Diversity and discrimination of large ornithopods revealed through their tracks (Lower Cretaceous, Spain): A phenetic correlation approach

Abstract

Large ornithopods were frequent in the alluvial-to-coastal sedimentary systems developed during the Barremian in the Iberian Peninsula. Here, several pes track casts of large ornithopods from the Barremian Camarillas Formation of the southwestern sector of the Maestrazgo Basin (Iberian Basin, Spain) are analyzed. Two morphotypes of tracks from three different size classes are differentiated based on some morphological features, such as length of digits, the width of the “heel” pad impression, and width of the proximal part of digit III. These casts are assigned to Caririchnium isp. Morphotype 1 is represented by small to large-sized tracks, whereas Morphotype 2 by medium-sized tracks. Morphological variation seen among tracks of Morphotype 1 is interpreted to be possibly an ontogenetic variation. On the contrary, the differences seen between Morphotype 1 and 2 cannot be explained only by extramorphological causes, so might represent different trackmakers, especially when taking into account the considerable Barremian ornithopod diversity. A modified phenetic correlation is carried out between several well-preserved casts and the identified feet of Barremian ornithopods from Iberia, in order to infer the putative trackmakers. Morphotype 1 fits with the foot skeletal structure of the large styracosternan Iguanodon, including small-sized tracks that are perfectly accommodated to that of perinates of I. galvensis. Morphotype 2 matches better to the pes skeletal structure of the medium-sized Mantellisaurus. Similar two morphotypes can be also identified in other areas of the Lower Cretaceous of Europe, so, they could be tentatively produced by the two sized groups of styracosternans typical in the Early Cretaceous of Europe. Therefore, the ichnological analysis may allow differentiation of the diversity known through the direct record. In addition, the paleoecological analysis indicates a spatial niche partitioning between two size groups of styracosternans, because large-sized tracks and bones of these dinosaurs are the most common in such sediments.