Distribution of Architectonicidae (Heterobranchia, Gastropoda) of the Western Mediterranean Pliocene: Ecological and historical considerations

PALAEO ELSEVIER

Palaeogeography, Palaeoclimatology, Palaeoecology 126 (1996) 281 290

Distribution of Architectonicidae (Heterobranchia, Gastropoda) of the Western Mediterranean Pliocene: ecological and historical considerations Marta Solsona, Jordi Martinell 1 , , Departament de Geologia Dindmica, Geofisiea i Paleontologia, Facultat de Geologia, Universitat de Barcelona,

08071 Barcelona, Spain Received 9 November 1995; accepted 26 April 1996

Abstract

The palaeobiogeography of Pliocene Architectonicidae from the Western Mediterranean and adjacent Atlantic areas is analyzed. Quantitative analyses (calculation of faunal affinity indices and clustering) are presented. The distribution shows a heterogeneous pattern: three species (A. simplex, A. monilifera and A. obtusa) extend over the entire area, the others show a more restricted distribution. The interpretation of the causal factors of this distribution is based on ecological aspects (related to available outcrop data and to the biology of these species) and on historical aspects. Depth and type of sediment are major controls in this distribution. Commensalism with coelenterates could also be a determinating factor.

Keywords: Mollusca; Gastropoda; Architectonicidae; Pliocene; Mediterranean; palaeoecology; palaeobiogeography

1. Introduction

The Architectonicidae are a small and welldefined marine gastropod family. Their shell shape ranges from trochoidal to discoidal with a basalcentred umbilicus, but occasionally it is planispiral with disjunct whorls. They are characterized by a hyperstrophic protoconch with the apex projected into the teleoconch umbilicus. These protoconchs are sinistral, though their larval soft parts have a dextral organization like the adults (Bieler, 1993). Architectonicidae stratigraphic distribution ranges from the Late Cretaceous (Co ssman, 1915; * Corresponding author. 1 FAX (3) 402 13 40. E-mail [email protected] 0031-0182/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved PII S0031-0182 (96) 00044-2

Wenz, 1938) to the present. At present approximately 140 species of this family are known throughout all the tropical and temperate seas, though mainly in tropical seas (Bieler, 1993). The Mediterranean living species are found in the infra and circalittoral zones, rarely at greater depths (Melone and Taviani, 1984). Little is known about the biology of this family. The present species of the Architectonicidae undergo a remarkable larval development. They have planktotrophic larvae which are able to live in plankton for long periods, from several weeks to 7 months or even longer (Robertson, 1967), allowing the larvae to be displaced over large distances by currents. This explains the wide geographical distribution of some species of this

282

M. Solsona, J. Martinell/Palaeogeography, Palaeoclimatology, Palaeoecology 126 (1996) 281~90

family. For instance, of the eleven species found in the Mediterranean, six are amphiatlantic (Garcia-Talavera, 1982; Fernandes and Rol~in, 1994). The present species of this family are carnivorous. It is important to notice that some species of this family are commensalists with coelenterates (Robertson, 1967, 1973): some species of genus Heliacus are commensalists with Zoanthus and Palythoa, colonial sea anemones of the Zoanthidae family; species of Philippia (Psilaxis) have a commensalist relationship with scleractinians, and species of Philippia (Philippia) with actiniarians. Robertson (1973) speculated that the commensalism with coelenterates could be a characteristic of this family, even though the feeding habits of some species are still not known. The aim of this article is, firstly, to describe the palaeogeographical distribution of the Pliocene Architectonicidae in the Western Mediterranean and, secondly, to interpret the factors that caused this distribution.

2. Material and methods

The area we studied the palaeogeographical distribution of these species is shown in Fig. 1. It includes the main Western Mediterranean Pliocene basins and some adjacent Atlantic basins. There are eleven known species and one hitherto unidentified form in the Pliocene basins of this area. A systematic revision of these species was carried out using our own collections kept in the Paleontological Laboratory at the University of Barcelona. These species come mainly from the NW Mediterranean Pliocene basins (Catalan, South of France and Liguria basins), and from the Southern Spanish basins. The following nine species were identified: (1) Architectonica simplex (Bronn, 1831) (2) Architectonica monilifera (Bronn, 1831) (3) Architectonica semisquamosa (Bronn, 1831 ) (4) Architectonica contexta (Seguenza, 1902) (5) Architeetonica pseudoperspectiva (Brocchi, 1814) (6) Architectonica obtusa (Bronn, 1831) (7) Architeetoniea planulata (Grateloup, 1832)

