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New microconchiate Hybonoticeras from Mexico
NEW MICROCONCHIATE HYBONOTICERAS FROM MEXICO
FEDERICOOLORIZ & ANA BERTHAVILLASEI~OR OL()RIZ F. & VILLASENOR A.B. 1999. New microconchiate Hybonoticeras from Mexico. [Nouveaux Hybonoticeras microconques du Mexique]. GEOBIOS, 32, 4: 561-573. Villeurbanne, le 31.08.1999. Manuscrit d@pos@le 08.12.1997; accept@ d@finitivement le 02.06.1998. ABSTRACT - A diversified and previously unknown record of microconchiate Hybonoticeras is analysed on the basis of the material collected in sections sampled bed-by-bed in the Mexican Altiplano. Five new species are described and compared to species traditionally recorded from the uppermost Kimmeridgian and the lowermost Tithonian in Europe. A general trend to smoothing in Mexican and European species during the earliest Tithonian is outlined. KEY-WORDS: HYBONOTICERAS, AMMONITINA, UPPER KIMMERIDGIAN, LOWER TITHONIAN, MEXICO. RESUMt~ - Une nouvelle faune diversifi~e de microconques d'Hybonoticeras recueillies banc par banc sur !'Altiplano Mexicain est dtudi@e. Cinq nouvelles esp~ces sont d@crites et compar@es avec des esp~ces classiqnes du Kimmeridgien terminal et du Tithonien basal d'Europe. Une tendance g@n@rale h l'effacement de l'ornementation chez les esp~ces mexicaines et europ@ennes durant le Tithonien inf@rieur est raise en 4vidence. MOTS-CLt~S: HYBONOTICERAS, AMMONITINA, KIMMERIDGIEN SUPI~RIEUR, TITHONIEN INFI~RIEUR, MEXIQUE RESUMEN - Se analiza una variada y previamente desconocida fauna de Hybonoticeras microconchas procedentes de perfiles estratigraficamente controlados en el Altiplano Mexicano. Se describen cinco nuevas especies y se comparan con las especies tradicionalmente reconocidas en el Kimmeridgiense terminal y Tithonico basal de Europa. Se reconoce una tendencia al alisamiento de la concha en las especies mexicanas y europeas durante el Tith6nico temprano. PALABRAS-CLAVE: HYBONOTICERAS, AMMONITINA, KIMMERIDGIENSE SUPERIOR, TITHONICO INFERIOR, M]~XICO
INTRODUCTION Microconchiate hybonoticeratids were first reported by Oppel 1865, who described a f r a g m e n t of t h e body ch am be r of Hybonoticeras mundulum from Rogoznik (Poland). With or without illustration, microconchiate Hybonoticeras have been cited from Bulgaria, France, southern Germany, Hungary, Iberia, Italy and Poland, in Europe; Mad ag as car and Morocco, in Africa, and recently Mexico in the Americas (Berckhemer & HOlder 1959; HOlder & Ziegler 1959; Collignon 1960; Christ 1960; Benetti 1975; E n a y & Geyssant 1975; Ftilop 1976; Sapunov 1976, 1977, 1979; Sarti 1988; O15riz & Rivas 1977-78; O15riz 1978, 1981; G e y e r & P e l l e d u h n 1979; K u t e k & W i e r z b o w s k i 1979, 1986; Wierzbowski 1990; B en etti et al. 1990; Villasefior 1991; O16riz et al. 1993, 1999; Caracuel & Olbriz 1994; Villasefior et al. 1995; B en z aggagh 1996; Schweigert et al. 1996).
T he best k n o w n
and w i d e s p r e a d
species is
Hybonoticeras mundulum (OPPEL), which shows the widest stratigraphic range (Beckeridower Hybonotum Zones) and has been considered as the central species encompassing a r a t h e r limited group of forms. Among t hese forms are Hybonoticeras striatulum and H. attenuatum, described by Berckhem er & H~lder 1959, which have been traditionally t r e a t e d as morpho-subspecies of the species created by Oppel 1865.
Hybonoticeras attenuatum BERCKHEMER& HC)LDER was originally recorded in Crussol (France) as Waagenia pressula NEUMAY~by F o n t a n n e s (1879), who considered this species to be r a r e in his "assises sup@rieures"; B e r c k h e m e r & H(ilder 1959 established its age as early Beckeri Chron (= Subeum el a Subzone in Swabia). Sarti 1988 cited this species from the Beckeri/Pressulum Zone in the Venetian Alps, as did O16riz 1978 from the Beckeri Zone in association with H. verestoicum
562 HERBICH in southern Spain. Sapunov (1977, 1979) reported this species from the upper Kimmeridgian (Beckeri Zone) from Bulgaria, but the specimen illustrated (Sapunov 1979, pl. 53, fig. 3) does not belong to this species. Benzaggaggh (1996) cited this species from the lower Tithonian (Hybonotum Zone) in the Rif (Morocco).
Venetian Alps. Two specimens morphologically close to this species were described by O15riz (1978) from the Beckeri Zone in southern Spain, associated with Hybonoticeras pressulum NEUMAYRand H. verestoicum HERBICH. Benzaggagh (1996) cited H. (Hybonotella) mundulum striatellum (OPPEL)from the lower Tithonian (Hybonotum Zone) in the Rif, but the specimen illustrated (Benzaggagh 1996; pl. 2, fig. 6) is not related to the species created by Berckhemer & HSlder.
The holotype ofHybonoticeras striatuIum BERCKttEM~R & HOLD~R was recorded from the UlmensisSchichten (lower Tithonian, Hybonotum Zone) in Swabia. Berckhemer & HSlder (1959) also figured a paratype from the Zementmergel (~2) at Allmendingen. HSlder & Ziegler (1959) illustrated this species from the ~1 (lower Tithonian) near Ehingen, in Franconia. Sarti (1988) cited H. striatulum, together with attenuatum and mundulum pars, from the Beckeri/Pressulum Zone in the
Hybonoticeras nov. sp.(?) "gracillimum', from the Tithonian Hangende Bankalke (~3) in Riedlingen, is insufficiently known at present. The only records alluding to this species and reported outside from epicontinental Europe are those of H. sp. nov. cf. gracillimum (Benetti et al. 1990; pl. 1, fig. 3), from the Hybonotum Zone of the western Lessinian
Phosphoritic limestone ]
I
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~with ! [
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Hybonoticeras
AD H. m u n d u l u m FAD
(M & m) (m)
< . gonzalezi
'1 C -J
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Location and stratigraphical sections with horizons of described species. Localisation des F~CURE 1 -
affleurements et profils stratigraphiques avec les horizons des esp~ces dgcrites.
ga7
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-
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.
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cuencamensis
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563 Following such a treatment, this paper provides new data about five new species of microconchiate hybonoticeratids from Mexico (Fig. 1) on the basis of a new and rare f a u n a which represents less t h a n 2% among 1,322 ammonites and fragments collected in the sections studied. Macroconchiate H y b o n o t i c e r a s are also rare in the material collected, their preservation is defective (crushed fragments) and belong to shells of small to moderate size (< 100ram). Although a conclusive interpretation is unappropiate, fragmented remains of macroconchiate H y b o n o t i c e r a s are better represented by " b e c k e r i - t y p e " forms, some bitubercula-
Alps, and the citation of H. ( H y b o n o t e l l a ) nov. sp. " g r a c i l l i m u m " , from the upper Beckeri-lower Hybonotum Zones of the Rif (Morocco) made by Benzaggagh (1996) without illustration. Recently, new forms have been notified from Mexico (O15riz et al. 1993) and Swabia (Schweigert et al. (1996), without description). O15riz et al. (1999), recorded the existence of Mexican populations of H . r n u n d u l u r n (OPPEL) S. str. (m) and revised this species separating it from its subspecies, as originally interpreted by Berckhemer & HSlder (1959), favouring its interpretation as biogeographic r a t h e r t h a n morphologic subspecies.
20t
15
---4=.... cuencamensis ....¢._, nov. sp. aft. striatulum
•
attenuatum S & H
•
striatulum
S & H
striatulum
H & Z
striatulum
S. et al.
5
0
10
15 diameter
20
25
30
20" 15.
°o o
10-
o <~ 0 ° <~
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~,
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._.+
5. u cuencamensis
0
1~0
5
15~
2+0
diameter
o mundulum
*
.L25
attenuatum S & H
30
35
C --4U---- cuencarnefTsis --..e.-. nov, sp. aft. striatulum
attenuatum B & H
Bivariate plots for quoted species of microconchiateHybonoticeras. Measurementsin millimeters. Diagrammes de variation des esp#ces indiqudes d'Hybonoticeras microconques. Dimensions en FIGURE 2 -
striatulum
B & H
striatulum
H & Z
striatuium
S. et al,
1()
15 diameter
20
2'5
30
millim~tres.
Abbreviations as used in the tables of measurements (ram): Diameter (Dm), umbilicus (U), whorl height (H), umbilicus vs. diameter (U/D), whorl height vs. diameter (H/D), and number ofribs per half a whorl (R/2). Abrdviations utilisdes dans les tableaux de mesures (ram): Diam~tre (Din), ombilie (U), hauteur (H), rapport de l'ombilic vs. diam~tre (U/D), rapport de la hauteur vs. diam#tre (H/D), et nombre de cStes sur un demi-tour (R/2).
0.6 ¸
o
0.50.4.
•
c~
•
o o <~ a . . . . . . . . . +' . . . . . . . . . . . . o
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o o
o
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~ + + ,o, -....++++ B~ . . . . . . . . . . ~,- + . = . . +
+
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o - -+,
~o
d+'~
o
o
- i -
¢
._.-.-.-
Or3"
0.2 0.1 0
0
c~ cuencamensis
1'0
1'5
diameter
20
o mundulum
25
• attenuaturn S & H
3'0
35
564 3 - Bivariate plots for quoted species of microconchiate Hybonoticeras. M e a s u r e m e n t s in millimeters. Diagrammes de FIGURE
12
A --..~--.. cuencamensis ....o.-. nov. sp. aft. striatulum
variation des esp~ces indiqudes d ' H y b o n o t i c e r a s microconques. Dimensions en miUim~tres.
s~
4+
.L
attenuatum B & H
•
striatu/um B & H
•
striatulum H & Z
•
striatulum B et al.
2{ 01 5
10
15
20
25
30
diameter
B
.o--~+:::~"++~
•
,ip.-~_,o.. ~ o. ~
~
•
+ o 2 + •
•
4+ o
2+
•cuencamensis
o
mundulurn • attenuatum B & H
Oi 0
0.6 0.5
10
15
25
30
35
•..,¢.- nov. sp. aft. striatulum attenuaturn B & H
o.2t
20
---..~-... cuencamensis
0.4 ~0.3~
diameter
c..................................... ,,i........ L ...........................
striatulum 8 & H .
striatulum H & Z
•
striatulum B et al.
o.lt 0J 5
10
15
20
25
30
diameter
0.~
D
0.~
•
•
0.,I
•
~
0.~
o o" ....................... o
: o a
,
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~
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......
+ ~o
...........
~ .....
~
o
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v
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•
0.~
o
~
o
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[] cuencamensis o mundulurn • attenuaturn B & H
0 5
10
te fragments could belong to "hybonotum-type" and/or "extraspinatum-type"forms, and their respective stratigraphic horizons are in accordance with our interpretation of the uppermost Kimmeridgian and lowermost Tithonian in the sections studied (Fig. 1). SYSTEMATIC PALAEONTOLOGY Order AMMONOIDEAZittel, 1884 Family ASPIDOCERATIDAEZittel, 1895, emend Checa, 1985
15 diameter
20
25
30
35
Subfamily HYBONOTICERATINAEO16riz, 1978 Genus Hybonoticeras BREISTROFFER,1947 T y p e - s p e c i e s -Ammonites hybonotus OPPEL,1863
Hybonoticeras cuencamensis
nov. sp. Figs. 2a-d; 3a-d, 4a; 8.1
(m)
H o l o t y p e - IGM 6017, from the CU-6 section at Sierra de Palotes, near Cuencam~ (Durango). Upper Kimmeridgian, upper Beckeri Zone. Derivatio nominis
-
From the town of Cuencam~ (Durango).
M a t e r i a l - One specimen.
565 reinforcements do not determine real periumbilical tuberculations. Very fine striations develop on the remainder of the shell. Nine radial and rather regularly separated ribs, which show enlarged ventral extremes, are typical on the outer flank of the last whorl. Some of these prominent ribs support curved prorsiradiate spines, 9 mm in length; at the pre-peristomal shell the last three ribs are approximated. The peristome is moderately projected into lappets, which are 4 mm long and 2 mm wide. No traces of suture lines are preserved.
