User:Abyssal/Aquatilavipes

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 Grande 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.

The first discovery of Late Cretaceous bird tracks in Alberta was made by Dr. Charles R. Stelck in Dunvegan Formation strata along the Pouce Coupe River in 1951. These tracks had not been formally described in the scientific literature and the authors doubted that they were even left by birds. In 1981 the first record of Cretaceous bird tracks from western Canada was published by Phil Currie to document tracks recovered from the Aptian age Gething Formation strata of eastern British Columbia's Peace River Canyon. The tracks were small, with lengths ranging from 2 to 4.4 cm and formally christened Aquatilavipes swiboldae. In 1991 Darren Tanke of the Royal Tyrell Museum of Paleontology discovered natural casts of three-toed footprints. The rocks containing the tracks were exposed at a road cut near the Smoky River Coal Mine now known as the Highway 40 site. A block of rock containing the tracks was cut out and interred at the Royal Tyrell Museum of Paleontology under the catalogue number TMP 90.30.1. Richard McCrea, upon reexamining the block found that a previously unnoticed, faint, third Aquatilavipes swiboldae track was present therein. Confusion plagued early attempts to identify the strata containing the Highway 40 bird tracks. In 1992 some researchers concluded that the tracks originated in the Gates Formation. However, an examination of local geological maps found that no Gates Formation outcrops existed near the Highway 40 site. Lead author Richard T. McCrea collaborated with Phillip Currie in 1998 to publish a paper arguing that the tracks had come from the Cadomin Formation, which dates to the Aptian-Albian. However, a more recent visit allowed the authors to pinpoint the stratum yielding the tracks discovered by Tanke in 1991; the Gladstone Formation. This formation correlated to the Gething Formation where the original Aquatilavipes swiboldae type specimens had been recovered. The Highway 40 A. swiboldae specimens were the first confirmed fossil bird footprints from Alberta.

During the summer of 1998 several expeditions were undertaken to the W3 Main site at the W3 footwall in the Smoky River Coal Mine, as dinosaur tracks had been reported from there in the early 1990s. On July 15th, Richard McCrea observed three-toed tracks preserved in a rippled sandstone layer in the footway. These tracks ranged from 6.4-10.1 cm, smaller than the smallest dinosaur tracks which themselves ranged from 13.5 to 19 cm long. At first only individual specimens of these bird tracks were discovered, but after some time a trackway document six consecutive steps was discovered near a Irenesauripus mclearni trackway left by a large theropod. The smaller tracks had wider digital divarications than those left by dinosaurs on the same footwall. These tracks reveal a pigeon-toed trackmaker whose gait seemed to be accelerating, as evidenced by the progressively increasing stride lengths within the trackway.

The W3 Main tracksite is at about 1700 meters of altitude. Common meteorological conditions like fog or overcasts skies inhibit the study of the tracksite. Compounding the problem, the footwall itself is oriented in such a way that the sun only shines on it for part of the day. However, despite these obstacles, McCrea and Sarjeant determined that the bird tracks were very abundant, comprising about 750 of the 1200 or more tracks in the 500 m^2 study area. Not all of the bird tracks were complete, in some prints only one or two toe marks were preserved. The trackmakers seems to have been a long legged bird, probably built like a modern heron. The prints are all wider than they are long and bear short claws. The authors regarded them as a new ichnospecies in the ichnogenus Aquatilavipes, although noted that this referral necessitated an emendation to the genus's diagnosis as well as the diagnosis of a related form, Fuscinapeda.

Aquatilavipes swiboldae tracks were discovered by Dr. Donald Brinkman of the Royal Tyrell Museum of Paleontology in two talus blocks positioned at the base of the W3 footwall. Both specimens are housed in the Royal Tyrell Musuem, one as TMP 98.89.21, a natural cast, the other as TMP 98.89.20, a natural mold. It is not certain that these specimens originate from where they were found at the W3 footwall because that part of the mine had been subjected to backfill operations that introduced foreign material from other parts of the mine.

Recently as of the time of writing, a large quantity of three toed bird footprints preserved as both molds and casts were discovered on the W3 footwall, but at a site now known as W3 Bird, away from the main area of study. The authors opine that this is the point of origin for the bird tracks, and that the tracks are of the ichnospecies Aquatilavipes swiboldae. A different kind of bird track was discovered at the W3 extension site, but as of yet there is too little material to describe them as a new ichnospecies. They are smaller and have more slender digits than those of A. swiboldae.

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.

Confusion has persisted in ichnology due to differences in terminology used by European and North American ichnologists. North Americans frequently use the word track to denote a single print. However, this differs both from how European scientists and North American big game hunters use the term; to denote a series of prints. This makes the "track" of European and hunting usage essentially cognate with the "trackway" of North Americans. The authors advised writers to avoid using the word track to stop perpetuating this confusion. The total interdigital span is also known as total divarication and divarication of digits. This measures the angle between the axes of the outermost digits. In the case of fossil birds this means digits II and IV. Different scientists have used different standards for measuring the digit lengths of fossil bird prints. In the original description of Aquatilavipes swiboldae Currie measured digits to the point of contact with the metatarsal pad. The authors of this paper instead opted to measure all the way to the rear of the metatarsal pad. The pace angle is the meaure of the angle of the foot in relation to the midline of the trackway.

