Orthrozanclus
Encyclopedia
Orthrozanclus reburrus ("Dawn scythe with bristling hair") is a sea creature known from the Middle Cambrian (~) Burgess shale
, about one centimeter long, with long spikes protruding from its armored body
The describers of this fossil animal, Simon Conway Morris
and Jean-Bernard Caron, say Orthrozanclus may have formed a link between the halkieriid
and the wiwaxiid
families, uniting them tentatively in a group called "Halwaxiida
", characterized by a similar type of body armor; these organisms might have been stem group molluscs, or fall as a stem group to the larger lophotrochozoan clade (containing molluscs, annelid
s and brachiopod
s). However, the status of the Halwaxiid grouping is not universally accepted.
and in 2006 referred to it as "scleritomorph C" without a detailed description. In 2007 Caron and Simon Conway Morris
published a description and named the fossil Orthrozanclus reburrus, which means "Dawn scythe with bristling hair".
s", which fitted close to the body and one of which ran all the way round the animal; 16 to 20 long, upwards-curving spines on each side of the body. The sclerites and spines were unmineralized, and had internal cavities that appear to have been circular in cross-section
. The shell was convex and shaped like a triangle with rounded corners. It had a bulge at the front, a raised rear edge and a ridge along the middle that flared out towards the rear. It also had finely spaced rings that indicate growth by addition of material round the edges, and coarser ridges which may indicate that the animal was metameric
, in other words built out of repeated "modules". The function of the shell at the front is unknown.
The animal clearly lived on the sea-floor, and is thought to have had a muscular foot rather like that of a snail.
Orthrozanclus′ sclerites are very similar to those of its Burgess Shale
contemporary Wiwaxia
. Its shell is very similar to: one of the two Burgess Shale shell types labelled Oikozetetes
; the forward shell of Halkieriids, most of which are dated to the Early Cambrian
; and those of other Early Cambrian
fossils such as Ocruranus and Eohalobia. These similarities suggest that Orthrozanclus was an intermediate form between Wiwaxia and the Halkieriids and that all three of these taxa
formed a monophyletic clade
, in other words a group that consists of a common ancestor and all of its descendants. However this draws Orthrozanclus into a complex debate that has gone on since 1990 about whether Wiwaxia is more closely related to molluscs or to polychaete
worms, and therefore about the entire "family tree" of the Lophotrochozoa
, a "super-phylum
" that is thought to contain modern molluscs, annelids and brachiopod
s as well as some extinct groups. The main opponent of this view, Nicholas Butterfield, proposes that Wiwaxia is more closely related to annelids while Halkieriids are very close to molluscs.
Cladograms: Conway Morris and Caron (2007) — Hypothesis 1 (left) and 2 (right).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...
, about one centimeter long, with long spikes protruding from its armored body
The describers of this fossil animal, Simon Conway Morris
Simon Conway Morris
Simon Conway Morris FRS is an English paleontologist made known by his detailed and careful study of the Burgess Shale fossils, an exploit celebrated in Wonderful Life by Stephen Jay Gould...
and Jean-Bernard Caron, say Orthrozanclus may have formed a link between the halkieriid
Halkieria
Halkieria is a genus of fossil organisms from the Lower to Middle Cambrian. It has been found on almost every continent in Lower to Mid Cambrian deposits, forming a large component of the small shelly fossil assemblages...
and the wiwaxiid
Wiwaxia
Wiwaxia is a genus of soft-bodied, scale-covered animals known from Burgess shale type Lagerstätte dating from the upper Lower Cambrian to Middle Cambrian. The organisms are mainly known from dispersed sclerites; articulated specimens, where found, range from to a little over 50.8 millimeters in...
families, uniting them tentatively in a group called "Halwaxiida
Halwaxiida
Halwaxiida or halwaxiids is a proposed clade loosely uniting scale-bearing Cambrian animals, which may lie in the stem group to molluscs or lophotrochozoa. Some palaeontologists question the validity of the Halwaxiida clade.-Definition and members:...
