User:Abyssal/Irenichnites

Irenichnites is an ichnogenus of dinosaur footprint. In 2001, McCrea and Sarjeant used the ratio of an Irenichnites 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. Irenichnites gracilis had an average length-to-width ratio of 1.19 among eleven 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. Instead, Irenichnites was probably left by a small ornithomimid, or ostrich dinosaur.

W3 Main
Irenichnites 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, Gypsichnites. 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
Sternberg diagnosed Irenichnites as being a three-toed track left by a digitigrade biped. The toes are spaced reasonably widely and are the same thickness throughout their length. The toe impressions end in dull claws. The impressions left by the second digit was disconnected from the rest of the track. Toe pads are present, but their impressions are "fain[t]". The heel didn't leave a complete impression. Irenichnites trackways are narrow, with small footprints but comparatively long strides.

The type ichnospecies of IRenichnites is Irenichnites gracilis. I. gracilis was erected by Sternberg in 1932 from the Aptian Gething Formation of eastern British Columbia.

Sternberg diagnosed Irenichnites gracilis as being a three-toed track left by a digitigrade biped. The toes are spaced reasonably widely and are the same thickness throughout their length. The toe impressions end in dull claws. Irenichnites gracilis tracks are proportionally short and wide. Sternberg observed the presence of toe pad impressions of the third and fourth digits in one I. gracilis track. The trackway forms a nearly straight line. Its long stride lengths suggests a long-legged trackmaker was responsible for the traces. The claw marks are shallow.

The best preserved Irenichnites gracilis trackway that Sternberg examined had digits with a divarication of 38 degrees between digits II and III and 40 degrees between digit III and IV.

McCrea emended Sternberg's diagnosis to take into account some specimens having "well-impressed" claw marks. There were two toe pads on the second digit, three on the third, and at least three on the fourth. The fourth toe's claw had a Triangular shape. Most Irenichnites gracilis footprints are longer than wide. The length to width ratio of individual tracks is between 1.1 and 1.31, but the average is 1.19. The holotype had a significantly lower length to width ratio of 0.90. The total divarication ranges from 60 to 80 degrees. The tracks are angled parallel to the direction of the trackway rather than angled in relation to it like in some ichnotaxa.

McCrea examined Irenichnites gracilis from both W3 Main and the South Pit Lake Site.

The holotype of Irenichnites gracilis is a series of five footprints catalogued by the Canadian Museum of Nature in Ottawa as NMC 8552.

McCrea emended the diagnosis for Irenichnites gracilis to take into account the toe pad impressions seen in some specimens he examined.

McCrea's Irenichnites tracks were smaller than many of the other non-avian theropod ichnotaxa he examined, but their long strides suggest that their trackmaker had long legs. In 1990 Thulborn proposed that ornithomimids were responsible for Irenichnites. He noted that Irenichnites preserves digit II impressions that are separate from the rest of the track. This suggests that the trackmaker's second toe was located higher up on its foot than the other toes. Irenichnites preserves toe pad impressions in similar quantities across its toes to the number of phalanges in ornithomimid toes (2:3:4:5:0). Irenichnites tracks from both Peace River and Grande Cache compare well with the foot size of an ornithomimid like Dromeceiomimus. However, Dromeceiomimus lived in the Late Cretaceous. Ornithomimid bones have been discovered in the Aptian to Albian Cloverly Formation in Montana and Wyoming. In 1970, John Ostrom proposed that these may have been left by Ornithomimus velox, another Late Cretaceous ornithomimid. Similar ornithomimid bones are known from other geologic formations in the US from the same time period.

Hook
... that fossil dinosaur footprints called Irenichnites have been discovered in an Alberta coal mine?

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.