Dinoflagellata (Dinozoa)

Taxa on This Page

1. Dinoflagellata

Summary

This page briefly describes the morphology, origins and taxonomy of the dinoflagellates.

Introduction

Dinoflagellates are Alveoles: single celled organisms (protists) which are neither animals nor plants though, for nomenclatural purposes they are treated as if they were plants. They are found in most aquatic environments and form a major part of the modern plankton.

"Living dinoflagellates may be autotrophs, phagotrophs, symbionts or parasites. Photosynthetic species (autotrophs) account for about half the number of living dinoflagellate genera. Some species have more than one nutritional strategy; for example, species of Protoodinium are both parasitic and photosynthetic. Free living dinoflagellates are a major component of the marine phytoplankton and thus important primary producers. Some toxic marine species cause paralytic shellfish poisoning Shimuzu 1987; Taylor 1987c), particularly when forming red tides. Symbiodinium and its allies ("zooxanthellae") are photosynthetic symbionts of other protists and invertebrates, notably corals, and play a major role in reef and other marine ecosystems. Dinoflagellates, although most common in marine environments, also inhabit fresh water environments Pollingher 1987), snow, and the interstices of wet sand" (Fensome et al. 1996: 108).

"Many genera are sensitive to such conditions as water salinity and nutrients, and some genera are characteristic of latitudinal oceanic temperature zones; hence, the geographic distributions of dinoflagellates can be important indicators of environmental conditions Dale 1996), not only for present day environments but also for ancient ones. Fossilized dinoflagellate cysts are widespread in Mesozoic-Cenozoic sedimentary rocks" (Moldowan & Talyzina 1998, p. 1168).

Life Cycle

"Among protists, life cycles may be:

1. haplontic, in which the vegetative (i.e. actively feeding and asexually reproducing) cells are haploid, the zygote being the only diploid cell in the life cycle;

2. diplontic, in which the vegetative cells are diploid, the gametes being the only haploid cells in the life cycle; or

3. diplohaplontic, in which there is an alternation of diploid and haploid vegetative generations.

With rare exceptions, dinoflagellates are known, or believed, to have haplontic life cycles.

"The life cycle of most dinoflagellate species involves relatively simple asexual division of one cell into two daughter cells, the process commonly including a shedding of part or all of the parent cell wall. However, more complex life cycles occur, especially among parasitic and symbiotic species, and many free-living dinoflagellates are known to produce cysts ... . A cyst is any nonmotile cell possessing a cell wall (see next section). Some cysts have walls composed of cellulose and are not preservable as fossils; others are fossilizable, having walls composed of a complex organic polymer similar to sporopollenin (see Brooks et al. 1971), termed dinosporin Fensome et al. 1993b). Cysts can be categorized in terms of their function. Among living dinoflagellates, three functional types of cyst are prominent (Dale 1983; Taylor 1990):

1. resting cysts. Resting cysts represent a dormant stage in which normal life processes are greatly reduced. Dinoflagellate resting cysts have, so far, been found to result from sexual fusion; they are thus zygotic resting cysts, termed hypnozygotes. Walls of resting cysts are commonly strengthened by a sporopollenin-like material (dinosporin) and may comprise several layers. Most fossil dinoflagellates are probably hypnozygotes, although this is not directly demonstrable for extinct species.

2. temporary cysts. A motile dinoflagellate cell with a well developed pellicle may, under adverse conditions, shed its flagella and outer wall (including plates, where present) and form a temporary cyst surrounded by the pellicle...

3. vegetative cysts. Vegetative cysts are nonmotile cells surrounded by a continuous wall, probably the pellicle. These cells are metabolically and/or reproductively active, in contrast to resting and temporary cysts. In some dinoflagellates, especially parasitic and symbiotic taxa such as Blastodinium and Symbiodinium, the principal life cycle stage is represented by vegetative cysts. Pyrocystis is an example of a free-living dinoflagellate that passes most of its life cycle as a vegetative cyst.

The sexual process, which can result in a hypnozygote, is known for only one percent of living dinoflagellates (Pfiester & Anderson 1987). However, it may be more widespread than currently observed. As Pfiester& Anderson pointed out, the sexual process has probably been overlooked in many species because: 1) gametes resemble normal cells; 2) fusion is slow and readily confused with division; 3) fusion occurs at night in photosynthetic species; and 4) warty zygotes have been misinterpreted as aberrant cells."

