Asaphus expansus

Overview

Asaphus expansus is a trilobite with a segmented thorax connecting rounded head and tail shields. The central head region and eyes are also described as prominent features. Its three major divisions can be explored alongside small surface structures.^[1]

Key facts

The central head region is described as reaching the anterior margin broadly. Even within a rounded head outline, the central shape and eye positions provide different clues.^[1]

Time and place

It is discussed in comparative research on Ordovician trilobites from Sweden. The Ordovician spanned approximately 490–440 million years ago. Differences in head shields and thoracic organization provide the basis for species comparisons.^[1][2]

The time range describes source context, not the complete duration of this species.

Size, form and skeleton

The thorax between the rounded head and tail shields has parallel sides. A prominent central region and eyes are described on the head, with the central region extending broadly to the anterior margin. Form can be read by moving from the outline toward the center.^[1]

Asaphus expansus specimen RM Ar. 51076 is described as 7 cm long in a 2003 paper. It reveals at a glance an oval body with connected head, thorax, and tail shield.^[3]

Habitat and reproduction

What information does a hard exoskeleton preserve? Head, segmented thorax, and tail shield reveal the broad arrangement, while the central head region and eyes provide details for comparing species. The same exoskeleton shows different features at different observational scales.^[1]

Fossil discovery

A 2003 comparative study of Asaphus examined specimens and illustrations of several species side by side. Material of Asaphus expansus reveals rounded shields and the form of the central head region. This research seeks differences among similar trilobites.^[1]

References

1. Bergström et al. (2003) — Comparisons of Asaphus from the Ordovician of Sweden
2. International Commission on Stratigraphy (2024) — Geological time chart
3. Bergström et al. (2003) — Reassessment of the trilobite Asaphus
4. Rudkin et al. (2003) — Description of the new species Isotelus rex

Research questions

How can trilobites with similar rounded head shields be distinguished? Looking at the central head region, eyes, and thoracic connections provides more comparative features than the outline alone. Alternating between broad outlines and small parts is useful.^[1]

Observation

Follow the rounded head shield through the segmented thorax to the tail shield. Return to the central head region and eyes to locate details within the overall outline.^[1]