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. ^[1] Numerous bird trackway had recently been discovered associated with non-avian dinosaur footprints in the Smoky River Coal Mine near Grande Cache, Alberta. ^[1] This paper was the second report of birds tracks from the region but the first for the Gates Formation, specifically. ^[1] Two avian ichnotaxa are known from the W3 Main site, but only one is common. ^[1] A third ichnotaxon has been reported from talus blocks at another site, the W3 Bird locale. ^[1] W3 Extension is another Gates Formation site that has yielded fossil bird footprints. ^[1] The authors characterize the bird fauna of the Gates Formation as being "diverse" but known only from trace fossils. ^[1] The authors also reported the discovery of three-toed mammal footprints bearing sharp forward pointing claws. ^[1] These mammal tracks were found on a single talus block that also contained bird prints. ^[1] 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. ^[2]

The W3 Main tracksite is at about 1700 meters of altitude. ^[3] Common meteorological conditions like fog or overcasts skies inhibit the study of the tracksite. ^[3] Compounding the problem, the footwall itself is oriented in such a way that the sun only shines on it for part of the day. ^[3] 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. ^[3] Not all of the bird tracks were complete, in some prints only one or two toe marks were preserved. ^[3] The trackmakers seems to have been a long legged bird, probably built like a modern heron. ^[3] The prints are all wider than they are long and bear short claws. ^[3] 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. ^[3]

McCrea and Sarjeant emdended the diagnosis of the ichnogenus Aquatilavipes. ^[4] 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. ^[4] The authors made this emendation to more clearly distinguish between Aquatilavipes and the similar ichnogenus Fuscinapeda, which they also emended later in the paper. ^[4] The two taxa differ in that Aquatilavipes has more slender digits. ^[4] Another ichnotaxa resembling Aquatilavipes is Aviadactyla, although in this ichnogenus the toeprints fuse farther from the body and have a "stick-like" character. ^[4] Ludicharadripodiscus differs by consistently leaving impressions of the hallux. ^[4] 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. ^[4]

The authors emended the diagnosis for Fuscinapeda in order to clarify the distinction between the two ichnogenera, which was that Fuscinapeda had thicker digits. ^[5]

Diagnosis for Fuscinapeda:

Small to large size ^[6] Three toes, moderately thick digits ^[6] No webbing or hallux ^[6] Digit III 25% longer than II and IV ^[6] Digital span > 95 degrees, frequently > 120 ^[6] Digits II and IV similar length ^[6] All digits clawed, with claws often flexing inward ^[7] "Length of digits II and III may be similar, but digit IV is frequently somewhat larger." ^[5] Better preserved specimens show digital pads, 3 or 4 on digit III, two on digits II and IV ^[5]

Footnotes

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.