Typhloesus: The 'Alien Goldfish' of Bear Gulch

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Above: Typhloesus wellsi, external appearance, and the same with major anatomical details shown. Based on figure of specimen U.M. 6027 in Conway Morris (1990).

Recently, the interwebs became all agog at the suggestion that the hitherto-mysterious Carboniferous fossil Tullimonstrum gregarium could possibly represent a vertebrate, distantly related to modern lampreys. But there are other fossil animals whose relationships remain inexplicable and one of these is another child of the Carboniferous, the so-called 'alien goldfish' Typhloesus wellsi.

When I announced my plan to write this post, I referred to Typhloesus as coming from Mazon Creek, the fossil deposit from whence comes Tullimonstrum. This, as it turns out, was a mistake on my part: Typhloesus actually comes from a different deposit, Bear Gulch in Montana. Bear Gulch is perhaps most famous for its fossils of early fish, such as symmoriiform sharks (the ones with the weird shoebrush headgear) and heavily armoured palaeoniscoids. Indeed, compared to other Carboniferous deposits, Bear Gulch is unusual for its preponderance of swimming rather than benthic animals. Typhloesus is represented in the deposit by a number of individuals in varying states of preservation.

In some ways, Typhloesus is more famous for what it is not than for what it is. It was one of the first body fossils found in association with conodonts, minute teeth-like fossils that had been subject to much speculation as to what sort of animal they might have come from. Initially, there was much excitement that the conodont animal may have finally been found, but it did not take very long for questions to be raised about the nature of this association. By the time Typhloesus was reviewed in detail by Conway Morris (1990), it was clear that the conodont fossils had been preserved within its gut, not its mouth, and Typhloesus was a conodont-eater rather than a conodont-bearer (it has since been found that conodont animals were eel-like chordates).

Externally, Typhloesus was a fairly simple, cigar-shaped animal, with its body laterally compressed and higher than wide. It grew to a decent size, with the largest specimens being a little under ten centimetres in length. There is no sign of eyes or any other prominent sensory structure, and so far as is known the external skin or cuticle was smooth and unornamented. The most distinctive external feature is a large 'tail-fin' at the rear. This fin was supported by an arrangement of criss-crossing rods or fibres, and would have been fairly stiff in life. Another pair of folds or fins ran along most of the underside of the body with a noticeable gap towards the rear. Typhloesus probably swam in a not dissimilar manner to an active modern fish, using sweeps of the tail-fin to provide thrust; the ventral fins may have provided stability and steerage. The visible line of the foregut comes to a halt slightly before reaching the front of the body, and it seems that the mouth would have been slightly ventral and contained within a 'hood'. Though its overall conformation and known gut-contents (most commonly conodonts, but sometimes worm jaws or fish scales) suggest an active predator, I am at a loss to understand how it located its prey without eyes. Perhaps the hood contained some sort of chemical sensors in life.

When it was first found, it was thought that its overall appearance suggested a relationship of Typhloesus to the chordates. However, Conway Morris (1990) saw its internal anatomy as incompatible with this view. Fossils of this animal show a narrow foregut leading into a voluminous, sack-like midgut. Below the midgut is a pair of dark, disc-shaped organs showing a concentration of iron deposits called the ferrodiscus; though a striking element of all Typhloesus fossils, the function of this structure is completely unknown. What Conway Morris found conspicuous by its absence, however, was an anus: there appeared to be no sign of any gut structures in the rear of the animal. The gut was a blind sack, with the only way out being the same as the way in. The absence of a through-gut would be unprecedented in a chordate, or indeed in many animals except jellyfish or flatworms. Conway Morris was also unable to identify other chordate-specific structures such as muscle-blocks, gill openings or a notochord; though he confessed that the first two might be obscured by the vagaries of decay, he felt that the third at least should have left more of a sign. It was this combination of an overall fish-like appearance with a very un-fish-like anatomy that led Conway Morris to later dub Typhloesus the 'alien goldfish'.

With the exclusion of a chordate connection as a possibility, Conway Morris found himself at a loss as to just where Typhloesus fitted into animal evolutionary history. Finned swimmers are also known among molluscs, nemerteans and chaetognaths, but Typhloesus is no more like any of these than it is like a chordate. Conway Morris felt himself compelled to declare the affinities of Typhloesus completely unknown. Personally, though, I can't help wondering if the 'alien goldfish' might not be so alien after all: maybe it is a chordate. The overall similarities of Typhloesus to a chordate are remarkable; in particular, the hooded mouth is very similar to that of a lancelet. But what about that missing anus, you say? Where is that all-important butthole? To which I respond, is it really missing? Looking at the figures of Typhloesus fossils in Conway Morris (1990) (which is of course a poor competitor to Conway Morris' ability to look directly at the fossils themselves), I see that directly below the midgut is the ferrodiscus. And directly below that is a streak running between the ferrodiscus and the animal's venter. Conway Morris saw this structure (which he called the 'midventral strand') as some sort of connection between the ferrodiscus and the exterior, but could it in fact be the tail-end of the reargut? It is certainly not unknown for the anus in chordates to not be right at the very rear of the animal; in some fish (such as the scorpionfish-like Aploactinidae) it is even moved so far forward as to be almost underneath the head. And the missing notochord? Considering that despite the presence of specimens numbering in the thousands, a notochord was only announced in Tullimonstrum within the past year, maybe on that front Typhloesus could reward a second look.

REFERENCE

Conway Morris, S. 1990. Typhloesus wellsi (Melton and Scott, 1973), a bizarre metazoan from the Carboniferous of Montana, U.S.A. Philosophical Transactions of the Royal Society of London Series B 327: 595–624.

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.

Hagfish: Probably the World's Most Disgusting Vertebrates

South African hagfish Myxine capensis, copyright Andy Murch.

