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Fuxianhuia protensa
Topic Started: Apr 8 2014, 05:42 PM (1,794 Views)
Taipan
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Fuxianhuia protensa

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Temporal range: Lower Cambrian

Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Chelicerata?
Class: Yunnanata†
Order: Fuxianhuiida
Family: Fuxianhuiidae
Genus: Fuxianhuia
Species: Fuxianhuia protensa

Fuxianhuia protensa is a Lower Cambrian fossil arthropod known from the Chengjiang Fauna in China. Its purportedly primitive features have led to its playing a pivotal role in discussions about the euarthropod stem group. Nevertheless, despite being known from many specimens, disputes about its morphology, in particular its head appendages, have made it one of the most controversial of the Chengjiang taxa, and it has been discussed extensively in the context of the arthropod head problem.

Morphology
Complete Fuxianhuia specimens are approximately 3 centimetres long. The anterior of Fuxianhuia is encased in an oval sclerite, from which two stalked eyes emerge. Inserting directly behind this sclerite, on the head shield proper, are six stout antennae. When the head of Fuxianhuia was originally described, twelve additional head appendages, the "sub-chelate" pair were also described. These are geniculate, backward-pointing appendages that lie in a highly stereotypical position (i.e., their position does not vary much from one specimen to another). Partly because of this, and partly because of their rather indistinct morphology, their status as appendages has been questioned. Indeed, on the grounds that these structures seem to lie between two cuticular layers, Waloszek and colleagues have suggested that they are not appendages at all, but rather gut diverticula; a reassignment that has however not been universally accepted. Ventrally, a large plate has been interpreted as a hypostome.

The head shield overlaps a tapering series of 12–17 trunk tergites, which lead into a set of limb-bearing segments comprising the thorax. The limbs are simple in form, consisting of a smooth oval exopod and a stout, annulated endopod. There is no one-to-one correspondence between the thoracic tergites and the limbs, but, rather, there appear to be two or three limbs per tergite.

Behind the thorax is a narrower abdominal region consisting of 14 tergites that bears no appendages. The abdomen is terminated by a telson-like spine.

Brain anatomy
In 2012, a Fuxianhuia fossil has been described with exceptional preservation of brain and optic lobes. The shape and complexity is roughly corresponding to that of a modern malacostracan brain. In general, the Fuxianhuia brain shows the same tripartite morphology of Malacostraca, Chilopoda and Insecta, indicating that such an organization could be precedent to the divergence between these clades.

Cardiovascular anatomy
In 2014 a fossil was described in the journal Nature Communications that preserved in exquisite, unequaled detail the tubular heart and blood vessels, which represent the oldest cardiovascular system yet identified. "The rich vascularization in the head... suggests that the brain of this species required a good supply of oxygen for its performance," said University of Arizona neuroscientist Nicholas Strausfeld, one of the researchers.

Classification
Fuxianhuia was first described from incomplete material, and its true nature did not become apparent until the head and limbs were discovered. Its articulated head region, lack of tergite-segment correspondence and undifferentiated limbs have all been taken to indicate a very basal position in the arthropods, even though an early cladistic analysis suggested, rather, that it was a stem-group chelicerate. The presence of a distinct anterior sclerite bearing the eyes has been taken to suggest that a distinct acron once existed in front of the euarthropod head.

Fuxianhuia is not a unique arthropod: two other taxa, Chengjiangocaris and Shankouia are clearly closely related, although they differ in some details, such as the limbs. A relationship with the Burgess Shale taxon Canadaspis has also been suggested.
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Taipan
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Ancient Fossils Had Heart and Brain

By Becky Oskin, Senior Writer | April 07, 2014 02:26pm ET

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Fuxianhuia protensa, a three-inch-long fossil found in sediments dating from 520 million years ago in China. This is an image of the animal's blood vessel system.

The fossil of an extinct marine predator that lay entombed in an ancient seafloor for 520 million years reveals the creature had a sophisticated heart and blood-vessel system similar to those of its distant modern relatives, arthropods such as lobsters and ants, researchers report today (April 7).

