Phytosaur Skull Preparation II

I last featured this phytosaur skull undergoing preparation on February 24th and since that time more work has been done. Some of the right side of the skull has now been exposed. Preparation has been slow due to a plethora of other projects combined with a really tough sandstone matrix. Separation is poor in places and in parts the bone is impregnated with hematite, a common condition of bone encased in Chinle sandstones.

The ventral portion of the right side of the skull projects laterally a little too far suggesting that some dorsoventral crushing of the skull has occurred. So far we have exposed the lateral surface of the posterior projection of the squamosal (it is still a pseudopalatine), both frontals, the right jugal and much of the right maxilla. This picture shows the right side of the skull as currently exposed. Anterior (forward) is to the right, posterior (back) is to the left.




Preparation continues and we will follow it here until it is complete.

Phytosaur Skull Preparation

Back in November of 2008 I mentioned a new phytosaur skull collected from Petrified Forest National Park during the summer. Encased in a sandstone horizon of the Jim Camp Wash beds of the Sonsela Member (Chinle Formation), the skull is from the type locality of Pseudopalatus jablonskiae and was recovered about 100 meters from the spot where the type skull was collected. As I stated this skull is important because it may represent a second specimen of P. jablonskiae, but more complete than the type skull. Alternatively it could represent another phytosaur taxon which was a contemporary of P. jablonskiae. Because the skull is from the upper Jim Camp Wash beds, above the Jasper Forest bed it should be a pseudopalatine.

















The picture on the left shows the initial discovery. The skull is dorsal side up and beneath the rock hammer. Squamosals showing on the left [below the hammer handle] and the external nares showing on the right just below the hammer head. The picture on the right shows the beginning of the excavation. The sandstone was very well indurated and we needed a rock saw and hammers and chisels to block out the skull for removal. The resulting jacket weighed a few hundred pounds.

Late last week we cracked open the jacket and PEFO preparator Matt Brown started removing the rock. As of Tuesday morning this is what was exposed:

As you can see the upper left corner of the skull is now partially exposed. The squamosals are broad in dorsal view, thus the skull does belong to a pseudopalatine. The squamosals are more posteriorly extended than in the type of P. jablonskiae; however, this appears to be a larger specimen. Unfortunately the posterodorsal surfaces of the squamosals were what was exposed at discovery and thus were somewhat eroded. Also, the rock is much harder than we expected and the separation from the bone is not always clean. Some of the bone within the rock is actually fractured as it is softer than the surrounding matrix. Still, during excavation we located the tip of the premaxilla and both quadratojugals, so we are fairly certain that we have a complete and undistorted skull. Only time will tell. Over the next month or two I'm hoping to document the entire preparation process here and we will discover exactly what this skull is and how well it is preserved. Stay tuned.

Pseudopalatus jablonskiae

rutgerjansma answered both quickly and correctly. Mystery fossil #3 is the holotype skull of the phytosaur Pseudopalatus jablonskiae named by myself and Randall Irmis in 2006. This skull was collected from the Sonsela Member of the Chinle Formation in Petrified Forest National Park. It was lying palate side up in a path used by archaeologists to access a ruined pueblo on a cliff top. Fortunately, Pat Jablonsky (a long time volunteer at the Denver Museum of Natural History, who was working as a ranger in the park at the time) recognized the specimen in the path and brought it to my attention. Unfortunately, it was pretty badly eroded (and trampled) and only the skull roof was still in-situ. The upper portion of the brain case was able to be reassembled by collecting the float. Preparation showed that it was a pseudopalatine phytosaur; however, it differed from other known pseudopalatines in possessing very anteroposteriorly short squamosal processes (seen in the photo below projecting from the back of the skull). In addition, the squamosal tips were not pointed and 'knob-like' as in other pseudopalatines, but more like the older Leptosuchus. This was supported by a character of the braincase, the entrance of an anterior projection of the squamosal into the lateral wall, another character of Leptosuchus (Camp, 1930). Finally, there is a small fossa around the supratemporal fenestra that is only seen in this specimen (autapomorphy). These characters and discussion with Axel Hungerbuehler (probably THE expert on phytosaurs) made us realize that this specimen was unique. Unfortunately, unique was definitely the right word here, because this incomplete skull is the only known specimen of P. jabloskiae.
However, earlier this summer a trip back to the quarry resulted in the discovery of the lower portion of the braincase of the holotype specimen. It had rolled away from the exposed skull and been buried, surfacing only this year. Subsequently another trip was made to the site to try to find more of the holotype and another, this time complete, skull of a phytosaur was found about 40 meters away and at the same horizon. This specimen has not been prepared yet but preliminary work during the excavation suggests that it may belong to P. jablonskiae. If so this would provide a description of the rest of the skull and further support the taxonomic validity of the species.

One more note. In our 2006 paper we state that P. jablonskiae is from just above the base of the Sonsela Member and therefore represents the lowest occurrence of Pseudopalatus in Petrified Forest National Park. However, reexamination of the Sonsela Member this summer by Jeff Martz figured out that the type locality for P. jablonskiae is higher in the Sonsela than previously believed, and that it occurs just above other localities that have provided specimens of another species of Pseudopalatus, P. pristinus. Thus, the newly recovered skull becomes even more important to test whether or not P. jablonskiae is restricted to a narrow horizon or represents a species that co-existed with P. pristinus.

The full description of P. jablonskiae can be found at Randall Irmis' CV page.

REFERENCES

Camp, C. L. 1930. A study of the phytosaurs with description of
new material from western North America. Memoirs of the University
of California, 10:1-174.

Parker, W. G., and R. B. Irmis. 2006. A new species of the Late Triassic phytosaur Pseudopalatus (Archosauria: Pseudosuchia) from Petrified Forest National Park, Arizona. Museum of Northern Arizona Bulletin 62:126-143.