Cambroraster: documents how some flatworms became trilobites

Moysiuk J and Caron JB 2019
brought us a new semi-segmented ‘radiodont’ that bridges the gap between unsegmented flatworrms and segmented trilobites: Cambroraster (Fig. 1), a middle Cambrian late survivor of an Early Cambrian or earlier radiation.

Figure 1. Cambroraster falcatus, here compared to a trilobite, a flatworm and Anomalocaris. The flatworm is unsegmented. The trilobite is segmented. The other two are semi-segmented, but otherwise share a long list of traits. Note the extant swimming flatworm moves in much the same way as imagined in anomalocarids, but without semi-segmentation.

Maysiuk and Caron report,
“Cambroraster’s morphology is consistent with a nektobenthic lifestyle. The broad, vaulted H-element is convergent with the head armature of limulids [34], carcinized crustaceans and ‘filter chamber’ resuspension feeding trilobites, but also the head shields of some ostracoderm fish.”

the authors omit segmented trilobites and unsegmented flatworms from their cladogram.

the authors include several segmented, multi-legged, velvet worm-like taxa, including Hallucigenia and Opabinia. Their cladogram indicated these two Cambrian legged worms were ancestral to semi-segmented, legless anomalocarids. Thus, with regard to anomalocarids, the authors presented an upside-down cladogram, with primitive legless taxa at derived nodes. This comes from taxon exclusion: excluding flatworms, in this case.

The authors conclude,
“The inferred ecology of Cambroraster indicates that the evolution of large nektobenthic consumers, alongside smaller carnivores like trilobites, occurred in tandem with the radiation of these prey, in line with hypotheses emphasizing the catalysing effects of escalation during this radiation. As large and abundant nektobenthic carnivores, hurdiids like Cambroraster likely had a considerable impact on the local benthic community through both predation and bioturbation.”

The authors mistakenly assume
that Cambroraster (and other anomolacarids) used its oral cone as a predatory organ for biting large prey. Others have considered this unlikely given the largely immobile circular construction of the oral cone ventral to the cephalon, as in flatworms and trilobites (Fig. 1). Rather than a predator, Cambroraster likely fed like a trilobite on immobile, ubiquitous and defenseless algal mats.

What and how did trilobites eat?
According to “There has been a long history of speculation about the feeding habits of trilobites, ranging from predators, scavengers, filter-feeders, free-swimming planktivores, and even parasites or hosts of chemoautrophic symbionts. Using modern-day crustaceans as an analog, it is reasonable to suggest that the majority of trilobites may have been predator-scavengers, as the majority of marine crustaceans are today. Fortey and Owens suggest indicate a shift away from predation and into particle feeding, which includes scavenging for bits of benthic detritus (as the group of olenids below might be doing), or perhaps grazing on beds of algae.” On their well-researched webpage, provides other forms of feeding on tiny and/or buried prey or particles.

Carapace before limbs
Cambroraster displayed a carapace and began to produce segments before it had legs. That means Cambroraster was producing chitin, the material that covered the limbs of trilobites. That’s the next transitional taxon to look for: a Cambroraster with tiny segment buds arising near the ventral midline (Fig. 1).

We first looked at the origin of Anomalocaris
and trilobites via flatworms earlier here.

The traditional origin of trilobites according to
The Early Cambrian included several orders of trilobites, so their genesis among artrhopods must have been earlier, in the Ediacaran. “Probably the key distinguishing character, one that also allowed trilobites to be preserved so well (and which accounts for their sudden prominence in the Cambrian), is calcification of the exoskeleton.”

Two opposing questions arise:
1. Did trilobites arise from slender velvet worm-types, essentially outer tubes over internal tubes with one leg pair per segment and a terminal mouth, and thereafter widen and flatten and rotate the oral cone ventrally to become a trilobite? Or…

2. Did trilobites start off as wide flatworms with a ventral oral cone and thereafter segment their bodies and grow legs and gills arising from each segment?

