Henodus, filter-feeding Triassic marine reptile

ResearchBlogging.org

Long-time readers will know that I've been planning to cover placodonts - a group of marine, armour-plated Triassic sauropterygian reptiles - for a long, long time. Still haven't gotten round to it (though there is this one picture). But here's something, at least: a piece of text on the weird, fascinating German placodont Henodus. The text is a (slightly modified) excerpt from Naish (2004); most of the discussion on possible feeding behaviour is based on Rieppel's (2002) conclusions.

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Henodus chelyops is known only from the Gipskeuper deposit of Tübingen-Lustnau (Carnian, early Upper Triassic), southern Germany. Because the Gipskeuper is believed to represent a semi-enclosed brackish lagoon, Henodus is the only placodont known to have inhabited a non-marine environment [life restoration shown above by Jim Robbins; the more peculiar one shown below is by me].

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The skull of Henodus [shown here, photographed on a mobile phone, hence poor quality] looks rather surreal with its broad, squared-off rostrum, hypershortened pre-orbital region and broad flat skull table. The nostrils and orbits are located close to the snout tip, and because the skull is curved when viewed from the side, the nostrils and orbits face forwards. Sub-triangular scutes are located on the posterior border of the skull (the number and position of these scutes is variable across individuals). The supratemporal fenestrae that are normally present in placodont skulls have been secondarily roofed over, so the skull roof is therefore solid and unfenestrated.

The dentition is strongly reduced with only one pair of small teeth located far to the rear on the palatine bones and another on the dentaries (more usually in placodonts, large, flattened teeth cover much of the palatal and lower jaw surface). Gutter-like grooves run along the edges of the upper and lower jaws. The lower jaw is surprisingly deep and heavy, but its low coronoid process suggests that Henodus did not have the awesome crushing bite of other cyamodontoid placodonts. However, features indicative of musculature allowing rapid jaw opening are present while prominent hyoid bones imply that Henodus had a distensible throat. Compared to other cyamodontoids, Henodus has an even wider carapace; it also has a plastron and a lateral wall (a structure that links the carapace to the plastron, thereby fully enclosing the body within the shell).

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Given these features, how and what Henodus ate has always been controversial. A popular idea has been that it grubbed around in the mud for crustaceans. However, the rediscovery in the 1990s of striations within the jaw grooves (originally reported in 1936) indicate that Henodus had baleen-like material fringing its jaws. Furthermore, long overlooked is that the premaxillae support a ventrally projecting flange with a sharp cutting edge. Even more surprising is that this cutting edge exhibits a row of toothlike denticles along its anterior surface. When considered together and compared with the feeding adaptations of turtles, these features suggest that Henodus was a filter feeder that employed depression of the heavy lower jaw and expansion of the throat to suck in food particles. It may have engulfed swimming invertebrates, filtered burrowing forms out from the substrate, or used the premaxillary flange and denticles to scrape algae off rocks or to cut off pieces of other kinds of aquatic vegetation. Clearly, reappraisal of this bizarre placodont has revealed hitherto unsuspected diversity in the ecology of Mesozoic marine reptiles.

Too few people know that Mesozoic marine reptiles seem to have evolved filter-feeding at least once. On a somewhat related note, some neat, very interesting news on the feeding behaviour of Triassic marine reptiles is due to appear in print quite soon.

For previous Tet Zoo articles on sauropterygians, see...

And for other Mesozoic marine reptiles, see...

Refs - -

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

Rieppel, O. (2002). 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

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The skull doesn't look so weird to me. Look at the rostrum of our Bonnerichthys gladius that came out in Feb. Bar shaped and perpendicular to the axis of the animal, very similar to what we see here.

I already knew about this placodont but I had no idea it was a filter-feeder. Interesting to see more extinct mesozoic reptiles that used this way of feeding.

"The rediscovery in the 1990s of striations within the jaw grooves (originally reported in 1936) indicate that Henodus had baleen-like material fringing its jaws."

Plausible, but I don't buy it just on face value alone. The fact that one clade of marine artiodactyls (mysticetes) have this feature and it correlates to baleen has very little bearing on the soft-tissue/functional significance of striated grooves in a member of an extinct clade of diapsid reptiles! A single groove-baleen co-occurrence in the modern world does not make for a strong inference for an unrelated extinct taxon. Wouldn't you think a more reasonable interpretation is that the jaws of Henodus supported rhamphotheca?

