Wiwaxia is a group of ancient, soft-bodied animals that had scales and spines made of carbon. These features helped protect them from predators. Fossils of Wiwaxia, mostly separate scales but sometimes full, connected fossils, have been found in early and middle Cambrian rock layers around the world. The full-grown animal was about 5 centimeters (2 inches) long, but smaller, juvenile specimens as short as 2 millimeters (0.08 inches) have also been discovered.
Scientists have long debated which group of animals Wiwaxia is related to. Some thought its rows of scales resembled those of certain segmented worms (annelids). However, its mouthparts and overall body shape suggested a connection to shell-less molluscs. Recent studies, based on new details about its mouthparts, scales, and growth patterns, have provided stronger support for a relationship to molluscs. The name "Wiwaxia" comes from Wiwaxy Peak in British Columbia.
Wiwaxia is part of a proposed group called Halwaxiida, which includes Wiwaxia and several other similar Cambrian animals.
Description
This article focuses on the species Wiwaxia corrugata, which is known from many complete fossils found in the Burgess Shale. Other species are known only from broken pieces or small numbers of fossils.
Wiwaxia had a body that was symmetrical on both sides. From above, its body was oval-shaped with no clear head or tail. From the front or back, it looked almost rectangular. It was about 5 centimeters (2 inches) long. Scientists have difficulty estimating its height because the fossils were flattened after death. A typical specimen may have been about 1 centimeter (0.39 inches) tall, not including the spines on its back. The width-to-length ratio of its body remained the same as it grew.
The bottom of Wiwaxia was flat, soft, and not covered in armor. Most of its underside was covered by a single, slug-like foot. Little is known about its internal organs, but its gut appears to have run straight from the front to the back. At the front end of the gut, about 5 millimeters (0 inches) from the front in an average specimen about 2.5 centimeters (0.98 inches) long, there was a feeding structure. This structure had two rows of backward-pointing cone-shaped teeth, or three rows in some larger specimens. The feeding structure was tough and often preserved, but it was not made of minerals and was flexible.
The body of Wiwaxia was covered in eight rows of small, ribbed armor plates called sclerites. These plates lay flat against the body, overlapping so that the back of one covered the front of the one behind. They formed five main sections: the top, the upper sides, the lower sides, the front, and the bottom. Most sclerites were shaped like oval leaves, but those on the lower sides, near the seafloor, were crescent-shaped, like flattened bananas, and formed a single row. Larger specimens (greater than about 15 mm) had two rows of ribbed spines running from front to back, one on each side of the top surface. These spines pointed outward and slightly upward, with a slight upward curve near their tips. The middle spines in each row were usually the longest, up to 5 centimeters (2 inches), but some specimens had shorter middle spines, which may have been partially grown replacements.
Each sclerite was attached separately to the body. The roots of body sclerites were about 40% of their external length, while the roots of the spines were slightly over 25% of their external length. Both types of roots were attached in pockets in the skin, similar to the follicles of mammalian hair. The roots of body sclerites were narrower than the sclerites themselves, but the spines had roots about as wide as their bases. Both types of roots were made of soft tissue. The sclerites and spines had protruding ribs on their upper and lower surfaces. They were not made of minerals but of a tough organic (carbon-based) material. Butterfield (1990) studied some sclerites using optical and scanning electron microscopes and found they were not hollow. Their bases split and spread to form blade-like structures, a pattern also seen in monocot leaves. The sclerites had internal structures of longitudinal chambers, suggesting they were secreted from their bases, similar to sclerites in Lophotrochozoan organisms.
As seen in other species like Marrella and Canadia, evidence suggests Wiwaxia’s sclerites may have been iridescent due to a structure called a diffraction grating, though later studies have questioned this.