(8) Arehitectoniea millegrana (Lamarck, 1822) (9) Architectoniea emiliae (Semper, 1861) There are two other species in the Western Mediterranean Pliocene of which we only have bibliographic information: Architectonica formosa (Cristofori and Jan, 1832) Pseudomalaxis aldovandrii ( Foresti, 1868) Recent revisions of genera of living Architectonicidae (Melone and Taviani, 1984; Bieler, 1993) are based principally on relative position of spiral ribs of the teleoconch, and on characters of radula and operculum which in most cases are not preserved in the fossil record. Because not all criteria used to define Architectonicidae genera are applicable, we decided to use the genus Architectonica s.1. The use of a single large genus instead of various genera has been applied to other families in the fossil record, for instance Nassaridae (Adam et Glibert, 1974; Gili, 1991). In this case, the same criteria was also applied later to living forms (Cernohorsky, 1984). The work was carried out in two stages. First, qualitative and quantitative analyses were carried out. Our own data on the presence or absence of each species in each basin was complemented with extensive reference to the available literature. Using this data, a map of distribution and a presence/absence table was compiled; then, an affinity matrix and a cluster was calculated to evaluate the possible existence of patterns in this distribution. The second stage involved the interpretation of the ecological and historical factors that caused this distribution. To avoid mistakes, data from the different zones needed to be as homogeneous as possible. Much of the location data in the literature are biased in favour of the Italian basins, where many studies have been carried out, for more than two centuries. There is, on the other hand, a substantial lack of information concerning the North African basins. Because of this, only those basins we studied directly were taken into account when interpreting the results. In these basins, both homogeneous sampling and taphonomical studies were carried out. Only data coming from outcrops where fossil assemblages correspond to a palaeo-

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Fig. 1. Palaeogeographical distribution of Pliocene Architectonicidae of the Western Mediterranean and adjacent Atlantic. Basins: M = Mfilaga, B L = Baix Llobregat, E = Empord/~, R = Roussillon, R H = Rhrne, A M = Alpes-Maritimes, L I - T I = Liguria and Tirreno, S I = Sicilia, N I = Northern Italy, T U = Tunisia, A = Algeria, E M = Mediterranean Morocco, W M = Atlantic Morocco, H = Huelva, P = Portugal Species: s = A. simplex, m = A. monilifera, o = A. obtusa, se = A. semisquamosa, p = A. pseudoperpectiva, m i = A. millegrana, e = A. emiliae, p l = A. planulata, c = A. contexta, f = A. formosa, a = P. aldovandrff. Palaeogeography modified from Pomerol (1973).

community were useful, and those outcrops showing important taphonomical processes were rejected.

3. Results

Fig. 1 and Table 1 show the palaeogeographical distribution of the studied species during the Pliocene. The following groups of species based on geographical distribution were observed: (1) A. simplex, A. monilifera and A. obtusa. This first group of species extends over the entire geographical region. We must bear in mind that Architectonicidae form populations with few individuals. Hence, if some of these species have not been found yet in the studied basins, this does not necessarily mean that they did not exist in those basins. For instance, A. simplex has been found in all basins except the Rhrne, but this does not imply it never ocurred there. (2) A. formosa and Architectonica sp. This is another group of species found in a restricted area. They are both limited to the N. Italian basin.

(3) The remaining species have a more irregular distribution. We must point out that all of them, with the exception of A. contexta, are found in the N. Italian basin. On the whole, we observed one group of basins (the Italian and Mfilaga basins) with a relatively high number of species, 6 or more, and another group (the N o r t h Western Mediterranean, the N o r t h African, and the Atlantic basins) with few species, 4 or less. This implies a heterogeneous distribution. By means of a presence/absence table of species in the different basins (Table 1), an affinity matrix was calculated (Fig. 2). We used the DICE coefficient of faunistical affinity, expressed as i = 2c/nl + n2, where c is the number of shared species between two basins and nl and n2 the total number of species in each basin (Cheetman and Hazel, 1969). This is a useful coefficient for palaeogeographical analysis (Gili and Martinell, 1993, 1994) because it emphasizes the similarity between the compared basins. Its value is 0 when there are no shared species, and 1 when two basins share the same species.

