REVIEW OF

PALAEOBOTANY AND

PALYNOLOGY ELS E V IE R Review of Palaeobotany and Palynology 94 (1996) 1-9

Reassessment of Palaeothamnium Conti, 1946 (Corallinales, Rhodophyta)

Ju l io A g u i r r e a , , , J u a n C. B r a g a a W e r n e r E. P i l l e r b

a Departamento de Estratigrafia y Paleontologia, Universidad de Granada, Campus Fuentenueva, 18002 Granada, Spain b Institutfar Paltiontologie, Universiti~t Wien, Geozentrum, Althanstrasse 14, A-I090 Vienna, Austria

Received 19 June 1995; revision 22 January 1996; accepted 25 March 1996

Abstract

Palaeothamnium has been identified in many algal associations from the Late Cretaceous to the Miocene since its definition by Conti (1946b). Revision of Conti's original collection and of new material from the type localities in the Vienna Basin indicate that the developmental stages of the conceptacles of the type species P. archaeotypum are common in many other present-day and fossil coralline algae belonging to different genera of the subfamily Melobesioideae and therefore cannot be considered a diagnostic generic character. The presence of flat epithallial cells and long subepithallial initials together with a well-developed peripheral region in the type species leads us to consider Palaeothamnium as a younger heterotypic synonym of Lithothamnion Heydrich, 1897. However, according to modem melobesioid taxonomy, its attribution to Synarthrophyton Townsend, 1979 cannot be completely excluded. In addition, the type species of Palaeothamnium must be treated as a younger heterotypic synonym of Lithothamnion ramosissimum (Reuss, 1847) Piller, 1994.

1. Introduction

Conti (1946b) established the genus Palaeo- thamnium, based on P. archaeotypum Conti, 1946b (P. archaeotipum and P. arcaeotypum are ortho- graphic variants used by Conti in the same description), from Middle Miocene limestones (Leithakalk/Leitha Limestone) from the Vienna Basin. According to Conti (1946b, p. 46) the only diagnostic character of Palaeothamnium is the nature of the conceptacles. When describing the conceptacles of P. archaeotypum, Conti (1946b) especially remarks .an several developmental stages in the formation of the sporangial conceptacles. He deduced these stages from independent concep-

* Corresponding author.

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tacles in a single plant, interpreting the preserved structures in terms of aborted stages of a sequence of cytological change:

(1) In a first stage, some cells in an area of the perithallium elongate, giving way to a "fusiform" structure.

(2) Inside this structure, two or three cells from each filament fuse by reabsorption of the cell walls, producing long single "cells" ("bastoncelli").

(3) These long "cells" become arranged in 3-5 contiguous groups, increasingly differentiating into small, spherical to ovoid bodies.

(4) The walls separating the long cells in each group disappear.

(5) Finally, the cells separating the small bodies disappear as well, leaving an empty conceptacle. The location of pore canals and irregularities in

2 J. Aguirre et al./Review ofPalaeobotany andPalynology 94 (1996) 1-9

the conceptacle roof and bottom are reminders of the position of the former small bodies.

Conti (1946b, pp. 45, 46) considered this devel- opmental process to be intermediate between sim- ilar processes in Lithothamnion and Sporolithon (as Lithothamnium and Archaeolithothamnium, respectively).

Later, Maslov (1956, 1962) described two new species of Palaeothamnium from the Paleogene of Crimea (P. iorii Maslov, 1956; P. kossovense Maslov, 1962) and one from the Miocene of Ukraine (Palaeothamnium sp.). Two other species (P. oligocenicum and P. ravanascoi) were established by Mastrorilli (1967) from the Oligocene of north- western Italy.

Segonzac (1969) observed tetra/bisporangial conceptacle developmental stages giving way to cytological structures which, when fossilized, would produce the structures described by Conti (1946b) in other present-day melobesioid corallines belong- ing to different genera, such as Lithothamnion (as Lithothamnium) and Mesophyllum. Segonzac (1969) concluded that this developmental pattern cannot be accepted as a diagnostic generic character. However, she retained Palaeothamnium as a subge- nus of Lithothamnion (as Lithothamnium) without proposing any definite diagnostic characters for it. In the same paper, Lithothamnium (Palaeo- thamnium) bodellei Segonzac, 1969 was established for coralline algae from the Eocene of the Alpes- Maritimes (southern France).

