ADVERTISEMENT
  About the SA Blog Network













Tetrapod Zoology

Tetrapod Zoology


Amphibians, reptiles, birds and mammals - living and extinct
Tetrapod Zoology Home

The confusing diplospondylous tupilakosaurids


Email   PrintPrint



Sherri DeFauw's temnospondyl reconstructions from 1989 (DeFauw 1989). Some of them might be inaccurate (the trematosaurid, for example, should probably be longer-bodied and longer-tailed), but how many times has this sort of thing been done before? Not nearly enough.

Time for a quick look at another temnospondyl group. Today, we focus on the tupilakosaurids, a group of short-limbed, blunt-skulled, long-bodied Permo-Triassic temnos. Ossified ceratobranchials, poorly ossified limbs and long and flexible bodies all suggest that they were fully aquatic though – like some other aquatic temnospondyl groups – their bones lack lateral line sulci. Distinctive features of this group include a unique pterygoid incisure on the palate (a V-shaped notch adjacent to the braincase) and a cranial bone configuration that involves large prefrontals, small postfrontals and large postorbitals. None were very big, with total lengths of 20-40 cm.

Nielsen's reconstructions of different parts of the vertebral column of Tupilakosaurus heilmani (Nielsen 1954). What - neural arch straddling two centra? Yes, it's called diplospondyly.

Fans of palaeoart and of the history of ideas concerning bird origins will be interested to know that Eigil Nielsen named the type species of the group – Tupilakosaurus heilmani­ from the Lower Triassic of Greenland – after artist and author Gerhard Heilmann (Nielsen 1954), best known for his seminal The Origin of Birds. However, Nielsen incorrectly spelt Heilmann with only one ‘n’!

Nielsen (1954) was originally “convinced that [he was dealing with] the earliest Triassic ichthyosaurian so far known” thanks to the short, amphicoelous centra and tall neural arches of Tupilakosaurus. More specifically, the vertebrae of Tupilakosaurus are what we term diplospondylous: this is the weird condition (well, weird to us) where each vertebral ‘segment’ actually consists of two, equally-sized centra, with the neural arch straddling both. However, the discovery during preparation of skull roof elements revealed to Nielsen that he was dealing with a ‘labyrinthodont’ that he considered related to trematosaurs (despite this, much of his 1954 paper on Tupilakosaurus is devoted to a discussion of ichthyosaur origins; had he known of the short-faced, blunt-snouted appearance of the entire skull, he might not have taken the idea of ichthyosaur ancestry that seriously). The sediments that yield Tupilakosaurus are marine in origin and contain ammonoids, apparently showing that this was another temnospondyl clade that dwelt – for at least part of its history – in the sea.

Diplospondylous vertebrae in three tupilakosaurids: anterior is to the left. From Werneburg et al. (2007).

Dvinosauroids or plagiosauroids?

Tupilakosaurids share enlarged retroarticular processes, shallow (or absent) ‘tympanic’ notches, a distinctive palate where the pterygoids descend ventrally around the sides of the interpterygoid vacuities, and short and poorly ossified limbs with several other temnospondyl groups including the trimerorhachids we looked at previously (see links below) and the dvinosaurids.

Temnospondyl phylogeny from Yates & Warren (2000). You should be able to see that Tupilakosaurus is close to Dvinosaurus, and hence well away from Batrachosuchus and kin (the Brachyopidae, very bottom of diagram).

Shared similarities with trimerorhachids have led some authors to propose a sister-group relationship between trimerorhachids and tupilakosaurids (Milner 1990). However, tupilakosaurids are also linked to dvinosaurids due to their absence of both the lacrimal and supratemporal bones otherwise common to temnospondyls, their possession of a transverse toothrow on the vomers, and other characters (Yates & Warren 2000). Tupilakosaurids are thus part of Dvinosauroidea according to this view.

McHugh (2012), however, recovered the supposed tupilakosaurid Thabanchuia as part of Brachyopidae, and brachyopids are considered part of Trematosauria; Trematosauria is part of the great clade Stereospondyli. Without explaining the full and complex details of the temnospondyl tree, all I need do is tell you that this position places Thabanchuia very far away from dvinosauroids in McHugh’s topology. Is it that Thabanchuia is not a tupilakosaurid, or is it that there really are strongly competing views on where tupilakosaurids might fit within temnospondyl phylogeny?