D i m e n s i o n s (en ram)
Horizons
Specimens
Dm
U
H
U/D
H/D R/2
CU-6 lm above t00 IGM 6017 26,00 10,10 9,20 0,39 0,35 0 17,20 7,20 7,25 0,42 0,42 2 11,50 4 , 4 0 5,00 0,38 0,43 15
D e s c r i p t i o n - A crushed but well preserved specimen which permits the analysis of the complete ontogeny in detail. The umbilicus is shallow and wide (0,38-0,42), and the coiling seems to increase at 17 mm and then to decrease slowly (Fig. 2a-c). Due to flattening, a reduced umbilical wall is inferred from the locally crescentic umbilical edge. The whorl section is presumably ovate and the relative whorl height i s nearly constant, slightly decreasing from 17 mm onwards according to the data obtained. The sculpture shows a significant change between 8 and 12 mm. Below 8 ram, the inner whorls show simple straight ribs, which characterize the first ornamental phase shared by the majority of micro- and macroconchiate Hybonoticeras; but no tubercles are present. Ribs enlarge to the periphery. Between 8 and 12 mm, extremely fine and slightly sinuous ribs bunch at the umbilical edge, but the resulting
- As usual in microconchiate Hybono~iceras, Hybonoticeras cuencamensis nov. sp. (m) does not show major differences in shell structure in comparison with other species. As shown in figures 2a-d and 3a-d, the ontogenetic course for coiling and the whorl height clearly separates this species from H. m u n d u l u m (OPPEL) S. str. and H. attenuatum Remarks
BERCKHEMER & HOLDER; a d u l t s ef H. striatulum BERCKHEMER & HOLDERand H. gracillimum BERCKHEMER 8~ HOLDER have wider umbilicus (Figs. 2a,c). In contrast, the tightly coiled H. attenuatum BERCKHEMER& HOLDER is closer in the sculptural pattern and relative whorl height. The ontogenetic course of the ornamentation (Fig. 4a) permits easy separation of Hybonoticeras cuencamensis nov. sp. (m)
from H. munduIum. H. attenuatum is a closer species among smoothed microconchiate Hybonoticeras, but smoothing is earA
18 16 14 12
•..4~.... cuer~camensis
c~
.-.~.-. nov. sp. aft. striatulum attenuaturn S & H striatulum B & H
4t
striatulurn H & Z striatulum B et al.
131-5
10
15
diameter
20
25
30
35
B
1E
o
14
o
• •
o
A
'~
...... ...x
o
Q
o
4
2
o mundulum ~ gonza/ei
)c geminaturn
0 5
FIGURE4 - a. Rib curves for quoted species of microconchiate Hybono. ticeras. Dimensions in millimeters. b-c. Bivariate plots for quoted species of microconchiate Hybonoticeras. Measurements in millimeters. a. Courbes de costulation des esp.~ces indiquges dTIybonoticeras m~croconques. Dimensions en millim~tres, b-c. Diagrarnmes de variation des esp~ces indiqudes dTIybonoticeras microconques. Dimensions en millim~tres.
10
15
diameter
20
25
30
35
C 0.6 •
•
0.5 6
0
•
0
O
0.4 0.3 0.2 0.1
o mundulum ~ gonzalei
~ gerninatum
0 5
10
15
diameter
20
25
30
35
566 lier a n d comparatively accentuated in H. cuencamensis nov. sp. t h a n in the older H. attenuatum; moreover, the base of the external tubercles in a t t e n u a t u m is longitudinally elongated instead of radially as in H. cuencamensis nov. sp. Thus, the Mexican H. cuencamensis nov. sp. is separate from the morphologically closest species, the E u r o p e a n attenuaturn, on the basis of phenotypic and stratigraphic data.
Bio-chronostratigraphy and biogeography H. cuencamensis nov. sp. has been recorded below the first record of Pseudodiscosphinctes siliceus
(QUENSTEDT),which was interpreted to indicate the base of the Tithonian at the CU-6 section, near Cuencam~ (Durango), by O15riz et al. (1993). Thus, assuming both the traditional interpretation of the Kimmeridgian-Tithonian boundary (i. e., the Beckeri-Hybonotum Zone boundary) and the absence of Hybonoticeras of the pressulumverestoicum groups, the latter being a general trait in the Mexican Altiplano, H. cuencamensis nov. sp. (m) is interpreted as belonging to the top of the Kimmeridgian, i.e., the upper Beckeri Zone. At present, H. cuencamensis nov. sp. (m) is known from only one section near Cuencam6 (Durango) and, therefore, is considered to be endemic to Mexico. Hybonoticeras gonzalezi nov. sp. (m) Figs. 4b, c; 5a-d; 8.6-9 H o l o t y p e - IGM 6023, from the MZ-3 section at the Cafihn de S a n Matias, S i e r r a de S a n t a Rosa, Zacatecas. U p p e r Kimmeridgian, upper Beckeri Zone. P a r a t y p e s -IGM 6020-6023, from the MZ-3 section at the Cafihn de San Matlas, Sierra de S a n t a Rosa (Zacatecas); IGM 6025 and IGM 6026, from the MZ-1 section at Vereda del Quemado, Sierra de Mazapfl (Zacatecas). Upper Kimmeridgian, upper Beckei Zone. D e r i v a t i o n o m i n i s - This species is dedicated to C. Gonz~lezArreola, ammonitologist of the UNAM (Mexico). M a t e r i a l - Six specimens moderately preserved. D i m e n s i o n s ( e n ram) Horizons Specimens Dm MZ-3.449 top
IGM 6020
MZ-3.449 top
IGM 6021
MZ-3.449 top
IGM 6022
MZ-3.449 top
IGM 6023
MZ-l(22 to 24) IGM 6025 MZ-1(22 to 24) IGM 6026
32,10 27,20 17,80 29,30 26,40 20,50 30,30 21,00 30,30 24,60 20,20 15,70 12,50 28,00 19,50 22,30 18,30 15,70
U 12,70 11,00 6,60 13,00 9,90 7,50 12,60 9,00 12,80 10,80 8,00 6,10 5,00 10,30 6,00 9,00 8,20 5,50
H
U/D
H/D
PJ2
9,50 10,90 7,90 9,00 8,60 7,00 9,50 8,20 10,00 7,40 6,80 6,10 5,00 8,50 8,20 7,30 5,30 6,20
0,40 0,30 14 0,40 0,40 .... 0,37 0,44 .... 0,44 0,31 13 0,38 0,33 14-15 0,37 0,34 14 0,42 0,31 .... 0,43 0,39 .... 0,42 0,33 15 0,44 0,30 14 0,40 0,34 12 0,39 0,39 14 0,40 0,40 13 0,37 0,30 13 0,31 0,42 11-12 0,40 0,33 12-13 0,45 0,29 12-13 0,35 0,39 12
D i a g n o s i s - Microconchiate, spiny Hybonoticeras, with persistent ribbing and lappeted peristome.
D e s c r i p t i o n - The material analysed consists of crushed but complete specimens, 22,3 to 32,1 mm in size. In adults, the umbilicus is wide (0,37 to 0,44), shallow, and limited by a low and more or less sharp umbilical wall. The umbilical edge is gently rounded. Values of coiling slightly increase during ontogeny (Fig. 4b-c). The whorl section is ovate and the whorl height slowly increases during ontogeny (Fig. 5a). The relative whorl height decreases towards greater sizes (Fig. 5b). The sculpture is of ribs which persist to the peristome. No smoothing is observed on the body chamber. From the nucleus to the beginning of the outer whorl, ribs are simple, straight, and slightly enlarged towards the periphery. Bifurcation close to the umbilical edge occurs rarely. An incipient sinuosity of ribs can be observed at the beginning of the outer whorl. The last whorl shows more irregular ribbing, some ribs tend to joint at the umbilical edge while, occasionally, others coalesce to peripheral spines. External tubercles are present in the outer whorl and some connect with moderate and slightly prorsiradiate spines. Prorsiradiate curved ribs are common on the adoral quarter of the body chamber where intercalatories seem, rarely, to induce a zig-zag "Simocosmoceras-like" sculpture. Pre-peristomal ribs show a more or less acute adoral convexity, which corresponds to the base of the lappets. The peristome is elongated into narrow projections without distal expansion, up to 8 mm in length. R e m a r k s - The most striking traits in this species are the persistence of ribbing, without weakening towards the peristome, the occasional presence of zig-zag sculpture n e a r the aperture, and the spinous tubercles at the periphery of the outer whorl. Hybonoticeras mundulum (OPPEL) s. str. (m) shows a similar structure of the shell, more constant and higher values of coiling and the opposite for the relative whorl height (Fig. 4b-c; 5a-b). Moreover, the ribbing style indicates a clear separation on the basis of the more conservative and dense ribbing, which is typical in Oppel'species. Schweigert et al. (1996) reported, without illustration, the existence of a new spiny microconchiate Hybonoticeras in the lower Beckeri Zone in Swabia; therefore, the new species from southern Germany is older. Due to persistent ribbing, Hybonoticeras gonzalezi nov. sp. is more closely related to the new Mexican species geminatum. However, these two species differ, mainly, in ribbing density, the development of geminate-spiny sculpture (Figs. 5c-d) and the structure of lappets. Smoothed microconchiate Hybonoticeras are clearly different in both, the course of the o r n a m e n t a t i o n and, sometimes, the shell structure.
Bio-chronostratigraphy and biogeography Hybonoticeras gonzalezi nov. sp. (m) has been recorded from the upper Beckeri Zone in the region of Mazapfl, Zacatecas (Sierra de Santa Rosa and Sierra de Mazapil). At present, this species is considered to be endemic to Mexico.
567 FIGURE 5 - Bivariate plots for quoted species of microconchiate Hybonoticeras. Measurements in millimeters. Diagrammes de variation des esp~ces indiqu~es d'Hybonoticeras rnicroconques. Dimen-
A
1 1
. +....++~." p
10
o~
x
...............
o'
a
~s ~
sions en millim~tres. /
o
o
o
~
..m~-~..-
' ' ~
o
o
o
0
0 mundulum
5
10
15
20 diameter
A gonzatei
x geminalorn
25
30
35
B 0.61
o.5{
•
o
0.4+
oo
A
o
~r._-z.--~_
x
•
x
A
............... : .......
~. o.3t
•
•
o
o.1{
o mundulum
5
10
15
20
diameter
20I
,
o
z~ 9onzalei
25
x
geminatum
30
35
....x x .....
15
~ .
.
.
.
.
.
x ..........
.
,~- ............... ; .=:.=~~--....~-~..~-~_~ . . . . . . .
2
,,.
x
+
0.2;
OI 0
•
•
a
-
- ~ - gonzalei
5
10
15
20
25
....x....ge;ninatum
30
35
diameter e D
2o1
•
•
e
o
<~
~
e
o
e
. + , ,~. . . . . . . . . . x . . ~
.............
15t e
A
~ &
.~1ot 51 ¢ mundulum
z~ gonzalei
x gemlnatum
01
5
Hybonoticeras gerninatum nov.
sp. (m)
Figs. 4b-c; 5a-d; 8.2-5 Synonymy 1993 Hybonoticeras (Hybonotella) sp.: O15riz et al. p. 280, pl. 2, fig. 6. H o l o t y p e - IGM 6190, from the CU-6 section at Sierra de Palotes, Cuencam~ (Durango). Upper Kirnmeridgian, upper Beckeri Zone. P a r a t y p e s - IGM 6019, IGM 6024a and IGM 6024b, from the MZ-3 section at the CafiSn de San Matlas, Sierra de S a n t a Rosa (Zacatecas). Upper Kimmeridgian, upper Beckeri Zone.
10
15
20
25
30
35
M a t e r i a l - Four specimens preserved as impressions.
D i m e n s i o n s (en r a m ) Horizons Specimens CU-6.122
MZ-3.452a MZ-3.449 mid.
MZ-3.449 mid.
D e r i v a t i o n o m i n i s - In allusion to the presence of geminate ribs.
diameter
Dm
U
30,50
12,40
10,80 0,41
0,35
17
19,60 12,20 IGM 6019 23,00 21,00 IGM 6024a 27,00 22,00 19,00 IGM 6024b 34,00 22,70 17,00
6,80 4,90 i0,00 9,00 12,00 8,00 7,30 14,30 9,30 6,30
7,60 4,90 9,50 6,60 7,20 7,30 7,30 12,50 7,00 7,00
0,39 0,40 0,41 0,31 0,27 0,33 0,38 0,37 0,31 0,41
13 13 16' .... 16 15 16 .... .... ....
IGM 6190
H
U~
0,35 0,40 0,43 0,43 0,44 0,36 0,38 0,42 0,41 0,37
H/D
R/2
568 D i a g n o s i s - Microconchiate Hybonoticeras with variable development of near-geminate to geminate ribs and spines. Lappeted peristome.
