The Tragic Tale of Machaeroprosopus and Acompsosaurus

In 1914 Maurice Mehl (then at the University of Wisconsin-Madison) collected vertebrate material from the Chinle Formation near Cameron Arizona and near Fort Wingate New Mexico. Work that resulted in the discovery of a nearly complete phytosaur skull and the large sacrum of a "new form" (UW 3811), which he named Acompsosaurus ("sturdy lizard")wingatensis (Mehl, 1915). A full description of this material followed a year later in which Mehl et al. (1916) erected a new genus of phytosaur, Machaeroprosopus ("knife face"), for the skull from near Cameron, with the type species of M. validus. The descriptions of these specimens are good; however, whereas the 1916 paper contains a photographic plate (albeit very poor) of the type specimen of A. wingatensis; the holotype skull (UW 3807) of M. validus is only represented by line drawings. Furthermore, although Mehl et al. (1916) mention the presence of osteoderms with the type of A. wingatensis, unfortunately they do not describe or figure these elements as they can be diagnostic for aetosaurs.

In 1922, Mehl assigned another new species of phytosaur from New Mexico (although he erroneously says Arizona in the article title), M. andersoni, to the genus Machaeroprosopus and in 1928 erected another new genus, Pseudopalatus ("false palate"), for a skull (P. pristinus) from the Petrified Forest of Arizona (Mehl, 1922; 1928).

In his classic 1930 monograph on the phytosaurs, Charles Camp assigned five new species to Machaeroprosopus (M. adamanensis, M. lithodendrorum, M. zunii, M. tenuis, and M. gregorii). He also argued that Pseudopalatus pristinus was very similar to M. tenuis and assigned it to the genus Machaeroprosopus although as a distinct species (Camp, 1930).

Despite that no new species were assigned to the genus after 1930, the name Machaeroprosopus was heavily used in the literature until Joseph Gregory's revision of the phytosaurs in 1962. Gregory (1962) considered Machaeroprosopus to be a junior synonym of the east coast taxon Rutiodon and assigned all of the species, with the exception of M. gregorii, which he assigned to Phytosaurus, to that genus; however, he also noted that he was not able to examine the holotype of M. validus as it could not be located.

Westphal (1979) brought attention to the missing holotype of M. validus, noting that it had been missing since about 1958 (presumably when Joe Gregory went to see it), and asking if anyone had any information on the specimen.

Previously Gregory (1953) had also addressed the taxonomic status of Acompsosaurus, which he recognized to represent the pelvis of an aetosaur; however, he notes that has "not been able to examine the specimen). Gregory (1953) suggested that Acompsosaurus may be a synonym of Typothorax (comparing to T. meadei, which is now Longosuchus).

However, one researcher who had examined the material was E.C. Case, who in 1929 wrote that he "had the opportunity to study the specimen [A. wingatensis holotype] in the museum of the University of Wisconsin". Unfortunately this is the last documentation of a worker examining the holotype. I assume that Case studied the skull of M. validus as well as he never mentions it as missing in any of his subsequent phytosaur work.

The type specimen of M. validus became of importance again in 1989 when Karen Ballew removed R. pristinus and R. tenuis and restored the genus Pseudopalatus with P. tenuis as a junior synonm of P. pristinus. This is because Ballew (1989) and subsequent authors consider M. validus, based on the description and line drawings, to be very similar (if not identical) to Pseudopalatus. Thus, if the missing holotype were found, Pseudopalatus would be a junior synonym of Machaeroprosopus and that genus name would once again be valid. M. validus is also unique and thus important in that is is the only phytosaur with a purported 'overbite' in that the upper jaws protrude much more anteriorly than the lower jaws; however, this condition can only be ascertained by direct inspection of the specimen. Drawing below from Mehl et al. (1916).

The holotype of Acompsosaurus is also of importance because based on existing descriptions and its stratigraphic position it probably represents an aetosaur similar if not identical to Calyptosuchus (Stagonolepis) wellesi, named by Long and Ballew (1985). Thus, it too would be a valid taxon, although possibly as Stagonolepis wingatensis. Drawing below from Mehl et al. (1916).