Warren et al. (2011) drew attention to the tupilakosaurid-like, diplospondylous vertebrae of the Tasmanian brachyopid Bothriceps australis. In their phylogenetic analysis, Warren et al. (2011) found dvinosauroids (dvinosaurians of their usage) to be nested within Stereospondyli, but they didn’t support any special relationship between tupilakosaurids and brachyopids. It would seem, therefore, that those distinctive diplospondylous, tupilakosaurid-style vertebrae evolved more than once. Indeed, diplospondyly is not present in all tupilakosaurids: Slaugenhopia from the Lower Permian and Kourerpeton (read on) both lack it, and it may be that it didn’t evolve in the group until the Late Permian (Werneburg et al. 2007, Warren et al. 2011).

Diplospondylous vetrebrae of the brachyopid Bothriceps, from Warren et al. (2011). (B) shows an enlargement of the right half of the neural spine. Scale bar = 10 mm.

Slaugenhopia, Thabanchuia and the ‘barber’s shop reptile’

Until recently, tupilakosaurids were only known from the Lower Triassic of Greenland and Russia. However, the presence of their probable sister-taxon (Dvinosauridae) in the Upper Permian means that their presence is expected in the Permian too. This was confirmed with the 2004 realisation that Slaugenhopia from the Lower Permian of Texas, originally named in 1962, was actually an overlooked tupilakosaurid (Milner & Sequeira 2004). Milner (1990) also reported Upper Permian tupilakosaurid vertebrae from South Africa but (as revealed by A. R. Milner’s hand-written notes on a reprint I own) these turned out to be dicynodont remains. An Upper Permian tupilakosaurid is also known from France (Werneburg et al. 2007). It’s the earliest known diplospondylous member of the group.

Partial vertebral column of a diplospondylous tupilakosaurid from the Upper Permian of France. From Werneburg et al. (2007).

Thabanchuia oomie from the Lower Triassic of South Africa was described as a good member of this clade (Warren 1998), meaning that tupilakosaurids were apparently present on Gondwana (though see above). Warren (1998) suggested that tupilakosaurids survived the end-Permian extinction event in Gondwana, and later re-invaded the north from this southern refugium. Diplospondylous vertebrae from the Lower Triassic of India, suggested as times to be from a tupilakosaurid, might actually be from a diplospondylous brachyopid.

Skull of the South African taxon Thabanchuia (total length c. 25 mm), from Milner & Sequeira (2004).

The Permian tupilakosaurid Kourerpeton was famously ‘discovered’ in the window of a barber’s shop in Arizona, later added to the collections of the University of Arizona, and formally ‘withdrawn’ from the collections in 1967 due to lack of provenance data (the barber was never 100% sure where it had come from and there have been recent suggestions that the specimen may not even have been American). It was later accessioned at the Manitoba Museum of Man and Nature in Winnipeg, and only then named and described: Kourerpeton, meaning ‘barber’s shop reptile’ (Olson & Lammers 1976). The latter authors gave Kourerpeton its own ‘family’, Kourerpetidae (which I think is etymologically incorrect: shouldn’t it have been Kourerpetontidae? I think the authors knew this – they used Kourerpetidae on p. 46 and p. 54 of the article, but Kourerpetontidae in the abstract and on p. 56). Kourerpeton lacks the pterygoid incisure present in other tupilakosaurids, so might be outside the clade that contains other taxa (Milner & Sequeira 2004).

And that… is the end of that; I look forward to much exciting, insightful, controversial tupilakosaurid-themed debate and discussion in the comments below. Many, many more temnospondyls to come.

For previous Tet Zoo articles on temnospondyls, see…

Refs – -

DeFauw, S. L. 1989. Temnospondyl amphibians: a new perspective on the last phases in the evolution of the Labyrinthodontia. Michigan Academician 21, 7-32

Milner, A. R. 1990. The radiations of temnospondyl amphibians. In Taylor, P. D. & Larwood, G. P. (eds) Major Evolutionary Radiations. Systematics Association Special Volume 42. Clarendon Pres (Oxford), pp. 321-349.

- . & Sequeira, S. E. K. 2004. Slaugenhopia texensis (Amphibia: Temnospondyli) from the Permian of Texas is a primitive tupilakosaurid. Journal of Vertebrate Paleontology, 24, 320-325.

