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Trimerorhachid temnospondyls: numerous scale layers and… gill-pouch brooding?

The views expressed are those of the author and are not necessarily those of Scientific American.


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Trimerorhachis insignis, best known of the trimerorhachids (named by Cope in 1878). Image by Ghedoghedo, licensed under Creative Commons Attribution-Share Alike 3.0 Unported license.

Continuing with the theme of the previous article on trematosauroid temnospondyls, I thought I may as well publish another randomly chosen chunk of my grand, super-long temnospondyl review. This time we look at the trimerorhachids. While there are diverse and often conflicting opinions on the phylogenetic affinities of the many temnospondyl lineages, it’s generally agreed that trimerorhachids are part of Dvinosauroidea (a clade easily recognized by their inflated, enlarged proximal end of the humerus, vaulted palate and other characters); in turn, dvinosauroids are generally (but not universally: see Ruta et al. 2007) agreed to be part of Limnarchia (Yates & Warren 2000, McHugh 2012), the great temnospondyl clade, best known for its Permian and Mesozoic representatives, that – as a generalization – can be imagined as including the mostly aquatic and amphibious forms with poorly ossified limbs.

Life restoration of the trimerorhachid Neldasaurus wrightae, by Smokeybjb, licensed under Creative Commons Attribution-Share Alike 3.0 Unported license.

A more inclusive group recognized by some authors for dvinosauroids and supposed relatives – termed Dvinosauria by Yates & Warren (2000) – was argued to be polyphyletic by McHugh (2012).

Anyway, trimerorhachids were dvinosauroids of the Carboniferous and Early Permian of North America, characterized by a distinctive palatal anatomy and an unusual vertical ilium where the dorsal part of the bone is strongly flared in the parasagittal plane. The best known member of the group is Trimerorhachis (of which two species are currently recognised; T. insignis and T. sandovalensis); other taxa include Lafonius, Nannospondylus, Doragnathus and Neldasaurus [photo of Trimerorhachis above by Ghegoghedo; swimming Neldasaurus image by Smokybjb]. Reaching about 1 m in total length, they were blunt-snouted and with closely set, dorsally facing orbits and a long skull table which, at the level of the occiput, was broader than the snout. Small and very small teeth line the jaw edges while several sets of substantially larger fangs are located around the edges of the palate; a row of small, conical teeth also decorate the edge of the palate in between the  interpterygoid vacuities (remember, a key peculiarity of temnospondyls) and the lateral edges of the upper jaw. Branchial elements preserved in-situ at the back of the skull show that trimerorhachids were perennibranchiate [UPDATE: see comments below for clarification on this], just like dvinosaurids, and thus certainly fully aquatic. Their skulls are also covered by lateral line sulci.

Overlapping body scales of Trimerorhachis, as figured by Colbert (1955). Note the concentric growth lines, and also that the scales actually have two layers: a ventral one with the concentric lines, and a dorsal one with parallel, slightly wavy ridges that would have formed a corrugated external texture.

Trimerorhachid bodies were long and their limbs small, and articulated specimens show that they were covered in multiple overlapping layers (between 3 and 20, and perhaps more) of small scales (Colbert 1955, Chase 1965, Olson 1979). Olson noted that these scales would have added considerably to the weight of the animal, and he estimated that, in a specimen weighing 5 kg, the scales alone would have added another 0.8-0.85 kg. Presumably this increased weight would have made these animals more negatively buoyant than they already were, and thus perhaps it was an adaptation to assist the animal in staying under water. A defensive function was also considered for the scales, and Olson (1979) also noted that it seems unlikely that the scales would have made the animal stiff-bodied.

Colbert's (1955) photos of body scalation in Trimerorhachis. Fishy-looking, no?

One particularly bizarre discovery makes trimerorhachids really interesting: one specimen of Trimerorhachis, today held at the University of California, preserves a cluster of tiny Trimerorhachis bones in the same approximate area as its gills. The bones seem to belong to baby animals that would have been 12-15 cm long.

Olson (1979) considered two possible explanations for their presence; (1) that they were cannibalized, and had fortuitously found their way into the gill region from the mouth, throat or elsewhere; or (2) that Trimerorhachis was a pharyngeal brooder – an animal that held its babies inside special gill pouches. Regarding the latter idea, Olson had been inspired by the internal brooding of fry practiced by some fishes, and by the retention of some frog larvae within dermal pouches and throat sacs. It’s a fascinating, speculative area, and to my knowledge no further information has been published on it. As is so often the case, we need more and better fossils before we can say any more!

