Elrathia kingi

Overview

Elrathia kingi is a trilobite known for a narrow central axis and short genal spines. Its adult-form thorax consists of thirteen segments. Growth stages and differences between body regions can be explored within its small body.^[1]

Key facts

The thirteen thoracic segments of the adult form and the four axial rings of the pygidium belong to different regions. Although both look like repeated rings, their positions and organization within the body differ.^[1]

Time and place

Cambrian material from the Wheeler and Marjum formations has been studied. The research context spans the late Wuliuan into the Drumian. The entire Cambrian spanned approximately 540–490 million years ago.^[1][2]

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

Size, form and skeleton

The head bears short genal spines, and the thoracic central axis is described as narrow. The adult form has thirteen thoracic segments; the pygidium has four axial rings and a terminal piece. A small notch at the rear center of the pygidium is also common.^[1]

The adult-form, holaspid-stage specimens examined in the study ranged from 4.9–36.8 mm in total length.^[1]

Habitat and reproduction

How does the number of body segments change during growth? Growth studies examine specimens at different stages separately from the adult-form arrangement. Distinguishing thorax from pygidium matters even in a small trilobite.^[1]

Fossil discovery

In 2021, Hopkins studied the ontogeny of Elrathia. Thoracic segments, pygidia, and head shapes of adult-form specimens were compared. This research explores one organism through several stages of growth rather than a single finished form.^[1]

References

1. Ontogeny of Elrathia kingii
2. International Commission on Stratigraphy (2024) — Geological time chart
3. The Burgess Shale: Pagetia bootes

Research questions

How can thoracic segments be distinguished from rings within the pygidium? First locating major regional boundaries, then following repetitions within them, reveals their organization. Similar ring-like shapes convey different information depending on position.^[1]

Observation

Follow the short genal spines of the head toward the narrow thoracic axis. Examining the thirteen thoracic segments separately from the pygidial rings connects major boundaries with small repetitions.^[1]