(After Fensome et al. 1996: 108-109.)

Habit

"Although generally motile and biflagellate dinoflagellates may also occur as coccoid cells, amoeboid cells, multinucleate cells, tentacle bearing cells, and filamentous and ribbon-like colonies of cells. Coccoid cells (including most cysts) are nonmotile, thus lacking flagella, and have a continuous wall. Amoeboid cells (e.g. in Stylodinium) may all represent parasitic life cycle stages. Polykrikos is unique among dinoflagellates in having multinucleate cells, each cell bearing several sets of flagella and flagellar furrows. Cells of Noctiluca are also unusual in having a single, small, inconspicuous flagellum and a prominent, food procuring tentacle; these cells contain extensive vacuoles separated by strands of cytoplasm, and are best described as buoyancy regulating, rather than motile. The non-parasitic Dinoclonium and Dinothrix and the tapeworm-like parasitic Haplozoon exist as filamentous and ribbon-like multicellular forms, respectively, during prominent parts of their life cycles." Fensome et al. 1996: 107 (figure references omitted).

Encystment

(...) purpose of encystment... (Evitt p. 13)

Interpretation of Fossil Cysts

Evitt p. 13b

(...) on the other hand...

Fensome's bit about Nannoceratopsiales etc. (p. 155)

"Evitt (1981) cautioned against a literal interpretation of the dinoflagellate fossil record on the basis that few living dinoflagellates produce fossilizable cysts. He concluded that fossil dinoflagellates have only a limited relevance in elucidating the pattern of dinoflagellate phylogeny. However, if there were no dinoflagellate fossils, we would be unaware of the Nannoceratopsiales - the “missing link” between the Peridiniphycidae and Dinophysiphycidae; we would not know that peridinialean and gonyaulacalean tabulations have been separate since Jurassic times; we would know nothing of the early Mesozoic Rhaetogonyaulacineae - a precursor of later gonyaulacaleans and possibly also of the Peridiniales; we would not know that Ceratium-like dinoflagellates existed in the Late Jurassic and that Balechina-like ptychodiscaleans (Dinogymnium and its allies) were present in the Late Cretaceous" (Fensome et al. 1996, p. 155).

Morphology

Anatomical Features

"Living dinoflagellates exhibit a great diversity in form, habit, and habitat that belies their systematic position near the base of the phylogenetic tree of the eukaryotes. Their primitiveness is shown especially by properties of the nucleus, mitotic apparatus, and chloroplast. The nuclear structure (typically with chromosomes permanently condensed) and the mitotic apparatus (with spindles external to the nuclear membrane) are perhaps the most primitive in any eukaryote. The chloroplast structure and the pigment assortment that includes chlorophyll a and c2, but not c1, suggest that only the red algae may be more primitive. However, the general organization of the dinoflagellate cell and extreme specializations to be found in certain taxa hardly match the usual concept of primitive. As an example of a highly specialized organelle, consider the light-sensitive structure, with eye-like succession of lens, fluid-filled "camera", retinoid, and pigment backing, which occurs in a few species (Francis, 1967; Greuet, 1970). Less spectacular but interesting for their widespread occurrence are the vacuole-like pusules, fluid-filled bodies which occur two per cell and possibly have an excretory or assimilative function.

"Chloroplasts may be present or absent, and holophytic, phagotrophic, saprophytic, symbiotic, and parasitic nutritional regimes occur. Planktonic forms inhabit the open sea, coastal and estuarine waters, and rivers and lakes-environments which, collectively, encompass extreme ranges in temperature, salinity, and other aspects of water chemistry" (Evitt 1985, p. 7).

"Dinoflagellates are primarily single-celled organisms variously considered algae, protozoans or, nowadays preferably, protists) that occur typically as motile cells with two flagella Text-Fig. 1). The transverse flagellum is ribbon-like, encircles the cell, is usually within a transverse furrow known as the cingulum or girdle, and is thrown into many waves. The longitudinal flagellum is whip-like, trails posteriorly, is thrown only into a few waves and, proximally, is usually within a longitudinal furrow known as the sulcus. The flagella, together with the unique forward rotating motion which they impart..." (Fensome et al. 1996, p. 107).

