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Home , Calliphylloceras, Lytoceratina, Nannolytoceras, Phylloceras, Phylloceratidae, Phylloceratina

ammonoids in the Iberian Basin during the Middle Iurassic: a model of biogeographical and taphonomic dispersal related to relative sea-level changes

s. Femandez-L6peza, G. Melendez b

a Departamento de Paleontologia, Facultad de Ciencias Geologicas (UCM) e Instituto de Geologia Economica (CSIC-UCM), 28040 Madrid, Sp ain b Area de Paleontologia, Departamento de Ciencias de la Tierra, Facultad de Ciencias, Universidad de Zaragoza, 50009 Zaragoza, Sp ain

Abstract

Phylloceratina and annnonoids in the Middle of the Iberian Chain (E. Spain) represent less than I % of the whole of Annnonoidea. There are two intervals, however, in which their occurrence is remarkably constant: within the Upper and at the LowerlMiddle Callovian boundary. These two dispersal episodes of Phylloceratina and Lytoceratina into the Iberian Basin are regarded to reflect changes in their palaeoecological and taphonomical behaviour, as a consequence of regional sea-level changes during the Middle Jurassic. A relative rise during the Late Bajocian favoured the immigration of juvenile phyllocerataceans. and Adabofoloceras immigrations gave rise to monospecific assemblages, where they soon died. They did not breed or ontogenically develop in this basin. In contrast, phyllocerataceans recorded at the LowerlMiddle Callovian boundary constitute polyspecific assemblages, dominated by adult individuals. These Callovian assemblages were formed by nekroplanktonic drift, related to a relative sea-level fall and general homogenization of the shelf bottom, hence favouring the concentration of remains of organisms from more open marine and oceanic areas. "

1. Introduction in the Middle Jurassic. Shells of these ammonoids, classically considered as characteristic inhabitants Ammonites of the suborder Phylloceratina have of oceanic environments, are more abundant and traditionally been considered as absent from the geographically persistent within the Upper palaeontological record in the Middle Jurassic of Bajocian sediments and at the Lower-Middle the Iberian Chain (Neumayr, 1885 fide Dereims, Callovian boundary. The purpose of this paper is 1898; Haug, 1908; Bataller, 1922; Elmi, 1985; to show that the distribution of Phylloceratina Sequeiros, 1987; Thierry and Wilde, 1990). shells in the Iberian Basin during these two epi­ However, some remains of Phylloceratina and sodes was mainly controlled by relative changes of Lytoceratina are in fact present and appear to be sea level, favouring their biogeographical and relatively constant at some strati graphic intervals taphonomic dispersal. 2. Distribution of Phylloceratina and Lytoceratina (140 mm) incomplete phragmocone from the in the Middle Jurassic of the Iberian Chain Lower Aalenian, Opalinum Zone, of Arifio. Callovian shells are also large-sized, some of them Representatives of superfamilies Phyllo- over 150 mm diameter. In contrast, Bajocian speci­ cerataceae and Lytocerataceae have been reported mens are mostly millimetric in size, showing a by several authors in the Middle Jurassic of the corroded suture line, and it cannot be excluded Iberian Chain. Eighty specimens of Phylloceratina that they belong to the genus Adabofoloceras. have been so far reported from the Aalenian, Several Lower Bajocian representatives of Bajocian, and Callovian, in sixteen Adabofoloceras have been identified in La Olmeda localities of the Iberian Chain (Fig. 1). All these and Barranco La Canaleja (Humphriesianum specimens, though, represent less than 1 % of the Zone, Femandez-L6pez, 1985c) and they are also whole of in the fossil assemblages. common in the Upper Bajocian from Tivenys, Some specimens of the genus Phylloceras have Ribarroja and Barranco La Canaleja (Niortense been reported from the Lower Bajocian of Villel and Garantiana Zones, Femandez-L6pez, 1980, (Sauzei and Humphriesianum zones, Hinkelbein, 1983, 1985c). 1975), and from the Upper Bajocian of Tivenys Some representatives of come (Niortense and Garantiana Zones, Fallot and from the Upper Bajocian of Albarracin (Niortense Blanchet, 1923; Femandez-L6pez, 1983; Zone, Hinkelbein, 1975) and Ricla (Garantiana Femandez-L6pez and Mouterde, 1985), Hontoria Zone, Femandez-L6pez and Aurell, 1988). (Westermann, 1955), Ricla (Mensink, 1966), However, specimens of Calliphylloceras appear Tuejar, Riodeva and Albarracin (Niortense and more common and constant in the Callovian sedi­ Garantiana Zones, Hinkelbein, 1975), Ribarroja ments, as regards the number of specimens and Rambla La Gotera (Niortense Zone, reported from the Arifio region (Marin and Femandez-L6pez, 1980), as well as from the Toulouse, 1972); Tuejar, Riodeva and Villel Callovian of Frias de Albarracin (Riba Arderiu, (Hinkelbein, 1975), Moscard6n and Frias de 1959), Tuejar, Riodeva, Villel (Hinkelbein, 1975) Albarracin (Femandez-L6pez et aI., 1978). The and Ricla (Sequeiros and Cariou, 1985). A C. Callovian specimen of Phylloceras kudernatschi species disputabile (Zittel) is the most abundant (Hauer) is also reported here. The only record so among the Callovian associations. far reported from the Aalenian is a large size Some references to Holcophylloceras have been reported from the Lower Bajocian of Albarracin (Sauzei Zone, Hinkelbein, 1975) and from the