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).

Nielsen, E. 1954. Tupilakosaurus heilmani n. g. et n. sp. an interesting batrachomorph from the Triassic of East Greenland. Meddelelser om Grønland udgivne af Kommissionen for Videnskabelige Undersøgelser I Grønland 72 (8), 1-33.

Olson, E. C. & Lammers, G. E. 1976. A new brachyopoid amphibian. In Churcher, C. S. (ed) Athlon, Essays in Palaeontology in Honour of Loris Shano Russell. Royal Ontario Museum (Toronto), pp. 45-57.

Warren, A. A 1999. Karoo tupilakosaurid: a relict from Gondwana. Transactions of the Royal Society of Edinburgh 89, 145-160.

- ., Rozefelds, A. C. & Bull, S. 2011. Tupilakosaur-like vertebrae in Bothriceps australis, an Australian brachyopid stereospondyl. Journal of Vertebrate Paleontology 31, 738-753.

Werneburg, J., Steyer, S., Sommer, G., Gand, G., Schneider, J. W. & Vianey-Liaud, M. 2007. The earliest tupilakosaurid amphibian with diplospondylous vertebrae from the Late Permian of southern France. Journal of Vertebrate Paleontology 27, 26-30.

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!

Nature Blog Network

Follow on Twitter @TetZoo.

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





Rights & Permissions

Comments 27 Comments

Add Comment
  1. 1. David Marjanović 10:15 am 04/30/2013

    Yay yay yay! ^_^

    Sherri DeFauw’s temnospondyl reconstructions from 1989 (DeFauw 1989). Some of them might be inaccurate (the trematosaurid, for example, should probably be longer-bodied and longer-tailed), but how many times has this sort of thing been done before? Not nearly enough.

    Word. Temnospondyl skeletons are generally rarely reconstructed; temnospondyl research is totally craniocentric.

    tympanic notches

    *cringe* You’ve waded deep into controversy here!

    Temnospondyl phylogeny from Yates & Warren (2000).

    I’m pretty sure the “undescribed genus” is Thabanchuia. Yates & Warren (2000) belongs to an edited book, and those (as you’ve experienced) always take years to be published.

    Kourerpetidae (which I think is etymologically incorrect: shouldn’t it have been Kourerpetontidae?

    No. The o of herpetón isn’t an omega, it’s an omicron; the -on is simply the neuter adjective ending, the whole word is an adjective derived from “to creep”. There are masculine nouns in -on with omega whose stem is -ont-; that’s different.

    (I had to deal with this in the manuscript for my next paper.)

    Somebody please act as First Reviser and choose Kourerpetidae as the correct form.

    Link to this
  2. 2. naishd 10:28 am 04/30/2013

    Thanks indeed, David. Dammit – I forgot the tympanic/spiracular notch debate (reminder: this text is old, and updated very hurriedly). Neverthless, will go correct. Thanks for thoughts on Kourerpetidae, my bad.

    Darren

    Link to this
  3. 3. ectodysplasin 11:50 am 04/30/2013

    Worth pointing out that the intercentra in the more basal dvinosaur Acroplous approach the diplospondylous condition, so there’s a bit of a continuum between your typical “rhachitomous” vertebrae and “diplospondylous” vertebrae. Then, if you take that incipiently diplospondylous vertebra and reduce the pleurocentra, you have a stereospondylous vertebra.

    Also speaking of diplospondyly, the problematic “lepospondyl” Acherontiscus is also diplospondylous.

    Link to this
  4. 4. ectodysplasin 12:20 pm 04/30/2013

    Also while we’re at it…

    a distinctive palate where the pterygoids descend ventrally around the sides of the interpterygoid vacuities

    This morphology seems to be a variant on the starkly downturned quadrate ramus of the pterygoid seen in tupilakosaurids, brachyopoids, and plagiosauroids (and interestingly enough, diplocaulid nectrideans). This morphology is also seen in some other more basal dvinosaurs but isn’t seen elsewhere among temnospondyls.

    Link to this
  5. 5. vagnry 3:18 pm 04/30/2013

    It is interesting that the tupilakosaurids are named after the inuit Tupilaq, tupilak in Danish, a witchcraft monster made of bone, narwhal or walruss teeth, etc.