Hybonoticeras nov. sp. aff. striatulum
Description - The specimens are complete to nearly complete. The whorl section is ovate, s u b r e c t a n g u l a r at the level of the external tubercles. The umbilicus is wide (0,41 to 0,44 in adults) and moderately shallow due to the existence of a small, gently convex, umbilical wall. The umbilical edge is rounded. Slightly increasing uncoiling characterizes the ontogeny (Fig. 4b-c). The whorl height increases regularly during ontogeny, b u t the relative whorl height decreases very slowly (Fig. 5a-b). On the inner whorls, ribs are simple, more or less straight, and show enlarged to incipiently tuberculate ventral extremes, to which some paired costulae m a y connect (neargeminate structures). This is shown in the holotype as early as at 5 mm in size, and in paratypes IGM 6024a and IGM 6024b between 6 and 9 mm., and 10 mm, respectively (Fig. 8.2-5). In the penult i m a t e whorl, g e m i n a t e ribs with spinous tubercles can be more or less apparent. The same affects the outer whorl, where ribbing density and sinousity are variable. In the holotype, near-geminate structures made by up to three ribs, two of them aproximated near the umbilicus, can unite in the same external tubercle. Pre-peristomal ribs show adoral convexity in accordance with the location of the beginning of lappets. The most complete peristome includes short and broad lappets, 4,5 m m in length and 3 mm in width.
Synonymy aff. 1959 Hybonoticeras mundulum striatulum BERCKHEMER
Remarks - Hybonoticeras geminatum nov. sp. (m) shows coiling values very similar to Hybonoticeras gonzalezi nov. sp. (m) a n d
lower t h a n
those
in Mexican p o l u l a t i o n s
of
Hybonoticeras mundulum (OPPEL) S. str (m) (Fig. 4b-c). The relative whorl h e i g h t is h i g h e r t h a n in Hybonoticeras rnundulure (OPPEL) s. s t r (m), which shares the same t r e n d during ontogeny (Fig. 5a-b). Ribbing density is variable in H. gerninature nov. sp. (m) (Fig. 5c) a n d i n t e r m e d i a t e between the more densely ribbed Mexican populations of H. mundulum (OPPEL) S. str. (m) a n d the less densely ribbed H. gonzalezi nov. sp. (m) (Fig. 5d). Moreover, H. geminatum nov. sp. develops geminate ribbing which is u n k n o w n in the other two species. The persistence of ribbing is a clear difference w h e n compared to species belonging to more or less smoothed microconchiate Hybonoticeras.
Bio-chronostratigraphy and biogeography T h e holotype ofHybonoticeras geminatum nov. sp.
(m) has been recorded j u s t below the first record of Pseudodiscosphinctes siliceus (QuENSTEDT) at the CU-6 section in Sierra de Palotes (Cuencam6, Durango). The paratypes come from the upper Beckeri Zone from the region of Mazapfl, Zacatecas (Sierra de Santa Rosa and Sierra de Mazapfl). Thus, this endemic species characterizes the upper Beckeri Zone in the studied sections of the Mexican Altiplano.
BERCKHEMER • HOLDER (m) Figs. 2a, c; 3a, c; 4a; 6a-d; 7; 8.10-11
aff. aff.
aff. non non
non
8g HOLDER, p. 36, textfig. 21, pl. 5, figs. 20, 21, pl. 6, fig. 31. 1959 Hybonoticeras mundulum striatulum BERCKHEMER gg HOLDER: HSlder & Ziegler, p. 213, pl. 20, figs. 6. 1978 Hybonoticeras (Hybonotella) mundulum striatuIum BERCKHEMER & HOLDER: O15riz, p. 365, pl. 33, fig. 7. 1988 Hybonoticeras striatulum BERCKHEMER & HOLDER: Sarti, p. 468. 1990 Hybonoticeras mundulum striatulum HOELDER: Benetti et al., p. 34, pl. 1, fig. 38. 1991 Hybonoticeras (Hybonotella) rnundulurn (OPPEL) striatulum BERCKHEMER • HOLDER: Villasefior, p. 72, pl. 6, fig. 10. 1996 Hybonoticeras (H.) mundulum aff. striatellum (OPPEL): Benzaggagh, p. 47, fig. 39, pl. 2, fig. 6.
Holotype - IGM 6018, from the MZ-3 section at the CafiSn de San Matias, Sierra de S a n t a Rosa, Zacatecas. Upper Kimmeridgian, upper Beckeri Zone.
Material - One specimen. Dimensions (en mm) Horizons
Specimen
Dm
MZ-3 449 mid.
IGM 6018 26,00 21,50 16,20 14,50
U
H
U/D
H/D R/2
10,50 9,50 5,60 5,20
9,00 7,10 5,70 5,20
0,40 0,45 0,35 0,36
0,35 0,33 0,35 0,36
7 9 13 14
- Small sized Hybonoticeras with fine striations bunching at the umbilical edge on the outer whorl.
Diagnosis
- Shell impression without peristome. The umbilicus is moderately wide (0,36-0,45), shallow, and limited by a small but identifiable umbilical wall, which is emphasized by crushing. The coiling gently increases during ontogeny (Fig. 6a-b). The whorl section is presumably ovate. The relative whorl height decreases up to around 20 mm and then increases slowly (Fig. 6c-d). The sculpture shows the pattern typical in Hybonoticeras striatulum BERCKHEMER & HOLDER, with simple ribs present from the nucleus to a variable extent onto the outer whorl, and then striations which are more or less reinforced near the umbilical edge. In the specimen described, the sculptural phase of simple, straight ribs ends at the beginning of the outer whorl, at around 15 mm in shell size. Among striations, some stronger costulae seem to joint at the periphery, sometimes coinciding with the base of straight "mummy" spines. Increasing smoothing is related to looser and more widely spaced primaries from 17 mm onwards (Fig. 7). Description
Remarks - The umbilicus in IGM 6018 shows values close to the upper range of Hybonoticeras striatulum BERCKHEMER HOLDER, according to meassurements of the specimen illustrated by Berckhemer & HSlder (1959) and HSlder & Ziegler
569
16ii
FIGURE 6 - Bivariate plots for quoted species of microconchiate Hybonoticeras. Measurements in millimeters. Diagrammes de variation des esp~ces indiquges d'Hyboneticeras microeonques. Dimensions en millim~tres.
evanidum
14
--.c-- nov. sp. aff, str/atu/um
12
#l
NlO
"',
-
-* -
attenuatum S & H
..,~..... striatulum S & H • . 4~ - striatulum H & Z •. . ~ . - , gracifl/mum
w"
#
O~ 0
B et aL
wm
~
10
15
20 diameter
25
30
35
graci/I/mum B & H
40
.--+--
evanidurn
0.6
nov, sp. aft. striatulum
0.5
- ~ -
attenuatum
0.4
-.~....
striatulum
0.3
• . , ~ - striatutun'l H & Z
#8
S & H B & H
S e~ al,
. , . , ~ . - . gracJllimum
0.2
#
g,racilfimunl
B & H
0.1 0 5
0
10
1B
20 diameter
25
30
35
40
c
14
--4,--
ev~nidum
-.--o-- nov. sp. aft, striatulum - ~ -
10
~,~:#'~
."
~
.--'~.-. gr~cillimum #
0 40
1
5
10
I
15
S & I-t
-.-,~ - striatulum H & Z
.~,-
• ~-e
attenuatum
2 % a m e t e r 25
30
SetaL
gracitfirnum B & H
35
40
D
0.6 evanidum
0.5.
,--o.-- nov. sp. aft. stfiatulum
0.4
- - " " J " " " = - - # " - + - " - - - - - J~-
1
"
10.3
..................... ,_...~--x.
m
- '~ - attenu~tum B & H
,
...4-..
striatulurn S & H
.. ~ - striatulum H & Z
0.2,
•..-~--, gravillimum
0.1• O-
#
10
(1959), but the coiling does not increase during the final ontogeny (Fig. 6a-b). Whorl height values and the relative whorl height are close to the upper range in Hybonotieeras striatulum BERCKHEMER& HOLDER(Fig. 6c-d). The most striking difference between the German and the Mexican species concerns rib curves, which largely vary between the more densely ribbed holotype and the paratypes illustrated from southern Germany. The rib density in the German species is clearly higher from 17 mm onwards (Fig. 7). Curiously, Hybonoticeras nov. sp. aff. striatulum is closer to the smoothed H. attenuatum BEECKHEMER8~ HOLDER (Fig. 7). However, the Mexican specimen IGM 6018 is older since comes from the uppermost Kimmeridgian (upper Beckeri Zone). Villasefior (1991) reported Hybonoticeras (Hybonotella) mundulum (OPPEL) striatulum BERCKHEMER& HOLDER from the upper Beckeri Zone of the Sierras de Mazapfl and Santa Rosa.
15
20 diameter
25
30
35
B et al.
gracitlirnum B & H
40
This author noted differences in sculpture in comparison with the German species, and recognized the possibility of differentiating her material at the subspecies level (see below). Specimens morphologically close to Hybonoticeras striatulum BERCt~IEMER& HOLDERhave been cited, and/or figured, by O16riz (1978) and Sarti (1988), from the lower Beckeri Zone in association with Hybonoticeras of the group pressulum-verestoicum. At present, there are no illustration of true Hybonoticeras striatulure BERCKHEMER • HOLDER from the lower Tithonian outside southern Germany.
Among smoothed microconchiate Hybonoticeras, Hybonoticeras nov. sp. aff. striatulum BERCKHEMER& HOLDER(lri) seems to be a morphological transient betweem the groups of the European attenuatum and striatulum, being closer to striatulum in ceiling (Fig. 6a-b), the incipient periumbitical reinforcement of costulae, and stratigraphy, while the whorl height and the den-
570 sity of ribs are closer to attenuatum (Figs. 6c-d, 7). Thus, Hybonoticeras nov. sp. aff. striatulum BERCKHEMER& HOLDER (m) is considered as both, the predecessor of Lower Tithonian microconchiate smoothed Hynonoticeras in Mexico, and morphologically related to the group of Hybonoticeras striatulum BERCKHEMER& HOLDER, which is unknown from the southern North American Plate. Bio-chronostratigraphy and biogeography H y b o n o t i c e r a s nov. sp. aff. s t r i a t u l u m BERCKHEMER HOLDER (m) c o m e s f r o m t h e u p p e r m o s t Kimmeridgian, upper Beckeri Zone of the Cafi6n de S a n M a t i a s ( S i e r r a de S a n t a Rosa, Z a c a t e c a s ) .
H y b o n o t i c e r a s e v a n i d u m nov. sp. (m) Figs. 6a-d; 7; 8.12 Synonymy 1991 Hybonoticeras (Hybonotella) mundulum (OPPEL)striatulum BERCKHEMER• HOLDER:Villasefior, p. 72, pl. 6, fig. i0. Holotype - IGM 6027, from the MZ-3 section of the Cafi6n de San Matias, Sierra de Santa Rosa (Zacatecas). Lowermost Tithonian, lower Hybonotum Zone. Paratype Quemado,
- IGM 6028, from the MZ-I section at Vereda del Sierra de Mazapfl (Zacatecas).
