Fuxianhuia
Encyclopedia
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
.
, 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 appendage
s 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 diverticulae
; 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.
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.
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
known from the Chengjiang Fauna in China
China
Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture...
. 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
Arthropod head problem
The arthropod head problem is a long-standing zoological dispute concerning the segmental composition of the heads of the various arthropod groups, and how they are evolutionarily related to each other...
.
Morphology
Complete Fuxianhuia specimens are approximately 3 centimetres long. The anterior of Fuxianhuia is marked by an oval scleriteSclerite
A sclerite is a hardened body part. The term is used in various branches of biology for various structures including hardened portions of sponges, but it is most commonly used for the hardened portions of arthropod exoskeletons....
, 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 appendage
Appendage
In invertebrate biology, an appendage is an external body part, or natural prolongation, that protrudes from an organism's body . It is a general term that covers any of the homologous body parts that may extend from a body segment...
s 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
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
, 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 diverticulae
Diverticulum
A diverticulum is medical or biological term for an outpouching of a hollow structure in the body. Depending upon which layers of the structure are involved, they are described as being either true or false....
; a reassignment that has however not been universally accepted. Ventrally, a large plate has been interpreted as a hypostome
Hypostome
A hypostome , is a calcified harpoon-like structure near the mouth area of certain parasitic arthropods including ticks and mites , that allows them to anchor themselves firmly in place on a host mammal while sucking blood...
.
The head shield overlaps a tapering series of 12–17 trunk tergites
Tergum
A tergum is the dorsal portion of an arthropod segment other than the head. The anterior edge is called the base and posterior edge is called the apex or margin. A given tergum may be divided into hardened plates or sclerites commonly referred to as tergites...
, which lead into a set of limb-bearing segments comprising the thorax
Thorax
The thorax is a division of an animal's body that lies between the head and the abdomen.-In tetrapods:...
. 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
Telson
The telson is the last division of the body of a crustacean. It is not considered a true segment because it does not arise in the embryo from teloblast areas as do real segments. It never carries any appendages, but a forked "tail" called the caudal furca is often present. Together with the...
-like spine.
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 acronProstomium
Prostomium is the first body segment in annelid worms. It is in front of the mouth, being usually a small shelf- or lip-like extension over the dorsal side of the mouth. It sometimes bears antennae and eyes. It often functions like a kind of overlip when the animal is feeding...
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
Burgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...
taxon Canadaspis
Canadaspis
Canadaspis was a Cambrian genus of crustacean or euarthropod, a benthic feeder that moved mainly by walking and possibly used its biramous appendages to stir mud in search of food...
has also been suggested.
Further reading
- Hou, Xian-Guang; Aldridge, Richard J., Bengstrom, Jan; Siveter, David J.; Feng, Xiang-Hong 2004; The Cambrian Fossils of Chengjang, China, Blackwell Science Ltd, 233 pp.