284

hi. Solsona, J. Martinell/Palaeogeography, Palaeoclimatology, Palaeoecology 126 (1996) 281-290

Table 1 Presence/absence table of the studied species in the different basins. M = M/daga, B L = Baix Llobregat, E = Empord~i, R = Roussillon, R H = Rh6ne, A M = Alpes-Maritimes, L I = Liguria, T I = Tirreno, S I = Sicilia, N I = Northern Italy, T U = Tunisia, A = Algeria, E M = Mediterranean Morocco, W M = Atlantic Morocco, H = Huelva, P = Portugal NW ~ M

IlL

E

R

RH

A.simplex

,

,

,

,

A.monilifera

,

,

,

,

,

A.obtusa

,

,

,

,

A.semisquam.

,

A.pseudopers.

,

A.millegrana

,

LIGURIANTYRRHENIAN SEA AM LI TI SI

NI

TU

*

*

N AFRICA A EM W M *

,

*

*

*

H

P

*

*

*

*

*

*

*

*

*

*

A.emiliae A4)lanulata

A.contexta A formosa A.aldovandrii

cases, there are greater affinity indexes between distant basins than between nearer ones, (for instance, between M and SI i=0.933, whereas between M and EM i=0.727). The highest affinity indexes are to be found between basins with a large number of species (SI-M i=0.933, SI-LI and SI-TI i=0.875, NI-LI and NI-TI i = 0.842), or between basins with a low number of species, (R-BL, H-BL, EM-R, EM-A, WM-EM and H-EM i=0.857).This second case

In the resulting affinity matrix we observed the following traits: the obtained affinities show a great variability, ranging from 0 (for example, the TU-RH group having few and non-shared species) to 1 (the LI-TI and the H-R groups both with exactly the same species). Correlations between faunal affinity indexes and latitudinal position or geographical distance were not observed. The affinity indexes do not decrease systematically when geographical distance increases. In some

M BL E R Rtt AM LI T1 SI NI TU A EM WM H P

M

BL

E

R

RH

AM

LI

1.000 0.545 0.444 0.600 0.444 0.769 0.800 0.800 0.933 0.667 0.444 0.600 0.727 0.600 0.600 0.444

1.000 0.667 0.857 0.667 0.400 0.500 0.500 0,500 0.533 0.333 0.571 0.750 0.571 0.857 0.667

1.000 0.800 0.500 0.500 0.400 0.400 0.400 0.308 0.500 0.800 0.667 0.400 0.800 0.500

1.000 0.800 0.444 0.545 0.545 0.545 0.429 0.400 0.667 0.857 0.667 1.000 0.800

1,000 0.250 0.400 0.400 0.400 0.308 0.000 0.400 0.667 0.400 0.800 0.500

1.000 0.714 0.714 0.714 0.588 0.500 0.667 0.600 0.444 0.444 0.250

1.000 1.000 0.875 0.842 0.400 0.545 0.667 0.545 0.545 0.400

M

BL

E

R

RH

AM

LI

TI

SI

NI

TU

A

0.875 0.842 0.400 0.545 0.667 0.545 0.545 0.400

1.000 0.737 0.400 0.545 0.667 0.545 0.545 0.400

1.000 0.308 0.429 0.533 0.429 0.429 0.308

1.000 0.800 0.667 0.800 0.400 0.500

1.000 0.857 0.667 0.667 0.400

TI

Sl

NI

TU

A

EM

WM

H

P

1.000

Fig. 2. Faunal affinity matrix of the studied species.

1.000 0.857 1.000 0.857 0.667 1.000 0.667 0.800 0.800 1.000 EM

WM

H

P

M. Solsona, J. Martinell/Palaeogeography, Palaeoclimatology, Palaeoecology 126 (1996) 281-290

indicates that species of these basins are always the same. This distribution in two groups of basins can be better appreciated by means of clustering analysis. We used three different clustering methods (program NTSYS version 1.80): (1) the single linkage method which takes the maximum affinity between basins; (2) the unweighted pair group method which takes the arithmetic average of affinities; and (3) the complete linkage method which takes the minimum affinitiy (Sokal and Sneath, 1963). Clusters obtained from different methods exhibit very similar patterns. In Fig. 3 we show the results from the unweighted pair group method where two well defined clusters were obtained with a joint point of relatively low affinity (/--0.480): --The first group consists of the following basins: Alpes-Maritimes, Liguria, Tirreno, Sicilia, N. Italy, and Mfilaga. All of them characterized by a high number of species. This is the best defined group of basins in the three methods of clustering, and it includes the highest affinities. --The other group includes the basins of Baix Llobregat, Alt Empord/l, Roussillon, Rh6ne, Tunisia, Algeria, Atlantic and Mediterranean Morocco, Huelva and Portugal, all of them with few species. We must bear in mind that we have little information about the N African basins and this fact could influence the results. This group is always clearly defined but lower rank clusters are -0.200 I

