Herrerasaurus ischigualastensis is a basal theropod known only from the Upper Triassic Ischigualasto Formation of NW Argentina. Herrerasaurus is found in the lower third of the formation, which is Middle Carnian (~225-235mya) in age. The original specimen was discovered in 1959 by Osvaldo A. Reig and Victorino Herrera, and later named by Reig in 1963.

Herrerasaurus ischigualastensis was a 4 meter long, 300 pound biped. Some of the features that place it within Saurischia and Theropoda are the presence of a subnarial foramen, and an intramandibular joint. Despite its classification as a theropod by Sereno and Novas (1993), Thomas Holtz (1995) completed a phylogenetic analysis which placed Herrerasauridae as the sister group to the theropod-sauropodomorph clade, or even as the sister group to all Dinosauria.

The skull used for this project was collected by Paul Sereno and Alfredo Monetto in 1988 from the Valle Fertil, San Juan. This specimen (PVSJ 407) is part of the collections at the Museo de Ciencias Naturales, Universidad Nacional de San Juan, San Juan. It was found partially contained within a hematite precipitate in a medium grained sandstone lens. The prepared skull was brought to the University of Texas at Austin for scanning by Oscar Alcober in 1997.

For each specimen, different parameters and settings are needed to give the desired results. For the Herrerasaurus skull which was scanned by Richard Ketcham on 26 February 1998, the most important parameters and information used were: Image dimensions- 512X512 pixels; Pantak X-ray source- 420kV, 4.8mA; X-ray detector- P250D; garnet silt wedge; slice thickness- 1mm, interslice spacing- 0.8mm; field of reconstruction- 151mm.

The image dimensions tell us that the original images are 512X512 pixels. The Pantak X-ray source is one of two ways that X-rays can be created with the CT Scanner. 420kV is the potential of the scanner's electric field, and 4.8mA is its filament setting. The X-ray detector is P250D. This detector uses 512 detectors made of hand crafted cadmium tungsten crystals that are 0.25mm wide, 5mm high, and 5mm deep. The specimen was scanned in a garnet silt wedge to help filter out any low energy X-rays and produce a well-calibrated, clear scan. A slice thickness of 1mm means just what it says, each slice was 1mm thick. The interslice spacing and field of reconstruction were used with the image dimensions to get the interslice spacing in pixels needed to reslice the data. The formula used was: (512 pixels/151mm) X 0.8mm= 2.712582 pixels. The Herrerasaurus dataset has ring artifacts. In the original plane, which is coronal, there are concentric rings caused by detectors drifting off line. In this dataset, the rings are very faint in the affected slices. This can be corrected by running frequent recalibrations.

Image Processing

The animations and slices seen on this web site were processed using the following software programs: Adobe Photoshop 4.01PPC, NIH Image 1.62b17/PPC, File Buddy 5.01 PPC, and Movieplayer Pro. The first step in preparing the images for this site was getting the image contrast at an optimum level. This was done in Photoshop's Image Adjusting Levels window, and performed as a Batch Action on all the images. The Input numbers used were 50,1,255, and the Output numbers were 0,239. This was done to 8-bit modified images. The images were then stacked in NIH Image, and resliced into the other two slice planes (sagittal and horizontal) after calculating the interslice spacing of 2.712582 pixels. The images then had to be reduced in size for the web. This was done with a Photoshop Batch Action to a size of 320X320 pixels for the coronal plane, and 320 pixels width X154 pixels height for the sagittal and horizontal planes. These were then saved as TIFF files. Next, the images had to be numbered. The first step in numbering the images was to rename the TIFF images in File Buddy. The Basename was deleted, the Start numbering was set at 000, the Increment was kept at 1, and the Append to each name was left empty. The renamed images were then all opened in NIH Image. The macro used to number the images was an NIH Editing macro called Draw File Name in Each Image. To get white numbers on the black background, the color value for the macro was changed from 255 to 0. After applying this macro, all the images were numbered. The numbered files were then stacked, and saved to files as PICT images. This was done for all three slice planes. The images were then renamed in File Buddy: starting with COR000.PIC, SAG000.PIC, and HORI000.PIC. Next, the images needed to be put in GIF format for use on the Web. This was done with a Batch Action in Photoshop. After this, the new GIF images were renamed in File Buddy with the appendix GIF.

Quicktime movies were also made from the images. This was done by opening the TIFF images in Movieplayer Pro. In Open Image Sequence, a frame rate of 15 frames per second was chosen. The images were used to make a master movie of PICT or TIFF files. The movie was named with a .mov extension, and the radio button that says make movie self-contained was checked. The movie was then exported and appended with (web).mov. The Compression settings were Graphics and Grayscale, and the keyframe every box was unchecked. The coronal slice plane was too large to do directly in Movieplayer Pro. First, the images had to be changed to an MooV- Quicktime Movie in Graphic Converter. Once this was done, the movie was taken into Movieplayer Pro and completed just like the other two planes were when exported. The coronal movie could also have been made by making two movies, one for each half of the data, and then pasting them together.

Farlow, J. O., and M. K. Brett-Surman. 1997. The Complete Dinosaur. Indiana University Press, Bloomington and Indianapolis. pgs. 207-210.

Glut, D. F. 1997. Dinosaurs: The Encyclopedia. McFarland & Company Inc., Jefferson, North Carolina. pgs. 462-467.

Romer, A. S. 1956. Osteology of the Reptiles. University of Chicago Press, Chicago. (used for abbreviations for skull elements)

Sereno, P. C., and F. E. Novas. 1993. The skull and neck of the basal theropod Herrerasaurus ischigualastensis. Journal of Vertebrate Paleontology 13:451-476.

Weishampel, D. B., P. Dodson, and H. Osmolska. 1990. The Dinosauria. University of California Press, Berkeley. pgs. 169-177, 322-327.

Links

More info on Herrerasaurus from the Texas Memorial Museum of Science and History

More info on Herrerasaurus from the Museum of Paleontology, UC Berkeley

Dinosauria On-Line: General information on dinosaurs

Dinosaur Resource Directory

The skull of Eoraptor lunensis

None available.