I say "probably" not the title not because there's any question about whether hagfish are disgusting–they are, they really are–but because there's been some debate in the past about whether hagfish are vertebrates. Hagfish, as you may already know, are superficially eel-like marine animals that, together with the lampreys, are one of the two living lineages of 'jawless fish'. Their skeleton is both completely cartilaginous decidedly rudimentary: they even lack a developed spine, instead retaining the fluid-filled notochord throughout their life. They do possess a brain-case, as well as some appendicular cartilages that provide support for the fins. Around the mouth are a set of muscularly-controlled tooth-plates together with short sensory tentacles. Hagfish have no eyes; instead, they find their way about primarily through the use of a single large nostril in the middle of the head. Along the underside of the body run a series of glands capable of producing a truly mind-bending amount of mucus. As noted by Martini & Flescher (2002), "A single live individual hagfish can turn a 2 gallon pail of water into a gelatinous mass within a few minutes". Most hagfish seem to be in the one or two feet range size-wise, but the New Zealand species Eptatretus goliath was described from a single monster specimen a bit over 1.25 metres long (Mincarone & Stewart 2006). In contrast, the hydrothermal vent inhabitant Eptatretus strickrotti is only just over a foot long and built like a swimming shoelace (Møller & Jones 2007).

A demonstration of a hagfish's slime-producing capabilities, copyright Andra Zommers.

Martini & Flescher (2002) summarised the lifestyle of the Atlantic hagfish Myxine glutinosa (or probably the western Atlantic hagfish M. limosa which they regarded as synonymous with the eastern Atlantic M. glutinosa), which I'm guessing is fairly typical of the group. Atlantic hagfish spend most of their lives buried in burrows in muddy sea-bottoms (the technical term for the type of sediment they prefer is 'flocculent', which is a wonderful word to say), emerging primarily to feed. A large part of their diet is obtained by predating small animals such as crustaceans. They are most notorious, though, as scavengers. Hagfish will emerge in large numbers to feed on any animal corpses that sink within their range. Though they are capable of tearing off external chunks of flesh (more on that in a moment), they are not able to do so efficiently so they prefer to focus on the softer internal organs when they can. This they do by worming their way into the carcasse through a convenient orifice such as the mouth or anus and enjoying the laid-on buffet within. The reproduction of hagfish is poorly known. The Royal Academy of Copenhagen offered an award in 1864 to the first person to describe the details of hagfish nooky; the offer was withdrawn in the 1980s, still unclaimed. Female hagfish have been caught with developing eggs, up to 30 at a time, connected in a string by velcro-like hooks. The absence of any sort of obvious intromittent organ in the male suggests that fertilisation is external, but anything beyond that is a mystery.

Their lack of a rigid skeleton makes hagfish capable of some behaviours that would be beyond other vertebrates. One of these is referred to as 'knotting' and it is exactly what it sounds like. The hagfish makes a loop with its body through with it sticks its tail, quite literally tying itself in a knot. By pulling itself through itself, it can move the knot up the body until the head pops out at the other end. One reason it may do this is to clean itself; for instance, a hagfish may drown in its own mucus if not given the opportunity to remove it (so that single live individual in the two-gallon bucket is probably not live any more). Another reason is that the knot can be used to push against something, such as when the hagfish wants to escape from an enclosed space. When feeding on something large and solid (such as the aforementioned external scavenging), the hagfish will latch on with its tooth-plates and then form a knot to push against it until eventually it tears away with a mouthful of food.

Hagfish can be abundant in some areas, make them an important part of the local ecosystem. They may be regarded as a nuisance in fisheries, attacking fish caught on lines and traps and reducing their commercial value. However, hagfish are also caught for food in some parts of the world (particularly in east Asia) and their skins are cured to produce a soft textile known somewhat euphemistically as 'eelskin'.

Pacific hagfish Eptatretus stoutii, photographed by Linda Snook.

About sixty species of hagfish are currently recognised around the world, usually classified in a single family Myxinidae. Most are divided between two subfamilies (sometimes recognised as separate families), the Myxininae and Eptatretinae. Myxininae have a single external gill opening whereas Eptatretinae have multiple gill openings. A phylogenetic analysis of the hagfish by Fernhom et al. (2013) found a couple of species previously assigned to Eptatretus to probably sit outside the Myxininae-Eptatretinae clade and transferred them to a new genus Rubicundus in its own small subfamily, differing from other hagfish in having the single nostril on a short tubular snout.

As alluded to above, there has been some debate about the affinities of hagfish. Though superficially similar to the other living group of 'jawless fishes', the lampreys (largely through both being eel-like in form), hagfish are very different in the anatomical details, and at the very least the two lineages have been separate for a very long time. Because of their lack of a number of derived features, hagfish were suggested to be the sister lineage of all other vertebrates, leading to the observation that it was not really appropriate to classify a lineage that did not have and probably never had vertebrae as 'vertebrates'. As such, hagfish became regarded as the closest relatives of vertebrates rather than vertebrates themselves. However, molecular studies of vertebrate phylogeny have pretty much universally identified hagfish as forming a clade with lampreys after all, implying that the 'primitive' features of hagfish probably represent secondary losses. When constrained as a clade in morphological analyses, nevertheless, the hagfish-lamprey group remains basal in vertebrates: most if not all of the fossil groups of 'jawless fish', particularly those with an outer covering of bony plates, are more closely related to the jawed fishes than to hagfish or lampreys.

The most likely fossil hagfish (and even then it's not much), Myxinikela siroka, copyright RCFossils.

