̽��ֱ�� of Cambridge - Aaron Hunter

New starfish-like fossil reveals evolution in action

cmm201 — Wed, 20 Jan 2021 00:01:40 +0000

̽��ֱ��prototype starfish, which has features in common with both sea lilies and modern-day starfish, is a missing link for scientists trying to piece together its early evolutionary history.

̽��ֱ��exceptionally preserved fossil, named Cantabrigiaster fezouataensis, was discovered in Morocco’s Anti-Atlas mountain range. Its intricate design – with feathery arms akin to a lacework – has been frozen in time for roughly 480 million years.

̽��ֱ��new species is unusual because it doesn’t have many of the key features of its contemporary relatives, lacking roughly 60% of a modern starfish’s body plan.

̽��ֱ��fossil’s features are instead a hybrid between those of a starfish and a sea lily or crinoid - not a plant but a wavy-armed filter feeder which fixes itself to the seabed via a cylindrical ‘stem’.

̽��ֱ��discovery, reported in Biology Letters, captures the early evolutionary steps of the animal at a time in Earth’s history when life suddenly expanded, a period known as the Ordovician Biodiversification Event.

̽��ֱ��find also means scientists can now use the new find as a template to work out how it evolved from this more basic form to the complexity of their contemporaries.

“Finding this missing link to their ancestors is incredibly exciting. If you went back in time and put your head under the sea in the Ordovician then you wouldn’t recognize any of the marine organisms - except the starfish, they are one of the first modern animals,” said lead author Dr Aaron Hunter, a visiting postdoctoral researcher in the Department of Earth Sciences.

Modern starfish and brittle stars are part of a family of spiny-skinned animals called the echinoderms which, although they don’t have a backbone, are one of the closest group of animals to vertebrates. Crinoids, and otherworldly creatures like the sea urchins and sea cucumbers are all echinoderms.

̽��ֱ��origin of starfish has eluded scientists for decades. But the new species is so well preserved that its body can finally be mapped in detail and its evolution understood. “ ̽��ֱ��level of detail in the fossil is amazing – its structure is so complex that it took us a while to unravel its significance,” said Hunter.

It was Hunter’s work on both living and fossil echinoderms that helped him spot its hybrid features. “I was looking at a modern crinoid in one of the collections at the Western Australian Museum and I realised the arms looked really familiar, they reminded me of this unusual fossil that I had found years earlier in Morocco but had found difficult to work with,” he said.

Fezouata in Morocco is something of a holy grail for palaeontologists - the new fossil is just one of the many remarkably well preserved soft-bodied animals uncovered from the site.

Hunter and co-author Dr Javier Ortega-Hernández, who was previously based at Cambridge’s Department of Zoology and is now based at Harvard ̽��ֱ��, named the species Cantabrigiaster in honour of the long history of echinoderm research at their respective institutions.

Hunter and Ortega-Hernández examined their new species alongside a catalogue of hundreds starfish-like animals. They indexed all of their body structures and features, building a road map of the echinoderm skeleton which they could use to assess how Cantabrigiaster was related to other family members.

Modern echinoderms come in many shapes and sizes, so it can be difficult to work out how they are related to one another. ̽��ֱ��new analysis, which uses extra-axial theory – a biology model usually only applied to living species – meant that Hunter and Ortega-Hernández could identify similarities and differences between the body plan of modern echinoderms and then figure out how each family member was linked to their Cambrian ancestors.

They found that only the key or axial part of the body, the food groove – which funnels food along each of the starfish’s arms – was present in Cantabrigiaster. Everything outside this, the extra-axial body parts, were added later.

̽��ֱ��authors plan to expand their work in search of early echinoderms. “One thing we hope to answer in the future is why starfish developed their five arms,” said Hunter. “It seems to be a stable shape for them to adopt - but we don’t yet know why. We still need to keep searching for the fossil that gives us that particular connection, but by going right back to the early ancestors like Cantabrigiaster we are getting closer to that answer.”

Reference:
Aaron W. Hunter and Javier Ortega-Hernández. ‘A new somasteroid from the Fezouata Lagerstätte in Morocco and the Early Ordovician origin of Asterozoa.’ Bioloigy Letters (2021).

Researchers from the ̽��ֱ�� of Cambridge have discovered a fossil of the earliest starfish-like animal, which helps us understand the origins of the nimble-armed creature.

