Anomalocaris: how flatworms transition to trilobites

According to Wikipedia,
Anomalocaris (“unlike other shrimp”, or “abnormal shrimp”) is an extinct genus of radiodont (anomalocaridid), an order of animals thought to be closely related to ancestral arthropods.”

That is confirmed below (Fig. 1). But where did Anomalocaris come from?

According to Wikipedia,
Stephen Jay Gould cites Anomalocaris as one of the fossilized extinct species he believed to be evidence of a much more diverse set of phyla that existed in the Cambrian Period, discussed in his book Wonderful Life, a conclusion disputed by other paleontologists.”

That is not confirmed below. Based on phylogenetic bracketing, Anomalocaris evolved from flatworms and into trilobites. Thus, these do not increase the diversity of phyla in the Cambrian, but blend one into another.

Figure 1. Possible evolution of Anomalocaris after phylogenetic bracketing between flatworms and trilobites.

Figure 1. Possible evolution of Anomalocaris after phylogenetic bracketing between flatworms and trilobites.

Where else do we find such a mouth?
That’s the first clue to the origin of anomalocarids.

Figure x. Added April 7. As predicted Kiisortoqia bridges the gap between Anomalocaris and Triarthrus, the trilobite.

Figure x. Added April 7. As predicted Kiisortoqia bridges the gap between Anomalocaris and Triarthrus, the trilobite.

Evidently overlooked until now,
certain flatworms have a similar concentric ventral mouth (Fig. 1).

Anomalocarids apparently had the fluidity of motion
of a large swimming flatworm (see video below), combined with the segmentation of trilobites (= arthropod ancestors).

Distinct from flatworms, but like trilobites,
an anus appears posteriorly.

Like tentacled flatworms and trilobites with antennae,
two armored tentacles appear on anomalocarids,

Unlike flatworms, but like trilobites,
a pair of lateral eyes on short stalks appear.

A YouTube video
featuring Burgess Shale expert Professor Des Collins explains how the bits and pieces of Anomalocaris came together historically over several years as he holds a model of a large specimen.

According to the Des Collins website:
“The fossil Anomalacaris was unlike any living animal and was misidentified over a period of 100 years revealing the false starts that can happen in scientific research. Collins set out to piece together the entire animal by looking at the vast trove of Burgess Shale fossils at the Royal Museum of Ontario where he worked. He discovered more pieces of the puzzle and realized that previous fossils that were described as separate organisms were, in fact, part of the animal Anomalacaris. Once he had assembled the entire animal, he had a model built to show what a fearsome predator it must have been.”

Or not. Anomalocaris was large, but its mouth was not ideally suited to crack open and attack the hard-shelled animals that were evolving in the Cambrian. According to the article cited below, “We found that it’s extremely unlikely Anomalocaris could eat most trilobites,” said James Whitey Hagadorn, the research team’s leader and a paleontologist at the Denver Museum of Nature and Science. “It couldn’t close its mouth all of the way, its mouth was too soft to crush trilobite shells.”

Anomalocaris arising from large free-swimming flatworms
appears to be a novel hypothesis of interrelationships. If not, please provide a citation so I can promote it here.

This just in (March 11, 2021):
Tests show Anomalocaris was not a trilobite eater, but preferred mush, like modern flatworms do.

Daley AC, Paterson JR, Edgecombe GD, García-Bellido DC and Jago JB 2013. Donoghue P (ed.). New anatomical information on Anomalocaris from the Cambrian Emu Bay Shale of South Australia and a reassessment of its inferred predatory habits. Palaeontology: n/a. doi:10.1111/pala.12029
Whiteaves JF 1892. Description of a new genus and species of phyllocarid Crustacea from the Middle Cambrian of Mount Stephen, BC. The Canadian Record of Science. 5 (4).
Whittington HB and Briggs DE 1985. The largest Cambrian animal, Anomalocaris, Burgess Shale, British Columbia. Philosophical Transactions of the Royal Society B. 309 (1141): 569–609.


12 thoughts on “Anomalocaris: how flatworms transition to trilobites

  1. The evolution of Anomalocaris is actually rather well understood in comparison to how you have presented it. Radiodonts evolved from a Gilled lobopodian ancestor, which evolved from the Siberiids, a group of stocky, carnivorous Lobopodians on a different “branch” from animals like the Luolishaniids or Onychophorans (velvet worms). Essentially, Radiodonts did not evolve into arthropods, but we’re rather a very, very closely related clade of animals – radiodonts evolved from lobopodians, which themselves evolved from more simple worms. However, these worms were not flatworms. Lobopodians would have been closely related to Cycloneuralians, animals like nematodes, kinorhynchs, and priapulids. This is well understood in more recent literature, which I encourage you to read. Unfortunately, your flatworm hypothesis holds no weight. As an introduction to these animals, I would recommend reading Cong et al, 2014 (doi:10.1038/nature13486), or Smith and Ortega-Hernández, 2014 (

    • Thank you, Christian M for your kind and deeply appreciated reply. Lyrarapax is particularly interesting. Will discuss this critter on the next post after further study. Velvet worms appear to be all cylindrical crawlers, no undulating swimmers. Even so, this possibility will not be glossed over as I return to this topic, but I note neither flatworms nor trilobites are mentioned in the texts or cladograms, except to note trilobites were also present in the strata. Possible taxon exclusion issues here.

      • I will let you know that I have spent the greater part of two years studying these animals intensely, and I will promise you they are not related to flatworms. You claim that velvet worms are all crawling animals – no one is arguing against this. Velvet worms are more closely related to lobopods like Hallucigenia, rather than gilled lobopods and siberiids. Trilobites are certainly not excluded, but they lie further in the arthropods part of the tree, rather than as a descendant of the radiodonts. Some of the most basal arthropods include the “Great Appendage” arthropods like Parapeytoia or Isoxys. So many theories for the origins of lobopods and radiodonts have been tested, decades if not centuries of ideas and testing, and through all those decades of testing, the result we have today is essentially agreed upon unanimously, though it is always being tested again and again with better understanding and more fossils.

        You argue that taxon exclusion could be the reason, but I beg to differ. Rather, it is your “taxon exclusion” which has given you your hypothesis. You already assume that Anomalocaris must be related to these two animals closely, and have thusly excluded the greater part of over 180 different species of Lobopod, Radiodont, arthropod and worm. Before you make any other posts on lobopods or radiodonts, I encourage you to spend a bit of time reading the primary sources, and not go in with the preconception that everyone else in the field is wrong, and that this is a problem for you to solve. For further reading, try reading Van Roy, Daley, & Briggs, 2015, on the quad flaps of Aegirocassis (

      • Thank you, again, Christian. I will report back when I have a better understanding. Am steeped in study at present and appreciate every taxon you care to mention to bring to my attention.

      • I would be happy to address any concerns you have on the topic of lobopodians and radiodonts, the two mostly go hand in hand. I hope you will find interest in more recent contributions to the field. You will hopefully find it true that our understanding of these animals, from Anomalocaris to Opabinia to Hallucigenia, is more fleshed out than you portray in your initial blog post.

      • Thank you for your offer, Christian. I will be returning to vertebrates after this short detour. The purpose here is to suggest previously omitted taxa to be included in the next analysis of pertinent taxa and characters.

  2. This is called “Pulling a Larry Martin”: Making a hypothesis phylogenetic relationships based on a few selected traits rather than performing a phylogenetic analysis

    • While it is true that no analysis was performed here, the overall similarity (most traits are matches) falsifies the claim of “few”. The purpose here is to suggest previously omitted taxa to be included in the next analysis of pertinent taxa and characters.

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