Protists in the fossil record

Systematists today do not treat Protista as a formal taxon, but the term "protist" is still commonly used for convenience in two ways.

[25] The other definition describes protists primarily by functional or biological criteria: protists are essentially those eukaryotes that are never multicellular,[22] that either exist as independent cells, or if they occur in colonies, do not show differentiation into tissues (but vegetative cell differentiation may occur restricted to sexual reproduction, alternate vegetative morphology, and quiescent or resistant stages, such as cysts);[26] this definition excludes many brown, multicellular red and green algae, which may have tissues.

Newer classifications attempt to present monophyletic groups based on morphological (especially ultrastructural),[27][28][29] biochemical (chemotaxonomy)[30][31] and DNA sequence (molecular research) information.

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.

[37] Two kinds of fossils resembling red algae were found sometime between 2006 and 2011 in well-preserved sedimentary rocks in Chitrakoot, central India.

[40] The occurrence of Phaeophyceae as fossils is rare due to their generally soft-bodied nature,[41] and scientists continue to debate the identification of some finds.

Among the brown algae, only species of the genus Padina deposit significant quantities of minerals in or around their cell walls.

[49] While many carbonaceous fossils have been described from the Precambrian, they are typically preserved as flattened outlines or fragments measuring only millimeters long.

[41] The Devonian megafossil Prototaxites, which consists of masses of filaments grouped into trunk-like axes, has been considered a possible brown alga.

[53] Likewise, the fossil Protosalvinia was once considered a possible brown alga, but is now thought to be an early land plant.

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.

[43] 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[43] or Dictyota.

The earliest known fossil diatoms date from the early Jurassic (~185 Ma ago),[59] although the molecular clock[59] and sedimentary[60] evidence suggests an earlier origin.

Prior to the Phanerozoic (before 544 Ma), it is believed that microbial or inorganic processes weakly regulated the ocean's silicon cycle.

[63][64][65] Subsequently, the cycle appears dominated (and more strongly regulated) by the radiolarians and siliceous sponges, the former as zooplankton, the latter as sedentary filter-feeders primarily on the continental shelves.

[68] 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.

The duration of diatom species ranges have been documented through the study of ocean cores and rock sequences exposed on land.

[73] Where diatom biozones are well established and calibrated to the geomagnetic polarity time scale (e.g., Southern Ocean, North Pacific, eastern equatorial Pacific), 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.

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.

[83] The earliest stages of dinoflagellate evolution appear to be dominated by parasitic lineages, such as perkinsids and syndinians (e.g. Amoebophrya and Hematodinium).

[95] Dinoflagellate evolution has been summarized into five principal organizational types: prorocentroid, dinophysoid, gonyaulacoid, peridinioid, and gymnodinoid.

[96] The transitions of marine species into fresh water have been infrequent events during the diversification of dinoflagellates and in most cases have not occurred recently, possibly as late as the Cretaceous.

[97] Recently, the "living fossil" Dapsilidinium pastielsii was found inhabiting the Indo-Pacific Warm Pool, which served as a refugium for thermophilic dinoflagellates.

[101] However, the discovery of diagenetically altered C27 sterols associated with the remains of Dickinsonia cast doubt on this identification and suggest it may instead be an animal.

The earliest known definite foraminifera appear in the fossil record towards the very end of the Ediacaran; these forms all have agglutinated tests and are unilocular.

[109][110] Later spirillinids would evolve multilocularity and calcitic tests, with the first such forms appearing during the Triassic; the group saw little effects on diversity due to the K-Pg extinction.

The group remained at low diversity throughout its fossil history; all living representatives belong to the Robertinidae, which first appeared during the Paleocene.

[116][117] The earliest known radiolaria date to the very start of the Cambrian period,[119][120][121][122] appearing in the same beds as the first small shelly fauna—they may even be terminal Precambrian in age.

[124] (in part) Because many of these organisms had a silica capsule, they have a relatively complete fossil record, allowing modern biologists to confirm that they are, in fact, not derived from cyanobacteria, but rather an ancestor that did not possess the capability to photosynthesize.