The oldest occurrence of Lepisosteus (Ginglymodi: Lepisosteiformes: Lepisosteidae) in Gondwana

Lepisosteiformes fishes diversified during the Aptian-Cenomanian into two lineages: Obaichthyidae, restricted to South America, Africa and Europe; and Lepisosteidae in North America. However, the recent description of enigmatic lepisosteid specimens from the Albian–Cenomanian Açu Formation (Potiguar Basin, Northeast Brazil) challenged the understanding of the paleobiogeographic trajectories of the clade during the early Late Cretaceous. The Açu 4 Subunit records an estuarine paleoenvironment characterised by a rich vertebrate diversity in central Gondwana, with fish remains predominantly represented by scales and fragmentary bone elements. Here, we describe three ganoid scales aiming to refine the taxonomy of the local paleoicthyofauna and to explore their paleobiogeographic significance. Scanning Electron Microscopy revealed the scale micro-ornamentations, allowing the measurement of their tubercle diameter (4.49 μm) and spacing (3.74 μm). Based on this morphometric data, the specimens MCC 3268.1-V, MCC 3268.2-V and MCC 3268.3-V are tentatively assigned to the genus Lepisosteus. This represents the oldest record of Lepisosteus in the Southern Hemisphere. Our findings challenge previous studies that proposed the emergence of crown-group lepisosteids around 100 Ma (early Cenomanian) in North America. Furthermore, the data support a scenario of rapid dispersal for the clade and a Cenomanian age for the Açu 4 Subunit.