McCrea and Sarjeant emdended the diagnosis of the ichnogenus Aquatilavipes. Their emendation incorporated another emendation published in 1992 by Martin G. Lockley and others who noted that some Aquatilavipes tracks bear faint impressions of digital pads in certain sedimentary contexts. The authors made this emendation to more clearly distinguish between Aquatilavipes and the similar ichnogenus Fuscinapeda, which they also emended later in the paper. The two taxa differ in that Aquatilavipes has more slender digits. Another ichnotaxa resembling Aquatilavipes is Aviadactyla, although in this ichnogenus the toeprints fuse farther from the body and have a "stick-like" character. Ludicharadripodiscus differs by consistently leaving impressions of the hallux. Other similar ichnotaxa include Avipeda, which has shorter and thicker digit impressions than Aquatilavipes and Ornithotarnocia, which has a thicker digit III and greater asymmetry.

Emended Diagnosis for Aquatilavipes swiboldae:

Small size for Aquatilavipes Digits are thin, with digit III being thinnest; its digital width is about 8% of its length. The other two digits are about 12.5% of their respective lengths. All digits bear claws, with digit III bearing an especially sharp one. Flexible digits; II and III typically curved inward near the tip while digit IV typically curved outward. Digit III was longest, about 50% longer than digit II and 40% longer than digit IV Total interdigital span varies from 90 to 130, averaging at 113.

The type secimen for Aquatilavipes swiboldae is Footprint 76, which is both a mold and a cast. The mold is being held in the Royal Tyrell Museum of Paleontology catalogues as TMP 79.23.37. The cast is interred in the British Columbia Provincial Museum as BCPM 744.

Aquatilavipes curriei was named in honor of Phil Currie "in recognition of his contributions to vertebrate paleontology and paleoichnology in western Canada."

Diagnosis for Aquatilavipes curriei: Moderately large size for Aquatilavipes Digital thickness about 10% of length Claws present and sharp; those of digits II and IV tilt towards digit III Total digital span ranges from 120 to 135 in response to "gate and substrate hardness." The angle between digits II and III are "consistently" larger than that between III and IV Digital pads frequently preserved, with two on digit II, three on digit III and two on digit IV. Some tracks show a longitudinal groove; sometimes it is discontinuous and on shallower traces may be absent Prints consistently exhibit slight inward angle Trackway is broad Pace length varies but is typically short

A cast of an isolated left print catalogued at the Royal Tyrell Museum of Paleontology as TMP 98.89.11 was chosen as the type specimen. The paratype was a trackway cast catalogued as TMP 98.89.10.

The type horizon was the Grand Cache Member of the Gates Formation, which dates back to the early Albian. The type locale was the Smoky River Coal Mine, which lies about 21km to the northwest of Grand Cache, Alberta. The interdigital angles of the holotype were 68 degrees between digits II and III, and 52 degrees between digits III and IV.

The W3 footwall bears A. curriei prints in at least three layers at the W3 Main tracksite. The first is a sandstone layer known as Layer C, which is of medium thickness, ranging from 19 to 21 cm. This layer is also characterized by the presence of "a variety" of dinosaur prints. Layer C has a grain size of .15 mm in diameter topped by a layer of fine silt. This silt layer indicates "the settling of a body of water during lowstand." The second A. curriei-bearing layer is called Layer B and is much thinner than Layer C, being on 1 to 2 cm thick. It shares the same sandstone-transitioning-to-silt structure as Layer C. Its siltstone portion has better preserved tracks. Layer A is of similar thickness to Layer B, but has only produced a single track. These track containing layers also contain plant fossils preserved by carbonization including tree trunks, stumps, and cones. The environment wherein the tracks were imprinted was gradually replaced by a coal swamp that left behind a number 4 coal seam 2 to 3 meters in thickness.

More than 750 individual A. curriei prints from ten trackways were examined. The paratype specimen bears the tracks of two birds moving in opposite direction at moderate speed with a moderately long stride. One print on the slab, labelled A6 exhibits "craterlike swellings" on the left side of digit III and on the rear part of the metatarsal pad. These structures may reflect a pathological condition of the trackmaker as similar structures are produced on the feet of poultry with a disease called bumblefoot. However, they may also have been produced by the action of small animals living within the sediment itself. The researchers observed that the quality of the other prints in the series were too low to "distinguish between these alternative hypotheses."

Another notable trackway shows a meandering pattern of prints. McCrea and Sarjeant speculated that this animal may have been wandering at the edge of a drying pool looking for prey, as there were many fossil invertebrate burrows present in the sediment.