I thought I'd mention that there's a type of plesiosaur from Antarctica I've heard of that may have evolved a filter feeding apparatus (tons of tiny teeth, IIRC). I can't remember the name for the life of me. This is, of course, in addition to other Cretaceous filter feeders such as Pterodaustro, pachycormids, and Megachasma.

And WRT "baleen" like structures - keep in mind that they've evolved already within archosaurs - anseriforms, as well as the possible occurrence in Gallimimus (don't do dinos, don't care, don't know if that's still taken seriously). I would by no means discount it just because its not an artiodactyl. Besides, what Darren described sound much different from nutrient foramina from mysticete palates anyway (that's a whole other discussion).

Bosse said, "I would by no means discount it just because its not an artiodactyl."

Thus the reason why I said that it is "plausible", but that I'm not going to accept the interpretation uncritically.

With regards to anseriforms and ornithomimosaurs, are the 'baleen-like' structures connected in anyway with grooves/striations on the palatal surface of the jaws?

Finally - the discussion is not about filter-feeding; rather, the interpretation of 'baleen-like' structures. There are a number of extinct reptiles that were probably filter feeders with many many small teeth, but this does not mean they had "baleen." Also, Henodus is a diapsid, but not an archosaur.

Uhh... I'm well aware they're not archosaurs. Archosaurs are a hell of a lot closer to diapsids than cetaceans... My point is that keratinous filter feeding structures have evolved multiple times, not just within artiodactyls.

My point regarding filter feeding dental modifications- to make it painfully obvious - the fact that there are non-mammalian tetrapods in multiple groups exploiting filter-feeding as a viable feeding method indicates that there is least some selection toward this type of strategy during parts of the Mesozoic. Given this, it could be expected that some critters evolved to possess keratinous baleen like structures.

That being said - rhamphotheca is only a step away from a baleen-like structure any way. Unfortunately, I don't know about osteological correlates in anseriforms and ornithomimosaurs - but I'm sure

Hmmmm!

I remember seeing an illustration of this creature in Spinar's "Life Before Man" and thinking how strange and fascinating it appeared.

"Life Before Man" was my first ever book on prehistoric creatures - now hopelessly out of date, it's still a treasured thing of beauty.

If Henodus was indeed a filter-feeder, then a "what if" speculation of its continued evolution is truly mind blowing. Giant long necked turtle-whales?

By C. M. Kosemen (not verified) on 05 Nov 2010 #permalink

Darren:

the rediscovery in the 1990s of striations within the jaw grooves (originally reported in 1936)

What do you mean by 'rediscovery'? Did people use to think that von Huene (it's his 1936 paper you're referring to, right?) was mistaken regarding his description of those striations?

Robert:

I remember seeing an illustration of this creature in Spinar's "Life Before Man" and thinking how strange and fascinating it appeared.

Ah yes, you mean this Burian picture. Quite a few people, I suppose, have a 'WTF?' moment the first time they see a picture of Henodus...

It's good to be sceptical, especially about remarkable claims. Huene (1936) described non-ossified structures that formed a striated pattern _within_ one of the jaw grooves: that is, he actually described structures that (apparently) looked like short hairs within a jaw groove. He likened this to baleen, but his observations can't be corroborated because the material was later prepped away! Reif & Stein (1999), however, reported similar structures within the jaw grooves. So, the possibility that Henodus possessed a baleen analogue is not based on the presence of the grooves alone (and note that grooves have not been regarded as an osteological correlate of baleen/a baleen analogue given that grooves do not correlate with baleen in whales).

The plesiosaur that Boesse (comment 5) has in mind is either Morturneria or Aristonectes, both of which are regarded as synonymous by some authors: whether these plesiosaurs are elasmosaurids or belong within an Aristonectidae (formerly termed Cimoliasauridae) closer to cryptoclidids has been controversial. The slim, closely spaced teeth of these animals might have been used to retain small prey; a form of filter-feeding analogous to that used by some lobodontin seals is plausible.

