Unusual primal arthropod discovered

More than 500 million years old fossil dazzled with huge head and sieve legs

An enormous head-armor, a ring-shaped, shinguled mouth and two sieve-like forelegs - that's what the great arthropod Cambroraster falcatus might have looked like during his lifetime. © Lars Fields / Royal Ontario Museum
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Surprisingly different: paleontologists have discovered an unusual representative of the great primal arthropods of the Cambrian. For the 506 million year old Cambroraster falcatus possessed a mighty tank, a mouth surrounded by horn plates and front legs, which were shaped like rakes. The animal was probably combing the bottom of the sea for prey. This is further evidence of the amazing ecological diversity of this group of animals in the Cambrian, according to the researchers.

They were the first large predators of the oceans: more than 500 million years ago, the radiodonten, early ancestors of the arthropods, dominated the oceans of the Cambrian. These included the up to two-meter-long anomalocarids, carcino-like animals with segmented trunk, two large gripping arms and powerful complex eyes. Most of them hunted their prey while swimming, but there were also species that might filter plankton out of the water.

However, fossils of these prehistoric creatures are so far very rare. "There are very few fossils from this group of animals, typically we find only scattered pieces and fragments, " explains Jean-Bernard Caron of the Royal Ontario Museum in Toronto.

Fossil of Cambroraster falcatus. © Jean-Bernard Caron / Royal Ontario Museum

Primal arthropod with a huge head

All the more surprised Caron and his colleague Joe Moysiuk were when they discovered more than a hundred specimens of a new type of radio record in the famed Burgess Shale formation in the Canadian Rocky Mountains. "Finding so many pieces and uncommonly complete fossils in one place is really a coup, " says Caron. "Because that helps us to better understand how these animals looked and how they lived."

The Cambroraster falcatus baptized species lived about 506 million years ago and became about 30 inches long. With that, she was one of the greatest of her time in the primeval sea. "His height was certainly even more impressive during his lifetime, because at that time almost all animals were smaller than my little finger, " says Moysiuk. Equally impressive, however, were the proportions of this primal arthropod: his head accounted for more than half of the entire body length. display

Backhoes like horseshoe crabs

Even more unusual, however, is a broad, shield-like shell that covered the fossil animal from the head to far above the back. At the front, this head shield was rounded and punctuated by two notches that provided space for the stalked complex eyes. At the back the tank ran out into two lateral and one middle spur. Cambroraster owes his "Nachnahme" to this form of armor: it reminded the researchers of the "Millennium Falcon" spacecraft from the Star Wars series, so they gave it the falcatus style.

More important, however: "With this broad headgear and the deep incisions for the upward-facing eyes, Cambroraster resembles a little the horseshoe crabs living on the ocean floor today, " explains Moysiuk. "It represents a remarkable case of convergent evolution." The researchers suspect that Cambroraster, like today, lived the horseshoe crabs at the bottom of the sea.

Rake made of spines instead of gripping legs

But what is this original arthropod? Indications of this are another peculiarity of Cambroraster: its two front body appendages do not form gripping arms like in Anomalocaris, but rather resemble two broad rakes. Because the appendages wear long, curved spines, which are closely spaced at a distance of only one millimeter.

"We suspect that Cambroraster used these claws to sift the sediment, " says Caron. Probably Cambroraster first chose the mud with his leg lobes and then spread out the "Siebrechen". "Next, these appendices were withdrawn. This filtered feed from the muddy sediment and moved it towards the mouth, "explain the researchers. Then, when Cambroraster brought his front attachments together, they formed a basket that locked in all the trapped animals and facilitated their absorption.

Surprising ecological diversity

"Cambroraster was a distant cousin of Anomalocaris, the top predator of the oceans of the time. But he evidently fed on a completely different way than this, "says Moysiuk. For while Anomalocaris primarily hunted larger prey and pursued it swimming, Cambroraster apparently had a wide prey spectrum: He sifted small animals from the sediment, but could devour and crush large prey his robust teeth and chiming plates indicate.

"Cambroraster is thus contributing to a growing body of fossil evidence that the radiodontents and, in general, arctropod precursors were by no means 'primitive', " emphasize Caron and Moysiuk. "These organisms displayed a high level of ecological diversity and played a number of important roles in the world's first complex animal communities." (Proceedings of the Royal Society B, 2019; doi: 10.1098 / rspb.2019.1079)

Source: Royal Ontario Museum, Royal Society

- Nadja Podbregar