Not surprisingly for something without much of a skeleton, the fossil record of hagfish is pretty minimal. A species from the Carboniferous Mazon Creek lagerstätte, Myxinikela siroka, is likely to be a stem-hagfish; a couple of other fossils from the same formation have also been suggested as candidates. Myxinikela was broadly similar to a modern hagfish, the most obvious difference being that it was shorter and more cigar- or banana-shaped than eel-like (I can't really imagine it being able to tie itself in knots). Some authors have also suggested similarities between the braincase of hagfish and that of Palaeospondylus, an unusual eel-like vertebrate from the Middle Devonian of Scotland whose confusing assortment of features has lead to it being seen at one time or another as a jawless fish, a degenerate bony fish that failed to develop bone, or even a larval amphibian (Janvier 2015). The most obvious difference between Palaeospondylus and a hagfish is that Palaeospondylus possessed a complete cartilaginous skeleton, but the molecular phylogenies suggest that may not be the problem it would have previously been assumed to be...

REFERENCES

Fernholm, B., M. Norén, S. O. Kullander, A. M. Quattrini, V. Zintzen, C. D. Roberts, H.-K. Mok & C.-H. Kuo. 2013. Hagfish phylogeny and taxonomy, with description of the new genus Rubicundus (Craniata, Myxinidae). Journal of Zoological Systematics and Evolutionary Research 51 (4): 296–307.

Janvier, P. 2015. Facts and fancies about early fossil chordates and vertebrates. Nature 520: 483–489.

Martini, F. H., & D. Flescher. 2002. Hagfishes. Family Myxinidae. In: Collette, B. B., & G. Klein-MacPhee (eds) Bigelow and Schroeder's Fishes of the Gulf of Maine 3rd ed. pp. 9–16. Smithsonian Institution Press: London.

Mincarone, M. M., & A. L. Stewart. 2006. A new species of giant seven-gilled hagfish (Myxinidae: Eptatretus) from New Zealand. Copeia 2006 (2): 225–229.

Møller, P. R., & W. J. Jones. 2007. Eptatretus strickrotti n. sp. (Myxinidae): first hagfish captured from a hydrothermal vent. Biol. Bull. 212: 55–66.

Dream-fish, Coelacanths and Super-Predators: The Sarcopterygians

For the subject of today's post, I drew the Sarcopterygii, the 'lobe-finned fishes'. Though something of a poor relation to their considerably more diverse sister-group, the ray-finned fishes of the Actinopterygii, this is a group most of my readers will have probably encountered in some capacity. As their names both formal and vernacular indicate, the Sarcopterygii were originally characterised by the development of the fins as fleshy lobes, with at least some fins possessing an internal skeleton of serial bones. Living sarcopterygians belong to three major groups, the coelacanths, lungfishes and tetrapods (in which, of course, the ancestral fins have been modified into walking limbs). The majority of recent studies have placed the coelacanths as the most divergent of these groups, with lungfishes and tetrapods as sister taxa. As the tetrapods are a particularly tedious group of organisms, with little to interest the casual observer, I'll put them aside for this post (you can go to Tetrapod Zoology if you must). The lungfishes, too, warrant a more detailed look at another time.

The oldest known sarcopterygian (and, indeed, the oldest known crown-group bony fish) is the Guiyu oneiros (shown above in a reconstruction by Brian Choo for Zhu et al. 2009), whose species name suggests the vernacular name of 'dream fish'. The dream-fish is known from the late Silurian of China, with a number of other stem-sarcopterygians such as Psarolepis and Meemannia known from the early Devonian of the same region. These taxa retained a number of ancestral features such as heavy ganoid scales (a type of scale also found in basal actinopterygians), and strong spines in front of the fins. However, crown-group sarcopterygians had also evolved and diverged by the early Devonian, as shown by the presence of the stem-lungfish Youngolepis.

Congregation of West Indian Ocean coelacanths Latimeria chalumnae, photographed by Hans Fricke.

The coelacanths are, of course, best known to most people for the discovery of the living Latimeria chalumnae in 1938 in South Africa, after the lineage had been thought to have become extinct in the Cretaceous. The subsequent media frenzy must have been interesting to fishermen in the area who had long known the coelacanth primarily as an infernal nuisance. Though only captured occasionally as bycatch, a landed coelacanth represents two metres or more of snap-jawed bad temper, while the oily flesh is inedible. More recently, a second species of living coelacanth, Latimeria menadoensis has been described from near Sulawesi in Indonesia.

Because of the circumstances of its discovery, Latimeria became a textbook example of a 'living fossil'. However, all fossil coelacanths were not mere duplicates of Latimeria. To begin with, Latimeria is quite a bit larger than the majority of its fossil relatives (Casane & Laurenti 2013). These included such distinctive forms as the fork-tailed speedster Rebellatrix and the eel-like Holopterygius. And then there was Allenypterus montanus, a Carboniferous taxon that... well, just look at the thing (photo from here):

Though Latimeria may lord it over its immediate relatives, it is far from the largest sarcopterygian (even excluding the tetrapods). The tetrapod stem-group also included a number of large predators, including the famous Eusthenopteron (how many other fossil fish have been referred to by name in an episode of Doraemon?). Particularly dramatic were the Rhizodontida, freshwater ambush predators of the Devonian and Carboniferous. Though probably very low on the tetrapod stem (and hence not directly related to limbed tetrapods), rhizodontids developed enlarged pectoral fins that articulated with the body in a not dissimilar manner to tetrapod forelegs. Like tetrapods, rhizodontids probably used their pectoral fins to push against the substrate and provide explosive propulsion (Davis et al. 2004). The jaw of rhizodontids contained enlarged tusks interspersed among smaller teeth that would have hooked into struggling prey. The largest rhizodontids have been estimated to be about seven metres in length, and were the sort of predator that the term 'apex' was invented for.

Reconstruction of Rhizodus by Mike Coates.