As for ornithomimids, the suggestion that they might have been filter-feeders was based on the presence of vertical, regularly spaced, pillar-like structures observed on the medial surface of preserved rhamphotheca (Norell et al. 2001). These authors suggested that these vertical structures were similar to the flexible lamellae present in waterfowl, used by them for straining food from water. However, as was argued by Barrett (2005), the structures in the ornithomimids appear to have been stiff columns that are merely part of the rhamphothecal architecture: indeed, very similar âvertical pillarsâ are seen on the rhamphothecal tissue of modern turtles and fossil hadrosaurs (see figures in Barrett 2005). Waterfowl lamellae are not just pillar-like structures on the rhamphothecae: theyâre flexible and connected only to the rhamphothecae at their base (though, some waterfowl â notably, geese â have stiffened and partially fused the lamellae).

Refs - -

Barrett, P. M. 2005. The diet of ostrich dinosaurs (Theropoda: Ornithomimosauria). Palaeontology 48, 347-358.

Huene, F. von 1936. Henodus chelyops, ein neuer Placodontier. Palaeontographica A 84, 99-148.

Norell, M. A., Makovicky, P. & Currie, P. J. 2001. The beaks of ostrich dinosaurs. Nature 412, 873-874.

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

You pretty much answered my question in your comment, Darren; thanks. But now I have another:

Waterfowl lamellae are not just pillar-like structures on the rhamphothecae: theyâre flexible and connected only to the rhamphothecae at their base (though, some waterfowl â notably, geese â have stiffened and partially fused the lamellae).

What about flamingo lamellae?

By happy coincidence, I have a flamingo skull with me here now. The lamellae are not all that different from those of waterfowl (which is partly why some authors have proposed a phylogenetic link between waterfowl and flamingos): in the upper jaw, they're regularly spaced bristle-like structures that extend ventrally from the jaw margins. These are the marginal lamellae (about 4 mm tall anteriorly, separated from each other by gaps of about 1 mm): they're lateral to larger, blade-like structures (inner maxillary lamellae) covered in tiny, hair-like structures that extend transversely across the whole of the palatal surface. Structures similar to the inner maxillary lamellae - the mandibular lamellae - are present in the lower jaw. For loads more detail, do check out...

Zweers, G., de Jong, F, Berkhoudt, H. & Vanden Berge, J. C. 1995. Filter feeding in flamingos (Phoenicopterus ruber). The Condor 97, 297-324.

I thought the "denticles" on the premaxilla were real teeth? I'll need to dig up my copy of Reif & Stein (1999)... which may be physically impossible...

other Cretaceous filter feeders such as Pterodaustro, pachycormids, and Megachasma.

Megachasma, the megamouth shark, is extant.

Archosaurs are a hell of a lot closer to diapsids than cetaceans...

Archosaurs are diapsids.

By David MarjanoviÄ (not verified) on 05 Nov 2010 #permalink

I like turtles.

David:

Megachasma, the megamouth shark, is extant.

Shimada (2007) described a fossil Megachasma from the Late Cretaceous of Colorado:

Shimada, K. 2007. Mesozoic origin for megamouth shark (Lamniformes: Megachasmidae). Journal of Vertebrate Paleontology 27, 512-516.

Very interesting. I can´t believe I don´t even have a photo of the specimen from Tübingen despite the fact that I have already looked at it so many times...It is really exceptionally well preserved, and even without any ideas about filter-feeding behavior, it already looks extremely grotesque.
BTW, the name of the location where it was found is Lustnau, not Lüstnau.

"Ah yes, you mean this Burian picture. Quite a few people, I suppose, have a 'WTF?' moment the first time they see a picture of Henodus..."

Indeed yes! I was looking for that picture to accompany my post, but couldn't find it.

I also know it from Burian's painting buty didn't know it was a filter feeder.

Darren,

Thanks for the clarification. That is much better evidence for some sort of soft-tissue structure. Just too bad it was mostly prepped away!!

Few people realize it, but Gamera is an overgrown descendant of Henodus, not a turtle.

Howard - have you demonstrated this with a phylogenetic analysis?

By Bradley F (not verified) on 05 Nov 2010 #permalink

I find the similarities to the shells of Henodus (which is totally unlike other placodonts) and turtles to be striking. Has anybody theorized on the evolutionary pathway from more basal placodonts to Henodus, and what might that say about how true turtles developed their shell?

I know that turtle relationships are always up in the air, but what are the chances that they share some close ancestry with placodonts?

