Extinct species of trilobite
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Ptychagnostus atavus is a species of agnostid trilobite. It was originally described by Swedish paleontologist Sven Axel Tullberg as "Agnostus atavus" in 1880. It is used in biostratigraphy as an index fossil. Its first appearance at the GSSP section in the Wheeler Shale of Utah is defined as the beginning of the Drumian Age (around million years ago) of the Miaolingian (Middle Cambrian).
Laurie (2008) grouped "punctuosus" and "affinis" within "Ptychagnostus", but preferred to place the closely related "atavus" within "Acidusus".
References.
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