Stylaria, Opabinia and Tullimonstrom side-by-side

Yesterday we took a detour into the realm of invertebrates
comparing large, extant, colorful, free-swimming marine flatworms and trilobites to the giant of the Cambrian, Anomalocaris. Although the three taxa show a gradual accumulation of traits and are overall similar in shape, readers suggested some ‘further reading’ of the academic literature was needed.

That reading has been fascinating
and a defense of that minor thesis is forthcoming. So far that argument looks like it will need several building blocks.

Figure 1. Opabinia in situ, enlarged several times.

Figure 1. Opabinia in situ, enlarged several times.

Meanwhile… today brings one of those building blocks.
Opabinia (Figs. 1, 2) is yet another strange Cambrian taxon often found in the same cladogram as Anomalocaris (Figs. 3, 4). Whittington’s (1975) first interpretation of Opabinia was met with laughter due to the implausible ‘strangeness’ of the fossil.

Perhaps the following
will take some of the strangeness out of Opabinia. It has some overlooked relatives, one still living, Stylaria, a tiny segmented worm with a mobile proboscis (Fig. 2). Note the presence of eyes, a head section, a tail section and precursors to swimming lobes present as needle-like lateral spines, one per segment. Opabinia only stands out as odd (Fig. 3)  until you add a few similar taxa.

Figure 1. Comparing the extant worm, Stylaria to Cambrian Opabinia and Carboniferous Tullimontrom. All three share a similar morphology that has not been fully explored yet.

Figure 2. Comparing the extant worm, Stylaria to Cambrian Opabinia and Carboniferous Tullimontrom. All three share a similar morphology that has not been fully explored yet. The proboscis does not aid in feeding Stylaria and it probably acted as a simple probe and/or holdfast for Opabinia and Tullimonstrom. Note the lack of legs in any of these taxa.

According to Wikipedia,
“When the first thorough examination of Opabinia in 1975 revealed its unusual features, it was thought to be unrelated to any known phylum, although possibly related to a hypothetical ancestor of arthropods and of annelid worms. However other finds, most notably Anomalocaris, suggested that it belonged to a group of animals that were closely related to the ancestors of arthropods and of which the living animals onychophorans (velvet worms) and tardigrades may also be members.”

Given the many shared traits with Stylaria, the proboscis in Opabinia does not appear to be used elephant-like, as Whittington 1975 suggested, for carrying prey items back to the ventral mouth. Rather the proboscis was more likely used as a sand probe and/or a poison delivery system. Feeding would have taken place in a more typical flatworm fashion, by settling over a docile prey item and everting soft ventral mouth parts to haul in food that had stopped struggling.

Smith and Ortega-Hernández 2014
offered a competing hypothesis of interrelationships (Fig. 3). In their study focusing on the terminal claws of another Burgess Shale former enigma, Hallucigenia, the authors included Anomalocaris and two related anomalocarids. Opabinia was the outgroup. Stylaria and Tullimonstrum were excluded taxa.

I have no issues with the presence of walking velvet worms nesting with walking tardigrades and walking Hallucigenia. However, I think legless, swimming Opabinia and Anomalocaris do not belong here. An omitted primitive legless taxon, Stylaria (Fig. 2) needs to be added to the Smith and Ortega-Hernandez taxon list to ascertain or modify relationships. More outgroups are probably needed. Or delete the inappropriate swimmers.

Figure 2. Illustrated cladogram from Smith and Ortega-Hernández 2014 (colors, arrows, gray taxa added here) inserts flat, swimming anomalocardids in a claodogram that otherwise features cylindrical lobe-footed crawling worms.

Figure 3. Illustrated cladogram from Smith and Ortega-Hernández 2014 (colors, arrows, gray taxa added here) inserts flat, swimming anomalocardids in a claodogram that otherwise features cylindrical lobe-footed crawling worms as basal taxa. Note how Opabinia stands out as the oddball here. 

Cong et al. 2014 offer a second competing hypothesis
based on their study of Lyrarapax (Fig. 3), a tiny genus clearly related to Anomalocaris. In the Cong et al. cladogram (Fig. 4) Opabinia nests a few nodes further away from several specimens attributed to Anomalocaris. Stylaria and Tullimonstrum were once again omitted from the Cong et al. taxon list.

Figure 3. Cladogram from Cong et al. 2014 nest Anomalocaris in the clade Radiodonta derived from nematodes and penis worms.

Figure 4. Cladogram from Cong et al. 2014 nest Anomalocaris in the clade Radiodonta derived from nematodes and penis worm in which the mouth and anus are both terminal.

The case is building that 
Anomalocaris and Opabinia never went through an evolutionary phase in which the body was cylindrical with the short clawed feet of velvet worms and tardigrades. Instead these two appear to have evolved directly from free-swimming segmented worms without legs. Let’s keep adding taxa to figure this out.