Sooo many other temnospondyl groups to cover, I might carry on like this for a while. For previous Tet Zoo articles on temnospondyls, see…

Refs – -

Chase, J. N. 1965. Neldasaurus wrightae, a new rhachitomous labyrinthodont from the Texas Lower Permian. Bulletin of the Museum of Comparative Zoology 133, 153-225.

Colbert, E. H. 1955. Scales in the Permian amphibian Trimerorhachis. American Museum Novitates 1740, 1-17.

McHugh, J. B. 2012. Temnospondyl ontogeny and phylogeny, a window into terrestrial ecosystems during the Permian-Triassic mass extinction. University of Iowa, dissertation (available at http://ir.uiowa.edu/etd/2942).

Olson, E. C. 1979. Aspects of the biology of Trimerorhachis (Amphibia, Temnospondyli). Journal of Paleontology 53, 1-17.

Ruta, M., Pisani, D., Lloyd, G. T. & Benton, M. J. 2007. A supertree of Temnospondyli: cladogenetic patterns in the most species-rich group of early tetrapods. Proceedings of the Royal Society of London B 274, 3087-3095.

Yates, A. M. & Warren, A. A. 2000. The phylogeny of the “higher” temnospondyls (Vertebrata: Choanata) and its implications for the monophyly and origins of the Stereospondyli. Zoological Journal of the Linnean Society 128, 77-121.

Darren Naish About the Author: Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at tetzoo.com! Follow on Twitter @TetZoo.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. Andreas Johansson 12:49 pm 04/13/2013

    Neat. There can never be too many TetZoo articles on obscure extinct critters.

    Link to this
  2. 2. Heteromeles 1:40 pm 04/13/2013

    I’m trying to get my head around how you have 20 overlapping layers of scales without it looking like a pine cone. I’m also trying to get my head around concentric growth rings. In plants, this works because the vascular cambium is on the outside, laying down new rings as the girth expands on a trunk. In animals, the rapidly dividing embryonic cells producing the scales are in the back, right? I’ll admit I don’t get how those rings are produced.

    Weird.

    Link to this
  3. 3. naishd 5:17 pm 04/13/2013

    Thanks for comments. Haven’t dealt with those at the trematosauroid article yet (nor even the newest Squamozoic ones), too little time.

    Could numerous overlapping scale layers be more feasible if the scales were super-thin; like, just a few fractions of a mm each? As for concentric growth rings on a scale, this isn’t unusual – it’s normal for fish at least.

    Darren

    Link to this
  4. 4. David Marjanović 5:55 pm 04/13/2013

    Have you seen Schoch’s (2013) temnospondyl phylogeny yet? It’s quite different from McHugh’s.

    Doragnathus as a trimerorhachid is an… intriguing concept. In my work based on the matrix of Ruta & Coates (2007), I find it in a totally different place, most importantly not as a temnospondyl.

    A third species of Trimerorhachis is going to be published at some point; I’ve seen the future holotype skull and the label with the future name.

    perennibranchiate

    I don’t like that word in this context. It implies that the larval external gills persist “through the year” (per annum). Witzmann & Schoch (2010) instead think that Trimerorhachis had internal gills as an adult, because the ceratobranchials bear grooves for arteries that don’t occur together with external gills (and because T. has a postbranchial lamina on the clavicle). External gills are preserved in larvae of Isodectes; a soft gill lid may be preserved in an older specimen, according to the same paper.

    I’m trying to get my head around how you have 20 overlapping layers of scales without it looking like a pine cone.

    I wonder how much of that is taphonomic (the scales just got pushed together). But nobody has published on that since 1955. Scales are much neglected in general.

    I’m also trying to get my head around concentric growth rings.

    The center isn’t the center of the body, it’s the center of the scale in question. Scales are bone plates that grow at their margins.

    In animals, the rapidly dividing embryonic cells producing the scales are in the back, right?

    They’re in the dermis of the entire body except the head, and not in the embryo (scales appear later).

    Link to this
  5. 5. naishd 6:09 pm 04/13/2013

    Thanks, David. Nope, haven’t seen Schoch (2013) – which paper is this, please? The temnospondyl text I’m using was written a few years ago and, while I tried to update the section used here, I did it in a hurry. I should have been more cautious about Doragnathus: I think I copied the idea of it being a trimerorhachid from wikipedia or some such source.

