What is Triopticus? (It’s not what they think it is…)

riIt’s been a long time
since an interesting ‘reptile’ showed up in the literature. Especially an enigma like this one.

A recent paper by Stocker et al. 2016
reports on a domed and expanded Late Triassic cranium that they identify as an archosaur, but it’s unlike that of any other archosaur. Triopticus primus was named for its three eyes, with a big one on top (Fig. 1). The authors compared the domed appearance of the cranium in Triopticus with a Cretaceous dome-headed ornithischian dinosaur, Stegoceras.  They also discussed convergence in general and provided a CT scan brain endocast of Triopticus.

Unfortunately 
the authors employed a prior cladogram (Fig. 2) by Nesbit et al. 2015, expanded from Pritchard et al. 2015. that was shown to not recover sister taxa that looked alike and did not provide a gradual accumulation of derived traits at several nodes. In their cladogram Triopticus nested without resolution among basal archosauriforms like Proterosuchus, which looks nothing like it. By contrast, the LRT was able nest and fully resolve Triopticus elsewhere.

Figure 1. In round 1 I added characters shown here to the LRT in two passes. One recovered a sisterhood with mesosaurs. The other nested with Tanytrachelos, among the tanystropheid tritosaur lepidosaurs. Both shown here for comparison.

Figure 1. In round 1 I added characters shown here to the LRT in two passes. One recovered a sisterhood with mesosaurs. The other nested with Tanytrachelos, among the tanystropheid tritosaur lepidosaurs. Both shown here for comparison. Triopticus would be 2x the size of the giant Tanyrachelos from New Mexico.

From the Stocker et al. abstract:  “Exemplifying this extreme morphological convergence, we present here a new dome-headed taxon from the assemblage, which further illustrates the extraordinary range of morphological disparity present early in the Late Triassic.” That ‘extraordinary range’ should be — and will be — chopped down substantially with the right sister taxa.

A few problems with the archosauriform hypothesis include:

  1. No other archosauriforms, until you get to pachycelphalosaurs in the Cretaceous, expand the cranium deleting the upper temporal fenestra.
  2. The entire rostrum and mandible is absent, so no naris, antorbital fenestra or teeth are known, even in part.
  3. They dubiously identified an antorbital fenestra and fossa at the edge of the fossil.
  4. And they were not aware that Tanytrachelos and kin, including pterosaurs within the – Lepidosauria -, also have an antorbital fenestra, but without a fossa.
  5. A large pineal opening is present, but never present at such a size in archosauriforms.
  6. Large and expanded supratemporals (overlooked originally) are present.
  7. The extreme angle of the rostrum coupled with the large orbit are traits not found in basal archosauriforms that typically have a long boxy rostrum.
Figure 2. Stocker et al. 2106 cladogram nesting Triopticus uncertainly within a set of unresolved basal archosauriforms. The LRT completely resolves that node.

Figure 2. Stocker et al. 2106 cladogram nesting Triopticus uncertainly within a set of unresolved basal archosauriforms and far from the Tanystropheidae. The LRT completely resolves all nodes. Note how this cladogram mixes Lepidosauromorpha with Archosauromorpha and separates the protorosaurus, Protorosaurus and Prolacerta.

This is a perfect problem
for the large reptile tree (LRT) which now provides 820 (chart needs to be updated from 812) opportunities for Triopticus to nest in. With that large number of taxa, unfortunately I had to split the matrix in two, even for a simple Heuristic Search. By contrast, the Stocker et al. matrix included 30 taxa and 247 characters.

Stocker et al. report,
“We chose this dataset because the following combination of character states in Triopticus are also present in some archosauromorph taxa:

  1. presence of a single occipital condyle;
  2. ossified laterosphenoid;
  3. presence of a metotic strut of the otoccipital;
  4. presence of upper and lower temporal fenestrae;
  5. presence of an antorbital fenestra and fossa formed by the lacrimal.”

They provided no reconstructions of included taxa.

First,
I tested Triopticus against basal tetrapods and the new Lepidosaurmorpha and found that Triopticus nested with the aquatic, long-necked tritosaur Tanytrachelos (Fig. 1), large specimens of which were recently found in New Mexico (Fig. 3). Like Triopticus the rostrum descends at a high angle from a tall cranium in Tanytrachelos, which also shares a large orbit and a large pineal foramen (at present known only from sister taxa). Like related fenestrasaurs and langobardisaurs, Tanytrachelos also had a small antorbital fenestra without a fossa, but that would have been beyond the rim of the broken skull in Triopticus (Fig. 5).

Figure 2. A large incomplete Tanytrachelos from New Mexico compared to the smaller more complete East Coast specimen. Triopticus would be twice as large as the New Mexico specimen.