"Most dinoflagellates are distinguished by a dinokaryon, a special eukaryotic nucleus involving, among other distinctive features, fibrillar chromosomes that remain condensed during the mitotic cycle. The dinokaryon and other internal cell structures have been recently reviewed in detail by Taylor (1990) and Fensome, Taylor et al. (1993)" (Fensome et al. 1996, p. 107).

Armoured and Unarmoured Forms

(Evitt p. 14)

Orientation and Terminology

"In terms of orientation of the motile cell, that part towards the direction of movement is anterior, while the trailing part of the cell is posterior. The anterior end is the apex and the posterior end is the antapex. The two flagella usually emanate from a single pore, commonly in the equatorial region of the cell. That side of the cell from which the flagella arise is ventral, the opposite side is dorsal. Left and right sides of the cell are then determined by biological convention, as in humans. Although other shapes occur, many motile dinoflagellates have a more or less streamlined configuration, commonly with a single protrusion or horn at the apex (apical horn) and an antapex that may be broadly rounded, or that may have two, commonly unequal, antapical horns. Motile cells may be spheroidal (e.g. Protoceratium), dorsoventrally compressed (e.g. Ceratium), anteroposteriorly compressed (e.g. Ostreopsis), or laterally compressed (e.g. Dinophysis)" Fensome et al. 1996, p. 108).

"That part of the cell (whether cyst, thecate motile cell or athecate motile cell) anterior to the cingulum is termed the episome; that part of the cell posterior to the cingulum is termed the hyposome. Equivalent terms specifically for the cyst are epitract (or epicyst) and hypotract (or hypocyst); equivalent terms specifically for a thecate motile cell are epitheca and hypotheca; and equivalent terms for an a thecate cell are epicone and hypocone" (Fensome et al. 1996, p. 108).

Tabulation

"The complex outer region of dinoflagellate cells (Text-Fig. 2) is termed the amphiesma (see Morrill & Loeblich III 1983) or cortex Netzel & Dürr 1984). Dinoflagellate motile cells are bounded by the cell membrane (plasmalemma). Beneath the plasmalemma, a single layer of vesicles (amphiesmal vesicles) is almost invariably present. The vesicles may contain cellulosic plates (thecal plates) in taxa that are thus termed thecate (or armored); or the vesicles may lack thecal plates, such taxa being termed athecate (unarmored or naked). In athecate taxa, the amphiesmal vesicles playa structural role. In thecate taxa, thecal plates, one of which occurs in each amphiesmal vesicle, fit tightly together (Text-Fig. 5). Thecal plates vary from being thin and difficult to observe under the light microscope to thick and heavily ornamented. Collectively, the thecal plates of a single cell constitute a theca.

"In some athecate dinoflagellates there is a thin discontinuous layer within the amphiesmal vesicles that resembles the plate precursor layer in thecate species. According to Morrill & Loeblich III 1983), the membrane bounding the amphiesmal vesicles may partially break down and this discontinuous layer develops into a continuous layer, the pellicle. Perhaps more commonly, the pellicle develops as a separate layer internal to the amphiesmal vesicles. The pellicle, however formed, consists primarily of cellulose, sometimes with a dinosporin component. In some athecate genera (e.g. Balechina, Ptychodiscus and Noctiluca), the pellicle forms the principal strengthening layer of the amphiesma, and the cells are termed pelliculate. The pellicle is sometimes present beneath the theca (e.g. of Alexandrium and Scrippsiella) and forms the wall of temporary cysts. The pellicle may also be the layer represented by the wall of fossilizable resting cysts. A dinoflagellate is said to have a cell wall if a cellulosic or otherwise strengthened layer - i.e. a theca or pellicle - is present in the amphiesma. Hence, athecate, nonpelliculate cells lack a cell wall whereas thecate motile cells and pelliculate motile and nonmotile cells (including fossil resting cysts) possess a cell wall.

"Conventionally, the term tabulation has been used to refer to the arrangement of thecal plates. However, as thecal plates occur within amphiesmal vesicles, and since there is a morphological continuum between taxa that have thecal plates and those that do not, tabulation can also be conceived of as the arrangement of amphiesmal vesicles, with or without thecal plates. Although each thecal plate occurs within an amphiesmal vesicle (Text-Fig. 2, 6), the plates adjoin one another tightly along linear plate sutures (Text-Fig. 5), usually with the margin of one plate overlapping the margin of the adjacent plate. It is generally assumed that thecal plates are composed of cellulose. Most plates are penetrated by trichocyst pores (see Dodge 1987) which may lie in pits (areolae). The plates may be ornamented, for example, by a reticulum (Text-Fig. 5) or by striae.

