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The associations of Brachiopodsfrom the Toarcian-Aalenian transition in the Fuentelsaz section (Iberian Range, Spain)
THE ASSOCIATIONS OF BRACHIOPODS FROM THE TOARCIAN-AAI,ENIAN TRANSITION IN TI-IE FUENTELSAZ SECTION (IBERIAN RANGE, SPAIN)
FERNANDO GARCIA J O R A L & ANTONIO GOY
GARCIA JORAL F. & GOY A. 1994. The associations of Brachiopods from the Toarcian-Aalenian transition in the Fuentelsaz section (Iberian Range, Spain). [Les associations de brachiopodes au passage Toarcien-Aal6nien dans la section de Fuentelsaz (Chaine Ib6rique, Espagne)]. GEOBIOS, M.S. 17 : 223-228.
ABSTRACT The stratigraphic continuity and good preservation of the Brachiopods in the section of Fuentelsaz (Province of Guadalajara, Spain), permit to carry out a detailed study of these fossils in the Toarcian-Aalenian transition. This study puts on evidence an episode of species renewal in the Aalensis Subzone, and another episode of morphological modification at the base of the Buckmani Subzone, the latter being more important from the stratigraphical point of view. In the transition from the Buckmani Subzone to the Opalinum Subzone no significative change is observed in the associations. KEY WORDS : JURASSIC, BIOSTRATIGRAPHY, LIAS-DOGGER TRANSITION, BRACHIOPODS, IBERIAN RANGE (SPAIN).
Rt~SUME La continuit~ stratigraphique et la bonne conservation des brachiopodes recueillis dans la section de Fuentelsaz ont permis d'6tndier dans le d6tail les modifications des associations ~ la transition Toarcien-Aal6nien. On a reconnu dans cette section un 6pisode de renouvellement au cours de la sous-zone ~ Aalensis et un autre 6pisode de modification morphologique au d6but de la sous-zone ~ Buckmani. Ce dernier est envisag6 comme plus important du point de rue biostratigraphique. La limite entre la zone ~ Aalensis et la zone ~ Opalinum n'est marqu6e Fuentelsaz par aucun changement important des associations. MOTS CLI~S : JURASSIQUE, BIOSTRATIGRAPHIE, PASSAGE LIAS-DOGGER, BRACHIOPODES, CHAJNE IBl~RIQUE (ESPAGNE).
INTRODUCTION T h e section o f F u e n t e l s a z is located a b o u t 500 m N o r t h f r o m t h e village of F u e n t e l s a z ( G u a d a l a j a ra), n e a r t h e b o r d e r w i t h t h e p r o v i n c e of Z a r a g o za (Fig. 1). It h a s b e e n described b y C o m a s Rengifo & Goy (1983), G a r c i a - J o r a l (1986) a n d Goy & U r e t a (1987, 1991). T h i s section p r e s e n t s a n
i m p o r t a n t d e v e l o p m e n t a n d a r e m a r k a b l e contin u i t y in t h e m a t e r i a l s belonging to t h e T o a r c i a n A a l e n i a n t r a n s i t i o n . T h e s e m a t e r i a l s consist basically of m a r l i c - c a l c a r e o u s a l t e r n a n c e s , w h i c h yield a b u n d a n t r e m a i n s of s e v e r a l g r o u p s of fossils, g e n e r a l l y w e l l - p r e s e r v e d . T h e s e c h a r a c t e r i s t i c s of c o n t i n u i t y a n d good p r e s e r v a t i o n enable t h e d e t a i l e d s t u d y of t h e c h a n g e s o b s e r v e d in
224
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Figure i - Geographical situation of the Fuentelsaz Section (modified after Goy & Ureta 1991). Situation gdographique de la Coupe de Fuentelsaz.
the Lias-Dogger transition. In this paper some observations in this sense dealing with the fossil brachiopods are presented. This fossil group has been studied by one of us (F. G. J.) in the Doctoral Thesis (1986, unpubl.). Nevertheless, new material has been collected for the development of this paper, enabling to widen certain stratigraphic distributions. The biostratigraphic zonation used is based on Goy & Ureta (1991) for the Ammonites and on Goy et al. (1984), Garcia Joral (1986) and Garcia Joral et al. (1990) for the Brachiopods. This research is included within a wider project which deals with several fossil groups from the Lias-Dogger transition in the Iberian Basin, which represent different forms of life.
