Late Jurassic–Early Cretaceous trackways of small-sized sauropods from China: New discoveries, ichnotaxonomy and sauropod manus morphology

The growing database on sauropod tracksites, particularly from China, raises questions about hypotheses that wide-gauge sauropod trackways with low heteropody (Brontopodus) dominate the global sauropod track record in the Cretaceous. It also raises questions about the definition of narrow-, medium- and wide-gauge trackways and the quality of preservation needed to use such labels reliably. A number of Lower Cretaceous sauropod tracksites from China have yielded trackways of small-sized sauropods with pronounced heteropody that have been named Parabrontopodus. These co-occur with medium-sized to large Brontopodus trackways giving rise to at least two possible interpretations regarding sauropod trackmakers at these sites: 1) trackways were left by two different, smaller narrow gauge and larger wide gauge, taxonomic groups, 2) trackmakers belong to the same taxonomic group and were narrow gauge when smaller and wide gauge when larger, therefore not maintaining a constant gauge during ontogeny as inferred from some assemblages. The presence of different taxonomic groups is further evidence that narrow gauge trackmakers, previously considered typical of the Jurassic, persisted into the Early Cretaceous. This could be part of a regional trend in East Asia and some other regions such as the Iberian peninsula in Europe, where similar trackways have been found. Alternatively, this could reflect a previously unrecognized global trend. More track and skeletal data are needed to corroborate these hypotheses, because presently essential parameters are missing.