Rend. Fis. Acc. Linceis. 9, v. 9:201-211 (1998)

Paleontologia. — Biostratigraphic, paleobiogeographic and paleoecological implicationsin the taxonomic review of Corallinaceae. Nota (*) di Daniela Basso, Patrizia Fravega,Michele Piazza e Grazia Vannucci, presentata dal Socio M. B. Cita.

Abstract. — Some fossil species of nongeniculate coralline algae are synonyms of living species.These synonimies allow us to use stratigraphic distributions of Phymatolithon calcareum (Pallas) Adey andMcKibbin 1970 (Oligocene - Recent) and of the Mediterranean endemics Lithophyllum racemus (Lamarck)Foslie 1901 (Aquitanian - Recent) and Lithothamnion valens Foslie 1909 (Priabonian - Recent) to clarifytheir paleobiogeography. It also illuminates the bionomy and paleoenvironments of benthic Mediterraneanbottoms dominated by unattached corallines. The changes in species and morphologies in the coatingsequence of rhodoliths, from nucleus to periphery, document sequential environmental changes.

Key words: Biostratigraphy; Paleobiogeography; Paleoecology; Corallinaceae; Taxonomy.

Riassunto. — Implicazioni biostratigrafiche, paleobiogeografiche e paleoecologiche nelle revisioni tasso-nomiche di Corallinaceae. Alcune specie fossili di Corallinaceae non genicolate sono sinonimi di specieviventi. Queste sinonimie permettono di utilizzare la distribuzione stratigrafica di Phymatolithon calcareum(Pallas) Adey and McKibbin 1970 (Oligocene-Recente), come anche di Lithophyllum racemus (Lamarck)Foslie 1901 (Aquitaniano-Recente) e Lithothamnion valens Foslie 1909 (Priaboniano-Recente), due specieendemiche del Mediterraneo, per chiarirne l’evoluzione paleobiogeografica. Queste sinonimie tra speciefossili ed attuali e quindi il significato attribuibile ai fossili nell’ambito della bionomia bentonica mediter-ranea (almeno dal tardo Miocene) consentono di ricostruire con maggior dettaglio alcuni paleoambienti difondo mobile dominati da queste corallinacee. In presenza di rodoliti multispecifiche, e possibile tracciarel’evoluzione del paleoambiente attraverso la sequenza di accrescimento delle rodoliti stesse.

Introduction

The application of modern botanic methods (i.e., decalcification of the thalli, studyof the characters of the reproductive organs and of the vegetative thallus with opticand scanning electron microscopy, etc.) in revising the type collections (e.g., Philippi’s,Kutzing’s) of red calcareous algae (Rhodophyceae, Corallinales) has necessitated consid-erable changes in systematics, including the definition of several genera. However, thedelimitation of most species is still undefined (Wilks and Woelkerling, 1994; Womer-sley, 1996: pp. 158,159; Woelkerling and Campbell, 1992; Woelkerling and Harvey,1993; Irvine and Chamberlain, 1994: p. VI).

This revision process involved the paleobotany, that has been induced to use thesame diagnostic characters accepted by the botanists, whenever they are preserved andobservable in the fossil (Braga et al., 1993). The studies of fossil corallines, theircomparisons with extant species and the revision of types are very important in thiscontext (Braga and Aguirre, 1995; Basso et al., 1996).

(*) Pervenuta in forma definitiva all’Accademia il 23 giugno 1998.

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Many species of fossil Corallinaceae were newly described by the algologists of theUniversity of Genova (i.e., M. Airoldi, S. Conti, V. I. Mastrorilli) from 1930 to 1970.Studies and revisions on the fossil material of the original collections are in progress(Basso et al., 1997, 1998); the first results, concerning the conspecificity of some fossiland living taxa, have relevant biostratigraphic, paleobiogeographic and paleoecologicalimplications. In this paper, the biostratigraphy of Lithophyllum racemus (Lamarck) Foslie1901, Phymatolithon calcareum (Pallas) Adey and McKibbin 1970, and Lithothamnionvalens Foslie 1909 and synonyms are summarized from published data (Basso et al.,1996, 1997). On the basis of the resulting stratigraphic and geographic distributions,a first attempt is made to clarify the paleobiogeography of these species (hypothesison origin and diffusion). Moreover, the application of the autoecology of the livingspecies to the fossil synonyms is shown to provide a greater detail in paleoecologicalreconstructions.