SPECIES
DISTRIBUTION
Eleven Brachiopod species have been identified in the Aalensis and Opalinum Zones. Figure 2 represents the detailed stratigraphic distribution of these species in the different levels of the profile. Seven species of Rhynchonellids have been identified, RhynchoneUoidea goyi GARCIA JORAL, 1983, Homoeorhynchia cynocephala (RICHARD, 1840), Praemonticlarella nov. sp. [this species was described formally as Praemonticlarella distercica by one of the authors (Garcia-Joral 1993) same time after this manuscript was sent, on september 1991, to Geobios (see references)], Prionorhynchia r u b r i s a x e n s i s (ROTHPLETZ, 1886), Stolrnorhynchia roblesi GARCIA JORAL, 1986, Parvirhynchia sp. y Globirhynchia subobsoleta (DAVIDSON, 1852). The
Terebratulids are represented in this section only by two species, Stroudithyris stephanoides ALMERAS & MOULAN, 1982 and S. pisolithica (BUCKMAN, 1886). The Zeillerids is the group of Brachiopods with the lowest representation if the number of specimens is considered, there being only two species of a relatively short distribution, Aulacothyris blakei (DAVIDSON, 1878) and Zeilleria (Cincta) anglica (OPPEL, 1858). This distribution of Brachiopod species basically agrees with the distribution observed in other sections of the Iberian Range (cf. Garcia Joral 1986), though the more marlic nature of the sedimentation during this time interval in the section described in this paper probably has some sort of influence in the distribution if compared with other areas in the basin (cf. Garcia Joral et al. 1990). For instance, R. goyi, which appears at the top of Pseudoradiosa Zone, or in the base of the Aalensis Zone all throughout the basin, disappears in Fuentelsaz at the beginning of the Aalensis Subzone, whereas in other sections it persists up to the Buckmani Subzone. Probably related to this is the earlier appearance of H. cynocephala in Fuentelsaz in comparison with other sections from the basin. The latter species substitutes in a conspicuous w a y the former one in the lowermost Aalensis Subzone, coincident with a very clayish episode. In these same levels Praemonticlarella nov. sp. appears, a species which is only found in marlic levels in the Iberian Range. P. rubrisaxensis is a species rarely
225 found in other localities of the Iberian Range and relatively scarce in Fuentelsaz. For this reason its presence does not give much information, though it is important in the correlation with other regions of the Mediterranean realm. The remaining Rhynchonellid species present a shorter distribution at Fuentelsaz compared with the total range recognised in the basin (cf. Garcia Joral 1986). ~]
In the Terebratulids, the evolutive transition from S. stephanoides to S. pisolithica (cf. GARCIA JORAL, 1986), recorded in different sections of the Iberian Range, seems to take place somewhat earlier in Fuentelsaz, where the first specimens that can be described as S. pisolithica are found in the base of the Aalensis Subzone, just in the same levels in which R. goyi is substituted by H.
cynocephala.
Limestones Marls and marly limestones
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F i g u r e 2 - S t r a t i g r a p h i c a l distribution of the Brachiopod species recorded in t h e F u e n telsaz Section. Rdpartition
stratigraphique des esp~ces de brachiopodes enregistrdes dans la Section de Fuentelsaz.
226 The Zeillerids (A. blakei and Z. (C.) anglica) present in Fuentelsaz a similar distribution as observed in other localities from the Iberian Range, and are. not common in this section.