The cardiovascular system was discovered in the 3-inch-long (8 centimeters) fossilized marine animal species called Fuxianhuia protensa, which is an arthropod from the Chengjiang fossil site in China's Yunnan province. It is the oldest example of an arthropod heart and blood vessel system ever found.

"It's really quite extraordinary," said study co-author Nicholas Strausfeld, a neuroscientist at the University of Arizona in Tucson.

The cardiovascular network is the latest evidence that arthropods had developed a complex organ system 520 million years ago, in the Cambrian Period, the researchers said. Arthropods come in a wide range of shapes and sizes today, but the animals have kept some aspects of their basic body plan since the Cambrian. For instance, the brain in living crustaceans is very similar to that of F. protensa, which is a distant relative — but not a direct ancestor of — modern species, Strausfeld said. "The brain has not changed much over 520 million years," he said.

In contrast, blood vessel networks have become both simpler and more complex in the ensuing millennia, in response to changing bodies. The modern relatives of F. protensa are arthropods with mandible jaws, and include everything from insects such as beetles and flies to crustaceans such as shrimp and crabs.

"What we're seeing in the arterial system is the ground pattern, the basic body pattern from which all these modern variations could have arisen," Strausfeld told Live Science.

In the fossil, the creature's organs were preserved like a carbon 'copy.' Its hard exoskeleton is extremely faint, but the soft, internal organs became a dark-brown carbon imprint on fine-grained rock called mudstone.

The animal had a tube-shaped heart positioned near its back, rather than toward the front. The blood vessels extended from the heart along its body segments and clustered near the eyes and brain, which suggests these organs required a rich oxygen supply. The fossil also has eyestalks, antennae, legs and a brain, the researchers reported.

The heart and blood vessels were identified in a fossil in the collection at the Yunnan Key Laboratory for Palaeobiology in China, by an international team of researchers led by London Natural History Museum paleontologist Xiaoya Ma. The findings were published in the April 7 issue of the journal Nature Communications.

Posted Image
Reconstructions of Fuxianhuia protensa, with the cardiovascular system in red, the gut in green and the central nervous system in blue.

In 2012, the same team also reported the oldest example of an arthropod brain, in a different Chengjiang F. protensa fossil.

Strausfeld described the Chengjiang fossil depositas a seafloor "Pompeii" — akin to the Roman city buried in volcanic ash — because of its remarkable preservation of soft body parts such as eyes, guts and brains. The abundance of fossil species in Chengjiang rivals that of the Burgess Shale in Canada, and provides the oldest glimpse into the Cambrian Explosion, when life rapidly diversified into the wide range of body plans known today.

"520 million years ago, we had these basic [body] patterns appear that have been maintained over time," Strausfeld said. "The search is on for the ancestors of these early animals. The question is, what came before?"

http://www.livescience.com/44654-first-fossil-blood-vessel-arthropod.html




Journal Reference:
Xiaoya Ma, Peiyun Cong, Xianguang Hou, Gregory D. Edgecombe, Nicholas J. Strausfeld. An exceptionally preserved arthropod cardiovascular system from the early Cambrian. Nature Communications, 2014; 5 DOI: 10.1038/ncomms4560

Abstract
The assumption that amongst internal organs of early arthropods only the digestive system withstands fossilization is challenged by the identification of brain and ganglia in early Cambrian fuxianhuiids and megacheirans from southwest China. Here we document in the 520-million-year-old Chengjiang arthropod Fuxianhuia protensa an exceptionally preserved bilaterally symmetrical organ system corresponding to the vascular system of extant arthropods. Preserved primarily as carbon, this system includes a broad dorsal vessel extending through the thorax to the brain where anastomosing branches overlap brain segments and supply the eyes and antennae. The dorsal vessel provides segmentally paired branches to lateral vessels, an arthropod ground pattern character, and extends into the anterior part of the abdomen. The addition of its vascular system to documented digestive and nervous systems resolves the internal organization of F. protensa as the most completely understood of any Cambrian arthropod, emphasizing complexity that had evolved by the early Cambrian.

http://www.nature.com/ncomms/2014/140407/ncomms4560/full/ncomms4560.html
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