Mysterious trilobite killer: diving into the Cambrian ocean and a crime mystery from half a billion years ago

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The largest sea monster from the Cambrian oceans (538.8 ± 0.2 – 485.4 ± 1.9 million years ago), Anomalocaris canadensis has so far been considered an apex predator of its ecosystem and the culprit behind the trilobite armor damage visible in fossils. However, new analyses of its grasping legs showed that it would not have been able to split hard armor. Anomalocaris canadensis grew up to a meter and had to rely more on speed and feed on soft-tissue creatures. The investigation into the Cambrian trilobite killings has thus begun in earnest.

Bez tytulu 1
pic. Przemyslaw Trześniowski, Adventurephotography

Diving into the ocean depths of the past….

Although the name Cambrian comes from the original Welsh Celtic name Cymru (read: Kemri), the richest evidence of life from that period is found in the mountains of today’s Western Canada. That’s where we will go together to dive deep and backwards in time by some 515 million years. Are you able to imagine diving in the Cambrian oceans? Without the air cylinder taken from our time, you would have lost consciousness even before entering the water, because the oxygen content in the atmosphere at the time was only 13 percent. Carbon dioxide levels, on the other hand, were many times higher than today. Hypoxia and hypercapnia even today take a deadly toll on closed circuit divers.

It was warm – carbon dioxide probably caused the greenhouse effect. Most of the Cambrian landmasses are the bare rocks of Laurence, Angara, Gondwana and the Baltics. Small patches of rachitic greenery in wetter areas are mostly simple mosses and photosynthesizing bacteria. The sun does not shine very brightly. There are no animals or trees. No ice caps. The Moon, being much closer to Earth than it is now, causes tides whose amplitude reaches as much as 15 m! Almost all of Poland was under water at the time, and our souvenirs from that time are the Vojtosh limestones in the Sudetes and the quartz sandstones in the Swietokrzyskie Mountains, where Anomalocaris was found.

So let’s go back to the sea, because life, after all, was born in water. We can dive into the oceans of Iapetus, Panthalassa or Prototetida [1]. Let it be the Burgess Shale on the shelf off the coast of Laurence. This is the place to which Stephen Jay Gould, co-author of the concept of punctualism and tireless popularizer of evolutionary theory (he believed that contingency plans – contingency, taking the lead in drastic changes in the environment, which Richard Dawkins strongly opposed), devoted an entire book. But be warned – we are diving to a hundred meters, and this requires much more than one cylinder or rebrether….

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pic. A contemporary vision of diving in the Cambrian ocean (CC BY 4.0 license)

Burgess Shale is one of the richest paleontological sites from the Cambrian period. It lies on the border of the Canadian states of Alberta and British Columbia. It was discovered in 1909. Charles Doolittle Walcott of the Smithsonian Institution, who returned there until his death in 1927, collecting more than 65,000 fossils during that time. Since 1967, they were intensively studied by Cambridge paleontologists Harry Blackmore Whittington, Simon Conway Morris and Derek Ernest Gilmor Briggs. Burgess Shale has been a UNESCO World Heritage Site since 1980. There we find mainly arthropods, but also, and this brings the greatest value to our knowledge of the planet’s past, soft-bodied lilliforms, centipedes, worms and unicellular organisms that, having no hard parts, are very poorly preserved in the fossil record elsewhere. In the Burgess Shale, they were unexpectedly covered with silt and deprived of oxygen, resulting in immediate death, and today allows us to study even the remains of viscera or muscles. Many of these wonders can be viewed at the Yoho National Park or in museum collections.

How unlike today was Cambrian life? Stephen Jay Gould believed that life at that time was much more diverse than it has ever been since, although Simon Conway Morris, favored by him, took a different view [2]. We cross the edge of a shallow plateau and drop down along a vertical cliff. In the water column float the flashing lights of the ribworms and jellyfish, which drag their menacing, armored arms behind them. The wall and later the bottom are overgrown with sponges, existing since the Ediacaran (635 – 538.8 ± 0.2 million years ago), when multicellular life first appeared on Earth.

