Tooth and consequences: Heterodonty and dental replacement in piranhas and pacus (Serrasalmidae)

Abstract

Tooth replacement in piranhas is unusual: all teeth on one side of the head are lost as a unit, then replaced simultaneously. We used histology and microCT to examine tooth‐replacement modes across carnivorous piranhas and their herbivorous pacu cousins (Serrasalmidae) and then mapped replacement patterns onto a molecular phylogeny. Pacu teeth develop and are replaced in a manner like piranhas. For serrasalmids, unilateral tooth replacement is not an “all or nothing” phenomenon; we demonstrate that both sides of the jaws have developing tooth rows within them, albeit with one side more mineralized than the other. All serrasalmids (except one) share unilateral tooth replacement, so this is not an adaptation for carnivory. All serrasalmids have interlocking teeth; piranhas interdigitate lateral tooth cusps with adjacent teeth, forming a singular saw‐like blade, whereas lateral cusps in pacus clasp together. For serrasalmids to have an interlocking dentition, their teeth need to develop and erupt at the same time. We propose that interlocking mechanisms prevent tooth loss and ensure continued functionality of the feeding apparatus. Serrasalmid dentitions are ubiquitously heterodont, having incisiform and molariform dentitions reminiscent of mammals. Finally, we propose that simultaneous tooth replacement be considered as a synapomorphy for the family.

Phylogenomics of piranhas and pacus (Serrasalmidae) uncovers how convergent diets obfuscate traditional morphological taxonomy

“…Tooth shape is not predictive of diet among most herbivorous and carnivorous piranhas. The specialized scale-feeding wimple piranha, Catoprion mento , has teeth unlike any of its relatives (Kolmann et al, 2018a, 2019), while Catoprion ’s sister taxon Pygopristis , the only serrasalmid with pentacuspid teeth, feeds on plants and insects as an adult (Fink, 1989; Nico, 1991). But there are several notable cases of morphological convergence.…”

“…Paleontologists studying heterodont elasmobranchs have a long-established history of skepticism when designating extinct species or evaluating taxonomic affinity based solely on isolated teeth (Shimada, 2005; Whitenack & Godfried, 2010; Marrama & Kriwet, 2017). Similarly, convergent and heterodont tooth morphologies are quite prevalent among serrasalmids, but also most characiforms in general (Murray et al, 2004a; Kolmann et al, 2019), and this phenomenon may lead taxonomic classification of fossils astray. Without a quantitative evaluation of tooth characters across pacus and relevant outgroups, we argue that assignment of isolated teeth to certain lineages or taxa is fraught with uncertainty.…”

“…The oldest fossils associated with Serrasalmidae are isolated pacu-like teeth from the Bolivian El Molino Formation (∼73-60 mya; Gayet, 1991; Gayet & Meunier, 1998). Although often used to date the origin of the family, these fossil teeth are unusual for serrasalmids for several reasons: (1) their small size of < 0.75-1.0 mm (Gayet, 1991; Gayet & Meunier, 1998; Gayet et al, 2001) despite most serrasalmid teeth being far larger (>1 cm; Shellis & Berkovitz, 1976; Kolmann et al, 2019); (2) pacu teeth, while complex in shape, lack the lingual cusp visible in Gayet et al (2001, Fig. 7d); (3) these fossil teeth lack any of the interlocking morphologies typical of extant serrasalmid dentitions (Kolmann et al, 2019); (4) the timeline of Bolivian serrasalmid fossil teeth leave a 25-32 mya gap in the fossil record until the first confidently-identified serrasalmid tooth (∼38 mya; DeCelles & Morton, 2002; Dahdul, 2007; Lundberg et al, 2010).…”

“…Cione et al (2009) and more recent studies have expanded the breadth of ecological and morphological diversity among serrasalmids. For example, some species of pacus have heterodont dentitions which vary along numerous phenotypic axes, from molariform to incisiform, spatulate to crenulate (Huie et al, 2019; Kolmann et al, 2019). In fact, tooth characteristics and arrangement remain the best characters for distinguishing pacu taxa (Andrade et al, 2013, 2016; Nico et al, 2018).…”

The Natural Historian's Guide to the CT Galaxy: Step-by-Step Instructions for Preparing and Analyzing Computed Tomographic (CT) Data Using Cross-Platform, Open Access Software

“…2018 ), and track changes in the orientation of anatomical structures across specimens ( Kolmann et al. 2016 , 2019 ; see also Workflow Step 9.d below). Volume renderings cannot be used for 3D printing other downstream processes that take place outside of 3D Slicer such as FEA.…”