REFERENCES

Casane, D., & P. Laurenti. 2013. Why coelacanths are not 'living fossils'. BioEssays 35: 332-338.

Davis, M. C., N. Shubin & E. B. Daeschler. 2004. A new specimen of Sauripterus taylori (Sarcopterygii, Osteichthyes) from the Famennian Catskill Formation of North America. Journal of Vertebrate Paleontology 24 (1): 26-40.

Zhu, M., W. Zhao, L. Jia, J. Lu, T. Qiao & Q. Qu. 2009. The oldest articulated osteichthyan reveals mosaic gnathostome characters. Nature 458: 469-474.

Linguipolygnathus Redux

Upper (left in each case) and lower (right, do.) views of representative Pa elements of polygnathids from Bardashev et al. (2000): (upper left) 'Linguipolygnathus' anastasiae; (upper right) 'Eolinguipolygnathus' nothoperbonus; (lower) 'Costapolygnathus' inversus. Bardashev et al. (2002) classify L. anastasiae closer to C. inversus despite regarding it as phylogenetically closer to E. nothoperbonus.

There are some things that you find yourself returning to like an itching scab. Yes, it's time for me to once again wade into the unsettling world of polygnathid conodont taxonomy.

Previous comments on the subject can be found here and here. To briefly recap: Bardashev et al. (2002) divided the Devonian conodonts of the Polygnathidae, most of them previously assigned to a single genus Polygnathus, between a number of families and genera. However, they explicitly represented a number of both families and genera as extensively polyphyletic. Later, Weddige (2005) responded to criticisms of the divided taxonomy by claiming that it represented a form taxonomy only. In my first post on the subject, I expressed confusion at what exactly Weddige meant by that claim.

On closer examination, I'm somewhat more inclined to take Weddige's claim at face value. Despite proposing detailed phylogenetic relationships between the species studied, Bardashev et al.'s (2002) taxonomy is supposed to prioritise identification above all. The primary division, Polygnathidae vs 'Eopolygnathidae', is based on a single character: the development of the basal cavity on the underside of the Pa element of the polygnathid apparatus. Presence of a basal cavity is the ancestral condition; within the 'eopolygnathids', the margins of the basal cavity become progressively closed in a number of lineages until, in the 'polygnathids', there is only a small basal pit remaining. So, for instance, the genera Eolinguipolygnathus and Linguipolygnathus are placed in separate families, despite the facts that (a) they differ in no other characters (the diagnoses provided for the two genera by Bardashev et al. are effectively identical), (b) 'Linguipolygnathus' is proposed to have arisen no less than five separate times from 'Eolinguipolygnathus' ancestors (and is not directly connected phylogenetically to other genera in its own family), and (c) relative to the other polygnathids examined, the two 'genera' supposedly share a clear and (more significantly) phylogenetically coherent character in the formation of the posterior part of the Pa element into a transversely ridged tongue.

Bardashev et al. argued that this division was necessary because the restricted basal cavity was the original character used to establish the Polygnathidae, so the inclusion of taxa with an open basal cavity violated the original diagnosis of the family. The possibility of revising the family diagnosis is not raised, despite their own research apparently showing that it does not diagnose a coherent group. The underlying motivation for this prioritisation of diagnostic characters over phylogeny seems to be the use of conodonts as markers in biostratigraphy. For instance, the type of Eolinguipolygnathus, Polygnathus dehiscens, has been proposed as the marker for the beginning of the section of the Devonian known as the Emsian. But does this emphasis on diagnostic features truly serve even biostratigraphy? Carls et al. (2008), for instance, claim that emphasis on characters of the ventral side of the conodont Pa element has lead to a number of distinct taxa being confused under the name 'Polygnathus dehiscens', leading to misdiagnosis of the Emsian boundary.

Just as I have stated before that a key should not be a taxonomy, a taxonomy should not be a key. Both are important, but both have their own roles to play.

REFERENCES

Bardashev, I. A., K. Weddige & W. Ziegler. 2002. The phylomorphogenesis of some Early Devonian platform conodonts. Senckenbergiana Lethaea 82 (2): 375-451.

Carls, P., L. Slavík & J. I. Valenzuela-Ríos. 2008. Comments on the GSSP for the basal Emsian stage boundary: the need for its redefinition. Bulletin of Geosciences 83 (4): 383–390.

Weddige, K. 2005. Contra Ruth Mawson’s critizising Bardashev, Weddige & Ziegler 2002, e.g. in SDS Newsletters 20 (2004). Subcommission on Devonian Stratigraphy Newsletter 21: 51-52.

A Bizarre New Shark

Live goblin shark Mitsukurina owstoni, from here.

It's a bit unusual for me to be posting anything on a Sunday, but I've just received notice of something so incredibly cool that I couldn't wait to tell you all about it. A new paper has just come out describing a truly remarkable new species of shark:

Takahashi, N., & N. Yuasa. 2012. First recorded use of weaponised light by an elasmobranch. National Daiei Journal 7: 17-87.

The new species, Neomitsukurina nodai, is most closely related to the unusual goblin shark Mitsukurina owstoni, and the resemblance between the two is clearly visible in the head region:

Photo of the new shark species from Takahashi & Yuasa.

Nevertheless, it possesses several remarkable differences. First there is the distinctive fin array, somewhat more extensive than that found in most shark species. The denticles in the skin are much reduced, giving the body an almost rubbery appearance. Furthermore, in a remarkable case of life imitating art, Neomitsukurina differs in its jaw morphology. The vast majority of depictions of goblin sharks show it with protruding jaws but, as can be seen in the photo at the top of the post, this is not the usual appearance of this species: the jaws are generally only protruded when the shark is picking up food. In Neomitsukurina, however, the jaws are seemingly permanently protruded, and the upper jaw has been modified into a sharpened beak. The most interesting distinction of all, however, is the presence of a massively enlarged photophore on the underside of the rostrum, above the jaws:

Close-up of the head of Neomitsukurina nodai, from Takahashi & Yuasa.

