Tetrapod Zoology

ResearchBlogging.org

Having written (briefly) about the turtle-like shelled placodont Henodus chelyops, it’s as good a time as any to provide some more information. For starters, here’s a close-up photo (kindly provided by Markus Bühler) showing one of the grooves in the left lower jaw. These gutter-like structures (reportedly) contained a baleen-like apparatus, possibly used in filter-feeding (for more discussion of this, you’ll need to see the previous Henodus article, or Reif & Stein (1999), Rieppel (2002a) or Naish (2004)). If you’re struggling to interpret the skull as shown in the photo, anterior is to the left; the orbit is at top left, and part of the left external nostril is just visible at far left.

i-377e88ccef5705d393fad53527360f9d-Henodus-skull-Markus-B-groove-Nov-2010-resized.jpg

Last time, I also mentioned the strip of small denticles present across the anterior surfaces of the premaxillae. These denticles [shown below; from Rieppel (2001)] appear to be partially fused at their bases, or is it that they’re part of a ‘strip’ which has become split into near-individualised, regularly spaced units? It’s been inferred that these structures might have been used to scrape algae off rocks (Rieppel 2002a). They don’t seem to be true teeth (despite the apparent presence of pulp cavities): the possibility that they might be is highly unlikely in phylogenetic terms, given the reduction of premaxillary teeth in other cyamodontoid placodonts…. but then again, we do know that teeth can re-appear in some lineages (some discussion here: sixth paragraph).

i-06294611925f800bd1123ffa0e52ed8b-Henodus-premax-denticles-Rieppel-2001-Nov-2010.jpg

Polymorphism in skull roof anatomy? Naaah.

I think I also mentioned the fact that Henodus is unusual among placodonts in having a solid (as in, unfenestrated) skull roof. It seems that the ancestral supratemporal fenestrae were roofed over as the parietals expanded laterally. However, it’s been claimed that not all individuals are like this, and that at least one specimen – ‘specimen III’, shown below, at right [image provided by Markus Bühler] – has supratemporal fenestrae (Huene 1938). I initially got quite excited about this, as polymorphism of this sort would (potentially) result in all kinds of freaky, intraspecific variation in how the jaw muscles might work, and so on. However, Rieppel (2001) argued that the weak, thin bone here might be prone to breakage; he also noted that the alleged fenestrae of ‘specimen III’ have irregular margins (unlike true fenestrae), and that some of the other specimens have damage, or regions of depressed bone, in the same area. So, no supratemporal fenestrae in some Henodus specimens, just damage.

i-18476ee693e0a2b737c7fbe071cba30d-Henodus-skull-variation-Nov-2010-resized.jpg

There still is, however, a bit of shape variation in Henodus skulls. In some specimens, the ornamented region at the back of the skull has straight lateral margins, while in others the same margins are concave. Having referred to ‘ornament’ (the ornamentation I’m referring to is the collection of subconical osteoderms that line the borders of the posterior part of the skull), the number and position of these osteoderms is variable across individuals. Some of the osteoderms have fused to the squamosal and quadratojugal, giving these bones a ‘lumpy’ texture.

You might like to speculate on the function of these little hornlets – might they be sexually dimorphic, or used in display or some kind of combat? We don’t know, and nor (to my knowledge) has anybody tried to find out. A role in self-defense is perhaps plausible, but note that Henodus inhabited an environment where predators seem to have been mostly absent (only a single Nothosaurus tooth is known from the sediments that have yielded all the Henodus specimens).

