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

 

 

 

Thoracic transverse processes – here and there

In reptiles sometimes the dorsal (thoracic) vertebrae develop elongate transverse processes (Fig. 1). The phylogenetic pattern of these appearances is today’s topic, inspired by Hirasawa 2013.

Note (Fig. 1) that the turtle-like enaliosaur, Sinosaurophargis has elongate thoracic transverse processes. Turtles and near-turtles, like Odontochelys, don’t. So why did Hirasawa et al. (2013) add Sinosaurosphargis to their turtle family tree? Were they influenced by the convergent carapace?

The large reptile tree found the two clades (turtles and saurosphargids) were not related. Turtles nested with the new lepidosauromorphs, while saurosphargids nested with the new archosauromorphs.

from Hirasawa et al. 2013, pink arrow points to elongate transverse processes on Sinosaurosphargis. These are not present on Odontochelys and turtles.

Figure 1. from Hirasawa et al. 2013, pink arrow points to elongate transverse processes on Sinosaurosphargis. These are not present on Odontochelys and turtles. We’ll look at where in the tree such processes do appear  by convergence. 

The appearance of thoracic transverse processes within the Reptilia
You’ll recall that reptiles are essentially diphyletic. We’ll start with one of these clades, then look at the other in the large reptile tree.

The pattern of appearance within the new Lepidosauromorpha
The first appearance of transverse processes in the new Lepidosauromorpha is at the Kuehneosauridae, the gliding reptiles of the Permian to Cretaceous.

The only other clade is the Fenestrasauria (including the Pterosauria) of the Triassic to Cretaceous.

So, no turtles or near-turtles have elongate transverse processes.

The pattern of appearance within the new Archosauromorpha
The entire Synapsida develop elongate transverse processes in the new Archosauromorpha.

The next appearance includes the turtle-like basal enaliosaurs, Sinosaurosphargis (Fig. 1) + Largocephalosaurus.

Eusaurosphargis alone among thalattosaurs develops elongate transverse processes. It also has a wide, flattened torso, but gracile ribs.

Placodonts have elongate transverse processes. So do plesiosaurs, but not nothosaurs or pachypleurosaurs.

The pararchosauriforms from Doswellia to Tropidosuchus all have elongate transverse processes.  (Does Lagerpeton follow this pattern?)

Basal euarchosauriforms up to and including rauisuchids do not have elongate transverse processes. Derived rauisuchia from Yarasuchus and Ticinosuchus through all crocs and dinos (including birds and poposaurs) do have elongate dorsal transverse processes.

Pattern?
Wide flat taxa tend to have elongate transverse processes, whether they are trying to increase their width to glide or to flatten out on the ground or underwater. Even so, many flattened taxa do not have elongate transverse processes.

The stiffening of the torso (less undulating) appears to be the second reason, seen in synapsids, fenestrasaurs, pararchosauriforms and derived rauisuchians.

References
Hirasawa T, Nagashima H and Kuratani S 2013. The endoskeletal origin of the turtle carapace. Nature Communications 4:2107. online here.

Cleaning up mistakes – Sinosaurosphargis now nests (almost) in the Enaliosauria

Sinosaurosphargis.

Figure 1. Sinosaurosphargis. Click for more information. Like other enaliosaurs, the naris is high, the humerus is bent and the transverse processes  are elongated, anchors for laterally-directed ribs beneath a turtle-like shell.

Sinosaurosphargis, and its sister, Largocephalosaurus, are two more turtle-like forms I earlier nested with basal enaliosaurs, close to Claudiosaurus. That was correct. Nestings with basal placodonts were incorrect. Largocephalosaurus is better known now.

This nesting means the shell of Sinosaurosphargis was derived independently, convergent with those of turtles, Henodus and Cyamodus. None of these are homologous structures! Thankfully PAUP can see through such convergence.

