ALLOMETRIC SCALING OF FOOT SIZE RELATIVE TO BODY SIZE IN A JURASSIC THERAPSID IS RECORDED IN FOOTPRINT AND TRACKWAY DIMENSIONS OF THE TRACE FOSSIL BRASILICHNIUM

Among animals generally, it is common for different skeletal elements to grow at somewhat different rates relative to body size during ontogeny. In humans, for example, babies are born with a relatively large head, but during subsequent growth the baby’s head increases in size more slowly than does the rest of the body. Another example of allometric growth is foot size in some breeds of domestic dogs. In puppies the foot bones grow faster than the leg bones, creating a puppy with oversize feet; later the foot growth rate slows, and the dogs “grow into their feet.”

In this study we explore the use of fossil trackways to test hypotheses concerning allometric growth in vertebrate species for which the bone record is meager or nonexistent. The trace fossil Brasilichnium is one of the most common vertebrate ichnogenera in the Jurassic Aztec-Navajo-Nugget erg deposits of southwestern North America. The Brasilichnium trackmaker was most probably a cynodont therapsid (protomammal). In this study we use Brasilichnium trackways to test the hypothesis that the Brasilichnium trackmaker experienced allometric growth in its feet, relative to overall body size.

We have compiled a database of several hundred individual Brasilichnium foot impressions occurring in 84 distinct trackways studied at three locations: (1) the Dewey Bridge site near Moab, Utah (Navajo Ss), (2) Valley of Fire State Park, Nevada (Aztec Ss), and (3) the Gold Butte area, Nevada (Aztec Ss). Not being able to measure body size directly from trace fossils, we use the width of a trackway as a proxy for body size, and we use the width of individual foot impressions as a measure of foot size.

When foot-impression width is plotted against trackway width on logarithmic coordinates, trackways with a width of 9 cm or less yield a linear-regression line with a slope of 1.3, while trackways with a width greater than 9 cm yield a linear regression line with a much lower slope. We conclude that the Brasilichnium trackmaker did indeed experience allometric growth in its feet. The feet grew relatively quickly up until the animal’s trackway width was about 9 cm, at which point the rate of growth decreased. As far as we are aware, this is the first study of ontogenetic growth using trace fossils, and the first report of allometric growth in therapsids.