The History and Significance of Anomalocaris

As part of the 350 Year of Scientific Publishing celebration from the Royal Society, top palaeontologist Derek Briggs wrote a paper describing the history of study of known Cambrian freak, Opabinia regalis, one of the first fossils to have been redescribed by Harry Whittington in the 1970s revival of the Burgess Shale. This paper is a must-read for anybody interested in the history of palaeontology and in the Cambrian Radiation, using Opabinia as a lens to look at the phylogenetic and palaeobiological revolutions that took place in the past decades. Check it out:

Briggs DEG. 2015Extraordinary fossils reveal the nature of Cambrian life: a commentary on Whittington (1975) ‘The enigmatic animal Opabinia regalis, Middle Cambrian, Burgess Shale, British Columbia’Phil Trans R Soc B 370, 20140313.

You can write a similar paper by looking at any of the iconic Burgess Shale fossils, and it is a good exercise for palaeontology students to attempt to do this, since tracing the history of descriptions and redescriptions of taxa and localities is part and parcel of regular, daily palaeontological work. Here is an example using Anomalocaris, arguably the most famous Burgess Shale fossil.

In 1886, Richard McConnell, a member of the Geological Survey of Canada, discovered the Trilobite Beds of the  Stephen Formation from the Canadian Rockies in British Columbia. From the fossils he collected, all were trilobites… except for two strange shrimp-like fossils.

At the time, biodiversity was thought to have evolved gradually to a Recent peak, and there was no such thing as phylogenetics.

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The leading geologist of the Geological Survey, Joseph Whiteaves, described these strange fossils as phyllocarid crustaceans in an 1892 paper, naming them Anomalocaris canadensis in the process (Whiteaves, 1892). His drawing of the animal is shown above. Without any sort of concept of stem groups and a preconceived idea of the Cambrian as having very little diversity, it was only natural that he would place them into existing taxonomic groups.

In 1902, the British Museum’s Richard Woodward got his hands on another collection of Anomalocaris specimens and reconstructed the animal pretty much the same way, except he magicked a head for the animal, despite none having been discovered yet (Woodward, 1902). Standards were lower back then.

In 1909, Charles Doolittle Walcott discovered the Burgess Shale while working in the Stephen Formation.  He would spend the summers thereafter collecting hundreds of specimens from the Phyllopod Bed.Unlike in the Trilobite Bed, most specimens from the Burgess Shale represented unique, heretofore unknown and unimaginable species or body parts, what palaeontologists colloquially refer to as UFOs: Unidentified Fossil Organisms. He wasn’t just dealing with vaguely shrimp-like animals anymore… but the state of evolutionary biology was such that he had no choice but to do the same as Whiteaves before him by doing his best to ally these forms to modern animals.

Three fossils described by Walcott are of particular interest to Anomalocaris. Two are described by Walcott (1911):

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Laggania cambria, a putative holothurian (sea cucumber); and

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Peytoia nathorstii, a putative medusoid.

Remember their names and appearances, they will become important later.

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The third fossil was correctly identified by Walcott as an anomalocarid, which he named Anomalocaris gigantea, a larger version of Anomalocaris canadensis (Walcott, 1912).

One more puzzle piece came in 1928, when Danish zoologist Kai Ludvig Henriksen glued Tuzoia – what he thought was an “odd shrimp’s” head – to Anomalocaris (Henricksen, 1928).

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This interpretation remained for several decades, and its result can be seen in the famous 1942 National Geographic illustration by Charles Knight, chosen by Stephen J. Gould as a cover for his Wonderful Life. If you’re having trouble spotting it because of your modern imagination, Anomalocaris is the shrimp, an obvious crustacean with nothing really so strange about it.

Things remained static until 1966, when Cambridge palaeontologist Harry Whittington and two of his students, Derek Briggs and Simon Conway Morris, were hired by the Geological Survey of Canada to review the Burgess Shale. This trio started a revolution in our understanding of the history of life on Earth, a revolution that took place through exceptionally detailed and imaginative redescriptions of the specimens Walcott and his predecessors had collected decades ago.

Whittington was chosen for this due to a 1959 revision of trilobites, in which he placed all the Burgess Shale animals as a sister group to the trilobites (Whittington, 1959). We can already see the rudimentary application of both a phylogenetic framework and a critical eye.

The most important trait that Whittington brought to the project, and what really turned a run-of-the-mill systematic revision to a true revolution, was Whittington’s methodological approach. He realised that the key to understanding the Burgess Shale fossils was understanding how the fossils were deposited and preserved.

The Burgess Shale was the result of a massive underwater mudslide that buried the ecosystem in an anoxic environment. The animals did not just plop down to the sea floor and die. They were buried in all sorts of positions, contorted and jumbled up. Whittington realised that in order to truly get to the nature of these animals, you have to look at things in 3D, try to straighten the crooked, and reform the crushed. This is how we now work with all fossils, and while Whittington surely wasn’t the first to do this, the workflow he started and the spectacular results it yielded were instrumental in cementing it.