Since the outgroups of Sinosaurosphargis are all slender carnivorous or piscivorous speedsters we should expect to see more variation in this already very wide radiation at the moment these reptiles returned to the water. When the differences are this great, the number of intervening or transitional taxa is great, all undiscovered at this point. Look for more basal enaliosaurs to fill this gap, probably buried along with the sediments of the ancient Tethys Sea.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Li C, Rieppel O, Wu X-C, Zhao L-J and Wang LT 2011. A new Triassic marine reptile from southwestern China. Journal of Vertebrate Paleontology 31 (2): 303-312. doi:10.1080/02724634.2011.550368.
Cheng L, Chen X-H, Zeng X-W and Ca Y-J 2012. A new eosauropterygian (Diapsida: Sauropterygia) from the Middle Triassic of Luoping, Yunnan Province. Journal of Earth Science 23 (1): 33-40.

wiki/Sinosaurosphargis

wiki/Largocephalosaurus

 

What is Omphalosaurus? A sister to Largocephalosaurus.

Updated October 28, 2016 with a nesting of Omphalosaurus as a sister to Largocephalosaurus, an aquatic diapsid, basal to enaliosauria. 

Omphalosaurus nevadanus is a formerly enigmatic marine reptile.
Known from only a few scraps (ribs, vertebrae) plus a few skull pieces that few paleontologists can confidently identify (and some very strange ones they can!), Omphalosaurus has baffled workers for over a hundred years. Considered “aberrant and fragmentary” by Maisch (2010), Omphalosaurus has been nested in and out of the Ichthyopterygia because there are no closely related sister taxa. It really was ‘out there,’ pretty much all alone. Here in the large reptile tree (subset Fig. 4) Omphalosaurus nests with Largocephalosaurus outside the base of the Enaliosauria.

Omphalosaurus palate with elements colorized.

Figure 1. Omphalosaurus palate with elements colorized. Note the huge expanded splenials from Motani 2000. The palate is broad like that in Claudiosaurus, but toothless.

Meriam (1906, 1908, 1911) erected the genus based on a fragmentary skull with two associated vertebrae from the middle Triassic of Nevada. He considered it a distinct sort of reptile possibly related to placodonts or rhynchosaurs based on the button-like and disordered dentition.

Shortly thereafter, Wiman (1910) described similar teeth form the Lower Triassic of Spitzbergen. The postcranial bones included discoidal vertebrae and humeri that resembled those of the ichthyosaur, Shastasaurus. Wiman (1910) erected a new genus and three species based on fragmentary remains.

Later papers by both workers split the fossil generically, considering the vertebrae ichthyopterygian, and the dentition something else. More recent papers lumped Omphalosaurus with the ichthyosaur, Grippia, typically without citing reasons for doing so.

Mazin (1983) stated reasons for including Omphalosaurus within the Ichthyopterygia. Tichy (1995) described a more complete specimen from Austria. Sander and Faber (1998) added data to this enigma.

The holotype (UCMP 8121, University of California Museum of Paleontology, Berkeley) is difficult to interpret, according to Motani (2000). The key feature of Omphalosaurus is the presence of rounded teeth that do not form a single tooth row, but are established, almost randomly, along the premaxilla and dentary. The maxilla (Fig. 2) appears edentulous. The extent of the splenials is also unmatched, including an elongated symphysis. Motani (2004) concluded that the Wiman (1911) Spitzbergen material consisted of several distinct genera.

Motani (2004) nested Omphalosaurus outside of the Ichthyopterygia, more basal than Utatsusaurus. This is always a problem nesting enigmas at the base of any clade without including further outgroups.

Omphalosaurus nevadanus,

Figure 2. Omphalosaurus nevadanus, MBG 1500, the Austria specimen, skull portion in situ, mandible parts, skull parts and reconstruction with matching colors based on Sinosaurosphargis, the only other taxon with such a large set of splenials. The premaxillae are wider than the dentaries. Distinct from Sinosaurosphargis, the premaxillae were much enlarged. Sander and Faber (2003) extended the splenials to the tips of the jaws, but this evidence suggests they did not extend as far forward as the dentaries.