Today, the cluster of pink-bellied sponges(Demospongiae) includes most of the representatives in the Earth’s oceans, and the most spectacular are the magnificent barrel sponge(Xestospongia testudinaria), quite common in the dives of the archipelagos of Indonesia or the Philippines, or the golden-green Aplysina fistularis, which lives in the Caribbean. An attentive diver will spot brachiopods similar to clams, and still present today as a relic group. You may encounter them contemporarily while snorkeling in the Philippines – look for the nicely colored shells of Frenulina sanguinolenta. There are also many other brachiopods in different parts of the Earth [3]. Further on, the echinoderms (Edrioasteroidea, Eocrinoidea and Helicoplacoidea), although they had radial symmetry, were rather unlike today’s serpentines, starfish and sea urchins. Further blooming anemones and perhaps rare lilacs? From the bottom, small non-mogonites are poking out. Their descendants are also encountered on diving sites today. Then it only gets more exotic….

If only the grotesque hallucigens, marching along the bottom and eating spikes. They are unlike anything living on Earth today. Perhaps that’s why their reconstruction took more than a century? The first hallucigeni fossils were still discovered by Walcott, who, however, mistook them for canadia [4]. The new species in the fossil was spotted by Conway Morris, but he reconstructed it spiny downward and without legs, which were simply less well preserved [5]. It was only a few years ago that it was possible to determine where these amazing creatures had a back and a front body [6]. Sneaking through the water in the bottom zone, several-centimeter-long Canadia spinosa try to deter attackers with their iridescent bristles [7]. The bottom-dwelling vivax have chosen scaly armor – caracenas enriched with spikes [8]. Indeed, the main invention of the time was the exoskeleton – inevitably the soft-tissue creatures’ response to the threat. With the emergence of active predators, an arms race between hunters and prey in Earth’s oceans must have begun. A race that, after all, continues to this day. Potential Cambrian victims could choose between hiding in the silt or producing armor – the so-called “armor”. Verdun Syndrome. The Cambrian oceans are thus ruled by arthropods: the somewhat horsetail-like Cambroraster [9], the armored Saperion or the ubiquitous five-eyed Opabinia.

We have plunged into the Cambrian explosion of life that took place 538.8 – 515 million years ago, although some believe it lasted 12 million years less. Almost all the types of animals known today evolved then, which Charles Darwin considered quite a challenge to evolution [10]. The question remains, however, was this really an explosion of life, or just the fossil explosion we observe in the fossil record today? The soft-tissue life of the ediacar is much less preserved, so we may not find enough traces of it. Here’s how Peter D. Ward – a paleontologist, astrobiologist and diver – imagined such a dive:

Swimming lazily, a few meters above the vibrant bottom, we see a meter-long Anomalocaris […]. The fallout. A powerful paddle-like tail and numerous legs cushion the shock. Cutting with large pincers, he begins foraging, catching smaller, diverging arthropods. Suddenly he notices a larger invertebrate, heavily armored […] The lobopod’s phosphate plates offer some protection, but soon it too succumbs, and Anomalocaris positions itself over the body, beginning to feed with a peculiar […] mouth apparatus.[11] (author’s own translation).

The vision of one of my idols has not stood the test of time. Also wrong earlier was Simon Conway Morris, who described Cambrian diving in what is now western Canada in his book on the Burgess Shale. Indeed, he included an engraving of an anomalocaris hunting a defenseless trilobite.

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pic. Model of a trilobite of the order Redlichia, considered the oldest of all orders of trilobites ©Museums Victoria, reg. no. P 315739, excerpted by Tom Davies, photo: Benjami n Healley (CC BY 4.0 license)

Trilobites

Trilobites are some of the most interesting and imagination-provoking Paleozoic fossils. Charles Darwin’s generation considered them to be the oldest animals ever to have lived on Earth, and indeed they were the first Cambrian fossils found. They had heavily scaled armor, which can be clearly seen in the fossil record – these are the so-called “scaled armor”. guide fossils. Trilobites were a cluster of highly differentiated arthropods (Arthropoda). More than 22,000 are known to science. species! They appeared at the end of the Cambrian explosion (about 521 million years ago) and survived two great extinctions: the Ordovician ~445 million years ago, when 85 percent became extinct. species known to science, and the Devonian ~372-359 million years ago, when half of the known genera became extinct, including 70 percent. Trilobite genera – all orders except Proetida! They were defeated only by the third and largest, at the Permian-Triassic boundary 251.9 million years ago – the Mother of All Extinctions. Thus, they lived on Earth some 270 million years!

There are indications that even in the early Cambrian, trilobites tried to go out on land. As Richard Fortey writes in Trilobite: Eyewitness to Evolution, it is hard to deny that the world is there to be seen. Trilobites exceptionally took this idea to heart – their eyes were made up of up to a dozen thousand calcite lenses. They helped them fish or look out for predators. Did they have to beware of Anomalacaris?

