A new ichnospecies of Nihilichnus Mikuláš et al., 2006 from the Palaeogene of South America: trace makers and ichnological and palaeobiological implications

Dentalites on bones represent important evidence for revealing prey preferences and feeding ecology of extinct animals. Here, we present a new ichnospecies of Nihilichnus based on perforations on mammal jaw-based specimens from the Palaeogene of South America. The new ichnotaxon, Nihilichnus fissuratus isp. nov. is identified as a perforation with associated longitudinal bone fractures. We also present new concepts for ichnological studies in bone substrates: discrete and composite surface patterns and ‘antagonistic marks’. A discrete pattern represents a bone specimen with a single or a few organized machichnid traces, resulting in little modification of its surface. A composite pattern represents a bone specimen with many machichnid traces, with organized and non-organized (chaotically dispersed) machichnid traces, resulting in visible alteration of bone natural surface. These patterns correlate well with mammalian and mesoeucrocodylian tracemakers, considering Palaeogene (Palaeocene and Eocene) South American land faunas. The ‘antagonistic marks’ represent traces produced by occlusally antagonistic teeth (upper and lower teeth), sharing an opposed placement on a substrate. For these traces, the mark made by a jaw teeth-substrate contact is more developed than that of a skull teeth-substrate contact. Two of the studied specimens, medium-sized South American native ungulates (SANUs), display ‘antagonistic’ perforations assigned to Nihilichnus fissuratus isp. nov. on both lateral sides on the symphyseal part of their jaws, mandatorily identifying their predators as medium-sized mammals.