evanidus-a-um. In allusion to the extreme fading of the sculpture. D e r i v a t i o n o m i n i s - From Latin
M a t e r i a l - Two specimens partially flattened. D i m e n s i o n s ( e n ram) Horizons Specimen Dm MZ-3 465 mid. IGM 6027 35,90 34,00 26,40 22,60 18,00 MZ-1 26 IGM 6028 26,50 23,00 19,00 17,00
U 15,50 15,00 10,70 9,30
H U/D 11,00 0,43 10,50 0,44 8,70 0,41 7,40 0,41
10,80 9,80 6,90 6,50
8,80 6,90 6,70 6,50
0,41 0,43 0,36 0,38
H/D R/2 0,31 2 0,31 i0 0,33 22-23 0,33 25* 18 0,33 9 0,29 17-18 0,35 15-16 0,38 ~13-14
- S m a l l , e v o l u t e a n d l a p p e t e d Hybonoticeras w i t h e x t r e m e f a d i n g of t h e s c u l p t u r e . Diagnosis
- The analysed specimens are nearly complete, especially the holotype which shows preperistomal structures indicating the location of t h e b e g i n n i n g of a l a p p e t e d p e r i s t o m e . T h e shell size v a r i e s b e t w e e n 26,5 a n d 35,9 m m . I n a d u l t s , Description
t h e u m b i l i c u s is s h a l l o w a n d w i d e (0,41 to 0,44; Fig. 6a). T h e u m b i l i c a l w a l l is s m a l l , distinct, a n d t h e u m b i l i c a l edge r o u n d e d . O n a v e r a g e , coiling v a l u e s i n c r e a s e v e r y s l o w l y d u r i n g o n t o g e n y (Fig. 6b). T h e w h o r l s e c t i o n is o v a t e a n d t h e o n t o g e n e t i c course of the relative whorl height gently d e c r e a s e s (Fig. 6d). T h e s c u l p t u r e is o f m a i n l y simple, s t r a i g h t ribs, w h i c h m o r e or less e n l a r g e t o w a r d s t h e p e r i p h e r y . R a r e b i f u r c a t i o n is p r e s e n t w i t h divisions l o c a t e d s l i g h t l y a b o v e t h e u m b i l i c a l edge. Rib d e n s i t y v a r i e s i n v e r s e l y to rib c o a r s e n e s s (Fig. 7). E x t r e m e f a d i n g of t h e s c u l p t u r e a p p e a r s s u d d e n l y i n t h e a d u l t b o d y c h a m b e r giving, t h u s , a t y p i c a l a p p e a r a n c e to t h i s species. I n t h e h o l o t y p e , s o m e s i n u o s e a n d v e r y s u b t l e u n d u l a t i o n s c a n be r e c o g n i s e d on t h e b o d y c h a m b e r . N e a r t h e p e r i s t o me, a m a r k e d u n d u l a t i o n is i n t e r r u p t e d s l i g h t l y a b o v e t h e m i d - f l a n k , p r o b a b l y i n d i c a t i n g t h e locat i o n of t h e b a s e of l a p p e t s ; t h u s , t w o b u l k y a n d adorally concave branches precede the aperture. N o s u t u r e lines a r e p r e s e r v e d . R e m a r k s - Hybonoticeras
evanidum nov. sp. has a shell structure (coiling and relative whorl height) with no major differences from other microconchiate Hybonoticeras. According to measurements taken from the illustration in Benetti et al. (1990), the shell structure in Hybonoticeras sp. nov. c£ gracillimum HOLDERis especially close at the same diameters (Fig. 6ad), but the specimen from the western Lessinian Alps shows more distant ribs in the inner whorls and coarse spines which connect to large undulations on the flank in the outer whorl. Dimensions of the holotype of Hybonoticeras nov. sp.? ("gracillimum") BERCKHEMER& HOLDER, given by Berckhemer & H61der (1959, p. 37), show that the German specimen has a more evolute shell with whorl height lower than in Hybonoticeras evanidum nov. sp. (Fig. 6a-d). As far as the counting of ribs is possible in the specimen illustrated by Berckhemer & H61der (1959, pl. 5, fig. 22), smoothing in this specimen is weaker, and density of ribs is lower (# in Fig. 7); moreover, fine and longitudinally elongated tubercles are well developed at the periphery. Since other smoothed microconchiate Hybonoticeras have clearly different ribbing curves (Fig. 7), irrespective of wether they are older (group of attenuatum) or more or less contemporaneus (group of striatulum), it seems that the two specimens described from Mexico can be separated from contemporaneous species on the basis of both, the type and ontogenetic course of the ornamentation. According to the above, smoothing in microconchiate Hybonoticeras was an ontogenetic trend shared by populations inhabiting widely separate areas in epicontinental Europe and Mexico, as well as in the epioceanic western Tethys, during the
24t 2O
evanidum
j16
nov, sp. aft. striatulun 112f
FIGURE 7 - Rib curves for quoted species of m i c r o c o n c h i a t e Hybonoticeras. D i m e n s i o n s in m i l l i m e t e r s .
Courbes de costulation des esp~ces indiqades d ' H y b o n o t i c e r a s microconques. Dimensions en millim~tres.
- ~ , - - attenuaturn B & H
....m----striatulum
st
B& H
. . . ~ - striatulum H & Z
4~ #
OI 0
10
15
20 d}ameter
25
30
35
gracillirnum S & H
40
571
1
2
3 4
6
7
9
10
11
FIGURE 8 1. Hybonoticeras cuencamensis nov. sp. (m), IGM 6017, holotype, Upper Kimmeridgian, upper Beckeri Zone, CU-6 section (Sierra de Palotes, Durango). 2-5. Hybonoticeras geminatum nov. sp. (m). 2-3, IGM 6190, holotype, Upper Kimmeridgian, upper Beckeri Zone, CU-6 section (Sierra de Palotes, Durango).4, IGM 6024b, paratype, Upper Kimmeridgian, upper Beckeri Zone, MZ3 section (Sierra de Santa Rosa, Zacatecas). 5, IGM 6024a, from the same horizon and section as figure 4. 6-9. Hybonoticeras gonzalezi nov. sp. (m). 6, IGM 6023, holotype, Upper Kimmeridgian, upper Beckeri Zone, MZ-3 section (Sierra de Santa Rosa, Zacatecas). 7, IGM 6021, paratype, Upper Kimmeridgian, upper Beckeri Zone, MZ-3 section (Sierra de Santa Rosa, Zacatecas). 8, IGM 6020, paratype, from the same horizon and section as figure 7.9, IGM 6025, paratype, Upper Kimmeridgian, upper Beckeri Zone, MZ-1 section (Sierra de Mazapfl, Zacateeas). 10-11. Nybonoticeras nov. sp. aff. striatulum BERCKHEMER& HOLDER (m). IGM 6018, holotype, Upper Kimmerid~an, upper Beckeri Zone, MZ-3 section (Sierra de Santa Rosa, Zacateeas). 12. Hybonoticeras evanidum nov sp. (m). IGM 6027, holotype, lowermost Tithonian, lower Hybonotum Zone, MZ-3 section (Sierra de Santa Rosa, Zaeatecas). All figures at natural size. -
early Tithonian. Thus, in Mexico, as in Europe, extremely smoothed microconchiate Hybonoticeras (evanidum and gracillimun and related forms, respectively) were presumably derived from local ancestors (endemic ?) in which the fading of the sculpture was less accentuated, i.e., Hybonoticeras nov. sp. aff. striatulurn, in Mexico, and Hybonotyceras striatulum in southern Germany.
Bio-chronostratigraphy and biogeography Hybonoticeras evanidum nov. sp. (m) comes from the lowermost Tithonian, lower Hybonotum Zone of the
Cafidn de San Matias, Sierra de Santa Rosa (Zacatecas) and Vereda del Quemado, Sierra de Mazapil (Zacatecas). CONCLUSIONS
A valuable fauna of microconchiate Hybonoticeras is first described from the uppermost Kimmeridgian and the lowermost Tithonian in the Mexican Altiplano.
572
Paying attention to stratigraphy, the evaluation of the morphological diversity provides the basis for the recognition of five new species. Significant differences between these species are found in the patterns of ornamentation rather than in shell structure. Thus, differences in coiling and relative whorl height are considered to be secondary, comparatively. Within impoverished assemblages of ammonites, the taxonomic diversification, interpreted at the species level, is in accordance with the endemic character of the fauna, a feature previously envisaged by the authors (O16riz 1992; O16riz et al. 1990, 1999) The information obtained supports the assumption that general trends in morphologic evolution (e.g., smoothing) were shared by microconchiate Hybonoticeras inhabiting separate areas during the latest Kimmeridgian and the earliest Tithonian. A c k n o w l e d g e m e n t s - This research was supported by the Direcci6n General de Asuntos del Personal Acad~mico (DGAPA) and the Instituto de Geologia, UNAM, Mexico, and the EMMI Group (RNM 0178 Junta de Andalucia) Spain. We are grateful to G. Schweigert (Staatliches Museum ffir Naturkunde, Stuttgart) who provided valuable biostratigraphic information and comments on ammonite types from old German collections, as well as to R. Enay (URA 11 CNRS et Centre des Sciences de la Terre, Universit~ Claude-Bernard, Lyon I) and N. Morton (Department of Geology, Birkbeck College, University of London) for their interesting remarks on the submitted manuscript.
REFERENCES BENETTI A. 1975 - Note biostratigrafiche sul giurassico del "Covolo di Camposilvano" nei Monti Lessini Veronesi. Studi Trentini di Scienze Naturale, Acta Geologica, Trento, 54: 227-245. Benetti A., Pezzoni N & Zeiss A. 1990 - A small, but interesting new ammonite fauna from the Western Lessinian Alps. In PALLINI G., CECCAF., CRESTAS. & SANTANTONIOM. (eds), Atti
del Secondo Convengo Internazionale Fossili, Evoluzione, Ambiente Pergola: 33-37. BENZAGGAGH M. 1996 - Le Malm Sup~rieur et le Berriasien dans le Pr6rif interne et Ie M~sorif (Rif, Maroc): Stratigraphie, S~dimentologie, Pal~og~ographie et Evolution Tectono-Sedimentaire. Th~se Universit6 Mohamed V, Rabat, 403 p. (unpublished). BERCKHERMER F. & HOLDER H. 1959 - Ammoniten aus dem Oberen Weissen J u r a Suddeutschlands. Beihefte zum Geologischen Jahrbuch, 35, 135 p. BREISTROFFER M. 1947 - Notes de Nomenclature pal~ozoologique. I. A m m o n i t e s j u r a s s i q u e s et cr~tac~es. Proc~s Verbaux mensuels, Societd scientifique Dauphind, 26, 195 p. CARACUEL J.E. & OLORIZ F. 1994 - La Formaci6n Aumedr& Nuevos datos sobre el Jur~sico Superior en la Sierra Norte de Mallorca. X Jornadas de Paleontologfa, Madrid, Abstracts: 40 CHECA A. 1985 - Los aspidoceratiformes en Europa (Ammonitina, Fam. Aspidoceratidae: Subfamilias Aspidoceratinae y Physodoceratinae). Tesis Doctorales de la Universidad de Granada, 413 p.
CHRIST H.A. 1960 - Beitrage zur Stratigraphie und Palaontologie des Malm von Westsizilien. Schweizerischen Pal~tontologischen Abhandlungen, 77: 5-141. COLI~IGNON M. 1960 - Atlas des fossiles caract~ristiques de Madagascar, Fascicle 6 (Tithonique). Tananarive, Minist~re
des Mines et de l'Energie, Service gdologique, Rdpublique Malgaehe. ENAY R. & GEYSSANT J. 1975 - Faunes des chaines b6tiques (Espagne meridionale). Colloqne sur la limite JurassiqueCr~tac~, Lyon, Neuchatel. M4moires du Bureau des Recherches gdologiques et mini~res, 86: 39-55. FONTANNES F. 1879 - Description des ammonites des Calcaires du Ch(tteau de Crussol. F. Saby, (ed.), Paris, 122 p. FULOP J. 1976 - The mesozoic b a s e m e n t horst blocks of Tara. Geologica Hungarica, Series Geologica, 16: 5-228. GEYER O.F. & PELLEDUHN R. 1979 - Sobre la estratigra~a y la facies espongiolitica del K i m m e r i d g i e n s e de Calanda (Provincia de Teruel). Cuadernos de Geologfa de la Universidad de Granada, 10: 67-72. HOLDER H. • ZIEGLER B. 1959 - Stratigraphische und faunistische Beziehungen im Weissen J u r a (Kimeridgien) zwischen Suddeutschland und Ard~che. Neues Jahrbuch fi~r Geologie und Pal~ontologie, Abhandlungen, 108: 150-214. KUTEK J & WIERZBOWSKIA. 1979 - A Tithoflian ammonite succession at Rogoznik, Pienifiy Klippen Belt, Poland. Acta Geologica Polonica, 29 (2): 195-205. KUTEK J & WIERZBOWSKIA. 1986 - A new account of the Upper Jurassic stratigraphy and ammonite of the Czorzstyn succession, Pieniny Klippen Belt, Poland. Acta Geologica Polonica, 36 (4): 289-316 OL(~RIZ f. 1978 - Kimmeridgiense-Tithonico inferior en el Sector Central de las Cordilleras B~ticas (Zona Subb~tica). Paleontologia.Bioestratigrafia. Tesis Doctorales de la Universidad de Granada, 184 (1-2), 758 p. OLORIZ F. 1981 - Introduction to the knowledge of the Kimmeridgian and lower Tithonian in the Central part of the Subbetic Cordilleras (Betic Mountains), Southern Spain. Geologica Balcanica, 11 (1): 27-50. OLORIZF. 1992 - North-central and eastern Mexico. In WESTERMANNG.E.G. (ed.), The Jurassic of the Circum-Pacific, part
3, Regional Geology and Stratigraphy, Meso-America. Cambridge University Press, New York: 100-107. OLORIZ F & RIVAS P. 1977-78 - La serie jur~sica de la unidad Casa Blanca-Puerto de las Palomas (Zona Subb~tiea). Cuadernos de Geologfa de la Universidad de Granada, 8-9: 205-224. OLORIZF., VILLASENORA.B., GONZALEZ-ARREOLAC. & WESTERMANN G.E.G. 1990 - Las plataformas mexicanas d u r a n t e el Jur~sico Superior. Un ejemplo de ~reas de recepci6n en biogeografia, Actas de Paleontologfa, IV Jornadas de Paleontologla, Salamanca, 68: 277-287. OLORIZF., LARAL., DE LA MORAA., VILLASENORA.B. & GONZALEZARREOLA C. 1993 - The Kimmeridgian/Tithonian boundary in the "Barranquito del Alacr~n" section at Cuencam~ (Durango, Mexico); its biostratigraphy and ecostratigraphic interpretation. Acta Geologica Polonica, 43 (3-4): 273-288. OL(~RIZ F., VILLASENOR A.B. & GONZALEZ-ARREOLAC. 1999 Hybonoticeras mundulum (OPPEL) from the Mexican Altiplano. A case of biogeography controlling phenotype expression. Lethaia, (in litt.). OPPEL A. 1863 - Uber jurassische Cephalopoden. Palaeontolo-
gische Mittheilungen aus den Museum des Koeniglischen Bayer Staates, Stuttgart: 127-266. OPPEL A. 1865 - Die titonische Etage. Zeitschrift der deutschen geologischen Gesellschaft, 17 (3): 535-558. SARTI C. 1988 - Biostratigraphic subdivision for the Upper Jurassic of the Venetian Alps (Northern Italy) on the base of ammonites. In ROCHA R.B. & SOARES F. (eds), 2nd Symposium on Jurassic Stratigraphy, Lisboa: 459-476.