Ode to the placodont shell

One more thing needs to be said about Henodus… its shell is very odd. In fact, the armour of placodonts in general is very odd. I was going to launch here into a discussion of placodont armour, but it’s a complex subject and I’ve run out of time. For now all I’ll say is that placodont armour is really peculiar in that it contains cartilaginous tissue (Scheyer 2007), and that parallel longitudinal dorsal ridges were present in some species. These ridges might have served a hydrodynamic role (Westphal 1976) [nod to the incredible Dermochelys]. Henodus is a very ‘special’ placodont in having a shell composed of a geometrically complex, mosaic-like arrangement of scutes and identically arranged underlying osteoderms (Westphal 1976, Rieppel 2002b). I tried to draw its carapace once, here’s the attempt (it’s incorrect in a few details, sorry. I also screwed up when doing the life restoration shown below.. just cannot get that carapace right)…

i-7c380347f5101deb14f7c76698525545-Henodus-carapace-Naish-Nov-2010.jpg

i-f3f72be2dd12e2836cb481a3f97110a2-Henodus-life-Naish-Nov-2010.jpg

I said last time that Henodus is also peculiar in that it inhabited a partially enclosed lagoon where salinity must have fluctuated a lot due to evaporation and influx of both marine and fresh water. This was an extreme environment; perhaps “the most severe environment ever successfully invaded by a sauropterygian” (Rieppel 2002b, p. 38). It supports the view that shelled placodonts were extremely tolerant of fluctuating salinities, and that it was their extensive, turtle-like dermal armour that allowed this. In this, they were behaviourally convergent with those tough, adaptable turtles that can inhabit fresh, brackish and salt water.

For previous Tet Zoo articles on sauropterygians, see…

And for other Mesozoic marine reptiles, see…

Refs – -

Huene, F. von 1938. Der dritte Henodus, Erganzungen zur Kenntnis des Placodontiers Henodus chelyops Huene. Palaeontographica A 89, 105-114.

Naish, D. 2004. Fossils explained 48. Placodonts. Geology Today 20 (4), 153-158.

Reif, W.-E. & Stein, F. 1999. Morphology and function of the dentition of Henodus chelyops (Huene, 1936 (Placodontia, Triassic). Neues Jahrbuch fur Geologie und Palaontologie, Monatshefte 1999, 65-80.

Rieppel, O. 2001. The cranial anatomy of Placochelys placodonta Jaeckel, 1902, and a review of the Cyamodontoidea (Reptilia, Placodonta [sic]). Fieldiana, Geology (New Series) 45, 1-104.

- . 2002a. Feeding mechanisms in Triassic stem-group sauropterygians: the anatomy of a successful invasion of Mesozoic seas. Zoological Journal of the Linnean Society 135, 33-63.

- . 2002b. The dermal armour of the cyamodontoid placodonts (Reptilia, Sauropterygia): morphology and systematic value. Fieldiana, Geology (New Series) 46, 1-41.

Scheyer TM (2007). Skeletal histology of the dermal armor of Placodontia: the occurrence of ‘postcranial fibro-cartilaginous bone’ and its developmental implications. Journal of anatomy, 211 (6), 737-53 PMID: 17944862

Westphal, F. 1976. The dermal armour of some Triassic placodont reptiles. In Bellairs, A. d’A. and Cox, C. B. (eds) Morphology and Biology of Reptiles. Academic Press (London), pp. 31-41.

Comments

  1. #1 Zach Miller
    November 12, 2010

    Thanks for more info on this fascinating critter. A few questions pop to mind:

    1) Have younger individuals been found? This might shed light on the “roofed supratemporal fenestrae” idea. I say this because the “damaged” areas of ‘specimen 3′ are very supratemporally-placed.

    2) How does Henodus relate to other placodonts? Its shell, aside from being a SHELL, seems completely at odds with that of cyamodontoids, which have two-part shells that do not flare laterally to the same extent (also, do cyamodontoids have “plastrons?”).

    3) How would Henodus’ extensive dermal armor have helped it survive in brackish waters?

    4) Did Henodus’ elbows bend so that the forearm points away from the body, like a sea turtle? How was the skeleton arranged within the shell?

  2. #2 Sven DIMilo
    November 12, 2010

    What strikes me are the gigantic nares.

    a partially enclosed lagoon where salinity must have fluctuated a lot due to evaporation and influx of both marine and fresh water.

    Why “must have”? Data?

    it was their extensive, turtle-like dermal armour that allowed this.

    by what mechanism?