    I also knew that I should have been more cautious about use of ‘perennibranchiate’: you are correct. So, internal gills in adults and probable gill lids. I’m going to modify the article to bring attention to this.

    More temnos here real soon!

    Darren

    Link to this
  6. 6. Heteromeles 7:01 pm 04/13/2013

    Thanks David. I should have defined that I was thinking of cell division on the scale in particular, not the body in general. I thought that scales grew like human fingernails, from the base, not the outer edge. Weird that scales grow at the outer edge without getting damaged (rapidly dividing cells tend to be fairly fragile, AFAIK), but if that’s the way it works, that’s the way it works.

    As for staking scales, take a bunch of punched out circles from a three-hole punch, and try gluing them so that each one overlaps 20 others, with their basal edges on another piece of paper. That’s what has me puzzled. If that’s too much work, do it with post-its.

    Link to this
  7. 7. AlHazen 8:58 pm 04/13/2013

    As a member of a clade that, for … I’d guess over a hundred million generations… has had teeth only along the margins of the jaws, I find the places more basal tetrapods (and non-tetrapod osteichthyans) have teeth a bit weird. But suppose if you are trying to catch and hold onto slippery prey under water, the more teeth the merrier!

    (David– “perenni-” doesn’t necessarily mean “through the year”: it can also mean “through multiple years,” as when gardeners distinguish “perennials” from annual plants. So the use of “perennibranchiate” to mean “keeping gils through its whole life” doesn’t bother me. But you make a useful distinction: if someone were backward enough to think that basal tetrapods were “amphibians” in the same sense that salamanders are, they might be misled to think of temnospondyls as having axolotl-like “neck feathers,” and this would be a mistake. One more reason, I suppose, to avoid the word “amphibian”!)

    Link to this
  8. 8. Therizinosaurus 11:58 pm 04/13/2013

    I gotta say that 20 overlapping scales confuses me too, unless they were very long so that we’re only seeing the tips. In which case that would be one crazy-looking creature.

    Very cool about the possible mooth-brooding. I had no idea such a thing was preserved in fossils.

    Link to this
  9. 9. AlHazen 1:07 am 04/14/2013

    Have just re-read “Temnospondylsthe early years, I” and had a thought. Most of the known members of one clade supposedly lacked lateral line stuff, but the biggest did have it. How, developmentally, does the “invasion” of the dermal bones of the skull by the lateral line organ work? Is it possible that
    —early in ontogeny the lateral line stuff is entirely in the soft tissue overlying the bone
    —later in ontogeny it starts digging canals and tunnels to house itself in the bone surface
    —”later” for this clade became quite late, so that most fossil skulls show no sign of l.l.
    —visible lateral line markings on the skull only show up in old individuals of a particularly long-lived, and big, species?

    More generally: presence of lateral line organ can be inferred from appropriate canals or tunnels in the skull surface, but, to coin a phrase, absence of evidence is not evidence of absence. Or is there some way of being confident that a paleozoic thing-we-once-would-call-amphibian really lacked lateral line organs?

    Link to this
  10. 10. RaptorX 1:09 am 04/14/2013

    Awesome! Indeed this ties in with what I was doing, as I just finished reading Mallon and Holmes’ re-description of CMN 8547, which suggested a semi-aquatic lifestyle might be implied by the apparent robustness of the limb bones.

    I’ve heard about these trimerorhachid’s scaly limbs before, but until now I hadn’t learned just how thick they were. With Heteromeles’ comment, I’m also unable to get the image of a poor Axolotl with pinecones strapped to its feet to leave my mind. I’m guessing we can imply by the sheer weight of their limbs that these were predators of river bottoms?

    Gill-brooding is also interesting to hear about. Didn’t know that could get fossilized, but then again, neither did I think feather colors could either!

    Link to this
  11. 11. David Marjanović 8:49 am 04/14/2013

    Oops. “Witzmann & Schoch 2010″ is actually Schoch & Witzmann:

    Schoch, R. R. & Witzmann, F. 2010. Bystrow’s Paradox – gills, fossils, and the fish-to-tetrapod transition. Acta Zoologica (Stockholm). 15 pp. I’m too lazy to look up the volume & page numbers now, the pdf I have is from the online-early edition.

