Paleontologists have found the fossilized remains of Devonian brittle stars in the ‘upper unit’ of the Baviaanskloof Formation in South Africa. Their analysis revealed the presence of two distinct species: the new species Krommaster spinosus and the previously described species Hexuraster weitzi.
Krommaster spinosus. Image credit: C. Reddy.
Brittle stars, scientifically known as ophiuroids, are members of the phylum Echinodermata.
They are represented by over 2,000 species today and are known from a considerable fossil record, particularly in the northern hemisphere.
Sometimes confused with their cousins, the starfish, they differ in having long whip-like arms that may reach up to 60 cm long.
The Paleozoic era (541-252 million years ago) was dominated by what is called ‘archaic’ type brittle stars.
These brittle stars had an arm morphology very different from that of ‘modern’ type or crown group brittle stars.
‘Archaic’ and ‘modern’ brittle stars diverged in the Early Ordovician epoch (485-470 million years ago) and until recently, ‘archaic’ brittle stars were thought to have become extinct by the latest Carboniferous period (around 300 million years ago).
Recent extension of their range, however, indicates coexistence of ‘modern’ and ‘archaic’ forms until at least the Triassic period (251-201 million years ago).
The decline of the archaic forms is likely related to the mid-Paleozoic Marine Revolution which involved an increase in the diversity of predatory strategies within shallow water environments in low latitudes.
‘Archaic’ brittle stars are argued to have been more vulnerable to this diversified predation and thus became restricted to communities affiliated with lower predation levels such as marine environments at high latitudes.
“Currently, the relationship between ‘archaic’ and ‘modern’ type brittle stars is poorly understood,” said Rhodes University paleontologist Caitlin Reddy and her colleagues.
In their new research, the researchers examined the fossilized remains of ‘archaic’ brittle stars from South Africa.
These comprise natural moulds and casts collected from two localities if the ‘Upper Unit’ of the Baviaanskloof Formation.
All brittle star remains are preserved as empty voids, with all traces of the calcitic skeleton dissolved.
In order to study the skeletal details, Reddy and co-authors prepared silicone moulds of selected specimens.
Their morphologic analysis revealed the presence of two species: Krommaster spinosus, a new species of encrinasterid brittle star characterized by very large spines, and Hexuraster weitzi, a previously described species of cheiropterasterid brittle star.
The specimens are nearly 410 million years old — the earliest known record of brittle stars from the entire supercontinent of Gondwana (which later broke up into Africa, South America, Antarctica, India, Australia and Madagascar).
They are also some of very few known from ancient polar regions, as southern Africa was then within the Antarctic circle.
This is important in understanding the early diversification of brittle stars as, so far, the majority of early known species are recovered from rocks formed nearer the equator — largely from the northern supercontinent of Laurasia (which later split into Europe, Greenland, North America and parts of Asia).
“I always find it completely incredible, almost magical, that we can find frozen in time, records of such passing moments,” said Dr. Rob Gess, a researcher at Rhodes University and Albany Museum.
The team’s paper was published in the journal PLoS ONE.
_____
C. Reddy et al. 2023. Earliest known ophiuroids from high palaeolatitude, southern Gondwana, recovered from the Pragian to earliest Emsian Baviaanskloof Formation (Table Mountain Group, Cape Supergroup) South Africa. PLoS ONE 18 (10): e0292636; doi: 10.1371/journal.pone.0292636
