User:Abyssal/Tricorynopus

Tricorynopus is an ichnogenus of prehistoric mammals, possibly a rodent, from the middle Albian Gates Formation of Canada. Fossils have been found at the tracksite of the Smoky River Coal Mine in Alberta. The trace fossils are very small digitigrade tracks with only three digits (probably digits II–IV). The digits diverge and broaden distally and all have distinct claw impressions. Manus and pes are almost similar in size.

<!-- ==Cretaceous bird and mammal tracks== Richard T. McCrea and William A. S. Sarjeant published a paper describing new bird and mammal ichnotaxa from the Gates Formation in Alberta, Canada. Numerous bird trackway had recently been discovered associated with non-avian dinosaur footprints in the Smoky River Coal Mine near Grand Cache, Alberta. This paper was the second report of birds tracks from the region but the first for the Gates Formation, specifically. Two avian ichnotaxa are known from the W3 Main site, but only one is common. A third ichnotaxon has been reported from talus blocks at another site, the W3 Bird locale. W3 Extension is another Gates Formation site that has yielded fossil bird footprints. The authors characterize the bird fauna of the Gates Formation as being "diverse" but known only from trace fossils. The authors also reported the discovery of three-toed mammal footprints bearing sharp forward pointing claws. These mammal tracks were found on a single talus block that also contained bird prints. Significantly, these fossil mammal tracks were the first known from the Cretaceous period that were not left by marsupials. The authors amended the definitions of Aquatilavipes, Aquatilavipes swiboldae, and Fuscinapeda.

During the course of examining the "natural mold talus block (TMP 98.89.20)" William Sarjeant discovered "extremely shallowly impressed" mammal tracks preserved in the lower center part of the specimen. Mammal footprints had previously documented from the Gething Formation strata of the Peace River Canyon. These tracks were given the name Duquettichnus kooli by second author William Sarjeant and Thulborn in 1986. They were so large and so closely resembled the tracks of the modern Australian brush tail possum that the authors concluded that they were left by a marsupial. The Gates Formation tracks, by contrast, were the smallest mammal tracks reported from Mesozoic strata, and had a significantly different morphology.

The mammal prints were referred to a new ichnospecies, Tricorynopus brinkmani, named after Dr. Donald Brinkman who discovered the holotype specimen.

Diagnosis for Tricorynopus brinkmani: Very small Digitigrade to semi-digitigrade Three toes Presumed pes trace twice as large as presumed manus Manual digits radiate symmetrically from base Interdigital span 15 degrees Manual digits moderately thick at base and become narrower outwards. All digits bear sharp forward pointing claws Pedal prints bore more flexible and widely separated digits. Interdigital span 60 degrees Digit III longest in both manus and pes Manual digits II and IV of aproximately similar length Pedal digit IV longer than digit III and curves outward

The holotype of this ichnospecies are the prints left near the lower central part of a slab bearing Aquatilavipes swiboldae, TMP 98.89.20 interred in the Royal Tyrell Museum of Palaeontology. The presumed manus prints are 3.5 mm long and 3 mm wide. The pes prints are 7.5 mm long and 6.5 mm wide. Other prints from the slab were not complete enough to measure. The holotype was discovered in the talus at the base of the W3 footwall from the Grand Cache Member of the Gates Formation. The researchers could not determine the trackmakers gate or superposition. Therefore manus and pes prints were identified on the basis that the pes was likely the larger print. The authors admitted this was a tentative assumption because very little is known about the postcranial anatomy of Mesozoic mammals. Another problem is that Mesozoic were so small, typically weighing only tens of grams, that they left very shallow footprints that had a difficult time fossilizing. Tricorynopus brinkmani contradicts the diagnosis for its own genus in that the manus and pes seemed to differ strongly in size. The authors opined that the discovery of more information about the trackway pattern may result in the species having a new ichnogenus erected for it. The size of the prints are not helpful for identifying what kind of mammal made them since during the Mesozoic almost all mammals were in the shrew to rat size range. Their small size renders them difficult to fossilize since their bones are fragile as to be extremely vulnerable to destruction by digestion or erosion. Only their teeth commonly survive these forces well enough to end up in the fossil record.

When trying to identify the trackmakers that left behind ancient trace fossils paleontologists attempt to compare the morphology of the traces with the morphology of fossils known from skeletal material. A second method involves attempting to correlate the traces with known taxa from the same time and place. Both are difficult with Mesozoic mammals because the lack of good fossil material. With the former method there is very little known material to serve as a comparison. With the second, the low probability of fossilization devalues the worth of negative evidence in trying to understand the regional mammal fauna; a mere absence of fossils does not imply the mammals were not there because even if they were there is little chance they would be preserved as fossils.

A large quantity of Early Cretaceous mammal bones have been recovered from the Cloverly Formation in Montana and Wyoming. These specimens were typically preserved in concretionary nodules. This fauna was dominated by members of the families Amphilestidae and Triconodontidae. The amphilestids were too large to be likely trackmakers as a typical body length excluding tail would be in excess of 35 cm. The triconodontid Gobiconodon was also probably too large. However, smaller triconodonts known from jaw fragments, like Corviconodon would be in the necessary size range to be candidate track makers. The Paluxian Land Mammal Age of mid Albian strata contained in the Trinity Group of Texas and the Antlers Formation of Oklahoma is characterized by the presence triconodontids, multituberculates, spalacotheriids, aegialodontids, pappotheriids. Teeth and lower jaw remains left by Triconodonts have been recovered from the Arundel Clay of the Patuxent Formation. The Cedar Mountain Formation of Utah, dating back to the Albian to Cenomanian contains teeth and fragments of jaws left behind by triconodonts. Few of these mammals have left remains of parts from behind the head to the rear of the animal. Most were known solely from teeth. As of the time the paper was written, in fact, no Lower Cretaceous mammal remains had been found in Western Canada. The authors admitted that the lack of adequate remains for comparison defeated any attempt to ascribe a specific trackmaker to the prints left in the Grand Cache trackway. McCrea and Sarjeant expressed hope that further trackway discoveries in the Gates Formation would provide more information about its native mammal ichnofauna.

The mammal tracks are "partially superimposed" on bird tracks although which trackway was left first isn't clear because "they are only seen on a talus block."