0.000 I

0.200 I

0.400 /

0.600 I

I i

-0.200

0.000

0.200'

0.,~00 0.~00

0.800

1' iO00 Level [---- M 0.933 SI 0.792 FL'I 1.000 0.842 I NI 0.700 AM 0.480 - 8L 0.857 F R 1.000 / H 0.756 [ RH 0.684 - EM 0.857 ] WM 0.733 P 0555 AE 0.800 A 0.650 TU 0.000 0.800' 1000 Level

Fig. 3. Faunal affinity cluster between the studied basins based on the unweighted pair group method of clustering.

285

slightly different using the three methods despite the fact that only three species occur in these basins. This can be easily explained because the affinity coefficient we used (DICE's coefficient) does not take into account the shared absences, and resolution is low because there are few species.

4. Discussion

4.1. Ecological factors In order to study the causes that produced this distribution, we have to consider some ecological factors. These factors can be related to the biology of the studied species (larval development, commensalism with coelenterates), and/or related to the physico-chemical conditions of the environment (temperature, salinity, type of substrate, depth,...). In relation to larval development, we compared the planktotrophic protoconchs of two Recent amphiatlantic species (A. nobilis and H. bisulcatus) with those of the Pliocene species studied in this survey (Table 2). Pliocene protoconchs are qualitatively like the present ones: they have smooth and globular whorls, a rectilinear suture and a sharp boundary with the teleoconch. They also have approximately the same number of whorls, between 2.5 and 3. The protoconch diameters of the two Recent amphiatlantic species are quite different: 1.16 mm in A. nobilis, but only 0.68 mm in H. bisulcatus. Those of the fossil species are approximately between these two values. A. planulata is the only Pliocene species which presents a relatively low number of protoconch whorls. According to Scheltema (in Jablonski, 1985), the best way to assign a kind of larval development to a gastropod fossil species is to compare its protoconch with that of a present species of the same family which we have direct knowledge about. In the present case, due to the great similarity between fossil and Recent protoconchs, we can say that Pliocene species of this family had a planktotrophic development. This, however, is not so clear for A. planulata. Therefore, we can affirm that the larval develop-

286

M. Solsona, J. Martinell/Palaeogeography, Palaeoclimatology, Palaeoecology 126 (1996) 281~90

Table 2 Comparison between the number of protoconch whorls and diameters of two Recent amphiatlantic species (A. nobilis and/-L bisulcatus) and the Pliocene species studied in this survey. Protoconch of A. contexta was not available PROTOCONCH

A. nobilis Heliacus bisulcatus A. simplex A. obtusa A. semisquamosa A. pseudogerspec. A. mille~rana A. emiliae A. planulata A. monilifera a. contexta

PROTOCONCH

WHORLS 3 2.5 3-3.25 2.5-2.75 2.75 2.5 2.5 2.5-2.75 1.5

DIAMETER ~mm} 1.15 0.68 1.2-1.5 1-1.15 1.45 0.78 1.15-1.25 0.90 0.55

2.5 [

1.3 [

ment of the Pliocene Architectonicidae allowed them to extend widely. This factor did not limit the geographical distribution of these species. Limitation, as we observed, are caused by other factors. The main abiotic factors that can influence the geographical distribution of marine organisms are: temperature, salinity, depth, and type of substrate. Paleontological and sedimentological data from the studied basins point towards conditions of a fully marine environment. Hence, salinity is not a relevant factor for this analysis. The climate was tropical to subtropical during the Early Pliocene and slightly cooler during the Late Pliocene. The whole of the malacological fauna from the Pliocene basins of the Western Mediterranean shows latitudinal variations with more tropical species to the South (Martinell, 1995; Martinell et al., 1995). This pattern was not observed, however, in Architectonicidae. Consequently, temperature was not a main influential factor for the geographical distribution of these species, either. Table 3 represents the assumed depths of deposition, and the type of sediment for the different studied outcrops. It is important to notice that we only took into account those outcrops where a taphonomic analysis indicates that transport was not important, and the fossil fauna corresponds to a palaeocommunity. The majority of the outcrops present a similar lithology consisting of blue clays. Only those from the Huelva and Roussillon basins present a different lithology, with yellow sands and

