Bacteria and Protists from Kansas

Waggoner Bacteria and Protists from Cretaceous Amber Page 24

DISCUSSION

Recent species and strains of Leptothrix range from unpolluted water to activated sludge habitats. Most strains do not require high organic concentrations but are common in slow-moving water containing iron (Buchanan and Gibbons, 1975; Waggoner, personal observations). Although conclusions based on so few specimens must be extremely tentative, the testate amoebae identified seem to characterize a similar habitat. In general, testate amoebae characterize habitats with slow decomposition (Bamforth, 1980). The fossil testate amoebae are both elongated "acrostomes"; this morphotype is most common in very moist environments such as mosses and aquatic vegetation masses (Bamforth, 1980; Bovee, 1985a, 1985b). Testate amoebae may also favor mineral-rich habitats; the brown color of many tests of lobose amoebae, including Pontigulasia, is often due to iron uptake, and euglyphids generally take up silicon as silicate to construct their tests (Hedley and Ogden, 1980). The presence of Nebela indirectly demonstrates a trophic relationship: Nebela ingests smaller siliceous testate amoebae and uses their plates to construct its test (Ogden and Hedley, 1980). Naegleria, which has previously been reported from this amber (Waggoner, 1993), is quite eurytopic.

In general, the paleoenvironment suggested by these microbes was aquatic, probably only moderately organically enriched, but enriched in iron and other minerals. Judging by modern analogues and fossil evidence, the organisms so far found all possessed the ability to encyst, suggesting that the paleohabitat was relatively transient or variable. The lack of soil particles or plant debris in the jelinite studied so far does not suggest a soil or litter paleohabitat. The microorganisms were presumably trapped in resin flowing into a freshwater puddle, lake or pond, a scenario which also accounts for the numerous air and water bubbles in the amber. The water then would have diffused out of the resin as it solidified, a process known as inert dehydration and known to happen in insect fossils (Poinar, 1992). Alternatively, the microorganisms might have grown in a small pocket of water, perhaps in a crevice in the tree bark, as was suggested for the microbiota of the late Triassic amber from Bavaria (Poinar et al., 1993a). However, this does not account for the large, fairly homogeneous pieces of cloudy jelinite; these suggest that the aquatic habitat was not a small crevice. It has been suggested that aquatic microfossils in amber indicate resin flows from swamp trees analogous to modern bald cypress (Cooper, 1964). On the basis of the fossil inclusions, this is certainly plausible in the case of jelinite. The association of jelinite with lignite beds further suggests swampy conditions in the area. Unfortunately no new data can now be collected on the depositional environment of the amber.

The microfossil assemblage of this amber most closely resembles that of the Triassic amber of Bavaria (Poinar et al., 1993a) of those studied so far; both contain testate amoebae and filamentous iron bacteria. The nearly contemporaneous Cenomanian amber from northwestern France also contains an aquatic microfossil assemblage, but the assemblagesmall ciliates, small colorless flagellates, and cyanobacteriaindicates a more eutrophic paleohabitat (Waggoner, 1994). Cretaceous amber from Canada also contains an aquatic microfossil assemblage: alga-like inclusions, the only known fossil Paramecium, and a fossil tardigrade (Cooper, 1964; Poinar, 1992). Other microfossil assemblages in Dominican, Baltic, and Washington state amber are much more "terrestrial", containing fungi, actinomycetes, and myxomycetes (Larsson, 1978; Poinar, 1992; Waggoner, 1993a, b, 1996; Waggoner, unpublished observations).

As Waggoner (1993a) pointed out, the reported cysts of Naegleria in Kansas amber differ little from extant ones, which fits the morphological stasis inferred from molecular data on Naegleria (Baverstock et al., 1989). This has also been suggested for some testate amoebae, on the basis of their present-day biogeographical distribution (Ogden and Hedley, 1980; Bamforth, 1981). The