No valve perforations (save a central pore on the valve face of some species), altogether quite similar to the design of incunabular scales on diatom auxospore walls. All this evidence taken together is consistent with the antiquity of the Leptocylindrus and Paralia lineages and places them in the position where their life histories may indeed inform about the habit of the earliest diatoms. The initial cells of both Leptocylindrus [31] and the Paralia species shown here are strongly structurally similar to resting spores, and we hypothesize that these diatoms conserved archaic, spore-like morphology in the frustules of their initial cells which we apply here to infer the habit of the early diatoms. If the above is true, we further hypothesize that following the pre-diatom stages possibly involving the “multiplate diploid spore” (called a “multiplate diploid cyst” in [95]), diatom frustules underwent three phases of evolution. During Phase 1 (possibly Triassic-Jurassic), archaic diatom frustules (both vegetative and initial), might have been spore-like: heterovalvate, relatively poorly perforated and without processes. These archaic frustule-bearing cells (e.g., some possibly similar to frustules of the Korean Calyptosporium-like diatom or initial frustules of Leptocylindrus) may not have been readily recognizable as belonging to diatoms because they were too similar to “generic” resting spores. These frustules would have been somewhat different from the hypothetical Ur-diatoms in that they were hetero- rather than homovalvate (compare Fig 5J to [95]; fig 16.1d). From some of these spore-like frustule bearing archaic diatoms there evolved more porous vegetative valves without processes. Jurassic specimens of Phyxidicua and initial frustules of P. guyana may reflect such a transition. Thus, in Phase ijerph7041855 2, more familiar vegetative valve designs (many homovalvate, chambered and non-chambered; [16]) with various, now extinct valve portulae such as those known from the Lower MG-132 site Cretaceous (ca. 110 Ma) became most common while the sparsely perforated, spore-like frustules might have become limited to the initial cell stage in their life-cycles. In Phase 3, the modern-valve phase of diatom frustule evolution (at least since Upper Cretaceous, but possibly even since Lower Cretaceous, if Bilingua and Kerkis represent early polar diatoms), modern frustule designs dominate. These contained generally only nearly identical vegetative valves, modern portulae, and most diatoms developed simpler initial cell frustules, more similar to their vegetative counterparts. Spore-like initial frustules remained conserved in a few representatives of the most ancient lineages, such as those containing Leptocylindrus and Paralia. Should this scenario prove true, then diatom emergence in the Lower Mesozoic suggested by some molecular clocks becomes plausible. j.jebo.2013.04.005 This hypothesis can be tested by examination of silica rich sediments inPLOS ONE | DOI:10.1371/journal.pone.0141150 October 20,21 /Auxosporulation in Paraliathin section, rather than following chemical dissolution (with strong acids, including HF) which is effective in the recovery of organic material, but is destructive to siliceous remains. However, whether pre-diatom ancestral stocks originated within haploid, siliceous-scale covered flagellated chrysophycea-like cells [95, 96], filamentous 3-MethyladenineMedChemExpress 3-MA organisms similar to oomycete/xanthophycean-like organisms (wrapped in a sheath with a hexagonal pseudoloculate pattern; [87].No valve perforations (save a central pore on the valve face of some species), altogether quite similar to the design of incunabular scales on diatom auxospore walls. All this evidence taken together is consistent with the antiquity of the Leptocylindrus and Paralia lineages and places them in the position where their life histories may indeed inform about the habit of the earliest diatoms. The initial cells of both Leptocylindrus [31] and the Paralia species shown here are strongly structurally similar to resting spores, and we hypothesize that these diatoms conserved archaic, spore-like morphology in the frustules of their initial cells which we apply here to infer the habit of the early diatoms. If the above is true, we further hypothesize that following the pre-diatom stages possibly involving the “multiplate diploid spore” (called a “multiplate diploid cyst” in [95]), diatom frustules underwent three phases of evolution. During Phase 1 (possibly Triassic-Jurassic), archaic diatom frustules (both vegetative and initial), might have been spore-like: heterovalvate, relatively poorly perforated and without processes. These archaic frustule-bearing cells (e.g., some possibly similar to frustules of the Korean Calyptosporium-like diatom or initial frustules of Leptocylindrus) may not have been readily recognizable as belonging to diatoms because they were too similar to “generic” resting spores. These frustules would have been somewhat different from the hypothetical Ur-diatoms in that they were hetero- rather than homovalvate (compare Fig 5J to [95]; fig 16.1d). From some of these spore-like frustule bearing archaic diatoms there evolved more porous vegetative valves without processes. Jurassic specimens of Phyxidicua and initial frustules of P. guyana may reflect such a transition. Thus, in Phase ijerph7041855 2, more familiar vegetative valve designs (many homovalvate, chambered and non-chambered; [16]) with various, now extinct valve portulae such as those known from the Lower Cretaceous (ca. 110 Ma) became most common while the sparsely perforated, spore-like frustules might have become limited to the initial cell stage in their life-cycles. In Phase 3, the modern-valve phase of diatom frustule evolution (at least since Upper Cretaceous, but possibly even since Lower Cretaceous, if Bilingua and Kerkis represent early polar diatoms), modern frustule designs dominate. These contained generally only nearly identical vegetative valves, modern portulae, and most diatoms developed simpler initial cell frustules, more similar to their vegetative counterparts. Spore-like initial frustules remained conserved in a few representatives of the most ancient lineages, such as those containing Leptocylindrus and Paralia. Should this scenario prove true, then diatom emergence in the Lower Mesozoic suggested by some molecular clocks becomes plausible. j.jebo.2013.04.005 This hypothesis can be tested by examination of silica rich sediments inPLOS ONE | DOI:10.1371/journal.pone.0141150 October 20,21 /Auxosporulation in Paraliathin section, rather than following chemical dissolution (with strong acids, including HF) which is effective in the recovery of organic material, but is destructive to siliceous remains. However, whether pre-diatom ancestral stocks originated within haploid, siliceous-scale covered flagellated chrysophycea-like cells [95, 96], filamentous organisms similar to oomycete/xanthophycean-like organisms (wrapped in a sheath with a hexagonal pseudoloculate pattern; [87].