    But, then again, hardly surprising, as the first? find was in Greenland, described by the dane Nielsen!

    Link to this
  6. 6. Therizinosaurus 4:33 pm 04/30/2013

    How stupid of a museum to withdraw a specimen purely due to a lack of provenence data. Obviously Kourerpeton’s completely worthless if we don’t know where or when it lived…. *rolls eyes*

    Link to this
  7. 7. Halbred 7:51 pm 04/30/2013

    Get Jack Horner in there–he’ll smash the fossil without remorse.

    Link to this
  8. 8. ectodysplasin 8:36 pm 04/30/2013

    Kourerpeton has a pretty sordid history.

    It’s been suggested to be from the Chickasha Formation (early Wordian) on the basis of matrix sedimentology, but honestly there’s very little to that. The specimen itself hasn’t been restudied since Olson described it in 1976, and could use some revisitation, especially given the lack of attention Olson gave the postcrania.

    Someone with a lot of money and time and without a lot of ambition could probably play around with rare earth elements and try to work out provenance that way; you might be able to (at the very least) narrow down formation or region, but you’d have to ask yourself whether this would actually be worth it (answer: probably not)…there are a ton of undescribed temnospondyls sitting around in collections that probably deserve greater attention and ultimately have greater potential to tell us about the evolution of major groups than Kourerpeton does.

    Could use a redescription though.

    Maybe I’ll see if I can get someone to pick it up from Winnipeg sometime.

    Link to this
  9. 9. ectodysplasin 8:51 pm 04/30/2013

    @Darren;

    I forgot the tympanic/spiracular notch debate

    In the case of “eobrachyopids”+dvinosauids+tupilakosaurids, there’s no otic notch present in the lateral skull; the entire stapedial canal is enclosed within the cheek (convergent with what we see in pipids, by the way). We can probably comfortably call this an adaptation to subaqueous sound transduction in “eobrachyopids”+dvinosauroids. Whether this represents a secondarily aquatic middle ear or not is phylogeny-dependent and still largely up for debate (detailed anatomy of that region remains to be described for any member of this group). Brachyopoids and plagiosauroids have a similar anatomy (moreso brachyopoids….plagiosauroids are just bizarre), as do diplocaulid nectrideans (in contrast to essentially all other “lepospondyls”), but when it comes down to it, we really don’t have a good enough handle on temnospondyl middle ear morphology to really say whether we’re seeing secondary adaptation of an impedance-matching tympanic ear to a secondarily aquatic lifestyle in these groups (although I’d favor this interpretation) or variation in stapedial morphology within atympanic spiracles associated with buccopharyngeal pumping (which I think David tends to favor).

    Wow, that’s a lot of word salad. Maybe I can summarize that in plainspeak if anyone is interested.

    Link to this
  10. 10. Andreas Johansson 4:37 am 05/1/2013

    No. The o of herpetón isn’t an omega, it’s an omicron; the -on is simply the neuter adjective ending, the whole word is an adjective derived from “to creep”. There are masculine nouns in -on with omega whose stem is -ont-; that’s different.

    Which is why it’s “herpetology” and not “herpetontology” or whatever.

    Link to this
  11. 11. naishd 8:16 am 05/1/2013

    Finally, I can log back in again. Huh. Thanks for comments, great stuff. Some quick responses…

    Regarding temnospondyl skeletal reconstructions (comment # 1), a noted palaeontologist tells me that Rainer Schoch’s papers frequently include far superior skeletal reconstructions. I have quite a few of Rainer’s papers but evidently don’t have enough, since I’ve only seen one or two such illustrations (e.g., Mastodonsaurus giganteus in his 1999 monograph, Sclerothorax in the 2007 APP paper). Any ideas which reconstructions I’m missing?

    On the distribution of diplospondyly (comment # 3): the condition is supposed to be present in some cynodonts (and maybe other non-mammalian synapsids) as well!

    Darren

    Link to this
  12. 12. ectodysplasin 5:50 pm 05/1/2013

    @Darren,

    On the distribution of diplospondyly (comment # 3): the condition is supposed to be present in some cynodonts (and maybe other non-mammalian synapsids) as well!

    Yep. My guess is that it’s highly influenced by function. Embolomeres are also essentially diplospondylous (well, most of them are).