    And Schoch 2013 is:

    Schoch, R. R. 2013. The evolution of major temnospondyl clades: an inclusive phylogenetic analysis. Journal of Systematic Palaeontology online-early (“iFirst”), 33 pp.

    The only outgroup is Proterogyrinus. Similarly, McHugh’s only outgroup is Colosteidae. I think that’s not enough – and once you have enough outgroups, you have a pretty complete tetrapod analysis like Ruta & Coates 2007.

    I thought that scales grew like human fingernails, from the base, not the outer edge. Weird that scales grow at the outer edge without getting damaged (rapidly dividing cells tend to be fairly fragile, AFAIK), but if that’s the way it works, that’s the way it works.

    Ah, now I see where the confusion comes from: two very different things are called “scales” in vertebrates.

    Epidermal scales found in amniotes are thickened, keratinized folds/outgrowths of the epidermis.

    Dermal scales, found in “fish”, in plenty of Paleozoic tetrapods and (poorly mineralized) in some extant caecilians, are bone plates in the dermis, under the epidermis.* They’re not at all homologous to epidermal scales. They don’t have a base, they’re circular or rhombic and grow all around the edge. Scale growth isn’t particularly fast either; the scales just grow with the rest of the animal.

    Of course, I guess, you can have several layers of scales in the dermis. 20 such layers aren’t a problem in theory at least.

    * Unless the bone layer is overlain by a dentine layer and an enamel-or-similar layer that erupts just like a tooth. Indeed, that’s where teeth come from.

    So the use of “perennibranchiate” to mean “keeping gils through its whole life” doesn’t bother me.

    But they didn’t keep their external gills. They lost them and grew internal gills instead. External gills are distal outgrowths of the walls that separate the gill slits in which the internal gills (later) lie.

    Internal gills don’t occur in any extant tetrapod, unless the Tet Zoo article on Amphiuma is right. Even the gills of tadpoles, covered by a lid, are homologous to external gills (Schoch & Witzmann 2010).

    How, developmentally, does the “invasion” of the dermal bones of the skull by the lateral line organ work?

    The dermis on top of the skull gradually turns into bone. The farther outward the bone grows, and the thinner the dermis was in the first place, the deeper the lateral-line canals are.

    presence of lateral line organ can be inferred from appropriate canals or tunnels in the skull surface, but, to coin a phrase, absence of evidence is not evidence of absence.

    Correct. In lissamphibians, the lateral-line organ is present in all aquatic life stages but never leaves traces on the skull – with the sole exception of the Late Jurassic crown-group salamander Beiyanerpeton.

    Incidentally, the postcranial lateral line runs in a canal in the ventralmost row of dorsal scales (dorsal and ventral scales often have different shapes).

    Link to this
  12. 12. naishd 9:00 am 04/14/2013

    Thanks for all the great comments and discussion, very useful. Just want to note that the “20 overlapping scales” thing comes from Olson 1979: I need to dig out my copy to check and I don’t have time to do that at the moment. It’s wholly possible that I misunderstood or made a mistake. I’ll post the answer here eventually.

    Darren

    Link to this
  13. 13. Heteromeles 10:12 am 04/14/2013

    Thanks David. That clarifies it quite a bit. I now see where 20 layers of dermal scales could come from. Why they’d do it this way, I have no idea. It seems like a vascular nightmare, for one thing, getting capillaries to all those scales, but at least it’s more structurally feasible.

    Link to this
  14. 14. Andreas Johansson 10:38 am 04/14/2013

    Where do crocodilian etc osteoderms come into the picture? Given the distribution, a bunch of independent origins within Amniota?

    Link to this
  15. 15. AlHazen 5:57 pm 04/14/2013

    Thanks, David, for the details on lateral lines.
    (And, as for not being bothered by “perennibranchiate” for keeping INTERNAL gills– nothing in the word specifies externality! Perhaps we should say that some temnospondyls were perennibranchiate but that axolotls are perenni-ectobranchiate? (Grin!)) (Anyway, “perenni” is Latin and “branchia” is Greek, so the word is a barbarism!)

    Link to this
  16. 16. Heteromeles 6:28 pm 04/14/2013

    So you want perenniendoramate and perenniectoramate gills? Sure, why not?