EVOLUTION OF THE BRACHIOPOD ASSOCIATION IN CONNECTION WITH THE BIOSTRATIGRAPHIC LIMITS As often pointed out, the distribution of the Brachiopod species is frequently conditioned by the n a t u r e of the facies, or at least, is influenced by it in a higher degree t h a n in other groups, such as the Ammonites. This fact has been considered a disadvantage for the use of these fossils in biostratigraphy. However, this sensibility to environment permits to relate their distribution with episodes of global or regional environmental changes, which are not necessarily revealed by discontinuities or by the distributions of other groups. A section with the characteristics of sedim e n t a r y continuity as Fuentelsaz can be a good example. In the stratigraphic profile of this locality, several perturbation episodes in the morphology of the species of Brachiopods or in the lineages of these same fossils can be pointed out. As several authors have already mentioned (Garcia Joral & Goy 1984 ; Garcia Joral et al. 1990 ; Alm~ras & Faure 1990), the diversity of the Brachiopods shows an important minimum in Insigne and Pseudoradiosa Zones of the Upper Toarcian. In this way, at the beginning of the Aalensis Zone practically only one genus from each of the three major groups persist : Homoeorhynchia, Stroudithyris and Aulacothyris. In the Mactra Subzone from the Iberian Range and other regions, a remarkable increase in the morphologic variability of the Rhynchonellids and Terebratulids is observed. In the Rhynchonellids this increase is put in evidence by the appearance of a markedly polimorphic species (R. goyi), which probably corresponds to an offshot from the main stock of Homoeorhynchia. In the Terebratulids a similar increase in the morphologic variability is observed, notably in the populations of S. stephanoides, giving place to the morphotypes named by Almeras and Moulan (1982) Monsardithyris catzigrasae, Stroudithyris infraoolithica or S. infraooIithica "transient renfi6". Both species present a series of common characters already pointed out in a previous paper (Garcia Joral & Goy 1984), namedly generalized morphology, large populations and big size. The crowning of this process of increase in the
variability is a renewal episode in which H. cynocephala, P. nov. sp., S. pisolithica and possibly more new species appear. In general, these new species are less generalized and with a more restricted variability, a fact t h a t can be related to a relative stabilization of the environment. From this renewal onwards, a gradual increase in the diversity takes place, reaching its highest value in the Comptum Subzone. In the section of Fuentelsaz, the renewal episode takes place at the moment when layers FZ64 to FZ68 are deposited, and might be used as a biostratigraphic limit. However, it has already been mentioned t h a t the correlation of this event of renewal with the scale of Ammonites does not adjust perfectly, taking place at different subzones depending on the nature of the sediments in the different sections of the basin (cf. Garcia Joral 1986 ; Garcia Joral et al. 1990). Another episode is observed in Fuentelsaz, less obvious, but more useful in this sense. At the moment of the sedimentation of layers FZ86 to FZ88, an event t h a t affects the fauna of Brachiopods takes place. In the Terebratulids, though it means the final disappearance of the S. stephanoides morphotypes, this event is not too evident, and might easily be misinterpreted because of the gradual substitution of S. stephanoides with S. pisolithica. But in the Rhynchonellids this episode is expressed by a remarkable reduction in size in the individuals from layer FZ88 onwards, which affects both H. cynocephala and Praemonticlarella nov. sp. (Fig. 3). The size reduction is almost to a half, and implies other morphological changes (such as the number of ribs) as described in Laurin & Garcia Joral (1990) for H. cynocephala. In the paper referred to, this phenomenon of miniaturization has been related to developmental heterochronies that might permit a reduction in the generation spans, perhaps related to an "r" strategy due to adverse conditions of the environment. Agreeing with this, the miniaturization of the shells m a y be related to an external process of regional or global nature, and therefore fit for correlations. On the other hand, no important modification in the associations of Fuentelsaz at the Toarcian Aalenian limit is observed. Only one species appears for the first time at this limit, P. rubrisaxensis, a species which, as already mentioned, is present only in a sporadic way in the Iberian Range. Its stratigraphic distribution in the basin remains difficult at this moment to define exactly.