Piller (1991) discovered the coralline algal mate- rial from the Middle Miocene Leitha Limestone of the Vienna Basin, described by Reuss (1847) as Nullipora ramosissima. Among the different coralline taxa included in the original Reuss samples, Piller (1994) lectotypified N. ramosissima Reuss, 1847 with a specimen ascribable to Lithothamnion. In the systematic description of Lithothamnion ramosissimum (Reuss, 1847), Piller (1994) includes P. archaeotypum Conti, 1946b as a younger heterotypic synonym of this species. However, he does not discuss in detail the status of the genus Palaeothamnium.

Since the establishment of Palaeothamnium, it has been identified in many coralline algal associa- tions from the Late Cretaceous to the Neogene (Moussavian, 1991). In most cases the presence of

fossilized developmental stages of conceptacles has been used as the single diagnostic criterion. In this paper we show that such developmental stages occur in different genera of present-day melobesi- oid corallines and therefore cannot be considered as a diagnostic generic character. We have re-examined Conti's original P. archaeotypum material and studied samples from the Leitha Limestone in the Vienna Basin in order to reassess the status of Palaeothamnium in the light of the recent taxonomy of melobesioid coralline algae (Woelkerling, 1988; Chamberlain and Keats, 1994).

2. Material and methods

2.1. Palaeothamnium archaeotypum--original collection

Conti's original collection from the Leitha Limestone (Middle Miocene, Vienna Basin) is housed at the Dipartimento di Scienze della Terra, Universit~i di Genova, Genoa (Italy). Two thin sections (Collezione Conti--Miocene del Leithakalk, Sez. 92/3E and 91/3D) have been pre- served from the original material of P. archaeoty- pum. Only one, however, contains coralline plants with recognizable characters: sample 92/3E, figured by Conti (1946b, plate III, 3a-c, drawings; plate VIII, la-c, photographs) and here designated as the lectotype of the species.

2.2. Palaeothamnium archaeotypum from type localities

The lectotype of P. archaeotypum comes from the locality of Kalksburg (Fig. 1) close to the southern city limit of Vienna (Site 3 in Conti, 1946a, p. 22). At this locality, however, no Leitha Limestone outcrop is currently available. In the course of the coralline algal flora study of the Vienna Basin most of the available Leitha Limestone outcrops have been sampled by one of the authors (W. E. P.). The outcrop with the best record of P. archaeotypum at the moment is the abandoned upper quarry at Rauchstall- brunngraben (Fig. 1) southwest of Baden (PiUer, 1993, stop 12). This locality corresponds to Conti's

J. Aguirre et al./Review of Palaeobotany and Palynology 94 (1996) 1-9 3

Kalksburg

BADEN []

Rauchstallbrunngraben Lake Neusiedl

i EISENSTADT/ I ' - - I 1"3• 10kin Fenk Quarry J l f I

Fig. 1. Geographical location of the localities Kalksburg, Rauchstallbrtmngraben and Fenk Quarry in the Vienna Basin.

sites 7 and 8, although he did not mention the occurrence of P. archaeotypum at these sites. His samples, provided by Pia, probably came from the conglomeratic part (Baden Conglomerate) where this species is missing. Additional material comes from the Fenk Qual.a'y near Eisenstadt (Fig. 1).

2.3. M e ~ o ~

We have studied numerous thin sections of P. archaeotypum from these localities and carried out SEM examination of fourteen samples following the procedure described by Braga et al. (1993).

Growth-form terminology follows Woelkerling et al. (1993). Nomenclature and orientation of cell and conceptacle dimensions follow Chamberlain et al. (1988).

In accordance with the interpretation of Piller (1994), we will formally describe the P. archaeoty- pum material from its type formation and localities as Lithothamnion ramosissimum (Reuss, 1847) Piller, 1994.

3. Taxonomy

Division RHODOPHYTA Wettstein, 1901 Class m-IODOPnYCBt,~ Rabenhorst, 1863

Table 1 Plumose core, cell and coneeptacle dimensions of L. ramossissi- mum. Examples of P. archaeotypum from its type formation and localities Rauchstallbrunngraben and Fenk Quarry

Rauchstallbrtmngraben Fenk Quarry

Epi tha l l ia l ce l l s n = 14 Length range 4.6-6.6 Mean ± s.d. 5.46 ± 0.63 Diameter range 9.2-13.3 Mean ± s.d. 11.11 ± 1.31 Length/diameter 0.5 ± 0.1

Subepifludlml ceih n = 14 Length range 12.5-20 Mean +__ s.d. 15.22 ± 1.9 Diameter range 9.2-13.3 Mean ± s.d. 11.32 + 1.31 Length/diameter 1.37_+ 0.26

