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How crustaceans help bridge the gaps of knowledge in early evolution

The team of researchers led by Dieter Waloßek, zoologist at Ulm University, is working on the organisation of knowledge in their discipline. The taxonomists are documenting living creatures, in particular arthropods, which also includes issues of evolution. Only recently, Waloßek along with Andreas Maas and their foreign colleagues have hit the headlines with a 520-million-year-old fossil find.

The researchers identified a fossil from the Chinese province Yunnan as the world’s oldest fossil find to date when they discovered that the fossil was approximately 10 million years older than previously thought (520 million years). Waloßek is sure that this fossil will contribute to the debate on the evolution of aliferous (winged) arthropods, which represent the majority of all animals.

What is the relationship between arthropods?

Photo of a group of people, a women on the left an four men.
The Biosystematic Documentation working group (from left to right): Bianca Schulz, Joachim Haug, Andreas Maas, Dieter Waloßek and Christopher Castellani © Pytlik/BioRegionUlm
It is still not clear how the four major arthropod groups – insects, crustaceans, centipedes and spiders – are related to each other. Waloßek believes that this is because analysis mainly concentrates on the living representatives of these groups. Moreover, only fragmentary and inaccurate fossil records are available.

Lucky discovery in China

Electron-microscopical photo, compares three different cancers.
Comparison of (from left to right) Bredocaris, Rehbachiella and Martinssonia; the anterior body segments resemble that of Yicaris and the species are clearly related to each other. © University of Ulm, Biosystemaic Documentation working group
The Chinese fossil stands out from other fossil finds in that the fossils of the tiny crustaceans are extraordinarily well preserved, down to the eyes and the bristles on the limbs. Different developmental stages are represented, which enable deep insights into the life of these crustaceans, said Waloßek, who is one of the four authors of the study that was published in “Nature” in October 2007.

The oldest of the modern crustaceans

The phylogenetic group of crustaceans includes nearly 100,000 species, including relatively big creatures such as prawns, lobsters and crabs, but also smaller specimens such as sea fleas and bay barnacles. In geological terms, the group is as much as 480 million years old; some rare fossils even belong to the late Cambrian and date as far back as 505 million years ago. The new fossil is part of the Lower Cambrian and is no doubt the oldest representative of modern crustaceans.
Electron microscopial photo of Yacaris dianensis. The arrows show special parts of the animal.
Yicaris dianensis, oldest representative of modern crustaceans. The Chinese crustacean fossil is unique in that the three external gills are preserved, which are lacking in Cephalocarida but are present in other crustaceous organisms. © University of Ulm, Biosystemaic Documentation working group
Small leaf-shaped structures on the outer limb base of the new species provide evidence related to the much-debated issue on the origin of epipodites. Some biologists believe that these structures also gave rise to insect wings. Waloßek is sure that the discovery of the Cambrian fossil with these extremely soft structures, which have not previously been found in other fossils, makes an important contribution to the much-debated issue on the evolution of winged arthropods. Even more so as the development of wings from such tiny epipods seems to go against currently accepted hypotheses.

Contradicting the Cambrian explosion

The age of Yicaris also gives substantial support to the proposition that the main cladogenetic event that gave rise to arthropods was long before the Cambrian. This contradicts the “Cambrian explosion” hypothesis which holds that most major groups of complex animals appeared around 530 years ago, and that they developed relatively quickly within a time span of only 50 million years.

The paths leading to the publication of the study in Nature ((doi:10.1038/nature06138) were rather tortuous. They date back to the academic beginnings of Waloßek who joined the paleontologist Jürgen Müller’s group in Bonn after his graduation in zoology.
Electron-microscopopical photo of cancers.
Examples of Orsten-type fossils: top left: Martinssonia elongata, a representative of the crustacean stem lineage, and its earliest larva (top left). Row below: representative of Phosphatocopina, one of the largest groups of Orsten animals found. These animals possessed shield covers similar to mussel ones. Damaged shell (left) enabling a view of the legs and the extraordinarily well preserved bristles. Very early larva (right), completely unfolded revealing the legs. Third row: Bredocaris admirabilis and first-stage larva (right). Bottom row: 12-stage Rehbachiella kuinnekullensis and the salt gland on the head shield of an early larva. © University of Ulm, Biosystemaic Documentation working group

“Window into the early development of life”

Jürgen Müller once discovered well preserved fossils in the south of Sweden, termed Orsten fossils, which are about 500 million years old. The arthropod fossils were well preserved and the material was three-dimensional. Scanning microscope images opened “a window onto the early development of life”. At that time unique in the world, Orsten fossils have in the meantime also been discovered in Poland, Canada, Australia, Russia, England, and now also in China.

Waloßek was absolutely fascinated by the arthropods. He travelled the world, to Sweden, the Australian outback, northeast Siberia and China. However, since it was difficult for zoologists like himself to gain access to Orsten material, Waloßek established the C.O.R.E. Group (see link in the top right hand corner), a research network consisting of researchers from six countries. This network helps researchers to gain direct access to Orsten fossils in remote regions of the world. Andreas Maas, biologist from Ulm, is also part of the C.O.R.E. Group.