227 0 II
5 I
lO mm 'i
Figure 3 - Individual variation in size, referred to t h e length (L), in Homoeorhynchia eynocephala (dots) and in Praemontielarella nov. sp. (triangles) in the Fuentelsaz Section. Variations en taille, reprgsentge par la longeur (L), des exemplaires de
25
24
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Homoeorhynchia cynocephala (points) et de Praemonticlarella nov. sp. (trian-
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CONCLUSIONS The detailed study of the Brachiopod associations recorded at the Toarcian - Aalenian transition in the area of Fuentelsaz, permits to make the following observations : - the Rhynchonellids are the Brachiopods more frequently represented in the different materials from the Aalensis and Opalinum Zones, both in t h e number of species as in the number of specimens, showing the highest value of diversity at the Comptum Subzone (H. cynocephala, Praemonticlarella nov. sp., P. rubrisaxensis, Parv i r h y n c h i a sp., S. roblesi, G. subobsoleta). On the contrary, these species have not been recorded in the Opalinum Subzone, - the Terebratulids are relatively frequent in the Mactra Subzone. L a t e r on, only one species persists, S. pisolithica, generally with a poor but continuous record, - the Zeillerids are found in a small proportion and in a discontinuous way, A. blakei recorded in
the Mactra Subzone and Z. (C.) anglica in the Comptum Subzone. The analysis of the succession of associations, together with the study of the characteristics of the populations of the different species lead to the following conclusions. The most important renewal in the Brachiopods takes place in the Aalensis Subzone. Nevertheless, the most important change from the biostratigraphic point of view occurs problably in the transition from the Aalensis Subzone to the Buckmani Subzone. This can be inferred from the important morphological modifications t h a t the populations of Rhynchonellids undergo, shown by an important change in the size of the adult specimens. The Terebratulids also present modifications, although these are more difficult to appreciate. In the transition from the Buckmani to the Opalinum Subzone in Fuentelsaz no important chan-
228 a s s o c i a t i o n s a r e d e t e c t e d . P. rubrisaxensis is r e c o r d e d for t h e f i r s t t i m e i n t h e final part of the Buckmani Subzone, this being t h e o n l y s p e c i e s r e l a t e d to t h i s t r a n s i t i o n . O n t h e other hand, no important modifications are detect e d i n S. pisolithica, a s p e c i e s w h i c h is p r e s e n t i n both subzones. ges
in
the
REFERENCES ALMI~RAS Y. & FAURE P. 1990 - H i s t o i r e des Brachiopod e s liasiques d a n s la T h e t h y s Occidentale : Les crises et l'~cologie. Cahiers de l'Universitg Catholique de Lyon (sdrie science), 4 : 1-12. ALM]~RAS Y. & MOULAN G. 1982 - Les t~r~bratulid~s l i a s i q u e s de Provence (Pal~ontologie, b i o s t r a t i g r a phie, pal~o~cologie, phylog~nie). Documents du Laboratoire de Geologie de Lyon, 86 : 365 p. COMAS RENGIFO M.J. & GoY A. 1983 - Macropaleontologia. In PORTERO J.M., GUTIERREZ M., AGUmAR M.J., LEAL M.C. & GINER J. : Explicacidn de la Hoja n ° 464. M a p a GeolSgico de Espafia. Escala I : 50.000. I.G.M.E. : 74 p. GARCIA JORAL F. 1986 - Los BraquiSpodos del Aalen i e n s e de la C o r d i l l e r a Ib~rica. (Unpubl. Doctoral Thesis). U n i v e r s i d a d Complutense de M a d r i d : 326 p. GARCIA-JORAL F. 1993 - Praemonticlarella distercica nov. gen. nov. sp. (Brachiopoda, Rhynchonellida) del Toarciense s u p e r i o r y el A a l e n i e n s e inferior de la Cordillera Ib~rica (NE de Espafia). Boletin de la
Real Sociedad Espa~ola de Historia Natural (Secci6n GeolSgica), 88, 1-4 : 73-83. GARCIA JORAL F. & GOY A. 1984 - Caracter~sticas de la f a u n a de BraquiSpodos del Toarciense superior en
el Sector Central de la Cordillera Ib~rica (Noreste de Espafia). Estudios geoldgieos, 40 : 55-59. GARCL~ JORAL F., GOY A. & URETA S. 1990 - Las sucesiones de BraquiSpodos en el tr~nsito Lfas-Dogger en la Cordillera Ib~rica. Cuadernos de Geologfa Ibdrica, 14 : 55-65. GoY A. COMAS RENGIFO M.J. & GARCIA JORAL F. 1984 The Liassic Brachiopods of the I b e r i a n Range (Spain) : s t r a t i g r a p h i c d i s t r i b u t i o n a n d biozonation. In MICHELSEN O. • ZEISS A. (edit.) : International
Symposium on Jurassic Stratigraphy ; Erlangen (Germany). Symposium volume. Geological Survey of Denmark. Copenhagen, 1 : 227-250. GoY A. & URETA S. 1987 - Leioceratinae ( A m m o n i t i n a ) de] A a l e n i e n s e inferior de F u e n t e l s a z (Cordillera Ib~rica, Espafia). Bolletino de la Societd Paleontologica Italiana, 25 : 213-236. GoY A. & URETA S. 1991 - The Lower B o u n d a r y of the A a l e n i a n in the F u e n t e l s a z Section ( I b e r i a n Range, Spain). In MORTON N. (edit.) : Conference on Aale-
nian - Bajocian Stratigraphy. Isle of Skye (Scotland). Birkbeck College, U n i v e r s i t y of London : 33-47. LAURIN B. & GARC~AJORAL F. 1990 - M i n i a t u r i z a t i o n and heterochrony in Homoeorhynchia meridionalis and H. cynocephala (Brachiopoda, Rhynchonellidae) from the J u r a s s i c of the I b e r i a n Range, Spain. Paleobiology, 16, (1) : 62-76.