    Link to this
  13. 13. David Marjanović 6:30 pm 05/1/2013

    Diplospondylous cynodonts??? What the vertical gene transfer???

    Rainer Schoch makes wonderful skull reconstructions, but I can’t remember seeing him go much beyond that – an exception is the great big redescription of Sclerocephalus (Witzmann & Schoch 2009).

    which I think David tends to favor

    Definitely; we’re talking about animals with internal gills – I have real trouble imagining they ever had ancestors that habitually stuck their heads out of the water, let alone had any reason to care about the noise out there. Also, Archegosaurus, IIRC Mastodonsaurus, and Dvinosaurus have extra processes on their stapedes that point towards the quadrates. As far as I can see, the temnospondyl stapes is by default a plank or pillar that buttresses the ventral part of the braincase against the tabular (and, via the usual extension in cartilage, the quadrate or squamosal or so), the same way that the paroccipital process buttresses the dorsal part of the braincase against the tabular. The stapes doesn’t have a “shaft” that is homologous between temnospondyls and (say) crown-group diapsids, it has a quadrate process and a dorsal process which ossify to varying extents.

    The stapes famously fuses to the parasphenoid in adult Mastodonsaurus and probably other temnospondyls (in many Russian specimens it’s still in place even though you’d expect it to have fallen off very early).

    The stapes is also really large in all temnospondyls I’ve seen. I’ve seen the skull of a goliath frog (on exhibit in the National Zoo in DC, IIRC); those stapedes are tiny, much smaller than in a temnospondyl of the same skull size.

    In short, I think these animals simply didn’t have a middle ear. Frogs, turtles, crown-group diapsids and eucynodonts or so have a middle ear.

    Rant over. What exactly do you mean by “stapedial canal”?

    Link to this
  14. 14. David Marjanović 6:34 pm 05/1/2013

    Uh, salamanders have a middle ear, too; I forgot about the operculum* apparatus (shared with frogs and possibly Eocaecilia… and there are several “microsaurs” with assorted weirdness around the fenestra ovalis that has yet to be sorted out).

    * Not homologous to the gill lid, and probably never been thought to be so.

    Link to this
  15. 15. naishd 8:08 pm 05/1/2013

    Ok, turns out it’s alleged diplospondyly in alleged cynodonts (see comment # 13). I quote from Warren et al. (2011) (the Bothriceps paper cited above): “It is possible that other brachyopids were diplospondylous; tupilakosaur-like centra have been found in the Panchet Formation of India (Huxley, 1865), the origin of the brachyopid, Brachyops laticeps. In the Arcadia Formation of Queensland, and the Beaufort Beds of South Africa, both of which include brachyopids without associated centra, tupilakosaur-like centra have been attributed to cynodonts (Thulborn, 1990).”

    Thulborn (1990) is…

    Thulborn, R. A. 1990. Mammal-like reptiles of Australia. Memoirs of the Queensland Museum 28, 169.

    That article is here and there’s no clear reference to (or illustration of) diplospondyly. The condition is of course well known for various actinopterygians.

    Darren

    Link to this
  16. 16. naishd 8:10 pm 05/1/2013

    … and I just realised that I’ve missed the point of that quote. Warren et al. (2011) are saying that those alleged cynodont centra are perhaps from brachyopids, not from cynodonts at all.

    Darren

    Link to this
  17. 17. ectodysplasin 4:37 am 05/2/2013

    @david,

    In no particular order…

    I have real trouble imagining they ever had ancestors that habitually stuck their heads out of the water, let alone had any reason to care about the noise out there.

    Well, IIRC, Brazeau & Ahlberg (2006) were recognizing adaptations for hearing in the stapes of Panderichthys and there’s other relevant anatomy in Ichthyostega, so there’s probably good reason to believe that the tetrapod stem was using the hyomandibula for dual purposes, even within aquatic taxa. The question then becomes whether you need a tympanum in order to be making use of this ear, and the answer in this case is no; aquatic tetrapods tend to lose the tympanum, because vibrations pass through soft tissue more or less unobstructed in water. However, you’re still using the stapes to transmit sound via the fenestra vestibularis.

