A New Basal Placodont with Sharp Teeth! Palatodonta bleekeri

This post revises an earlier one on March 31.
A note from lead author, Dr. Neenan (see below), reminded me that I had overlooked the CT scans and encouraged me to reevaluate earlier identifications. In science mistakes are corrected as they are here. 

It has sharp teeth,
but  Palatodonta bleekeri (Neenan et al. 2013) is a basal placodont. Only the skull was described and the authors considered this a juvenile due to its small size and large orbit (but see below). This is a great find! Thanks to co-author, Torsten Scheyer, for forwarding the paper and its supplementary materials.

The Enaliosauria, including Palatodonta, nested at the base of the Placodontia.

Figure 1. The Enaliosauria, including Palatodonta, nested at the base of the Placodontia in the large reptile tree. With a greater number of taxa, this tree differs from that offered by Neenan et al. (2012), but the nesting of Palatodonta at the base of the Plaocodontia is the same.

The original reconstruction is very good.
However, according to Neenan et al. (2013, Fig. 2)) the lower temporal arch is missing. Digital Graphic Segregation (DGS) has helped reveal a little bit more.

The other parietal is the other pterygoid and the QJ is not missing
The disarticulated “parietal” (Fig. 1) does not match the articulated one, but exactly matches the CG pterygoid.

The identified squamosal also includes the quadratojugal (Fig. 3). The top of the identified quadrate includes a descending flange of the squamosal. The unidentified bone (“?”) is the other quadrate. The quadratojugal is the same morphology as in StereosternumMesosaurus and Claudiosaurus. In Claudisaurus the quadratojugal was not recognized  by Carroll (1981). but it is present.

Figure 1. Palatodonta in situ. Only the skull has been published. This is a basal placodont and perhaps a juvenile. Above, in situ fossil. Middle a CT scan to reveal hidden elements. Below, reconstruction by Neenan et al. (2013). Scale bar = 3mm. See new reconstruction below (Fig. 3).

Figure 2. Palatodonta in situ. Only the skull has been published. This is a basal placodont and perhaps a juvenile. Above, in situ fossil. Middle a CT scan to reveal hidden elements. Below, reconstruction by Neenan et al. (2013). Scale bar = 3mm. See new reconstruction below (Fig. 3).

Key postorbital bones color-coded. Jaw dorsal outline with black line.

Figure 3. Key postorbital bones color-coded and reidentified. Jaw dorsal outline with black line. Quadrates in red. Squamosal in green. Quadratojugal in brown. Jugal in blue. Lacrimal in pink. Postfrontal in orange. Postorbital in aqua. Parietal in pink. Supratemporal in purple.

The nasals
The large purported ‘nasal’ identified by Neenan et al. (2013) appears to be the top of the broken maxilla, but the actual nasal appears to be peeking out from below the upper side. Descendant taxa, like Paraplacodus and Henodus, have a taller maxilla and a smaller nasal to match this morphology. However, ancestral taxa, like Claudiosaurus, have a larger prefrontal and I would not be surprised if this triangular bone were a broken anterior flange of the prefrontal as indicated in the original reconstruction (Fig. 1). Some descendant taxa, like Henodus and Placodus, reduce the premaxillary ascending process. Others, like Paraplacodus (Fig. 4), increase the size of the premaxillary ascending process, further dividing the nasals.

The supratemporal on Palatodonta is exactly where we find it on Claudiosaurus. Here it appears to be nearly completely fused to the squamosal.

Figure 4. The supratemporal on Palatodonta is exactly where we find it on Claudiosaurus. Here it appears to be nearly completely fused to the squamosal.

The lacrimal
is present and distinct from the prefrontal in Paraplacodus and Henodus. Neenan et al. (2013) insist the lacrimal is absent. I colorize what appear to be its borders here (Fig. 3). matching Paraplacodus. The lacrimal is not distinct in illustrations of Placodus, but examinations of photographs indicates some sutures below the prefrontal that indicate a small one is present.

The supratemporal
is present (Fig. 4, light green). However, it is nearly completely fused to the squamosal here and is not distinct in descendant taxa. The morphology matches that of Claudiosaurus. This brings up an age-old problem, when to score for missing (when the bone is reduced to nothing) or fused (when a smaller bone joins a larger one. losing its identity). Note, the CT scan shows some bones that are not visible on the surface.

Palatodonta reconstructed according to new tracings. The quadratojgual is brown.

Figure 5. Palatodonta reconstructed according to new tracings. The quadratojgual is brown.

The frontal
appears to be much longer than the nasal and longer than originally reconstructed in accord with all descendant taxa.