We already know
that all segmented worms arose from unsegmented round worms (nematodes with a mouth on one end and an anus on the other), and all nematodes arose from flatworms (with a more primitive mouth = anus). The transition from flatworm to trilobite via anomalocarid skips the roundworm and segmented (annelid) worm steps.

So, is this true?
Was there a third flatworm radiation that went directly to segmentation without rotating the mouth anteriorly while developing a separate anal opening posteriorly. The present evidence indicates this is a strong possibility.

Unfortunately, all trilobite clade members are now extinct,
but that fact may be part of today’s solution. If the present hypothesis (that trilobites, anomalocarids and Cambroraster were all gentle algal mat grazers) is correct, then the extinction of this clade can be explained by the near extinction of the algal mat during a planet-wide extinction event, like the end Permian. Thereafter algae could have slowly returned from distant and remote refugia not inhabited by now extinct trilobites and their relatives. concludes:
“The likely scenario is that trilobites arose from Precambrian bilaterians, arguably arthropods, that gave rise to Cambrian arachnomorphs, among them trilobites. The evidence is neither clear nor unambiguous. The fossil record is spotty, but suggestive. Perhaps it is the simple, dorsally unsegmented Precambrian fossil, Parvancorina, that offers the most reasonable link to arachnomorphs.”

Figure 2. Bigotinella is an Early Cambrian trilobite lacking a post-cephalon. Parvancornina is from the Ediacaran, but that may be too soon for trilobite legs. Neither of these taxa preserve segments. These two may be alga-eating semi-segmented flatworm descendants with a ventral mouth and a dorsal shield, like Cambroraster.

Parvancorina bears a distinct resemblance to Cambroraster.
Moreover, Early Cambrian trilobites seem to be known better from head shields lacking ‘post-cranial’ thorax material (Figs. 2, 3). Note: This is what we see in Cambroraster (Fig. 1).

Figure 3. Early Cambrian trilobites are often, as shown here, known only by their hard parts, their cephalon. That means the post-cephalon was soft, as in Cambroraster and flatworms.

If the evidence points in a certain direction
maybe that’s where we should be looking for more data. Perhaps someone else can dive a little more deeply into this issue given this new direction.

The takeaway:
As usual, add a few taxa and see where it take you.

Hagadorn JW 2009. Taking a Bite out of Anomalocaris. In Smith MR, O’Brien LJ, Caron J (eds.). Abstract Volume. International Conference on the Cambrian Explosion (Walcott 2009). Toronto, Ontario, Canada: The Burgess Shale Consortium (published 31 July 2009).
Moysiuk J and Caron JB 2019. A new hurdiid radiodont from the Burgess Shale evinces the exploitation of Cambrian infaunal food sources. Proc. R. Soc. B 286:20191079.


Publicity for Cambroraster:
‘Millennium Falcon’ predator soared across ocean floor at dawn of animal life
By Joshua Sokol Jul. 30, 2019 , 7:01 PM

“The ‘ship’ was one of the largest known animals of its day to churn up the sea floor. It sailed in fleets over muddy ocean sediment, plying its unusual claws in the hunt for small prey.

“Cambroraster had a round mouth lined with toothlike plates, fronted with comblike claws it could hold out like a basket. Its eyes sat in deep notches that give the carapace its signature “spaceship” look.

“Hagadorn said the most likely diet of Anomalocaris was similar to that of modern arthropods such as crabs, lobsters and shrimps, which mostly eat soft items such as worms in the mud or microorganisms or plankton in the water. It could have eaten very small trilobites and recently molted trilobites whose new shells had not yet hardened, but the vast majority of trilobites would have broken Anomalocaris’ mouth parts.”

15 thoughts on “Cambroraster: documents how some flatworms became trilobites

  1. Hello Peters,

    I’ve commented on your blog posts relating to Radiodonts and Lobopods previously, advising you to be more well read before making unfounded claims about them in your posts. This post is no exception, except for that I cannot be sure you actually read the paper in question at all.

