Turtle tracks morphology: A neoichnological approach to fossil interpretation

Tetrapod tracks record not only the morphological features of autopods but also critical insights into locomotion, behavior, and paleoecology. Since the behavior of extinct trackmakers cannot be observed directly, neoichnological experiments using extant analogues are essential for interpreting the fossil record.

In this study, we investigate the relationship between gait, substrate conditions, and track morphology, employing the red-eared slider (Trachemys scripta elegans) as a model organism. Controlled experiments were conducted across three size classes of trackmakers on various substrates with differing grain sizes and moisture levels. Tracks and trackways produced during distinct behaviors—including steady locomotion, pausing, climbing, and paddling—were documented and analyzed via high-resolution 3D digitization to extract quantitative morphological parameters.

Our results demonstrate that substrate moisture is the primary determinant of overall track morphology and preservation potential, with preservation quality exhibiting a hump-shaped relationship with increasing moisture content. While grain size primarily influences the resolution of fine anatomical details (e.g., digit and claw marks), dry or near-saturated substrates are prone to collapse or flow, resulting in shallow, poorly defined impressions. In contrast, moderately moist, fine-grained, and cohesive substrates are optimal for preserving clear outlines. Notably, very high-moisture or subaqueous tracks may partially recover detail upon drying. While track size correlates positively with body size on similar substrates, preservation quality is not strictly size-dependent: larger individuals produce clearer tracks on firm ground but may yield inconsistent results on loose sediments. Furthermore, behavioral variations induce distinct morphological signatures even under identical substrate conditions.

Comparison with fossil records suggests that Chelonipus parvus is inconsistent with turtle locomotion and should likely be reassigned to a non-turtle trackmaker, whereas Chelonipus liui shows greater affinity with Emydhipus. These findings provide experimental benchmarks for identifying turtle tracks and underscore the utility of neoichnological experiments in reconstructing ancient environments and vertebrate communities.