. | 2001 | Rhytidoplites robertsi, , p. 192, Fig. 4, 3-4. |
| non v. | 2003 | Rhytidoplites robertsi, , p. 80, Pl. 8, figs. 1-3. |
Discussion: Our analysis of Rhytidoplites robertsi and 'Gargasiceras' adkinsi indicates that there were several errors in previous identifications of these taxa, as noted above. One of these misidentifications ( et al., 1996: p. 275, based on the misidentification of , 1976) has important consequences on the identification of the genus Rhytidoplites. An amended concept of this genus is as follows:
Rhytidoplites is characterized by a shell with a sub-rectangular whorl section with flat flanks and flattened venter. The ornamentation consists of primary and secondary ribs. Primary ribs are well developed along the flanks, especially on the lower third where they are quite prominent. The secondary ribs are weaker than the primaries, and appear approximately in the middle of the flank. In the ventral region, the primary and secondary ribs are indistinguishable. Three or four secondary ribs are intercalated between the primary ribs. This genus is represented by a single species, Rhytidoplites robertsi, from the lower Albian of Texas which is the type species by original designation.
According to (1940), a second species belongs to this genus: Rhytidoplites fasciculatus. However, the general form of the shell and the ornamentation of this taxon are not appropriate to the genus, Rhytidoplites. The ventral zone is clearly different, R. fasciculatus having a rounded venter and genus Rhytidoplites a flat venter. R. fasciculatus has an umbilical nodes branching into two or three low ribs. The ribs, which are quite straight, continue to the ventral region. Secondary ribs are few and appear in the upper part of the flank where the primary ribs bifurcate. Rhytidoplites has very unusual features in the prominence of the primary ribs in the middle of the flank and in their slight sinuosity. In addition, Rhytidoplites has well develop intercalated secondary ribs that appears approximately at midflank.
The flattened ventral region of the genus Rhytidoplites is similar to those of the European genus Hypacanthoplites (, 1923) and the American genus Immunitoceras (, 1949).
Click on thumbnail to enlarge the image.
Figure 4: A-D: 'Gargasiceras' adkinsi. Lateral, ventral and frontal views of UMMP 22681, Rincon de los Potreritos, San Jose, Coahuila. E, I: 'Gargasiceras' adkinsi. Ventral and lateral views of UMMP 21845. F-H: Rhytidhoplites robertsi. Lateral and ventral views of BEG 34825, Mayfield Canyon, Hudspeth Country, Texas. J-K: 'Gargasiceras' adkinsi. Lateral and ventral views of UMMP 24297, Rincon de los Potreritos, San Jose, Coahuila. L-N 'Gargasiceras' adkinsi. Lateral and frontal views of UMMP 41880, Rincon de los Potreritos, San Jose, Coahuila [Scale bar 1 cm].
Click on thumbnail to enlarge the image.
Figure 5: A-C: 'Gargasiceras' adkinsi. Lateral and ventral views of IGM 1881, Rancho el Mulato, Durango State. D-E: 'Gargasiceras' adkinsi. Lateral and ventral views of IGM 1882, Rancho el Mulato, Durango State [Scale bar 1 cm].
We report an interesting lower Aptian ammonite record in the Sierra de Parras, Coahuila State, Mexico, that contains a macroconch of Dufrenoyia, which constitutes the first such report from Mexico, and the largest known specimen of 'Gargasiceras' adkinsi. This assemblage, containing one macroconch and several large ammonites, is associated with a proximal region within a marine external platform environment. This type of Aptian ammonite assemblage is poorly known in Mexico, prompting us to return to the locality to conduct a systematic bed-by-bed sampling of the section.
The species 'Gargasiceras' adkinsi, which has been misidentified in the literature generally as Rhytidoplites robertsi and Penaceras rursiradiatus, exhibits intraspecific variability, especially in the secondary ribs. 'Gargasiceras' adkinsi includes the nominal species Acanthoplites ? sandidgei. We analyze in detail the differences between 'Gargasiceras' adkinsi and Rhytidoplites robertsi, permitting us to emend the concept of the genus Rhytidoplites. These taxonomic improvements of 'Gargasiceras' adkinsi, and related forms, will also allow effective biostratigraphic use of this taxon.
We appreciate the help of Violeta for providing access to the Mexican ammonoids studied by in 1925 which are housed in the Museo María del Carmen , Colección Nacional de Paleontología, Instituto de Geología, UNAM, México D.F., Mexico. We appreciate the assistance and access to facilities to review the Mexican Aptian ammonoids housed in the University of Michigan Museum of Paleontology provided by Dr. Dan , Coordinator Invertebrate Collection of the Museum of Paleontology, and Dr. William , Preparator of the Vertebrate Fossil Preparation Laboratory, both at the University of Michigan, Ann Arbor, Michigan, USA. Finally, we would like to thank Dr. Ann , Collections Manager, Non-Vertebrate Paleontology Laboratory, University of Texas, for giving us permission to photograph BEG 34825 in the Gayle Collection. We are very grateful for the helpful corrections and suggestions made by J.-L. , H. , one anonymous reviewer and the editors Michel and Bruno . We are also very grateful to the Language Editor, Stephen , for his corrections which allow significant improvements to the manuscript.
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