Biostratigraphic synthesis

The classification of fossil corallines species has been troubled by historical andartificial demarcation between the biologic and paleontological taxonomy, along withthe difficulty in the identification of sexual specimens. The validity of any speciesneeds to be confirmed through the study of relevant type material and comparisonwith other species (living and fossil) of the same genus. Following this procedureBasso et al. (1996) could establish that the fossil species Lithophyllum viennotii Lemoine1929 (tetrasporangial plant) and Lithophyllum microsporum Maslov 1962 (carpogonial/carposporangial plant), and the living Lithophyllum duckeri Woelkerling 1983 are con-specifics of the living Mediterranean endemic species Lithophyllum racemus (Lamarck)Foslie 1901. Therefore L. racemus is distributed from Aquitanian to Recent (fig. 1; seeBasso et al., 1996 and Vannucci et al., 1997 for details on stratigraphic distribution ofeach species).

The species Lithothamnion operculatum (Conti) Conti 1950 and Lithothamnion ramo-sissimum (Gumbel non Reuss) Conti 1946, both known only from the fossil record,resulted to be conspecific with the living species Phymatolithon calcareum (Pallas) Adeyand McKibbin 1970 and Lithothamnion valens Foslie 1909, respectively (Basso et al.,1997). Therefore, P. calcareum is distributed from the Oligocene to the Recent (fig. 1,see also Vannucci et al., 1997) and the age of L. valens spans from the Priabonian toRecent (fig. 1; see Basso et al., 1997 and Fravega et al., 1997 for details).

A long stratigraphic range is common among corallines. As already pointed out(Braga and Aguirre, 1995), available information and in-progress researches confirmthat corallines are of little age value.

Paleobiogeographic remarks

According to Basso et al. (1996) the living Lithophyllum racemus seems to be aMediterranean endemic species, since the quotation of Traill (1892, in Irvine and Cham-

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Fig. 1. – Stratigraphic distribution of the species Lithophyllum racemus, Lithothamnion valens andPhymatolithon calcareum.

berlain, 1994) from the Atlantic requires further confirmation. As far as we know, noAtlantic species have affinities of habitus and anatomy with the Mediterranean L. race-mus. On the contrary, some Indo-Pacific species of Lithophyllum (such as L. okamuraiFoslie 1900 and L. kotschyanum Unger 1858) show many analogies with both anatom-ical and macroscopic features of L. racemus (habitus, shape and dimension of branches,single layer of primigenous filaments of cells; Masaki, 1968, for L. okamurai; Lemoine,1911 for L. kaiseri Heydrich 1897 = L. kotschyanum; see also specimens of L. kotschyanumillustrated by Piller and Rasser, 1996). The similarity of these species with the Mediter-ranean L. racemus has been observed by several authors (Foslie, 1903; Weber-van Bosseand Foslie, 1904; Lemoine, 1911; Segonzac, 1984). After a geographic separation of anatural marine population, as it happened after the mid-Burdigalian closure of the east-ward connection between the western Tethys and the Indo-Pacific area, the separatedpopulations started to diverge genetically from each other, finally resulting in vicariantspecies. Some common anatomical characters remain as evidence of the common an-cestor. On the basis of the available information concerning the modern distribution ofliving species inside and outside the Mediterranean, and on paleontological literature,it is possible to hypothesize that the living Lithophyllum okamurai and/or Lithophyllumkotschyanum might be regarded as vicariant species of L. racemus in the Indo-Pacific area(Basso et al., 1996). Since the closure of the connection between the Mediterranean andthe Atlantic, the species L. racemus also sporadically occurred along the eastern coastof the Atlantic (Orszag-Sperber et al., 1977; Civis et al., 1994, and Braga and Aguirre,

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1995 as L. incrustans, see Basso et al., 1996, p. 286 for discussion) (1). L. racemus sur-vived the Messinian salinity crisis and in the Pliocene was still present on both sidesof Gibraltar (for the Atlantic, see Braga and Aguirre, 1995, as L. incrustans). In thePleistocene we could not find any report of L. racemus in the Atlantic, that is to say thatthe species had the same geographic distribution than the present. In conclusion, theavailable information points toward a paleomediterranean-tethyan origin of L. racemus.

The living Lithothamnion valens is also a Mediterranean endemic species. The oldestoccurrence reported for its fossil representative (= Lithothamnion ramosissimum) is fromthe Priabonian of Northern Calcareous Alps (Moussavian, 1984). In the lower-middleMiocene its East-West distribution ranges from Iraq to the Tertiary Piedmont Basin andCorse. From the upper Miocene on, the species seems to be already restricted to theMediterranean area (Basso et al., 1997), since no data exist for the Atlantic. Data arealso completely missing for the Pleistocene, however, this does not affect the hypothesisof a paleomediterranean-tethyan origin of L. valens.