If you went back in time and put your head under the sea in the Ordovician then you wouldn’t recognize any of the marine organisms - except the starfish, they are one of the first modern animals

Aaron Hunter

Collection of Yale ̽��ֱ��

Cantabrigiaster fezouataensis from the Lower Ordovician (Tremadocian) Fezouata Shale, Zagora Morocco

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Meadow of dancing brittle stars shows evolution at work

sc604 — Sun, 13 Aug 2017 23:01:51 +0000

Researchers have described a new species of brittle star, which are closely related to starfish, and showed how these sea creatures evolved in response to the rise of shell-crushing predators during the late Palaeozoic Era. ̽��ֱ��results, reported in the Journal of Systematic Palaeontology, also suggest that brittle stars evolved new traits before the largest mass extinction event in Earth’s history, and not after, as was the case with many other forms of life.

A fossilised ‘meadow’ of dancing brittle stars – frozen in time in the very spot that they lived – was found in Western Australia and dates from 275 million years ago. It contains several remarkably preserved ‘archaic’ brittle stars, a newly-described genus and species called Teleosaster creasyi. They are the last known complete brittle stars of their kind, an evolutionary hangover pushed to the margins of the world’s oceans by the threat from predators.

̽��ֱ��researchers, from the ̽��ֱ�� of Cambridge, suggest that while other species of brittle stars evolved in response to predators such as early forms of rays and crabs, these archaic forms simply moved to where the predators weren’t – namely the seas around Australia, which during the Palaeozoic era was pushed up against Antarctica. In these cold, predator-free waters, the archaic forms were able to grow much larger, and lived at the same time as the modern forms of brittle star, which still exist today.

Brittle stars consist of a central disc and five whip-like appendages, which are used for locomotion. They first appear in the fossil record about 500 million years ago, in the Ordovician Period, and today there are about 2,100 different species, mostly found in the deep ocean.

Early brittle stars were just that: brittle. During the Palaeozoic Era, when early shell-crushing predators first appeared, brittle stars made for easy prey. At this point, a split in the evolutionary tree appears to have occurred: the archaic, clunky brittle stars moved south to polar waters, while the modern form first began to emerge in response to the threat from predators, and was able to continue to live in the warmer waters closer to the equator. Both forms existed at the same time, but in different parts of the ocean.

“ ̽��ֱ��threat from predation is an under-appreciated driver of evolutionary change,” said study co-author Dr Kenneth McNamara of Cambridge’s Department of Earth Sciences. “As more predators began to appear, the brittle stars started to evolve more flexible bodies, which enabled them to either burrow into the sediment, or to move more rapidly to escape.”

About 250 million years ago, the greatest mass extinction in Earth’s history – the Permian-Triassic extinction event, or the “Great Dying” – occurred. More than 90% of marine species and 70% of terrestrial species went extinct, and as a result, most surviving species underwent major evolutionary changes as a result.

“Brittle stars appear to have bucked this trend, however,” said co-author Dr Aaron Hunter, a visiting postdoctoral researcher in the Department of Earth Sciences. “They seem to have evolved before the Great Dying, into a form which we still see today.”

Meadows of brittle stars and other invertebrates such as sea urchins and starfish can still be seen today in the seas around Antarctica. As was the case during the Palaeozoic, the threat from predators is fairly low, although the warming of the Antarctic seas due to climate change has been linked to the recent arrival of armies of king crabs, which represent a real threat to these star-filled meadows.

Reference:
Aaron W. Hunter and Kenneth J. McNamara. ‘Prolonged co-existence of “Archaic” and “Modern” Palaeozoic ophiuroids – evidence from the early Permian, Southern Carnarvon Basin, Western Australia.’ Journal of Systematic Palaeontology (2017). DOI: 10.1080/14772019.2017.1353549

Inset image: Brittle stars, by Ratha Grimes.

Newly-described fossil shows how brittle stars evolved in response to pressure from predators, and how an ‘evolutionary hangover’ managed to escape them.

̽��ֱ��threat from predation is an under-appreciated driver of evolutionary change.

Kenneth McNamara

Aaron Hunter

Fossilised Teleosaster creasyi, from the Cundlefo Formation, Gascoyne Junction, Western Australia

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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