A morphometric approach for addressing tooth-based species delimitation in fossil mako sharks,Isurus(Elasmobranchii: Lamniformes)

Abstract

ABSTRACT-Morphological studies of fossil and extant shark teeth have typically been qualitative in nature, with resulting taxonomic problems due to the complicated forms of heterodonty exhibited by many sharks. This is apparent in the designation of fossil species assigned to Isurus (Lamniformes), where the status of the putative Neogene fossil species Isurus xiphodon and I. hastalis is solely based upon supposed differences in tooth morphology. Here we apply the geometric morphometric techniques of Procrustes superimposition, resampling-based Goodall's F-test, and canonical variates analysis to extant Isurus teeth, with the goal of assessing whether these quantitative analytical techniques provide a more objective basis for taxonomic decisions than do traditional qualitative morphological descriptions. These quantitative techniques are then applied to I. hastalis and I. xiphodon to examine whether I. xiphodon should be considered a junior synonym of I. hastalis or a separate species. Results show that geometric morphometric techniques can differentiate between the two extant species as well as the two extinct species, supporting I. xiphodon as a valid taxon. We suggest that this type of analysis is applicable for examining tooth-based species delimitations of sharks with both extant and extinct members, and has the potential to be applied to other fossil shark species as well.

Origin of the white shark Carcharodon (Lamniformes: Lamnidae) based on recalibration of the Upper Neogene Pisco Formation of Peru

“…xiphodon as a nomen dubium based on the arguments of Leriche (1926), the absence of any type specimens, and an unlikely provenance. Recent morphometric analysis of broad‐toothed ‘mako’ specimens by Whitenack and Gottfried (2010) supports a morphological difference based on the broadness of the crown in broad‐toothed ‘makos’. While this study might accurately separate these species, this could also represent sexual dimorphism or ontogenetic changes in tooth morphology in one taxon.…”

Do sharks exhibit heterodonty by tooth position and over ontogeny? A comparison using elliptic Fourier analysis

“…In general, studies of shark tooth morphology have often been conducted using linear or geometric morphometrics (French et al, ; Marramà & Kriwet, ; Nyberg, Ciampaglio, & Wray, ; Whitenack & Gottfried, ; Whitenack & Motta, ), but these methods do not fully capture the complexity of tooth morphology in most instances (Crampton, ). Unlike linear and geometric morphometrics, elliptic Fourier analysis (EFA) is able to create a more accurate representation of complex organismal morphologies by characterizing the whole outline of the structure of interest (Ferson, Rohlf, & Koehn, ; Kuhl & Giardina, ).…”

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

“…Isolated fossil material, particularly unarticulated teeth, are difficult to assign to specific taxonomic groups. 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.…”