Antapical horns

Clngular lagellum

Clngular lagellum

(9) Crest

Clngulum

Crest and clngulum

Central body

Fig. 10.6 Examples of cyst from the Prorocentroidia and Bilidinea. (a) Recent Prorocentrum (Prorocentroidia), about x350. (b) Recent Gymnodinium (Gymnodinoidia), about x350. (c) Fossil Dinogymnodinium (Gymnodinoidia). (d) Recent Polykrikos (Bilidinea), about x500. (e) Recent Noctiluca (Noctilucea), about x180. (f) Fossil Nannoceratopsis (Bilidinea), about x680. (g) Recent Ornithocercus (Bilidinea), about x275. ((a) From Chapman & Chapman 1973; (b) from Kofoid & Swezy 1921; (d) after Dodge 1985; (f) based on Sarjeant 1974; (g) based on Barnes 1968.)

food chains of the marine realm. The autotrophic forms thrive in areas of upwelling currents that are rich in nutrients such as nitrates and phosphates, whilst they are rarely found alive below 50 m depth because of their need for light. Flagella locomotion is employed in bringing them to the surface at night and withdrawing them to greater depths in the day because they must avoid harmful ultraviolet light. In Mesozoic, Cenozoic and Recent dinoflagellate assemblages only a few taxa appear to have palaeoecological or palaeo-biogeographical significance. Dale (1976) described dinoflagellate cyst ecology and discussed the geological implications. Of the primary ecological factors one of the most important for controlling cyst assemblages is sea surface temperature. As a whole, the group has a wide temperature tolerance (1-35°C) with an optimum for most species of 18-25°C. Ceratium (Fig.10.7d) shows temperature-related morphological variability particularly in the length and angle between the antapical horns.

Dale (1976) noted a change of only a few degrees might be sufficient to cause differentiation into bio-geographical provinces. One of the most important temperature boundaries controlling the distribution of dinoflagellate cysts in the Northern Hemisphere occurs between the main bodies of cooler and warmer water in the North Atlantic Ocean. This boundary lies between Cape Cod and Nova Scotia (42-43°N) and between the English Channel and southwestern Norway (Dale 1983; Taylor 1987). Dale (in Jansonius & McGregor 1996, vol. 3, pp. 1249-1275) described the distribution of selected cyst assemblages compared to the modern biogeographical zones (polar, subpolar, temperate and equatorial) for the Atlantic Ocean. In this some species range from pole to pole, whilst others are restricted to the zones and have obvious applications in biogeographical and climate studies. On a global scale modern dinoflagellates occupy broad latitudinal low-, middle- and high-latitude zones (Taylor 1987).

Fig. 10.7 Various peridinoid dinoflagellates mentioned in the text.

(a) Protoperidinium theca, about x350.

(b) Protoperidinium cyst, about x350.

(c) Operculodinium cyst, about x80.

(d) Ceratium cyst, about x500.

(e) Wetzeliella cyst, about x350.

(f) Suessia cyst, about x350. ((a), (b), (c), (e) After Edwards 1993 in Lipps 1993; (d) from Evitt 1985; (f) after Tappan, 1980.)

Fig. 10.7 Various peridinoid dinoflagellates mentioned in the text.

(a) Protoperidinium theca, about x350.

(b) Protoperidinium cyst, about x350.

(c) Operculodinium cyst, about x80.

(d) Ceratium cyst, about x500.

(e) Wetzeliella cyst, about x350.

(f) Suessia cyst, about x350. ((a), (b), (c), (e) After Edwards 1993 in Lipps 1993; (d) from Evitt 1985; (f) after Tappan, 1980.)

Dinoflagellates can tolerate a wide range of salinities and are found in lakes, ponds and rivers. Certain genera, such as Gymnodinium (Fig. 10.6b) and Peridinium (Fig. 10.2e,f), are found in both fresh and salt water, although the majority of species are marine and show optimal growth at salinities of 10-20%o. Recent experiments on dinoflagellate cultures show that, for a single species, the size and morphology of the cyst may vary considerably with salinity. The greatest variation lies in the number, density and structure of the processes. Dale (1983) described similar effects in the morphology of the resting cyst of Lingulodinium from the Black Sea. Other examples can be found in Ellegaard (2000) and Hallett & Lewis (2001).

