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.
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).
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.
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 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…
- Tet Zoo picture of the day # 25 (on elasmosaurid plesiosaurs)
- Tet Zoo picture of the day # 26 (brief text and pic on placodonts)
- Voracious snub-nosed robber
- The Cumnor monster mandible
- In which Bob Nicholls exceeds expectations and produces some jolly good artwork
- Who made the giant Jurassic sea-floor gutters?
- Henodus, filter-feeding Triassic marine reptile
And for other Mesozoic marine reptiles, see…
- One of so many bizarre Triassic marine reptiles (Helveticosaurus)
- The skin of ichthyosaurs
- Sea Dragons of Avalon, an Arthurian adventure (part I)
- An Arthurian adventure, part II: more fossil marine reptiles than are good for your health
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.