573 SAPUNOV J.G. 1976 - Ammonite stratigraphy of the Upper Jurassic in Bulgaria. IV Tithonian: Substages, Zones and Subzones. Geologica Balcanica, 7 (2): 43-64 SAPUNOV J.G. 1977 - Ammonite s t r a t i g r a p h y of the Upper J u r a s s i c in Bulgaria. III Kimmeridgian: Substages, Zones and Subzones. Geologica Balcanica, 7 (1): 19-36. SAPUNOV J.G. 1979. Les fossiles de Bulgarie III.3 Jurassique Superieur. Ammonoidea. Acaddmie Bulgare des Sciences, 263 p. SCHWEIGERT G., KRISHNA J., PANDEY B. • PATHAK D.B. 1996 - A new approach to the correlation of the Upper Kimmeridgian Beckeri Zone across the Tethyan Sea. Neues Jahrbuch fi~r Geologie und Pal(tontologie, Abhandlungen 202 (3): 345373 VILLASF,~ORA.B. 1991 - Aportaciones a la bioestratigraffa, basada en f a u n a de ammonites, de la susesion del Jurfisico Superior (Kimmeridgiano-Tithoniano) del ~rea de Mazapil, Zacatecas, M~xico. Tesis, Facultad de Ciencias, Universidad Naeional Aut6moma de M~xico, 156 p. (unpublished). VILLASE~OR A.B., OLORIZ F. & DE LA MORA A. 1995 - Consideraciones sobre el registro de Hybonotieeras (Ammonitina) en M4xico. Memoria de resdmenes del V Congreso Nacional de Paleontologfa, M~xico: 46-47. WmRZBOWSKIA. 1990 - The taxonony and phylogenetic significance of Early Tithonian ammonites of the genus Prota-
ncyloceras SPATH from the Pieniny Klippen Belt (Carpathians, Poland). Atti del Secondo Convengo Internazionale Fossili, Evoluzione, Ambiente, Pergola: 479-489. ZITTEL K.A. 1870 - Die fauna der Aeltern Cephalopoden fuehrender tithonbildungen. Suplement, Pal~ontographica~ 89 p. ZITTEL K.A. 1884 - Handbuch der PaIaeontologie. Abt. 1 (2): 329-522. ZITTEL K.A. 1895 - Grundzi~ge der Palaeontologie (PalaeozooIogie). Munich and Leipzig.
F. OLORIZ Departamento de Estratigrafia y Paleontoiogia Facultad de Ciencias, Universidad de G r a n a d a Av. F u e n t e n u e v a s/n E-18002, G r a n a d a e-mail: [email protected]
A.B. VILLASEI~IOR Departamento de Paleontologia, Instituto de Geologia Universidad Nacional Aut6noma de Mgxico Cd. Universitaria 45010 M~xico, D.F., M~xico e-mail: [email protected]
FEDERICOOLORIZ & ANA BERTHAVILLASEI~OR OL()RIZ F. & VILLASENOR A.B. 1999. New microconchiate Hybonoticeras from Mexico. [Nouveaux Hybonoticeras microconques du Mexique]. GEOBIOS, 32, 4: 561-573. Villeurbanne, le 31.08.1999. Manuscrit d@pos@le 08.12.1997; accept@ d@finitivement le 02.06.1998. ABSTRACT - A diversified and previously unknown record of microconchiate Hybonoticeras is analysed on the basis of the material collected in sections sampled bed-by-bed in the Mexican Altiplano. Five new species are described and compared to species traditionally recorded from the uppermost Kimmeridgian and the lowermost Tithonian in Europe. A general trend to smoothing in Mexican and European species during the earliest Tithonian is outlined. KEY-WORDS: HYBONOTICERAS, AMMONITINA, UPPER KIMMERIDGIAN, LOWER TITHONIAN, MEXICO. RESUMt~ - Une nouvelle faune diversifi~e de microconques d'Hybonoticeras recueillies banc par banc sur !'Altiplano Mexicain est dtudi@e. Cinq nouvelles esp~ces sont d@crites et compar@es avec des esp~ces classiqnes du Kimmeridgien terminal et du Tithonien basal d'Europe. Une tendance g@n@rale h l'effacement de l'ornementation chez les esp~ces mexicaines et europ@ennes durant le Tithonien inf@rieur est raise en 4vidence. MOTS-CLt~S: HYBONOTICERAS, AMMONITINA, KIMMERIDGIEN SUPI~RIEUR, TITHONIEN INFI~RIEUR, MEXIQUE RESUMEN - Se analiza una variada y previamente desconocida fauna de Hybonoticeras microconchas procedentes de perfiles estratigraficamente controlados en el Altiplano Mexicano. Se describen cinco nuevas especies y se comparan con las especies tradicionalmente reconocidas en el Kimmeridgiense terminal y Tithonico basal de Europa. Se reconoce una tendencia al alisamiento de la concha en las especies mexicanas y europeas durante el Tith6nico temprano. PALABRAS-CLAVE: HYBONOTICERAS, AMMONITINA, KIMMERIDGIENSE SUPERIOR, TITHONICO INFERIOR, M]~XICO
INTRODUCTION Microconchiate hybonoticeratids were first reported by Oppel 1865, who described a f r a g m e n t of t h e body ch am be r of Hybonoticeras mundulum from Rogoznik (Poland). With or without illustration, microconchiate Hybonoticeras have been cited from Bulgaria, France, southern Germany, Hungary, Iberia, Italy and Poland, in Europe; Mad ag as car and Morocco, in Africa, and recently Mexico in the Americas (Berckhemer & HOlder 1959; HOlder & Ziegler 1959; Collignon 1960; Christ 1960; Benetti 1975; E n a y & Geyssant 1975; Ftilop 1976; Sapunov 1976, 1977, 1979; Sarti 1988; O15riz & Rivas 1977-78; O15riz 1978, 1981; G e y e r & P e l l e d u h n 1979; K u t e k & W i e r z b o w s k i 1979, 1986; Wierzbowski 1990; B en etti et al. 1990; Villasefior 1991; O16riz et al. 1993, 1999; Caracuel & Olbriz 1994; Villasefior et al. 1995; B en z aggagh 1996; Schweigert et al. 1996).
T he best k n o w n
and w i d e s p r e a d
species is
Hybonoticeras mundulum (OPPEL), which shows the widest stratigraphic range (Beckeridower Hybonotum Zones) and has been considered as the central species encompassing a r a t h e r limited group of forms. Among t hese forms are Hybonoticeras striatulum and H. attenuatum, described by Berckhem er & H~lder 1959, which have been traditionally t r e a t e d as morpho-subspecies of the species created by Oppel 1865.
Hybonoticeras attenuatum BERCKHEMER& HC)LDER was originally recorded in Crussol (France) as Waagenia pressula NEUMAY~by F o n t a n n e s (1879), who considered this species to be r a r e in his "assises sup@rieures"; B e r c k h e m e r & H(ilder 1959 established its age as early Beckeri Chron (= Subeum el a Subzone in Swabia). Sarti 1988 cited this species from the Beckeri/Pressulum Zone in the Venetian Alps, as did O16riz 1978 from the Beckeri Zone in association with H. verestoicum
562 HERBICH in southern Spain. Sapunov (1977, 1979) reported this species from the upper Kimmeridgian (Beckeri Zone) from Bulgaria, but the specimen illustrated (Sapunov 1979, pl. 53, fig. 3) does not belong to this species. Benzaggaggh (1996) cited this species from the lower Tithonian (Hybonotum Zone) in the Rif (Morocco).
Venetian Alps. Two specimens morphologically close to this species were described by O15riz (1978) from the Beckeri Zone in southern Spain, associated with Hybonoticeras pressulum NEUMAYRand H. verestoicum HERBICH. Benzaggagh (1996) cited H. (Hybonotella) mundulum striatellum (OPPEL)from the lower Tithonian (Hybonotum Zone) in the Rif, but the specimen illustrated (Benzaggagh 1996; pl. 2, fig. 6) is not related to the species created by Berckhemer & HSlder.
The holotype ofHybonoticeras striatuIum BERCKttEM~R & HOLD~R was recorded from the UlmensisSchichten (lower Tithonian, Hybonotum Zone) in Swabia. Berckhemer & HSlder (1959) also figured a paratype from the Zementmergel (~2) at Allmendingen. HSlder & Ziegler (1959) illustrated this species from the ~1 (lower Tithonian) near Ehingen, in Franconia. Sarti (1988) cited H. striatulum, together with attenuatum and mundulum pars, from the Beckeri/Pressulum Zone in the
Hybonoticeras nov. sp.(?) "gracillimum', from the Tithonian Hangende Bankalke (~3) in Riedlingen, is insufficiently known at present. The only records alluding to this species and reported outside from epicontinental Europe are those of H. sp. nov. cf. gracillimum (Benetti et al. 1990; pl. 1, fig. 3), from the Hybonotum Zone of the western Lessinian
Phosphoritic limestone ]
I
I
i marly limestone
1
, siltstone
i
r
q
r marly limestone
~with ! [
A Cuencam~ CU (Durango) Mazapil MZ (Zaca~cas)
'i.~
concretions -4~
fo.,
! siltstone '::~, with concretions
X-Zapatecas\
I
Gulf of
\ \ exico
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condensed limestone m=:=::~====
Om
4m
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r
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/
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01" M7~ Concepci;~~/- ~125 Mazap ilii~'" del Oro~ ! "--;,ZC>~ - -C:q""~ ',
~
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rr O u_
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-
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evinidum
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<
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rr
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,
Hybonoticeras
AD H. m u n d u l u m FAD
(M & m) (m)
< . gonzalezi
'1 C -J
/ I
Location and stratigraphical sections with horizons of described species. Localisation des F~CURE 1 -
affleurements et profils stratigraphiques avec les horizons des esp~ces dgcrites.
ga7
,, ,, .
~£ N= D=E
-
geminatum
.
gonzalezi geminatum & nov. sp. aff. striatulum
~, cu-6
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cuencamensis
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563 Following such a treatment, this paper provides new data about five new species of microconchiate hybonoticeratids from Mexico (Fig. 1) on the basis of a new and rare f a u n a which represents less t h a n 2% among 1,322 ammonites and fragments collected in the sections studied. Macroconchiate H y b o n o t i c e r a s are also rare in the material collected, their preservation is defective (crushed fragments) and belong to shells of small to moderate size (< 100ram). Although a conclusive interpretation is unappropiate, fragmented remains of macroconchiate H y b o n o t i c e r a s are better represented by " b e c k e r i - t y p e " forms, some bitubercula-
Alps, and the citation of H. ( H y b o n o t e l l a ) nov. sp. " g r a c i l l i m u m " , from the upper Beckeri-lower Hybonotum Zones of the Rif (Morocco) made by Benzaggagh (1996) without illustration. Recently, new forms have been notified from Mexico (O15riz et al. 1993) and Swabia (Schweigert et al. (1996), without description). O15riz et al. (1999), recorded the existence of Mexican populations of H . r n u n d u l u r n (OPPEL) S. str. (m) and revised this species separating it from its subspecies, as originally interpreted by Berckhemer & HSlder (1959), favouring its interpretation as biogeographic r a t h e r t h a n morphologic subspecies.
20t
15
---4=.... cuencamensis ....¢._, nov. sp. aft. striatulum
•
attenuatum S & H
•
striatulum
S & H
striatulum
H & Z
striatulum
S. et al.
5
0
10
15 diameter
20
25
30
20" 15.