However, it is a very time-intensive method. With 13000 specimens of just Marella splendens, he realised that doing it alone would be impossible, so he turned the project into a collaborative one, sending some fossils to colleagues, and having his two grad students, Simon Conway Morris and Derek Briggs, take care of one group each. Conway Morris got the “worms”, Briggs got the bivalved crustaceans, and so began the revolution. Anomalocaris‘s story picks up again in the midst of this.

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Recall that Walcott had described Laggania cambria as a sea cucumber. Simon Conway Morris got the only specimen of it for redescription, which he did in 1978 (Conway Morris, 1978). He came to the conclusion that Laggania cambria is actually two animals: the body is a sponge, and the circular bit is none other than Peytoia nathorsti, the medusoid described by Walcott. Both animals were swept in the mudslide and landed on top of each other.

The title of this paper was a to-the-point “Laggania cambria Walcott: A Composite Fossil“. One year later, Briggs came out with another paper, with just as pointy a title: “Anomalocaris, The Largest Cambrian Arthropod“.

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Briggs (1979) analysed Anomalocaris canadensis, and came tot he conclusion that everything that had been assumed was a complete animal was in fact a limb. A limb so big, the animal that possessed it must have been gigantic. This paper was a pivotal step in understanding Anomalocaris‘s true form, so a more detailed look at it is warranted.

Briggs found that the fossils he was investigating fell into two categories. Most were similar to the A. canadensis type specimen, the same one studied by Whiteaves, as drawn above. He reconstructed them as having fourteen segments, with thirteen of them possessing ventral triangular blades. These blades were formerly assumed to have been the walking limbs of the animal… but they were not segmented as an arthropod limb must be. What was considered to have been a segmented animal was in fact a segmented limb, with no body attached. Given that some fossil slabs had several A. canadensis specimens lying in parallel was further evidence of this interpretation, and also allowed Briggs to tentatively conclude that these were the walking limbs.

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The other category of fossil Briggs discovered was what he referred to as Appendage F. These fossils were much rarer, and had so far been assigned to another arthropod, Sidneyia inexpectans. However, the other palaeontologist in charge of redescribing S. expectans, David Bruton, had already found that Appendage F does not belong there. His results would be published in Bruton (1981), Briggs only knew this earlier by personal communication.

Anyway, Appendage F is very different to the other Anomalocaris appendages. It has only eleven segments, with five blades in total: one ventral, two dorsal, and two laterally, witht he ventral blades being the longest and having spines that extend towards the body, overall resembling a comb. In fact, Briggs hypothesised that they were used as combs to sweep the sea floor and find food – the F stands for feeding, as in feeding appendage.

What remains is any sign of a body. I will digress and give some first-person insight into how these large redescription projects usually happen. You usually have crates and storage cabinets full of specimens collected long ago, but never examined, simply because of lack of time and effort. Unless you have something specific you’re looking for, you can’t really pick and choose which specimens you start to work with – you just move from one specimen to the next; if it’s not interesting, you put it back on the pile and move on to the next one.

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This is why a specimen collected by the Geological Survey of Canada in 1966 took fifteen years to be noticed by Whittington. The slab contained two A. canadensis limbs and a circular structure pretty much identical to a Peytoia nathorsti. The drawing above is their composite drawing of this slab, taking away fractures.

The combination of the two species is intriguing enough that Whittington and Briggs spent time searching through Walcott’s undescribed specimens, looking for other Peytoia associations… and finding several puzzling ones with PeytoiaOpabinia-like bodies, and Appendage Fs.

These set the ball rolling towards the defining publication on Anomalocaris: Whittington & Briggs (1985).

This paper describes two species of Anomalocaris. The first is yet another incarnation of Anomalocaris canadensis, based on a reinterpretation of the same specimen Briggs (1979) described. The walking appendages that Briggs though were attached to the sides for locomotion turned out to be attached to the head. Next to the head are a pair of yellow yellow minerals – these are the eyes. The body has lobes sticking out of the side. The body has a line of apatite running through it, which in other Burgess arthropods represents the gut.

The other species they described was Anomalocaris nathorsti, based on the body findings, the Appendage Fs, and isolated Peytoia specimens. The body specimens, upon much closer inspection, revealed that they also had Appendage Fs preserved with them, only they were very compacted. If Whittington and Briggs weren’t specifically looking for appendages near the head, they wouldn’t have found them – this is the trickiness of working with Burgess Shale material.

They also managed to clear up the status of Peytoia. Recall that until now, Peytoia was thought to be some sort of medusoid, as described by Conway Morris (1979). The examination of these specimens made it clear that Peytoia is in fact the mouth: a “pineapple slice” with 32 plates connected by a membrane. With this discovery, Laggania cambria ended up being A. nathorsti, and Peytoia was nothing more than its mouth.

To recap:

  • Anomalocaris canadensis started off as a complete animal. When it was discovered to be just an appendage, it was described as the walking limb of the Anomalocaris canadensis animal. Finally it was discovered that it was a feeding appendage in the head.
  • Peytoia nathorsti was thought to be some form of very weird coral or medusoid. It ended up being the mouth of the Anomalocaris animal.
  • Appendage F was correctly identified all along, as a feeding appendage.