 

Palatal elements, etc.
The “unidentified” elements in the orbit include a possible ectopterygoid (brown) that rather nicely fits an open area just its size (Fig. 5). With the current palatal reconstruction, the suborbital fenestra is nearly the size of the internal naris (choana), which is atypical for related taxa.

The skull turns out to be taller than wide, which is atypical for sister taxa, except Paraplacodus (Fig. 4), its closest kin.

Figure 4. Palatodonta is closest to Paraplacodus, a much larger basal placodont.

Figure 7. Paraplacodus, a much larger basal placodont and the one closest to Palatodonta.

While derived from a sister to Claudiosaurus (Fig. 5) and closest to Paraplacodus (Fig. 4) in the large reptile tree.

Problems with the Neenan et al. (2013) reptile tree.
The published tree of Neenan et al. 2013 (Fig. 6) includes many taxa related to each other (in blue) and several others (in white) that are actually on the other branch of the reptile tree and so do not belong here. Those errant inclusions produced several “odd bedfellows” that don’t look like or share many traits with their sisters. This is a result of not having a wide gamut reptile tree, like the large reptile tree to start with, to ascertain overall relationships.

The published reptile tree by Neenan et al. (2013) showing a patchwork of unrelated reptiles and "odd bedfellows" (red dots) sharing few traits.

Figure 8. The published reptile tree by Neenan et al. (2013) showing a patchwork of unrelated reptiles and “odd bedfellows” (red dots) sharing few traits.

At the base of the marine reptiles (Fig. 7) beginning with Ichthyopterygia should be Stereosternum, a mesosaur, but it is missing here. Claudiosaurus also nests at the base of the Enaliosauria, but then you have to question how turtles, the lepidosauria and the “old” archosauromorpha nest between them. They just don’t belong, but nest in the Neenan et al. tree (Fig. 7) by default.

The use of suprageneric taxa provides further problems as the Thalattosauria do not include Helveticosaurus and Eusaurosphargis, both of which nest as thalattosaurs in the large reptile tree. Hanosaurus (Fig. 6) was shown (Rieppel 1998) to be a pachypleurosaur. It looks nothing like Eusaurosphargis.

The nesting of Rhychocephalia apart from the Rhynchosauria also speaks to the problem of suprageneric taxa. The large reptile tree, which uses specimens as taxa, unites them in a clade.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again. Thanks again Dr. Neenan, for encouraging another look at the literature.

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

References
Carroll RL 1981. Plesiosaur ancestors from the Upper Permian of Madagascar. Philosophical Transactions of the Royal Society London B 293: 315-383.
Neenan JM, Klein N and Scheyer TM 2013. European origin of placodont marine reptiles and the evolution of crushing dentition in Placodontia. Nature Communications 4:1621. – DOI: 10.1038/ncomms2633 |www.nature.com/naturecommunications.
Rieppel O 1998. The systematic status of Hanosaurus hupehensis (Reptilia, Sauropterygia) from the Triassic of China. Journal of Vertebrate Paleontology 18(3):545-557.

2 thoughts on “A New Basal Placodont with Sharp Teeth! Palatodonta bleekeri

  1. Dr. Neenan wrote: Hi I appreciate your interpretation of our paper. However, your osteological discrepancies from ours are completely erroneous. I would encourage you to follow your own advice and examine the specimen itself before criticising work which took many months of painstaking examination. Don’t forget we used CT data to confirm individial elements (see supplemental movie). There is no quadratojugal or supratemporal. The lacrimal is not missing… there isn’t one at all. And the disassociated parietal in one bone. Don’t be so quick to dismiss it out of hand and presume you know more than the experts without even being decent enough to examine the specimen first.
    James Neenan

  2. As I mentioned in my private apology, I completely overlooked the CT scan portion of the paper and made other blunders, here, hopefully corrected (the beauty of the web comes through as mistakes can be erased). Given the second chance, a better quadratojugal came to light (via nothing more than a burst of insight and playing the game of test fitting the puzzle pieces) and hopefully other discrepancies are further resolved in this updated post. Contra Dr. Neena’s assertions, I don’t presume to know more than the experts. I do provide a second opinion based on the evidence, which during the course of discussions had some value and some error, which I am more than happy to take both credit and chastisement for. I also do have the advantage of a wider gamut family tree of data to work with. Even so, our nestings were identical. Examining the specimen first hand appears to be key to the psyche of all paleontologists, but everyone, including yours truly, overlooks something sometime. In this case there was an overlooked qj on the one hand and overlooked CT scans on the other. If more mistakes are still present, I will endeavor to correct them once alerted to their presence.

    This is the process of testing. Most of our results were duplicated. It’s a fantastic and important fossil.

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