°o o
10-
o <~ 0 ° <~
• • ...... o .. - J l - + " + ' o + + + :.:4t~ +
~,
c + •
._.+
5. u cuencamensis
0
1~0
5
15~
2+0
diameter
o mundulum
*
.L25
attenuatum S & H
30
35
C --4U---- cuencarnefTsis --..e.-. nov, sp. aft. striatulum
attenuatum B & H
Bivariate plots for quoted species of microconchiateHybonoticeras. Measurementsin millimeters. Diagrammes de variation des esp#ces indiqudes d'Hybonoticeras microconques. Dimensions en FIGURE 2 -
striatulum
B & H
striatulum
H & Z
striatuium
S. et al,
1()
15 diameter
20
2'5
30
millim~tres.
Abbreviations as used in the tables of measurements (ram): Diameter (Dm), umbilicus (U), whorl height (H), umbilicus vs. diameter (U/D), whorl height vs. diameter (H/D), and number ofribs per half a whorl (R/2). Abrdviations utilisdes dans les tableaux de mesures (ram): Diam~tre (Din), ombilie (U), hauteur (H), rapport de l'ombilic vs. diam~tre (U/D), rapport de la hauteur vs. diam#tre (H/D), et nombre de cStes sur un demi-tour (R/2).
0.6 ¸
o
0.50.4.
•
c~
•
o o <~ a . . . . . . . . . +' . . . . . . . . . . . . o
t~
o
o o
o
•
~ + + ,o, -....++++ B~ . . . . . . . . . . ~,- + . = . . +
+
__+
.
o - -+,
~o
d+'~
o
o
- i -
¢
._.-.-.-
Or3"
0.2 0.1 0
0
c~ cuencamensis
1'0
1'5
diameter
20
o mundulum
25
• attenuaturn S & H
3'0
35
564 3 - Bivariate plots for quoted species of microconchiate Hybonoticeras. M e a s u r e m e n t s in millimeters. Diagrammes de FIGURE
12
A --..~--.. cuencamensis ....o.-. nov. sp. aft. striatulum
variation des esp~ces indiqudes d ' H y b o n o t i c e r a s microconques. Dimensions en miUim~tres.
s~
4+
.L
attenuatum B & H
•
striatu/um B & H
•
striatulum H & Z
•
striatulum B et al.
2{ 01 5
10
15
20
25
30
diameter
B
.o--~+:::~"++~
•
,ip.-~_,o.. ~ o. ~
~
•
+ o 2 + •
•
4+ o
2+
•cuencamensis
o
mundulurn • attenuatum B & H
Oi 0
0.6 0.5
10
15
25
30
35
•..,¢.- nov. sp. aft. striatulum attenuaturn B & H
o.2t
20
---..~-... cuencamensis
0.4 ~0.3~
diameter
c..................................... ,,i........ L ...........................
striatulum 8 & H .
striatulum H & Z
•
striatulum B et al.
o.lt 0J 5
10
15
20
25
30
diameter
0.~
D
0.~
•
•
0.,I
•
~
0.~
o o" ....................... o
: o a
,
6"'"-~
~
•
o v °
......
+ ~o
...........
~ .....
~
o
o
o
v
o
o
o
•
0.~
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~
o
~ -
o
o
0.1
[] cuencamensis o mundulurn • attenuaturn B & H
0 5
10
te fragments could belong to "hybonotum-type" and/or "extraspinatum-type"forms, and their respective stratigraphic horizons are in accordance with our interpretation of the uppermost Kimmeridgian and lowermost Tithonian in the sections studied (Fig. 1). SYSTEMATIC PALAEONTOLOGY Order AMMONOIDEAZittel, 1884 Family ASPIDOCERATIDAEZittel, 1895, emend Checa, 1985
15 diameter
20
25
30
35
Subfamily HYBONOTICERATINAEO16riz, 1978 Genus Hybonoticeras BREISTROFFER,1947 T y p e - s p e c i e s -Ammonites hybonotus OPPEL,1863
Hybonoticeras cuencamensis
nov. sp. Figs. 2a-d; 3a-d, 4a; 8.1
(m)
H o l o t y p e - IGM 6017, from the CU-6 section at Sierra de Palotes, near Cuencam~ (Durango). Upper Kimmeridgian, upper Beckeri Zone. Derivatio nominis
-
From the town of Cuencam~ (Durango).
M a t e r i a l - One specimen.
565 reinforcements do not determine real periumbilical tuberculations. Very fine striations develop on the remainder of the shell. Nine radial and rather regularly separated ribs, which show enlarged ventral extremes, are typical on the outer flank of the last whorl. Some of these prominent ribs support curved prorsiradiate spines, 9 mm in length; at the pre-peristomal shell the last three ribs are approximated. The peristome is moderately projected into lappets, which are 4 mm long and 2 mm wide. No traces of suture lines are preserved.
D i m e n s i o n s (en ram)
Horizons
Specimens
Dm
U
H
U/D
H/D R/2
CU-6 lm above t00 IGM 6017 26,00 10,10 9,20 0,39 0,35 0 17,20 7,20 7,25 0,42 0,42 2 11,50 4 , 4 0 5,00 0,38 0,43 15
D e s c r i p t i o n - A crushed but well preserved specimen which permits the analysis of the complete ontogeny in detail. The umbilicus is shallow and wide (0,38-0,42), and the coiling seems to increase at 17 mm and then to decrease slowly (Fig. 2a-c). Due to flattening, a reduced umbilical wall is inferred from the locally crescentic umbilical edge. The whorl section is presumably ovate and the relative whorl height i s nearly constant, slightly decreasing from 17 mm onwards according to the data obtained. The sculpture shows a significant change between 8 and 12 mm. Below 8 ram, the inner whorls show simple straight ribs, which characterize the first ornamental phase shared by the majority of micro- and macroconchiate Hybonoticeras; but no tubercles are present. Ribs enlarge to the periphery. Between 8 and 12 mm, extremely fine and slightly sinuous ribs bunch at the umbilical edge, but the resulting
- As usual in microconchiate Hybono~iceras, Hybonoticeras cuencamensis nov. sp. (m) does not show major differences in shell structure in comparison with other species. As shown in figures 2a-d and 3a-d, the ontogenetic course for coiling and the whorl height clearly separates this species from H. m u n d u l u m (OPPEL) S. str. and H. attenuatum Remarks
BERCKHEMER & HOLDER; a d u l t s ef H. striatulum BERCKHEMER & HOLDERand H. gracillimum BERCKHEMER 8~ HOLDER have wider umbilicus (Figs. 2a,c). In contrast, the tightly coiled H. attenuatum BERCKHEMER& HOLDER is closer in the sculptural pattern and relative whorl height. The ontogenetic course of the ornamentation (Fig. 4a) permits easy separation of Hybonoticeras cuencamensis nov. sp. (m)
from H. munduIum. H. attenuatum is a closer species among smoothed microconchiate Hybonoticeras, but smoothing is earA
18 16 14 12
•..4~.... cuer~camensis
c~
.-.~.-. nov. sp. aft. striatulum attenuaturn S & H striatulum B & H
4t
striatulurn H & Z striatulum B et al.
131-5
10
15
diameter
20
25
30
35
B
1E
o
14
o
• •
o
A
'~
...... ...x
o
Q
o
4
2
o mundulum ~ gonza/ei
)c geminaturn
0 5
FIGURE4 - a. Rib curves for quoted species of microconchiate Hybono. ticeras. Dimensions in millimeters. b-c. Bivariate plots for quoted species of microconchiate Hybonoticeras. Measurements in millimeters. a. Courbes de costulation des esp.~ces indiquges dTIybonoticeras m~croconques. Dimensions en millim~tres, b-c. Diagrarnmes de variation des esp~ces indiqudes dTIybonoticeras microconques. Dimensions en millim~tres.
10
15
diameter
20
25
30
35
C 0.6 •
•
0.5 6
0
•
0
O
0.4 0.3 0.2 0.1
o mundulum ~ gonzalei
~ gerninatum
0 5
10
15
diameter
20
25
30
35
566 lier a n d comparatively accentuated in H. cuencamensis nov. sp. t h a n in the older H. attenuatum; moreover, the base of the external tubercles in a t t e n u a t u m is longitudinally elongated instead of radially as in H. cuencamensis nov. sp. Thus, the Mexican H. cuencamensis nov. sp. is separate from the morphologically closest species, the E u r o p e a n attenuaturn, on the basis of phenotypic and stratigraphic data.
Bio-chronostratigraphy and biogeography H. cuencamensis nov. sp. has been recorded below the first record of Pseudodiscosphinctes siliceus
(QUENSTEDT),which was interpreted to indicate the base of the Tithonian at the CU-6 section, near Cuencam~ (Durango), by O15riz et al. (1993). Thus, assuming both the traditional interpretation of the Kimmeridgian-Tithonian boundary (i. e., the Beckeri-Hybonotum Zone boundary) and the absence of Hybonoticeras of the pressulumverestoicum groups, the latter being a general trait in the Mexican Altiplano, H. cuencamensis nov. sp. (m) is interpreted as belonging to the top of the Kimmeridgian, i.e., the upper Beckeri Zone. At present, H. cuencamensis nov. sp. (m) is known from only one section near Cuencam6 (Durango) and, therefore, is considered to be endemic to Mexico. Hybonoticeras gonzalezi nov. sp. (m) Figs. 4b, c; 5a-d; 8.6-9 H o l o t y p e - IGM 6023, from the MZ-3 section at the Cafihn de S a n Matias, S i e r r a de S a n t a Rosa, Zacatecas. U p p e r Kimmeridgian, upper Beckeri Zone. P a r a t y p e s -IGM 6020-6023, from the MZ-3 section at the Cafihn de San Matlas, Sierra de S a n t a Rosa (Zacatecas); IGM 6025 and IGM 6026, from the MZ-1 section at Vereda del Quemado, Sierra de Mazapfl (Zacatecas). Upper Kimmeridgian, upper Beckei Zone. D e r i v a t i o n o m i n i s - This species is dedicated to C. Gonz~lezArreola, ammonitologist of the UNAM (Mexico). M a t e r i a l - Six specimens moderately preserved. D i m e n s i o n s ( e n ram) Horizons Specimens Dm MZ-3.449 top
IGM 6020
MZ-3.449 top
IGM 6021
MZ-3.449 top
IGM 6022
MZ-3.449 top
IGM 6023
MZ-l(22 to 24) IGM 6025 MZ-1(22 to 24) IGM 6026
32,10 27,20 17,80 29,30 26,40 20,50 30,30 21,00 30,30 24,60 20,20 15,70 12,50 28,00 19,50 22,30 18,30 15,70
U 12,70 11,00 6,60 13,00 9,90 7,50 12,60 9,00 12,80 10,80 8,00 6,10 5,00 10,30 6,00 9,00 8,20 5,50
H
U/D
H/D
PJ2
9,50 10,90 7,90 9,00 8,60 7,00 9,50 8,20 10,00 7,40 6,80 6,10 5,00 8,50 8,20 7,30 5,30 6,20
0,40 0,30 14 0,40 0,40 .... 0,37 0,44 .... 0,44 0,31 13 0,38 0,33 14-15 0,37 0,34 14 0,42 0,31 .... 0,43 0,39 .... 0,42 0,33 15 0,44 0,30 14 0,40 0,34 12 0,39 0,39 14 0,40 0,40 13 0,37 0,30 13 0,31 0,42 11-12 0,40 0,33 12-13 0,45 0,29 12-13 0,35 0,39 12
D i a g n o s i s - Microconchiate, spiny Hybonoticeras, with persistent ribbing and lappeted peristome.
D e s c r i p t i o n - The material analysed consists of crushed but complete specimens, 22,3 to 32,1 mm in size. In adults, the umbilicus is wide (0,37 to 0,44), shallow, and limited by a low and more or less sharp umbilical wall. The umbilical edge is gently rounded. Values of coiling slightly increase during ontogeny (Fig. 4b-c). The whorl section is ovate and the whorl height slowly increases during ontogeny (Fig. 5a). The relative whorl height decreases towards greater sizes (Fig. 5b). The sculpture is of ribs which persist to the peristome. No smoothing is observed on the body chamber. From the nucleus to the beginning of the outer whorl, ribs are simple, straight, and slightly enlarged towards the periphery. Bifurcation close to the umbilical edge occurs rarely. An incipient sinuosity of ribs can be observed at the beginning of the outer whorl. The last whorl shows more irregular ribbing, some ribs tend to joint at the umbilical edge while, occasionally, others coalesce to peripheral spines. External tubercles are present in the outer whorl and some connect with moderate and slightly prorsiradiate spines. Prorsiradiate curved ribs are common on the adoral quarter of the body chamber where intercalatories seem, rarely, to induce a zig-zag "Simocosmoceras-like" sculpture. Pre-peristomal ribs show a more or less acute adoral convexity, which corresponds to the base of the lappets. The peristome is elongated into narrow projections without distal expansion, up to 8 mm in length. R e m a r k s - The most striking traits in this species are the persistence of ribbing, without weakening towards the peristome, the occasional presence of zig-zag sculpture n e a r the aperture, and the spinous tubercles at the periphery of the outer whorl. Hybonoticeras mundulum (OPPEL) s. str. (m) shows a similar structure of the shell, more constant and higher values of coiling and the opposite for the relative whorl height (Fig. 4b-c; 5a-b). Moreover, the ribbing style indicates a clear separation on the basis of the more conservative and dense ribbing, which is typical in Oppel'species. Schweigert et al. (1996) reported, without illustration, the existence of a new spiny microconchiate Hybonoticeras in the lower Beckeri Zone in Swabia; therefore, the new species from southern Germany is older. Due to persistent ribbing, Hybonoticeras gonzalezi nov. sp. is more closely related to the new Mexican species geminatum. However, these two species differ, mainly, in ribbing density, the development of geminate-spiny sculpture (Figs. 5c-d) and the structure of lappets. Smoothed microconchiate Hybonoticeras are clearly different in both, the course of the o r n a m e n t a t i o n and, sometimes, the shell structure.