"Cell growth, and hence increasing surface area, is accommodated by secondary growth of the plates at one or more of the plate margins. The growth bands thus produced are usually striated at right angles to the adjacent suture (Text- Fig. 5) and have been termed "intercalary bands". However, the term" growth band" avoids confusion with the unrelated term "intercalary plate". Growth bands lack trichocyst pores. Dinoflagellate tabulations can be grouped into six types (Text-Fig. 7), each of which is discussed below."

(After Fensome et al. 1996, pp. 110-111.)

Tabulation Notation Systems

Most popular of the tabulation notation systems is Kofoid's, which is a strictly descriptive notation system. There are others, such as the Evitt-Taylor and Edwards systems. Each has some advantages but they share a common failing in attempting to codify presumed plate homologies within the notation itself. While there may yet come a day when these homologies are so well-understood that they acquire a near-factual status, for the present they remain interpretive and interpretation has no place in a descriptive notation.

Phylogeny and Evolution

Affinities

Some dinoflagellates photosynthesise; they generally possess chlorophyll a and variants of c, and other pigments including carotenes and xanthins. Others, however, are heterotrophic.

Two other phyla thought to be closely related to dinoflagellates are the Ciliophora and the Apicomplexa.

"Ideas on dinoflagellate evolution have been developed by, or summarized in, Taylor (1980), Tappan (1980), Bujak & Williams (1981), Loeblich III (1984) and Goodman (1987). A possible scenario for dinoflagellates, proposed by Fensome, Taylor et at. (1993) is shown in Text-Figure 60.

"From cytological and biochemical evidence, dinoflagellates appear to be an ancient group of protists, most authorities now believing them to have originated in the Late Precambrian (Taylor 1978, 1980; Loeblich III 1984). These earliest dinoflagellates either produced no preservable cysts or generated cysts (acritarchs) whose morphology does not demonstrate their affinity (see Downie 1973; Sarjeant 1974). A study of openings and process distribution in Early Paleozoic acritarchs led Lister (1970) to conclude that some may be the cysts of thecate dinoflagellates. However, the tabulations produced by Lister were speculative, and not convincingly similar to any Mesozoic-Cenozoic or modem tabulations.

"Most workers have accepted that the Late Silurian genus Arpylorus is a dinoflagellate cyst (Calandra 1964; Evitt in van Oyen 1964; Sarjeant 1978b; Stover & Evitt 1978; Lentin & Williams 1981; for a contrary view, see Bujak & Williams 1981). It clearly has plates that can reasonably be interpreted as thecal. However, like Lister’s tabulations, they do not closely resemble any Mesozoic-Cenozoic or modem tabulations’ and the cingulum and sulcus are not prominent as they are in later dinoflagellates. Perhaps Arpylorus offers a fleeting glimpse of an earlier, Paleozoic radiation of dinoflagellates. Possibly more closely comparable with a group of modem dinoflagellates is the Devonian genus Palaeodinophysis Vozzhennikova & Sheshegova 1989). There is at least a superficial similarity between Palaeodinophysis and living dinophysialeans (as well as fossil nannoceratopsialeans) and, if its dinophysialean affinity and stratigraphic distribution are confirmed by future studies, the evolutionary scenario for Mesozoic to Recent dinoflagellates as provided below will require modification;

"Early dinoflagellates may have had a temporary dinokaryon (see Fensome, Taylor et al. 1993), but there is, of course, no proof of this in the fossil record. The temporary dinokaryon of blastodinialeans (e.g. Text-Fig. 3K) and noctilucaleans (e.g. Text-Fig. 3Q, R, T, U) is possibly a relict feature, but living blastodinialeans and noctilucaleans are highly specialized and, apart from their nucleus, are not good models for primitive dinoflagellates" (Fensome et al. 1996, p. 157).