The great extent of the splenial is a key trait
when looking for the closest known sister taxon. Fortunately this trait is extremely rare, virtually unknown elsewhere within the Reptilia. Look for any other marine taxa that has a greatly extended/expanded splenial and you are left with just two taxa that were not previously considered in any discussions of Omphalosaurus.

Largocephalosaurus nests between long-necked tiny Claudiosaurus and short-necked turtle-like Sinosaurosphargis. These represent an entirely new clade of marine reptiles.

Figure 3 Largocephalosaurus nests between long-necked tiny Claudiosaurus and short-necked turtle-like Sinosaurosphargis. These represent an entirely new clade of marine reptiles.

While
Sinosaurosphargis is clearly greatly derived with a turtle-like shell and and Largocephalosaurus is not so derived, these are the only other taxa with an extensive set of splenials that form a symphysis anteriorly, brief though it may be. The teeth are not so derived. The ribs were not so derived. Even so, the splenials were similar. So were other skull bones, like the much greater width of the premaxillae compared to the dentaries. And when you’re dealing with an enigma like Omphalosaurus, so different from any other fossil currently known, you grasp at straws. Others have not attempted to match these taxa to Omphalosaurus.

A phylogenetic analysis of Omphalosaurus (Fig. 4), agrees with Motani (2004) who nested it outside the Ichthyopterygia. Way outside. It is indeed a sister to Sinosaurosphargis and Largocephalosaurus, as it appears based on the extent of the splenials, then Omphalosaurus nested just outside the Enaliosauria derived from a sister to Hovasaurus and Claudiosaurus. That means it probably had a similar elongated post-cranial morphology. There are no indications of the unusually elongated transverse processes and carapace that characterize Largocephalosaurus and Sinosaurosphargis. Even so, Omphalosaurus and its close sisters form a completely distinct clade apart from the Enaliosauria.

Figure 3. Aquatic younginiform subset of the LRT demonstrating relationships within the Enaliosauria (=Sauropterygia + Ichthyosauria)

Figure 3. Aquatic younginiform subset of the LRT demonstrating relationships within the Enaliosauria (=Sauropterygia + Ichthyosauria)

References
Maisch, M 2010. Phylogeny, systematics, and origin of the Ichthyosauria – the state of the art. Palaeodiversiry 3: 151-214. (Full text PDF)
Merriam JC 1906. Preliminary note on a new marine reptile from the Middle Triassic of Nevada. University of California Publications, Bulletin of the Department of Geology 5:71–79.
Merriam JC 1908. Triassic Ichthyosauria, with special reference to the American forms. Memoirs of the University of California 1:1–196.
Merriam JC 1911. Notes on the relationships of the marine saurian fauna described from the Triassic of Spitzbergen by Wiman. University of California Publications, Bulletin of the Department of Geology 6:317–327.
Merriam JC and H. C. Bryant. 1911. Notes on the dentition of Omphalosaurus.University of California Publications, Bulletin of the Department
of Geology 6:329–332.
Motani R 2000. Is Omphalosaurus ichthyopterygian? — A phylogenetic perspective. Journal of Vertebrate Paleontology 20(2): 295-301.
Sander PM and Faber C 1998. New fi nds of Omphalosaurus and a review of Triassic ichthyosaur paleobiogeography. – Paläontologische Zeitschrift, 72: 149–162.
Sander PM and Faber C 2003. The Triassic marine reptile Omphalosaurus: Osteology, jaw anatomy, and evidence for ichthyosaurian affi nities. – Journal of Vertebrate Paleontology, 23: 799–816.
Wiman C. 1910. Ichthyosaurier aus der Trias Spitzbergens. – Bulletin of the Geological Institution of the University of Upsala, 10: 125–148.

wiki/Omphalosaurus