User:Abyssal/Tetrapodosaurus

Tetrapodosaurus is an ichnogenus of dinosaur footprint.

W3 Main
Tetrapodosaurus 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. Tetrapodosaurus occurs alongside other dinosaur tracks like the theropod ichnotaxa Aquatilavipes curriei, Irenesauripus, Ornithomimipus, Irenichnites, and Gypsichnites. 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
Tetrapodosaurus is in the ichnofamily Tetrapodosauridae. Sternberg erected the Tetrapodosauridae in 1932. His original diagnosis described tracks of the family as those left by medium sized quadrupeds with dull, hoof-like ungual impressions. They left wide trackways but had short stride lengths. He interpreted them as the tracks of quadrupedal ornithischians.

Sternberg originally diagnosed Tetrapodosaurus as follows. The toes were linked by padding or webbing. The impressions left by the forepaws are directly in front of those left by the hind feet but there is no ohysical overlap of the traces. The forepaw was short and wide, with five digitigrade toes. The hindfoot had a "medium length", foure toes, and was semiplantigrade.

McCrea emended Sternberg's diagnosis. He disputed Sternberg's assessment that the toes of either the forepaws or hindfeet were webbed or linked by padding. Tracks with structure suggestive of this webbing or padding were probably caused the trackmaker's weight warping the sediment it was walking over. The back edge of the forepaw impressions is concave.

The type ichnospecies of Tetrapodosaurus is Tetrapodosaurus borealis, erected by Sternberg in 1932 for tracks found in the Gething Formation of eastern British Columbia.

McCrea studied many footprints at several different sites in the Smoky River Coal Mine near Grande Cache, Alberta.

McCrea emended Sternberg's diagnosis to contest the latter's claim that the toes of the Tetrapodosaurus were webbed or linked by padding.

Tetrapodosaurus borealis was erected by Sternberg in 1932. The impressions left by the forepaws are directly in front of those left by the hind feet but there is no physical overlap of the traces. The forepaw was short and wide, with five digitigrade toes. Three of these toes were more likely to leave impressions than the others so they were probably shorter. Or, the uneven impressions may be related to the sediment the trackmaker was walking on. In ceratopsians only the three inner toes had hooves. The fourth and fifth digits of the foot probably wouldn't have left impressions extending beyond that left by the heel. The outer toes of the Tetrapodosaurus trackmaker may likewise have been too short to leave impressions if the inner toes were supporting too uch of the animal's weight. The trackmaker's toes seemed to Sternberg to be bound by padding with only their tips free. The forepaw impressions seem to preserve only the animal's toes while the hindfoot impressions preserved its heel. The digits of the forepaw spread at wider angles than those of the hindfoot. The total divarication of the forepaws' digits exceeded 180 degrees. I and II had a divarication angle of 73, II and III 42, III and IV 33, IV and V, 50 degrees.

The hindfoot is longer than it was wide. The pad cushioning the metatarsals supported much of the trackmaker's weight. The metatarsal and toe pads seem to have been a single unified pad. The toes were short and ended in hooves. The impressions left by the tips of the toes were deeper than those left by the bottom of the foot. The back edges of the tracks tended not to be well-preserved. The outermost of the four toes had the highest divarication angle. The rear edge of some tracks preserved triangluar impressions likely caused by the animals hooves dragging over the sediment when they were coming to rest.

Digits I and II had divarication angles of 14 degrees, II and III, 21 degrees, III and IV were 32 degrees.

McCrea emended Sternberg's 1932 diagnosis for Tetrapodosaurus borealis to take into consideration the absence of some of the five toes of the foreprint in the less-well preserved specimens. Further, he disputed Sternberg's interpretation that the toes were bound by padding. The forepaw's padding at the base of the toes was actually minimal. Digits II-IV faced forward, but Digits I and V angled out to the side, or backward in some cases. The first digit is long and thin. McCrea frequently observed it facing at a right angle to the trackway. This digit is so distinctive that it's useful for helping ichnologists distinguish between left and right Tetrapodosaurus borealis tracks. The total divarication of the foreprint ranges from 100 to over 200 degrees.