F. GARCIA J O R A L & A. GOY
Departamento de Paleontologia Universidad Complutense de Madrid and Instituto de Geologia Econdmica C.S.I.C. Ciudad Universitaria 28040 Madrid, Spain
FERNANDO GARCIA J O R A L & ANTONIO GOY
GARCIA JORAL F. & GOY A. 1994. The associations of Brachiopods from the Toarcian-Aalenian transition in the Fuentelsaz section (Iberian Range, Spain). [Les associations de brachiopodes au passage Toarcien-Aal6nien dans la section de Fuentelsaz (Chaine Ib6rique, Espagne)]. GEOBIOS, M.S. 17 : 223-228.
ABSTRACT The stratigraphic continuity and good preservation of the Brachiopods in the section of Fuentelsaz (Province of Guadalajara, Spain), permit to carry out a detailed study of these fossils in the Toarcian-Aalenian transition. This study puts on evidence an episode of species renewal in the Aalensis Subzone, and another episode of morphological modification at the base of the Buckmani Subzone, the latter being more important from the stratigraphical point of view. In the transition from the Buckmani Subzone to the Opalinum Subzone no significative change is observed in the associations. KEY WORDS : JURASSIC, BIOSTRATIGRAPHY, LIAS-DOGGER TRANSITION, BRACHIOPODS, IBERIAN RANGE (SPAIN).
Rt~SUME La continuit~ stratigraphique et la bonne conservation des brachiopodes recueillis dans la section de Fuentelsaz ont permis d'6tndier dans le d6tail les modifications des associations ~ la transition Toarcien-Aal6nien. On a reconnu dans cette section un 6pisode de renouvellement au cours de la sous-zone ~ Aalensis et un autre 6pisode de modification morphologique au d6but de la sous-zone ~ Buckmani. Ce dernier est envisag6 comme plus important du point de rue biostratigraphique. La limite entre la zone ~ Aalensis et la zone ~ Opalinum n'est marqu6e Fuentelsaz par aucun changement important des associations. MOTS CLI~S : JURASSIQUE, BIOSTRATIGRAPHIE, PASSAGE LIAS-DOGGER, BRACHIOPODES, CHAJNE IBl~RIQUE (ESPAGNE).
INTRODUCTION T h e section o f F u e n t e l s a z is located a b o u t 500 m N o r t h f r o m t h e village of F u e n t e l s a z ( G u a d a l a j a ra), n e a r t h e b o r d e r w i t h t h e p r o v i n c e of Z a r a g o za (Fig. 1). It h a s b e e n described b y C o m a s Rengifo & Goy (1983), G a r c i a - J o r a l (1986) a n d Goy & U r e t a (1987, 1991). T h i s section p r e s e n t s a n
i m p o r t a n t d e v e l o p m e n t a n d a r e m a r k a b l e contin u i t y in t h e m a t e r i a l s belonging to t h e T o a r c i a n A a l e n i a n t r a n s i t i o n . T h e s e m a t e r i a l s consist basically of m a r l i c - c a l c a r e o u s a l t e r n a n c e s , w h i c h yield a b u n d a n t r e m a i n s of s e v e r a l g r o u p s of fossils, g e n e r a l l y w e l l - p r e s e r v e d . T h e s e c h a r a c t e r i s t i c s of c o n t i n u i t y a n d good p r e s e r v a t i o n enable t h e d e t a i l e d s t u d y of t h e c h a n g e s o b s e r v e d in
224
~~
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/
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\.,2zo
mw
•
lIF9., '~'/2-2~,_I//-,,,
I)
I
,,' ."
