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January 5, 2013
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Nectrideans are a Carboniferous and Permian group of tetrapods, typically regarded as ‘amphibians’ and classified alongside microsaurs and lysorophians within the group Lepospondyli. However, close relationships with the long-bodied aïstopods have been supported by some: indeed, nectrideans have been found to be paraphyletic to aïstopods in some studies (Ruta et al. 2003). Thomson & Bossy (1970) gave the name Holospondyli to a hypothesised nectridean + aïstopod clade. Diplocaulus – the most famous nectridean – is typically portrayed as here. However, fossils show that these ‘horned’ nectrideans actually had skin webs connecting the tips of their ‘horns’ to their bodies.
Next: plesiosaurs!
Refs – -
Ruta, M., Coates, M. I. & Quicke, D. L. J. 2003. Early tetrapod relationships revisited. Biological Reviews 78, 251-345.
Thomson, K. S. & Bossy, K. H. 1970. Adaptive trends and relationships in early Amphibia. Forma et Functio 3, 7–31.
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. He has been blogging at Tetrapod Zoology since 2006.
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interesting on the skin flaps,
how far back the body did they extend and what shape did they had? has anyone looked at their possible functions?
natation assistance? camouflage (by breaking the body shape outline)? or even *gasp* thermoregulation/breathing assistance if they where richly vascularised?
Link to thisCheeky buggers, aren’t they?
Link to this@accipter,
There isn’t really a satisfactory answer to the question of skull shape in diplocaulids. Cruickshank & Skews (1980) argued that the tabular horns may have served as some sort of hydrofoil, but that doesn’t explain the short-horned forms (Peronedon and Ductilodon as well as some skulls attributed to Diplocaulus). Skulls of Diploceraspis, which is the longest-horned form from Western Pennsylvania, Ohio, and West Virginia, show some manner of ornament at the tips of the horns, but what this indicates is also unclear.
I doubt that diplocaulids would have used the horns for thermoregulation, as all these animals are essentially obligately aquatic with gills, lateral line canals, reduced limbs, deep paddle-like tails (this, by the way, is one of the few common characters that unites the Nectridea), and so on.
There’s some manner of new information coming together on some of these animals, but you’ll have to wait for the papers.
Link to this@Darren,
The Nectridean-Aistopod relationship is probably only a persistent feature of some phylogenetic analyses (although not all) due to convergences associated with small size and elongation of the trunk and tail, and due to poor knowledge of key parts of the skull in both groups. In fact, Carroll has argued as recently as 1998 that aistopods are probably not closely related to any other ‘Lepospondyl’ groups, and provided some pretty intriguing evidence in favor of this view, albeit without a cladogram to back it up. There’s some new research on the matter that’s currently forthcoming. Keep an eye out, it’s pretty exciting stuff, if you’re into early tetrapods.
Link to thisWow – great comments, thanks, everyone (smiley).
Skin webs: I should add that these are inferred from resting traces, not preserved on body fossils. They were reported in…
Walter, H. v, & Wernerberg, R. 1988. Über Liegespuren (Cubichnia) aquatischer Tetrapoden (?Diplocauliden, Nectridea) aus den Rotteröder Schichten (Rotliegendes, Thüringer Wald/DDR). Freiberger Forschungsheft 419, 96-106.
Darren
Link to thisDiplocaulid horns: as we said in our recent TREE article on sexual selection, the shape of the allometric slope in these animals is similar to what you see in extant animals that use elaborate structures in sociosexual display (Knell et al. 2013). Such an interpretation would not, we note, be inconsistent with a role in generating lift as well, as proposed by Cruickshank & Skews (1980).
ectodysplasin: thanks for these interesting teasers, we all look forward to seeing the work concerned. Love your work, by the way (some of you may have guessed that ectodysplasin has a special, technical interest in anamniote phylogeny).
Darren
Ref – -
Knell, R. J., Naish, D., Tomkins, J. L. & Hone, D. W. E. 2013. Sexual selection in prehistoric animals: detection and implications. Trends in Ecology and Evolution 28, 38-47.
Link to thisNot all “nectrideans” are diplocaulids or urocordylids! There are also the probably amphibious (rather than aquatic) scincosaurids!
Random puzzling fact: Urocordylus and the diplocaulid Diceratosaurus had five fingers per hand, rather than the four otherwise found in lepospondyls, temnospondyls and lissamphibians.
nectrideans have been found to be paraphyletic to aïstopods in some studies (Ruta et al. 2003)
As well as version 2 of that matrix (Ruta & Coates 2007, J of Syst Pal) and all the unpublished successors – I hope to submit a version for publication this month or next.
The Nectridean-Aistopod relationship is probably only a persistent feature of some phylogenetic analyses (although not all) due to convergences associated with small size and elongation of the trunk and tail
But that’s it: aïstopods have stupidly long tails, unlike any other limbless or limb-reduced tetrapod with the exception of a few climbing snakes that were mentioned here on Tet Zoo not long ago. The only way this makes sense is if they’re descended from very long-tailed limbed animals… urocordylid “nectrideans” have such tails*, and they tend to come out as the aïstopod sister-group (they always do in the matrix series cited above).