By Zach Miller (not verified) on 05 Nov 2010 #permalink

David,

That's exactly my point (albeit not stated well) - that those structures are already documented in other diapsids (and gasp! things other than cetaceans).

And Megachasma, as mentioned above, is now known from the Cretaceous.

I know that turtle relationships are always up in the air, but what are the chances that they share some close ancestry with placodonts?

Can't be very close. The sister-group relationship of Placodontia and Sauropterygia is beyond reasonable doubt, and the most turtle-like armour is limited to the most highly nested placodonts -- Paraplacodus and Placodus lack armour.

What's more, the plastron of turtles appears to contain the gastralia, while that of placodonts (where present) lies external to the gastralia; the carapace of turtles touches the ribs directly, while the osteoderms of placodonts aren't connected to the ribs; and placodont armour consists of lots and lots of separate osteoderms, while in turtles there's little more than one plate per rib.

Placodonts and sauropterygians together (Euryapsida) could be the sister-group of the turtles, but I like the resurrected Eunotosaurus hypothesis a lot better.

By David MarjanoviÄ (not verified) on 06 Nov 2010 #permalink

There are other differences between turtle and placodont armour. In placodonts, each osteoderm was covered by one epidermal scale; in turtles, they don't line up at all (this can be seen in fossils, too, because the margins of the epidermal scales leave very well visible grooves on the bones).

By David MarjanoviÄ (not verified) on 06 Nov 2010 #permalink

"By happy coincidence, I have a flamingo skull with me here now." - as one does - :)

By Mark Lees (not verified) on 06 Nov 2010 #permalink

It is probably just the effect of the picture, but I am reminded a bit of Manta rays. As many more standard rays feed ona similar kind of shellfish diet to 'standard' placodionts, could it be that henodus was a kind of reptilian version of a Manta ray? Obviously with a different feeding method though.

Alan - the superficial similarity that cyamodontoid (= armoured, turtle-like) placodonts have with rays has been mentioned on several occasions. In fact, some workers have suggested that these placodonts were convergent with eagle rays. Others have argued that this supposed resemblance is merely superficial and misleading: placodonts have stiff bodies and couldn't bury themselves in the substrate as the rays do, plus they had to make regular trips to the surface etc. (ignoring for now the possibility of arsegills). I don't think a manta-like lifestyle is plausible, in part because Henodus is unlikely to have been a continuous, efficient cruiser in open water (it was probably clumsy, and like all placodonts it was negatively buoyant).

While I'm here... I said above that Henodus lacks supratemporal fenestrae. I forgot that it's polymorphic for this character, and that some individuals possess the fenestrae while others lack them.

In fact, some workers have suggested that these placodonts were convergent with eagle rays.

The end-Triassic placodont Psephoderma alpinum was remarkably similar to rays in size, shape and probably lifestyle. This goes so far that propulsion seems to have come from the long, strong hindlimbs, not the long but thin and stiff tail.

(ignoring for now the possibility of arsegills)

X-)

By David MarjanoviÄ (not verified) on 08 Nov 2010 #permalink

How buoyant was it? I had always heard that gray whales spent at least some time grubbing along the bottom in order to filter tasty things out of disturbed mud. Could this beast have done something like that?

By Jenny Islander (not verified) on 08 Nov 2010 #permalink

Armor, ballast, camouflage, anti-dehydration could have been shared functions; jellyfish snacks & algae salad(cf marine iguana) and small crustaceans and detritus flotsam might have been typical diet. The flat body may have related to shallow water and slow hydrodynamics. Large nares might give an advantage in both submerged-body breathing and vocalization?
Golden jellyfish at marine lake/lagoon:
http://www.wired.com/wiredscience/2010/11/on-a-hoof-a-wing-and-a-fin-na…

It seems to have parallels with the Galapagos marine iguana, the snout flat/squared-off. While the iguana clings to slippery rock in tidal currents with long sharp claws, this beast may have been so heavily ballasted that it didn't need to. Note the iguanas's nasal salt sneezing
from eating salty food, maybe a parallel to the large nares? Could there have been a temperature-valvable vascularised 'solar panel' tissue layer in between the osteoderms and the external shell, for warmth between dives?

http://en.wikipedia.org/wiki/Marine_iguana

The osteoderms are the external shell... the horn layer of the epidermis lay directly atop them.

By David MarjanoviÄ (not verified) on 13 Nov 2010 #permalink