The photophore contains a unique lens structure that focuses the light it produces. So strongly focused is the light, in fact, that it can be used in prey capture by the shark. Through a mechanism not yet fully understood, but possibly a shock reaction to its brightness, the light causes potential prey animals to become stunned, after which they can be easily picked off. Preliminary observations of Neomitsukurina suggest that it may be willing to take on quite large prey: even turtles have not proven immune to stunning, though the shark did not always immediately ingest stunned prey animals. Neomitsukurina has also been observed gliding above the surface of the water through the use of its enlarged pectoral fins.

It might be wondered how such a distinctive and mobile predator eluded discovery until the present, but Neomitsukurina's strict nocturnality might have something to do with it. It is also worth noting that sightings of what may, in hindsight, have been Neomitsukurina have been described in the past (a particularly famous sighting occurred in 1971, near the island of Niemonjima), but attempts to follow up such records have so far only collected other animals such as sea bass.

Just When You Thought It Was Safe

Smalltooth cookiecutter shark Isistius brasiliensis, photographed by Joshua Lambus.

Sometimes, you can get pretty much everything you need to know from the title of an article alone. To whit:

First documented attack on a live human by a cookiecutter shark (Squaliformes, Dalatiidae: Isistius sp.)

The article itself is in a journal I don't have access to, but I can read the abstract: the person attacked was a long-distance swimmer in Hawaii and was bitten twice. The bite was treated with skin grafts, but still took nine months to finish healing.

Cookiecutter sharks are one of the more fascinatingly evil fish out there. They are small, as sharks go (up to about 50 cm, tops) but have proportionately oversized teeth that are arranged in a tight, single-row array that can be protruded outwards to take a neat plug out of the flesh of a larger animal: hence the name of 'cookiecutter'. The effectiveness of the cutting tooth row is maintained by being replaced all at once, rather than individual teeth being replaced piecemeal as in other sharks. Cookiecutters are rarely encountered by humans as they are generally deep sea fish, living below the light zone, but like many mesopelagic animals they appear to migrate closer to the surface at night (Papastamatiou et al. 2010). Bioluminescent photophores behind the head have been suggested to function as a lure, drawing larger fish, dolphins, etc. into range of an ambush. Cookiecutters have very catholic tastes, and evidence of bites has been recorded from just about any decent-sized pelagic animal. They will even bite the external insulation on submarines.

Fish with cookiecutter bites, from Rick Macpherson (who, it turns out, covered this event when it was first happened).

Given their lack of pickiness, it is hardly surprising that a cookiecutter would take a bite out of a human. Of course, humans very rarely venture into the pelagic environment in which cookiecutters can be found. The very fact that the Hawaii indicent is the first confirmed attack indicates how extremely rare this would be expected to be. The Wikipedia page on cookiecutters refers to possible attacks on shipwreck survivors (though the source page linked to does not provide citations for such reports), and the body of a drowned fisherman was recovered in Hawaii with cookiecutter bites. But unless you happen to be swimming in the open ocean at night, your chances of being bitten by a cookiecutter are low.

REFERENCE

Papastamatiou, Y. P., B. M. Wetherbee, J. O’Sullivan, G. D. Goodmanlowe & C. G. Lowe. 2010. Foraging ecology of cookiecutter sharks (Isistius brasiliensis) on pelagic fishes in Hawaii, inferred from prey bite wounds. Environmental Biology of Fishes 88 (4): 361-368.

A little Linguipolygnathus

Variants of Linguipolygnathus linguiformis over time, from Bardashev et al. (2002).

Three points for this ID challenge go to Adam Yates who recognised the objects in the figure as P-elements of an ozarkodinid conodont (the first person to identify them as a conodont looses out on points because they didn't supply any supporting comments). Linguipolygnathus linguiformis is the type species of Linguipolygnathus, one of the genera carved by Bardashev et al. (2002) out of the large older genus Polygnathus. I've commented on the taxonomic insanity of Bardashev et al. in a previous post, though the idea of subdividing Polygnathus is not in itself a bad one (and note that if Linguipolygnathus were synonymised with its supposed polyphyletically-ancestral genus Eolinguipolygnathus we'd be left with a single monophyletic genus).

Many discussions of conodonts make reference to their minuteness (I've done it myself in the past) and the preserved conodont fossils are certainly minute. However, I must confess to only realising fairly recently that, just because the preserved fossils are minute, doesn't necessarily mean that the (largely soft-bodied and hence rarely preserved) animals themselves were. Of the two best-preserved body fossils of conodonts available to us, the remains of Promissum are those of an animal about 20 cm long. Even the more modestly sized Clydagnathus, which is apparently more like the usual run of conodonts, would have been about 6 cm long in life: not huge, but still comparable in size to a modern anchovy.

REFERENCES

Bardashev, I. A., K. Weddige & W. Ziegler. 2002. The phylomorphogenesis of some Early Devonian platform conodonts. Senckenbergiana Lethaea 82 (2): 375-451.

Conodonts: They Just Got Scarier

Reconstructed apparatus of Besselodus arcticus, from Dzik (1991).

I've told you before about conodonts, Palaeozoic microcarnivores with impressive tooth arrays. In the earlier post, I referred mostly to ozarkodinids, later conodonts that had grasping teeth in the front of their mouths and crushing plates towards the back. In this post, I'll be referring to panderodontids, an earlier group that lacked the crushing plates of ozarkodinids and had a tooth apparatus made up of simpler fang-like elements, similar to the reconstruction above. Apparatus of panderodontids have been found preserved in association, but we don't yet have preserved examples as good as available for the ozarkodinids.



