Field of Science

Tully as a Vertebrate

Reconstruction of Tullimonstrum gregarium by Sean McMahon, from McCoy et al. (2016).

McCoy, V. E., E. E. Saupe, J. C. Lamsdell, L. G. Tarhan, S. McMahon, S. Lidgard, P. Mayer, C. D. Whalen, C. Soriano, L. Finney, S. Vogt, E. G. Clark, R. P. Anderson, H. Petermann, E. R. Locatelli & D. E. G. Briggs (in press, 2016) The ‘Tully monster’ is a vertebrate. Nature.

Several years ago, I included the 'Tully monster' Tullimonstrum gregarium in a list of some of the most phylogenetically mysterious organisms on the planet. Multiple suggestions have been made as to its affinities: mollusc, annelid, nemertean (nemerteans and sea cuumbers both having weird histories of problematic fossils assigned to them for little apparent reason), some sort of de-chitinised arthropod relative by way of Opabinia, the Loch Ness monster... A new publication just out by McCoy et al. (2016) adds a further interpretation to the mix.

Tullimonstrum is represented by literally thousands of specimens from the Carboniferous Mazon Creek deposit of Illinois. The organisms preserved in this deposit are contained within nodules, each individual at the centre of a mineral ball that precipitated around it after its death. It had a somewhat elongate, torpedo-shaped body, at the front of which was an elongate proboscis ending in a pincer-like structure. Towards the front of the main body was a dorsal cross-bar with a dark round body at each end; these bodies have most commonly been seen as eyes on the end of stalks but alternative interpretations include statocysts, solid structures that many aquatic animals possess for sensing balance. A fin-like structure was present at the tail end of the animal. Many specimens also show regularly spaced dark cross-lines suggesting some sort of segmental division of the body.

Another structure commonly visible in the Tullimonstrum fossils is a pale, flattened linear structure running down the length of the animal. Most authors have presumed that this represents the gut but McCoy et al. argue that it does not resemble the gut as preserved in other Mazon Creek fossils. In these other fossils, the gut is dark-coloured and is not flattened. Some authors have tried to explain this difference between the 'gut' of Tullimonstrum and that of its associates by suggesting that the Tully monster fed on soft prey such as jellyfish whose remains did not preserve after death, but the dark colour in most Mazon Creek guts does not represent the actual gut contents themselves but minerals that precipitated around the gut contents during the fossilisation process. Presumably, such minerals would be just as likely to condense around jellyfish remains as any other organic tissue. Even more damning, McCoy et al. identified a handful of Tullimonstrum specimens in which the gut was indeed preserved as in other Mazon Creek fossils, and as a separate structure from the pale line that was also present in these same specimens.

An actual fossil of Tullimonstrum in the Museo di Storia Naturale di Milano, copyright Ghedoghedo.

So what was this structure, if not a gut? McCoy et al. note that at least one other fossil from the Mazon Creek preserves a similar structure: the hagfish-like Gilpichthys, in which it represents the notochord. The structure's preservation is consistent with this interpretation: being a fluid-filled tube, the notochord would flatten readily during fossilisation, and it does not accumulate minerals like the gut because it lacks an external connection. And if Tullimonstrum also possesses a notochord, then that makes it also a chordate. And with that in mind, McCoy et al. interpret other structures as supporting chordate, and specifically vertebrate, affinities: the fin-like structures are indeed fins, paired stains bordering the notochord in a few specimens appear to be gill pouches, tooth-like structures within the 'pincer' at the end of the proboscis are keratinous teeth similar to those of lampreys and hagfish, and the apparent 'segments' in some specimens represent vertebrate myomeres (muscle blocks). Including Tullimonstrum in a phylogenetic analysis of basal vertebrates, coded according to these and other interpretations, places it within the stem-lineage of modern lampreys.

So how strong is this re-assignment? The problem with the structural analysis of any problematic fossil is that it is ultimately dependent on finding the right comparative framework, and the more distinct the problematicum is from any living organism the harder it is to be sure you're making the right comparison. That's not a criticism of this particular paper; that's simply the limitation its authors have to work with. In this case, I kind of suspect that the identification of Tullimonstrum as a vertebrate all hinges on whether they've correctly identified that notochord. None of the other 'vertebrate' features identified is sufficiently distinct to clinch the deal on their own. A tail-fin could indicate a vertebrate, or it could indicate a mollusc like a squid. The famous Tullimonstrum proboscis (which, offhand, McCoy et al. interpret as a cartilage-supported structure rigidly bending at set points like an arm rather than curling like a tentacle, based on the regular aspect of its preservation) is unlike anything known from any other vertebrate, but nor does it strongly resemble anything found in any other animal (the aforementioned Opabinia suggestion is right out: as I mentioned in an earlier post on Nectocaris, the Opabinia proboscis contains no direct part of the digestive tract itself). Certainly the placement of Tullimonstrum as a stem-lamprey is the weakest part of the whole deal, as the specific features cited as synapomorphies are either convergently present in other vertebrates (e.g. keratinous teeth) and/or dependent on some admittedly more tentative structural interpretations (e.g. tectal cartilages). There may be a certain element here of Tullimonstrum's intractable weirdness conflicting with the phylogenetic analysis' need to put it somewhere. I also wonder if I should be criticising Sean McMahon's reconstruction (reproduced at the top of this post) for presenting Tullimonstrum as somewhat laterally flattened: the majority of Tullimonstrum specimens are preserved dorsoventrally rather than laterally, which I would suspect indicates that they were probably flatter top-to-bottom than side-to-side.