The authors emended the diagnosis for Fuscinapeda in order to clarify the distinction between the two ichnogenera, which was that Fuscinapeda had thicker digits.

Diagnosis for Fuscinapeda:

Small to large size Three toes, moderately thick digits No webbing or hallux Digit III 25% longer than II and IV Digital span > 95 degrees, frequently > 120 Digits II and IV similar length All digits clawed, with claws often flexing inward "Length of digits II and III may be similar, but digit IV is frequently somewhat larger." Better preserved specimens show digital pads, 3 or 4 on digit III, two on digits II and IV

Preserved in Layer B are large incomplete bird footprints. No complete prints have yet been discovered. Two imprints left by individual digits and one print bearing two connected digit prints were discovered on the Aquatilavipes curriei paratype. Two more prints, each bearing two connected digits were observed on the W3 footwall. The morphology and angles of these prints are similar to those of A. curriei, but much larger. McCrea and Sarjeant estimated that the trackmaker was a bird one and a half to three times as large as the A. curriei trackmaker. They are of similar dimensions to the ichnospecies Magnoavipes lowei from Cenomanian deposits in Texas and Archaeornithopus meijidei from Spanish Berriasian strata. These mystery prints have thicker digits than either of those ichnotaxa. Although the unidentified prints probably represent a new ichnospecies the researchers declined to name them on the basis of their poor quality.

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.

As of the publication of the paper, six occurences of Cretaceous fossil bird track sites from western Canada had been documented. They were:

Indeterminate ichnospecies from the Maastrichtian Horseshoe Canyon Formation of eastern Alberta Indeterminate ichnospecies from the Maastrichtian St. Mary River Formation of Southern Alberta Jindongornipes-like prints from the Dunvegan Formation of British Columbia Aquatilavipes swiboldae, A. curriei, and indeterminate ichnospecies from the Grand Cache Member of the Gates Formation of southern Alberta A. swiboldae from the Gladstone Formation of western Alberta A. swiboldae from the Aptian Gething Formation of the Peace River Canyon in northeastern British Columbia.

The Aquatilavipes swiboldae tracks of the Peace River Canyon were not discovered in association with dinosaur prints, although they are known from other locations in the canyon. Neither dinosaur nor bird bones are known from strata adjacent to the canyon. In the W3 track site the A. swiboldae tracks are also not associated with any dinosaur tracks. The A. curriei tracks present a strong contrast in that they were associated with "a rich late Early Cretaceous fauna." Associated ichnotaxa include tracks left by ankylosaurs (Tetrapodosaurus) and theropods of varying sizes (Irenesauripus, Ornithomimipus, Gypsichnites, and Irenichnites). These tracks are preserved in rippled sandstone in the presence of many trace fossils left by both large and small invertebrates. Logs and "widely spaced" tree stumps are among the formation's plant remains. Other constituents of the Gates flora include ferns, conifers, cycads, ginkgoes and two species of angiosperm. These are the remains of an ancient coastal plain or delta. The track maker of Aquatilavipes curriei was a large wading bird. The tracks may have been left as its prowled for invertebrates to feed on, although no dabbling traces have been recognized as a result of this activity. 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 because the exposed sediment was highly saturated when imprinted upon. The slab containing the paratype tracks exhibits two trackways, A and B. Trackway A contains better defined prints overlain by the less clear Trackway B impressions. The authors hypothesized that Trackway B was left sometime after Trackway A and during the intervening period the sediment had dried slightly, become "slightly more resistant" to track formation.

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."

McCrea and Sarjeant used the ratio of a 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. The length to width ratios of the W3 Main dinosaur track site were found to be:

Irenesauripus mclearni 1.20, ten tracks examined Ornithomimipus angustus? 1.09, twelve tracks examined Irenichnites gracilis 1.19, eleven tracks examined Gypsichnites pascensis 1.19, twenty seven tracks examined

All of these tracks had length-to-width ratios of less than 1.25, the cut-off point used for identifying ornithopod tracks. The tracks referred to Aquatilavipes curriei have an average ratio of 0.73 for forty seven tracks. This ratio is much smaller than is typical for non-avian theropods. The type species A. swiboldae has an average ratio of 0.80 for forty four tracks, similar to that of A. curriei. Moratalla and his research partners also studied footprints left by paludicolous birds like the killdeer and great blue heron. They compared these tracks left by extant birds with the A. swiboldae tracks and found that the extant species had higher but similar ratios to the Grand Cache tracks. Killdeer tracks have an average length to width ratio of 0.88 for forty prints. Great blue herons had a length to width ratio of 0.90 for fourteen tracks. McCrea and Sarjeant saw these results as supporting the utility of footprint length to width ratios in distinguishing tracks left by birds from those of non-avian dinosaurs. The authors encouraged further research in the field, especially research focusing on tracks left by extant bird species.

Reference

 * 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.