Problems in building trust

Until 2004, the German researchers worked in close cooperation with Chinese researchers on flattened fossils from the famous Chengjiang fauna of the Lower Cambrian, which were discovered close to the city of Kunming in the southwest of China, recalls the zoologist from Ulm. This cooperation is being continued through a Kunming student who is now working in the Ulm research group.

For Waloßek and Maas, this type of cooperation is extremely important because direct access to the Chinese fossils is impossible. The Chinese are afraid that other colleagues might take something from them. Waloßek explains that several Chinese research groups with whom the Ulm scientists work are in direct competition with each other. That’s why it is important to be seen as a reliable partner. It is difficult to gain the trust of the people there.

Work on photos

That explains why the Ulm scientists previously only had access to photos of the famous crustacean. However, they now have full use of the three-dimensional Orsten fossils, which are only slightly younger than the Yicaris specimen. This has enabled them to carry out morphological comparisons, group the fossils and gain an in-depth understanding of the evolution and development of crustaceans.
Electron-microscopial photo
Modern crustacean from the group of Cephalocarida, which consists of only a few species. The group, which has numerous features that resemble those of fossil records and is very similar to Orsten crustaceans, is of key importance in the investigation of the evolutionary relationships among crustaceans. © University of Ulm, Biosystemaic Documentation working group
Waloßek explained this using the example of the crustaceous manible, which the crustaceans use to break down their food, i.e. as part of the eating process. This mouthpart was originally only a tiny flap with two bristles. (Waloßek found an early form of this apparatus back in 1990).

These small outgrowths evolved on the limbs of the crustaceans and different stages can be found on different legs, sometimes several such outgrowths can be found next to each other. Since the Cambrian 500 million years ago, these outgrowths developed separately in each of the modern crustaceous groups. That is why early crustaceous representatives like Yicaris are so important. The relationship between the crustaceans cannot be deduced solely from modern crustaceans, explains Waloßek.

The identification of a strange pattern was not easy

Based on the prestages of such structures found on the individual limbs, the Ulm scientists produced evidence that this outgrowth was originally missing on all limbs and over time developed on one particular leg, later on developing on legs further along the body in a kind of zigzag pattern. The Ulm scientists had access to the details of this strange pattern over two years ago, and were assisted in their investigations by an extra researcher and observations made on previous finds. This clearly shows how long it takes to clarify ancient structures.

Fossils are facts

For Waloßek fossils such as that of the Chinese crustacean have the value of “facts”; they are “previously living evidence and information”. These fossils are the only ones that provide true evidence about the evolution of species, they tell us much more than relationships assumed on the basis of statistical data or molecular systematics, said Waloßek criticising a trend in his discipline, which started about ten years ago, and which attaches very little importance to morphological features when trying to elucidate evolutionary relationships.

Evolution does not make statistical compromises

“Statistics are important in biology, but have nothing to do with evolution. Molecular changes do not necessarily lead to findings from which evolution can be directly deduced,” said Waloßek explaining that “evolution is a one-time event along the entire time axis, and does not make compromises in the form of ‘consensus trees’”.

The scientists generally agree on the axioms of evolution (the changeability of genetic information that is transferred to subsequent generations through reproduction), said the Ulm researchers, explaining that their criticism mainly refers to “actualistic” science which they consider to lack an overview, quoting for example the global view of Peter Ax who is regarded as one of the most important representatives of phylogenetic systematics.

Hennig’s strict science methodology

The scientific method developed by the German Willi Hennig in the mid-20th century is the “only one that is based on consistent logics and intersubjectively testable hypotheses,” said Waloßek and Maas affirming that they have strongly internalised this type of systematics. Their method does not even spare the biological species concept put forward by Ernst Mayr, an evolutionary biologist who is well known in his own and other disciplines.
Electron-microscopial photo of Rehbachiella kinnekullensis (left) and a recent representative of Cyclestheria hislopi
The early representative of Rehbachiella kinnekullensis (left) and a recent representative of Cyclestheria hislopi have obvious similarities. View on the inner side of the thoracic limbs with small outgrowths (endites) and bristles. On the right, behind the endites, another limb is developing. In Rehbachiella, the bristles had developed but broke off; in Cyclestheria, the bristles are in the larval stage. © University of Ulm, Biosystemaic Documentation working group
The Ulm scientists know that they do not represent the mainstream, because they favour the concept of the “phylogenetic species” following the work of the scientist Rainer Willmann from Göttingen, which considers species development along a time axis. Waloßek: “With our ideas, we are truly ploughing a lonely furrow.”

A bigger picture

The biologists’ lecture notes provide an informative view of the science of systematics (The basics of evolution, phylogenetics, systematics and the evolution of animals), in which they discuss Hennigean evolution (phylogenetic systematics), and try to put their observations into a wider systematic context: “The ability to explain what the living world looks like at time x and from what it developed, brings us closer to understanding the interactions between organisms and their environment. Knowledge of the relationship between species, the phylogenies, enables basic statements in areas such as ecology, biogeography or pharmacology to be made, for example.”

“They will hate us for this“

The two Ulm scientists not only criticise other hypotheses within their scientific discipline, they also enjoy critically discussing seven arguments that are well accepted by well-known evolutionary biologists and opponents of creationism: “Some of our colleagues will hate us for this,” said Dieter Waloßek with a smile. 

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