    Link to this
  17. 17. ectodysplasin 11:26 pm 04/14/2013

    Olson (1979) considered two possible explanations for their presence; (1) that they were cannibalized, and had fortuitously found their way into the gill region from the mouth, throat or elsewhere; or (2) that Trimerorhachis was a pharyngeal brooder – an animal that held its babies inside special gill pouches.

    Worth pointing out that these specimens are basically packed like sardines in these sediments, which typically represent dried-out ephemeral wetland systems. So you’re going to have the same sort of problem in with the Trimerorhachis assemblages as you have in the Coelophysis bauri bonebeds, where you have lots of overlay of specimens of different size classes, and it’s not always safe to assume that individuals of one size class necessarily were brooding or eating individuals of another just because one fossil is crammed up against another.

    Link to this
  18. 18. ectodysplasin 11:34 pm 04/14/2013

    Also I’m surprised that David didn’t make mention of Erpetosaurus radiatus, which is a bizarre trimerorhachoid from Linton, Ohio, and was recently redescribed by Andrew Milner and Sandra Sequiera (2011). E. radiatus is a bizarre little animal with very anteriorly-displaced orbits and huge anterior teeth, which it probably used for snagging fish.

    The citation is here:

    Milner, A. R., & Sequeira, S. E. K. (2011). The amphibian Erpetosaurus radiatus (Temnospondyli, Dvinosauria) from the Middle Pennsylvanian of Linton, Ohio: morphology and systematic position. Special Papers in Palaeontology 86, 57-73.

    Link to this
  19. 19. ectodysplasin 11:51 pm 04/14/2013

    Also vis a vis the discussion of the placement of trimerorhachids amongst the Temnospondyli, temnospondyl phylogeny is largely still a disaster. Schoch (2013) finds a topology similar to that of Yates & Warren (2000), but there are aspects of the Julia McHugh’s phylogeny that are really very compelling at well. Some of the problems may be associated with the nature of skull roof patterning and plasticity in general, and others may be due to gaps in our understanding of temnospondyl morphology. Braincase data really is turning out to be the most immediately informative morphological bin we have for these early tetrapods, and the braincase of temnospondyls is poorly known for a variety of reasons, not limited to the facts that most temnospondyls are flattened (e.g. most ‘branchiosaurs’) or are represented by solid three-dimensional skulls where it’s difficult to justify destructive sampling to expose braincase (e.g. most other temnospondyls).

    At least temnospondyls are probably still a natural group, unlike some of the other Paleozoic tetrapod clades we’re still muddling through.

    Link to this
  20. 20. ectodysplasin 1:46 am 04/15/2013

    @AlHazen;

    Most of the known members of one clade supposedly lacked lateral line stuff, but the biggest did have it. How, developmentally, does the “invasion” of the dermal bones of the skull by the lateral line organ work? Is it possible that
    —early in ontogeny the lateral line stuff is entirely in the soft tissue overlying the bone
    —later in ontogeny it starts digging canals and tunnels to house itself in the bone surface
    —”later” for this clade became quite late, so that most fossil skulls show no sign of l.l.
    —visible lateral line markings on the skull only show up in old individuals of a particularly long-lived, and big, species?

    More generally: presence of lateral line organ can be inferred from appropriate canals or tunnels in the skull surface, but, to coin a phrase, absence of evidence is not evidence of absence. Or is there some way of being confident that a paleozoic thing-we-once-would-call-amphibian really lacked lateral line organs?

    For starters, depending on your phylogenetic analysis, it would probably be safe to say that temnospondyls belong to the amphibian total group (David will disagree with me on this, of course).

    As for the ontogeny of the lateral line system, it’s important to take into account both phylogeny and ontogeny. The earliest tetrapods had lateral lines that were enclosed within the bone and communicated with the surface via a series of pores. This is incidentally also seen in a variety of fishes, so I’ll talk specifically about ontogeny of the structure in fishes and then make some comparisons with what you see in modern amphibians and what we can probably infer about the ontogeny of the structure in temnospondyls and other early tetrapods.

    Okay.

    Lateral line canals are placodal structures (much like teeth and many other dermal structures). First, the outer cell layer (epithelium) thickens into a placode, then it’s innervated. Then the placode (which starts out as a small essentially oval to round thickening) starts to proliferate along specific trajectories to create those radiating networks of lines. Then the epithelium and the inner tissue layer (mesenchyme) invaginate to create an enclosed trough, which then closes up except for pores that communicate with the surface. So you have this tube of innervated epithelium and it’s more or less enclosed in mesenchyme. Then, afterwards, the mesenchyme will ossify intramembraneously, enclosing the lateral line system.