", characterized by a similar type of body armor; these organisms might have been stem group molluscs, or fall as a stem group to the larger lophotrochozoan clade (containing molluscs, annelid
Annelid
The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...
s and brachiopod
Brachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...
s). However, the status of the Halwaxiid grouping is not universally accepted.
History of discovery
J.B. Caron and D.A. Jackson found a specimen in the Burgess ShaleBurgess 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...
and in 2006 referred to it as "scleritomorph C" without a detailed description. In 2007 Caron and Simon Conway Morris
Simon Conway Morris
Simon Conway Morris FRS is an English paleontologist made known by his detailed and careful study of the Burgess Shale fossils, an exploit celebrated in Wonderful Life by Stephen Jay Gould...
published a description and named the fossil Orthrozanclus reburrus, which means "Dawn scythe with bristling hair".
Description
Orthrozanclus reburrus was 6–10.3 mm (0.236220472440945–0.405511811023622 in) long including its long spines, and had a fairly slim, roughly oval body that tapered towards the rear, was distinctly convex on top but was surrounded by a flatter rim. The underside was soft and unarmored, but the upward-facing surfaces were armored by: a small shell, near the front end; three zones of armor plates called "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....
s", which fitted close to the body and one of which ran all the way round the animal; 16 to 20 long, upwards-curving spines on each side of the body. The sclerites and spines were unmineralized, and had internal cavities that appear to have been circular in cross-section
Cross section (geometry)
In geometry, a cross-section is the intersection of a figure in 2-dimensional space with a line, or of a body in 3-dimensional space with a plane, etc...
. The shell was convex and shaped like a triangle with rounded corners. It had a bulge at the front, a raised rear edge and a ridge along the middle that flared out towards the rear. It also had finely spaced rings that indicate growth by addition of material round the edges, and coarser ridges which may indicate that the animal was metameric
Metamerism (biology)
In biology, metamerism is a linear series of body segments fundamentally similar in structure, though not all such structures are entirely alike in any single life form because some of them perform special functions....
, in other words built out of repeated "modules". The function of the shell at the front is unknown.
The animal clearly lived on the sea-floor, and is thought to have had a muscular foot rather like that of a snail.
Classification
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...
contemporary Wiwaxia
Wiwaxia
Wiwaxia is a genus of soft-bodied, scale-covered animals known from Burgess shale type Lagerstätte dating from the upper Lower Cambrian to Middle Cambrian. The organisms are mainly known from dispersed sclerites; articulated specimens, where found, range from to a little over 50.8 millimeters in...
. Its shell is very similar to: one of the two Burgess Shale shell types labelled Oikozetetes
Oikozetetes
Oikozetetes is a scleritome-bearing Cambrian organism which may be related to the halkieriids.They bore two morphologically different shells, which were probably calcareous while the organism was alive...
; the forward shell of Halkieriids, most of which are dated to the Early Cambrian
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...
; and those of other Early Cambrian
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...
fossils such as Ocruranus and Eohalobia. These similarities suggest that Orthrozanclus was an intermediate form between Wiwaxia and the Halkieriids and that all three of these taxa
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
formed a monophyletic clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
, in other words a group that consists of a common ancestor and all of its descendants. However this draws Orthrozanclus into a complex debate that has gone on since 1990 about whether Wiwaxia is more closely related to molluscs or to polychaete
Polychaete
The Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Indeed, polychaetes are sometimes referred to as bristle worms. More than 10,000...
worms, and therefore about the entire "family tree" of the Lophotrochozoa
Lophotrochozoa
The Lophotrochozoa are a major grouping of protostome animals. The taxon was discovered based on molecular data. Molecular evidence such as a result of studies of the evolution of small-subunit ribosomal RNA supports the monophyly of the phyla listed in the infobox shown at right.-Terminology:The...
, a "super-phylum
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
" that is thought to contain modern molluscs, annelids and brachiopod
Brachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...
s as well as some extinct groups. The main opponent of this view, Nicholas Butterfield, proposes that Wiwaxia is more closely related to annelids while Halkieriids are very close to molluscs.
| crown group Crown group A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms... ) }} >2= }} }} }} |
The authors regard Hypothesis 1 as better supported by the evidence, but not robust.
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