On the need of providing tooth morphology in descriptions of extant elasmobranch species

Abstract

Elasmobranchii is a clade of chondrichthyans (cartilaginous fishes) that comprises sharks, skates and rays represented today by approximately 1,200 species. Chondrichthyans have a long evolutionary history dating back to the Late Ordovician (ca. 450 million years ago [Mya]) based on isolated dermal denticles (Janvier 1996). Other remains such as articulated skeletons and teeth are known from the Lower Devonian (ca. 410 Mya: Mader 1986; Miller et al. 2003). The fossil record of modern elasmobranchs (Neoselachii) can be traced back to the Early Permian (ca. 290 Mya) and is represented by isolated teeth (Ivanov 2005), with fossils of crown group sharks and rays appearing in Lower Jurassic (ca. 200 Mya) rocks (e.g., Cappetta 2012). Since their appearance in the geological record, elasmobranchs are mainly represented by isolated teeth, whereas articulated skeletons are very rare and restricted to a small number of fossil localities (e.g., Cappetta 2012). The scarcity of skeletal remains in their fossil record is due to their poorly mineralized cartilaginous skeleton that requires special taphonomical conditions to be preserved. Elasmobranch teeth, in contrast, are composed of highly mineralized tissues (hydroxyapatite) that have a strong preservation potential (Shimada 2006). In addition, elasmobranchs replace their teeth continuously over the course of their life span (polyphyodonty) and therefore shed thousands of teeth in their lifetime (Reif et al. 1978; Schnetz et al. 2016) leading to large numbers of potential fossils. These morphologically highly diverse isolated teeth constitute much of the rich fossil record of elasmobranchs, and largely form the basis of our understanding of elasmobranch diversity and evolution through geological time.

Climate cooling and clade competition likely drove the decline of lamniform sharks

“…The elasmobranch fossil record is mainly based on isolated teeth, resulting in a particularly good preservation potential compared to other marine vertebrates (21). However, lack of information on tooth morphology of some living taxa may hamper the identification of some of their fossil representatives (28). As opposed to some elasmobranch groups, lamniforms have most of their living species represented in the fossil record (12 over 15) and their dentition and tooth morphology are well known.…”

A new genus and species of extinct ground shark, †Diprosopovenator hilperti, gen. et sp. nov. (Carcharhiniformes, †Pseudoscyliorhinidae, fam. nov.), from the Upper Cretaceous of Germany

“…However, controversial issues concerning the taxonomic content and phylogenetic relationships of Scyliorhinidae, as well as their longevity, still prevail because the family Scyliorhinidae is repeatedly assumed to represent an assemblage of unrelated taxa based on molecular data (e.g., Winchell et al, 2004;Iglésias et al, 2005;Human et al, 2006;Vélez-Zuazo and Agnarsson, 2011;Naylor et al, 2012), thus rendering the familial allocation of fossil carcharhiniforms characterized by scyliorhinid-like dentitions difficult (Maisey, 2012). In addition, our knowledge of extant carcharhiniforms (and elasmobranchs in general) is strongly biased toward external body anatomy and proportions as well as color patterns, whereas detailed dental and endoskeletal information often is omitted from species diagnoses provided by elasmobranch neontologists, resulting in significant negative effects in assessing the taxonomic and systematic affinities of fossil species properly (Guinot et al, 2018).…”

“…The unifying argument for inclusion of fossil taxa in this group is based on rather generalized tooth morphologies that, however, show distinct characters that could help to better understand their systematic placement within carcharhiniforms. However, a better understanding of dental patterns in extant carcharhiniform sharks also is mandatory (see also Guinot et al, 2018). The discovery of a new carcharhiniform shark, †Diprosopovenator hilperti, gen. et sp.…”

Scyland3D: Processing 3D landmarks

“…Living organisms display a high diversity of shapes and sizes that are used to characterise ontogenetic stages and phylogenies (Guinot et al, 2018;Jones, Smaers, & Goswami, 2015;Musa, Czachur, & Shiels, 2018). Shape similarities are used as phylogenetic signal -mostly in extinct species-and, when combined with ecological data, allow describing trophic habits of species (Randau, Goswami, Hutchinson, Cuff, & Pierce, 2016).…”