    I’m going through this piece by piece because there are several important features of ontogeny here:

    1. The lateral line forms before the dermal bones do, and resides within the tissue that the dermal bone develops out of. So even at relatively early stages of ossification, the lateral line canal is enclosed within the bone.

    2. The lateral lines grow from points and along specific trajectories. If you don’t form one of those placodes, you’ll never develop the lateral line canals that would have developed from it. If the lateral line canal stops growing at a certain point, you will lose certain late-forming canals. So the presence or absence of certain canals seems to follow pretty conservative patterns of loss.

    Now, tetrapods start reducing a number of features of their lateral line pretty quickly. This includes several lateral line canals and at least one placode (this is probably related both to the evolution of the cervical region as well as general paedomorphosis in the tetrapod lineage). They also seem to be situating their lateral line canals closer to the dermal surface, which means that there’s less mesenchyme available to enclose the canals. As a result, instead of being enclosed within the bone and communicating through a series of pores that pass through the bone, the lateral line canal rests within a sulcus on the surface of the bone.

    So when we look at early crown tetrapods, we see these networks of lateral line sulci around which the normal dermal ornament has formed. The extent of these sulci depends on the species and where it falls out in the phylogeny; eobrachyopids like Isodectes only have circumorbital lateral line sulci and part of the postorbital canal, for example, whereas trimerorhachids have a much more comprehensive distribution of lateral line sulci.

    Now, things get a little dicey as we get into modern amphibians, because modern amphibians generally only have lateral lines as larvae/neotenes (exceptions: cryptobranchids and Amphiuma). The thing is, though, the dermal skeleton is not exceptionally ossified in larval amphibians (there’s essentially no ossification of the dermal skull until metamorphosis in frogs, and dermal ossifications are very weak in larval salamanders). Full dermal ossification does not occur until after metamorphosis in these taxa, and even then, it doesn’t always occur at all (really well-ossified dermis is found in a handful of salamanders (mostly salamandrids, as David can attest) and a small diversity of frogs (generally leptodactylids, but also some assorted tree frogs, such as Osteopilus). Now, it’s important to understand that the dermis is ossifying in these species at/after metamorphosis, and that the lateral line canal is resorbed at metamorphosis as well. So in the species that do have extensive dermal sculpturing, there’s no trace of the lateral line canal in that sculpturing because there was no lateral line there when it ossified.

    So if we take this back to temnospondyls, essentially all temnospondyls (and this includes diminuative temnospondyls like amphibamids and ‘branchiosaurs’) have dermal sculpturing indicating pretty complete ossification of the dermis. So if the lateral lines were there, you’ll have canals. If they weren’t, you won’t. So, adult dissorophoids (except neotenic ‘branchiosaurs’) all lack lateral line canals in the skull, and that’s probably because dermal ossification didn’t really complete until after metamorphosis, at which point the lateral line canals had already been completely lost ontogenetically.

    Incidentally, lungfishes independently reduce their dermal ossifications to the extent that the lateral lines are no longer enclosed within the bone and lose a couple lateral line canals as well.

    Link to this
  21. 21. CS Shelton 3:52 am 04/15/2013

    OK, it’s amphibians so vaguely related. I know you said you wouldn’t get to rain frogs and time soon, but I just thought I’d let the frog speak for itself. http://www.youtube.com/watch?v=cBkWhkAZ9ds

    Link to this
  22. 22. David Marjanović 10:28 am 04/15/2013

    Just want to note that the “20 overlapping scales” thing comes from Olson 1979:

    Oh, so I misremembered the source. It does say that several layers of scales lie on top of each other.

    It seems like a vascular nightmare

    I’m so stealing this.

    Where do crocodilian etc osteoderms come into the picture? Given the distribution, a bunch of independent origins within Amniota?

    Yes. The dermis retained the ability to form bone, but the dorsal scales with all their details were lost in the ancestry of Amniota + Diadectomorpha. Ventral scales survive as gastralia – and are covered by new osteoderms in cyamodontoid placodonts as well as in plagiosaurid temnospondyls.

    And, as for not being bothered by “perennibranchiate” for keeping INTERNAL gills– nothing in the word specifies externality!

    I’m talking about the “keeping” part. Internal gills only form around the time the external ones are lost.

