Organism · Arthropods
Ptychagnostus atavus
Fossil record age bounds · ≈ 507–487 Ma
Inferred life reconstruction
Ptychagnostus atavus
Overview
Ptychagnostus atavus is an agnostid examined through small head and tail shields. Surfaces without spines or granules and the shape of the central tail axis provide clues to its identity.[1]
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Key facts
The first appearance of this species serves as a time marker for comparing the ages of rock layers. A 2015 study used Korean material to connect it with records of the beginning of the Drumian in other regions.[1]
Time and place
A 2015 study presented it as a species linked to the middle Cambrian timescale. The studied specimens came from the lower Machari Formation at the Deoksang section in Yeongwol, present-day Republic of Korea.[1]
The time range describes source context, not the complete duration of this species.
Size, form and skeleton
The isolated head and tail shields lack spines or granular surface ornament. A segment called M2 in the central tail axis is described as hexagonal. The lateral parts of the Deoksang specimens’ head shields are also smooth.[1]
Ptychagnostus atavus reveals Cambrian sea history with a body smaller than 1 cm. Three complete specimens studied from layers in present-day Newfoundland, Canada, measured 3.7 mm, 5.8 mm, and 8.5 mm, respectively. Their small bodies preserve distinct head and tail regions for comparison.[2]
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Habitat and reproduction
The study describes the Joseon Supergroup containing this material as mixed carbonate and clastic sediments of shallow-marine origin. Limestone and black shale in the Machari Formation together convey a record of the sea at that time.[1]
Fossil discovery
Researchers recorded 10 head shields and 22 tail shields from a horizon 7 m above the base of the Machari Formation. They compared separated shields to examine similarities and differences in form.[1]
References
- Hong·Choi (2015) — Agnostids from the Machari Formation, Yeongwol
- Hildenbrand et al. (2021) — A study of Drumian trilobites from Newfoundland
- Hildenbrand et al. (2021) — A study of Drumian trilobites from Newfoundland
- Terfelt (2003) — Late Cambrian trilobites from Kakeled
- International Commission on Stratigraphy (2024) — Geological time chart
- Eriksson·Horn (2017) — The mystery of pea-sized Agnostus
Research questions
Researchers distinguished this species by comparing its form with the similar Ptychagnostus sinicus. Comparing segments, grooves, and projections of the tail axis provided a basis for connecting records in widely separated layers.[1]
Observation
The image interprets a connected head, short thorax and tail in dorsal view instead of an isolated tail shield. It draws on complete-specimen records of this species and shield descriptions, with thoracic connections supplied by agnostid comparison. Exact segments, proportions, life colour and pose remain unresolved; legs, antennae and underside are not reconstructed.[1][2]
Related organisms
Agnostus pisiformisArthropodsSource time context (not species duration) · ≈ 539–487 MaHead and tail shields of Cambrian fossils
Examine the different records of bodies and time preserved in these two organisms. Ptychagnostus atavus: an agnostid examined through small head and tail shields; surfaces without spines or granules and the central tail axis provide identifying clues. A 2015 study linked it to the middle Cambrian timescale using specimens from the lower Machari Formation at the Deoksang section in Yeongwol, present-day Republic of Korea. Agnostus pisiformis: an organism with a very short trunk between rounded head and tail shields, known from nearly complete dorsal exoskeletons. It appears in research on Late Cambrian Kakeled material. The Cambrian as a whole spans approximately 540–490 million years ago.