Three age groups of ornithopods inferred from footprints in the mid-Cretaceous Dakota Group, eastern Colorado, North America

Ornithopod population structure is inferred from a large sample of footprints from the mid-Cretaceous Dakota Group of Colorado, New Mexico and Oklahoma. Allometric analyses and size-frequency distributions suggest size (age) classes or recruitments for tracks identified as Caririchnium leonardii (Ornithopoda) from sequence 3 of the mid-Cretaceous Dakota Group of the Rocky Mountain Front Range High Plains region. Five hundred ninety-eight footprints from 284 trackways were used in the statistical analysis. Tests were conducted that revealed no significant difference between analyses of right and left footprints or between casts and molds. Allometric analysis suggests a growth curve that can be subdivided into three segments, corresponding to foot lengths of 16.5–21.7, 21.7–29.3 and 29.3–55.0 cm, respectively. These are interpreted as juvenile (possibly yearlings), subadults and adults. The former two categories may correspond to animal recruitments (n, n+1), whereas the latter category may represent several recruitments (n+2, n+3, n+4, etc.). Once ornithopod growth curves of the type proposed for Maiasaura are established, there is considerable potential for estimating dinosaur age from footprints. In the meantime, however, large track samples can be analyzed for the size class information they contain.