    Worth pointing out that these specimens are basically packed like sardines in these sediments, which typically represent dried-out ephemeral wetland systems.

    Very good point.

    I didn’t mention Erpetosaurus because I didn’t see an opportunity. It’s fascinating for sure, and shows some convergence to colosteids, but nobody tried to list all dvinosaurs or supplied any other context where I’d have mentioned it.

    At least temnospondyls are probably still a natural group, unlike some of the other Paleozoic tetrapod clades we’re still muddling through.

    Terminology: if it’s not a natural (monophyletic) group, it’s not a clade. Clade = monophylum = an ancestor + all its descendants.

    Then the epithelium and the inner tissue layer (mesenchyme) invaginate to create an enclosed trough, which then closes up except for pores that communicate with the surface. So you have this tube of innervated epithelium and it’s more or less enclosed in mesenchyme. Then, afterwards, the mesenchyme will ossify intramembraneously, enclosing the lateral line system.

    Oh. I had no idea. My explanation was all wrong, then.

    So the presence or absence of certain canals seems to follow pretty conservative patterns of loss.

    Could you supply details? This smells like an ordered character that I’d want to use at some future point.

    modern amphibians generally only have lateral lines as larvae/neotenes (exceptions: cryptobranchids and Amphiuma)

    I thought pipids retain them, too? Anyway, there’s the remarkable example of the salamandrid Notophthalmus viridescens: the aquatic larva of course has a functional lateral-line organ; at metamorphosis, which produces a red terrestrial stage (the “red eft”), the organ sinks deep into the skin; at sexual maturity, the animal turns green and becomes aquatic again, and the lateral-line organ comes back out and resumes its function.

    BTW, what do you think of the mysterious tunnels through the quadratojugals of Eryops and Edops?

    Link to this
  23. 23. ectodysplasin 11:30 am 04/15/2013

    @David,

    BTW, what do you think of the mysterious tunnels through the quadratojugals of Eryops and Edops?

    Is this the tract that exits posteriorly through the paraquadrate foramen? If so, my guess is mandibular branch of the trigeminal.

    As for Notophthalmus, do the lateral line canals actually sink into crypts in the bone, or are they resorbed at metamorphosis, and then redeveloped from epithelial placodes upon reaching adulthood? I feel like the latter is more likely, but that’s something that would need to be studied in more detail.

    There’s probably health science money (and a lot of it) in that, actually, given that lateral line canals are homologous to semicircular canals in the ear, and regeneration of damage to the semicircular canals could help people with some forms of chronic vertigo. If it could be demonstrated that the LLCs are actually regenerating at the placode level, of course.

    As far as lateral line proliferation and growth, the classic paper is Holmgren & Pehrson (1949). Northcutt has done a lot of the morphogenesis and tissue-level work in the past 20 years.

    Citation is:

    Holmgren, N., & Pehrson, T. (1949). Some remarks on the ontogenetical development of the sensory lines on the cheek in fishes and amphibians. Acta Zoologica, 30(1‐2), 249-314.

    Link to this
  24. 24. AlHazen 1:07 am 04/16/2013

    David–
    O.k., maybe teasing you with linguistic trivia serves a purpose after all! It made you say
    “I’m talking about the “keeping” part. Internal gills only form around the time the external ones are lost.”

    Which finally got through to me: NOW I finally see the biological point that makes you think the p-word is inappropriate for these creatures.

    Link to this
  25. 25. AlHazen 1:21 am 04/16/2013

    Ectodysplasin–
    So, if I’ve understood you. The way LL developes is such that, if the animal (unlike typical Lissamphibians) ossifies its outer skull (= developes the intramembranal ossifications that we call the dermal bones of the skull) while retaining the LL, the LL will more or less inevitably show up in the form of channels/tunnels in the bone. Trying to pursue my idea: the mesenchymal layer that the dermal bone develops in has a finite thickness. Could some taxa innovate so that the dermal bone forms only in the deeper part of this layer, leaving the shallower part (containing the LL) unossified? Or does the way dermal ossification occurs make this (formation of bone only in one part of the mesenchymal layer) very unlikely?

    (And does anyone have a collection of Notophthalmus skulls representing different stages in its complex life history? It’s a small enough animal that you might need SEM to see details, but it would be interesting to know if the Red Eft — I’m assuming it has about as thoroughly ossified a skull as you are likely to get in a salamander, since it is a terrestrial animal — shows anything like canals/tunnels for LL?)