    The fact that the footplate of the stapes is resting against a vestibular fenestra in the first place suggests that this structure is being used for sound transmission. Lungfish and more basal stem-tetrapods don’t have a fenestra vestibularis in the first place; they have a series of facets in an unperforate lateral wall of the otic capsule where the hyomandibula articulates. The quadrate process of the hyomandibula braces against the quadrate, and the distal end of the hyomandibula (the processus opercularis) has a ligamentous attachment with the medial surface of the opercular bone. This whole apparatus is used for the opening and closing of the operculum during buccopharyngeal pumping. The inner ear in fishes is completely enclosed within the braincase (with the exception of the endolymphatic ducts) and hearing is accomplished solely via vibration of otoliths within the otic capsules. The hyomandibula/stapes has no role in sound transmission.

    Even if early crown-tetrapods were using their stapes for buccopharyngeal pumping (and I’m sure some were), you’re still looking at an ossicle that’s highly reduced compared to what we see in more basal tetrapodamorphs and in lungfishes, and we pretty quickly see the median surface of this ossicle attached to a membrane that connects to the lagena of the inner ear, both of which suggest some function in sound transmission.

    Rant over. What exactly do you mean by “stapedial canal”?

    The stapedial canal is a space between the dorsal lamina of the pterygoid and the parotic process of the occiput. Medially, this space terminates at the fenestra vestibularis and laterally this channel opens into the otic notch when the otic notch is present, or laterally towards the cheek when it isn’t. The stalk of the stapes in temnospondyls follows this canal pretty closely in most of the temnospondyls I’ve seen when the stapes is present. This anatomy is pretty well figured in Schoch’s monograph on Mastodonsaurus but once you know what you’re looking for, it’s pretty obvious.

    Also, Archegosaurus, IIRC Mastodonsaurus, and Dvinosaurus have extra processes on their stapedes that point towards the quadrates.

    This process is probably common to all tetrapods. Remember, ancestrally there’s an articulation between the hyomandibula and the quadrate, which also sends a process towards the stapes. The elongate stalk in temnospondyls is a separate structure probably associated with sound transmission.

    As far as I can see, the temnospondyl stapes is by default a plank or pillar that buttresses the ventral part of the braincase against the tabular (and, via the usual extension in cartilage, the quadrate or squamosal or so), the same way that the paroccipital process buttresses the dorsal part of the braincase against the tabular.

    Well, in dvinosaurs I can assure you the stapes doesn’t articulate with the skull roof. Same goes for plagiosaurids, and I believe the same goes for Mastodonsaurus. In dvinosaurs especially, the inner surface of the tabular is completely smooth except for the articulation between the tabular and the paroccipital process of the exoccipital. There’s essentially nowhere for the distal tip of the stapes to articulate with.

    In pipids, that distal tip of the stapes doesn’t terminate in a tympanic membrane; it terminates in a cartilaginous shield that remains in the pharyngeal pouch beneath the dermis, and is only loosely connected to the surrounding bone. This cartilaginous plate serves to pick up lower-frequency sounds subaqueously. I see no reason to believe that this sort of anatomy isn’t what we’re seeing in the large stapes of some of these aquatic temnospondyls.

    Uh, salamanders have a middle ear, too; I forgot about the operculum* apparatus (shared with frogs and possibly Eocaecilia… and there are several “microsaurs” with assorted weirdness around the fenestra ovalis that has yet to be sorted out).

    The opercular apparatus in the caudate ear is more or less equivalent to the round window in the mammalian ear; it’s an interface between the middle ear and inner ear that serves to offset pressure within the perilymph created by vibration of the stapes against the fenestra vestibularis. There’s typically some open space around the footplate of the stapes in most early tetrapods, which probably also served the same purpose. Whether the specific condition seen in salamanders and in amniotes is convergent or homologous is definitely still unclear, though, until we have better inner ear data for more early tetrapods.

    In short, I think these animals simply didn’t have a middle ear. Frogs, turtles, crown-group diapsids and eucynodonts or so have a middle ear.

    I guess this depends on how you define “middle ear” but really any use of the stapes to transmit sound to the fenestra vestibularis qualifies. These may not be impedance-matching middle ears, but they are middle ears nonetheless.

    Link to this
  18. 18. BrianL 5:39 am 05/2/2013

    I’m sorry fr asking a question that has zero to do with the discussion you’re having, but…is it possible to view/listen to the TetZoo podcasts without using Twitter?

    Link to this
  19. 19. naishd 7:25 am 05/2/2013

    The TetZoo podcasts are available here. Maybe I should announce their existence to the general readership (or, did I do this already?).