Fossil Record

Dinoflagellates have left a rich, if taxonomically selective, fossil record of organic-walled, calcareous and rare siliceous forms, almost exclusively cysts. Insofar as body fossils are concerned, the record begins with the single occurrence of Arpylorus antiquua, in the Silurian of Tunisia. After that, there is nothing until the Triassic, when fossils begin to become common. By the Jurassic, the group is well-known, well-established, and morphologically diverse.

"Fossil dinoflagellates occur primarily in strata of Late Triassic to Recent age. They are mostly of marine origin, but some fresh water fossils are known. As already noted, most fossil dinoflagellates appear to represent resting cysts or hypnozygotes (termed dinocysts by some workers and, in this work, hereafter referred to as cysts). A cyst becomes fossilizable if one or more wall layers are impregnated with a resistant organic or inorganic substance. Most fossil dinoflagellate cysts have organic walls comprising dinosporin. Calcareous and siliceous cysts may have a fossilizable organic component in their wall, and some "organic-walled" fossil cysts in palynological preparations may represent the organic linings of calcareous cysts (Lentin 1985; Hultberg 1985). Such fossils are thus somewhat analogous to the organic linings of foraminifera" (Fensome et al. 1996, p. 124).

"Cysts are produced inside the dinoflagellate theca (with one possible partial exception, Palaeoperidinium, which is discussed below). Cyst shape may approximate that of the motile cell, involving no long protrusions unrelated to thecal shape; such cysts are termed proximate see Sarjeant 1982c; Text-Fig. 22; PI. 1, Fig. 1-5). Alternatively, the cyst may comprise a more or less spherical central body with processes or crests (PI. 1, Fig. 6-16); such cysts are termed chorate or proximochorate, depending upon the height of the extensions relative to the central body. Although there is a morphological gradation between proximate, proximochorate and chorate cysts, these terms are useful in descriptions" (Fensome et al. 1996, p. 124).

Origins

Although body fossils of dinoflagellates are not recognised until the Silurian, several lines of evidence have indicated that dinoflagellates originated in the Neoproterozoic (Knoll 1996).

• RNA molecular sequencing and examination of mitochondrial cristae of modern organisms (Lipps 1993) suggest that dinoflagellates are older than Foraminifera and Radiolaria, which have been found in Cambrian rocks.

• Biochemical studies confirm the presence of dinoflagellate-specific biomarkers dinosteranes and 4α-methyl-24-ethylcholestane) at least as early as the earliest Cambrian. Reports include:

  • Proterozoic Bitter Springs and Pertatataka Formations, central Australia (Summons & Walter 1990);

  • Nonesuch Formation, North American mid-continent rift (Pratt et al. 1991);

  • lower part of the Upper Riphean (Neoproterozoic) Visingsö Beds, Sweden (ref?);

  • Atdabanian (Early Cambrian), in glauconitic clays from the Lükati Formation of Estonia (Moldowan & Talyzina 1998);

  • Buen Formation in northern Greenland (ref?).

(After Moldowan & Talyzina 1998.)

From the Emsian age (late Early Devonian) Battery Point Formation, Cap-aux-Os Member exposed at Gaspé Bay, Quebec, approximately ten species of acritarchs have been recovered, including Veryhachium, Helosphaeridium, Micrhystridium, Multiplicisphaeridium and Gorgonosphaeridium. "Most are thought to represent cysts of marine phytoplankton (Strother 1996); recent geochemical analyses suggest that many may represent dinoflagellates (Moldowan and Talyzina 1998)" Hotton et al. 2001, p. 195b).

"The presence of dinoflagellate relatives among acritarchs explains the continuous record of dinosteroids from Precambrian to Cenozoic source rocks from numerous localities world-wide" Moldowan & Talyzina 1998, p. 1170).

"Some acritarchs resemble dinoflagellate cysts Margulis & Schwartz 1982; Tappan 1980; Mendelson 1993), but they do not show paratabulation and they have excystments that are different from classical archeopyles of recognised Mesozoic and younger dinocysts. Many acritarch specimens have no excystment structure. However, most modern dinocysts reach sediments before germination (Anderson et al. 1985), and some of these can fossilize without excystment structure formation. Some Ordovician acanthomorphic acritarchs have a double-wall structure (Martin & Kjellström 1973) comparable with that of dinoflagellate cysts. Certain cysts of living dinoflagellates from the order Gymnodiniales lack clearly defined archeopyles or reflected tabulation (Wall & Dale 1968). ... [But, on balance,] the morphological evidence has not been sufficient to establish links between acritarchs and dinoflagellates" (Moldowan & Talyzina 1998, pp. 1168-1169).

