Wiwaxia

Wiwaxia was a unique genus of prehistoric, soft-bodied animals. These creatures lived during the early and middle Cambrian periods. They are famous for their complex protective armor. This armor consisted of carbonaceous scales and long spines. These structures helped protect the animal from predators in the ancient sea.

Today, scientists study Wiwaxia fossils to understand early animal evolution. Most of these fossils are found in deposits across the globe. Some fossils show only isolated scales. Others show complete, articulated bodies of the animals.

The body of Wiwaxia was bilaterally symmetrical. This means its left and right sides were mirror images. From above, the body looked like an ellipse. It had no distinct head or tail. From the front or rear, it appeared almost rectangular. The animal reached a specific length when fully grown. However, the smallest known juveniles were much shorter. The ratio of width to length stayed the same as they grew.

The underside of the animal was flat and unarmored. It used a single, slug-like foot to move along the sea floor.

Protection came from a system called a scleritome. This was a collection of eight rows of small, ribbed armor plates called sclerites. These sclerites overlapped like roof tiles. The rear of one plate covered the front of the next. They formed five main regions: the top, the upper sides, the lower sides, the front, and the bottom. Most sclerites were shaped like oval leaves. The ones nearest the sea floor were crescent-shaped, like flattened bananas. Larger specimens, over 15 mm long, also grew two rows of long, curved spines. These spines ran from the front to the rear along the top. Each sclerite was rooted in a skin pocket, similar to mammalian hair follicles. These roots were made of soft tissue. The armor itself was made of a tough, carbon-based biopolymer.

Growth in Wiwaxia involved a specific process called ontogeny. The animal periodically shed and replaced its sclerites as it grew. As the animal aged, the number of sclerites in each bundle increased. This created a thicker, stronger scleritome. Once the animal reached a certain size, it began to grow its long dorsal spines. For the species Wiwaxia corrugata, this happened at about 15 mm. In other species, like Wiwaxia taijengensis, this happened at a much smaller size. This shows that different species grew their defenses at different stages of life.

We can reconstruct how Wiwaxia lived by looking at its anatomy. It had a feeding apparatus at the front of its gut. This apparatus had two or three rows of backward-pointing, conical teeth. These teeth were tough but flexible. The animal likely used them as a rasp to scrape bacteria off microbial mats. It may have also used them like a rake to gather food particles.

Evidence suggests Wiwaxia was a solitary animal. It did not live in large groups. One specimen shows a small brachiopod attached to its side. This suggests Wiwaxia did not burrow deeply into the sea floor as it moved.

Determining the classification of Wiwaxia has caused intense scientific debate. Early researchers like Walcott thought it was a polychaete annelid worm. They believed the scales were similar to the scales of modern worms. Later, Simon Conway Morris suggested it was a mollusc. He was impressed by the feeding apparatus, which looked like a molluscan radula. Nick Butterfield studied the sclerites under microscopes. He found they were solid rather than hollow. This led him to believe they were similar to the bristles of annelids. He suggested Wiwaxia might be a "stem-group" annelid. This means it is an evolutionary relative of modern annelids.

Other scientists have offered different views. Amélie H. Scheltema and her team argued for a molluscan connection. They compared the feeding parts to those of shell-less molluscs. Danny Eibye-Jacobsen argued that no single feature firmly places Wiwaxia in one group. He noted that the number of sclerites did not match the patterns seen in segmented worms. Because of these conflicting details, Wiwaxia remains a puzzle. It sits at a crossroads of many different animal lineages. Studying these ancient creatures helps us map the complex tree of life.