    In this, they were behaviourally convergent with those tough, adaptable turtles that can inhabit fresh, brackish and salt water.

    uh…like what?
    Malaclemys probably, but that’s about it afaik.

  3. #3 Jerzy
    November 12, 2010

    I wonder if differences in skull were (besides sex/age polymorphism) a result of crushing/distortion?

    Hypersaline habitat for a filter feeder. Some sort of freaky flamingo niche?

  4. #4 neil
    November 12, 2010

    Ok, I can’t resist. The following comments are based on observations of the specimens along with refreshing my memory with some photos (like three decent ones out of 60 out-of focus, poorly lit shots). I’m not a great anatomist so I look forward to hearing from others who may have a different view.

    These denticles [shown below; from Rieppel (2001)] appear to be partially fused at their bases, or is it that they’re part of a ‘strip’ which has become split into near-individualised, regularly spaced units?

    I think the latter. There is a continuous spectrum between entirely individuated units and those with incipient divisions forming on the “apical” (lower) margin, but most seem to be about 3/4 or more split and fused at the base as you note.

    Rieppel (2001) argued that the weak, thin bone here might be prone to breakage; he also noted that the alleged fenestrae of ‘specimen III’ have irregular margins (unlike true fenestrae).

    I can confirm this from personal experience, unfortunately. When handling one of the skulls (specimen IV I think) a tiny, thin shard of the parietal fell off in my hands. I was mortified, but noticed that it had been broken previously and then been glued back in place. (A lesson to young paleos: If you handle enough specimens you will damage one eventually. You will feel miserable and ashamed. Suck it up and tell the curator. They are remarkably understanding about this sort of thing as long as it is brought to their attention in a timely manner.)

    On specimens where the parietal has broken away however, the squamosal appears to show a smooth curved margin below, probably corresponding the margin of a fenestra that becomes roofed-over during ontogeny. Unfortunately no very young specimens are known to my knowledge so we cannot be sure. But Rieppel (2001) suggests a small opening present in the left skull roof of specimen II may be a vestigial fenestra. Interestingly no trace of a similar opening is visible on the right side.

    There still is, however, a bit of shape variation in Henodus skulls. In some specimens, the ornamented region at the back of the skull has straight lateral margins.

    All of the skulls show some degree of both brittle and plastic deformation, so this apparent variability could be a preservational artefact.

    I think you mentioned the hyoids in the previous post but they are really remarkable and robust. I’ll send along a photo.

    As far as the “baleen” goes, I have to say that I remain a skeptic until we have tangible evidence beyond something Huene claimed to have prepped away. Not that I have any better interpretations, just my two cents.

    Regarding the freshwater/brackish habitat it bears mentioning that Macroplacus and Psephoderma occur in restricted limestone facies that may have occasionally been either highly saline or subject to fresh water influence. So it does seem plausible that armored placodonts were pretty flexible as far as salinity goes and that may have something to do with why they outlasted some other Triassic marine reptile lineages….

  5. #5 Art
    November 12, 2010

    That carapace reminds me of a similar structure on a common armadillo after it was flattened by a vehicle. The bottom sketch is a fair rendition of the post-vehicular corpse. Seems to me the structure makes more sense if it was wrapped around something.

  6. #6 William Miller
    November 12, 2010

    Sven DiMilo @2: Rieppel 2002b cited in the post says that: “Aigner (quoted in Reif & Stein, 1999) characterized the sedimentary facies of the upper Gipskeuper as a playa covered with brackish to hypersaline ponds that dried up seasonally.
    This is the most severe environment ever successfully invaded by sauropterygians, and it may testify to the role of the dermal armor as an osmotic barrier. Like the turtle shell, the dermal armor of cyamodontoids was covered by epidermal
    scutes, which, in combination with a wellossified carapace and plastron, provide a very efficient osmotic barrier in modern turtles. Experimental studies have shown a significantly smaller rate of gain of water (in fresh water) or loss of water (in sea water) in a slider turtle {Pseudemys
    scripta) with a well-ossified carapace and plastron than in a soft-shelled turtle (Apalone spiniferus) or a caiman {Caiman crocodilus) (Bentley, 1976).”

