Trackway evidence of herd structure among ornithopod dinosaurs from the cretaceous Dakota group of Northeastern New Mexico, USA

The Mosquero Creek dinosaur track site in New Mexico provides one of the best examples of social behavior among dinosaurs because many trackways exclusively of the same type (single ichnotaxon), with similar depth and quality of preservation, are aligned in parallel groupings. Previous studies based on ornithopod (Caririchnium) trackway directions and speed estimates proposed a large social group of quadrupedal subadults heading north, and a smaller group of bipedal adults heading south, but fell short of a complete description of the entire trackway sample. Our statistical analysis of trackway directions and speed estimates suggest that the 48 northwesterly oriented trackways (mean azimuth 326°) can be resolved into three groups (A, B and C) based on differential orientations, but that speed variations are not significant. Two southerly oriented groups (D and E) with an average direction of 174° and a randomly oriented group are also differentiated statistically.

We also infer a preponderance of juveniles and subadults heading north and subadults and adults heading south. The three intra‐herd groups that moved north may have been waves in a larger herd that wove, crisscrossed and veered somewhat from the hypothetical mean direction, presumably over a relatively short but unknown period of time. The concept of herd structure is examined and defined, ichnologically, as any orderly arrangement of trackway groupings within a larger group or herd that is clearly not the result of random movement of individuals. The concept refers to phenomena that are inherently variable and in some cases vague, but nonetheless amenable to statistical analysis.

Keywords:

• Cretaceous
• dinosaur footprints
• Caririchnium leonardii
• herd structure
• age structure
• New Mexico