Australohalkieria

Lore & Background

Australohalkieria was first described from sclerites found in Australia and Antarctica. The most complete species, Australohalkieria superstes, was named by Porter in 2004 and is the most abundant Australian halkieriid species. Its sclerites are convex on the upper surface and concave on the lower, and they overlap so that the concave side of each is partly covered by the convex side of the next. The internal cavity of the sclerites is more complex than in the related genus Halkieria, with a cylindrical tube that splits into a pair of longitudinal canals about halfway up; these canals do not appear to be connected.

In A. superstes, the central canals of the sclerites are flattened on their upper surfaces, creating a depression at the tip. This depression is not mineralized, suggesting it may have aided the animal's sense of smell by allowing chemicals in the water to penetrate exposed skin. The phosphatic coating on the sclerites shows features indicating they were originally covered by a thin organic skin, similar to an outer organic layer found on chancelloriids. The sclerites come in right- and left-handed variants that are equally abundant, suggesting the animal was bilaterally symmetrical.

A second species, Australohalkieria parva, was first described in 1990. Its sclerites have undivided longitudinal canals and a very similar wall structure to A. superstes, but the central canals are not flattened. Other halkieriid fossils from the Georgina Basin are different enough to be excluded from A. superstes but are not abundant enough to classify; one type is very similar to A. superstes but lacks the flattened central canal, while another has a flattened central canal but no longitudinal canals, possibly representing a distinct Middle Cambrian genus.

Reader's Guide

Australohalkieria is important for understanding the morphology and ecology of halkieriids, a group of early animals that may be related to molluscs. The sclerites of Australohalkieria are coelosclerites, meaning they have a mineralized shell around a space originally filled with organic tissue and show no evidence of growth by adding material around the outside. This genus shares with Halkieria three sclerite shapes: palmate, cultrate, and siculate. However, Australohalkieria differs in having more complex internal canals and, in A. superstes, a flattened central canal that may have been sensory.

The sclerites of A. superstes are about one-third the size of Early Cambrian halkieriid sclerites, and siculate sclerites are more abundant than palmate or cultrate ones, unlike Early Cambrian halkieriids. No shells that might be assigned to halkieriids have been found in the Georgina Basin, though this does not prove Australohalkieria lacked shells. The genus provides insight into the diversity of Middle Cambrian halkieriids and the possible evolution of sensory structures in early animals.

Did You Know?

• The internal cavity of Australohalkieria sclerites is more complex than the simple tube in Halkieria, splitting into a pair of longitudinal canals.
• The depression on the tip of A. superstes sclerites is not mineralized, which may have helped the animal's sense of smell.
• Siculate sclerites of A. superstes are more abundant than cultrate or palmate sclerites, unlike Early Cambrian halkieriids.