    Link to this
  26. 26. ectodysplasin 3:30 am 04/16/2013

    @AlHazen;

    So, if I’ve understood you. The way LL developes is such that, if the animal (unlike typical Lissamphibians) ossifies its outer skull (= developes the intramembranal ossifications that we call the dermal bones of the skull) while retaining the LL, the LL will more or less inevitably show up in the form of channels/tunnels in the bone. Trying to pursue my idea: the mesenchymal layer that the dermal bone develops in has a finite thickness. Could some taxa innovate so that the dermal bone forms only in the deeper part of this layer, leaving the shallower part (containing the LL) unossified? Or does the way dermal ossification occurs make this (formation of bone only in one part of the mesenchymal layer) very unlikely?

    Yes. This is precisely what happens in modern lissamphibians that retain the lateral line canals cranially as well as modern lungfishes. This is outside of the realm of possibility for Paleozoic temnospondyls, however, because the ossifications within the dermis are extensive and probably encompassed much/most of the dermis.

    (And does anyone have a collection of Notophthalmus skulls representing different stages in its complex life history? It’s a small enough animal that you might need SEM to see details, but it would be interesting to know if the Red Eft — I’m assuming it has about as thoroughly ossified a skull as you are likely to get in a salamander, since it is a terrestrial animal — shows anything like canals/tunnels for LL?)

    If not, it would be easy enough to assemble a collection of N. viridescens, because they are extremely common salamanders throughout much of eastern North America. It strikes me that it’d be easier to look at this with micro-CT than with SEM; you don’t need the sub-micron scale of resolution you’d get from an SEM, and micro-CT would allow for reconstruction of canals within the bone if there are any. I may have some scans around here somewhere, so I can potentially give you a quick and dirty answer as early as tomorrow.

    Link to this
  27. 27. David Marjanović 10:01 am 04/16/2013

    Is this the tract that exits posteriorly through the paraquadrate foramen?

    Probably. I’m talking about the one Anne Warren mentioned in her 2007 Ossinodus paper.

    As for Notophthalmus, do the lateral line canals actually sink into crypts in the bone, or are they resorbed at metamorphosis, and then redeveloped from epithelial placodes upon reaching adulthood?

    Schoch (2001 or 2002) said they sink deeper into the skin, IIRC. Definitely not into the bone.

    Placodes developing from adult epidermis would be something I’d want to see!

    David–
    O.k., maybe teasing you with linguistic trivia serves a purpose after all!

    Of course it does. :-| :-) As long as you don’t understand what I mean, I’ve failed to explain it; when you give up too early, I never find out I’ve failed and merrily keep failing – to the detriment of my readers and listeners, my hypothetical future students, and even myself when grant agencies and potential employers can’t properly appreciate my incomprehensible genius. :-) You’re doing me a favor by not giving up.

    Yes. This is precisely what happens in modern lissamphibians that retain the lateral line canals cranially

    …wait, what? Do you just mean the ones that retain the organ in the head?

    Link to this
  28. 28. ectodysplasin 12:32 pm 04/16/2013

    @David,

    …wait, what? Do you just mean the ones that retain the organ in the head?

    Yes. There are no canals in the skull of cryptobranchids or Amphiuma. The canals are superficial to the bone. Same goes for juvenile salamanders of all sorts.

    I found one reference that was suggesting that the LLC is retained in all adult caudates, but I’m having trouble finding references on that. It’s possible I suppose that the canals simply seal up but are present under the skin, but this would have to be demonstrated. In newts, however, the entire dermis is fully ossified, so any remaining lateral line canal should be present within the bone.

    Link to this
  29. 29. ectodysplasin 12:41 pm 04/16/2013

    Also,

    >Probably. I’m talking about the one Anne Warren mentioned in her 2007 Ossinodus paper.

    Hm.

    In which case it could be a LLC. Which would potentially offer some support for a more basal position of Eryops within the Temnospondyli.

    That’s a character to ponder over.

    Link to this
  30. 30. David Marjanović 1:20 pm 04/17/2013

    I found one reference that was suggesting that the LLC is retained in all adult caudates

    What. That can’t be right (Schoch 2001 and/or 2002 – tell me if you want the refs).

    Link to this
  31. 31. ectodysplasin 7:32 am 04/20/2013

    Well, I can tell from personal experience that it’s not the case!

    Link to this

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