̽��ֱ�� of Cambridge - Arthropod

Millipedes ‘as big as cars’ once roamed England

sc604 — Tue, 21 Dec 2021 05:50:30 +0000

̽��ֱ��largest-ever fossil of a giant millipede – as big as a car – has been found on a beach in the north of England.

Opinion: Our 500 million-year-old nervous system fossil shines a light on animal evolution

Anonymous — Mon, 29 Feb 2016 16:50:59 +0000

̽��ֱ��nervous system is the command centre of an animal’s body, carrying all the complex electrical signals for the actions that keep it alive, such as moving and eating. Because of its critical function, the nervous system also contains a lot of information about an animal’s evolution, and can even help us understand how different groups relate to each other. But preserved fossilised nervous systems from extinct creatures are extremely rare.

That’s why my colleagues and I were excited to discover one of the most detailed and well-preserved nervous system fossils ever found, from a crustacean-like animal known as a fuxianhuiid that lived more than 500m years ago. These fossils – which come from the Xiaoshiba biota in south China – are so well preserved that you can see individual nerve roots ten times thinner than a human hair. ̽��ֱ��findings offer the most detailed view of the nervous system in early animals available to date, and inform us about the early evolution of the nervous system in these creatures and their close relatives.

Ancient arthropod

̽��ֱ��fuxianhuiids (pronounced foo-see-an-who-eeds) were primitive animals known only to have lived during the early Cambrian period in China, some 515-520m years ago. Fuxianhuiids are widely regarded as being important for understanding the early evolution of the arthropods. This is a large group of animals with jointed limbs and hard exoskeletons that also includes insects, arachnids and crustaceans. So finding preserved nervous tissues in fuxianhuiids tells us a lot about their early evolution and that of their close relatives.

By painstakingly chipping away small pieces of rock from the fossil using a fine needle, my colleagues in China were able to reveal the ventral nerve cord running through their entire body. ̽��ֱ��ventral nerve cord is part of the nervous system, very much similar to our spinal cord, and it resembles a string of beads.

Each of the “beads” actually corresponds to a ganglion, a condensed mass of nerve cells whose function is to control the legs on each segment of the body in fuxianhuiids and other arthropods. Our fossils also preserve dozens of delicate nerves that emerge at either side of the ventral nerve cord and that would have been connected to the legs and other parts of the body.

Ventral cord. Jie Yang (Yunnan ̽��ֱ��, China) (left) and Javier Ortega-Hernández ( ̽��ֱ�� of Cambridge, UK)

Finding the fossilised remains of an animal’s nervous system is extremely unusual, as the brain and ventral nerve cord are mainly made of fatty tissues and decay very quickly under normal circumstances. But under exceptional conditions – such as very rapid burial in environments with little oxygen – these delicate structures can be preserved in the fossil record.

In the last five years, various studies, have reported the preservation of brains in Cambrian arthropods, which has greatly improved our understanding of their evolution. But in most cases, we can only recognise the broad outline of the brain and so there are limits to the information that can be extracted from the fossils. Our study is the first time that a complete ventral nerve cord has been described in such an extraordinary level of detail.

More importantly, the ventral nerve cord of fuxianhuiids is rather unique among arthropods. Whereas most arthropods also posses condensed ganglia, they generally lack the dozens of delicate nerve roots that are found in fuxianhuiids. However, this peculiar organisation can be found in velvet worms (or onychophorans), a group of animals resembling worms with legs that are cousins to the arthropods. So the fuxianhuiid ventral nerve cord is an intermediate between the nervous system of arthropods and velvet worms.

Common ancestral link

This means we can interpret the dozens of nerves in fuxianhuiids as an ancient trait inherited from the last common ancestor between velvet worms and arthropods. This is similar to how the the feet of modern birds resemble the feet of dinosaurs, because they were also inherited from their last common ancestor

By contrast, the presence of ganglia on the nerve cord of fuxianhuiids is an innovation that occurred in the evolution of arthropods. Keeping with the analogy, this is like how feathers are an innovation that occurred in the evolution of birds.

̽��ֱ��most interesting conclusion we can draw is that the origin of the arthropod nervous system required the dramatic reduction in the number of nerves, and that this event took place after the early Cambrian period. Without fuxianhuiids, it would have been impossible to attain this depth of knowledge on the evolution of the nervous system.