The scleritome of Wiwaxia consists of eight rows of sclerites arranged in bundles. These sclerites are shed and replaced as the animal grows, with the number of sclerites in each bundle increasing over time to create a thicker scleritome. Once specimens reach a certain size, spines are added to the scleritome. For W. corrugata, this size is about 15 mm, but it is smaller for W. taijengensis. One juvenile specimen was initially thought to be molting, but it was actually a single, folded individual.
Ecology
The long spines on Wiwaxia's back may have helped protect it from predators. Wiwaxia likely moved by contracting a foot that resembled a slug on its underside. In one fossil, a small creature called Diraphora bellicostata was found attached to one of Wiwaxia's hard, side-facing body parts. This suggests that adult Wiwaxia did not dig into the sea floor or plow through it as they moved. Two other fossils show Diraphora bellicostata attached to Wiwaxia's back body parts. Wiwaxia seems to have lived alone instead of in groups. Its feeding structure may have been used to scrape bacteria from the top of a microbial mat covering the sea floor or to collect food particles from the sea floor.
Classification
During the Cambrian period, many of the main animal groups that exist today were beginning to form. As a result, some ancient animal lineages (which later became extinct) had features that were similar to, but not exactly like, modern groups. These ancient animals often lacked traits common to all members of a modern group, placing them in the "stem group" of a modern taxon. Scientists are still debating whether Wiwaxia belongs to a modern group of animals called the "crown group," and if not, which stem group it might be part of.
When Walcott first described Wiwaxia, he thought it was a type of worm called a polychaete annelid, and he believed its hard, scale-like structures (called sclerites) were similar to the scales of annelids. Later, scientists have proposed many different classifications for Wiwaxia, including: a member of an extinct group related to mollusks; a member of the crown group of polychaetes; a stem group annelid; a group of animals called "problematic bilaterians"; or a stem or early member of the mollusk group.
In 1985, Simon Conway Morris noticed similarities between Wiwaxia and polychaetes but found its sclerites were different from those of annelids. He was more interested in the similarities between Wiwaxia’s feeding structure and the radula (a feeding tool) of mollusks. He placed Wiwaxia in a new group called Molluscata, which he said should include mollusks and another group called hyolithids. Later, he described Halkieria, a related animal, and suggested they were closely connected to Wiwaxia.
Nick Butterfield, a paleontologist inspired by Stephen Jay Gould, agreed that Wiwaxia’s sclerites were not like the soft scales of annelids. He argued that Wiwaxia could not belong to a group called Coeloscleritophora, which includes animals with hollow sclerites, because Wiwaxia’s sclerites were solid. Instead, he thought Wiwaxia’s sclerites were similar to the bristles (called setae) of modern annelids, which are made of the same material, have the same structure, and attach to the body in the same way. He also noted that Wiwaxia’s feeding structure might be on the sides of its head, a common feature in polychaetes. Butterfield classified Wiwaxia as part of a modern group called Phyllodocida and pointed out that Wiwaxia’s lack of visible body segments was not unusual, as some modern polychaetes also lack segments except during development. However, he later noted that Wiwaxia lacked certain features expected in polychaetes, suggesting it might be a stem group annelid, or an evolutionary "relative" of modern annelids.
Conway Morris and Peel (1995) largely supported Butterfield’s ideas, treating Wiwaxia as an ancestor of polychaetes. They confirmed that Wiwaxia’s sclerites matched those of two Burgess Shale polychaetes. They also noted that one Wiwaxia specimen had a small shell, possibly a leftover feature from an earlier stage of evolution, similar to a group of modern polychaetes. However, they still believed Wiwaxia’s feeding structure was more like a molluscan radula. They also argued that Wiwaxia was closely related to halkieriids, a group with similar sclerites, though smaller and more numerous. They noted that Butterfield had found hollow sclerites in Wiwaxia and presented a diagram showing its evolutionary relationships.