Phymatolithon calcareum is now present in the Atlantic and in the Mediterranean.Its first appearance is in the Oligocene of the Tertiary Piedmont Basin (Mastrorilli,1968 as Lithothamnium operculatum). The easternmost fossil record of L. operculatum isin the middle Miocene of Carpathian Mountains (Schalekova, 1973). In the Atlantic,it is recorded in the upper Pliocene of Cadiz, Spain (Aguirre et al., 1993), and dataare missing for the Pleistocene. Further investigations on Tertiary and Quaternary algalassociations of the eastern Atlantic region will hopefully clarify the origin of P. calcareum.

Paleoecological remarks

The following examples, based on the above mentioned synonymies, emphasize howthe conspecificity between fossil and living species improves the paleoecological recon-structions within rhodolith-rich stratigraphic successions.

In several modern environments, coralline algae are major components of benthicbiocoenoses and/or their facies (sensu Peres and Picard, 1964) of the modern Mediter-ranean. An overview of the subject can be found, among others, in Peres and Pi-card (1964); Peres (1967, 1982), and Bosence (1983a,b ). A strong abundance ofunattached branching thalli of Lithothamnion valens defines a facies of the Coastal De-tritic Bottom (DC), as well as abundant unattached branching thalli of Phymatolithoncalcareum and Lithothamnion corallioides characterize the maerl facies of DC. There-fore, an algal community dominated by branching thalli of L. operculatum (= P. cal-careum) and L. ramosissimum (= L. valens) could be confidently considered a fossil ana-logue of the maerl and L. valens, facies respectively. Similarly, monospecific rhodolithsmade up of Lithophyllum racemus are «characteristiques exlusives» (sensu Peres and Pi-card, 1964) of the SGCF Biocoenosis (Coarse Sand and fine Gravel Under Bottom

(1) A note of caution should be made for literature listing L. incrustans as synonym of L. viennotiiwithout providing illustration of material (Civis et al., 1994), since L. incrustans should be regarded asseparate species from L. viennotii, which is a synonym of L. racemus (Basso et al., 1996).

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Current) and consequently, a paleoenvironment characterized by abuntant, monospe-cific rhodoliths composed of Lithophyllum viennotii and/or Lithophyllum microsporum (=Lithophyllum racemus) can be interpreted as a fossil SGCF. Some other fossil corallineassemblages have been compared to modern benthic biocoenoses such as the Corallige-nous (C) (Bosence, 1985) and the pralines facies of the DC (Simone and Carannante,1988).

The ecological «optimum» inferred from the modern distribution of the genera hasbeen widely used for the paleoenvironmental reconstruction (e.g. Adey, 1979; Bosence,1983c , 1991; Braga and Martin, 1988; Martin et al., 1993; Perrin et al., 1995). Therisks of this taxonomic uniformitarianism have been already addressed (Bosence andAllison, 1995). The autoecology of a living species, when applied to its fossil repre-sentatives, allows more detailed paleoclimatic and paleobathymetric reconstructions thanthose exclusively based on the genera. A pertinent example is the Pliocene facies ofMasserano, NW Italy, where the abundance of Lithophyllum viennotii and Lithophyllummicrosporum was considered diagnostic of infralittoral environment on the basis of theirbelonging to the genus Lithophyllum (d’Atri and Piazza, 1988). After the identificationof L. viennotii and L. microsporum as L. racemus it is possible to recognize an SGCFaffinity for that paleoenvironment.

The coating sequence of rhodoliths can supply important information on the en-vironmental evolution of given sea bottom (Bosence, 1983c ; Braga and Martin, 1988;Bosence and Pedley, 1982). In tropical and temperate climates, the paleoenvironmentalevolution of the carbonate shelf (Carannante et al., 1988) can be easily traced with anactuopaleontological approach to the evolution of the rhodolith-rich facies (Basso, 1998),through the change in coralline species and morphology across the coating sequence ofrhodoliths.

The co-occurrence of Lithophyllum racemus, Lithothamnion valens and Phymatolithoncalcareum in the coating sequence of the rhodoliths from the Pliocene of Valle San Bar-tolomeo, Alessandria, NW Italy, can be compared with the growth of modern rhodolithsof the SGCF and of the DC (Peres and Picard, 1964). For example, L. racemus, charac-teristic species of the Mediterranean SGCF biocoenosis and common in the DC/SGCFecotone, occurs in the nucleus of modern boxwork rhodoliths in more stable DC bot-toms (fig. 2) (Basso, 1998) and is frequently recognized at the nucleus of multispecificrhodoliths described for the Serravallian of the Tertiary Piedmont Basin, NW Italy(Fravega et al., 1984), the Neogene of Almeria, Spain (Braga and Martin, 1988), andthe Pliocene of Valle San Bartolomeo, NW Italy (Vannucci et al., 1994). Commonly,in modern rhodoliths, these nuclei are overgrown by lumpy thalli of Lithothamnionand the outer coating layers are made of crustose thalli of Mesophyllum lichenoides (El-lis) Lemoine 1928 and Lithophyllum frondosum (Dufour) Furnari, Cormaci and Alongi1996. These successions of different species and algal morphologies from the nucleustoward the periphery of modern rhodoliths indicate that the marine bottoms evolvedfrom unstable (SGCF, maerl facies of DC) to more stable (DC, C) conditions, withthe appearance of Mesophyllum lichenoides and L. frondosum in the outermost layers ofmultispecific rhodoliths (fig. 2).