sandy clays respectively. As for bathymetry, most of the outcrops indicate relatively shallow conditions (less than 40 m), and only those from the Alpes-Maritimes basin show greater depths (100-200 m) (Gili and Martinell, 1993; Martinell and Marquina, 1984; Nolf and Cappetta, 1988). With regard to depth (Table 4), the greatest part of the Architectonicidae species are to be found in sediments indicating relatively great depths. There are only four species (A. simplex, A. monilifera, A. obtusa, and A. planulata) found in both deep and shallow sediments. If we consider the type of sediment (Table 4), it can be observed that three species (A. monilifera, A. obtusa and A. simplex) are to be found in all kinds of sediments, while the other species are only found in clay sediments. Depth and type of sediment, can therefore be considered major controls in the geographical distribution of these species. Commensalism with coelenterates is another factor that could influence the geographical distribution of Architectonicidae. This is a very difficult factor to detect in the fossil record because the majority of the hosts do not have a fossilizable skeleton. Besides, this commensual relationship is only certainly know for some recent species, hence, it is not possible to demonstrate that the same behaviour was undergone by all the fossil ones. However, if this relationship had really been common for Pliocene Architectonicidae, their distribution would have been more closely related

M. Solsona, £ Martinell/Palaeogeography, Palaeoclimatology, Palaeoecology 126 (1996) 281-290

287

Table 3 Lithologies and bathymetries for the different outcrops. Modified from Gili and Martinell (1993) LITHOLOGY Yellow sands Yellow sands Blue clays Blue clays Blue cla~,s Blue cla~rs Sandy clays Sandy clays Blue clays Blue clays Blue clays

OUTCROP HUELVA BAIXLLOBREGAT

Bona.rcs

Lucena E1Tarc St. Vicenq dels Horts

papiol

ALTEMPORDA

Siurana N6fiaeh Millas St. Isidore Costamagna St. Martin du Var

ROUSSILLON ALPES-MARITIMES

BATHYMETRY <30m. <30m. <40m. <40m. <40m. <25m. 10 to 25 m. <25 m. 100 to 200 m. I00 to 200 m. 100 to 200 m.

I

i

Table 4 S 9ecies found in the different bathymetries and lithologies of the outcrops studied BATHYMETRY SHALLOW DEEP A. simplex A. simplex A.monilifera A.monilifera A.obtusa A.obtusa A.planulata A.planulata A.semisquamosa A.pseudoperspect. A.millegrana A.emiliae A.contexta

BLUECLAYS A. simplex A.monilifera A.obtusa A.planulata A.semisquamosa A.pseudoperspeet. A .millegrana A.emiliae A.contexta

with coelenterates distribution rather than directly caused by depth a n d / o r type o f sediment.

4.2. Historical factors Table 5 shows the temporal distribution o f the studied species. This distribution presents the following patterns: Table 5 Temporal distribution of the species studied

LITHOLOGY SANDYCLAYS A. simplex A.monilifera A.obtusa

YELLOWSANDS A. simplex A.monilifera A.obtusa

• All studied species originated in the Miocene except A. contexta. • M o s t o f the studied species became extinct at the end o f the Pliocene, except A. obtusa and A. contexta. M o s t authors agree with the theory o f the Messinian crisis which occurred at the end o f the Miocene. D u r i n g this period, an i m p o r t a n t regres-

288

~ Solsona, J. Martinel(/Palaeogeography, Palaeoclimatology, Palaeoecology 126 (1996) 281-290