Evolution

"The fossilized matter available for paleontological investigation represents less than 1% of organisms that once existed on Earth. A high abundance of related specimens in a particular age suggests that there was an earlier radiation. Various kinds of simply structured, rounded acritarchs and dinoflagellate biomarkers coexist in upper Riphean rocks, although the dinoflagellate affinity of any particular Proterozoic genus requires further investigation.

"Dinosterane-containing acanthomorphic acritarchs are widespread in Lower Cambrian sediments. These results suggest the evolutionary sequence in which dinoflagellate ancestry originated by the Late Riphean ~800 million years ago); specimens with processes became abundant in the Early Cambrian; and finally, the branch of dinoflagellates with classical archeopyles and paratabulation developed in the Middle Triassic" (Moldowan & Talyzina 1998, p. 1170).

"The fossil record of dinoflagellates appears to show evolutionary patterns similar to those of other groups, such as a major adaptive radiation, which occurred in dinoflagellates in the Late Triassic and Early Jurassic. Should these patterns in the dinoflagellate record be taken as normal, or as curious coincidences? The initial Triassic-Jurassic rapid increase of diversity and its subsequent stability, as indicated by fossils, could be explained by the random or environmentally induced "switching on" and "switching off" of the ability to produce fossilizable cysts by long-ranging Phanerozoic taxa. Furthermore, the observed record does not include important taxa such as the Gymnodiniphycidae (except for Suessia and Dinogymnium, the latter appearing clearly to be a "switched on"-"switched off" ptychodiscalean), Dinophysiales (except possibly for Ternia and Palaeodinophysis), Prorocentrales, Noctilucales, Blastodiniales and Phytodiniales. However, the Mesozoic-Cenozoic fossil record shows a pattern that would be expected of a group undergoing an initial adaptive radiation and subsequent stabilization. It is, therefore, reasonable to believe that the observed pattern reflects a real phenomenon. The isolated Paleozoic occurrences of two possible dinoflagellate genera need to be considered in the context of dinoflagellate phylogeny (see below), but their existence does not diminish the striking nature, or disrupt the general pattern, of the Mesozoic-Cenozoic dinoflagellate fossil record.

"Within the dinoflagellate fossil record, examples of adaptive radiations or episodes of "experimentation" at lower taxonomic levels can be recognized. For example, in the early and middle Cretaceous, peridiniaceans had an "experimental" variety of mostly combination archeopyles; in contrast, most later Cretaceous peridiniaceans had a single plate archeopyle comprising the middorsal intercalary .In a second example, the archeopyle of Middle Jurassic gonyaulacaceans also appears to have undergone a period of experimentation. In the Aalenian and early Bajocian, many of the gonyaulacacean genera possessed multiplate precingular archeopyles: e.g. Durotrigia has a 1-5P archeopyle and Dissiliodinium has a 1-6P archeopyle. From late Bajocian onwards, gonyaulacaceans tended to have apical, single plate precingular, or epitractal archeopyles, the last of these being especially common in the Bathonian to early Oxfordian interval.

"The fossil record of dinoflagellates also reveals excellent examples of morphological stasis. For instance, the tabulation among fossil peridiniaceans shows great stability .The earliest known peridiniaceans have a bipesioid tabulation (Text-Fig. 52C’). The vast majority of Cretaceous and Cenozoic fossil organic-walled and calcareous peridiniaceans show not only the bipesioid stacking of the three middorsal plates, but also the same shapes and interrelationships of these plates. For example, the middorsal anterior intercalary plate (2a) is six-sided (hexa), except in the subfamily Wetzelielloideae, in which it is four-sided (quadra). This stability would perhaps not be so remarkable were it not for the great variation in the episomal tabulations of extant peridiniaceans.

"The question thus arises as to whether the stability in tabulation observed among fossil peridiniacean cysts is real or apparent. Is there more consistency in the tabulation of the cyst than of the theca? Did only past peridiniaceans with a bipesioid tabulation produce cysts (Goodman 1987), with the exception of the siliceous, cinctioid lithoperidinioideans? Or are extant peridiniaceans currently undergoing an episode of experimentation in their tabulation, perhaps stimulated by the environmental rigors or opportunities associated with the Quaternary glaciations? The family Congruentidiaceae, which includes Protoperidinium, and which appears to have arisen from peridiniaceans in Late Cretaceous or earliest Cenozoic times, also shows variation in episomal tabulation in the present day. However, the asymmetry of the archeopyle in such fossil genera as Selenopemphix (Text-Fig. 56K, L) indicates that this family has not had a stable bipesioid fossil history.