References
Bergström J 1986. Opabinia and Anomalocaris, unique Cambrian arthropods. Lethaia. 19 (3): 241–246.
Whittington HB 1975. The enigmatic animal Opabinia regalis, Middle Cambrian Burgess Shale, British Columbia. Philosophical Transactions of the Royal Society B. 271 (910): 1–43 271.

wiki/Opabinia

 

2 thoughts on “Stylaria, Opabinia and Tullimonstrom side-by-side

  1. I’m sorry David, but this is far from an improvement or fitting defence. You have moved onto Opabinia, and in doing so, *directly ignored all fossil evidence*. This is becoming a theme with the lobopodian related posts. Multiple studies have demonstrated the presence of lobopodous limbs just under the swimming flaps of Opabinia. You also seem to deny the existence of the mouth and gut, which, I honestly have no idea how to respond to. The mouth is visible in just about every fossil. To deny its presence, you may as well deny the existence of Opabinia altogether. All of these traits let Opabinia fit in alongside Gilled Lobopods. Accordingly, and easily demonstrably, the “proboscis” is the same structure as the feeding appendages of other gilled lobopods such as Pambdelurion, spines and all. The appendage did not end with a claw, but with two very short separated appendages, almost as if someone took the feeding arms of Pambdelurion and did them up like a zipper, leaving only the top undone.

    This post has the same issues as the previous post, and more. You ignore and contradict all available fossil evidence along with all previous work done on the animals, simply in favour of having an “edgy”, new hypothesis so that you may be perceived as an expert. I am not stopping you from making unfounded claims, but do not attest to being an expert in this field. I would advise, once again, taking some time to actually read through previous work, not under the assumption that everyone else is wrong and only you can see the truth.

    The radiodonts and the lobopods are not related to flatworms, and trilobites are arthropods. Your hypothesis can only exist by ignoring all fossil evidence and by excluding taxa from phylogenetic studies in favour of simply putting the tree together how you want it to look, the very thing you attest to solve.

    • Christian, it’s not as bad as you think and the urge to be ‘edgy’ or an ‘expert’ is not on my list. I don’t have to publish or perish. Perhaps too often I suggest taxa that seem to need to be included in analyses. That’s about it. Academically I have been ignored, including several peer-reviewed papers in prestigious journals. So don’t worry about this little blogpost. It won’t gain any traction. My slight detours into invertebrates are irregular and few.

      re: Opabinia — I did not attempt to deny the presence of a ventral mouth and terminal anus. I only objected to the suggestion that the proboscis was used to transport food to the mouth, as elephants do. When you say studies have found lobopodous limbs under the flaps of Opabinia, I have no trouble with that. I trust your citations all are newer than Whittington 1975, the source I have been working with. A citation or two on your data points would be appreciated.

      You wrote: “the “proboscis” is the same structure as the feeding appendages of other gilled lobopods such as Pambdelurion, spines and all.” This is difficult to agree to given that the structures on Pambdelurion are robust and laterally diverging while the proboscis on Opabinia is a weak, flexible medial tube. I simply looked for other taxa with a similar structure, then realized these taxa had more in common than just a proboscis, and presented them clearly without a phylogenetic analysis.

      I can’t imagine anyone claiming to be an expert in any field after a weekend of initial study. I also can’t imagine any expert or scientist objecting to the suggestion that the addition of a few more taxa might shed light on evolutionary pathways — and yet I’ve seen that happen often for the last 20 years of vertebrate studies. I’ve also been forward enough to report genetic studies deliver false positives over deep time when compared to trait studies. No one likes that.

      You wrote: “The radiodonts and the lobopods are not related to flatworms, and trilobites are arthropods.” I try to avoid the use of suprageneric taxa. Too often purported clade members are not clade members (e.g. traditional claims that pterosaurs are archosaurs, turtles are diapsids, mesosaurs are parareptiles, caseasauria are synapsids, etc.). At some point, all known animals with a separate mouth and anus had simpler ancestors that combined these two openings, as flatworms do. How radiodonts, lobopods and arthropods branched off from such ancestors is interesting, as you know. I simply wondered if some flat taxa evolved directly from flatworms that had developed a terminal anus, a coelom and then became segmented, rather than follow the traditional path of becoming first a ribbon worm and/or a cylindrical round worm with a terminal mouth and anus, then a segmented cylindrical round worm with a coelom and lobopods, etc.

      I’ve been writing about evolution since the publication by Little, Brown of “From the Beginning” in 1991, but every new taxon I look at is new to me, which makes me a high school freshman over and over. As I’ve told my readers many times, I have made over 100,000 corrections to the data and scores in the ReptileEvolution.com online study. So, I have no problem with making corrections, but I have to know they are, or seem to be, valid corrections.

      I appreciate your comments, Christian. I don’t plan to ever put an invertebrate tree together. I leave that to the experts. But I will raise a hand if something doesn’t make sense and another hypothesis or a few omitted taxa should be considered.

      Best regards

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