    Moysiuk and Caron, despite your claims, did include a trilobite (Eoredlichia) in their phylogenetic analyses, in addition to numerous other arthropods. Hallucigenia was selected as an out group because dozens of other phylogenetic analyses have recovered its basal position.

    Research and debate on how Lobopodians and Radiodonts relate to Arthropod evolution have been taking place for over a century, and yet, you have the confidence to not read any of the discussion and instead propose your own solution that conflicts with everything currently known about arthropods and radiodonts. This is something that is relatively well understood, and the character evolutions that lead to Euarthropoda can be mapped out relatively easily. The dorsal head sclerite possesses by all radiodonts is indeed homologous to the frontal most head shield of some euarthropods, including a putative structure in Helmetia and Odaraia ( Your tangent about Parvancorina being an arthropod ancestor was a valid theory decades ago, but has since been thoroughly disputed. The current basal-most euarthropod is thought to have looked like either Surusicaris or, according to some, Kylinxia (though it’s five eyes are only convergent with Opabinia, not implying direct relation). The cladograms you see are not just that – they represent years of robust phylogenetic analysis, including the one done by the authors for the paper. Trilobites, themselves Artiopods, arose from more primitive artiopods like Helmetia, Misszhouia, and Naraoia, as a few examples.

    Your claim that Cambroraster fed on defenceless prey on algal mats is, perhaps, among the least evidenced based in the post. It willfully ignores the massive, elongate frontal appendages that formed a basket around the mouth, the mobility of the mouth, the presence of numerous toothplates inside the mouth for mastication, and just as damningly, the literal rows of swimming flaps on either side of the animal.

    I cannot respond to everything here, because there is no actual evidence provided to respond to. Simply a claim that, on face value, Radiodonts look kind of like worms, and that Cambroraster has a big helmet.

    Once again, I would be willing to answer any questions you have about Radiodonts or lobopods, but must still strongly advise you not to make wild claims about them based on long-debunked hunches.

    • You wrote: “Hallucigenia was selected as an out group because dozens of other phylogenetic analyses have recovered its basal position.”

      Do any studies start with a mobile, benthic flatworm? Hallucigenia seems to be quite derived relative to flatworms. Some cladograms are literally upside-down because they make the mistake of choosing an outgroup taxon that is not as primitive as the workers imagine.

      The massive elongate frontal appendages on anomalocarids have precursor structures in certain mobile flatworms as shown in the above figures.

      • While Platyhelminths, flatworms, are part of Spiralia (like slugs, snails, cephalopods, et cetera) we know that lobopods, Radiodonts, and euarthropoda, are part of a group called the Ecdysozoans, all sharing the trait of going through the process Ecdysis, aka Moulting. Radiodonts did it (most of their fossils are probably moults, and the Cambroraster paper provided more evidence for massive gatherings of radiodonts in order to moult in synch), Lobopods like Hallucigenia did it (tardigrades and velvet worms, both living groups of Lobopodians, also do it). The most basal members of this group are the Cycloneuralians, like priapulid worms, nematodes, and kinorynchs. The Spiralians and the Ecdysozoans are both part of Protostomia. So, all-in-all, flatworms aren’t closely related at all.

        I promise you, the phylogenetic analyses are not upside down – and since we still have crown-group members of many of these animals, (living priapulids, living lobopods, living arthropods), it can be confirmed using genetic analyses as well as morphological, fossil-based ones – there’s a lot interesting science done there (

        I’m not sure what you mean by precursor structures – If you honestly care, you may be interested in researching the Arthropod Head Problem – a massive, ongoing debate about the evolution of radiodonts, lobopods, and euarthropods, which deals with the homology and evolution of the Radiodont frontal appendage. (

      • Thank you. The link directed me to “The origin and evolution of the euarthropod labrum” which I will read this weekend. I appreciate your guidance.