    Darren

    Link to this
  20. 20. BrianL 7:35 am 05/2/2013

    @Darren:

    Thank you but your link doesn’t seem to work. However, I’ve found them online now. I’m glad that they are available for watching.

    Link to this
  21. 21. naishd 7:38 am 05/2/2013

    Oops, sorry, will correct that link…

    Darren

    Link to this
  22. 22. llewelly 12:37 pm 05/2/2013

    “Maybe I should announce their existence to the general readership (or, did I do this already?).”

    What you need is a prominent link which always appears on every page; for example, in the header you presently have these links: [Tetrapod Zoology Home] [About] [Contact]
    and you need something like: [Tetrapod Zoology Home] [About] [Contact] [podcast]

    So that anyone who comes to any post on the blog can see that you have a podcast, and, have a way to get there. Without a link like that, newcomers to your blog often have any way to know about the podcast.

    Furthermore, just as you announce new podcast episodes on facebook, you should announce them here. Not everyone who reads the blog follows the tetzoo facebook page.

    Link to this
  23. 23. David Marjanović 10:56 am 05/4/2013

    Furthermore, just as you announce new podcast episodes on facebook, you should announce them here. Not everyone who reads the blog follows the tetzoo facebook page.

    For the record, even I don’t. I only notice it when it shows up in recent activities or suchlike.

    Link to this
  24. 24. David Marjanović 11:47 am 05/4/2013

    Well, IIRC, Brazeau & Ahlberg (2006) were recognizing adaptations for hearing in the stapes of Panderichthys and there’s other relevant anatomy in Ichthyostega, so there’s probably good reason to believe that the tetrapod stem was using the hyomandibula for dual purposes, even within aquatic taxa.

    Ichthyostega has a unique stapes with a sort of inbuilt tympanum. I buy that it was used for hearing. It’s just one more feature that makes this massively overgrown mudskipper so frigging weird. It’s an autapomorphy.

    Panderichthys? Seriously? I need to read the paper (again, I guess). All I know is that its hyomandibula lacked “the ventral part” and no longer articulated with the ceratohyal.

    The question then becomes whether you need a tympanum in order to be making use of this ear, and the answer in this case is no; aquatic tetrapods tend to lose the tympanum, because vibrations pass through soft tissue more or less unobstructed in water. However, you’re still using the stapes to transmit sound via the fenestra vestibularis.

    You don’t need the fenestra or anything for that, though; all you need is the inner ear, as your average extant fish demonstrates.

    The fact that the footplate of the stapes is resting against a vestibular fenestra in the first place suggests that this structure is being used for sound transmission. Lungfish and more basal stem-tetrapods don’t have a fenestra vestibularis in the first place; they have a series of facets in an unperforate lateral wall of the otic capsule where the hyomandibula articulates. The quadrate process of the hyomandibula braces against the quadrate, and the distal end of the hyomandibula (the processus opercularis) has a ligamentous attachment with the medial surface of the opercular bone. This whole apparatus is used for the opening and closing of the operculum during buccopharyngeal pumping. The inner ear in fishes is completely enclosed within the braincase (with the exception of the endolymphatic ducts) and hearing is accomplished solely via vibration of otoliths within the otic capsules. The hyomandibula/stapes has no role in sound transmission.

    So, once the skull was safely akinetic and the operculogular series was lost (as seen in Tiktaalik), the hyomandibula didn’t need to move anymore. It was able to become a stable brace between the ventral part of the braincase and the suspensorium. Because there wasn’t a joint between the braincase and the hyomandibula anymore, the braincase didn’t need to ossify where it was covered by the immobile head of the hyomandibula, and because that was the site of a fontanelle in earlier ontogeny, it was easy to just stop ossification and just round the edges. Thus was born the fenestra vestibularis, at first just as a weight- and material-saving feature. What do you think of this story? :-)

    The fenestra vestibularis is necessary for a middle ear. Thus, it had to evolve before it was used for sound conduction. Once it was there, a middle ear with a mobile stapes was able to evolve, and did so several times independently.

    Wings didn’t evolve for flying either. Flight became possible once wings were there.

    The stapedial canal is a space between the dorsal lamina of the pterygoid and the parotic process of the occiput.

