Akanthosuchus

History and description

Akanthosuchus is based on NMMNH NP-139, a partial skeleton lacking the skull. The specimen was found in a concretionary sandstone lens with the anterior portion eroding out. It was discovered in Torrejon Wash, northwestern Sandoval County, New Mexico. The hind legs, numerous back, hip, and tail vertebrae (28), and armor (about 200 scutes) were the primary elements preserved. F. Michael O'Neill et al. described the genus in 1981. The type species is A. langstoni, honoring paleontologist Wann Langston Jr., known for his work on fossil crocodilians.

Akanthosuchus was a moderately sized crocodilian. The thigh bone of NMMNH NP-139 was 137 mm long, and the shin was 102 mm long. The tail may have been relatively short, given the reduction of bony projections on known tail vertebrae. The armor was unusual in having elongate spikes or broad blades. Its scutes came in four basic types: square (13%, keeled or unkeeled), oval (27%, keeled), bladed (

Spiked and bladed scutes are not unknown among crocodilians, although no other known crocodilian had or has both. O'Neill and colleagues compared Akanthosuchus to Pinacosuchus, a crocodylomorph with spiked scutes known from the Late Cretaceous North Horn Formation of Utah, but it differed from Akanthosuchus in several ways: it was much smaller (three to four times smaller than NMMNH NP-139), it had less derived vertebrae, and it lacked bladed scutes.

Classification

O'Neill et al. suggested that Akanthosuchus was an alligatorine, based on the characteristics of a partial lower jaw bone (retroarticular). They considered the possibility that Akanthosuchus is a postcranial skeleton of either Ceratosuchus or Navajosuchus, contemporaneous crocodilians known primarily from skulls, but elected to name a new genus instead. More recently, Hill and Lucas, using a cladistic analysis, rejected the hypothesis that Akanthosuchus is the same as either Ceratosuchus or Navajosuchus, and found that Akanthosuchus is an alligatoroid. It tended to sort out with alligatorines, but Hill and Lucas found that this was weakly supported in their analysis.

Distribution

NMMNH NP-139 came from the upper part of the Torrejonian portion of the Nacimiento Formation. which gives the genus a range including much of the Nacimiento Formation, deposited between approximately 64.5 and 61.0 million years ago. A scute and femur from the De-Na-Zin Member of the late Campanian-age (Late Cretaceous) Kirtland Formation are similar to the corresponding elements of Akanthosuchus, but cannot be conclusively assigned to the genus at this time. This material is also from the San Juan Basin.

Paleobiology

The Nacimiento Formation is interpreted as having had a fluvial and lacustrine depositional setting. O'Neill et al. interpreted Akanthosuchus as either a terrestrial crocodilian or a specialized, modestly sized, heavily armored aquatic crocodilian.

References

References

1. O'Neill, F. Michael. (1981). "''Akanthosuchus langstoni'', a new crocodilian from the Nacimiento Formation (Paleocene, Torrejonian) of New Mexico". Journal of Paleontology.
2. Hill, Robert V.. (2006). "New data on the anatomy and relationships of the Paleocene crocodylian ''Akanthosuchus langstoni''". Acta Palaeontologica Polonica.
3. Williamson, Thomas E.. (1996). "The beginning of the age of mammals in the San Juan Basin, New Mexico: biostratigraphy and evolution of Paleocene mammals of the Nacimiento Formation". New Mexico Museum of Natural History and Science.
4. Lucas, Spencer G.. (2006). "Late Cretaceous vertebrates from the Western Interior". New Mexico Museum of Natural History and Science.