Figure 3. A large incomplete Tanytrachelos from New Mexico compared to the smaller more complete East Coast specimen. Triopticus would be twice as large as the New Mexico specimen.

Second,
I tested Triopticus with the rest of the matrix, the new Archosauromorpha, and found that Triopticus nested with the mesosaurs (Fig. 4), an aquatic enaliosaur clade close to thalattosaurs and ichthyosaurs, all derived from basal pachypleurosaurs. It did not nest with archosauriforms. While basal mesosaurs have typical diapsid temporal regions, Mesosaurus, like Triopticus, closes up the upper temporal fenestra, then the lateral temporal fenestra with bone expansion.  Mesosaurs also retain a relatively large pineal foramen and have large eyes, but they don’t have a sharply descending preorbital region.

Mesosaurus

Figure 4. Mesosaurus, like Triopticus, has a large pineal foramen and expands the skull bones to obliterate former temporal fenestrae.

Digital Graphic Segregation
was applied to the cranial lump that is Triopticus (Fig. 5) and the skull suture patterns, perhaps overlooked by those with firsthand access due to the expansion of the cranial bones, revealed a Tanytrachelos-like morphology (Fig. 2). I illustrate this interpretation here with the hope that this hypothesis can be either confirmed or falsified. This is a tough assignment.

Figure 4. Triopticus reconstructed along the bauplan of Tanytrachelos. Note the zone of weakness where the fossil split appears to be at the top of the narial/oral opening. The lateral temporal fenestra is the top half of a divided LTF. The UTF appears to be sealed over by expansion of the frontal over the postorbital. As in Tanytrachelos, the lacrimal has a dorsal process over the orbit separating the maxilla from the prefrontal.

Figure 5. Triopticus reconstructed along the bauplan of Tanytrachelos. Note the zone of weakness where the fossil split appears to be at the top of the narial/oral opening. The lateral temporal fenestra is the top half of a divided LTF. The UTF appears to be sealed over by expansion of the frontal over the postorbital. As in Tanytrachelos, the lacrimal has a dorsal process over the orbit separating the maxilla from the prefrontal. At 72 dpi this is 90 percent of actual size.

Tanystropheids
have been reported from the Hayden Quarry of northern New Mexico (Chinle Formation) far from the west central Texas location of Otis Chalk. Stocker et al. included Tanytrachelos in their study, even though they have not provided a reconstruction of it, so it is difficult to imagine how they interpreted it. Tanystropheids, in general, have widely varying skull shapes. Triopticus appears to have expanded the morphospace just a little, not a lot.

The loss of ventral material in the Triopticus fossil
appears to have occurred at the roof the narial/oral opening.

So what other long-necked animal
expands the cranium like Triopticus? Giraffa, the giraffe. Maybe it will turn out to be a better analogy than short-necked Stegoceras?

References
Nesbitt SJ, Flynn JJ, Pritchard AC, Parrish JM, Ranivoharimanana L and Wyss AR 2015. Postcranial osteology of Azendohsaurus madagaskarensis (?Middle to Upper Triassic, Isalo Group, Madagascar) and its systematic position among stem archosaur reptiles. Bulletin of the American Museum of Natural History 899, 1-125.
Pritchard AC, Turner AH, Nesbitt SJ, Irmis RB and Smith ND 2015. Late Triassic tanystropheid (Reptilia, Archosauromorpha) remains from northern New Mexico (Petrified Forest Member, Chinle Formation): insights into distribution, morphology, and paleoecology of Tanystropheidae. Journal of Vertebrate Paleontology, 10.1080/02724634.02722014.02911186.
Stocker MR, NesbittSJ, Criswell KE, Parker WG, Witmer LM, Rowe TB, Ridgely R  Brown MA 2016. A Dome-Headed Stem Archosaur Exemplifies Convergence among Dinosaurs and Their Distant Relatives. Current Biology (advance online publication)DOI: http://dx.doi.org/10.1016/j.cub.2016.07.066   pdf

10 thoughts on “What is Triopticus? (It’s not what they think it is…)

  1. “the authors employed a prior cladogram (Fig. 2) by Nesbit et al. 2015, expanded from Pritchard et al. 2015. that was shown to not recover sister taxa that looked alike and did not provide a gradual accumulation of derived traits at several nodes.”

    You’ve yet to say why we should expect a gradual accumulation of traits, when evolutionary biologists are in agreement there will be variation in the rate of accumulation. Nor have you tested how gradually your tree accumulates traits, or Nesbitt et al.’s. These have become buzzwords that you use to justify your results, when you’ve never shown why reality should be that way or objectively shown your tree even excels in that way compared to others’ trees.

    “With that large number of taxa, unfortunately I had to split the matrix in two, even for a simple Heuristic Search.”