So what happened to the the type specimens of M. validus and A. wingatensis? E. C. Case was the last documented worker to study them in the late 1920s and by the time Joe Gregory goes to see them in the late 1950s they are missing. Interestingly, Colbert (1947) assigned a beautiful skull from Arizona to Machaeroprosopus and in the paper provides skull measurements for M. validus that are not in Mehl et al. (1916). Did Colbert examine the specimen personally? Unfortunately he does not say, only stating in the acknowledgments that he examined the Berkeley material referred to the genus by Camp (1930). Thus all we can say is that the specimens were probably lost at some point between 1929 and 1958. Mehl moved to the University of Missouri in the 1920s and died in 1966.

A few years back I met a gentlemen at a meeting, the name escapes me now, who told me a story of being a graduate student at the University of Wisconsin-Madison in the 1950s - 1960s. He said that the museum hired a new curator who was a crinoid specialist. To make more room for his beloved crinoids, this gentleman disposed of some of the "old reptiles" in the dumpster! However, he did believe that the curator may have given the skull to the biology department. (Note: I am not trying to demean crinoids or people who study them, I am just relating the story as it was told to me).

I have since contacted the geology and biology departments inquiring about these specimens but to no avail and have not been able to corroborate this story; however, since researching this post I became aware that the UW Museum still exists and plan to contact them soon.


Ballew, K.L. 1989. A phylogenetic analysis of Phytosauria (Reptilia: Archosauria) from the Late Triassic of the western United States, p. 309-339. In S. G. Lucas and A. P. Hunt (eds.), Dawn of
The Age of Dinosaurs in the American Southwest. New Mexico
Museum of Natural History, Albuquerque.

Camp, C.L. 1930. A study of the phytosaurs with description of new material from western North America. University of California Memoir 10:1-161.
Colbert, E.H. 1947. Studies of the phytosaurs Machaeroprosopus and Rutiodon. Bulletin of the American Museum of Natural History 88:57-96.

Gregory, J. T. 1953. Typothorax and Desmatosuchus. Postilla 16:1-27.

Gregory, J.T. 1962. The genera of phytosaurs. American Journal of Science 260:652-690.

Long, R.A., and K.L. Ballew. 1985. Aetosaur dermal armor from the Late Triassic of southwestern North America, with special reference to material from the Chinle Formation of Petrified Forest National Park. Museum of Northern Arizona Bulletin 54:45-68.

Mehl, M.G. 1915. New reptiles from the Trias of Arizona and New Mexico. Science 41:735.

Mehl, M.G. 1922. A new phytosaur from the Trias of Arizona. Journal of Geology 30:144-157.

Mehl, M.G. 1928. Pseudopalatus pristinus, a new genus and species of phytosaur from Arizona. University of Missouri Studies 3:1-22.

Mehl, M.G., Toepelmann, W.C., and G.M. Schwartz. 1916. New or little known reptiles from the Trias of Arizona and New Mexico with notes from the fossil bearing horizons near Wingate, New Mexico. University of Oklahoma Bulletin 103:1-44.

Westphal, K.W. 1979. Missing holotype of Machaeroprosopus validus (Mehl, 1916). Journal of Paleontology 53:741.


  1. Hi Bill, I think this is a good time for me to stop lurking. I have been following this blog for a few months now after I ran across it googling something triassic. I am working on my phd at UW-Madison describing the depositional environments of the Chugwater Group of Wyoming. It needs some work! I am the prep lab manager here at the UW and was really excited at the first part of this post thinking 'wow we have phytosaur material! from Mehl no less!' and then... So sad. It seems that this has happened at a few repositories over time.
    So I will try to tack these down hopefully something will come to light! If you want you can reach me at lovelace@geology.wisc(dot)edu
    Thanks for the blogging, and the citations! They have proved most useful!

  2. Hi Dave,

    I'm glad that you enjoy the blog and that you stopped lurking! Just what we need to solve this mystery is someone on the inside. Anything you can find out is greatly appreciated.

    Thanks much!


  3. In his 1916 publication, Mehl actually reports several specimens that he refers to Machaeroprosopus. UW 3807 was the holotype material, but there was also UW 3808 and 3809 that he said were "generally identical" and consisted of the posterodorsal regions of the skull. I tried to find out about these specimens for my thesis, and I actually had correspondence with K. Westphal about it; unfortunately, all the specimens are missing now.


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