Redfish

Anomalocaris is one of the oldest arthropods (Arthropoda) known to science – a type that appeared on Earth after the Cambrian explosion. It belonged to the order of radiodonts, which did not survive as long as trilobites. Living in the post-Cambrian Ordovician Aegirocassis benmoulai grew up to 2 m! However, we don’t find deficient anomalocarids later than the early Devonian, making them about 120 million years old in total. Radiodonts were distinguished by a conically constructed mouth with tooth plates arranged around it. Their first fossils were discovered by Richard G. McConnell in 1886. Another Henri Ami in 1891. There were only fragments – grasping legs resembling shrimp, and the puzzling organism was classified as such. Fossils of other parts of Anomalacaris, meanwhile, lived their own lives as… strange jellyfish. After nearly a century, an attempt was made to unravel the paleo puzzle. In 1979. Derek Briggs realized that these were not fossils of giant shrimp, but rather legs of large organisms. The next steps were described by Simon Conway Morris himself:

“The road to understanding what Anomalocaris was turned out to be a kaleidoscope of confusion. What Derek Briggs took to be the legs of a giant centipede-like animal turned out to be a prominent pair of grasping arms located at the front. What Charles Walcott recognized the jellyfish in turned out to be an integral part of the animal, forming an unusual mouth and jaws, probably capable of grasping and piercing struggling prey. What I interpreted as a complex fossil, formed by the accidental combination of sponge and Peytoi, turned out to be a poorly preserved specimen of Anomalocaris.” (author’s own translation).

In 1981. Harry Blackmore Whittington identified a fossil of a nearly complete Anomalocaris, together with Derek Briggs, putting it together finically from elements of four different previously recognized organisms. Stephen Jay Gould called their joint book on anomalocaris a fitting culmination of the most important and outstanding series of monographs in 20th century paleontology. If it weren’t for their work, we probably wouldn’t have had any idea until today that we were dealing with a predator, an apex predator at that. Its carapace did not have enough mineral salts to preserve itself in the fossil record as well as was the case with trilobites. We could try to translate the Latin name Anomalocaris canadensis as Canadian shrimp. However, associating a monster of more than a meter in length with a shrimp may mislead a jury about its involvement in the mass murder of trilobites.

image 6
pic. Fossils of the grasping arms of Anomalocaris, identified for many years as independent creatures (Russell D. C. Bicknell et al. (2023) Raptorial appendages of the Cambrian apex predator Anomalocaris canadensis are built for soft prey and speed. “The Royal Society Publishing. DOI:10.1098/rspb.2023.0638 – CC-BY 4.0 license)

Non-mitigating circumstances

The redneck was at fault for its potential – its size and the fact that it was present at the scene. When a Christmas tree topples over, usually the cat flops, if not ours, then the neighbor’s. It’s nothing that, years later, it stopped holding a leg in the stand… In 2006, computer simulation of three-dimensional fluid mechanics showed that the Anomalocaris It floated brilliantly with the help of side lobe waving. Similar mechanics are used today by manta rays and perfectly maneuverable cuttlefish and squid. The gradual development of lobes can also be observed in the course of the evolution of anomalacaris.

In 2011. Anomalocaris eye fossils were analyzed. They were as small as 2-3 cm in diameter and each consisted of 16,700. omatidia (lens + receptors) – independent, hexagonal lenses, like those of today’s insects. However, the anomalocaris omatidia were composed of calcite. That is why they have survived for more than half a billion years. The long axes of a pair of anomalocaris eyes diverged at a 45° angle, providing a wide field of view. Even conservative estimates credit it with greater visual acuity than most living arthropods. In 2018. A team of engineers with a paleobiologist took on the mechanics of Anomalocaris again. It turned out that the peculiarly built tail enabled it to make sudden turns. Maneuverability is an important trait when hunting fast-moving prey. Guilty?