sion affected the Mediterranean, the connection with the Atlantic was interrupted and this produced the desiccation of the Mediterranean sea (Hsii et al., 1973; Clauzon et al., 1990). Some authors state that the Mediterranean did not desiccate totally, and that some small and localized basins remained (P6res, 1989). As can be seen in Table 5, there is no evidence indicating that studied species were affected by this crisis. By means of these observations, we can speculate that either Architectonicidae recolonized the Mediterranean after the Messinian crisis, or they remained in some residual basins, or both. Using data from the literature (Baluk, 1975; Cossman and Peyrot, 1919; Dollfus et al., 1904; Ferrero Mortara et al., 1984; Glibert, 1949, 1952; Janssen, 1984; Moroni and Ruggieri, 1984; Nordsieck, 1972), we have noticed that the majority of the studied species are to be found in basins in the Tethyan area during the Miocene; only three (A. simplex, A. obtusa and A. millegrana) are to be found in both Tethyan and Atlantic areas (Table 6). Therefore, species of the first group could remained in residual basins during the Messinian crisis. The others could also remained in residual basins or could recolonized the Mediterranean after the Messinian crisis. This is not unlikely given their long planktotrophic larval development. More data is needed to establish more concrete hypotheses, but it is important to observe that the Messinian crisis did not affect these Architectonicidae. This fact has also been Table 6 Miocene distribution of the studied species. Atlantic area includes the following Miocene basins: Tajo (Portugal), Loire and Aquitaine (France), Bolderberg (Belgium), Winterswijk (Netherlands). The Tethyan area includes Sicily, N. Italy and Korytnica (Poland) basins

noticed in Nassariidae (Gili, 1991; Gili and Martinell, 1993), Cirripeda (Riba-Vifias and Martinell, 1986), Bryozoa (Moissette and Pouyet, 1987), and others. During the Pliocene a gradual drop in temperature occurred, but it did not affect the studied species. The same species are to be found in the Early and in the Late Pliocene. But at the end of the Pliocene, sudden changes of temperatures ocurred with the beginning of the glacial and interglacial periods. These sudden changes seem to be the cause of the extinction of most of the studied species. Only two of them are to be found in the Mediterranean at present: A. eontexta and A. obtusa.

5. Conclusions

The geographical distribution of Pliocene Architectonicidae of the Western Mediterranean shows a heterogeneous pattern. There are: • Basins with a relatively high number of species (6 or more). • Basins with a relatively low number of species, (4 or less), with three species always present: A. simplex, A. obtusa and A. monilifera. These three species are to be found in all the basins of the studied area, while the others have a more restricted distribution. From the comparison between fossil and recent protoconchs we can postulate that Pliocene Architectonicidae underwent a planktotrophic larval development. This type of larval development allowed them to extend over the entire studied area, consequently the irregularities that we observed in the distribution seem to be caused by other factors. Considering the palaeoenvironmental conditions of the studied outcrops, we observed that two factors, depth and type of sediment, were major controls in the distribution. These factors can explain the observed heterogeneous distribution: • Basins with outcrops with clay sediments indicating relatively deep conditions (AlpesMaritimes) have a relatively high number of species.

Solsona, J. Martinell/Palaeogeography, Palaeoclimatology, Palaeoecology 126 (1996) 281-290 • Basins with shallow a n d s a n d y sediments (Huelva, Baix L l o b r e g a t , A l t E m p o r d ~ , R o u s s i l l o n , R h 6 n e ) have few a n d the same species: A. simplex, A. monilifera a n d A. obtusa. M o r e d a t a a b o u t the p a l a e o e c o l o g y o f r e m a i n i n g Pliocene M e d i t e r r a n e a n basins are n e e d e d to test these hypothesis. D i s t r i b u t i o n m i g h t also be d e t e r m i n e d by o t h e r factors n o t detected in this study, for instance the c o m m e n s u a l r e l a t i o n s h i p with coelenterates. I n r e l a t i o n to historical factors, we o b s e r v e d t h a t the M e s s i n i a n crisis d i d n o t affect the m a j o r i t y o f these species. T h e g r a d u a l d r o p in t e m p e r a t u r e t h a t o c c u r r e d in the Pliocene d i d n o t affect these species either. T h e s u d d e n changes o f t e m p e r a t u r e w h i c h o c c u r r e d at the e n d o f this p e r i o d p r o b a b l y c a u s e d the extinction o f the m a j o r i t y o f these species. O n l y two, A. contexta a n d A. obtusa, are to be f o u n d at p r e s e n t in the M e d i t e r r a n e a n .

Acknowledgements T h e a u t h o r s w o u l d like to express their g r a t i t u d e to Prof. R. R e y m e n t a n d an a n o n y m o u s referee for their constructive criticism, to D r . R. D o m b n e c h , Dr. C. Gili a n d Dr. J.M. de G i b e r t for c o m m e n t s a n d assistance with the draft, a n d to F r a n c e s L u t t i k h u i z e n for her help with the English draft. This research is within the r a n g e o f investigation o f the Projects D G I C Y T no. PB94-0946 a n d HP95-46.

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