Ptychagnostus atavus

Ptychagnostus atavus is a species of agnostid trilobite. It was originally described by Swedish paleontologist Sven Axel Tullberg as Agnostus atavus in 1880.^[1] 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 504.5 million years ago) of the Miaolingian (Middle Cambrian).^[2][3]

Ptychagnostus atavus Temporal range: Drumian PreꞒ Ꞓ O S D C P T J K Pg N ↓
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Clade:	†Artiopoda
Class:	†Trilobita (?)
Order:	†Agnostida
Family:	†Ptychagnostidae
Genus:	†Ptychagnostus
Species:	†P. atavus
Binomial name
†Ptychagnostus atavus (Sven Axel Tullberg, 1880)
Synonyms
Acidusus atavus (Tullberg, 1880) Agnostus atavus Tullberg 1880

Laurie (2008) ^[4] grouped punctuosus and affinis within Ptychagnostus, but preferred to place the closely related atavus within Acidusus.