~cl :
.69 •
,A2
L
Figure i - Geographical situation of the Fuentelsaz Section (modified after Goy & Ureta 1991). Situation gdographique de la Coupe de Fuentelsaz.
the Lias-Dogger transition. In this paper some observations in this sense dealing with the fossil brachiopods are presented. This fossil group has been studied by one of us (F. G. J.) in the Doctoral Thesis (1986, unpubl.). Nevertheless, new material has been collected for the development of this paper, enabling to widen certain stratigraphic distributions. The biostratigraphic zonation used is based on Goy & Ureta (1991) for the Ammonites and on Goy et al. (1984), Garcia Joral (1986) and Garcia Joral et al. (1990) for the Brachiopods. This research is included within a wider project which deals with several fossil groups from the Lias-Dogger transition in the Iberian Basin, which represent different forms of life.
SPECIES
DISTRIBUTION
Eleven Brachiopod species have been identified in the Aalensis and Opalinum Zones. Figure 2 represents the detailed stratigraphic distribution of these species in the different levels of the profile. Seven species of Rhynchonellids have been identified, RhynchoneUoidea goyi GARCIA JORAL, 1983, Homoeorhynchia cynocephala (RICHARD, 1840), Praemonticlarella nov. sp. [this species was described formally as Praemonticlarella distercica by one of the authors (Garcia-Joral 1993) same time after this manuscript was sent, on september 1991, to Geobios (see references)], Prionorhynchia r u b r i s a x e n s i s (ROTHPLETZ, 1886), Stolrnorhynchia roblesi GARCIA JORAL, 1986, Parvirhynchia sp. y Globirhynchia subobsoleta (DAVIDSON, 1852). The
Terebratulids are represented in this section only by two species, Stroudithyris stephanoides ALMERAS & MOULAN, 1982 and S. pisolithica (BUCKMAN, 1886). The Zeillerids is the group of Brachiopods with the lowest representation if the number of specimens is considered, there being only two species of a relatively short distribution, Aulacothyris blakei (DAVIDSON, 1878) and Zeilleria (Cincta) anglica (OPPEL, 1858). This distribution of Brachiopod species basically agrees with the distribution observed in other sections of the Iberian Range (cf. Garcia Joral 1986), though the more marlic nature of the sedimentation during this time interval in the section described in this paper probably has some sort of influence in the distribution if compared with other areas in the basin (cf. Garcia Joral et al. 1990). For instance, R. goyi, which appears at the top of Pseudoradiosa Zone, or in the base of the Aalensis Zone all throughout the basin, disappears in Fuentelsaz at the beginning of the Aalensis Subzone, whereas in other sections it persists up to the Buckmani Subzone. Probably related to this is the earlier appearance of H. cynocephala in Fuentelsaz in comparison with other sections from the basin. The latter species substitutes in a conspicuous w a y the former one in the lowermost Aalensis Subzone, coincident with a very clayish episode. In these same levels Praemonticlarella nov. sp. appears, a species which is only found in marlic levels in the Iberian Range. P. rubrisaxensis is a species rarely
225 found in other localities of the Iberian Range and relatively scarce in Fuentelsaz. For this reason its presence does not give much information, though it is important in the correlation with other regions of the Mediterranean realm. The remaining Rhynchonellid species present a shorter distribution at Fuentelsaz compared with the total range recognised in the basin (cf. Garcia Joral 1986). ~]
In the Terebratulids, the evolutive transition from S. stephanoides to S. pisolithica (cf. GARCIA JORAL, 1986), recorded in different sections of the Iberian Range, seems to take place somewhat earlier in Fuentelsaz, where the first specimens that can be described as S. pisolithica are found in the base of the Aalensis Subzone, just in the same levels in which R. goyi is substituted by H.
cynocephala.
Limestones Marls and marly limestones
• • [3
2
Rare Common Very common
.--
O
•- r O u
e-
O
0
I1
3O
I( o
!I ! !
24
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F i g u r e 2 - S t r a t i g r a p h i c a l distribution of the Brachiopod species recorded in t h e F u e n telsaz Section. Rdpartition
stratigraphique des esp~ces de brachiopodes enregistrdes dans la Section de Fuentelsaz.
226 The Zeillerids (A. blakei and Z. (C.) anglica) present in Fuentelsaz a similar distribution as observed in other localities from the Iberian Range, and are. not common in this section.