Javier Ortega-Hernandez, Research fellow in palaeobiology, ̽��ֱ�� of Cambridge

This article was originally published on ̽��ֱ��Conversation. Read the original article.

̽��ֱ��opinions expressed in this article are those of the individual author(s) and do not represent the views of the ̽��ֱ�� of Cambridge.

Javier Ortega-Hernández (Department of Zoology) discusses what the discovery of the earliest known fossilised nervous system could tell us about evolution.

Top: Jie Yang, Bottom: Yu Liu

Top: Complete specimen of Chengjiangocaris kunmingensis from the early Cambrian Xiaoshiba biota of South China. Bottom: Magnification of ventral nerve cord of Chengjiangocaris kunmingensis.

̽��ֱ��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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Amazing feet of science: Researchers sequence the centipede genome

cjb250 — Tue, 25 Nov 2014 19:00:01 +0000

An international team comprising more researchers than the arthropod has legs (106 researchers) has sequenced the genome of Strigamia maritima, a Northern European centipede, and found that its genome, while less than a tenth the size of a human’s, has around two-thirds the number of genes, distributed across one pair of large chromosomes and seven pairs of tiny ones, including X and Y sex chromosomes. ̽��ֱ��results are published today in the journal PLOS Biology.

Arthropods are the most species-rich group of animals on Earth. There are four classes of arthropods alive today: insects, crustaceans, chelicerates (which include spiders and scorpions) and myriapods. This latter class, which includes centipedes, is the only class for which no genome has yet been sequenced.

Myriapods arose most likely from marine ancestors that invaded the land more than 400 million years ago. All myriapods have a large number of near-identical segments, most bearing one or two pairs of legs. However, despite their name, centipedes never have a hundred legs. Strigamia maritima, which lives in coastal habitats, can have from 45 to 51 pairs – but the number of pairs is always odd, as it is in all centipedes.

̽��ֱ��team found that the centipede genome is more conserved than that of many other arthropods, such as the fruit fly, with less gene loss and scrambling. This suggests that the centipede has evolved slowly from their common ancestor and should allow researchers to draw comparisons between very different animals, which are not obvious when working with fruit flies or other fast evolving insects. For example, the researchers found parallels in the way that the brain is patterned between centipedes and other very distantly related animals such as marine worms. Such comparisons will enable scientists to build an overall picture of how genetic changes underlie the diversity of all animals.

“With genomes in hand from each of the four classes of living arthropod, we can now begin to build a picture of the genetic make-up of their common ancestor,” says Dr Frank Jiggins, of the Department of Genetics at the ̽��ֱ�� of Cambridge, one of the researchers involved “For example, by comparing flies and mosquitoes with centipedes, we have shown that the innate immune systems of insects are much older than previously appreciated.”

One of the most surprising findings is that these centipedes appear to have lost the genes encoding all of the known light receptors used by animals, as well as the genes controlling circadian rhythm, the body clock.

“Strigamia live underground and have no eyes, so it is not surprising that many of the genes for light receptors are missing, but they behave as if they are hiding from the light. They must have some alternative way of detecting when they are exposed,” says Professor Michael Akam, Head of the Department of Zoology at Cambridge and one of the lead researchers. “It’s curious, too, that this creature appears to have no body clock – or if it does, it must use a system very different to other animals.”

̽��ֱ��centipede’s genome sequence is of more than just scientific interest, argues Professor Akam. “Some of its genes may be of direct use. All centipedes inject venom to paralyse their prey,” he explains. “Components of venom often make powerful drugs, and the centipede genome will help researchers find these venom genes.”

Reference
Chipman, AD et al. ̽��ֱ��First Myriapod Genome Sequence Reveals Conservative Arthropod Gene Content and Genome Organisation In the Centipede Strigamia maritima. PLOS Biology; 25 Nov 2014

What it lacks in genes, it certainly makes up for in legs: the genome of the humble centipede has been found to have around 15,000 genes – around 7,000 fewer than a human.

Strigamia live underground and have no eyes, so it is not surprising that many of the genes for light receptors are missing, but they behave as if they are hiding from the light

Michael Akam

Anthony Barber

Strigamia maritima male, Loch Linnhe, Scotland

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