THE FOSSILISATION POTENTIAL OF DIAGNOSTIC FEATURES
by Michael Rasser
|Index:||Primigenous and core
Primigenous and core filaments
Except for the occurrence of palisade cells which helps to identify Lithoporella, the importance of the basal filament organisation for the generic identification has decreased during the last decade. This is mainly true for the occurrence of coaxial and non-coaxial core filaments in monomerous thalli. The decreasing importance mainly affects the definitions of Neogoniolithon and Mesophyllum, which have traditionally been characterised by a coaxial core (e. g., Woelkerling, 1988). Some authors do not accept the validity of this feature for the identification of both Neogoniolithon (see Penrose, 1992) and Mesophyllum (see Woelkerling & Harvey, 1993), although the type species of both genera show distinct coaxial cores (Woelkerling, 1988). This character is therefore still used in Recent (e. g., Irvine & Chamberlain, 1993) and fossil (Braga et al., 1993; Bassi, 1995a) taxonomy.
Interfilamental cell fusions are diagnostic criteria for separating Recent subfamilies of Corallinaceae in combination with conceptacle perforation (Woelkerling, 1988). Since Braga et al. (1993) showed that they are usually well preserved in fossil material, this character is applied by palaeontologists as well (Bassi, 1995a; 1995b; Braga & Aguirre, 1995; Basso et al., 1997). The recognition of this feature allowed Braga et al. (1993) to transfer Lithophyllum albanense Lemoine (1924) to Spongites albanensis and thus from the subfamily Lithophylloideae to the Mastophoroideae.
Trichocyte occurrence and arrangement have long been used to delimit genera within the Mastophoroideae, predominantly within the Spongites-complex (Woelkerling, 1985). Chamberlain (1983) and Jones & Woelkerling (1984), however, showed that trichocyte occurrence varies within species and is influenced by environmental conditions. Trichocyte occurrences in the peripheral filaments are still used by Braga et al. (1993) to define Neogoniolithon and Spongites.
The length of subepithallial initials with respect to the length of peripheral or postigenous cells subtending them is one of the most important characters used to separate genera within the Melobesioideae (Wilks & Woelkerling, 1994; 1995). As they are usually calcified, this feature can be applied to fossil material as well (Braga et al., 1993). The recognition of subepithallial initials, however, depends on the preservation of the overlying epithallial cells.
The shape of the epithallial cells is an important feature used to identify Lithothamnion and Sporolithon (Woelkerling, 1988). Both of them are characterised by flared but not rounded cell walls. Owing to the kind of preservation, the shape of cells are difficult to recognize. In our material the epithallium can easily be identified and separated from subepithallial initials by large crystals forming the cell walls in the SEM and by its transparency in thin section thin sections.
Conceptacle perforation is a well known traditional feature used for the identification of both Recent and fossil genera (Wray, 1977) and subfamilies (Woelkerling, 1988; Braga et al., 1993). The orientation of conceptacle roof filaments around conceptacle pores is an important character in separating genera within the Mastophoroideae in present-day taxonomy (Penrose & Woelkerling, 1992). Both Neogoniolithon and Spongites are characterised by filaments which are arranged subparallel to the roof surface (Braga et al., 1993; Penrose & Woelkerling, 1992).
Several fossil tetra/bisporangia were described from multiporate conceptacles (e. g., Conti, 1947; Johnson, 1957; Mastrorilli, 1973; Lemoine, 1977; Bosence, 1983). Preserved gametangia and tetra/bisporangia borne in uniporate conceptacles are, however, unknown. This fact forced palaeontologists to interpret uniporate conceptacles as tetra/bisporangial by the lack of multiporate conceptacles in the same thallus, only. In the current study we could indirectly exclude the gametangial nature of uniporate conceptacles in Neogoniolithon sp. Our designation of Spongites is, however, based on the lack of multiporate conceptacles. Consequently, we cannot exclude that both species we referred to Spongites have multiporate conceptacles which were not found owing to the low abundance of specimens in the studied material.
The kind of conceptacle roof formation is another important taxonomic feature. Roofs of multiporate conceptacles are formed by elongations of filaments interspersed between sporangia. After the release of sporangia these filaments in the conceptacle chambers are secondarily decalcified (Woelkerling, 1988) and thus usually unpreservable in the fossil record. Interspersed filaments in conceptacles of Phmyatolithon sp. are preserved in the studied material as the tetra/bisporangia are not released and the conceptacle is buried within the thallus. The current study and the recognition of conceptacle primordia in fossil material (Aguirre et al., 1996) give rise to the assumption that even more reproductive characters are potentially preservable. Further studies in taxonomy of fossil coralline algae will have to focus on this topic.
Created by Michael Rasser