Verdict

July 2023. A team of paleobiologists led by Russell D. C. Bicknell took a close look at the oldest known parts of Anomalocaris – its grasping legs, thought in the past to be bizarre shrimp, and its mouth apparatus, previously mistaken for a jellyfish. The giant whip scorpion(Mastigoproctus giganteus) and the whip spider(Heterophrynus elaphus) were used as modern arthropods for calibration. However, kinematic and biomechanical analyses showed that this iconic Cambrian species was unable to crush the armored prey [12]. The shrimp didn’t have anything to cut or crush, he could only grab fast dodging prey devoid of good armor! Investigators’ attention thus turned to other radiodonts, such as Ramskoeldia or Amplectobelua, and perhaps even Peytoia. The investigation reveals the existence of ecological niches among radiodonts, confirming the complexity of Cambrian food webs. This diversity tells us that there was an evolutionary race between the hunters’ weaponry and the armor and other defensive strategies of the victims at the time.

Research based on theoretical models is not the same as evidence from the field. As with the battle for supremacy of the heaviest of the cetaceans, will there be an appeal? Anomalocaris are the largest creatures known to science so far to have lived on earth in the Cambrian, but we judge this by fossils dating back half a billion years, and these are stop-frames plucked at random from a movie. It is possible to assemble a story from them, by no means necessarily reflecting its content. Perhaps somewhere in the darkness of Earth’s history lurks a larger, more dangerous, unknown predator – a serial killer of Cambrian trilobites. Perhaps someone will find new evidence incriminating Anomalocaris. We will never fully realize what we do not yet know.

Since the trilobites became extinct
The continent has risen, the mountains have grown,

In the depths new creatures tangle,
And man sits on Neptune’s ancient throne.
The human
race will become extinct, but the eyes of trilobites will remain,
Enchanted in stone for eternity,
They seem to look around with wild surprise
At changes greater than they have known before
.
Timothy Abbott Conrad, 1840


In the article, I used, among other things. From the works:

[1] Fabrizio Berra & Lucia Angiolini (2014) The evolution of the Tethys region throughoutthe Phanerozoic: A brief tectonic reconstruction (in:) L. Marlow, C. Kendall, L. Yose (eds.) Petroleum systems of the Tethyan region. “AAPG Memoir” 106:1-27.
[2] Simon Conway Morris (1998) The Crucible of Creation: the Burgess Shale and the Rise of Animals, Oxford University Press.
[3] Facheng Ye, G. R. Shi, Maria A. Bitner (2021) Global biogeography of living brachiopods: Bioregionalization patterns and possible controls. “PLoS One” 16(11): e0259004. DOI:10.1371/journal.pone.0259004.
[4] Charles Doolittle Walcott (1911) Cambrian Geology and Paleontology II. Middle Cambrian annelids. Smithsonian Miscellaneous Collections 57(5): 109-145.
[5] Simon Conway Morris(1977) A new metazoan from the Cambrian Burgess Shale of British Columbia. “Palaeontology” 20:623-640.
[6] Martin R. Smith, Jean-Bernard Caron (2015) Hallucigenia’s head and the pharyngeal armature of early ecdysozoans. „Nature” 523:75-78.DOI:10.1038/nature14573.
[7] Simon Conway Morris (1979) Middle Cambrian Polychaetes from the Burgess Shale of British Columbia. “Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences” 285(1007):227-274. DOI:10.1098/rstb.1979.0006
[8] Simon Conway Morris (1985) The Middle Cambrian metazoan Wiwaxia corrugata (Matthew) from the Burgess Shale and Ogygopsis Shale, British Columbia, Canada. “Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences” 307:507-582. DOI:10.1098/rstb.1985.0005.
[9] Joe Moysiuk and Jean-Bernard Caron (2019) A new hurdiid radiodont from the Burgess Shale evinces the exploitation of Cambrian infaunal food sources. “Proceedings of the Royal Society B: Biological Sciences” 286(1908). DOI:10.1098/rspb.2019.1079
[10] Charles Darwin (1859) On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life, John Murray, London.
[11] Peter Douglas Ward (2006) Out of Thin Air: Dinosaurs, Birds, and Earth’s Ancient Atmosphere. Joseph Henry Press, Washington. The book presents a thesis on how changing oxygen levels in the Earth’s atmosphere and respiratory needs have affected evolutionary pathways.
[12] Russell D. C. Bicknell, Michel Schmidt, Imran A. Rahman, Gregory D. Edgecombe, Susana Gutarra, Allison C. Daley, Roland R. Melzer, Stephen Wroe and John R. Paterson (2023) Raptorial appendages of the Cambrian apex predator Anomalocaris canadensis are built for soft prey and speed. “The Royal Society Publishing. DOI:10.1098/rspb.2023.0638.

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