        You wrote: ” So, all-in-all, flatworms aren’t closely related at all.” So you’re telling me that flatworms are not transitional from planulae to ribbon worms, round worms and their many descendants (all listed above)?

      • The term “worm” can be used to described an incredibly wide, disparate range of animals that are not necessarily related. Round worms are nematodes, making them Ecdysozoans – in fact, C. elegens, a famous model organism (one understood so thoroughly that we can simulate its entire brain construct and body within a computer program), is often used as outgroup for the radiodonts, lobopods, and euarthropods. Meanwhile, flatworms and ribbon worms are both part of spiralia, only related to nematodes in that they are protostomes. Even then, flatworms and ribbon worms are only distantly related, each being disparate crown groups. (

        Likewise, segmentation is also a bit of a messy term. Obviously, the exact segmentation of the head is the primary issue of the Arthropod Head Problem, but Chordates also exhibit segmentation of a kind. As a general term, it can also be used to describe morphological characters (“divisions of external body surface, annulated or segmented” is a commonly used character in panarthropod phylogenetic analyses).

      • Christian, Pre-flatworms have no body openings. Flatworms have one ventral opening. Ribbon worms have two openings, skin around an intestine. Round worms and higher taxa have a coelom between the intestine and skin. Note that flatworms are basal to all taxa that have at least one body opening. That being said, you can be very helpful if you could only find a paper that includes flatworms in the cladogram. If they don’t exist, just say so. Then let’s talk about creating a study that does include flatworms.

        Given that Spiralia is a clade of protostomes, and flatworms are not protostomes, but gave rise to both protostomes and deuterostomes, I’m wondering if they didn’t do it twice or thrice. We’ll never know until we run the test. I’ve already documented a dual origin of turtles and a triple origin of whales and an origin on pterosaurs apart from dinosaurs, and caseasaurs apart from synapsids, so there’s plenty of precedence for this sort of taxon exclusion protecting textbook traditions.

      • I hate to argue further, but what I’m telling you is not a matter of protecting textbook traditions, as you believe all of paleontology is. This can all be confirmed through extensive genome analysis, mapping the evolution of the whole of protostomia and beyond, based on as many lines of evidence as can be analyzed.

        This paper, liked below, analyzes numerous lines of molecular evidence to determine estimated divergence times for Protostomia, focusing on ecdysozoa, using fossil evidence from throughout the paleozoic and 5 different independent molecular/genomic datasets, “A total of 300 unique genes spanning 80,000 aligned sequence positions from 158 taxa were analyzed along with 67 ecdysozoan fossil calibrations”. (
        If you need further evidence, you can use Google Scholar to find a wealth of peer-reviewed research on the subject.

        If you wish to argue about this flatworm hypothesis further, you will have to somehow find and use even more robust lines of evidence than the collective sum of centuries of research and ongoing debate, and the field of genetics. Non-existent “taxon exclusion” will not fit the bill.

      • 1. Molecular evidence in deep time chordate studies recovers false positives. Whether or not this happens in shedding, segmented taxa (Ecdysozoa) is beyond my current knowledge, but I note the cladogram fails to nest non-segmented round worms basal to all segmented taxa.
        2. The keyword ‘flatworm’ is not found in the text.
        3. re: “if you need further evidence,” All I ask is a little taxon inclusion to make sure the cladogram is properly rooted. And I need a phenomic cladogram, not a genomic one.
        4. re: “the collective sum of centuries of research and ongoing debate, and the field of genetics.” That’s what I do. Thanks for trying, Christian, but all I want is what I asked for earlier and above. It’s okay that you were not able to find a phenomic cladogram or study that included flatworms. Science is all the more interesting when some doors have never been opened.