* The skeletal reconstruction above actually has a much shorter tail than some specimens I’ve seen. They look like pleurosaurs or the Cretaceous marine squamate Pontosaurus; it’s a sight to behold.
There are plenty of similarities in the unusual vertebrae and in what’s known of the skull, too, though I agree that additional research will help.
Analyses that fail to find Holospondyli tend to find aïstopods and adelospondyls as sister-groups. Now that looks like convergence due to elongation.
Carroll has argued as recently as 1998
If he’s right, he’s right for the wrong reasons. Have you read his 2007 review? Carroll has never understood phylogenetic analysis.
Freiberger Forschungsheft 419
Whoa. The museum I’m working in was in East Berlin, so I might be able to get a hold of that micro-journal without writing to the authors, but no promises.
…Wait. Is “R. Wernerberg” actually Ralf Werneburg? That would make writing to the author a lot easier.
ectodysplasin: thanks for these interesting teasers, we all look forward to seeing the work concerned. Love your work, by the way (some of you may have guessed that ectodysplasin has a special, technical interest in anamniote phylogeny).
Yeah, I think I’ve guessed who that is, even though I’m usually bad at that.
Link to thisSee? Set the bait, and you can lure him in
Darren
Link to thisOff topic, but I have been wondering recently about the phylogeny of alligators. How did one congener end up in China and one in America? I can’t seem to find the correct search terms for this. I wasn’t aware there were large reptiles in Beringia ever (except maybe the Eocene, although not sure if Beringia existed then), or did they cross the Atlantic?
Link to this@Darren,
You probably have guessed the right lab, but you may not be guessing the right grad student. Unless you caught my 2011 talk on, well, diplocaulids, in which case you probably are.
As far as horn use, there are some anatomical concerns that have not, to my knowledge, been stated yet in the literature (so I won’t state them here). I don’t think the Cruickshank & Skews hypothesis really makes a ton of sense; besides the fact that these animals show numerous anatomical features suggesting they were gape-and-suck ambush predators, diplocaulids are almost always at least a little asymmetrical, and are sometimes grotesquely so (see Germain, 2010, for a description of this in Diplocaulus from Morocco) which isn’t exactly conducive for a hydrofoil.
Link to this@David,
Hi David!
Scincosaurids are definitely also worth mentioning, because they’re a good example of the sorts of problems that are involved in nectridean phylogeny. Scincosaurids are unique among nectrideans for a variety of reasons, including a rather exaggerated olecranon process, suggesting that it was probably not only partly terrestrial, but was probably also engaged in some manner of burrowing. Some other features of the skeleton (e.g. the entepicondylar foramen) seem to be unique within nectrideans (albeit commonplace among most stem-amniotes and early amniotes). The palate is also pretty distinct from essentially all other nectrideans (although nectridean palates and occiputs seem to be extraordinarily diverse).
As for aistopods, they do indeed have stupid-long tails, but there’s quite a lot about them that doesn’t make sense no matter where you put them. The Anderson (2001) result, in which aistopods fall out with lysorophians and nectrideans) makes superficial sense, but is a disaster when you actually look at it in detail. The adelospondyl result is equally troublesome. So what it appears like to me is that aistopods are ending up in these places because they have to end up somewhere, and those are the least terrible places to put them, as opposed to reasonable to good places to put them. The new data, which is extensive, disagrees pretty strongly with both of these topologies, and raises a whole mess of new questions. As for Carroll’s interpretations in his 1998 paper, Carroll is certainly not a phylogeneticist, but he’s an excellent anatomist, and he’s right that there are anatomical features of these animals that don’t make a lot of sense.
And yes, you know exactly who I am!
Link to this@Metridia
Alligatoroids (e.g. Allognathosuchus) are found in the high arctic during the Paleocene-Eocene thermal maximum, in places like Ellesmere Island. Other Eocene alligatoroids (e.g. Diplocynodon) seem to have maintained distributions across both Eurasia and North America at higher latitudes.
Molecular clock divergence estimates for Alligator mississippiensis and A. sinensis suggest a divergence of these two genera in the Eocene, so it is not unlikely you’re looking at an arctic dispersal route, either via Beringia or via Greenland and Europe.
Dispersal of Crocodylus seems to have been marine, however.
Link to thisDiplocaulus from Morocco
(I forgot if Damien Germain mentions this in that paper, but D. minimus may actually belong to Diploceraspis instead; Diplocaulus needs a revision.)
Scincosaurids are unique among nectrideans for a variety of reasons, including a rather exaggerated olecranon process, suggesting that it was probably not only partly terrestrial, but was probably also engaged in some manner of burrowing. Some other features of the skeleton (e.g. the entepicondylar foramen) seem to be unique within nectrideans (albeit commonplace among most stem-amniotes and early amniotes).
