Our analyses focused on a specimen of S. secans (NMNS-PV20561, Figure 1A), collected from the Phosphorite Bed II (Thanetian age, Paleocene) in the Ouled Abdoun Basin, Morocco. We scanned the specimen by using X-ray micro-Computed Tomography scanning and segmented internal structures. The three-dimensional reconstruction of internal structures of NMNS-PV20561 shows the thecodont-like implantation in ankylosis attachment to the vomer bone (Figure 1C as already observed in a cross-section by Ref. ). Furthermore, this study visualized the internal structure of the vomer with dental implantations and teeth replacement system (Figure 1B, C). There is a pulp cavity containing a tooth germ in the center of each functional tooth, in contrast to common fishes, in which their tooth germs are formed on the lingual side of the functional teeth (e.g., Ref. ). NMNS-PV20561 lacks any trace of bone resorption, i.e., the replacement pore (see  for terminology), on the buccal nor lingual side of the bone, unlike other fishes with several teeth row (e.g., wolf fishes and blue fishes; see Ref. ). In archosaurians (e.g., crocodiles and dinosaurs), their replacement teeth are formed on the lingual side of their functional teeth, and then dissolved the wall of their functional teeth roots and move to a cavity below the functional teeth (see Figure 1 and Ref. ). In mammals, replacement teeth and tooth germs of mammals are formed directly below functional teeth (Figure 1). We interpret that, in S. secans, replacement teeth would have developed between tubular root structures of the functional teeth and erupted from its position directly below the teeth rather than the side of functional teeth, which closely resemble the vertical mode of tooth development in mammals. In addition, NMNS-PV20561 possesses only one generation of tooth germs under their functional teeth and no buds or caps, indicating that NMNS-PV20561 shed only one time in its life.
The concurrent combination of dental traits, carnassial teeth with one shed replacement and the thecodont implantation, is also seen in all carnivorans and some marsupials (Thylacoleo spp., and Sarcophilus harrisii). The latter carnivorous marsupials appeared in Pliocene, whereas the basal Carnivora first appeared around 60 Ma in the Paleocene. The latter is contemporaneous to the appearance of S. secans (Supplementary 3 and 4; Figure 2), meaning that carnassial teeth were coincidently acquired by predators in both marine and terrestrial realms.
The end Cretaceous (i.e., K-Pg) mass extinction event was a turning point for both terrestrial and marine ecosystems [10–13]. In marine ecosystems, top predators, such as ammonites, large predatory fish, and mosasaurs, went extinct, as did top predators in terrestrial ecosystems as well. It has been proposed that the absence of these top trophic predators left ecological niches that remained empty or unoccupied until the later Paleogene. The appearance of both S. secans and the earliest carnivorans around 60 Ma, 6–7 My after the K-Pg event, may not have been coincidental: their ages are broadly coincident with a global warming event, the Early Late Paleocene Event (ELPE) [15, 16] and ELPE, which triggered faunal turnover ,. Estimates of ecosystem recovery from K-Pg event required 100 years to 10 My both terrestrial and marine ecosystems[18–22]. These suggest that the acquisition of carnassial teeth must be closely related to filling the vacant ecological niche of top predators caused by the Cretaceous/Paleogene mass extinction and adjust with rapid global warming and faunal changes.
Although S. secans, carnivorans, and some marsupials have carnassial teeth, the teeth of fish and mammals do not share the same dental tissues or developmental processes. For example, the teeth of fishes consist of enameloid and dentin, while mammal teeth consist of enamel and dentin. Enameloid contains collagen and is made by odontoblasts and dental epithelial cells take place between a well-defined dental papilla (mesenchyme) and a dental organ (epithelium) , , while enamel is an inorganic material and made by epithelial cells interact with underlying mesenchymal cells in the underlying dental pulp [23, 25]. Therefore, carnassial teeth of S. secans are homoplastic convergence evolution because its origin and developmental process are distinctly different from that of mammals. In the evolutionary history of vertebrates, morphological homoplastic convergence appears in the appendicular skeletons (e.g., control or propulsion surfaces of limbs in volant tetrapods) but this is the first such evidence for convergence in the dentition between fish and mammals. This finding suggests that there may be more homoplastic convergence evolution between phylogenetically distant vertebrate groups, fishes and mammals, than the previously known.
There are some fishes that have functional teeth with the thecodont attachment type teeth. However, these fishes do not show the vertical mode of the replacement. For example, barracudas and parrotfishes adopt the replacement system that their legions of teeth fuse together and teeth constantly bursting from the soft tissue to replace old ones (e.g., ). As far as we know, S. secans is the only fish with vertical replacement mode in ankylothecodont teeth.
In this study, we not only provided the first evidence of tooth replacement in pycnodont fishes but also showed that flesh-eating specialist Serrasalmimus secans had a vertical mode of tooth replacement, which is previously characteristic exclusively in mammals. Both lineages with carnassial teeth first appeared less than eight million years after the K-Pg mass extinction event and likely reflects concurrent filling of empty top trophic niches in both the marine and terrestrial realms. Their materials and developmental process are clearly different, but they share same characteristics and functions. This is the first case of homoplastic convergence evolution between fish and mammals and shares aspects of gene expression and regulation like fins and limbs .