̽��ֱ�� of Cambridge - vertebrates

Fossil skull sheds new light on transition from water to land

fpjl2 — Mon, 16 Mar 2015 11:18:28 +0000

A new 3D reconstruction of skull of one of the earliest four-footed vertebrate – which differs from earlier 2D reconstructions – suggests such creatures, which lived their lives primarily in shallow water environments, were more like modern crocodiles than previously thought.

̽��ֱ��researchers applied high-resolution X-ray computed tomography (CT) scanning to several specimens of Acanthostega gunnari, one of the ‘four-footed’ vertebrates known as tetrapods which invaded the land during one of the great evolutionary transitions in Earth’s history, 380-360 million years ago. Tetrapods evolved from lobe-finned fishes and display a number of adaptations to help them survive on land.

An iconic fossil species, Acanthostega gunnari is crucial for understanding the anatomy and ecology of the earliest tetrapods. However, after hundreds of millions of years in the ground fossils are often damaged and deformed. No single specimen of Acanthostega preserves a skull that is complete and three-dimensional which has limited scientists’ understanding of how this key animal fed and breathed – until now.

Researchers from Cambridge and Bristol ̽��ֱ�� used specialist software to ‘digitally prepared’ a number of Acanthostega specimens from East Greenland, stripping away layers of rock to reveal the underlying bones.

They uncovered a number of bones deep within the skull, including some that had never before been seen or described, resulting in a detailed anatomical description of the Acanthostega skull.

Once all of the bones and teeth were digitally separated from each other, cracks were repaired and missing elements duplicated. Bones could then be manipulated individually in 3D space. Using information from other specimens, the bones were fitted together like puzzle pieces to produce the first 3D reconstruction of the skull of Acanthostega, with surprising results.

Co-author Dr Laura Porro, formerly of Cambridge’s Department of Zoology and Bristol’s School of Earth Sciences (now at the Royal Veterinary College) said: “Because early tetrapods skulls are often ‘pancaked’ during the fossilization process, these animals are usually reconstructed having very flat heads. Our new reconstruction suggests the skull of Acanthostega was taller and somewhat narrower than previously interpreted, more similar to the skull of a modern crocodile.”

̽��ֱ��researchers also found clues to how Acanthostega fed. ̽��ֱ��size and distribution of its teeth and the shape of contacts between individual bones of the skull (called sutures) suggest Acanthostega may have initially seized prey at the front of its jaws using its large front teeth and hook-shaped lower jaw.

̽��ֱ��team say that these new analyses provide fresh clues about the evolution of the jaws and feeding system as the earliest animals with limbs and digits began to conquer the land.

̽��ֱ��researchers plan to apply these methods to other flattened fossils of the earliest tetrapods to better understand how these early animals modified their bones and teeth to meet the challenges of living on land.

“This work is the first stage of a study towards understanding how the earliest tetrapods fed, and that might lead us to what they fed on, and give further clues as to when and how they started to feed on land,” said co-author Professor Jennifer Clack from Cambridge’s Zoology Department.

Digital models of the original fossils and the 3D reconstruction are also useful in scientific research and education. They can be accessed by researchers around the world, without risking damage to fragile original fossils and without scientists having to travel thousands of miles to see original specimens. Furthermore, digital models and 3D printouts can be easily and safely handled by students taking courses and by the public during outreach events. ̽��ֱ��study is published recently in the journal PLOS ONE.

Adapted from a Bristol ̽��ֱ�� press release.

Inset image: 3D model showing the complete skull on top with ‘exploded’ views of the upper and lower jaws below.

̽��ֱ��first 3D reconstruction of the skull of a 360 million-year-old near-ancestor of land vertebrates has been created by scientists.

This work is the first stage of a study towards understanding how the earliest tetrapods fed, and that might lead us to what they fed on

Jennifer Clack

Porro/Clack

Left: 3D model with the jaws open; the individual bones are colour-coded to show the boundaries between them. Right: Original fossil skull of Acanthostega gunnari

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New fossil find pinpoints the origin of jaws in vertebrates

sc604 — Wed, 11 Jun 2014 17:00:00 +0000

A key piece in the puzzle of the evolution of vertebrates has been identified, after the discovery of fossilised fish specimens, dating from the Cambrian period (around 505 million years old), in the Canadian Rockies. ̽��ֱ��fish, known as Metaspriggina, shows pairs of exceptionally well-preserved arches near the front of its body. ̽��ֱ��first of these pairs, closest to the head, eventually led to the evolution of jaws in vertebrates, the first time this feature has been seen so early in the fossil record.