  7. #7 Darren Naish
    November 12, 2010

    Thanks for comments: I see that some of the questions raised in the earlier comments have mostly been answered. The assertion that the salinity in the semi-enclosed lagoonal habitat inhabited by Henodus “must have” fluctuated a lot is based on sedimentological evidence indicating marginal desiccation, occasional hypersalinity, and occasional flooding by rain water. The relevant citations are in Rieppel (2001). As Rieppel discusses (and as William notes in comment 6), the presence of an ossified plastron and carapace appears consistent with ‘insulation’ from osmotic fluctuation, a hypothesis inspired by tolerance observed (Ernst & Barbour 1989) and demonstrated (Bently 1976) in some turtles.

    Sven (comment 2): Malaclemys appears resistant to osmotic changes, but it’s not unique. One of the best examples is Mauremys leprosa which can occur in fresh, brackish and sea water, and has even been recorded living in latrine pools. Tolerance to a wide range of salinities also seems to be the case in Caspian turtle Mauremys caspica, Painted terrapin Callagur borneoensis, Pacific pond turtle Clemmys marmorata and the snapping turtles.

    Refs – -

    Bently, P. J. 1976. Osmoregulation. In Gans, C. & Dawson, W. R. (eds) Biology of the Reptile, Volume 5. Academic Press (London), pp. 365-412.

    Ernst, C. H. & Barbour, R. W. 1989. Turtles of the World. Smithsonian Institution Press, Washington, D. C. & London

  8. #8 Marcus Good
    November 12, 2010

    The logo for the new, X-TREEM edition of the site will be Darren, downing a can of Mountain Dew, riding a placodont on the crest of a tsunami. This is entirely so the eventual book containing this article will be titled X-TETZOO: “RIDE THE HENODUS”

  9. #9 Cameron
    November 12, 2010

    Malaclemys appears resistant to osmotic changes, but it’s not unique.

    Turtles of the World lists over 40 species with stated or implied salt tolerance. IIRC Malaclemys can tolerate salinity considerably higher than seawater during low tide, I’m not sure how unique this ability is. I have an article in the works on this topic, although it is taking far longer than I’ve anticipated.

  10. #10 David Marjanović
    November 13, 2010

    If those bifid “denticles” have a pulp cavity, what makes them “not teeth”? Do they consist of dentine?

    How does Henodus relate to other placodonts?

    Good question. Next question?

    It’s so scary that it has never been included in a phylogenetic analysis AFAIK.

    Its shell, aside from being a SHELL, seems completely at odds with that of cyamodontoids, which have two-part shells that do not flare laterally to the same extent (also, do cyamodontoids have “plastrons?”).

    Yes, having a plastron is standard for cyamodontoids. Being flat and wide is also standard; Henodus is extreme, but take a look at Psephoderma if you can. So, I don’t see any problems.

  11. #11 Darren Naish
    November 13, 2010

    Why are those denticles on Henodus unlikely to be true teeth? As mentioned above, other cyamodontoids have a paltry dentition: Cyamodus and Protenodontosaurus only have two teeth in each premaxilla, and placochelyids lack premaxillary teeth entirely. Furthermore, you realise the structures in Henodus are arranged along the anterior face of a neomorphic anteroventral premaxillary flange? They aren’t on, or close to, the position of the ancestral alveoli. I know this is all circumstantial, but it points to a neomorphic origin for the denticles.

    The shell of Henodus is, as David says, not so different from that of other cyamodontoids, especially Cyamodus. A plastron is variably present in cyamodontoids (it’s absent in Cyamodus and Psephoderma): when present, so is a lateral wall, linking the carapace to the plastron. Compared to other cyamodontoids, Henodus is unusual in lacking the posterior/pelvic shield.