"Morphological stasis among fossil dinoflagellates is also exemplified by Gonyaulacysta jurassica, which maintained the same tabulation and general shape within a single species throughout the Middle and Late Jurassic. The related cyst Spiniferites ramosus endured even longer, from the Early Cretaceous to the present. Students of evolutionary theory (e.g. Vrba 1980; Eldredge 1985) have suggested that species with long histories are generalists, whereas those with short histories are more specifically tuned to their environment. Thus, Gonyaulacysta jurassica and Spiniferites ramosus could be visualized as generalists of Middle to Late Jurassic and Cretaceous to Recent seas respectively, whereas species with shorter histories, such as Spiniferites septatus and Alisocysta circumtabulata, may have been more specialized" (Fensome et al. 1996, pp. 156-157).

Systematics

Relationships within the dinoflagellates are ...

So, what are the apomorphies which we might use to classify fossil cysts?

We cannot say, for sure, though there are some characteristics which we can confidently say are not apomorphies. The nature of the ornamentation – whether chorate or whatever – has long been, for convenience, used to define form taxa at the generic level. Yet we see these characteristics recurring again and again, in lineages which are patently far removed. Intuitively, we realise that gross features like this, which clearly exercise a considerable effect on the life functions (e.g. flotation characteristics) of the organisms, are highly sensitive to evolutionary pressures, and are therefore likely to evolve quickly and repeatedly. Thus it is that the convenient, gross morphological features beloved of stratigraphers, and for so many years the underpinning of dinoflagellate taxonomy, are quite useless indicators of phylogeny.

... associations of characteristics ...

"Although it is a worthy objective, a widely accepted classification of fossil dinoflagellates at the family level has yet to be devised. Currently, divergent views on principles and criteria are more evident than is any general agreement on results. A comprehensive classification of fossil cysts that originated conceptually with Eisenack (1961) and was elaborated by Sarjeant and Downie (1966) has now been modified by them (Sarjeant and Downie, 1974; Sarjeant, 1974) and by others (Norris, 1978; Tappan, 1980) into several similar arrangements by which fossil cysts are distributed among about 40 families. While based mostly on cyst morphology, these families are regarded by Norris, at least, as approaching phylogenetically defensible entities. In contrast, Evitt (1975b) contended that a few modern genera collectively encompass the affinities of a majority of fossil cysts. In line with that view, but with modifications reflecting more recent interpretations, Table 1 .1 lists 13 families, including nine from the hierarchy of modern taxa, that would appear to accommodate the great majority of fossil cysts admitting that considerable uncertainty must attach to many fossils with highly "generalized" morphology). However, it is not the intent in this volume to pursue the problems of a phylogenetic classification. Instead, with obvious philosophical allegiance to the second approach mentioned above, we will focus attention in Chapter 8 on 17 morphological categories. While they will be defined without strict regard to family boundaries and will include cysts with "generalized" as well as "distinctive" morphology, their approximate correspondence to the families listed at the left in Table 1.1 is shown at the right" (Evitt 1985, p. 27).

Kingdom? Alveoles [Authority?]

Phylum* Dinoflagellata Bütschli 1885

* Dinoflagellates are protists - neither plants nor animals. Mercifully, taxonomy has not yet been cursed with an International Code of Protistan Nomenclature (given that the objective is the same, and the issues to be overcome nearly so, it is quite bad enough that there exists separate botanical and zoological codes) so it is necessary to treat dinoflagellates as one or the other, for the purposes of nomenclature. The botanical code has been settled upon, more or less by historical accident. Botanists frequently refer to the phylum-level taxonomic rank as a "division" - another absurd terminological distinction where there is no difference.

Type: [?] [Authority]

Original Diagnosis: xxx

Description: xxx

Habit: xxx

Distribution Occurrence: xxx

Discussion: xxx

Review of sub-ranks, if appropriate...