      • As stated earlier, the phylogenetic analysis in that paper is supported by the consensus of detailed fossil evidence, 5 independent genetic datasets (each one a different kind of molecular evidence), and our understanding of the crown groups. You may use google scholar to find molecular clocks including flatworms, I won’t stop you, but you will find essentially the same topology. (

        “Taxon exclusion” is not that great a problem as you believe it is. Before any character matrix is compiled, you must study the anatomy of the animals first, and determine which ones you will examine. Otherwise, you may not be able to draw a line anywhere. In the same sense you could include flatworms in a phylogenetic analysis of Radiodonta, you could just as well include Tyrannosaurus rex, “just to check”. This, of course, would only muddy the analysis, making it less accurate. The same applies to flatworms. Its okay to exclude Tyrannosaurus rex from an analysis of Radiodonta, it will not help to include it.

      • Hi Christian, The paper (above) used five deuterostomes for outgroups. That is inappropriate for a paper on protostomes. Using T-rex as a taxon in basal animal studies is also inappropriate. Skim through prior posts to find how poorly molecules too often recover untenable topologies relative to results from testing traits, which may also include fossils. Try to avoid hyperbole.

      • This will be my last comment here, but if you’re interested, I’ve just ran an analysis and included flatworms, Dugesia. To no great surprise, Dugesia is not related to any of the other taxa (an outgroup) because none of the morphological characters apply, because they are not at all similar. In fact, the Anomalocaris you included in your figure isn’t actually Anomalocaris.

      • Thank you for that experiment, Christian. I would have pulled a trick like that, too, if I were trying to dismantle a competing hypothesis. Dugesia is not the sort of flatworm illustrated in the post. It looks to be derived relative to more primitive taxa. Flatworms have been diversifying since the Ediacaran and they have so few traits. It’s probably a good idea to use one that looks most like an anomalocarid. Try a taxon from the clade Turbellaria, like the one pictured. They are free-swimming and move like anomalocarids are thought to have locomoted. By the way, odd things happen at the base of cladograms. It’s probably a good idea to include a planula and a few other flatworms, like Dugesia, to establish a valid extremely primitive outgroup for the testing you might want to attempt.

      • Christian… tell me about the split between segmented and unsegmented animals… At present it appears that molluscs and chordates +echinoderms are both unsegmented (though crinoids did produce segmented body parts by convergence). Annelids, priapulids, arthropods all others appear to be segmented. Given that segmented animals arose at least once, my question is: could there be a second radiation, perhaps led by semi-segmented anomalocarids?

      • Flatworms actually aren’t related to lobopods, radiodonts, or arthropods very much at all. Flatworms are platyhelminths, belonging to the group Spiralia (including snails, cephalopods, et cetera) – on the other hand, we know that lobopods, radiodonts, and euarthropods belong to a group called the Ecdysozoans. Ecdysozoans are connected by the process of Ecdysis, AKA moulting. Radiodonts do it, as do lobopods, and euarthropods of course. In fact, many if not most Radiodont fossils are moults, and they usually did it in massive gathers, group moulting events, like in Cambroraster. The most basal Ecdysozoans are the Cycloneuralian worms – Priapulids, nematomorphs (including nematodes), and Kinorynchs. Lobopods come immediately after the cycloneuralians. So, all in all, including platyhelminths would make no sense for the purposes of the analysis. We know they don’t have any place in Ecdysozoa, and would only interfere with the accuracy of the phylogeny.

        I promise you the cladograms are not upside down, and fit with all modern and fossil evidence. Because we have crown group lobopods (velvet worms and tardigrades are lobopods) actually, we are able to confirm our phylogenies through genetic and molecular testing, which confirms much of what we know from fossil evidence. It’s fascinating science. (

        Not sure what you mean by precursor structures – if you honestly do care about learning why these things are, I would read up on the Great Arthropod Head Problem, a big, ongoing debate about the evolution of the modern arthropod head tagma, and how the Radiodont frontal appendage fits into its development. (

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