Protists in the fossil record
A protist is any eukaryotic organism that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor, the last eukaryotic common ancestor, the exclusion of other eukaryotes means that protists do not form a natural group, or clade. Therefore, some protists may be more closely related to animals, plants, or fungi than they are to other protists. However, like algae, invertebrates and protozoans, the grouping is used for convenience.
Many protists have neither hard parts nor resistant spores, and their fossils are extremely rare or unknown. Examples of such groups include the apicomplexans, most ciliates, some green algae, choanoflagellates, oomycetes, brown algae, yellow-green algae, Excavata. Some of these have been found preserved in amber or under unusual conditions.
Others are relatively common in the fossil record, as the diatoms, golden algae, haptophytes, silicoflagellates, tintinnids, dinoflagellates, green algae, red algae, heliozoans, radiolarians, foraminiferans, ebriids and testate amoebae. Some are used as paleoecological indicators to reconstruct ancient environments.
More probable eukaryote fossils begin to appear at about 1.8 billion years ago, the acritarchs, spherical fossils of likely algal protists. Another possible representative of early fossil eukaryotes are the Gabonionta.
Modern classifications
s today do not treat Protista as a formal taxon, but the term "protist" is still commonly used for convenience in two ways. The most popular contemporary definition is a phylogenetic one, that identifies a paraphyletic group: a protist is any eukaryote that is not an animal, plant, or fungus; this definition excludes many unicellular groups, like the Microsporidia, many Chytridiomycetes, and yeasts, and also a non-unicellular group included in Protista in the past, the Myxozoa.The other definition describes protists primarily by functional or biological criteria: protists are essentially those eukaryotes that are never multicellular, that either exist as independent cells, or if they occur in colonies, do not show differentiation into tissues ; this definition excludes many brown, multicellular red and green algae, which may have tissues.
The taxonomy of protists is still changing. Newer classifications attempt to present monophyletic groups based on morphological, biochemical and DNA sequence information. However, there are sometimes discordances between molecular and morphological investigations; these can be categorized as two types: one morphology, multiple lineages and one lineage, multiple morphologies.
Because the protists as a whole are paraphyletic, new systems often split up or abandon the kingdom, instead treating the protist groups as separate lines of eukaryotes. The recent scheme by Adl et al. does not recognize formal ranks and instead treats groups as clades of phylogenetically related organisms. This is intended to make the classification more stable in the long term and easier to update. Some of the main groups of protists, which may be treated as phyla, are listed in the taxobox, upper right. Many are thought to be monophyletic, though there is still uncertainty. For instance, the Excavata are probably not monophyletic and the chromalveolates are probably only monophyletic if the haptophytes and cryptomonads are excluded.
Archaeplastida
Red algae
One of the oldest fossils identified as a red alga is also the oldest fossil eukaryote that belongs to a specific modern taxon. Bangiomorpha pubescens, a multicellular fossil from arctic Canada, strongly resembles the modern red alga Bangia and occurs in rocks dating to 1.05 billion years ago.Two kinds of fossils resembling red algae were found sometime between 2006 and 2011 in well-preserved sedimentary rocks in Chitrakoot, central India. The presumed red algae lie embedded in fossil mats of cyanobacteria, called stromatolites, in 1.6 billion-year-old Indian phosphorite – making them the oldest plant-like fossils ever found by about 400 million years.
Red algae are important builders of limestone reefs. The earliest such coralline algae, the solenopores, are known from the Cambrian period. Other algae of different origins filled a similar role in the late Paleozoic, and in more recent reefs.
Calcite crusts that have been interpreted as the remains of coralline red algae, date to the Ediacaran Period. Thallophytes resembling coralline red algae are known from the late Proterozoic Doushantuo formation.
Glaucophyta
SAR supergroup
Stramenopiles
Brown algae
The occurrence of Phaeophyceae as fossils is rare due to their generally soft-bodied nature, and scientists continue to debate the identification of some finds. Part of the problem with identification lies in the convergent evolution of morphologies between many brown and red algae. Most fossils of soft-tissue algae preserve only a flattened outline, without the microscopic features that permit the major groups of multicellular algae to be reliably distinguished. Among the brown algae, only species of the genus Padina deposit significant quantities of minerals in or around their cell walls. Other algal groups, such as the red algae and green algae, have a number of calcareous members. Because of this, they are more likely to leave evidence in the fossil record than the soft bodies of most brown algae and more often can be precisely classified.Fossils comparable in morphology to brown algae are known from strata as old as the Upper Ordovician, but the taxonomic affinity of these impression fossils is far from certain. Claims that earlier Ediacaran fossils are brown algae have since been dismissed. While many carbonaceous fossils have been described from the Precambrian, they are typically preserved as flattened outlines or fragments measuring only millimeters long. Because these fossils lack features diagnostic for identification at even the highest level, they are assigned to fossil form taxa according to their shape and other gross morphological features. A number of Devonian fossils termed fucoids, from their resemblance in outline to species in the genus Fucus, have proven to be inorganic rather than true fossils. The Devonian megafossil Prototaxites, which consists of masses of filaments grouped into trunk-like axes, has been considered a possible brown alga. However, modern research favors reinterpretation of this fossil as a terrestrial fungus or fungal-like organism. Likewise, the fossil Protosalvinia was once considered a possible brown alga, but is now thought to be an early land plant.
