Dinoflagellate Evidence in the Fossil Record

For many years dinoflagellates have been considered primitive and, therefore, ancient organisms, on the basis of their morphology, ultrastructure, and biochemistry (e.g., Margulis 1970; Evitt 1985; Taylor 1987; Withers 1987; Knoll and Lipps 1993). In the fossil record they are recognized by their acid-resistant organic-walled cysts in marine sediments. Unfortunately, the geologic record bears no undisputed fossil dinoflagellate cysts older than Middle Triassic (Goodman 1987; Helby et al. 1987), although there are 22 specimens of the enigmatic and controversial, thermally altered, organic-walled Late Silurian microfossil Arpylorus antiquus (Calandra) Sarjeant, from Tunisia (Calandra 1964; Sarjeant 1978; Bujak and Williams 1981; Evitt 1985; Goodman 1987), and the Devonian Palaeodinophysis altaica, which requires confirmation by further studies (Fensome et al. 1993). The inability of the fossil record to provide a thorough dinoflagellate history has been explained by noting that only 6 of 15 orders of living dinoflagellates produce fossilizable cysts (Goodman 1987). Head (1996)

Figure 21.3 Comparison of diversity of dino-flagellate and acritarch cysts with abundance and frequency of occurrence of triaromatic di-nosteroids over geologic time. A and B, Horizontal lines indicate triaromatic dinosteroid (1 in figure 21.1) amounts in 129 rock extracts in which methyltriaromatic steroids were detected. Values are expressed as percentage of triaromatic dinosteroids in the sum of triaromatic dinosteroids + 2- + 3-methyl-24-ethyl-cholesteroids (1/[1 + 8 + 9] in A) and triaromatic dinosteroids + 4-methyl-24-ethylcholes-teroids (1/[1 + 10] in B). Lower detection limit

Figure 21.3 Comparison of diversity of dino-flagellate and acritarch cysts with abundance and frequency of occurrence of triaromatic di-nosteroids over geologic time. A and B, Horizontal lines indicate triaromatic dinosteroid (1 in figure 21.1) amounts in 129 rock extracts in which methyltriaromatic steroids were detected. Values are expressed as percentage of triaromatic dinosteroids in the sum of triaromatic dinosteroids + 2- + 3-methyl-24-ethyl-cholesteroids (1/[1 + 8 + 9] in A) and triaromatic dinosteroids + 4-methyl-24-ethylcholes-teroids (1/[1 + 10] in B). Lower detection limit is ~10 percent for triaromatic dinosteroids, and samples with ~10 percent are indicated by horizontal marks stacked below the detection limit line. Nonzero amounts below 10 percent are not implied. C, Schematic representations of numbers of (a) dinoflagellate cyst genera (adapted from MacRae et al. 1996) and (b) acritarch genera (adapted from Strother 1996). Circles and dashed lines (c) give frequency of occurrence of detectable triaromatic dinosteroids in samples from each geologic time period.

noted that only 13-16 percent of living species produce preservable cysts. Therefore, absence of dinoflagellate cysts cannot prove that dinoflagellates did not exist (Evitt 1985). Some modern dinoflagellates form acritarchous cysts lacking morphologic features diagnostic of dinoflagellate cysts (e.g., Anderson and Wall 1978; Dale 1978). These taxonomically undiagnostic cysts reinforce the long and widely held hypothesis that at least some acritarchs (incertae sedis organic-walled microfossils that originate in the Precambrian) record the heritage of dinoflagellates.

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