.... Upper Bajocian of Tivenys and Ricla (Garantiana HN ZARAGOZA BARCELONA Zone, Femandez-L6pez, 1983; Femandez-L6pez "RC and Mouterde, 1985, Femandez-L6pez and Aurell,

"'AR 1988). However, representatives of this genus are SEA # more common at the Lower-Middle Callovian �� boundary. They have been recognized at the region l� of Arifio (Marin and Toulouse, 1972), Tuejar and iN Villel (Hinkelbein, 1975), Caudiel, Moscard6n and Frias de Albarracin (Femandez-L6pez et aI., 1978; 0 50 100 km VALENCIA Femandez-L6pez, 1985a). Representatives of Ptychophylloceras are very

Fig. 1. Geographical location of the referred outcrops (AL = scarce. However, they have been recognized in Albarracin,AR = Arii'io, EH = San Bias dam, C = La Canaleja the Upper Bajocian of Villel and Ribarroja = FS = G = cliff, CD Caudiel, Frias de Albarracin, Rambla La (Garantiana Zone, Hinkelbein, 1975, and Gotera, HN = Hontoria, U1= La Olmeda, M = Moscard6n, Niortense Zone, Femandez-L6pez, 1980). A few RE = Ribarroja, RC = Ricia, RD = Riodeva, TJ = Tuejar, TY = Tivenys, VL = Villel). Callovian specimens of this genus are known from Tuejar and Villel (Hinkelbein, 1975) and from 50% Moscard6n. No specimens fromthe genus have been reported so far from the Middle Jurassic of the Iberian Chain. Some recorded specimens, e.g. at the localities of Tuejar and Moscard6n, come from the iron oolite level at the Callovian­ Oxfordian boundary, with some other elements from the Middle and Upper Jurassic. They are a part of condensed associations containing F A c H T L N reworked (reelaborated sensu Femandez-L6pez, Fig. 2. Relative frequencies,at a generic level, of Phylloceratina 1985a, 1991; Femandez L6pez and Melendez, and Lytoceratina recorded from the Middle Jurassic of the 1994) Callovian specimens mixed with resedi­ Iberian Chain (F=Phylloceras, A = Adabofoloceras, C= mented Oxfordian specimens (Hinkelbein, 1975; Calliphylloceras, H= Holcophylloceras, T= Ptychophylloceras, Sequeiros et aI., 1985; Melendez, 1989), and are L = , N=). probably Oxfordian in age. Some adult specimens of Lytoceras have been $(S�S'S� Lytoceraflna found in several localities and stratigraphic Q m , , , intervals: Lower Bajocian of Ribarroja T (Humphriesianum Zone, Femandez-L6pez, 1980), Phyllocer.tlna , 50% 2I m. � Upper Bajocian of San Bias (Niortense Zone, ?� Femandez-L6pez, 1985c), Lower Bathonian of Ricla (Femandez-L6pez and Aurell, 1988), and Callovian of Riodeva, Villel, Moscard6n, Frias de Albarracin and Ricla (Hinkelbein, 1975; Fig. 3. Relative frequencies of Phylloceratina and Lytoceratina Femandez-L6pez et ai., 1978, Sequeiros and from the Iberian Chain in Middle Jurassic stages. Cariou, 1985). One specinlen of Nannolytoceras cf polyhelictum (Boeckh) comes from the Upper Upper Bathonian and Upper Callovian may result Bajocian of San Bias (Niortense Zone, Femandez­ from the poor record of these stratigraphic L6pez, 1985c). intervals. However, the lack of record of these In the Middle Jurassic of the Iberian Chain, taxonomic groups in the Middle and Upper phyllocerataceans (87,1%) are far more common Aalenian and in the Middle Bathonian has been than lytocerataceans (12,9%). They correspond, in checked with ammonite collections of over one decreasing order of abundance (Fig. 2), to the thousand specimens. following genera: Phylloceras (30.8%), In the Upper Bajocian (Fig. 4), the following Calliphylloceras (17.9%), Holcophylloceras genera, in decreasing order of abundance, are (17.�Io), Adabofoloceras (12.8%), Lytoceras recorded: Phylloceras s.1. (44.8%), Adabofoloceras (11.6%), Ptychophylloceras (7.7%), and Nannoly­ (20.7%), Holcophylloceras (10.3%), Calli­ toceras(1.3%). phylloceras (6.9%), Ptychophylloceras (6.9%), Despite their scarcity, shells of Phylloceratina Lytoceras (6.9%) and Nannolytoceras (3.5%). At and Lytoceratina show their highest frequency the Lower-Middle Callovian boundary (Fig. 5) (Fig. 3) at the Lower-Middle Callovian boundary the recorded genera are: Calliphylloceras (30%), level (51.3%) and in the Upper Bajocian (37.2%; Holcophylloceras (25%), Phylloceras (22.5%), Niortense Zone, Baculata Biohorizon, and Lytoceras (12.5%) and Ptychophylloceras (10%). Garantiana Zone). These two Middle Jurassic At the generic level, Phylloceratina an1ffionoids intervals also show the highest concentration of show a lower diversity and higher equitability in in the Iberian Chain. The virtual the Callovian than in the Bajocian. Those forms absence of phylloceratids and lytoceratids in the showing constrictions or pseudoconstrictions 50%