    Looks like it’s time for you to learn TNT.
    In the meantime, just use your full matrix in PAUP and constrain Triopticus as a mesosaur in one run, and as a tanystropheid in another, and see which MPTs are shorter.

    • @Mickey Mortimer: I find it weird that people keep recovering Triopticus as a tanystropheid or archaeosuchian-grade archosauriform, when the lacrimal and orbit orientation remind me more of primitive Allokotosauria like Pamelaria. But who knows, maybe it represents a completely novel group of archosaurs.

      • Well, it’s only been included in two analyses, so to say “people keep recovering” it as something is misleading. Also, Pamelaria is between tanystropheids and archosauriforms in the consensus, so Triopticus’ placement so far makes sense in your view. I’d like to see it added to Ezcurra’s (2016) archosauromorph matrix…

  2. A gradual accumulation of traits = your kids look like you, but not exactly. Sure there is variation in the rate, but between taxa, time is not a factor. We’re still looking for a minimum of change between sisters, which is what PAUP looks for and I’m preaching to the choir. I don’t know why you ask such A-B-C basic question. The variation between taxa varies, of course. Vancleavea is really different from its closest sister, but really really different from 820 other candidates. The juvenile Chasmosaurus, not so much from the adult. I have TNT. You’ve got to be kidding…Triopticus is not a mesosaur.

    • “Gradual” implies a certain rate, so that’s a misleading way of saying “a minimum of change between sisters.” What you really mean then is that your tree has low homoplasy, but that’s only compared to the other possible trees given your codings and taxa. I would say your trees have a lot _more_ homoplasy than standard ones due to characters you don’t use, misinterpreted anatomy, etc.. Which makes your claim of low homoplasy unevidenced at best, since to show your trees actually have less homoplasy, we would need to use yours and e.g. Nesbitt’s characters together, resolve the anatomical disagreements, etc..

      And this is why I ask such an “A-B-C basic question”, because on multiple occasions you’ve defended your tree or criticized others’ trees using this claim. Like above where you state “the authors employed a prior cladogram … that was shown to not recover sister taxa that looked alike and did not provide a gradual accumulation of derived traits at several nodes.” But that tree _did_ recover low homoplasy for its dataset (which is what you say ‘provide a gradual accumulation of derived traits at several nodes’ means) and thus its sister taxa _do_ look alike. Since you haven’t shown your trees have less homoplasy, due to the issues in the paragraph above (character and scoring differences, etc.), this is a claim you can’t factually make about Nesbitt’s or anyone elses trees. So to be honest, you should stop saying it in future posts.

      • Sorry for the late reply. Have been adding taxa and correcting errors.
        Mickey, you’re so brilliant otherwise. Not sure why you’re wasting your time with this monotonous diatribe and questioning my honesty. If you want to compare competing results, let’s put the candidate sister taxa up side-by-side and see what works. Every tree ever recovers the lowest homoplasy scores. At some point, you need to look at competing reconstructions and ask, “Does this make sense?” “Is there a more parsimonious solution among included taxa?” in other words, “Did I make a scoring error or two here?” I answer that last question thousands of time every year.

  3. The CT scan data didn’t show any evidence of cranial sutures, so I doubt that DGS of just the surface provides any additional power to identify features not observable by firsthand observation or CT data. In particular, what you identified as the nasal-frontal suture is clearly an asymmetric crack (see fig. 2L) and correctly identified as such by Stocker et al.

    How much braincase data do you have in your matrix?

  4. Sutures can be fused and still leave a mark of their presence, as in this case. If I misidentified a single suture, as you suggest, then do you think that will overturn all the other data? Let me know if so. Moreover, the simple lateral view outline of the skull, sans all sutures, provides a basis for guessing the missing parts. It will be up to critics, like yourself, to provide a new solution or to support an old solution with a similar graphic presentation. I’m happy to make the change… if warranted. The challenge is back to you. Which tetrapod skull do you think Triopticus most closely resembles? And thank you for your thoughtful comment.

    • Note here John this is a common method of David’s. He’ll code based on some uncertain data source, be it his guessed sutures, mounted material that is partially sculpted or cast from other taxa, etc., but consider that okay until specific codings are refuted by critics. Which introduces a lot of unsubstantiated data into his matrix, but he claims “In traits that are not real scores would tend to randomize, adding noise to the parsimony, not direction.” Of course most phylogeneticists would instead opt to exclude such uncertain data.

      As for the number of braincase characters Peters uses, if we exclude skull roof characters (frontal, parietal, etc.), I count eight.

  5. Sometimes eight is enough. Sometimes it is not. Remember, these are all ‘findings’. They are all up for discussion, as are the ‘findings’ of all other workers. If some workers are sacrosanct to you, give me your list and I’ll send you spoiler alerts when they arise.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s