Fish fossils from the Cambrian period are very rare and usually poorly preserved. This new discovery shows in unprecedented detail how some of the earliest vertebrates developed – the starting point of a story which led to animals such as later fish species, but also dinosaurs and mammals such as horses and even ourselves. ̽��ֱ��findings are published in the 11 June edition of the journal Nature.

Fossils of Metaspriggina were recovered from several locations including the Burgess Shale site in Canada’s Rocky Mountains, one of the richest Cambrian fossil deposits in the world. These fossils shed new light on the Cambrian ‘explosion’, a period of rapid evolution starting around 540 million years ago, when most major animal phyla originated.

Previously, only two incomplete specimens of Metaspriggina had been identified. During expeditions conducted by the Royal Ontario Museum in 2012, 44 new Burgess Shale fossils were collected near Marble Canyon in Kootenay National Park in British Columbia, which provide the basis for this study. Researchers from the ̽��ֱ�� of Cambridge and the Royal Ontario Museum/ ̽��ֱ�� of Toronto used these fossils, along with several more specimens from the eastern United States, to reclassify Metaspriggina as one of the first vertebrates.

̽��ֱ��fossils, which date from 505 million years ago, also show clearly for the first time how a series of rod-like structures, known as the gill or branchial arches, were arranged in the earliest vertebrates. These arches have long been known to have played a key role in the evolution of vertebrates, including the origin of jaws, and some of the tiny bones in the ear which transmit sound in mammals. Until now, however, a lack of quality fossils has meant that the arrangement of these arches in the first vertebrates had been hypothetical.

Vertebrates first appear in the fossil record slightly earlier than these finds, but pinpointing exactly how they developed is difficult. This is because fossils of such animals are rare, incomplete and open to varying interpretations, as they show soft tissues which are difficult to identify with complete certainty.

̽��ֱ��new fossils of Metaspriggina are remarkably well-preserved. ̽��ֱ��arrangement of the muscles shows these fish were active swimmers, not unlike a trout, and the animals saw the world through a pair of large eyes and sensed their surrounding environment with nasal structures.

“ ̽��ֱ��detail in this Metaspriggina fossil is stunning,” said lead author Professor Simon Conway Morris of Cambridge’s Department of Earth Sciences. “Even the eyes are beautifully preserved and clearly evident.”

But it is the branchial arches which makes this discovery so important. Previously, they were thought to exist as a series of single arches, but Metaspriggina now shows that they in fact existed in pairs. ̽��ֱ��anteriormost pair of arches is also slightly thicker than the remainder, and this subtle distinction may be the very first step in an evolutionary transformation that in due course led to the appearance of the jaw. “Once the jaws have developed, the whole world opens,” said Professor Conway Morris. “Having a hypothetical model swim into the fossil record like this is incredibly gratifying.”

“Obviously jawed fish came later, but this is like a starting post – everything is there and ready to go,” said the paper’s co-author Dr Jean-Bernard Caron, Curator of Invertebrate Palaeontology at the Royal Ontario Museum and and associate professor in the Departments of Earth Sciences and Ecology & Evolutionary Biology at the ̽��ֱ�� of Toronto. “Not only is this a major new discovery, one that will play a key role in understanding our own origins, but Marble Canyon, the new Burgess Shale locality itself has fantastic potential for revealing key insights into the early evolution of many other animal groups during this crucial time in the history of life.”

David Wilks, Member of Canadian Parliament for Kootenay-Columbia, noted, “ ̽��ֱ��Government of Canada is excited about this incredible fossil find. As an international leader in conservation and steward of the Burgess Shale, Parks Canada is pleased to provide its research partners with access to the fossils. Their remarkable discoveries inform the work we do to share this rich natural history through our popular guided hikes, and to protect this important Canadian heritage in a national park and UNESCO World Heritage Site.”

A major fossil discovery in Canada sheds new light on the development of the earliest vertebrates, including the origin of jaws, the first time this feature has been seen so early in the fossil record

Having a hypothetical model swim into the fossil record like this is incredibly gratifying

Simon Conway Morris

Left: Illustration of Metaspriggina swimming. Right: Fossil of Metaspriggina from Marble Canyon – head to the left with two eyes, and branchial arches at the top.

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