40 are characteristic of pelagic or oceanic environ­ ments. Conversely, "trachyostraca" ammonoids 30 are regarded as characteristic of neritic or shallow 20 1/V V'J '--_ marine habitats (Arkell et aI., 1957, p. L106). Supporting evidence for this interpretation is 10 varied: functional morphology, evolutionary behaviour, palaeogeographical distribution and facies analysis. The geometric shape of the shell, its microstruc­ ture, the greater thickness of the septa, the higher complexity of septal sutures and the lower relative diameter of the siphuncular tubes in Phylloceratina are significant differences in respect to other ammonoid groups, representing a greater ability Fig. 4. Relative frequencies,at a generic level,of Phylloceratina to inhabit deeper water environments. In the case and Lytoceratina from Upper Bajocian of the Iberian Chain. of adult ammonites it has been suggested that they could probably descend to some several hundred metres of depth. Resistance to implosion of ammo­ 50% nite shells would not exceed some 400-500 m 40 depth. Only a few phylloceratids could reach 30 greater depths than 1000 ill. Among them, cons- tricted forms do usually possess in addition septa 20 and connecting rings that would have been less 10 resistant to hydrostatic pressure, so they have been regarded as characteristic of relatively shallower water environments than non-constricted forms FA C HTLN (Ward and Westermann, 1985; Hewitt and . . � � Westermann, 1987; Westermann, 1990, 1993). Taking into account the Speciation and extinc­ tion rates of taxa and the mean time range of &��� different genera of Ammonoidea, it seems that phylloceratids and lytoceratids were less affected by environmental changes, most especially eustatic Fig. 5. Relative frequencies,at a generic level,of Phylloceratina oscillations, than other ammonoids during the and Lytoceratina from Lower to Middle Callovian of the Jurassic and . The longer specific Iberian Chain. and generic duration of Phylloceratina and Lytoceratina has also been interpreted as evidence (Holcophylloceras and Calliphylloceras) are also for a more oceanic character of this taxonomic more frequent at the Lower-Middle Callovian group (Ward and Signor, 1983). boundary than in the Upper Bajocian. Shells of Phylloceratina and Lytoceratina are abundant, both in number of specimens and species diversity, within sediments deposited in

