User:Abyssal/Gypsichnites

Gypsichnites is an ichnogenus of dinosaur footprint. In 2001, McCrea and Sarjeant used the ratio of an Gypsichnites print's length to its width in order to help identify what kind of animal produced it. Previous workers lead by Moratalla have used this method to distinguish tracks left by ornithopods and theropods. Gypsichnites had an average length-to-width ratio of 1.19 among twenty seven prints examined by McCrea and Sargent at the W3 Main tracksite. Their results excluded ornithopods from candidate trackmakers because the length-to-width ratios of their tracks were at least 1.25.

W3 Main
Gypsichnites is known from a fossil site called the W3 Main track site. This site forms part of a footwall in the Smoky River Coal Mine near Grande Cache, Alberta. The fossil footprints at W3 Main were first reported in the early 1990s. These reports were examined by several follow-up expeditions during the summer of 1998. McCrea and Sargent describe W3 Main as difficult to study because the tracksite is at about 1700 meters of altitude and frequently obscured by adverse weather conditions like fog or overcast skies. Compounding the problem, the footwall itself is oriented in such a way that the sun only shines on it for part of the day. occurs alongside other theropod tracks like Aquatilavipes curriei, Irenesauripus, Ornithomimipus, and Irenichnites. Ankylosaur tracks of the ichnogenus Tetrapodosaurus are also present. McCrea and Sargent have called this association of different dinosaur trackmakers as "a rich late Early Cretaceous fauna." These tracks are preserved in rippled sandstone in the presence of many trace fossils left by both large and small invertebrates. Plant fossils preserved in the same stratigraphic unit as the tracks, the Gates Formation, include ferns, conifers, cycads, ginkgoes, and two species of flowering plant. Larger plant remains include fossil logs and tree stumps that are spaced far apart from one another. The fossils of W3 Main paint a picture of an ancient coastal plain or delta. The lack of mud cracks in the track bearing sediments is evidence that they were never dehydrated fully before preservation, possibly because the tracks were left in water a few centimeters deep or just because the exposed sediment was very wet when stepped on.

Gates
Gypsichnites was left by a semi-digitigrade two-legged animal that walked on three toes. It had a wide round heel. It's foot was short and wide. The bases of the toe marks were connected by impressions of webbing or padding. Its toes ended with blunt hooves.

McCrea emended Sternberg's 1932 diagnosis. Gypsichnites was left by a semi-digitigrade two-legged animal that walked on three toes. It had a wide round heel. Its feet were longer than they were wide. In some Gypsichnites tracks the impressions left by the outside toes were not connected to the rest of the footprint. Its toes were wide and became even wider farther along their length. Digit III exhibited this trait to the most marked degree. McCrea verified the presence of claw marks on some tracks but disputed Sternberg's description of the traces as hoof-like.

The type ichnospecies of Gypsichnites is Gypsichnites pascensis. It was erected by Sternberg in 1932 for tracks from the Aptian Gething Formation of eastern British Columbia.

McCrea emended the diagnosis for this ichnogenus in order to account for the detachment of the outer toe impressions seen in some specimens and to describe the nature of the toe claws.

The type specimen did not bear impressions from toe pads. Sternberg characterized the toes as wide and their claws as hoof-like. The outer toes form a wide angle and their far ends face even further outwards. The bases of the toes were connected by padding or webbing. The impression left by the third digit is the deepest in the type specimen, but in the paratype the animal's weight seems to have been evenly distributed across all three toes. The widest point on the third digit is near the end but the other toes taper outward from their bases. The stridelength of the type specimen was measured from a poorly preserved second track as about three times the length of the track. The paratype specimen had a slightly longer stride length. The trail left by the Gypsichnites pascensis trackmaker is an almost perfectly straight line, although individual footprints face slightly outward. ^^^1/8/12 McCrea emended the diagnosis of Gypsichnites pascensis to note the presence of toe pads preserved in some specimens. The third toe tended to have the best preserved of these impressions. In some Gypsichnites tracks the impressions left by the outer toes may not connect with the impresion left by digit III. This makes some specimens resemble tracks left by theropods. However, these specimens with disconnected toe impressions were probably just made by the trackmaker walking over firmer sediments than fully connected traces. Some Gypsichnites tracks preserve claw impressions. The average Gypsichnites pascensis footprint has a length to width ratio of 1.19, but the full range of documented ratios is 1.1 to 1.52. The holotype specimen had a length to width ratio of 1.03. In 1988 Moratalla and other suggested that footprints with ratios like this were made by ornithopods.