EVOLUTION OF THE BRACHIOPOD ASSOCIATION IN CONNECTION WITH THE BIOSTRATIGRAPHIC LIMITS As often pointed out, the distribution of the Brachiopod species is frequently conditioned by the n a t u r e of the facies, or at least, is influenced by it in a higher degree t h a n in other groups, such as the Ammonites. This fact has been considered a disadvantage for the use of these fossils in biostratigraphy. However, this sensibility to environment permits to relate their distribution with episodes of global or regional environmental changes, which are not necessarily revealed by discontinuities or by the distributions of other groups. A section with the characteristics of sedim e n t a r y continuity as Fuentelsaz can be a good example. In the stratigraphic profile of this locality, several perturbation episodes in the morphology of the species of Brachiopods or in the lineages of these same fossils can be pointed out. As several authors have already mentioned (Garcia Joral & Goy 1984 ; Garcia Joral et al. 1990 ; Alm~ras & Faure 1990), the diversity of the Brachiopods shows an important minimum in Insigne and Pseudoradiosa Zones of the Upper Toarcian. In this way, at the beginning of the Aalensis Zone practically only one genus from each of the three major groups persist : Homoeorhynchia, Stroudithyris and Aulacothyris. In the Mactra Subzone from the Iberian Range and other regions, a remarkable increase in the morphologic variability of the Rhynchonellids and Terebratulids is observed. In the Rhynchonellids this increase is put in evidence by the appearance of a markedly polimorphic species (R. goyi), which probably corresponds to an offshot from the main stock of Homoeorhynchia. In the Terebratulids a similar increase in the morphologic variability is observed, notably in the populations of S. stephanoides, giving place to the morphotypes named by Almeras and Moulan (1982) Monsardithyris catzigrasae, Stroudithyris infraoolithica or S. infraooIithica "transient renfi6". Both species present a series of common characters already pointed out in a previous paper (Garcia Joral & Goy 1984), namedly generalized morphology, large populations and big size. The crowning of this process of increase in the
variability is a renewal episode in which H. cynocephala, P. nov. sp., S. pisolithica and possibly more new species appear. In general, these new species are less generalized and with a more restricted variability, a fact t h a t can be related to a relative stabilization of the environment. From this renewal onwards, a gradual increase in the diversity takes place, reaching its highest value in the Comptum Subzone. In the section of Fuentelsaz, the renewal episode takes place at the moment when layers FZ64 to FZ68 are deposited, and might be used as a biostratigraphic limit. However, it has already been mentioned t h a t the correlation of this event of renewal with the scale of Ammonites does not adjust perfectly, taking place at different subzones depending on the nature of the sediments in the different sections of the basin (cf. Garcia Joral 1986 ; Garcia Joral et al. 1990). Another episode is observed in Fuentelsaz, less obvious, but more useful in this sense. At the moment of the sedimentation of layers FZ86 to FZ88, an event t h a t affects the fauna of Brachiopods takes place. In the Terebratulids, though it means the final disappearance of the S. stephanoides morphotypes, this event is not too evident, and might easily be misinterpreted because of the gradual substitution of S. stephanoides with S. pisolithica. But in the Rhynchonellids this episode is expressed by a remarkable reduction in size in the individuals from layer FZ88 onwards, which affects both H. cynocephala and Praemonticlarella nov. sp. (Fig. 3). The size reduction is almost to a half, and implies other morphological changes (such as the number of ribs) as described in Laurin & Garcia Joral (1990) for H. cynocephala. In the paper referred to, this phenomenon of miniaturization has been related to developmental heterochronies that might permit a reduction in the generation spans, perhaps related to an "r" strategy due to adverse conditions of the environment. Agreeing with this, the miniaturization of the shells m a y be related to an external process of regional or global nature, and therefore fit for correlations. On the other hand, no important modification in the associations of Fuentelsaz at the Toarcian Aalenian limit is observed. Only one species appears for the first time at this limit, P. rubrisaxensis, a species which, as already mentioned, is present only in a sporadic way in the Iberian Range. Its stratigraphic distribution in the basin remains difficult at this moment to define exactly.