In 2003, Amélie H. Scheltema and others argued that Wiwaxia’s feeding structure was similar to the radulas of modern shell-less mollusks called aplacophorans, and that its sclerites were very similar to those of mollusks. They concluded Wiwaxia belonged to a group that includes mollusks. Scheltema also noted similarities between Wiwaxia and the larvae of certain mollusks, which have calcareous sclerites arranged in symmetrical patterns.
In 2004, Danny Eibye-Jacobsen argued that Wiwaxia lacked clear features that would place it as a polychaete or annelid. He noted that bristles are found in multiple groups, including mollusks, annelids, and brachiopods, so even if Wiwaxia’s sclerites resembled bristles, this would not prove it was closely related to annelids. He also pointed out that the number of sclerites in different body zones of Wiwaxia did not match any known segmentation patterns, making it harder to classify as a polychaete. He concluded there was no strong evidence to place Wiwaxia as a "proto-annelid" or "proto-mollusc," though he thought objections to the annelid classification were stronger.
In 2006, Butterfield returned to the debate, reiterating his earlier arguments that Wiwaxia was an early polychaete. He emphasized that bristles covering the back are a feature unique to polychaetes.
A 2012 study redescribing Wiwaxia’s mouthparts found strong similarities to the radula of mollusks, weakening arguments for an annelid classification and suggesting Wiwaxia was indeed a mollusc.
Occurrence
Wiwaxia was first described in 1899 by G. F. Matthew, based on a single spine found in the Ogyopsis Shale. It was initially named "Orthotheca corrugata." Later, in 1911, American paleontologist Charles Doolittle Walcott discovered more fossils during a field trip to the Burgess Shale in the Canadian Rocky Mountains. He classified the fossils as a type of annelid worm called Wiwaxia corrugata, noting similarities to other worm groups like the Aphroditidae and Polynoidae.
In 1966 and 1967, Harry B. Whittington led a team that revisited the Burgess Shale and collected many fossils. These fossils took years to study. From the Greater Phyllopod bed, 464 complete Wiwaxia fossils were found, making up 0.88% of the fossils in that area. In 1985, Simon Conway Morris, a member of Whittington’s team, published a detailed study and concluded that Wiwaxia was not a polychaete worm. Most fossils were found near the Burgess Shale until 1991, when partial fossils were discovered in Australia’s Georgina Basin. In 2004, additional fossils possibly representing two different species were found in the same area.
Articulated Wiwaxia fossils have been found in Cambrian Stage 3 deposits of Xiaoshiba, China. Fragmentary fossils have also been discovered in Chengjiang, Guizhou, China (Cambrian Series 2), and in the Miaolingian beds of the Kaili Formation, China. Other fossils have been found in the Tyrovice Member, Buchava Formation, Czech Republic; the Lower Cambrian Mount Cap Formation, Canada; the Emu Bay Shale, Kangaroo Island, South Australia; the upper Botomian Stage, Lower Cambrian; and the Middle Botomian Sinsk Biota, Siberia, Russia. Isolated sclerites are also common in small carbonaceous fossil records. These findings show that Wiwaxia lived in many parts of the world, across all ancient latitudes and most ancient continents.
Chinese fossils were once thought to belong to a different species. Like W. corrugata, they had spines and sclerites, but their sclerites had more ribs and two different rib thicknesses. Under a microscope, Chinese sclerites looked similar to those from the Burgess Shale and Mount Cap, but Chinese fossils developed spines earlier in life. Sclerites with knobs from three locations seem to belong to a different species, and another species was found in Xiaoshiba deposits. Surprisingly, the differences between species are small, as they all share a similar structure, showing little change over about 15 million years.
In areas where fossils are poorly preserved, isolated spines are more common than sclerites, suggesting spines were harder to destroy or more often collected. In well-preserved sites like the Phyllopod bed, the number of spines and sclerites matches the proportions seen in complete fossils. Younger spines possibly from Wiwaxia-like creatures have been found in the Valongo Formation (Middle Ordovician: Dapingian-Darriwilian) of northern Portugal and reported, though not fully described, in the Fezouata Biota.