Rare 410-Million-Year-Old Brittle Star Fossils Discovered in South Africa Presents Earliest Known Record From Supercontinent Gondwana

Paleontologists have discovered fossilized remains of Devonian brittle stars in the 'upper unit' of the Baviaanskloof Formation in South Africa.

Frozen in Time

The Paleozoic era was dominated by the presence of 'archaic' type brittle stars. These organisms had an arm morphology that is very different from that of 'modern' type or crown group brittle stars. The 'archaic' and 'modern' brittle stars diverged in the Early Ordovician epoch and until recently, the 'archaic' type were thought to have become extinct by the latest Carboniferous period.

The recent extension of their range indicates the coexistence of 'modern' and 'archaic' forms until at least the Triassic period. The decline of the archaic type is linked to the mid-Paleozoic Marine Revolution involving an increase in the diversity of predatory strategies within shallow water environments in low latitudes.

As described by paleontologist Caitlin Reddy from Rhodes University, the relationship between archaic and modern type brittle stars is poorly understood. In a new study, Reddy and her colleagues examined the fossilized remains of archaic brittle stars to gain more insights into its characteristics.

The team collected natural molds and casts which were found in near-perfect condition at the Kromme River Valley. All the remains are preserved as empty voids, with all traces of the calcitic skeleton dissolved.

Morphological analysis of the specimen revealed the presence of two distinct species: the new species of encrinasterid brittle star called Krommaster spinosus characterized by very large spines, and the previously described species of cheiropterasterid brittle star called Hexuraster weitzi.

The specimens were found to be almost 410 million years old, the oldest known record of brittle stars from the entire supercontinent of Gondwana which later broke up into South America, India, Africa, Antarctica, Madagascar, and Australia. There are also some of very few known from ancient polar regions, since southern Africa used to be within the Antarctic circle.

The findings of this study are crucial in understanding the early diversification of brittle stars since the majority of early known species are recovered from rocks formed near the equator. Most of them are found in the northern supercontinent of Laurasia which later split into North America, Europe, Greenland, and parts of Asia.

Seafloor Ecosystem Engineers

Brittle stars are any of the marine invertebrates under phylum Echinodermata in the class Ophiuroidea. Scientifically known as ophiuroids, most of them are scavengers which eat dead creatures and seaweed. Today, they are represented by more than 2,000 species and are known from a considerable fossil record particularly in the northern hemisphere.

These echinoderms live in a variety of habitats around the world, with 73 species living in the Arctic alone. They can also thrive in both deep and shallow ocean shelf waters.

Due to the presence of five arms, brittle stars are sometimes confused with starfish (Asteroidea). However, brittle stars have long whip-like limbs that could reach up to 23 inches (60 cm) long. They use these lengthy arms to feed themselves by snatching particles that fall in the water column. They stir up sediments as they dig, causing nutrients on the seafloor to be recycled which in turn affects the distribution of other seafloor species.