    Oh. Didn’t know it had a name.

    This process is probably common to all tetrapods. Remember, ancestrally there’s an articulation between the hyomandibula and the quadrate, which also sends a process towards the stapes. The elongate stalk in temnospondyls is a separate structure

    Oh, I agree. There are just too many papers that homologize this stalk with the “shaft” directed to the quadrate in other tetrapods.

    In dvinosaurs especially, the inner surface of the tabular is completely smooth except for the articulation between the tabular and the paroccipital process of the exoccipital. There’s essentially nowhere for the distal tip of the stapes to articulate with.

    Squamosal, postparietal…?

    Or maybe I’m wrong (Gosh! Say it ain’t so!) and the stalk of the stapes had some highly conserved function related to the spiracle, which was apparently missing in most dvinosaurs.

    In Dendrerpeton (Sigurdsen 2008: fig. 5), the paroccipital process has two ridges (“lateral processes”) that look like they held the stapes in place.

    In pipids, that distal tip of the stapes doesn’t terminate in a tympanic membrane; it terminates in a cartilaginous shield that remains in the pharyngeal pouch beneath the dermis, and is only loosely connected to the surrounding bone. This cartilaginous plate serves to pick up lower-frequency sounds subaqueously.

    This strongly reminds me of the ossified tympanum of mosasaurs and what looks like the same thing in, bizarrely enough, diadectids: apparently it’s sometimes easier to modify the distal end of a tympanic ear to make it work in water than it is to just lose the whole thing and return to using the inner ear alone.

    I see no reason to believe that this sort of anatomy isn’t what we’re seeing in the large stapes of some of these aquatic temnospondyls.

    It’s very large indeed, and it’s sutured (in some cases fused) to the parasphenoid instead of participating in a joint there. If bone conduction is enough, the skull roof can do it.

    …which is exactly what we see in lots and lots of small extant rainforest frogs that lack tympana but merrily vocalize away: they basically use the entire head as a tympanum that is small enough to pick up high frequencies.

    Link to this
  25. 25. ectodysplasin 2:23 am 05/7/2013

    @David,

    Wrote up a long response and then carelessly closed the window. tl;dr highlights are as follows:

    1. Various fish clades have evolved some sort of relationship between various gas bladders and the inner ear. This includes ostariophysans and mormyrids, most importantly, but also many other fish lineages. Fishes clearly benefit from complex sound transmission and amplification systems. Impedance-matching is less important in fishes, but sound amplification and transmission clearly is.

    2. Your description of otic capsule development does not fit with the published descriptions of otic capsule development for lungfishes. The fenestra vestibularis seems to be a true evolutionary novelty in tetrapods, rather than a paedomorphosis that exaggerates a juvenile morphology seen in other sarcopterygian lineages.

    3. A number of taxa lack a tympanum but still manage to hear relatively well with an enclosed middle-ear cavity (e.g. Lanthanotus, pipids, Sphenodon, crocodilians). This was likely the case with many early tetrapods. A tympanum is clearly not a requirement for terrestrial hearing.

    4. The lower jaw was probably basally associated with the sound transmission apparatus of early tetrapods. This was probably lost at some point in temnospondyls, but reacquired in salamanders and caecilians. This is incidentally also reacquired in various squamate lineages as well, generally related to fossoriality. The loss of a middle ear cavity is also repeatedly acquired in fossorial taxa and may have to do with functional morphology of the epaxial and mandibular musculature rather than any particular features of the ear.

    5. This would be a good topic to dissect in a review at some point. If you’re interested, we can talk about this more via email.

    Link to this
  26. 26. David Marjanović 10:22 am 05/7/2013

    I am interested. No time right now, though.

    But do you know what the middle ear cavity of frogs develops from? Is is the first gill pouch?

    Link to this
  27. 27. ectodysplasin 11:35 pm 05/8/2013

    I believe so, but I can check.

    Keep these thoughts on the back burner. I’m pretty busy right now as well working my way through the Lepospondyli with a machete, but I’ll start having more time in December or so.

    Link to this

Add a Comment
You must sign in or register as a ScientificAmerican.com member to submit a comment.

More from Scientific American

Scientific American MIND iPad

Give a Gift & Get a Gift - Free!

Give a 1 year subscription as low as $14.99

Subscribe Now >>

X

Email this Article

X