Micraster coranguinum

Overview

Micraster coranguinum is a sea urchin with a heart-shaped skeleton. Petal-like ambulacra are arranged across its gently domed body. We can distinguish top from bottom and front from rear within its rounded form.^[1]

Key facts

The mouth is described as lying beneath the front of the body, while the anus sits high on the rear surface. Even within a similarly rounded outline, organs occupy different positions at the front and back.^[1]

Time and place

It is known from Late Cretaceous material in the Upper Chalk of Britain. The Cretaceous as a whole spanned approximately 140–66 million years ago. This organism is also introduced as an index fossil used to compare strata.^[1][2]

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

Size, form and skeleton

A heart-shaped skeleton supports the broad outline, and the upper surface forms a gentle dome. Petal-like ambulacra lie on top. Knowing the positions of the mouth and anus helps explain the orientation of the body.^[1]

The heart-shaped test of Micraster coranguinum is approximately 6 cm long in a specimen presented by Britain’s Natural History Museum. Its original calcite test survives, allowing examination of the rounded exterior and surface patterns.^[9]

Habitat and reproduction

How do the top and bottom, or front and rear, differ? In Micraster, the mouth beneath the front and the anus high on the rear occupy different positions. Even in a rounded body, the placement of organs provides clues to orientation.^[1]

Fossil discovery

Research on Micraster describes this species as an index fossil of Britain’s Upper Chalk. It illustrates how the presence of a particular fossil can help compare strata. Studying an organism’s form leads to questions about reading time in rocks.^[1]

References

1. Micraster study — Microevolution, adaptation and predation
2. International Commission on Stratigraphy (2024) — Geological time chart
3. Veselská(2010). Revize vybraných skupin dekapodů z české křídové pánve. Zprávy o geologických výzkumech v roce 2009:172–174.
4. Wittler (1998) — Enoploclytia from Dortmund
5. International Commission on Stratigraphy (2024) — Geological time chart
6. City of Prague — Environmental report
7. Micraster study — Microevolution, adaptation and predation
8. International Commission on Stratigraphy (2024) — Geological time chart
9. Natural History Museum, UK — Micraster specimen description

Research questions

What can we compare when the same kind of fossil appears in several strata? Index fossils provide clues for correlating rock layers. The skeleton of Micraster offers evidence about geological time as well as an organism’s body.^[1]

Observation

First examine the heart-shaped outline and gentle dome. Then picture the petal-like ambulacra on top and the positions of organs at the front and rear to distinguish the body’s different directions.^[1]