Enigmatic Taxa I - Acallosuchus rectori

From time to time I'd like to introduce readers to some of the more 'interesting' Triassic critters. To start things off I will cover one of the more poorly known taxa, a mystery fossil in its own right, Acallosuchus rectori from the Chinle Formation of Arizona.

In his fieldnotes from May 22, 1923 Charles Camp discusses the discovery of what he described as the skull of "a small dinosaur or pterodactyl" from a quarry in what is today Petrified Forest National Park. His notes further state that he "plastered this and took it out but plaster broke off block and it had to be removed in pieces. Skull about 6 inches long. Part of rostrum broken" (Long and Murry, 1995). Accompanying this description is a field sketch of this "skull" (figure to the left taken from Irmis, 2005). According to Long and Murry (1995) when they rediscovered the specimen (in a cigar box) in the early 1980s the specimen bore almost no resemblance to Camp's field sketch. Still, what was preserved, although barely identifiable, is autopomorphic thus they felt justified to erecting a new taxon based on this material.

Previously the specimen was considered to represent a possible protrochampsid by Murry and Long (1989) who assigned a fragmentary postcranial skeleton (from probably the same locality) to their new taxon. However, by 1995 they had separated the material, naming the postcranial material Vancleavea campi, and the "skull" Acallosuchus rectori (Rector's ugly crocodile). We now know, based on new material (e.g., Hunt et al., 2002; Parker and Barton, 2008), what the skull of Vancleavea looks like and that this separation was indeed correct. Unfortunately, the skull is only known from a partial "mandible", and two other skull fragments which were interpreted as a portion of the frontal and postorbital, and a portion of the "postorbitojugal" bar by Long and Murry (1995).

Furthermore, Camp's sketch is unlike any known "reptile" skull, but as presented it would suggest that the element that Long and Murry (1995) identify as a dentary would actually be a maxilla. The large triangular opening would be an antorbital fenestra, above that a nasal and anteriorly (to the right on the drawing) possibly a premaxilla. The posterior opening could be an orbit, surrounded by a jugal/lacrimal bone anteriorly, and a postorbital dorsally? Unfortunately, the sketch does not provide clarification and was not labelled.

One of the autapomorphies of Acallosuchus is the presence of what Long and Murry (1995) described as osteoderms covering the surfaces of the skull. Irmis (2005) who further commented on this specimen and described these "osteoderms" as subtriangular knobs and noted that these knobs "are often arranged in rows running the length of the bone, and are themselves sculptured with longitudinal furrows. Other areas of bone not covered by these eminences are sculptured with additional grooves" (see photo below [from Irmis, 2005] of the dentary [occusal view] and another skull fragment [? view] showing these unique "knobs").



As discussed by Long and Murry (1995) and further commented on by Irmis (2005) the phylogenetic placement of this taxon is highly ambiguous. Long and Murry (1995) considered Acallosuchus to be a "neodiapsid" based on the presence of the "postorbitojugal bar"; however, even this identification is not unambiguous (Irmis, 2005) and will remain so without the discovery of more and better material.

One final note. Long and Murry (1995:193) named the species from ex-superintendent of Petrified Forest National Park Roger Rector, and his wife Betty, therefore the correct specific name would be A. rectororum; however, the current version of the ICZN does not require such emendations to be made, thus the name will stand as A. rectori. On a similar note I cannot believe that when such emendations were required that no none noticed that the 'rauisuchian' Postosuchus kirkpatricki was named by Chatterjee (1985) for the Kirkpatrick family and thus should probably have been P. kirkpatrickorum as well.

REFERENCES

Chatterjee, S. 1985. Postosuchus, a new thecodontian reptile from the Triassic of Texas and the origin of tyrannosaurs. Philosophical Transactions of the Royal Society of London B, 309:395-460.

Hunt, A. P., A. B. Heckert, S. G. Lucas, and A. Downs. 2002. The distribution of the enigmatic reptile Vancleavea in the Upper Triassic Chinle Group of the western United States. New Mexico Museum of Natural History & Science Bulletin, 21:269-273.

Irmis, R. B. 2005. The vertebrate fauna of the Upper Triassic Chinle Formation of Arizona. Mesa Southwest Museum Bulletin 9:63-88.

Long, R. A., and P. A. Murry. 1995. Late Triassic (Carnian and Norian) tetrapods from the southwestern United States. New Mexico Museum of Natural History and Science Bulletin, 4:1-254.

Murry, P. A., and R. A. Long. 1989. Geology and paleontology of the Chinle Formation, Petrified Forest National Park and vicinity, Arizona and a discussion of vertebrate fossils of the southwestern Upper Triassic, p. 29-64. 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, New Mexico.

Parker, W. G., and B. J. Barton. 2008. New information on the Upper Triassic archosauriform Vancleavea campi based on new material from the Chinle Formation of Arizona. Palaeontologia Electronica 11.3.14A.