Class Dinophyceae [Authority]

Type: [?] [Authority]

Original Diagnosis: xxx

Description: xxx

Habit: xxx

Distribution Occurrence: xxx

Discussion: xxx

Evitt pp. 26-27:

Class Dinophyceae - pyrrhophytes in which one flagellum is whiplike and extends longitudinally, while the second is ribbon-like and follows a circular path in a plane about at right angles to the first. .

Order Prorocentrales - dinoflagellates in which the flagella are inserted terminally (desmokont condition), the longitudinal one extending in advance of the cell, and the transverse one encircling the other anterior to the cell body. Some forms have a cellulosic theca of distinctive structure. Preservable resting cysts are unknown and there is no certain fossil record, although the order is conceivably represented by some of the organic-walled fossils currently regarded as acritarchs. Representative genera: Exuviaella (nonthecate), Prorocentrum thecate).

Order Dinophysiales - dinoflagellates having the transverse flagellar furrow near the anterior limit of the cell; cell normally shows moderate to strong lateral compression; two lateral plates of the cellulosic theca are much larger than any others. Preservable resting cysts are unknown and there is no unequivocal fossil record. Representative genera: Dinophysis, Ornithocercus .

Order Peridiniales - dinoflagellates having the transverse flagellar furrow normally located within the medial third of cell length; theca composed of several tens of cellulosic plates organized in several series paralleling the transverse furrow. Preservable resting cysts are found in some living species and there is an extensive fossil record. Representative living genera: Peridinium, Gonyaulax, Ceratium. Representative fossil genera: Deflandrea, Gonyaulacysta, Odontochitina. Silurian, Triassic-Holocene.

Order Gymnodiniales - dinoflagellates having the transverse flagellar furrow usually located within the medial third of cell length; cellulosic thecal plates lacking or (rarely) thin, but corresponding vesicles more numerous than typical for Peridiniales, small, and all of about similar size. Preservable resting cysts are known in a few living species. Moderate fossil record of cysts and distinctive sporopollenin coverings of possibly motile cells. Representative living genera: Gymnodinium, Polykrikos. Representative fossil genera: Dinogymnium, ?Distatodinium, ?Suessia. Triassic-Holocene.

Order Nannoceratopsiales - dinoflagellates having the transverse flagellar furrow near anterior extremity of cell; cell compressed laterally as in Dinophysiales; tabulation of inferred theca similar to Peridiniales in anterior part, similar to Dinophysiales in posterior part. Fossil; sole genus: Nannoceratopsis. Jurassic.

Class Ebriophyceae - nonphotosynthetic, biflagellate, free-living pyrrhophytes, lacking a resistant external covering but having a fossilizable internal siliceous skeleton. Representative genus: Ebria. Geologic range: Cretaceous to Holocene.

Class Ellobiophyceae - attached parasitic pyrrhophytes without known fossil record.

Class Syndiniophyceae - intracellular parasites without known fossil record.

Conclusion

Further Information

"Significant works on living dinoflagellates include books edited by Spector (1984) and Taylor (1987a) and monographs by Sournia 1986; an overview of marine taxa) and Popovsky & Pfiester (1990; an overview of nonmarine taxa). Dodge (1985) published an atlas of scanning electron photomicrographs of extant dinoflagellates. Fossil dinoflagellates have been discussed in detail by Evitt (1985). Sarjeant 1974) and Edwards (1993) provided overviews of living and fossil dinoflagellates, Williams (1977, 1978) surveyed fossil dinoflagellates, Dale (1983) and Sarjeant et al. (1987) reviewed the morphology and ecology of dinoflagellate cysts with emphasis on the fossil record, and Fensome, Taylor et al. (1993) treated the classification and evolution of both fossil and living dinoflagellates. Several catalogs and indices produced in recent decades include: the catalog series initiated by Eisenack & Klement (1964) , with subsequent issues by Eisenack (1967), Eisenack & Kjellström (1971, 1972, 1975a, b, 1981a, b) and Fensome, Gocht et al. (1991, 1993); the indexes of Lentin & Williams (1973, 1975, 1977, 1981,1985, 1989, 1993); and several compendia of genera, including Stover & Evitt (1978), Artzner et al. (1979), Wilson & Clowes (1980) and Stover & Williams (1987)" (Fensome et al. 1996, p. 107).

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Further Reading

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• Definition: Palynology

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