P l u m o s e c o r e

Thickness 154-184.8 Cells n = 32 n = 20 Length range 8-24 11.8-17.3 Mean + s.d. 15.63 ± 3.71 15.05 ± 1.7 Diameter range 6-16 8.2-11.4 Mean ± s.d. 8.67 ± 2.35 9.23 ± 0.66 Length/diameter 1.96 ± 0.37 1.64 + 0.2

Peripheral region Ceils n = 176 n = 68 Length range 4-36 9.1-22.7 Mean ± s.d. 12.99 _ 3.47 15.73 ± 2.1 Diameter range 5.6-14 7.7-12.7 Mean ± s.d. 9.02 ± 1.24 9.91 + 1.05 Length/diameter 1.48 -I- 0.48 1.6 ± 0.4

T / b c o n c e p t a c l e s n = 25 n = 6

Diameter range 123-940 203-460 Meand:s.d. 413±170 313.83+94.56 Height range 95-154 110-130 Mean ± s.d. 127.37 5:17.94 122.5 ± 8.8 Diameter/height 3.51 ± 1.75 2.55 ± 0.67

n = n u m b e r of measures; s,d,=standard deviation; T /b= tetra/bisporangial. All dimensions in gin.

Order CORALLINALES Silva et Johansen, 1986 Family CORALLINACEAE Lamouroux, 1816 Subfamily M~LOnESIOIDI~.~ Bi77ozero, 1885 Genus Litlwt&vnnion Heydrich, 1897 Lithothamnion ramosissimum (Reuss, 1847) Piller, 1994

4

PLATEI

J. Aguirre et at./~evtew oj rataeobotany and Palynology 94 (1996) 1-9

. . ~ r ~ " ~ • ~ \ ~

.1.. Aguirre et al./Review of Palaeobotany and Palynology 94 (1996) 1-9 5

Lectotype: Designated by Piller (1994, p. 184) from the original Reuss specimens, housed in the Geologische Bundesanstalt Wien, Inv. Nr. 1994/8/1. External appearance: This species occurs as small, warty to fruticose ]plants. They grew attached to other coralline plants, other bioclasts, or unat- tached on the sediment. Protuberances are up to 15 mm long and up to 5 mm in diameter. Vegetative anatomy: The thallus is monomerous. Plumose core, up to 185 lam thick (Plate I, 1). Cells have a slightly arched, rectangular shape and measure 8-24 Inn (mean + s.d. = 15.6 + 3.7 l~m) in length and 6-16pro (mean+s.d.=8.7+2.31xm) in diameter (Table 1).

Peripheral region well developed. Filaments bend from the core to run perpendicular to the dorsal surface (Plate I, 1) and are radially orga- nized inside the protuberances. Cell fusions are conspicuous (Plate I, 2). Peripheral region shows a well-developed zonation, especially inside the protuberances, with cells laterally aligned (Plate I, 3, 4). Maximum thickness of zones varies widely from 80 to 225 Bm. In each zone, cell length decreases from bottom to top and from the center to the sides of the protuberance. They vary in length from 36 I~m at the bottom of a zone in the center of a protuberance to 4 lxrn at the top. Cell diameter is less variable, from 5.6 to 141xm ( m e a n + s . d . = 9 _ 1.2) (Table 1).

Epithallial cells arranged in a single layer can be observed in some plants (Plate I, 5, 6).

They are flattened, 4.6-6.6~un (mean+s .d .= 5.5___0.61xm) in length and 9.2 to 13.31xm (mean _ s.d. = 11.1 ___ 1.3 lira) in diameter (Table 1). Subepithallial initials, immediately below the epi- thallial cells, are longer than cells located underneath (Plate I, 5, 6). Conceptacles: We found only multiporate, tetra/bisporangial conceptacles. They a r e / n o t prominent on the thallus surface although it does swell slightly where conceptacles are developed (Plate II, 1, 2). Void conceptacles are rectangular in section (Plate lI, 1), measuring 123-940~tm (mean + s.d. = 413_ 170 Bin) in diameter, and 95-154pm (mean+s.d.=127_+18~tm) in height (Table 1). Low values, however, may correspond to marginal sections of conceptacle chambers. Conceptacles preserved at different aborted stages of development are very common (Plate I, 3, 4; Plate II, 2-6). Elongated cells arranged in a fusi- form structure in section (Plate II, 2, 3, 5) can be interpreted as the remains of early stages of concep- tacle development. Similar structures with small ovoid spaces topped by a pore canal can be interpreted as further steps in conceptacle develop- ment where sporocytes and even tetra/bispores have presumably already formed and some elon- gated sterile cells were already decalcified (Plate I, 4; Plate II, 4-6).