With such different apparatus, panderodontids were obviously capturing and processing prey differently to ozarkodinids, and a paper just out by Szaniawski (2009) suggests one of those differences. Panderodontids and many other conodonts with coniform teeth had long grooves on the inner surface of some of their teeth (as seen in the photo of a Dapsilodus mutatus element above from Szaniawski, 2009) and Szaniawski points out that these grooves are extremely similar to those seen in the fangs of many venomous fish, lizards and snakes. He therefore infers that panderodontids were similarly venomous. As well as making conodont apparatus even more impressive than they already were, this would make panderodontids the earliest known venomous chordates*.

*Szaniawski refers to them as the "oldest known venomous animals". However, cnidarians had already been around for some time, and while the cnidarian venom delivery system doesn't fossilise, the fact that these were crown-group cnidarians makes it a pretty sure bet that they had it by then.

Earlier suggestions that the groove provided an anchoring point for muscles were couched in the belief that conodont elements were permanently internal, a view that is no longer standard*. Other forms of conodont lacked the venom groove, further evidence of the conodonts' ecological diversity.

*Conodont elements grew as new layers were put down over the outer surface, which is admittedly a little difficult to reconcile with their current interpretation as grasping teeth (which would require the absence of tissue cover). It seems likely that conodont teeth were only exposed when being actively used; at other times they would have been retracted into a covering pocket, in the same manner as the grasping spines of modern chaetognaths.

REFERENCES

Dzik, J. 1991. Evolution of oral apparatuses in the conodont chordates. Acta Palaeontologica Polonica 36 (3): 265-323.

Szaniawski, H. 2009. The earliest known venomous animals recognized among conodonts. Acta Palaeontologica Polonica 54 (4): 669-676.

Time For Teeth (Taxon of the Week: Polygnathus)

Lindström's (1974) hypothetical reconstruction of the then-unknown conodont animal as a barrel-shaped floater, with radially arranged conodont elements providing protection from predators dorsally and support for feeding tentacles ventrally.

Conodonts are among the iconic fossils of the Palaeozoic. Minute (in the millimetre size range) but extremely abundant, conodont elements* are tooth-like in appearance. The earliest forms were simple and fang-like; later forms were often blade-like with a median row of teeth. Their abundance and variety mean that conodonts are widely used in biostratigraphy, but for many years the identity of the animal they came from was unknown - whatever it was, it appeared to possess no other hard parts that would normally be preserved. It wasn't until the 1980s that the first unequivocal conodonts with preserved soft parts were discovered, revealing them to be stem- or basal vertebrates** (Sweet & Donoghue, 2001). Each of the conodont animals had a number of conodont elements arranged around the mouth and pharynx. Slender-pointed elements towards the front of the mouth would have seized or filtered prey, while many conodonts also possessed more robust elements further back in the pharynx to grind up their food. The figure below from Dzik (1991) gives a good idea of how it would have all worked, even if the result does look a bit like a carnivorous sock puppet (Dzik's arrangement of the elements has also since been superceeded - see Purnell & Donoghue, 1997, for details). Those full-body fossils of conodonts that have been identified to date are eel- or lamprey-like, but it is worth keeping in mind that only two species of this very speciose lineage are known from such remains and we may not be seeing a proper representation of conodont diversity.

*Before the nature of conodonts was understood, most authors restricted the name to the fossils themselves; the then-hypothetical animal that produced these structures was referred to as a "conodontophore". Since the current identification of conodonts has been accepted, this distinction has been abandoned.

**Conodonts had been found in association with soft body parts before, but the animals concerned are now agreed to have been predators or scavengers of conodonts (with conodont elements in their gut as a result) rather than the conodont animals themselves.


Conodont head in retroventral view as reconstructed by Dzik (1991), with mouth open to show the grasping elements in front and back of head removed to show the grinding elements in back.

Polygnathus has been recognised as one of the largest of conodont genera - some 545 Early Devonian to Early Carboniferous species and subspecies have been assigned to it over the years (Weddige, 2005). Polygnathus belonged to the conodont order Ozarkodinida, and would have had an apparatus of toothed elements similar to that shown below (not Polygnathus, but another ozarkodinidan genus). The lower saw-like S elements at the front of the mouth would have been the initial graspers; the act of opening the mouth would have rotated the curved upper M elements forward, and their rotating back as the mouth closed would have probably drawn the prey in; and the two pairs of large P elements in the back would have sliced and diced the prey.


Reconstructed model of the apparatus of the ozarkodinid Idiognathodus in lateral view, from Purnell & Donoghue (1997).

The Early Devonian members of Polygnathus were recently revised by Bardashev et al. (2002) in what I can only describe as one of the most taxonomically incredible papers it has ever been my misfortune to read. In the early days of conodont taxonomy, working purely from dissociated elements, different elements were treated as taxonomically separate entities. As the recognition developed that a single individual conodont animal would have possessed a number of differently formed elements (something that happened even before the discovery of conodont soft-body fossils as researchers noted that certain element types were always found in association, while specimens were occasionally found in which normally separate elements had become fused together), the older independent element taxonomy was replaced by a multi-element taxonomy based on the apparatus as a whole*. Bardashev et al. (2002), however, base their classification solely on the Pa or P₁ element, the large posteriormost element in the model above. All other elements, they seem to claim, are useless for distinguishing taxa (which could be a problem for dealing with basal conodonts that don't have P elements).

*At least ideally. In practice, of course, there are still a large number of cases in which the correct element associations cannot yet be reliably identified.