I agree that the shape of the humerus in general suggests lots of power, so probably some kind of digging. Scincosaurus and Sauravus (which needs to be redescribed) being unique among “nectrideans” may not mean much, though – ever since Ruta et al. (2003), Scincosaurus has come out as the sister-group of all other holospondyls that are in that matrix and its successors (Sauravus isn’t in there).
As for aistopods, they do indeed have stupid-long tails, but there’s quite a lot about them that doesn’t make sense no matter where you put them.
The Anderson (2001) result, in which aistopods fall out with lysorophians and nectrideans) makes superficial sense, but is a disaster when you actually look at it in detail.
Of course, when you look at that matrix in detail, you may find things you don’t like…
Carroll is certainly not a phylogeneticist, but he’s an excellent anatomist, and he’s right that there are anatomical features of these animals that don’t make a lot of sense.
But then, “nothing in biology makes sense except in the light of evolution”, and “nothing in evolution makes sense without a phylogeny”. If you put all the right anatomy into the wrong framework, let alone no framework at all, it won’t help much.
Molecular clock divergence estimates for Alligator mississippiensis and A. sinensis suggest a divergence of these two genera in the Eocene
Huh. I thought it was all Miocene. (The early and middle Miocene were quite warm, much warmer than the Oligocene.)
Dispersal of Crocodylus seems to have been marine, however.
Yes, they seem to have crossed the central Atlantic around the beginning of the Pliocene.
Link to thisectodysplasin: if your initials are JP, then I do know who you are
Loving the great comments, thanks, guys.
Darren
Link to this@Metridia
Alligatoroids (e.g. Allognathosuchus) are found in the high arctic during the Paleocene-Eocene thermal maximum, in places like Ellesmere Island. Other Eocene alligatoroids (e.g. Diplocynodon) seem to have maintained distributions across both Eurasia and North America at higher latitudes.
Molecular clock divergence estimates for Alligator mississippiensis and A. sinensis suggest a divergence of these two genera in the Eocene, so it is not unlikely you’re looking at an arctic dispersal route, either via Beringia or via Greenland and Europe.
Then, I believe rules of priority demand that all “Alligatoroid” fossils be folded into the genus Alligator? After all, if two widely dispersed and deeply diverging members of the genus are still in the genus, then everything in between automatically belongs there too, no?
Link to thisWho did the colour illustration? Gorgeous.
Link to this@David,
Damian definitely suggests similarities between D. minimus and Diploceraspis, but there are a number of characteristics not included in his dataset that would suggest D. minimus is indeed more closely related to classic Diplocaulus from Texas and Oklahoma. There are other populations of cf. Diplocaulus, such as the skulls from the Abo and Bursum, which should be critically reevaluated as well. Nomenclature within the Dilocaulidae is sort of outside my interest in the group, which is focused on anatomy, especially ear, braincase, and palate, and relationship of diplocaulids with other early tetrapods (and whether the small-horned species Diceratosaurus, Keraterpeton, and Batrachiderpeton are actually related to them).
The anatomy of Scincosaurus is problematic no matter where you put it in a clade of nectrideans+aistopods. Once again, we’re talking about “least terrible” rather than “best” phylogenetic position.
As for early tetrapod phylogenies and re: Carroll, phylogenetic analysis is a technology that is extremely important for understanding the evolution of characteristics, but it’s only as good as the dataset being analyzed, and the early tetrapod dataset is, across the board, at a very early stage in the refinement process. Refining the dataset necessarily requires extensive refinement of anatomy and in some cases refining the anatomy requires reference to assumed close relatives (which is, in my opinion, part of the problem we’ve been having with aistopods for years). So, yes, all phylogenetic hypotheses need to be evaluated and reevaluated in a rigorous systematic framework, but this needs to be part of a dialogue between anatomists and systematists with neither approach being given full veto power. My gut feeling is that once we have all this worked out, we’re going to have small-bodied holospondylous anamniotes dispersed all over the early tetrapod tree.
Link to this@Darren,
Yeah, that’s me.
Link to this@Heteromeles
Alligatoroids not contained in the genus Alligator are phylogenetically distinct. This includes all caimans and a bunch of other stuff. The genus Alligator only represents a subset of the Alligatorinae.
Link to thisMy pet theory is that those weird, wide skulls were somehow involved in some kind of electroreception – maybe locating small prey in the mud.
Unfortunately, it’s one of those hypotheses that probably will never be confirmed nor falsified.
Link to thisAlong with the richer mix of oxygen could there have been higher atmospheric pressure? Combine this with a roughage rich diet and a bad digestion, we’re talking about a methane rich flatulence. Combine that with an oxygen rich atmosphere and a correspondingly low ignition point and you’ve got a spontaneously igniting rocket. Combine that with the cheek flaps and you might have had rocketing amphibians swooping through the air from lagoon to marsh to swamp.
Link to this