Limuloid trackways from Permian-Triassic continental successions of North China

Forty-four well-preserved specimens of Kouphichnium, a typical limuloid (horseshoe crab) trackway, are reported and systematically studied for the first time from continental Upper Permian to Lower Triassic successions in North China. A new ichnotaxonomic system of Kouphichnium is proposed according to the ichnotaxobase of limuloid trackways, including simple foot imprints (T), pusher imprints (P), median impressions (M) and genal spine impressions (GS). In total, one ichnogenus and six ichnospecies are recognized, one of which is new: Kouphichnium liulinensis sp. nov. Ichnogenus Paramphibius is regarded as a synonym of Kouphichnium and the diagnoses of ichnogenus Kouphichnium and its two early ichnospecies, i.e. type ichnospecies Kouphichnium lithographicum Oppel, and Kouphichnium didactylum Willard, are revised. A morphological-palaeoethological pattern is proposed in reference to these ichnospecies from the continental Upper Permian to Lower Triassic successions in North China. These six ichnospecies are assigned to six types in the new ichnotaxonomic system and the locomotion rates of each type can be ranked by the possible speed index λ. The size of trackmaker is estimated from the external width of the trackway, and it shows a drastic decrease from the Late Permian to Early Triassic, followed by a gradual increase in the Early Triassic. This size variation of trackmakers (limuloids) during the Permian-Triassic transition, especially the drastic drop, probably reflects the intensive environmental stress during the biotic crisis. In addition, the analysis of the sizes of pusher imprints (P) from certain well-preserved trackways suggests a potential sexual dimorphism of limuloids.