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An assemblage of vertebrate and invertebrate tracks from the Union Chapel Mine (Lower Pennsylvanian: Westphalian A, 310 million years ago) in north-central Alabama constitutes an important new Carboniferous tracksite that is unrivaled in number and quality by any tracksite in the world. Hundreds of specimens recovered recently from the Union Chapel Mine (UCM) site by the Birmingham Paleontological Society (BPS) were cataloged cooperatively by this amateur group and scientists from the Geological Survey of Alabama, Alabama Museum of Natural History, and Emory University. Vertebrate tracks belong to the ichnospecies Cincosaurus cobbi; all other ichnospecies identified by Aldrich and Jones (1930) are synonyms of this ichnospecies. Tracks are preserved in a laminated gray shale, indicative of the muddy upper reaches of an ancient estuarine tidal flat. C. cobbi exhibit a wide variety of behaviors, including tail drags and changes in speed, direction, gait. Individual tracks range from 1-12 cm wide, although the majority of tracemakers were relatively small animals. Several trackways show detailed outlines of both manus and pes for the tracemaker that confirm an amphibian origin for the tracks. These tracks reveal five digits on each foot, bulbous terminations on those digits, and an elongated digit IV to the pes. The preponderance of amphibian tracks, presumably made by the same species of amphibian, enables an assessment of the vertebrate population structure that ordinarily could not be accomplished with skeletal remains of amphibians. Track width, trackway width, and glenoacetabular distance (an estimation of torso length derived from trackways) are parameters that indirectly measure a representative number and size ranges of amphibians from the UCM site. Although the collection bias of BPS may be a source of error in this attempt to determine the structure of a Carboniferous amphibian population, a detailed stratigraphic analysis of strata from the UCM site might better define depositional rates in the paleoenvironment, which in turn will provide better resolution of how many breeding cycles are represented by the track assemblage.
Ichnofossils, as opposed to body fossils like bones, teeth, feathers and hair, represent the remains of an ancient organism's behavior. Although an organism will only leave one skeleton, the same individual may produce millions of traces throughout its lifetime. In addition, ichnofossils directly reflect an organism's behavior during an interval of time - in this manner, ichnofossils may be thought of as the products of fossil behavior (Seilacher, as quoted by Bromley, 1990). Types of ichnofossils include fossil footprints, burrows, feeding traces like bite marks and bore holes, coprolites, and nests. Invertebrate ichnofossils are more abundant in the fossil record than vertebrate ichnofossils, and may be used for paleoenvironmental interpretations of facies. However, Paleozoic vertebrate ichnofossils are relatively uncommon and Carboniferous footprints are even more rare. Foot skeletons and recovered track specimens of Paleozoic tetrapods are characterized by a lack of a distinct morphology (Schult and Farlow, 1992). The paucity of well-preserved amphibian tracks elsewhere in the world emphasizes the importance of the excellent preservation of trackways at the UCM site - the quantity of vertebrate tracks from the UCM site (n= >1000) underscores the world-class significance of this site. Although population structures have been proposed using data from dinosaur tracksites (Lockley 1991), no such estimation has been conducted of amphibian trackways. Given the number and quality of UCM vertebrate tracks, it may be possible to elucidate the ontogeny of the UCM tetrapod population as well as its structure.
We attended Track Meet II (October 2000, in Oneonta AL) and Track Meet III (May 2001, in Anniston AL), both organized by the BPS. Members of BPS catalogued every specimen on display, and scientists from the Geological Survey of Alabama and Emory University described, analyzed and interpreted the ichnofossils. Measurements of the vertebrate ichnofossils included trackway length, trackway width, pes width, and pace length. Any behaviorally related features were also recorded. An analysis of the data using simple descriptive statistics was then applied.
Vertebrate trackmakers exhibited behavioral changes that include changes in speed, gait, and direction across trackways that show trends in an individual's behavior across an interval of time. The histograms of the most reliable measurements indicate an overwhelming presence of juvenile individuals in the UCM population. Median values of trackway width, glenoacetabular distance and pes width all suggest a skewed population structure towards the juvenile size range. Standard deviation values reveal particulary large sources of error for each measurement: trackway width values vary by over 20 cm from the mean, probably as a result of behavioral changes in the gait of the animals; glenoacetabular distance values vary according to the speed of the animal and thus the manus-pes placement in the trackway; pes width values vary by a comparatively small margin. Despite the variability inherent to all of the measurements, the amphibian population seems to have been composed, in the majority, of juvenile-sized animals. More data from the immense UCM assemblage will hopefully better elucidate the nature of the UCM amphibian population structure.
The authors would like to thank: Prescott Atkinson, Julie Bartley, David Bowie, Ron Buta, Neil Clark, Charles Darwin, David Dilcher, Harmut Haubold, Larry Herr, Sam Hood, Jim Lacefield, Steve Minkin, Jack Pashin, Andy Rindsberg Birmingham Paleontological Society Alabama Museum of Natural History And a few 310-million-year-old amphibians. This material is based upon work supported by the Howard Hughes Medical Institute under Grant No. 52003071 and funding from the University of California Board of Regents Also, a special thanks to Jere Lipps of the Department of Integrative Biology and Museum of Paleontology, University of California, Berkeley.
We studied a unique assemblage of trace fossils from the Union Chapel Mine (UCM) in northern Alabama, USA. While the UCM assemblage includes both vertebrate and invertebrate trace fossils, the vertebrate trackways recovered by the Birmingham Paleontological Society, an amateur group working with researchers from Geological Survey of Alabama and Emory Univeristy, are of world-class significance due to the remarkable quantity and quality of trackways. Paleozoic tetrapod tracks are very rare and nondescript; the quality and quantity of the UCM assemblage presents an opportunity to assess amphibian populations from the site. Using the most behaviorally invariant measurements of pes length, trackway width and glenoacetabular distance, it may possible to ascertain an estimation of the numbers and sizes of amphibians based on trackway evidence alone. Although many taphonomic, preservation and collecting biases present many challeneges and sources of error, the wealth of data from the UCM site (n =>1000 specimens) hopefully will aid in the elucidation of a Carboniferous amphibian population -- a feat only previously accomplished with dinosaur trackways.
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