206 d. basso et al.

Fig. 2. – Types of rhodoliths in the framework of the Mediterranean benthic zonation. The upper arrowrepresents the increasing water energy, the higher the density of the rhodolith, the stronger the currentrequired to move it. Frequent overturning under high water energy generates compact pralines, madealmost exclusively by L. racemus in the SGCF biocoenosis. The facies of the DC is represented by differentstructures of coated grains and rhodoliths. More stable facies can undergo burial under high sedimentaryrates (DE = Muddy Detritic Bottoms). Alternatively, under a low sedimentary rate, neighbouring rhodolithscoalesce and generate a rigid, highly porous, coralline algal framework (Coralligenous «de plateau»).

From Basso (1998).

Fossil materials from Valle San Bartolomeo exhibit very similar features: lumpy thalliof Lithothamnion giammarinoi Fravega, Piazza and Vannucci 1993 overgrow the nucleusof Lithophyllum racemus; crustose thalli of Mesophyllum pliocenum Segonzac 1972 andlaminar thalli of Lithophyllum cf. frondosum occur in the peripheral outer portions ofrhodoliths (fig. 3) (2).

Another example (Vannucci et al., 1994) is offered by some rhodoliths from thePliocene of Pavone (Alessandria, NW Italy). They are made of fragments of unattachedthalli of Lithophyllum racemus and Lithothamnion valens, frequently encrusted by Titano-derma dublancqui Lemoine 1917 and Lithophyllum cf. frondosum (fig. 4).

(2) Lithophyllum cf. frondosum was identified as Lithophyllum viennotii in Vannucci et al., 1994; seeBasso et al., 1996 for a discussion of the separation of L. frondosum = Lithophyllum grandiusculum (Montagne)Woelkerling, Penrose and Chamberlain, 1993 from Lithophyllum racemus.

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Fig. 3. – 1: Multispecific rhodolith. Pliocene, Valle San Bartolomeo (Central Piedmont, NW Italy).Sez. V2-I (scale bar = 1 cm). 2- 4: are details (from nucleus to outer surface) of the coating sequenceof this rhodolith. 2: L. racemus (previously identified as L. viennotii) forms the nucleus and is encrustedby Lithothamnion giammarinoi (scale bar = 200 µm). 3: L. giammarinoi overgrown by Titanodermadublancqui and Mesophyllum pliocenum (scale bar = 200 µm). 4: Carpogonial/carposporangial plant ofL. giammarinoi overgrown by T. dublancqui, which is encrusted by a crustose thallus of Lithophyllum cf.frondosum (previously identified as L. viennotii). Sexual thalli of M. pliocenum form the outermost layer

(scale bar = 200 µm).

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Fig. 4. – 1: Facies characterized by unattached thalli, dominated by L. racemus (previously identified asL. viennotii) and L. valens (previously identified as L. ramosissimum). Pliocene, Pavone (Alessandria, NWItaly). Sez. VT-10 (scale bar = 1 cm). 2: Detail of 1. Small rhodolith with nucleus made of a branchingthallus of L. valens overgrown by T. dublanqui which is encrusted by a thin thallus of L. cf. frondosum(previously identified as L. viennotii) (scale bar = 200 µm). 3: Particular of the area delimited in 2. T.

dublanqui encrusted by L. cf. frondosum (scale bar = 100 µm).

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The progress of studies on modern algal communities often enlightens peculiaraspects of modern marine environments that can be hardly placed in the bionomicschemes. Their better knowledge will greatly improve the accuracy of paleoenvironmen-tal reconstructions.

Acknowledgements

This paper has greately profited of critical reading by I. Premoli Silva, University of Milan, and A. G.Fischer, University of Southern California, Los Angeles.

This work was supported by MURST 60% funds.

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Pervenuta il 2 aprile 1998,in forma definitiva il 23 giugno 1998.

D. Basso:Dipartimento di Scienze della Terra

Universita degli Studi di MilanoVia Mangiagalli, 34 - 20133 Milano

P. Fravega, M. Piazza, G. Vannucci:Dipartimento di Scienze della Terra

Universita degli Studi di GenovaCorso Europa, 26 - 16132 Genova