3. Palaeoecological and biogeographical remarks deep tethyan environments. They are, however, on Phylloceratina and Lytoceratina scarce or absent in epicontinental deposits. This restricted palaeogeographical distribution has been Phylloceratina ammonoids have been often sepa­ repeatedly used as an argument against long post­ rated from other Ammonoidea by various cri­ mortem transport of ammonoid shells by ne­ teria. Phylloceratina, or "liostraca" ammonoids, kroplanktonic drift (Hall am, 1975), as well as an evidence of palaeoecological control rather than 4. Taphonomic remarks on Middle Jurassic of true provincialism (Geczy, 1967; Ziegler, 1967). Phylloceratina and Lytoceratina of the Iberian Similarly, the presence of representatives of Chain Phyllocerataceae (Phylloceras, Calliphylloceras, Holcophylloceras) in the boreal realm during the Shells of Phylloceratina and Lytoceratina, as Middle Jurassic has been taken as evidence of this described above, display some other taphonomic group being cosmopolitan (Imlay, 1962, 1964, and palaeoecological differences. Upper Bajocian 1973, 1982; Poulton, 1982, Poulton et aI., 1992). specimens are usually small-sized (D = 10-30 mm, On the other hand, some Toarcian and Aalenian excepting a single Lytoceras specimen, over Alocolytoceratinae are more common in some 300 mm diameter). They correspond to juvenile or sedimentary basins in northwestern Europe and immature individuals, showing no traces of were probably adapted to shallower environments dwarfmg or miniaturization (cf Mignot, 1993; (Ziegler, 1981). Some specimens of Mignot et aI., 1993). They are either accumulated Calliphylloceras invaded NW European areas or else resedimented elements (sensu Fernandez­ during its greatest diversity, e.g. in the Toarcian L6pez, 1991) showing no signs of colonization by and theCallovian (Gal25%) in benthos-rich sediments 1985a, 1991; Fernandez-L6pez and G6mez 1991; from the platform margin of epicontinental seas Fernandez-L6pez and Melendez, 1994, p. 32) (Geyer, 1971). Biogeographical dispersal and ne­ showing traces of internal and/or external coloni­ kroplanktonic drift of shells (a case of taphonomic zation on both sides of the shell. Among the dispersal) must have been the main determining colonizers, serpulids and bryozoans are most factors on the final distribution of Phylloceratina common. These specimens are commonly found shells. The present distribution of shells would, in recorded associations showing higher taxonomic therefore, reflect both their original and post­ diversity and equitability at specific level than mortem distribution (Fernandez-L6pez, 1983, those fromthe Upper Bajocian. Ammonite associa- 1985c; Tintant, 1984; Callomon, 1985; Fernandez­ tions are composed mostly by adult individuals L6pez and Mouterde, 1985; Westermann, 1990; and characterized by the absence of juvenile speci­ Enay, 1993). mens (i.e. taphonic populations of type 3 or 2 in Femandez-L6pez, 1985c, 1995; Femandez-L6pez Lytoceratina remains from the Upper Bajocian are and Mouterde, 1994). associated with transgressive facies, deepening From a taphonomic point of view, it is also upwards sequences and expanded successions, relevant to compare the values of relative fre­ developed under low turbulence conditions in rela­ quency (F) against those of the geographical persis­ tively deep, subtidal environments (F ermindez­ tence index (P). The value of this index for a L6pez, 1983, 1985c, 1987; Femandez-L6pez certain genus is expressed by the ratio between the and G6mez, 1991). Callovian remains of number of localities of each substage where this Phylloceratina and Lytoceratina, however, are genus is represented and the total number of associated with regressive facies, shallowing localities where shells of Phylloceratina and upwards sequences and condensed successions, Lytoceratina have been recorded. From the sixteen formed under high turbulence conditions in sha­ localities where these ammonoids are reported in llow subtidal environments, locally emerged and the Iberian Chain, ten are Upper Bajocian and submitted to subaerial alteration (cf Geyer et aI., eight Lower-Middle Callovian. In the Upper 1974; G6mez, 1979; G6mez and Goy, 1979; Benke, Bajocian sediments, representatives of the genus 1981; Mensink and Mertmann, 1984; Femandez­ Phylloceras s. l. (F=44.8%, P=7OOIo) and L6pez, 1985b; Wilde, 1988; Aurell, 1990; Aurell Adabofoloceras (F=20.7%, P=5001o) are the and Melendez, 1990; G6mez and Femandez­ most common and persistent (see Fig. 6). L6pez, 1992, 1994; Aurell et aI., 1995; Femandez­ Representativesof the genus Calliphylloceras (F= L6pez and Melendez, 1995). This relative sea-level 30%, P =75%) and H olcophylloce ras (F= 25%, P = fall, however, in the Iberian Basin is not incompati­ 75%) are dominant, instead, at the Lower-Middle ble with a global rise of the sea level during de Callovian boundary. They show a lower relative Early-Middle Callovian as proposed by several frequency but higher geographical persistence than authors (Hallam, 1988; Haq et aI., 1988). Upper Bajocian representatives of Phylloceras. Constricted phylloceratids (i.e. Holcophylloceras and Calliphylloceras) are the most common and 5. Interpretation of Phylloceratina and geographically persistent at the Lower-Middle Lytoceratina in the Iberian Chain Callovian boundary, but they are very scarce in