4. Discussion

As mentioned above, Conti (1946b) considered

PLATE I Lithothamnion ramossisimum (Reuss, 1847) Piller, 1994. Examples of P. archaeotypum from its type formation and localities. Leitha Limestone. Badenian (Middle Miocene). Vienna Basin. 1. Plumose core at ventral side and peripheral filaments bending from core to run perpendicular to dorsal surface. FC 14. Fenk Quarry (Faciostratotype of Leitha Limestone). Scale bar = 100 ~xn. 2. Perpendicular section of peripheral filaments. Cell fusions (arrows) are evidenced by cements filling original voids connecting adjacent filaments. ALPAB7.12. Rauchstallbrunngraben. Scale bar = 20 lain. 3. Perpendicular section of a thallus protuberance. Note zonation and lateral cell alignment. Conceptacles aborted at early developmen- tal stages (arrows) concentrate in the outer part of protuberance. R81/6. Rauchstallbrnnngraben. Scale bar= 500 pan. 4. Longitudinal section of peripheral filaments. Cell length decreases from bottom to top in each zone. Conceptacle fossilized at an intermediate stage of development. The void to its left probably represents non-calcified, already formed, sporocyte(s) or tetra/bispores. A 3. Fenk Quarry. Scale bar = 200 Inn. 5. Longitudinal section of peripheral filaments growing to the upper right topped by flat epithallial cells (arrows). Epithallial cells are overgrown by new filaments from the same or another plant. Note subepithallial cells (arrowheads) longer than cells underneath. ALPAB7.12. Rauchstallbrtmngraben. Scale bar= 50 pan. 6. Flat epithallial cells. Poor preservation of epithallial cells is due to weak calcification. Subepithallial cells are longer than cells underneath. ALPAB7.12. Rauehstallbruungraben. Scale bar = 10 Inn.

6

PLATE II

J. Aguirre et al./Review of Palaeobotany and Palynology 94 (1996) 1-9

J. Aguirre et al./Review of Palaeobdt(my ~tdPalynolOgy 94 (1996) 1-9 7

the conceptacle development that he deduced from his samples to be the diagnostic character for Palaeothamnium. Many authors, however, have described similar tetra/bisporangial conceptacle development in present-day melobesioid coralline algae belonging to different genera. The early stages of development recognized by Conti (1946b) [the elongation of ceils in an area that constitutes the conceptacle primordium (Plate II, 2, 3)] have been reported in many coralline species [i.e., in Lithothamnion (Adey and McKibbin, 1970, fig. 4), in Clathromorphum (Adey and Johansen, 1972, figs. 27 and 47), in Phymatolithon (Woelkerling and Irvine, 1986, figs. 13-14; Wilks and Woelkerling, 1994, fig. 5), in Mesophyllum (Woelkerling, 1988, fig. 46; Woelkerling and Harvey, 1992; figs. 32-34, 1993, figs. 5, 21, 27)] Further steps in the concepta- cle development in t: archaeotypum consist of non- calcified or decalcified portions which are pre- served as voids topped by a pore canal and calcified filaments beside or between these voids (Plate I, 4; Plate II, 4-6). In modern melobesioids, tetra/bisporocytes develop interspersed among sterile filaments in early stages of conceptacle development. In later stages, tetra/bisporocytes enlarge and give way to tetra/bispores, i.e., non- calcified elements, 'while interspersed sterile fila- ments degenerate and become decalcified. The fully formed conceptacle chamber may lack calcified elements and, therefore, is only preservable as a void (Plate II, 1). In many melobesioids, however, tetra/bispores coexist with groups of interspersed filaments for some time before the final decalcifica- tion of the latter, which can even remain as semide-

generate structures (Woelkerling, 1988). Good examples of this have been illustrated by Adey (1966, fig. 34) and Woelkerling (1988, figs. 199 and 200) in Lithothamnion, by Chamberlain (1991, fig. 14) in Phymatolithon, and by Woelkerling and Harvey (1993, figs. 6c and 30) in Mesophyllum. If the development of the conceptacle were aborted at this point, it would fossilize as voids topped by a pore canal, corresponding to the original tetra/bispores and their pore canals, between or beside cell filaments, corresponding to the sterile, not yet decalcified filaments (Plate I, 4; Plate II, 4-6). In summary, in many melobesioid species belonging to different genera, conceptacle fossiliza- tion at different stages of their development would result in structures similar to those considered to be diagnostic for Palaeothamnium by Conti (1946b). Therefore, these structures and the conceptacle development they represent cannot be used in delimiting a genus in the Melobesioideae (Aguirre and Braga, 1993), and Palaeothamnium cannot be held to be an independent genus.

