See: Tykoski, R.S., T.B. Rowe, R.A. Ketcham, and M.W. Colbert. 2002. Calsoyasuchus valliceps, a new crocodyliform from the Early Jurassic Kayenta Formation of Arizona. Journal of Vertebrate Paleontology 22:593-611. Get PDF

Calsoyasuchus is a new fossil crocodyliform archosaur from the Early Jurassic Kayenta Formation of the Navajo Nation that is surprisingly derived for so ancient a specimen. High-resolution X-ray CT analysis reveals that its long snout houses an extensive system of pneumatic paranasal cavities. These are among the most distinctive features of modern crocodylians, yet the evolutionary history of this unique system has been obscured by the inaccessibility of internal structures in most fossil crania.

Preliminary phylogenetic analysis presented in Tykoski et. al (2002) indicates that the new species is the oldest known member of a monophyletic Goniopholididae, and within this lineage to be the sister taxon of Eutretauranosuchus, from the Late Jurassic Morrison formation of Colorado. Goniopholididae became extinct at the end of the Cretaceous, but it is more closely related to living crocodylians than are several lineages known only from Cretaceous and younger fossils. The new taxon nearly doubles the known length of goniopholid history and implies a deep, as yet undiscovered, Mesozoic history for several crocodyliform lineages that were once thought to have relatively complete fossil records.

The holotype consists of a partial skull found lying palate-side up, wedged between two pieces of petrified wood at the base of a trough scour within a thick, crossbedded channel sandstone bed. The skull was preserved in light-green, medium to coarse-grained sandstone, with hematitic crust over much of its surface. The posterior end of the skull was reduced by erosion to several dozen weathered pieces of surface float, which included portions of the braincase, suspensorium, and palate. These fragments are too weathered to reassemble with confidence, and were not scanned. No postcranial elements were preserved.

X-ray energies were set to 420 kV and 4.7 mA, with a focal spot size of 1.8 mm. X-rays were pre-filtered to reduce beam-hardening artifacts using two brass plates with a total thickness of 3.175 mm. X-ray intensities were measured using an RLS detector with 2,048 channels spaced at 0.05 mm intervals. Forty channels were unsuitable for imaging, and the remaining channels were averaged into groups of 4 to reduce image noise, resulting in 502 effective channels with a spacing of 0.2 mm. Each slice was acquired using 1,800 views (angular orientations), each view having an acquisition time of 64 ms, and detector gain was set to 8 to maximize count rate. The resulting acquisition time was approximately 2 minutes per slice. The sample was scanned in a 190% offset mode (Ketcham and Carlson, 2001) with a slice thickness of 1.0 mm and an inter-slice spacing of 0.9 mm. The image field of reconstruction was 130 mm, and reconstruction parameters were calibrated to maximize usage of the 12-bit range of grayscales available in the output images.

At the time of the scan the detector showed behavior in which it would first drift rapidly out of calibration once data acquisition commenced, then drift more slowly. As a result it was decided to obtain the entire data set in one pass without recalibration during the scan. In order to minimize drift, the detector as exposed to X-rays for two hours before the scan began, and a very long signal calibration (3,600 views, or 3.84 minutes) was obtained. This procedure eliminated major problems, but nevertheless a drift in grayscale values of up to 7– 8% occurred over the course of the scan. In general, the drift resulted in darkening in the center and brightening of the margins of the images as scanning progressed. The majority of this effect was removed from the data set by comparing initial and final images and analyzing intervening images to discern the pattern and course of the drift. Routines to perform this analysis and apply the subsequent correction were written in IDL version 5.3.1. For easier handling, the 12-bit data stored in 16-bit TIFF-format data files were exported to 8-bit format by dividing all grayscale values by 16. The data are archived as individual slices.

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