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Thursday, February 22, 2007

now lets see what was in the oceans in the time of jurassic Period !IRVIN

There must surely have been something special about the Jurassic oceans. Of the dozen or so types of planktonic organisms with a fossil record, at least four either first evolved or experienced massive radiation during Jurassic: coccolithophorids (evolved latest Triassic), diatoms (evolved Late Jurassic), dinoflagellates (radiated Jurassic), planktonic foraminifera (?evolved or radiated Jurassic), and ostracodes (radiated Jurassic). Most plankton groups experienced greater prominence yet in the Cretaceous. However, there seems to have been -- literally -- something in the water during the Jurassic. That "something" may simply have been lots of free calcium with which to build shells and tests; but, that would not explain the ostracodes and microcrustaceans that also seem to have found the Jurassic seas particularly congenial.
Nor were these the only marine groups who left Jurassic microfossils. The "modern" coralline group of the Rhodophyta (red algae) evolved in the Jurassic. They are called modern to distinguish them from the Paleozoic corallines. The current view is that the modern and Paleozoic corallines are unrelated. However, the otherwise unidentifiable Middle to Late Jurassic Iberopora may be a late member of a transitional taxon. Schlagintweit (2004). It would be useful to be more certain, since it has been suggested that the "something in the water" was the rhodophytes themselves -- or, rather, their chloroplasts. All of the new photosynthetic forms of the Jurassic were "red," with chloroplasts of the red algae type, with chlorophyll c, rather than the chlorophyll b of green plants and green algae (Chlorophyta). Falkowski et al. (2004). It is almost certain that these chloroplasts were "adopted" from red algae by some secondary symbiosis, rather than by descent from the Rhodophyta. Grzebyk et al. (2003).
Both of the last cited papers are from the Coastal Ocean Observatory Laboratory at Rutgers so (although it pains us to fall victim to so obvious a linguistic ploy) we'll refer to them as the COOL group. The COOL workers offer two reasons to explain why the ocean is "red." First, the Mesozoic rhodophyte chloroplast was a sturdy, self-reliant sort of plastid which had retained a much greater amount of its own DNA, and thus had greater genetic independence from its host. Consequently, by the Mesozoic, it was much easier for the red chloroplast to trade symbionts than it might have been for some debased, decadent green chloroplasts, which had surrendered most of its genetic control to its hosts. Second, the COOL group notes that red and green species are associated with different trace element requirements. The "red" elements are cadmium, cobalt and manganese, while the "green" elements are copper and iron. This leads the COOL group into a somewhat confused discussion of ocean anoxia and its effects on trace element availability. We suggest that they are correct about trace elements, but for the wrong reasons. Marine iron concentrations are largely dependent on continental weathering. Iron is high when the winds drop iron-bearing dust weathered from barren, arid inland areas. The rising seas and humid, equable conditions of the Mesozoic strongly reduced the availability of marine iron. It is not really necessary to invoke anything more complicated; however, we may also note that sulfides produced at the highly active Mesozoic mid-ocean ridges would also draw down dissolved iron as insoluble pyrite.
The Jurassic was also, in a small way, a good time for acritarchs. Acritarchs are just curiously shaped organic casings, without any particular phylogenetic identity. The Jurassic variety are probably some type of radiolarian-like protist and may have nothing at all to do with the Paleozoic acritarchs. For radiolarians of the more conventional type, the Jurassic was also favorable. The Jurassic radiation of radiolarians was largely a radiation of the Spumellaria in the latter half of the Jurassic. Pantanellium, shown in the image, is a rather typical spumellarian. There is some speculation that this Late Jurassic recovery from a long period of decline may have been due to the availability of planktonic foraminifera as a food source. However, this remains speculation. Most radiolarian work in the Mesozoic is limited to identifying taxa for stratigraphic purposes. Surprisingly little has been done on their paleoecology or evolution.

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