P. archaeotypum, the type species of Palaeothamnium, is monomerous, with a noncoax- ial core, a single layer of flat epithallial cells, and long subepithallial initials (Plate I, 1, 5, 6). It is not possible from the preserved epithallial cells to discern whether they were originally flared or not. These vegetative characters would place arch- aeotypum in Lithothamnion, Clathromorphum or Synarthrophyton according to the characters delim- iting genera in the Melobesioideae proposed by Chamberlain and Keats (1994). Nevertheless, Woelkerling (1988, p. 82), following the definition

PLATE II Lithothamnion ramossisimum (Reuss, 1847) Piller, 1994. Examples of P. archaeotypura from its type formation and localities. Leitha Limestone. Badenian (Middle Miocene). Vienna Basin. 1. Void conceptacle with no remains of calcified ceils. R81/4. RauchstaUbrunngraben. Scale bar =200 pro. 2. Conceptaeles aborted, at early stages of development. Elongated ceils are grouped in fusiform conceptacle primordia. R81/6. Rauchstallbrunngraben. Scale bar = 200 ~trn. 3. Detail of elongated ceils in an aborted conceptacle primordium. ALPAB7.12. RauchstaUbrunngraben. Scale bar= 100 lain. 4. Conceptacles fossilized at intermediate stages of development. Voids topped by pore canals are already decalcified structures, probably representing sporocytes or tetra/bispores. R81/6. Rauchstallbrunngraben. Scale bar = 200 pro. 5. Conceptaeles aborted and fossilized at different developmental stages. Conceptacie primordia (bottom) coexist with a conceptacle where decalcified structures are already present (top). R81/6. Rauchstallbrunngraben. Scale bar=200 larn. 6. Horizontal cross section of a conceptacle at an intermediate developmental stage. Decalcified structures (voids) are surrounded by calcified cells. R81/6. Rauchstallbrunngraben. Scale bar=200 lain.

8 J. Aguirre et al./Review of Palaeobotany and Palynology 94 (1996) 1-9

of Adey (1970), includes in the key features of Clathromorphum the presence of more than one layer of epithaUial cells, which can be flat or rounded. Whether this is a consistent character in Clathromorphum probably requires further evalua- tion. On the other hand, the known species of Synarthrophyton include plants with a lamellar, discoidal to encrusting thallus producing some- times lamellate or protuberant branches (Harvey et al., 1994). The thallus consists of a plumose, occasionally coaxial core with filaments increas- ingly perpendicular to the thallus surface near the dorsal surface. These filaments terminate in rounded, occasionally slightly flattened epithallial cells. When branches are developed they show radial organization. Discoidal plants may have "thickenings", peripheral regions both in the ventral and dorsal sides of the core (Harvey et al., 1994). This type of thallus construction differs from that of archaeotypum, in which a plumose core gives way to a well-developed peripheral region with cell filaments terminating in regularly flat- tened epithallial cells. However, these differences in thallus construction, even if remarkable, are not considered diagnostic criteria in delimiting genera in the most recent botanical taxonomy of the subfamily Melobesioideae (Chamberlain and Keats, 1994, Harvey et al., 1994), which is mainly based upon reproductive characters not preserved in fossil corallines. Therefore, according to this modern generic taxonomy of melobesioids, which in any case "should be regarded as somewhat provisonal" (Chamberlain and Keats, 1994), the attribution of archaeotypum to Synarthrophyton cannot be completely excluded.

At the moment, however, we assume that the species archaeotypum Conti, 1946b should be assigned to Lithothamnion and therefore that Palaeothamnium Conti, 1946b has to be con- sidered as a younger heterotypic synonym of Lithothamnion Heydrich, 1897. Piller (1994) implic- itly realized this synonymization when including P. archaeotypum as a younger heterotypic synonym of Lithothamnion ramosissimum (Reuss, 1847) Piller, 1994. This inclusion was based on the coinci- dence of characters, both vegetative and repro- ductive, of the type material of P. archaeotypum and the lectotype of L. ramosissimum (Piller, 1994).

Acknowledgements

We are thankful to Drs. G. Vanucci, P. Fravega and M. Piazza, Genova, for providing access and information about Conti's original collection. F. Fern~indez helped with the SEM photographs and C. Laurin revised the English text. We thank Y. Chamberlain and D. Bosence for helpful comments on the manuscript. The work of J.A. and J.C.B. has been funded by the Spanish D G I C Y T (Project PB93-1113) and the Junta de Andalucia (Group 4076). The work of W.E.P. was supported by the Austrian Science Foundation (Project P 8090-GEO).

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