the Upper Bajocian. The presence of a fossil in a place does not Upper Bajocian and Lower-Middle Callovian mean that the producer organism lived there. rocks in the Iberian Basin also belong to two Organisms may be recorded outside of their living different facies, both characterized by the abun­ area. Consequently, the concepts about the prove­ dance of ammonites. Phylloceratina and nance of fossil remains may be distinguished from those about provenance of palaeobiological enti­ ties. The taphonomic concepts autochthonous/ F 50% allochthonous refer to remains recorded at the R E 40 very place in which they were produced (autoch­ Q thonous), or for those that have been transported U 30 laterally after their production and are recorded E now from a different place or region (allochtho­ N 20 C nous). Organisms may move by their own means 10 L Y N or by passive transport outside of their breeding area (migratory species or passive biodispersal) 10 20 30 40 50 60 70 80 90 100"/0 but inside of their living area (Fig. 7). Therefore, GEOGRAPHICAL PERSISTENCE demic species may be found in eudemic, miodemic Fig. 6. Relative values of frequencies and geographical persis­ or parademic conditions, depending on if they are tence, at a generic level, of Phylloceratina and Lytoceratina from Upper Bajocian (white dots) and from Lower to Middle inferred in theirbreeding area (eudemic; Callomon, Callovian (black dots). 1985), in an area normally occupied but where habitats (Phylloceras). The relative sea-level fall during the Early-Middle Callovian transition, associated with an increase of the homogeneity of the Iberian Basin, favoured the arrival of shells by nekroplanktonic drift and increased the accumula­ tion of phylloceratid shells from other, more open marine or oceanic areas (i.e., these shells represent ademic individuals; Fig. 8). In the case of Bajocian phylloceratids, the high proportion of small shells and the positive skew by the size-frequency distribution are taphonomic features disproving any processes of sorting by mechanical transport (i.e. arrival of shells to the burial area by surface transport or by nekroplank­ ADEMIC tonic drift from distant areas). Among Bajocian phylloceratids, poorly ornamented or smooth Fig. 7. Summary of palaeobiogeographical categories men­ forms (Phylloceras, Ptychophylloceras) are domi­ tioned in text. The palaeobiological entities are demic when nant and show higher geographical persistence their remains are found in their living area. Conversely, they are ademic species or taxa when they are inferred outside of than those showing heavy constrictions or pseudo­ their living area. Demic species may be inferred in their constrictions(Ho lcophylloceras, Calliphylloceras). breeding area (eudemic), in an area normally occupied but It follows that some juvenile specimens of where they do not breed (miodemic), or in a living area Phylloceras and Ptychophylloceras reached the occasionally reached by passive biodispersal (parademic). Iberian Basin as immigrants. However, they did not breed or ontogenically develop in this basin, they do not breed (miodemic), or in a living area so they are miodemic organisms. During the Late occasionally reached by passive biodispersal Bajocian the Iberian Basin probably acted as a (parademic; Fernandez-L6pez, 1990, 1991; L6pez­ migration route for the phylloceratids and for Martinez and Fernandez-L6pez, 1993; Fernandez­ some ammonite groups adapted to shallower envi­ L6pez and Mouterde, 1994). ronments (Fernandez-Lopez, 1985c), between the Callovian shells of Phylloceratina and western Tethys areas and the proto-Atlantic or Lytoceratina showing encrustation had to undergo the Northwestern Europe epicontinental seas. nekroplanktonic drift during the biostratinomic However, the scarce specimens ofBajocian lytocera- phase and after the loss of the soft parts. taceans recorded in the basin correspond to adult Associations are characterized by the high propor­ inOividuals and represent ademic (i.e., inferred out tion of large-sized shells, showing negative skew of the living area) or parademic organisms (trans­ in the distribution of size-frequencies, and by the ported within the living area). scarcity or even absence of juvenile individuals. Except for the Late Bajocian, this intracratonic, These taphonomic features, associated with very epicontinental migration route was not active shallow facies where reelaborated or reworked during the rest of the Middle Jurassic. internal moulds are common, result from the Stratigraphic intervals in which Ammonitina are higher capacity of nekroplanktonic drift in larger scarce and Phylloceratina or Lytoceratina are rare shells (C£ Chamberlain et aI., 1982; Fernandez­ to absent (Aalenian, Lower Bajocian, Bathonian L6pez, 1983, 1987, 1990). Phylloceratids display­ and Upper Callovian) do not represent episodes ing heavy pseudoconstrictions or constrictions of migration or taphonomic destruction but epi­ (Calliphylloceras, Holcophylloceras), characteristic sodes of poor communication between the Iberian of shallower habitats, should have lower pro­ Basin and open marine, or oceanic environments. bability of flooding after death and higher proba- Shallow conditions in these sedimentary palaeo­ bility of drift than the smooth forms of deeper environments probably prevented both the ammo------_. ------'

