Atopodentatus: fixing a mistake and Science marches on

Two years ago, when
Atopodentatus unicus was first described (Cheng et al. 2014) it (WIGM SPC V1107) was thought to have an odd downturned rostrum with medially facing premaxillae. That very odd (autapomorphic) interpretation was widely accepted.

Recently
Chun et al. 2016 found more fossils (IVPP V20291, IVPP V20292) that showed the crushing had introduced an illusion in the first Atopodentatus. The jaws were actually wide and flat, not deep (Fig. 2).

Figure 1. DGS tracings of the second and third specimens of Atopodentatus in several views.

Figure 1. DGS tracings of the second and third specimens of Atopodentatus in several views. Note the foramina in the nasals, a likely location for the salt gland. The mandibles were both rotated axially so that the glenoid/jaw joint was found on the former medial surface.

Chun et al. 2016 reported,
“The evidence indicates a novel feeding mechanism wherein the chisel-shaped teeth were used to scrape algae off the substrate, and the plant matter that was loosened was filtered from the water column through the more posteriorly positioned tooth mesh. This is the oldest record of herbivory within marine reptiles.”

Maybe — maybe not.
To me those pmx teeth look like mud/sand rakes/sievers. And IF so, Atopodentatus was seeking burrowing organisms that could be filtered by the posterior jaws. But on the other hand, we have a living analog…

Like a modern marine iguana?
This YouTube video shows a marine iguana grazing on algae coating the underwater rocks surrounding the Galapagos. Note the narrow-snouted iguana twists its head and uses its cheek teeth on the flat part of its face to nip the plants off the rocks. The video describes those teeth as ‘razor sharp’, but they are not. They are tricuspid, acting like rakes. In a way the duplicate having three times as many single-cusp teeth.

Figure x. Marine iguana teeth are tricuspid.

Figure x. Marine iguana teeth are tricuspid

Unfortunately
neither Cheng et al. nor Chun et al were able to decide what sort of marine reptile Atopodentatus was. Here, in the large reptile tree, even with the new changes to the skull, Atopodentatus nests outside the Sauropterygidae, as a marine younginiform, more derived than Claudiosaurus, nesting with the turtle-like Sinosaurosphargis and the longer but still wide, Largocephalosaurus (Fig. 2). Reconstructions help.

Figure 2. Atopodentatus nests with two other pre-sauropterygian marine younginforms, Sinosaurosphargis and Largocephalosaurus.

Figure 2. Atopodentatus nests with two other pre-sauropterygian marine younginforms, Sinosaurosphargis and Largocephalosaurus. Note the narrower postorbital skull compared to these sisters.

These three were bottom feeders. 
All were from the Middle Triassic. Onlly Atopodentatus had a vertical quadrate. Only Atopdentatus had a hole in its skull, presumably for a salt gland.

Some bones missed by Chun et al. 2016 include:

  1. supratemporals
  2. postparietals
  3. tabulars
  4. quadratojugals
  5. and the large lacrimals.
Figure 3. The two IVPP specimens of Atopodentatus both show the wide premaxillae ideals for scraping and raking. The pterygoids had a shagreen of tiny teeth. The maxillae also have filtering needle-like teeth. Note the presence of the lacrimal and supratemporal missed by first hand observation.

Figure 3. The two IVPP specimens of Atopodentatus both show the wide premaxillae ideals for scraping and raking. The pterygoids had a shagreen of tiny teeth. The maxillae also have filtering needle-like teeth. Note the presence of the lacrimal and supratemporal missed by first hand observation. Is that a salt gland location posterior to the nasals?

Was this a blunder? Or an honest mistake?
Let’s be professional about this and call it an honest mistake. And notice there is no reason to be embarrassed by such mistakes. We fix them and move on. Take these examples:

  1. Elasmosaurus had the head on the wrong end originally.
  2. Yi qi was thought to have an extra long wrist bone that turned out to be a displaced ulna on one side, a radius on the other side.
  3. Sordes was thought to have deep chord wings and a uropatagium between the legs, not including the tail.
  4. Longisquama was thought to have short hind limbs.
  5. The nesting of Vancleavea with archosauriformes.
  6. Or pterosaurs with dinosaurs and Scleromochlus.
  7. The wrong skull and dragging tail for Brontosaurus.
  8. … and all the little boo-boos that creep into everyone’s matrices (including yours truly)

Note that
one author on the original paper (Cheng L) is also an author on the new paper. Here’s how the new set of authors handled the prior mistake from the abstract, “The skull displays a pronounced hammerhead shape that was hitherto unknown.”

And from the text:
“Atopodentatus unicus was originally described as a putative sauropterygian filter feeder with a downturned rostrum, supposedly used to stir up invertebrates in soft sediment in a flamingo-like manner. Here, we describe two new specimens… that require a very different interpretation of skull morphology and provide evidence for an even more remarkable feeding strategy. The new specimens clearly demonstrate that rather than being downturned, the rostrum was developed into a “hammerhead” with pronounced lateral processes formed by the premaxillae and maxillae in the upper jaw and mirrored by the dentary in the lower jaw.”

That’s a nice way to do it. Don’t you agree?
We can all take a lesson from this.

While we’re on the subject of filter feeding marine reptiles…
Let’s not forget the oddly toothed thallattosaur, Helveticosaurus (Fig. 4, Middle Triassic). Helveticosaurus had such long cheek teeth they could not have been used for chewing or stabbing. Instead they look like baleen strainers. Helveticosaurus had long fangs anteriorly, perhaps useful for scraping rather than stabbing.

Figure 4. Helveticosaurus had cheek teeth that look like baleen strainers and long fangs anteriorly.

Figure 4. Helveticosaurus had cheek teeth that look like baleen strainers and long fangs anteriorly.

The proximal outgroup taxon for Atopodentatus
is the Late Permian marine younginiform, Adelosaurus, which doesn’t have any obvious marine traits. The skull is also unknown. And the phylogenetic difference between the Late Permian and Middle Triassic taxa are obvious (Fig. 5).

Figure 5. Atopodentatus compared to more primitive sister taxa, Adelosaurus and Claudiosaurus.

Figure 5. Atopodentatus compared to more primitive sister taxa, Adelosaurus and Claudiosaurus to scale.

References
Cheng L, Chen XH, Shang QH and Wu XC 2014. A new marine reptile from the Triassic of China, with a highly specialized feeding adaptation. Naturwissenschaften. doi:10.1007/s00114-014-1148-4.
Chun L, Rieppel O, Cheng L and Fraser NC 2016. The earliest herbivorous marine reptile and its remarkable jaw apparatus. Science Advances 06 May 2016: 2(5), e1501659
DOI: 10.1126/sciadv.1501659

 

 

 

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