Micro-CT imaging reveals the origin of the lamniform sharks

Sharks of the order Lamniformes comprise seven families with 15 extant species,including some of the mosticonic shark species, like the great white shark (Carcharodon carcharias) and †Otodus megalodon. Lamniform sharks have a rich fossil record dating back to the Cretaceous with the oldest confirmed members of thisgroup being known from the Barremian(125-130 mya). However, the origin of this group remains ambiguous, since sharksof the genus †Palaeocarcharias,small benthic sharks superficially resembling carpet sharks (Orectolobiformes)that are known from Middle Jurassic deposits(170 mya),are discussed to belong tothis order. We used high resolution micro-CT scans obtained with the micro-CT scanner SkyScan 1173, to examinethe tooth mineralization patterns of extantand fossil shark taxaand found three different tooth histologies: (1) orthodont teeth that have hollowpulp cavities surrounded by orthodentine, (2) osteodont teeth that have osteodentine andlack of orthodentine, (3) and pseudoosteodont teeth that have an osteodont core, which issurrounded by orthodentine.Our results show that exclusively osteodont teeth are only present in lamniform sharks and, therefore, suggest that this character represents a synapomorphic trait for this group. Additionally we could show that †Palaeocarchariasstromerishares this unique tooth histology pattern with all lamniform sharks, suggesting that this shark indeed is the oldest known ancestor of this group.