SEDIMENTARY PROXIMAL OUTER PLATFORM DISTAL OUTER PLATFORM PALAEOENVIRONMENTS CONFINED ENVIRONMENTS I THRESHOLD I OUTER ENVIRONMENTS E+ I E- I E+ I E+ I E-

r '3 '3 '3( '3 O ( SO S'3,,-z.;�Ii' (\'3� ( c'3-J : G('3G' \''3 (\\\'3(\ ( li'3(\ e.!a,,\ia : (\\\ \ (¥-i(\S <;\1 'i''3 �I(\ (\�C. SO \ � 6 ('3(\ 6'3('3 G ('3 \ G � ('3 'OC 'i''3 'OC· �I.- \'11" 'OC. O l'I . 'OC. '3 '?>C. '3 'OC. LOCALITIES . \l , 'O 'OC. ee. s, s, I " o �I 0"" 'OI.-"s, �"''' f>.Oo"" :-J�"''' 'i'0-V 0', ',J>.C "" Sc o 'oI .-, ",\ I\I\0JI', :-J '\\ \l G\l '\\ � and Biochrones sct' l'I,\".,.w .-'O"�\ \'110 ct' " s".,. I\lel' 50 k \oNo I . m 0'",,0 t' I TAPHONOMICGRADIENTSINPHYLLOCERATIDS: I BY I TAPHONOMIC DISPERSAL: I I (jjJ I I ,I I PJ ~ 1111all� Ii i i i' I Taphonic populations of type 3, I I dominated by constricted shells. I I BY PROCESSES OF ACCUMULATION AND RESEDIMENTATION: G I I I

I I I Taphonic populations of type 1, dominated by non-constricted shells. "- I I I )

