IV. RESULTADOS Y DISCUSIÓN
4.2. Familia y comportamiento entre estudiantes
The patchy pre-Eocene fossil record of diatoms has meant that they have attracted mainly scholarly interest and have, as yet, have few commercial applications. As such, their publication history consists mostly of single papers dealing with diatoms from an individual location. Little integrated work had been undertaken on a taxonomic level (exceptions to this include work by Sims, Ross, Harwood, Barron, Strelnikova and Fenner).
The first Palaeocene species were described in the late 19th century in Schmidt et a l (1874- 1959), Van Heurck (1880-85,1896), Grunow (1884), Witt (1886) and Kitton (1871) and these came almost exclusively from the well-known diatomites in Simbirsk, the Fur Formation (Mors), Franz-Joseph Land and Archangel (see Table 1.1). The first recorded Cretaceous flora was a study by Hanna (1927a, 1934) on the Maastrichtian diatoms of the Moreno Shale, California. This study included good descriptions and photographs of the species encountered and was followed up, in a similar vein, by Long et a l (1946). In the 1950's and 60's Russian diatomists worked on the considerable thicknesses of Palaeocene and Late Cretaceous age diatomites found in the Pre-Polar Ural areas of Siberia. Their chief workers on these deposits were Jouse (1948, 1949a, 1949b, 1951a, 1951b and other papers), Strelnikova (1960, 1964, 1965a, 1965b, 1966a, 1966b, 1968, 1971, 1974, 1975, 1988), Vekschina (1961a, 1961b) and Proschkina-Lavrenko (1949). Many new species were described from these deposits, mostly in short papers, but little integrated taxonomic work was done until Strelnikova (1974). This paper describes work done on sequences from Campanian boreholes and outcrops in Siberia and includes a thorough systematic study which is based on the taxonomic scheme outlined in Hustedt(1930).
From the 1970's onwards the DSDP/ODP programmes started to produce a number of cores from the oceans with Palaeocene and Cretaceous diatoms in them and diatoms of this age were also found in the cores Vityaz-6744, CESAR-6 and Fl-437 as well as in deposits from Seymour Island, the Tonga Trench and some other minor localities. Although many papers
were produced concerning these deposits, little in-depth examination of individual species or overall taxonomy was made.
The most detailed taxonomic studies of Cretaceous and Palaeocene diatoms are to be found in Sims (1986, 1989, 1990, 1994a, 1994b), Sims and Hasle (1987), Sims and Ross (1988), Ross (1972), Ross and Sims (1985, 1987) and Ross and Hasle (1977). These papers study individual species from problem genera, such as Trinacria and Hemiaulus, using picked specimens studied under light microscope and SEM. Many new genera have been erected from these studies and attempts have been made to classify these genera into existing schemes, as well as suggesting key evolutionary trends within certain genera. In addition to these papers, Harwood (1988) also provides a detailed taxonomic list based on the deposits of Seymour Island and Fenner (1991) does the same for several Palaeocene ODP cores as does Homann (1991) and Fenner (1994) for the Fur Formation.
3.2 - DIATOM MORPHOLOGY
Late Cretaceous and Palaeocene age diatom assemblages are almost exclusively dominated by the centric group of diatoms with only a small number of araphid pennate species known. Many of the genera, and some of the species, are still extant as are most of the morphological features seen on the fossil frustules. The following features are most used in identifying species and are illustrated in Figure 3.1.
VALVE SHAPE (Fig.3; A-Q:
All diatoms are bivalve in girdle view when alive. In Late Cretaceous and Palaeocene diatoms, there are three main valve shapes:
A. Actinomorphic diatoms are normally circular or sub-circular in valve view and rectangular with domed, flat or concave in girdle view. Genera in this group include
Coscinodiscus, Stellahma and Stephanopyxis.
B. Polygonal diatoms generally have flat central areas with pronounced elevations or elevated pads on the apices of the valve. In valve view most polygonal diatoms are elongate along one axis, triangular or square shaped. Polygonal diatoms include the genera Hemiaulus, Biddulphia and Trinacria.
— I X / I 1 c ______ / L / \
D>
B
0 6D E
F
G
H
I
J
K
M
CH
Q
R
FIGURE 3.1 - Diatom morphology. VALVE SHAPE: A=actinomorphic, B=
polygonal, C= solenoidal. PERFORATIONS: D= simple pore, E= internai loculate, F= external velum, G= pore field, H= external loculate, internai velum, 1= external velum, internal loculate, J= pore with external velum, K= complex external velum, L= ocellus. OTHER FEATURES: M= labiate process, N=linkirtg mechanism (biddulphid), 0= linking mechanism {Paralia), P= spines on Stephanopyxis, Q= sulcus, R= internal costae, S= undulating sections.
c.
Solenoidal diatoms have one long axis and in valve view the diatom usually tapers towards the ends. In girdle view the valve face is normally flat with no elevations. Pennate genera, including Rhaponeis, Sceptroneis and Incisoria, are characteristic of this group.PERFORATIONS (Fig.3.1; D-K):
These can be either simple pores, which are open holes or canals through the silica wall, or the more complex areolae. Areolae are characterised by the possession of a velum (plural = vela) which is an ornate siliceous plate or mesh covering the internal or external exit of a pore. Areolae may also have a thin siliceous plate with a narrow hole in the centre on the opposing exit to that of the velum. Such areolae are known as loculate. The various combinations of pores, vela and loculae are shown in Figure 3.1.
In addition to these features, there are some genera (e.g. Auliscus) that possess a tight circle of fine pores called an ocellus and others (e.g. Briggera and Sheshukovia) that have a looser pore- field on their elevations called a pseudoocellus.