On the basis of Pa morphology, Bardashev et al. divide species of Polygnathus between six genera in two families - and this is where things really start to go down the rabbit hole. Members of the family Polygnathidae are divided between the temporally successive families Eognathodidae, Eopolygnathidae and Polygnathidae. Eopolygnathidae are derived from Eognathodidae and Polygnathidae from Eopolygnathidae. Now, the use of paraphyletic taxa is nothing unusual in micropalaeontology. But explicitly polyphyletic taxa? In the phylogeny presented by Bardashev et al., Eognathodidae gave rise to Eopolygnathidae on two separate occasions, with Eoctenopolygnathus descended from a separate group of eognathodids from Eocostapolygnathus and Eolinguipolygnathus (note also that there is no genus 'Eopolygnathus', so 'Eopolygnathidae' is an invalid name under the ICZN). After that, Polygnathidae derives from 'Eopolygnathidae' eleven times - two separate origins of Ctenopolygnathus within Eoctenopolygnathus, four origins of Costapolygnathus from Eocostapolygnathus, five of Linguipolygnathus from Eolinguipolygnathus (the authors refer to these multiple origins as representing common 'trends' between the lineages). Bardashev et al. also name the type species of the new genus Costapolygnathus as Polygnathus dubius, which happens to be the type species of Polygnathus (a point that Bardashev et al. had commented upon themselves earlier in the paper). There are also cases where the type specimens of 'undiagnostic' species are assigned to new species named by Bardashev et al. - surely, if you can identify them to a species, they can't be undiagnostic?


Posterior and anterior views of the Pa element of Polygnathus costatus partitus. Photo from Palaeos.com.

Bardashev et al.'s (2002) reclassification was criticised and rejected by Mawson & Talent (2003), who maintained that because it only covered Early Devonian taxa, it created a strong disconnect in apparent diversity between Early and Late Devonian. This criticism, I must say, is unfair - all revisions have to start somewhere, and to demand an 'all or nothing at all' approach in such cases would be to effectively prevent much possibility of large taxonomic groups being revised at all. Potentially more problematic (but unfortunately not supported with specific examples) is Mawson & Talent's implication that some of the new 'species' recognised by Bardashev et al. are in fact variants of other species and not phylogenetically distinct entities.

Bardashev & Weddige (2003) published a brief note in which they corrected the objective synonymy of Polygnathus and Costapolygnathus by publishing a new genus Eucostapolygnathus that they said "includes the same species as Costapolygnathus - except the species dubius". In a reply to Mawson & Talent's comments, Weddige (2005) defended Bardashev et al.'s (2002) use of a high number of taxa on the basis that the latter had been a 'pure form-taxonomic study'. Or, more extensively:

The genus subdivision proposed by BARDASHEV, WEDDIGE & ZIEGLER (2002) might be regarded as a subgeneric subdivision. In form-taxonomy, however, and the paper represents a pure form-taxonomic study, subgenera are not in usage. Because of the pure form-taxonomy, moreover, resp. because of a more or less subgeneric level of the proposed subdivision, a multielement reference, e. g. by suspect statistics, is not needed, for the first. Thus, a distinctive serious discussion has to focus on (form-) taxonomic characters, i. e. the valuation and order of the diagnostic characters as they are used for the generic subdivision by BARDASHEV et al.. Admittedly, a broadly splitted form spectrum, often including revolutionary ideas, is a hard diet. On the other hand, a well known unchanged form spectrum is a usual and therefore easy diet that, moreover, becomes much easier to digest when the spectrum, or parts of it, is furthermore lumped. The differentiation in “splitters” and “lumpers” is an inadequate simplification -- since the study by BARDASHEV et al. is not only a splitting because of different new taxa, it has rather more the character of a synthesis because of its search for phylogenetic lines by which single species were “lumped”). Thus, the study is a lumping on a quality level, higher than a taxonomic lumping that resigns to differentiate and searches for a conservative comfortable easy diet. Conservatives bloc progress, that is their job – and it would be a total misunderstanding that a SDS commission or a Working Party is entitled to condemn per joint decision (that could not be the target of a discussion!).

So in reply to accusation of being splitters, Weddige replies that no, they were lumpers, but his definition of 'lumping' can only be described as an Inigo Montoya moment. There is also the problem that Bardashev et al. was self-evidently not a purely form-taxonomy study. Form taxa are those based on morphological distinctions only that cannot be confirmed as phylogenetically distinct units - but Bardashev et al. (2002) presented their readers with no less than nine representations of preferred phylogenetic hypotheses, as well as specifically commenting on the descent of every one of the taxa they described. If these were only 'form taxa', then those 'lineages' are completely meaningless, and you, my friend, have just been treated to seventy-seven pages of intellectual masturbation.

REFERENCES

Bardashev, I., & K. Weddige. 2003. The invalid genus name Costapolygnathus Bardashev, Weddige & Ziegler 2002 and the new conodont genus Eucostapolygnathus. Senckenbergiana Lethaea 83 (1-2): 1-2.

Bardashev, I. A., K. Weddige & W. Ziegler. 2002. The phylomorphogenesis of some Early Devonian platform conodonts. Senckenbergiana Lethaea 82 (2): 375-451.

Dzik, J. 1991. Evolution of oral apparatuses in the conodont chordates. Acta Palaeontologica Polonica 36 (3): 265-323.

Lindström, M. 1974. The conodont apparatus as a food-gathering mechanism. Palaeontology 17 (4): 729-744.

Mawson, R., & J. A. Talent. 2003. Conodont faunas from sequences on or marginal to the Anakie Inlier (Central Queensland, Australia) in relation to Devonian transgressions. Bulletin of Geosciences 78 (4): 335-358.

Purnell, M. A., & P. C. J. Donoghue. 1997. Architecture and functional morphology of the skeletal apparatus of ozarkodinid conodonts. Philosophical Transactions of the Royal Society of London B 352: 1545-1564.