Bio-chronostratigraphy and biogeography Hybonoticeras gonzalezi nov. sp. (m) has been recorded from the upper Beckeri Zone in the region of Mazapfl, Zacatecas (Sierra de Santa Rosa and Sierra de Mazapil). At present, this species is considered to be endemic to Mexico.
567 FIGURE 5 - Bivariate plots for quoted species of microconchiate Hybonoticeras. Measurements in millimeters. Diagrammes de variation des esp~ces indiqu~es d'Hybonoticeras rnicroconques. Dimen-
A
1 1
. +....++~." p
10
o~
x
...............
o'
a
~s ~
sions en millim~tres. /
o
o
o
~
..m~-~..-
' ' ~
o
o
o
0
0 mundulum
5
10
15
20 diameter
A gonzatei
x geminalorn
25
30
35
B 0.61
o.5{
•
o
0.4+
oo
A
o
~r._-z.--~_
x
•
x
A
............... : .......
~. o.3t
•
•
o
o.1{
o mundulum
5
10
15
20
diameter
20I
,
o
z~ 9onzalei
25
x
geminatum
30
35
....x x .....
15
~ .
.
.
.
.
.
x ..........
.
,~- ............... ; .=:.=~~--....~-~..~-~_~ . . . . . . .
2
,,.
x
+
0.2;
OI 0
•
•
a
-
- ~ - gonzalei
5
10
15
20
25
....x....ge;ninatum
30
35
diameter e D
2o1
•
•
e
o
<~
~
e
o
e
. + , ,~. . . . . . . . . . x . . ~
.............
15t e
A
~ &
.~1ot 51 ¢ mundulum
z~ gonzalei
x gemlnatum
01
5
Hybonoticeras gerninatum nov.
sp. (m)
Figs. 4b-c; 5a-d; 8.2-5 Synonymy 1993 Hybonoticeras (Hybonotella) sp.: O15riz et al. p. 280, pl. 2, fig. 6. H o l o t y p e - IGM 6190, from the CU-6 section at Sierra de Palotes, Cuencam~ (Durango). Upper Kirnmeridgian, upper Beckeri Zone. P a r a t y p e s - IGM 6019, IGM 6024a and IGM 6024b, from the MZ-3 section at the CafiSn de San Matlas, Sierra de S a n t a Rosa (Zacatecas). Upper Kimmeridgian, upper Beckeri Zone.
10
15
20
25
30
35
M a t e r i a l - Four specimens preserved as impressions.
D i m e n s i o n s (en r a m ) Horizons Specimens CU-6.122
MZ-3.452a MZ-3.449 mid.
MZ-3.449 mid.
D e r i v a t i o n o m i n i s - In allusion to the presence of geminate ribs.
diameter
Dm
U
30,50
12,40
10,80 0,41
0,35
17
19,60 12,20 IGM 6019 23,00 21,00 IGM 6024a 27,00 22,00 19,00 IGM 6024b 34,00 22,70 17,00
6,80 4,90 i0,00 9,00 12,00 8,00 7,30 14,30 9,30 6,30
7,60 4,90 9,50 6,60 7,20 7,30 7,30 12,50 7,00 7,00
0,39 0,40 0,41 0,31 0,27 0,33 0,38 0,37 0,31 0,41
13 13 16' .... 16 15 16 .... .... ....
IGM 6190
H
U~
0,35 0,40 0,43 0,43 0,44 0,36 0,38 0,42 0,41 0,37
H/D
R/2
568 D i a g n o s i s - Microconchiate Hybonoticeras with variable development of near-geminate to geminate ribs and spines. Lappeted peristome.
Hybonoticeras nov. sp. aff. striatulum
Description - The specimens are complete to nearly complete. The whorl section is ovate, s u b r e c t a n g u l a r at the level of the external tubercles. The umbilicus is wide (0,41 to 0,44 in adults) and moderately shallow due to the existence of a small, gently convex, umbilical wall. The umbilical edge is rounded. Slightly increasing uncoiling characterizes the ontogeny (Fig. 4b-c). The whorl height increases regularly during ontogeny, b u t the relative whorl height decreases very slowly (Fig. 5a-b). On the inner whorls, ribs are simple, more or less straight, and show enlarged to incipiently tuberculate ventral extremes, to which some paired costulae m a y connect (neargeminate structures). This is shown in the holotype as early as at 5 mm in size, and in paratypes IGM 6024a and IGM 6024b between 6 and 9 mm., and 10 mm, respectively (Fig. 8.2-5). In the penult i m a t e whorl, g e m i n a t e ribs with spinous tubercles can be more or less apparent. The same affects the outer whorl, where ribbing density and sinousity are variable. In the holotype, near-geminate structures made by up to three ribs, two of them aproximated near the umbilicus, can unite in the same external tubercle. Pre-peristomal ribs show adoral convexity in accordance with the location of the beginning of lappets. The most complete peristome includes short and broad lappets, 4,5 m m in length and 3 mm in width.
Synonymy aff. 1959 Hybonoticeras mundulum striatulum BERCKHEMER
Remarks - Hybonoticeras geminatum nov. sp. (m) shows coiling values very similar to Hybonoticeras gonzalezi nov. sp. (m) a n d
lower t h a n
those
in Mexican p o l u l a t i o n s
of
Hybonoticeras mundulum (OPPEL) S. str (m) (Fig. 4b-c). The relative whorl h e i g h t is h i g h e r t h a n in Hybonoticeras rnundulure (OPPEL) s. s t r (m), which shares the same t r e n d during ontogeny (Fig. 5a-b). Ribbing density is variable in H. gerninature nov. sp. (m) (Fig. 5c) a n d i n t e r m e d i a t e between the more densely ribbed Mexican populations of H. mundulum (OPPEL) S. str. (m) a n d the less densely ribbed H. gonzalezi nov. sp. (m) (Fig. 5d). Moreover, H. geminatum nov. sp. develops geminate ribbing which is u n k n o w n in the other two species. The persistence of ribbing is a clear difference w h e n compared to species belonging to more or less smoothed microconchiate Hybonoticeras.
Bio-chronostratigraphy and biogeography T h e holotype ofHybonoticeras geminatum nov. sp.
(m) has been recorded j u s t below the first record of Pseudodiscosphinctes siliceus (QuENSTEDT) at the CU-6 section in Sierra de Palotes (Cuencam6, Durango). The paratypes come from the upper Beckeri Zone from the region of Mazapfl, Zacatecas (Sierra de Santa Rosa and Sierra de Mazapfl). Thus, this endemic species characterizes the upper Beckeri Zone in the studied sections of the Mexican Altiplano.
BERCKHEMER • HOLDER (m) Figs. 2a, c; 3a, c; 4a; 6a-d; 7; 8.10-11
aff. aff.
aff. non non
non
8g HOLDER, p. 36, textfig. 21, pl. 5, figs. 20, 21, pl. 6, fig. 31. 1959 Hybonoticeras mundulum striatulum BERCKHEMER gg HOLDER: HSlder & Ziegler, p. 213, pl. 20, figs. 6. 1978 Hybonoticeras (Hybonotella) mundulum striatuIum BERCKHEMER & HOLDER: O15riz, p. 365, pl. 33, fig. 7. 1988 Hybonoticeras striatulum BERCKHEMER & HOLDER: Sarti, p. 468. 1990 Hybonoticeras mundulum striatulum HOELDER: Benetti et al., p. 34, pl. 1, fig. 38. 1991 Hybonoticeras (Hybonotella) rnundulurn (OPPEL) striatulum BERCKHEMER • HOLDER: Villasefior, p. 72, pl. 6, fig. 10. 1996 Hybonoticeras (H.) mundulum aff. striatellum (OPPEL): Benzaggagh, p. 47, fig. 39, pl. 2, fig. 6.
Holotype - IGM 6018, from the MZ-3 section at the CafiSn de San Matias, Sierra de S a n t a Rosa, Zacatecas. Upper Kimmeridgian, upper Beckeri Zone.
Material - One specimen. Dimensions (en mm) Horizons
Specimen
Dm
MZ-3 449 mid.
IGM 6018 26,00 21,50 16,20 14,50
U
H
U/D
H/D R/2
10,50 9,50 5,60 5,20
9,00 7,10 5,70 5,20
0,40 0,45 0,35 0,36
0,35 0,33 0,35 0,36
7 9 13 14
- Small sized Hybonoticeras with fine striations bunching at the umbilical edge on the outer whorl.
Diagnosis
- Shell impression without peristome. The umbilicus is moderately wide (0,36-0,45), shallow, and limited by a small but identifiable umbilical wall, which is emphasized by crushing. The coiling gently increases during ontogeny (Fig. 6a-b). The whorl section is presumably ovate. The relative whorl height decreases up to around 20 mm and then increases slowly (Fig. 6c-d). The sculpture shows the pattern typical in Hybonoticeras striatulum BERCKHEMER & HOLDER, with simple ribs present from the nucleus to a variable extent onto the outer whorl, and then striations which are more or less reinforced near the umbilical edge. In the specimen described, the sculptural phase of simple, straight ribs ends at the beginning of the outer whorl, at around 15 mm in shell size. Among striations, some stronger costulae seem to joint at the periphery, sometimes coinciding with the base of straight "mummy" spines. Increasing smoothing is related to looser and more widely spaced primaries from 17 mm onwards (Fig. 7). Description
Remarks - The umbilicus in IGM 6018 shows values close to the upper range of Hybonoticeras striatulum BERCKHEMER HOLDER, according to meassurements of the specimen illustrated by Berckhemer & HSlder (1959) and HSlder & Ziegler
569
16ii
FIGURE 6 - Bivariate plots for quoted species of microconchiate Hybonoticeras. Measurements in millimeters. Diagrammes de variation des esp~ces indiquges d'Hyboneticeras microeonques. Dimensions en millim~tres.
evanidum
14
--.c-- nov. sp. aff, str/atu/um
12
#l
NlO
"',
-
-* -
attenuatum S & H
..,~..... striatulum S & H • . 4~ - striatulum H & Z •. . ~ . - , gracifl/mum
w"
#
O~ 0
B et aL
wm
~
10
15
20 diameter
25
30
35
graci/I/mum B & H
40
.--+--
evanidurn
0.6
nov, sp. aft. striatulum
0.5
- ~ -
attenuatum
0.4
-.~....
striatulum
0.3
• . , ~ - striatutun'l H & Z
#8
S & H B & H
S e~ al,
. , . , ~ . - . gracJllimum
0.2
#
g,racilfimunl
B & H
0.1 0 5
0
10
1B
20 diameter
25
30
35
40
c
14
--4,--
ev~nidum
-.--o-- nov. sp. aft, striatulum - ~ -
10
~,~:#'~
."
~
.--'~.-. gr~cillimum #
0 40
1
5
10
I
15
S & I-t
-.-,~ - striatulum H & Z
.~,-
• ~-e
attenuatum
2 % a m e t e r 25
30
SetaL
gracitfirnum B & H
35
40
D
0.6 evanidum
0.5.
,--o.-- nov. sp. aft. stfiatulum
0.4
- - " " J " " " = - - # " - + - " - - - - - J~-
1
"
10.3
..................... ,_...~--x.
m
- '~ - attenu~tum B & H
,
...4-..
striatulurn S & H
.. ~ - striatulum H & Z
0.2,
•..-~--, gravillimum
0.1• O-
#
10
(1959), but the coiling does not increase during the final ontogeny (Fig. 6a-b). Whorl height values and the relative whorl height are close to the upper range in Hybonotieeras striatulum BERCKHEMER& HOLDER(Fig. 6c-d). The most striking difference between the German and the Mexican species concerns rib curves, which largely vary between the more densely ribbed holotype and the paratypes illustrated from southern Germany. The rib density in the German species is clearly higher from 17 mm onwards (Fig. 7). Curiously, Hybonoticeras nov. sp. aff. striatulum is closer to the smoothed H. attenuatum BEECKHEMER8~ HOLDER (Fig. 7). However, the Mexican specimen IGM 6018 is older since comes from the uppermost Kimmeridgian (upper Beckeri Zone). Villasefior (1991) reported Hybonoticeras (Hybonotella) mundulum (OPPEL) striatulum BERCKHEMER& HOLDER from the upper Beckeri Zone of the Sierras de Mazapfl and Santa Rosa.
