acritarchen

Acritarchen, in English literature commonly called acritarchs, are organic-walled microfossils of uncertain biological affinity found in sedimentary rocks from the Precambrian to the present. They form a broad, informal group defined by morphology rather than a single evolutionary lineage, and their exact biological sources are often unknown. This form-based approach means that acritarchs include cysts and other bodies produced by various major eukaryotic lineages, most notably groups of algae, but their precise affiliations remain debated.

Morphology and preservation: Acritarchs exhibit a wide range of shapes, from simple spheres to complex, ornamented, or multilayered bodies. Some bear processes, spines, or reticulate surfaces. Preservation occurs as carbonaceous compressions, phosphatized remains, or kerogenized walls in fine-grained sedimentary rocks, and their walls commonly resist diagenesis, aiding their recognition in deep-time sequences.

Stratigraphic use and significance: Acritarchs are integral to biostratigraphy, especially for Neoproterozoic and Paleozoic intervals prior to the diagnostic microfossils of other groups. Their diversity and morphological distinctiveness help correlate strata across regions, even when the exact taxonomic relationships are unclear. Consequently, they are treated largely as form-tosses rather than definitive evolutionary lineages.

Geographic and environmental context: They are found worldwide in marine sediments, with higher diversity in environments that preserve organic walls well, such as oxic to dysoxic shallow shelves and basinal settings, reflecting broad ecological roles of early eukaryotic phytoplankton.

History and interpretation: The concept emerged in the mid-20th century to describe enigmatic, organic-walled microfossils. Today, acritarchs remain a central tool for studying early eukaryote evolution and for reconstructing ancient marine ecosystems.