Those criticisms aside, McCoy et al. have certainly presented one of the more robust reconstructions of Tullimonstrum to date. Most of what I've said comes under the heading of intrigued enquiries rather than actual disagreements, and if they're right about that notochord then they're on pretty firm ground. After all, even if the Tully monster is not specifically a stem-lamprey doesn't exclude it from being any sort of chordate. There are few (if any) problematica as well represented in the fossil record as Tullimonstrum, and we have not heard the last word on it yet.


  1. They're not the first to suggest chordate affinities, right? I seem to recall someone saying it looked like an unholy hybrid between a vertebrate and a arthropod.

    Well, that's one down, nine to go from your list. Actually, that might make for a decent post topic: revisiting your 2008 list and summarizing what we've learnt (or unlearnt) since.

  2. Thank you for posting that! It was the first comment I saw that went beyond … beyond what might have been in a press release (media package?) put together by the investigators or their institution.
    If, as suggested in the study and shown in the picture, the "trunk" with the toothed mouth at its end was anointed structure with rigid internal cartilage "struts," it might have been capable of being shot forward (starting in a folded position, then extending) very rapidly: obviously valuable for a predator of small and mobile critters. I think there are analogous (though of course not homologous) feeding organs in some extant water creatures?
    Intriguing and fascinating.

  3. Hmmm… Ignore the colour, the gill pouches, and the fact that the spectacles are back on the torso instead near the mouth: the picture has a loon-like or cormorant-like gestalt. And maybe the necks of these animals are an extant analogue of Tully's trunk?

  4. Since my last post, I've skimmed the actual article (not the supporting stuff). One question about … Latin. The article uses "arcualium" as a singular for one of the, um, rudimentary vertebrae associated with the notochord. Is this right? I know the plural is "arcualia," but had always assumed that it was a third-declension noun declined something like "animal".

  5. Andreas: McCoy et al. themselves include 'conodonts' among a list of previously suggested allies, but I don't have access to the original reference cited to see how strong the comparison was. Considering said reference was published in 1991, I'm not certain that a comparison to conodonts would have automatically implied a comparison to vertebrates (I believe the first soft-body remains of conodonts would have been found by that time, but I don't know if their significance was yet universally agreed on). If there had been an earlier comparison to vertebrates as a whole, I think it would have hinged on the presence of fins. Or you might be thinking of Nectocaris, which before its recent re-description did seem to combine an arthropod fore-section with a vertebrate rear.

    Allen: Good point about the similarities to a shag; I can definitely see what you're saying. Could also tally with the point that, whether I'm correct in my criticism of the overall cross-section of the reconstruction or not, this does not look to have been a fast-moving animal overall. In the modern fauna, of course, the roles of marine pelagic ambush predators are dominated by ray-finned fishes and cephalopods, neither of which had yet evolved their modern facies in the Carboniferous.

    Re the derivation of 'arcualia', it looks like it might be a modern coining so I'm not sure what its correct formation should be. I find a number of Google results for 'arcualium' but the results for 'arcual' refer to something different. The closest classical Latinism that I can find on Perseus Tools is 'arcuarius', meaning 'pertaining to a bow'.

  6. The correct singular appears to be arcuale, which Merriam-Webster defines as "any of the primitive cartilages or structural elements of which a typical vertebra is formed". (Animal is an irregular form - it "should" be animale, from anima "vital principle" plus neuter of adjective-forming ending -alis.)

    Googling a bit, I can't seem to find any older suggestion of the vertebrateness of Tully, so I may indeed be misremembering. Nectocaris doesn't ring any bells for me on the subject of looking like a vertebrate-arthropod mashup however.

  7. Hmm… Thought I'd posted a reply yesterday, but must have clicked the wrong button… Thanks, both Christopher and Andreas, for replies.

    Andreas: I ***thought*** 'arcuale' sounded like a good singular, but then wondered why 'animal' wasn't 'animale'. It's the Romans being tricky and irregular to fool us, is it? (Sounds downright GREEK mutter grumble…)

    Zoological thought. The conodont apparatus has been compared to the … feeding gear? … of a hagfish, but the actual sharp bits are different in chemistry: keratinous (or chitinous?) in hagfish, phosphatic (like the bones and teeth of "higher" vertebrates) in Conodonts. Now, this "a-mazon" critter of Tully's … seems to have had sharp bits that were chemically more like a hagfish's than like a conodont's, but they don't seem to be arranged in anything resembling a conodont apparatus or the front end of an extend Cyclostome. If anything, the arrangement seems more like that of teeth in Gnathostomes: arranged on either side of a pair of jaws (and, given its asymmetry, I would think the "jaws" of a Tully monster are probably upper and lower rather than left and right). So it's all very SUGGESTIVE of chordate affinities, but it's hard to see clear homology at the front end. (Which is not to say I have any other suggestion, and the evidence of a notochord seems compelling.)

    1. There does seem to have been a fair bit of back-and-forthing about 'teeth' in early vertebrates: cylostomes, conodonts and gnathostomes may have evolved teeth independently (or teeth were lost in a number of early jawless groups), and I think phosphatisation of the teeth may have arisen on three separate occasions. Conodonts for one, and I think the phosphatic teeth of arthrodires might be independent from those of crown gnathostomes (unless the paraphyletisation of placoderms has removed that inconsistency). Certainly throwing Tully into the mix doesn't make it much more complicated than it already was.

      McCoy et al. suggest that Tully's cartilaginous proboscis may be homologous with the cartilage-supported 'hood' around the mouth of a lamprey, but I'm not familiar enough with the anatomy of the latter to comment.


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