227 0 II
5 I
lO mm 'i
Figure 3 - Individual variation in size, referred to t h e length (L), in Homoeorhynchia eynocephala (dots) and in Praemontielarella nov. sp. (triangles) in the Fuentelsaz Section. Variations en taille, reprgsentge par la longeur (L), des exemplaires de
25
24
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Homoeorhynchia cynocephala (points) et de Praemonticlarella nov. sp. (trian-
PO
gles) dans la coupe de Fuentelsaz.
ill
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iO
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."
AALE
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;
CONCLUSIONS The detailed study of the Brachiopod associations recorded at the Toarcian - Aalenian transition in the area of Fuentelsaz, permits to make the following observations : - the Rhynchonellids are the Brachiopods more frequently represented in the different materials from the Aalensis and Opalinum Zones, both in t h e number of species as in the number of specimens, showing the highest value of diversity at the Comptum Subzone (H. cynocephala, Praemonticlarella nov. sp., P. rubrisaxensis, Parv i r h y n c h i a sp., S. roblesi, G. subobsoleta). On the contrary, these species have not been recorded in the Opalinum Subzone, - the Terebratulids are relatively frequent in the Mactra Subzone. L a t e r on, only one species persists, S. pisolithica, generally with a poor but continuous record, - the Zeillerids are found in a small proportion and in a discontinuous way, A. blakei recorded in
the Mactra Subzone and Z. (C.) anglica in the Comptum Subzone. The analysis of the succession of associations, together with the study of the characteristics of the populations of the different species lead to the following conclusions. The most important renewal in the Brachiopods takes place in the Aalensis Subzone. Nevertheless, the most important change from the biostratigraphic point of view occurs problably in the transition from the Aalensis Subzone to the Buckmani Subzone. This can be inferred from the important morphological modifications t h a t the populations of Rhynchonellids undergo, shown by an important change in the size of the adult specimens. The Terebratulids also present modifications, although these are more difficult to appreciate. In the transition from the Buckmani to the Opalinum Subzone in Fuentelsaz no important chan-
228 a s s o c i a t i o n s a r e d e t e c t e d . P. rubrisaxensis is r e c o r d e d for t h e f i r s t t i m e i n t h e final part of the Buckmani Subzone, this being t h e o n l y s p e c i e s r e l a t e d to t h i s t r a n s i t i o n . O n t h e other hand, no important modifications are detect e d i n S. pisolithica, a s p e c i e s w h i c h is p r e s e n t i n both subzones. ges
in
the
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el Sector Central de la Cordillera Ib~rica (Noreste de Espafia). Estudios geoldgieos, 40 : 55-59. GARCL~ JORAL F., GOY A. & URETA S. 1990 - Las sucesiones de BraquiSpodos en el tr~nsito Lfas-Dogger en la Cordillera Ib~rica. Cuadernos de Geologfa Ibdrica, 14 : 55-65. GoY A. COMAS RENGIFO M.J. & GARCIA JORAL F. 1984 The Liassic Brachiopods of the I b e r i a n Range (Spain) : s t r a t i g r a p h i c d i s t r i b u t i o n a n d biozonation. In MICHELSEN O. • ZEISS A. (edit.) : International
Symposium on Jurassic Stratigraphy ; Erlangen (Germany). Symposium volume. Geological Survey of Denmark. Copenhagen, 1 : 227-250. GoY A. & URETA S. 1987 - Leioceratinae ( A m m o n i t i n a ) de] A a l e n i e n s e inferior de F u e n t e l s a z (Cordillera Ib~rica, Espafia). Bolletino de la Societd Paleontologica Italiana, 25 : 213-236. GoY A. & URETA S. 1991 - The Lower B o u n d a r y of the A a l e n i a n in the F u e n t e l s a z Section ( I b e r i a n Range, Spain). In MORTON N. (edit.) : Conference on Aale-
nian - Bajocian Stratigraphy. Isle of Skye (Scotland). Birkbeck College, U n i v e r s i t y of London : 33-47. LAURIN B. & GARC~AJORAL F. 1990 - M i n i a t u r i z a t i o n and heterochrony in Homoeorhynchia meridionalis and H. cynocephala (Brachiopoda, Rhynchonellidae) from the J u r a s s i c of the I b e r i a n Range, Spain. Paleobiology, 16, (1) : 62-76.
F. GARCIA J O R A L & A. GOY
Departamento de Paleontologia Universidad Complutense de Madrid and Instituto de Geologia Econdmica C.S.I.C. Ciudad Universitaria 28040 Madrid, Spain