A number of Paleozoic fossils have been tentatively classified with the brown algae, although most have also been compared to known red algae species. Phascolophyllaphycus possesses numerous elongate, inflated blades attached to a stipe. It is the most abundant of algal fossils found in a collection made from Carboniferous strata in Illinois. Each hollow blade bears up to eight pneumatocysts at its base, and the stipes appear to have been hollow and inflated as well. This combination of characteristics is similar to certain modern genera in the order Laminariales. Several fossils of Drydenia and a single specimen of Hungerfordia from the Upper Devonian of New York have also been compared to both brown and red algae. Fossils of Drydenia consist of an elliptical blade attached to a branching filamentous holdfast, not unlike some species of Laminaria, Porphyra, or Gigartina. The single known specimen of Hungerfordia branches dichotomously into lobes and resembles genera like Chondrus and Fucus or Dictyota.
The earliest known fossils that can be assigned reliably to the Phaeophyceae come from Miocene diatomite deposits of the Monterey Formation in California. Several soft-bodied brown macroalgae, such as Julescraneia, have been found.
Diatoms
Heterokont chloroplasts appear to derive from those of red algae, rather than directly from prokaryotes as occurred in plants. This suggests they had a more recent origin than many other algae. However, fossil evidence is scant, and only with the evolution of the diatoms themselves do the heterokonts make a serious impression on the fossil record.The earliest known fossil diatoms date from the early Jurassic, although the molecular clock and sedimentary evidence suggests an earlier origin. It has been suggested that their origin may be related to the end-Permian mass extinction, after which many marine niches were opened. The gap between this event and the time that fossil diatoms first appear may indicate a period when diatoms were unsilicified and their evolution was cryptic. Since the advent of silicification, diatoms have made a significant impression on the fossil record, with major fossil deposits found as far back as the early Cretaceous, and with some rocks such as diatomaceous earth, being composed almost entirely of them.
Although diatoms may have existed since the Triassic, the timing of their ascendancy and "take-over" of the silicon cycle occurred more recently. Prior to the Phanerozoic, it is believed that microbial or inorganic processes weakly regulated the ocean's silicon cycle. Subsequently, the cycle appears dominated by the radiolarians and siliceous sponges, the former as zooplankton, the latter as sedentary filter-feeders primarily on the continental shelves. Within the last 100 My, it is thought that the silicon cycle has come under even tighter control, and that this derives from the ecological ascendancy of the diatoms.
However, the precise timing of the "take-over" remains unclear, and different authors have conflicting interpretations of the fossil record. Some evidence, such as the displacement of siliceous sponges from the shelves, suggests that this takeover began in the Cretaceous, while evidence from radiolarians suggests "take-over" did not begin until the Cenozoic.
The expansion of grassland biomes and the evolutionary radiation of grasses during the Miocene is believed to have increased the flux of soluble silicon to the oceans, and it has been argued that this promoted the diatoms during the Cenozoic era. Recent work suggests that diatom success is decoupled from the evolution of grasses, although both diatom and grassland diversity increased strongly from the middle Miocene.
Diatom diversity over the Cenozoic has been very sensitive to global temperature, particularly to the equator-pole temperature gradient. Warmer oceans, particularly warmer polar regions, have in the past been shown to have had substantially lower diatom diversity. Future warm oceans with enhanced polar warming, as projected in global-warming scenarios, could thus in theory result in a significant loss of diatom diversity, although from current knowledge it is impossible to say if this would occur rapidly or only over many tens of thousands of years.
The fossil record of diatoms has largely been established through the recovery of their siliceous frustules in marine and non-marine sediments. Although diatoms have both a marine and non-marine stratigraphic record, diatom biostratigraphy, which is based on time-constrained evolutionary originations and extinctions of unique taxa, is only well developed and widely applicable in marine systems. The duration of diatom species ranges have been documented through the study of ocean cores and rock sequences exposed on land. Where diatom biozones are well established and calibrated to the geomagnetic polarity time scale, diatom-based age estimates may be resolved to within <100,000 years, although typical age resolution for Cenozoic diatom assemblages is several hundred thousand years.
Diatoms preserved in lake sediments are widely used for paleoenvironmental reconstructions of Quaternary climate, especially for closed-basin lakes which experience fluctuations in water depth and salinity.
The Cretaceous record of diatoms is limited, but recent studies reveal a progressive diversification of diatom types. The Cretaceous–Paleogene extinction event, which in the oceans dramatically affected organisms with calcareous skeletons, appears to have had relatively little impact on diatom evolution.
Although no mass extinctions of marine diatoms have been observed during the Cenozoic, times of relatively rapid evolutionary turnover in marine diatom species assemblages occurred near the Paleocene–Eocene boundary, and at the Eocene–Oligocene boundary. Further turnover of assemblages took place at various times between the middle Miocene and late Pliocene, in response to progressive cooling of polar regions and the development of more endemic diatom assemblages.
A global trend toward more delicate diatom frustules has been noted from the Oligocene to the Quaternary. This coincides with an increasingly more vigorous circulation of the ocean's surface and deep waters brought about by increasing latitudinal thermal gradients at the onset of major ice sheet expansion on Antarctica and progressive cooling through the Neogene and Quaternary towards a bipolar glaciated world. This caused diatoms to take in less silica for the formation of their frustules. Increased mixing of the oceans renews silica and other nutrients necessary for diatom growth in surface waters, especially in regions of coastal and oceanic upwelling.