A "Mammalian-like" Pycnodont Fish: Independent Acquisition of Thecodont Implantation, True Vertical Replacement, and Carnassial Dentitions in Carnivorous Mammals and a Peculiar Group of Pycnodont Fish.

Kumiko Matsui,Yuri Kimura

doi:10.3390/life12020250

Abstract

Vertebrates developed tooth replacement over 400 million years ago. Then, 200 million years later, the combination of vertical tooth replacement with the thecodont implantation (teeth in bone sockets) appeared a key morphological innovation in mammalian evolution. However, we discovered that an extinct fish taxon, Serrasalmimus secans, showed the same innovation in the lineage Serrasalmimidae, which survived the end Cretaceous mass extinction event. The carnassial teeth are known in both mammals and pycnodont fish, but these teeth do not share the same tissues or developmental processes. Therefore, this serrasalmimid pycnodont fish might have independently acquired mammal-like tooth replacement and implantation, indicating that the fish and mammals convergently evolved the carnassial dental morphologies at about the same time, approximately 60 My ago, in separate ecosystems.

Highlights

Vertebrates first acquired a tooth replacement mechanism during the latest Silurian, 424 million years ago [1], which played a key role in shaping the predator-prey relationships over time
The family Serrasalmimidae is characterized by the reduced tooth rows in their ankylothecodont-like dentition
Our analysis focused on a specimen of S. secans (Figure 1A, Figures S1–S4, Tables S1 and S2; Supplementary Material S1), which was collected from the Phosphorite Bed II (Thanetian age, Paleocene) in the Ouled Abdoun Basin, Morocco

Summary

Introduction

Vertebrates first acquired a tooth replacement mechanism during the latest Silurian, 424 million years ago [1], which played a key role in shaping the predator-prey relationships over time. The pycnodont fishes, Pycnodontiformes, are an extinct clade of Actinopterygii, which lived from the Late Triassic to the late Eocene [5] Within this clade, the family Serrasalmimidae is characterized by the reduced tooth rows in their ankylothecodont-like dentition (i.e., absence of true socket, but root-like structure firmly fused to the bone as in other pycnodonts). The compressed mammary-form teeth of S. secans show vertical wear facets by shearing on the labial side of the vomerine dentition and the lingual side of the prearticular dentition. This pattern resembles the carnassial teeth of mammals (though, in a reverse contact). We hypothesize that serrasalmimids independently acquired a vertical replacement in true thecodont implantation, which is a characteristic tooth replacement pattern of mammals

Methods

Results and Discussion

Conclusions