The holotype of Gypsichnites pascensis is an isolated footprint catalogued as NMC 8553 by the Canadian Museum of Nature in Ottawa, Ontario. McCrea found many Gypsichnite pascensis footprints at W3 Main. McCrea observed that many of the Gypsichnites pascensis at W3 Main closely resemble those that Sternberg originally described in 1932. However, he also noticed some specimens with significant variations. However, the variations appaear to be attributable to differences in the sediment and motions of the trackmaker. Sternberg originally suspected that the trackmaker for Gypsichnites pascensis was an ornithopod, however, he still hesitantly attributed it to the Eubrontidae, an ichnofamily of tracks left by theropods. The length to width ratios of Gypsichnites pascensis at W3 are similar to those though by Moratalla and others in 1988 to be attributable to ornithopods. However McCrea observed that every ichnotaxon left by bipedal three-toed trackmakers preserved at W3 main spanned both above and below the boundary of length to width ratio thought to distinguish theropods from ornithopods. McCrea concluded that length to width ratios were unreliable for distinguishing the local theropod tracks from those left by ornithopods.

Most of W3 Main's Gypsichnites pascensis footprints preserve toes with round blunt ends, which is an ornithopod like trait. Some however, preserve impressions left by toe claws, which would suggest a theropod trackmaker. This isn't enough to discount the ornithopod trackmaker hypothesis, however, since hypsilophodonts were a group of clawed ornithopods. Previous researchers have suspected that their footprints could be mistaken for those left by theropods. In 1990, Thulborn remarked that some ornithopod tracks preserve claws, but these were blunter and wider than theropod claw marks. Although claw marks associated with Gypsichnites pascensis tracks are rare, a sufficient number have been documented to build a case against their conventional interpretation as orntihopod tracks. Distinguishing theropod tracks from ornithopod tracks is complex and tricky.

The dinosaur most likely responsible for Gypichnites tracks is Tenontosaurus. Tenontosaurus is known from fossil skeletal remains uncovered in the Early Cretaceous Cloverly Formation in Montana and Wyoming. These are the closest known sources of skeletal fossils from the same age as the tracksites McCrea studied. Some researchers have interpreted Tenontosaurus as a habitual biped incapable of supporting itself with its front legs for long periods. No Gypsichnites trackways is known to preserve foreprints, which bolsters this interpretation. However, there are problems with attributing Gypsichnites to Tenontosaurus since Tenontosaurus had four large toes on its hind feet, while Gypsichnites only preserves three, However the attribution can't be completely discounted because some researchers have thought that the first digit of the Tenontosaurus hindfoot was too short to touch the ground while walking.

Reference

 * McCrea, Richard T. 2000. Vertebrate palaeoichnology of the lower cretaceous (lower Albian) gates formation of Alberta.
 * McCrea, Richard T. 2001. Tourism Opportunity Analysis: Dinosaur Tracks In the Grande Cache Area Tourism opportunity analysis dinosaur tracks in the Grande Cache area. Publisher: Edmonton : Alberta Economic Development. ISBN 0778512622.
 * McCrea, R. T. and W. A S. Sarjeant. 2001. New ichnotaxa of bird and mammal footprints from the Lower Cretaceous (Albian) Gates Formation of Alberta; pp. 453–478 in D. H. Tanke, and K. Carpenter, (eds.), Mesozoic Vertebrate Life. Indiana University Press, Bloomington and Indianapolis.