Autotrophic species live in the photic zone where trace element availability limits their productivity. Cyst-forming species live almost exclusively in marine environments, particularly in shallow coastal waters. Sudden blooms of dinoflagellates, called red tides, may occur under optimal conditions and the build up of toxins can kill great numbers of fish and invertebrates.

Planktonic forms with a predatory or parasitic mode of life are usually unarmoured and belong mostly to the Subclass Gymnodinoidia. Others of limited palaeontological interest contain immobile, benthic, colonial forms and the zooxanthellae that live symbi-otically in the tissues of reef-building corals and larger foraminifera.

At present, dinoflagellate cysts are most abundant in sediments from coastal to continental slope and rise settings, with 1000-3000 cysts per gram. There is also a tendency for specific diversity to increase with distance from shore and to be greatest in tropical waters, a pattern reflected in many groups of marine plankton. In modern sediments specific assemblages of dinoflagel-late cysts are known from estuarine, nearshore, neritic and oceanic environments. Ocean currents can be traced in cyst distribution patterns. Mudie (in Head & Wrenn 1992, pp. 347-390) documented inshore-offshore trends in transects across the temperate, subarctic and arctic margins of eastern Canada and mapped the distribution of selected dinoflagellate cysts in the northwestern Atlantic Ocean.

Modern ocean currents influence the distribution of dinoflagellate cysts, and the marine microplankton as a whole. Matthiessen (1995) reported the transport of cysts by currents in the Norwegian-Greenland Sea. Mudie & Harland (in Jansonius & McGregor 1996, vol. 2, pp. 843-877) noted the warm water of the North Atlantic Drift into the eastern Arctic was responsible for the mixing of dinoflagellate assemblages.

There are several inherent problems in interpreting the palaeoecology of fossil dinoflagellates. Firstly, those of pre-Quaternary age are not easy to relate to taxa of known habit, although lineages can be traced in a few cases. Secondly, many dinoflagellates do not encyst and therefore leave no fossil record. Thirdly, dinoflagellate cysts may sink and drift to be preserved at depths and conditions beyond the tolerance of the species. Some studies however suggest a strong correlation between cyst assemblages from the sea floor and the overlying water-mass, suggesting little post-mortem transport; however many contradictory examples are also known.

The distribution and ecology of Recent and Quaternary dinoflagellates are reviewed more fully by Williams (in Funnell & Reidel 1971, pp. 91-95, 231-243; in Ramsay 1977, pp. 1288-1292), Wall et al. (1977) and Harland (in Powell 1992, pp. 253-274). Additional useful modern syntheses can be found in Fensome et al. (in Jansonius & McGregor 1996, pp. 107-171) and Stover et al. (in Jansonius & McGregor 1996, pp. 641-787).

Classification

Kingdom PROTOZOA Subkingdom DICTYOZOA Phylum DINOZOA Subphylum DINOFLAGELLATA

At one time many dinoflagellate cysts were classed with the problematic hystrichospheres. Evitt (1961, 1963) demonstrated that some of these were true dinoflagellate cysts, designating the remaining problematica to the group Acritarcha.

The classification of dinoflagellates commonly represented in the fossil record is outlined in Box 10.1 and follows that proposed by Cavalier-Smith (1998). A major reclassification of the dinoflagellates by Fensome et al. (1993b) independently created the taxon Dinokaryota but differs in that it includes all dino-flagellates that have histones in at least one stage of their life cycle. Since six of the eight classes of dinoflagellates are totally non-photosynthetic, plus about half the species in the remaining two classes, it seems more appropriate to treat the whole phylum under the Zoological rather than Botanical Code of Nomenclature. The classification of living forms takes account of molecular sequence data, position of flagellar insertion, predominant habit (e.g. mobile and flagellate, mobile amoeboid, immobile solitary or immobile colonial), presence of armour, tabulation, shape and sculpture of the motile cell. Fossil dinoflagellate cysts are classified according to cyst type, reflected tabulation, archaeopyle position, shape and sculpture (see Fensome et al. 1993a).

Was this article helpful?

0 0
Freehand Sketching An Introduction

Freehand Sketching An Introduction

Learn to sketch by working through these quick, simple lessons. This Learn to Sketch course will help you learn to draw what you see and develop your skills.

Get My Free Ebook


Post a comment