    As for phylogenies, Henodus has indeed been incorporated in Rieppel’s parsimony analyses: in Rieppel & Zanon (1997) it was recovered as either the most basal cyamodontoid, or as the sister-taxon to Placochelys (the latter option was favoured). In more recent work it has emerged as the sister-taxon to Cyamodus (Rieppel 2000, 2001) within the cyamodontoid clade Cyamodontida.

    Refs – -

    Rieppel, O. 2000. Paraplacodus and the phylogeny of the Placodontia (Reptilia: Sauropterygia). Zoological Journal of the Linnean Society 130, 635-659.

    - . 2001. The cranial anatomy of Placochelys placodonta Jaeckel, 1902, and a review of the Cyamodontoidea (Reptilia, Placodonta). Fieldiana, Geology (New Series) 45, 1-104.

    - . & Zanon, R. T. 1997. The interrelationships of Placodontia. Historical Biology 12, 211-227.

  12. #12 David Marjanović
    November 14, 2010

    Furthermore, you realise the structures in Henodus are arranged along the anterior face of a neomorphic anteroventral premaxillary flange? They aren’t on, or close to, the position of the ancestral alveoli.

    Really? That’s not obvious from the (few) pictures I’ve seen, including your photos.

    Yes, of course pmx teeth in a derived cyamodontoid would be unexpected. But placodont phylogenetics isn’t in a terribly good state. The matrices are all small and craniocentric, and… for instance, ever since the mid-90s, everyone seems to suspect that Cyamodus as currently understood is polyphyletic, yet nobody does anything about it.

  13. #13 Darren Naish
    November 14, 2010

    It’s true that Cyamodus hildegardis has been said by some authors to be closer to placochelyids and Protenodontosaurus than to the rest of Cyamodus, but Rieppel (2001: same reference as above) found C. hildegardis, C. rostratus and C. kuhnschnyderi to form a clade. However, inclusion of C. muensteri made most of Cyamodontoidea collapse into a polytomy.

  14. #14 Yonk
    November 14, 2010

    I think it was one of your recent articles in which you described your fondness for hawksbills and their diet.

    http://news.yahoo.com/s/afp/20101112/wl_asia_afp/micronesiahealthturtleanimal

    maybe somebody should’ve explained the toxicity from eating spongiforms.

  15. #15 Darren Naish
    November 14, 2010

    Hi – thanks for that, Yonk. Yes, the original (2007) Tet Zoo article is Hard-shelled sea turtles and a diet of glass.

  16. #16 Jocelyn Falconnet
    November 14, 2010

    As for phylogenies, Henodus has indeed been incorporated in Rieppel’s parsimony analyses: in Rieppel & Zanon (1997)it was recovered as either the most basal cyamodontoid, or as the sister-taxon to Placochelys (the latter option was favoured). In more recent work it has emerged as the sister-taxon to Cyamodus (Rieppel 2000, 2001) within the cyamodontoid clade Cyamodontida.

    And don’t forget Mazin (1989) who recovered Cyamodontidae as the sister-group of (Placochelyidae + Henodus) on their dentition, even if there is little information regarding the latter genus.

    References:

    Mazin J.-M. 1989. La denture et la région palatine des placodontia (Reptilia, Trias).Implications phylogénétiques. Geobios 22(6):725-734.

    See also (but I didn’t have enough time to find it):
    Mazin J.-M. 1988. Paléobiogéographie des reptiles marins du Trias : phylogénie, systématique, écologie et implications paléobiogéographiques. Mémoires des Sciences de la Terre, Université Pierre et Marie Curie 8/88:1-312.

  17. #17 Dartian
    November 15, 2010

    Darren:

    Henodus inhabited an environment where predators seem to have been mostly absent

    Hmm. Then why did it have all that body armour? It seems like overkill to have a shell like that for any other purposes than those related to self-defence.

    Mauremys leprosa [...] has even been recorded living in latrine pools

    My word! You mean that someone, somewhere, in the real world (as opposed to Austin Powers movies) may have literally ‘shat on a turtle’?

The site is undergoing maintenance presently. Commenting has been disabled. Please check back later!