The toes of the hindfoot were long and thin. The first toe, though, was short and angled towards the midline of the track. The rest pointed straight forward. Thus, the first digit of the hindfoot is also useful for telling left tracks from right, even in tracks with low quality preservation. The hindfoot's total divarication was between 70 and 80 degrees and therefore much lower than that of the forefoot. The toe impressions are the deepest part of the footprint. Triangular dragmarks are located at the rear edge of some tracks.

The holotype of Tetrapodosaurus borealis is a series of 6 footprints, three front, and three hind, catalogued as NMC 8556 by the Canadian Museum of Nature in Ottawa.

McCrea emended Sternberg's original diagnosis for Tetrapodosaurus borealis to include the slender nature of the toes and more measurements of their divarication angles.

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Sternberg's original description of Tetrapodosaurus borealis is also applicable to many of the T. borealis tracks found at various sites near Grande Cache. However, McCrea observed that the Grande Cache T. borealis tracks come in a variety of forms. He found them comparable to the variations of Grallator documented in 1990 by Gatesy and others. These variant forms were not distinct kinds of tracks but were actually typicaly T. borealis tracks left in the same place by the same kinds of trackmaker exhibiting the same kind of behavior. McCrea attributed the observed differences to differences in the sediments being imprinted. Two T. borealis trackmakers could leave prints with differing levels of toe-print distinctness depending on how muddy the sediment was. Drier sediments would have fairly distinct toe prints while muddy sediments would have vague toe prints surrounded in bulges due to the softness of the mud. The distinct-toed tracks were probably left later when the muddy sediment had dried and become firmer.

McCrea classified the Tetrapodosaurus borealis variations he observed at Grande Cache into a series of "morphotypes".

The first of McCrea's morphotypes was morphotype A. This morphotype preserves both fore and hind prints as "shallow, oval-shaped depressions". The quality of preservation is so low that the fore and hindprints are difficult to distinguish from one another. This variation on T. borealis was probably left on firm sediment. The underlying layer preserves underprints which resulted from the weight of the trackmaker deforming the water-logged layer of sediment under the surface it was actually walking on.

The second of McCrea's T. borealis morphotypes is Morphotype B. These tracks are deep and surrounded by bulges from when the sediment was soft mud. The fore and hindprints are easy to tell apart, but the toe impressions are unclear.

Morphotype C have mud bulges, but are only "moderately" deep. These tracks look like they were made by a web-toed trackmaker because only the toe tips are distinct from the rest of the foot. The sediment was at least somewhat wet or the associated mud bulges would have formed.

Morphotype D also had some mud bulges, but these tend to be concentrated on the inner side of the tracks. The toe impressions of this morphotype are clearer than the others, and don't suggest a webbed foot was responsible for their formation. Some Morphotype D tracks even preserve skin impressions.

Morphotype E tracks are shallow but preserve clear impressions of the trackmaker's long toes. The bases of the toes in these impressions are clearly preserved and refute the interpretation of the trackmaker as web-toed.

In Morphotype F the outline of the tracks themselves are "very faint". The most conspicuous feature of these tracks are the impressions of the tips of the trackmaker's toes. This morphotype was left one rather dry sediment that was too hard to record more detail of the foot anatomy. McCrea contended that these traces were left during what Thulborn and Wade called the "'kick off-phase' of the step cycle" in 1989.

In 1932 Sternberg proposed that Tetrapodosaurus tracks were made by ceratopsians. However, in 1984 Kenneth Carpenter proposed that the armored dinosaur Sauropelta, which lived at the same time as the tracks were being left, was the real Tetrapodosaurus trackmaker. McCrea acknowledged that Sauropelta, specifically, may have been the trackmaker, but also noted that any similarly sized armored dinosaur with five toes on the front foot and four toes on the hindfeet. McCrea also conceded that some ceratopsians had five toes on the forefoot and four toes on the hindfoot and therefore would also be capable of leaving similar footprints. The best known ceratopsians were known from strata more recent than that which preserved Tetrapodosaurus, however more recent discoveries have pushed the age range of ceratopsians back into ages comparable to that of Tetrapodosaurus. Nevertheless, McCrea supported the ankylosaur interpretation.

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.

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