Fig. 8. Taphonomic gradients in Middle Jurassic Phylloceratina ofthe Iberian Basin, by taphonomic dispersal and by taphonomic accumulation and resedimentation. nite migration along this route and the Basin took place under two opposite bathymetric accumulation of drifted shells (Fig. 8). conditions: transgressive, deeper environments The appearance of shells of Phylloceratina and during the Late Bajocian (Garantiana Biochron) Lytoceratina at the Upper Bajocian and at the and regressive, shallow environments at the Early­ Lower-Middle Callovian boundary in the Iberian Middle Callovian boundary. Accessibility and Basin, associated with a greater abundance of taphonomic dispersal of drifted elements was Ammonitina, does not reflect two separate regional favoured as well during this second episode by the evolutionary events. These Middle Jurassic associa- greater homogeneity of the sea-bottom. tions, containing the highest concentration of As it has been shown for the Iberian Basin, the ammonoids (Ammonitina, Phylloceratina and maximum relative sea-level fall favoured thetapho­ Lytoceratina), resulted instead from environmental nomic dispersal of adult phylloceratids and changes leading to the increase of the taphonomic lytoceratids towards the epicontinental platforms potential of the basin. A higher amount of these increasing the abundance, richness and equitability shells was produced and imported into the basin of the recorded associations (Fig. 9). In contrast, during the Late Bajocian and EarlyIMiddle maximum sea-level rise episodes favoured the bio­ Callovian than during any other interval in the geographical dispersal of juvenile phylloceratids Middle Jurassic. The biogeographical and tapho­ towards the outer platforms leading to recorded nomic dispersal of anunonoids towards the Iberian associations with higher values of abundance and o RELATIVE SEA-LEVEL FALL NERITIC ------71 OCEANIC ENVIRONMENTS

PLATFORM

AREA OF TAPHONOMIC DISPERSAL

o RELATIVE SEA-LEVEL RISE

PLATFORM

AREA OF TAPHONOMIC DISPERSAL

Fig. 9. Model of biogeographical and taphonomic dispersal for Middle Jurassic Phylloceratina, in relation to relative sea-level changes. Demic phylloceratids were mainly, although not exclusively, Tethyan forms living in oceanic environments. Eudemic . phylloceratids inhabited basin areas, characterized by the occurrence ofjuvenile specimens. A. During episodes of relative sea­ level fall, taphonomic dispersal produced the concentration ofphylloceratid shells in shallow platforms. These shells represent ademic species, characterized by the absence of juvenile specimens and dominated by adult individuals. B. During episodes of relative sea-level rise, active and passive biodispersal led to the occurrence in outer platforms of immigrant and transported phylloceratids, which respectively represent miodemic and parademic species. These miodemic species are dominated by juvenile individuals. In contrast, taphonomic dispersal produced the concentration of adult shells in shallow neritic environments, representing ademic species.

lower values of richness and equitability. The and ecologically controlled by the regional particu­ relative frequency and geographical persistence of lar bathymetric conditions in each basin. phylloceratids can be used as an index to estimate therelative sea-level changes in epicontinental Peri­ Tethyan basins. It should be noted, however, that 6. Conclusions the appearance and distribution of shells of Phylloceratina and Lytoceratina, and probably In the Middle Jurassic, thetaphonomic potential other Ammonoidea as well, were taphonomically of the Iberian Basin with regard to ammonoid shells reached maximum values during two References episodes of extreme bathymetric conditions: transgressive, deep environments during the Arkell, W.I, Kummel, E. and Wright, CW., 1957. Mesozoic Late Bajocian (Niortense Biozone, Baculata Ammonoidea. In: RC. Moore (Editor), Treatise on Biohorizon, and Garantiana Biozone), and very Invertebrate Paleontology, (L), 4, Cephalopoda, Ammonoidea. Geo!. Soc. Am., Univ. Kansas Press, shallow, regressive conditions at the Early-Middle Lawrence,KS, pp. L80-L437. Callovian boundary. Aurell,M, 1990. El Junisico superior en la Cordillera Iberica The biogeographical dispersal of certain species Central (provincias de Zaragoza y Teruel). Amilisis de and the arrival of immigrant ammonoids was cuenca. Thesis. Dep. Geo!. Serv. Pub!. Univ. Zaragoza, pp. 1-389. favoured by a relative sea-level rise in the Late Aurell,M, Fernandez-L6pez,S. and Melendez,G., 1995. The Bajocian, particularly during the Garantiana Middle-Upper Jurassic oolitic ironstone level in the Iberian Biochron (Fema'ndez-Lopez, 1985; Hallam, 1988). Range (Spain). Eustatic implications. Geobios, in press. Some juvenile specimens of Phylloceras and Aurell, M and Melendez, G., 1990. Upper Jurassic of the Adabofoloceras reached the Iberian Basin during Northeastern Iberian Chain (E. Spain); a synthesis. Seminar. Paleonto!. Univ. Zaragoza (SEPAZ), 2: 5-33. this maximum depth interval. 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