15
20 diameter
25
30
35
B et al.
gracitlirnum B & H
40
This author noted differences in sculpture in comparison with the German species, and recognized the possibility of differentiating her material at the subspecies level (see below). Specimens morphologically close to Hybonoticeras striatulum BERCt~IEMER& HOLDERhave been cited, and/or figured, by O16riz (1978) and Sarti (1988), from the lower Beckeri Zone in association with Hybonoticeras of the group pressulum-verestoicum. At present, there are no illustration of true Hybonoticeras striatulure BERCKHEMER • HOLDER from the lower Tithonian outside southern Germany.
Among smoothed microconchiate Hybonoticeras, Hybonoticeras nov. sp. aff. striatulum BERCKHEMER& HOLDER(lri) seems to be a morphological transient betweem the groups of the European attenuatum and striatulum, being closer to striatulum in ceiling (Fig. 6a-b), the incipient periumbitical reinforcement of costulae, and stratigraphy, while the whorl height and the den-
570 sity of ribs are closer to attenuatum (Figs. 6c-d, 7). Thus, Hybonoticeras nov. sp. aff. striatulum BERCKHEMER& HOLDER (m) is considered as both, the predecessor of Lower Tithonian microconchiate smoothed Hynonoticeras in Mexico, and morphologically related to the group of Hybonoticeras striatulum BERCKHEMER& HOLDER, which is unknown from the southern North American Plate. Bio-chronostratigraphy and biogeography H y b o n o t i c e r a s nov. sp. aff. s t r i a t u l u m BERCKHEMER HOLDER (m) c o m e s f r o m t h e u p p e r m o s t Kimmeridgian, upper Beckeri Zone of the Cafi6n de S a n M a t i a s ( S i e r r a de S a n t a Rosa, Z a c a t e c a s ) .
H y b o n o t i c e r a s e v a n i d u m nov. sp. (m) Figs. 6a-d; 7; 8.12 Synonymy 1991 Hybonoticeras (Hybonotella) mundulum (OPPEL)striatulum BERCKHEMER• HOLDER:Villasefior, p. 72, pl. 6, fig. i0. Holotype - IGM 6027, from the MZ-3 section of the Cafi6n de San Matias, Sierra de Santa Rosa (Zacatecas). Lowermost Tithonian, lower Hybonotum Zone. Paratype Quemado,
- IGM 6028, from the MZ-I section at Vereda del Sierra de Mazapfl (Zacatecas).
evanidus-a-um. In allusion to the extreme fading of the sculpture. D e r i v a t i o n o m i n i s - From Latin
M a t e r i a l - Two specimens partially flattened. D i m e n s i o n s ( e n ram) Horizons Specimen Dm MZ-3 465 mid. IGM 6027 35,90 34,00 26,40 22,60 18,00 MZ-1 26 IGM 6028 26,50 23,00 19,00 17,00
U 15,50 15,00 10,70 9,30
H U/D 11,00 0,43 10,50 0,44 8,70 0,41 7,40 0,41
10,80 9,80 6,90 6,50
8,80 6,90 6,70 6,50
0,41 0,43 0,36 0,38
H/D R/2 0,31 2 0,31 i0 0,33 22-23 0,33 25* 18 0,33 9 0,29 17-18 0,35 15-16 0,38 ~13-14
- S m a l l , e v o l u t e a n d l a p p e t e d Hybonoticeras w i t h e x t r e m e f a d i n g of t h e s c u l p t u r e . Diagnosis
- The analysed specimens are nearly complete, especially the holotype which shows preperistomal structures indicating the location of t h e b e g i n n i n g of a l a p p e t e d p e r i s t o m e . T h e shell size v a r i e s b e t w e e n 26,5 a n d 35,9 m m . I n a d u l t s , Description
t h e u m b i l i c u s is s h a l l o w a n d w i d e (0,41 to 0,44; Fig. 6a). T h e u m b i l i c a l w a l l is s m a l l , distinct, a n d t h e u m b i l i c a l edge r o u n d e d . O n a v e r a g e , coiling v a l u e s i n c r e a s e v e r y s l o w l y d u r i n g o n t o g e n y (Fig. 6b). T h e w h o r l s e c t i o n is o v a t e a n d t h e o n t o g e n e t i c course of the relative whorl height gently d e c r e a s e s (Fig. 6d). T h e s c u l p t u r e is o f m a i n l y simple, s t r a i g h t ribs, w h i c h m o r e or less e n l a r g e t o w a r d s t h e p e r i p h e r y . R a r e b i f u r c a t i o n is p r e s e n t w i t h divisions l o c a t e d s l i g h t l y a b o v e t h e u m b i l i c a l edge. Rib d e n s i t y v a r i e s i n v e r s e l y to rib c o a r s e n e s s (Fig. 7). E x t r e m e f a d i n g of t h e s c u l p t u r e a p p e a r s s u d d e n l y i n t h e a d u l t b o d y c h a m b e r giving, t h u s , a t y p i c a l a p p e a r a n c e to t h i s species. I n t h e h o l o t y p e , s o m e s i n u o s e a n d v e r y s u b t l e u n d u l a t i o n s c a n be r e c o g n i s e d on t h e b o d y c h a m b e r . N e a r t h e p e r i s t o me, a m a r k e d u n d u l a t i o n is i n t e r r u p t e d s l i g h t l y a b o v e t h e m i d - f l a n k , p r o b a b l y i n d i c a t i n g t h e locat i o n of t h e b a s e of l a p p e t s ; t h u s , t w o b u l k y a n d adorally concave branches precede the aperture. N o s u t u r e lines a r e p r e s e r v e d . R e m a r k s - Hybonoticeras
evanidum nov. sp. has a shell structure (coiling and relative whorl height) with no major differences from other microconchiate Hybonoticeras. According to measurements taken from the illustration in Benetti et al. (1990), the shell structure in Hybonoticeras sp. nov. c£ gracillimum HOLDERis especially close at the same diameters (Fig. 6ad), but the specimen from the western Lessinian Alps shows more distant ribs in the inner whorls and coarse spines which connect to large undulations on the flank in the outer whorl. Dimensions of the holotype of Hybonoticeras nov. sp.? ("gracillimum") BERCKHEMER& HOLDER, given by Berckhemer & H61der (1959, p. 37), show that the German specimen has a more evolute shell with whorl height lower than in Hybonoticeras evanidum nov. sp. (Fig. 6a-d). As far as the counting of ribs is possible in the specimen illustrated by Berckhemer & H61der (1959, pl. 5, fig. 22), smoothing in this specimen is weaker, and density of ribs is lower (# in Fig. 7); moreover, fine and longitudinally elongated tubercles are well developed at the periphery. Since other smoothed microconchiate Hybonoticeras have clearly different ribbing curves (Fig. 7), irrespective of wether they are older (group of attenuatum) or more or less contemporaneus (group of striatulum), it seems that the two specimens described from Mexico can be separated from contemporaneous species on the basis of both, the type and ontogenetic course of the ornamentation. According to the above, smoothing in microconchiate Hybonoticeras was an ontogenetic trend shared by populations inhabiting widely separate areas in epicontinental Europe and Mexico, as well as in the epioceanic western Tethys, during the
24t 2O
evanidum
j16
nov, sp. aft. striatulun 112f
FIGURE 7 - Rib curves for quoted species of m i c r o c o n c h i a t e Hybonoticeras. D i m e n s i o n s in m i l l i m e t e r s .
Courbes de costulation des esp~ces indiqades d ' H y b o n o t i c e r a s microconques. Dimensions en millim~tres.
- ~ , - - attenuaturn B & H
....m----striatulum
st
B& H
. . . ~ - striatulum H & Z
4~ #
OI 0
10
15
20 d}ameter
25
30
35
gracillirnum S & H
40
571
1
2
3 4
6
7
9
10
11
FIGURE 8 1. Hybonoticeras cuencamensis nov. sp. (m), IGM 6017, holotype, Upper Kimmeridgian, upper Beckeri Zone, CU-6 section (Sierra de Palotes, Durango). 2-5. Hybonoticeras geminatum nov. sp. (m). 2-3, IGM 6190, holotype, Upper Kimmeridgian, upper Beckeri Zone, CU-6 section (Sierra de Palotes, Durango).4, IGM 6024b, paratype, Upper Kimmeridgian, upper Beckeri Zone, MZ3 section (Sierra de Santa Rosa, Zacatecas). 5, IGM 6024a, from the same horizon and section as figure 4. 6-9. Hybonoticeras gonzalezi nov. sp. (m). 6, IGM 6023, holotype, Upper Kimmeridgian, upper Beckeri Zone, MZ-3 section (Sierra de Santa Rosa, Zacatecas). 7, IGM 6021, paratype, Upper Kimmeridgian, upper Beckeri Zone, MZ-3 section (Sierra de Santa Rosa, Zacatecas). 8, IGM 6020, paratype, from the same horizon and section as figure 7.9, IGM 6025, paratype, Upper Kimmeridgian, upper Beckeri Zone, MZ-1 section (Sierra de Mazapfl, Zacateeas). 10-11. Nybonoticeras nov. sp. aff. striatulum BERCKHEMER& HOLDER (m). IGM 6018, holotype, Upper Kimmerid~an, upper Beckeri Zone, MZ-3 section (Sierra de Santa Rosa, Zacateeas). 12. Hybonoticeras evanidum nov sp. (m). IGM 6027, holotype, lowermost Tithonian, lower Hybonotum Zone, MZ-3 section (Sierra de Santa Rosa, Zaeatecas). All figures at natural size. -
early Tithonian. Thus, in Mexico, as in Europe, extremely smoothed microconchiate Hybonoticeras (evanidum and gracillimun and related forms, respectively) were presumably derived from local ancestors (endemic ?) in which the fading of the sculpture was less accentuated, i.e., Hybonoticeras nov. sp. aff. striatulurn, in Mexico, and Hybonotyceras striatulum in southern Germany.
Bio-chronostratigraphy and biogeography Hybonoticeras evanidum nov. sp. (m) comes from the lowermost Tithonian, lower Hybonotum Zone of the
Cafidn de San Matias, Sierra de Santa Rosa (Zacatecas) and Vereda del Quemado, Sierra de Mazapil (Zacatecas). CONCLUSIONS
A valuable fauna of microconchiate Hybonoticeras is first described from the uppermost Kimmeridgian and the lowermost Tithonian in the Mexican Altiplano.
572
Paying attention to stratigraphy, the evaluation of the morphological diversity provides the basis for the recognition of five new species. Significant differences between these species are found in the patterns of ornamentation rather than in shell structure. Thus, differences in coiling and relative whorl height are considered to be secondary, comparatively. Within impoverished assemblages of ammonites, the taxonomic diversification, interpreted at the species level, is in accordance with the endemic character of the fauna, a feature previously envisaged by the authors (O16riz 1992; O16riz et al. 1990, 1999) The information obtained supports the assumption that general trends in morphologic evolution (e.g., smoothing) were shared by microconchiate Hybonoticeras inhabiting separate areas during the latest Kimmeridgian and the earliest Tithonian. A c k n o w l e d g e m e n t s - This research was supported by the Direcci6n General de Asuntos del Personal Acad~mico (DGAPA) and the Instituto de Geologia, UNAM, Mexico, and the EMMI Group (RNM 0178 Junta de Andalucia) Spain. We are grateful to G. Schweigert (Staatliches Museum ffir Naturkunde, Stuttgart) who provided valuable biostratigraphic information and comments on ammonite types from old German collections, as well as to R. Enay (URA 11 CNRS et Centre des Sciences de la Terre, Universit~ Claude-Bernard, Lyon I) and N. Morton (Department of Geology, Birkbeck College, University of London) for their interesting remarks on the submitted manuscript.
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F. OLORIZ Departamento de Estratigrafia y Paleontoiogia Facultad de Ciencias, Universidad de G r a n a d a Av. F u e n t e n u e v a s/n E-18002, G r a n a d a e-mail: [email protected]
A.B. VILLASEI~IOR Departamento de Paleontologia, Instituto de Geologia Universidad Nacional Aut6noma de Mgxico Cd. Universitaria 45010 M~xico, D.F., M~xico e-mail: [email protected]