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Why did it take so long and so many errors through the ages to discover Anomalocaris‘s true form? Some may lay the blame on poor research work, but this is completely disingenuous. I have worked with such UFOs, and can tell you from first-hand experience that in one day spent with one of these fossils, you can sketch at least 5 different interpretations with equal amounts of support.

The fact is that that Cambrian animals are unlike anything we know from today. We are talking of animals who took an evolutionary road that is long-extinct. The fossilisation is very weird, with hard parts, soft parts, and much compression and contortion. An appendage could well have been a complete animal (think of the conodonts!).

The 1985 paper was pivotal in our study of Anomalocaris, but it was by no means the last say on the subject. Since then, we’ve discovered more anomalocaridids from more localities and, crucially, more times. We’ve expanded our knowledge of both their anatomy and their evolution, with each new paper delivering new morsels of information. Heck, we now know that Peytoia nathorsti is actually another anomalocarid, not a mouth or a composite fossil of a sponge and a medusoid. Nothing stays the same.

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Anomalocaridid phylogeny. Source: Vinther et al. (2014).

We now know that Anomalocaris was just one genus in a highly-successful order of animals, the Radiodonta, erected by Collins (1996), and we now usually refer to anomalocarids as a group. The phylogeny above, from Vinther et al. (2014), is testament to their evolutionary success.

nature14256-f3New species and genera being discovered frequently, making even that very recent phylogeny already outdated. The latest, drawn above, is both a new genus and a new species and is still in press: Aegirocassis benmoulae (Van Roy et al., in press).

Aegirocassis benmoulae is found neither in Canada nor in the Cambrian. It’s from the Fezouata Formation, from the Ordovician of Morocco.

The lifestyle reconstructed by Whittington and Briggs for Anomalocaris was a predatory one, with Anomalocaris taking its prey with its appendages and shoving them into its mouth. Aegirocassis benmoulae is not a predator. It’s a filter-feeder.

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Not only that, it evolved filter-feeding independently. Another filter-feeding anomalocaridid is Tamisiocaris borealis (appendage above), described by Vinther et al. (2014), from Sirius Passet, Greenland. Here’s another surprise: Sirius Passet is older than the Burgess Shale.

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Lest you think that discovering new genera and species means we have put Anomalocaris and Walcott’s specimens to rest, we actually keep going back to them and updating our description of them. Ae. benmoulae has an important trait heretofore undescribed in anomalocarids: dorsal flaps. Up until now, anomalocarids were known to have a single flap on the side, the lobes described by Whittington and Briggs. With the discovery of Ae. benmoulae, some specimens of Anomalocaris were reexamined, pictured above. Lo and behold: it turns out that there is “clear evidence” of two sets of flaps, meaning our reconstruction of Anomalocaris is out of date.

You might think that an extra set of flaps is a small discovery, maybe changing how we think of their swimming style but not much else. Wrong. That’s not how this business works.

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Anomalocarids are on the stem leading to the Euarthropoda, the “true arthropods”. They are more basal than crustaceans, chelicerates, and myriapods, and so anything we find out about their morphology will give us more clues about the evolution of the arthropods. The flaps are a key example of this, elucidating the evolution of the typical euarthropodan biramous limb. Before this knowledge, anomalocaridids were an anomaly in our current view of arthropod evolution; now they slot in very nicely on the road to euarthropods.

The continuing and constantly-updated story of the anomalocarids is a sterling example of palaeontological and palaeobiological science at work. If you check out my lecture on the scientific method, you will see that I used anomalocarids to demonstrate how falsifiability works.

 Until 2009, we thought that anomalocarids were a flash-in-the-pan success, reaching a great diversity in a short time and going extinct at the end of the Cambrian. But then came Schinderhannes bartelsi, a Devonian anomalocarid from the Hunsrück Slates of Germany that extended their fossil record by a staggering 100 million years (Kühl et al., 2009).

Every other principle of the scientific method can be demonstrated using anomalocarids. The fact that science cannot deal with absolute certainty is amply seen in the history of Anomalocaris canadensis. Its self-correcting nature is seen in the constant redescriptions of even the most ancient specimens. The constant evolution of knowledge is seen in how every new paper forces us to reconsider and update what we thought we knew (“this thing isn’t an animal, it’s just an appendage!”).

Most importantly, the story of the anomalocarids shows us how far we’ve come as a science. The very first Anomalocaris were thought to be weird shrimps, because it was inconceivable that anything so different existed so long ago. With the Whittington-led Burgess Shale revolution of the 1970s, we found out that the Cambrian was probably when the weirdest animals ever existed. Subsequent findings and developments in phylogenetic thinking finally led us to today’s realisation that the Cambrian Radiation was one of basal modern animals, not evolutionary one-off freaks, but successful animals that were merely more branches off the tree of animal life that just happened to have gone extinct. Anomalocarids were one part of that, whether as “gentle giants” filter-feeding their way through the oceans, or vicious apex predators, or slightly less vicious smaller predators.

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