Sweet, W. C., & P. C. J. Donoghue. 2001. Conodonts: past, present, future. Journal of Paleontology 75 (6): 1174-1184.

Weddige, K. 2005. Contra Ruth Mawson’s critizising Bardashev, Weddige & Ziegler 2002, e.g. in SDS Newsletters 20 (2004). Subcommission on Devonian Stratigraphy Newsletter 21: 51-52.

Some History of the History of Tetrapods

Titanophoneus potens, a Permian synapsid (image from Kheper).

Benton, M. J. (ed.) 1988. The Phylogeny and Classification of the Tetrapods. The Systematics Association Special Volume 35A & 35B. Clarendon Press: Oxford.

One interesting thing about comparing different fields of research is the different time-scales we work in when it comes to what constitutes a "recent" publication. As an invertebrate taxonomist, I think nothing of delving into stuff that was written in the 1950s or even earlier. A developmental geneticist is likely to regard anything more than a few years old as ancient history. Vertebrate palaeontology lies between these two extremes, but certainly 1988 was a long time ago for the tetrapods.

As a result, I suspect that Phylogeny and Classification of the Tetrapods can't really tell us much about the current state of tetrapod classification. What does make it interesting, though, is what it says about the state of vertebrate palaeontology at the time. The late 1980s were certainly interesting times, not just in vertebrate palaeontology but in systematics in general. The cladistic revolution was gathering speed. Molecular phylogeny was making its first faltering steps, and challenging a few orthodoxies.

The Phylogeny and Classification of the Tetrapods was published in two volumes, and even that says something about changes in focus since. The second volume was devoted entirely to the mammals (about 5,400 living species). Everything else - amphibians, reptiles, birds, about 24,000 living species - took up only the first volume. Birds in particular warrant a single chapter, as do living amphibians (that latter point possibly hasn't changed much). Dinosaurs (the non-birdy type, that is) barely rate a mention. The dinosaur renaissance was in its early stages at the time - for comparison, Bakker's The Dinosaur Heresies, a book I personally don't think much of but which became something of a focal point for changing views on the big lizards, was first published in 1986. (It was also in 1988 that Gregory S. Paul's Predatory Dinosaurs of the World first hit the shelves, in which Paul copped a certain degree of ridicule for his decidedly heterodox reconstructions of dinosaurs covered in feathers - Paul has since, of course, been able to carry around a big bag of harsh words and force his critics on this point to eat them.) Of course, it should be noted that despite its palaeontological bent, ultimately the main focus of Phylogeny and Classification of the Tetrapods is on the relationships between living tetrapods.

Despite all that has changed since then, some parts of Phylogeny and Classification of the Tetrapods seem somewhat prescient. Not so much the molecular chapters - that on molecular phylogenetics of tetrapods as a whole has the grim figure of the Haematothermia clade (birds and mammals to the exclusion of reptiles) rear its ugly head, though the authors at least had the sense to recognise this as probably convergence rather than the actual state of affairs. But the mammalian molecular phylogeny chapter gives us some of the early glimmerings of the Afrotheria hypothesis, though that clade was not to be formally recognised until some years later, while the Novacek et al. chapter on the morphological phylogeny of modern mammalian orders is noteworthy for not finding any support for Ungulata.

The prize for best statement in the book, however, has to go to Gaffney & Meylan's chapter on turtles, where, after a description of the apomorphies connecting turtles to their supposed nearest relatives (the captorhinids, in this case), the authors note "And so we reach God's noblest creature - the turtle".

Scleritome Week: Not just an invert thing

Unfortunately, I can't touch the chancelloriids until tomorrow, but they will be here, I promise.

So far, all the animals I've shown you in relation to Scleritome Week (see here, here and here) have all been definitely in the class of organisms dismissed by the sadly vertebrate-centric as "creepy-crawlies". Nevertheless, the disarticulated scleritome issue is not unique to invertebrates.

The figure at the top of the post (from Valiukevičius & Burrow, 2005) shows scales of Silurian fish of the family Tchunacanthidae. This family was originally described by Karatajute-Talimaa & Smith (2003) as a new order, distinct from all others previously described (while Valiukevičius & Burrow seem a little sceptical of such a high ranking, they do still maintain the family's distinctiveness). The interesting thing for this post is that, so far, tchunacanthids are known only from scales.



Tchunacanthidae are member of the Acanthodii, an extinct class of vertebrates found from the Silurian to the Permian (a couple of examples are shown above in an illustration from here). Acanthodians are sometimes referred as "spiny sharks", a name that probably survives more because it sounds neat than because of its appropriateness for the actual animals. While generally regarded as more closely related to modern bony fishes and tetrapods than actual sharks, acanthodians resembled sharks in having a cartilaginous rather than a bony skeleton. As a result, acanthodian skeletons were rarely fossilised, and usually only the hard mineralised parts survived - teeth, scales and spines. Without the soft tissue holding them together, however, the fossils became disarticulated, just like the sclerites of a scleritome animal. Tchunacanthids are far from being the only family of non-bony fish known only from scattered pieces of armation - articulated specimens (except in those taxa that developed large bony plates) are the exception rather than the rule. Beyond the general features shared by all acanthodians, we are doomed to ignorance about what a living tchunacanthid looked like unless some day we are lucky enough to find one of those rare articulated fossils.

REFERENCES

Karatajute-Talimaa, V., & M. M. Smith. 2003. Early acanthodians from the Lower Silurian of Asia. Transactions of the Royal Society of Edinburgh: Earth Sciences 93: 277-299.

Valiukevičius, J., & J. C. Burrow. 2005. Diversity of tissues in acanthodians with Nostolepis−type histological structure. Acta Palaeontologica Polonica 50 (3): 635-649.