Scanning electron microscope examination of the dental enameloid of the Cretaceous durophagous sharkPtychodussupports neoselachian classification

Abstract

Although Ptychodus teeth are well known in Late Cretaceous marine deposits in North America and Europe and a few specimens with jaw elements have been discovered, the taxonomic position of the shark genus Ptychodus is enigmatic due to the lack of preservation of diagnostic material other than teeth. These sharks possessed a pavement dentition suited to a diet of hard-shelled macroinvertebrates (durophagy), leading several studies to variously describe Ptychodus as a batoid, a hybodont shark, or a selachimorph. Members of the Selachimorpha share one dental synapomorphy, a triple-layered enameloid (TLE) consisting of an outer shiny-layered enameloid (SLE) of randomly oriented hydroxyapatite crystallites, a middle layer of parallel-bundled enameloid (PBE), and an inner layer of tangled-bundled enameloid (TBE). Batoids and hybodonts both have teeth with single crystallite enameloid (SCE). We examined teeth from Ptychodus collected from the Lincoln Limestone of the Greenhorn Formation of Barton County, Kansas, and compared their enameloid ultrastructure with that of a Carboniferous hybodontiform and the Cretaceous lamniform shark Squalicorax curvatus Williston, 1900. Scanning electron microscopic examination of Ptychodus shows that crystallite bundling in the form of a TLE is evident in these teeth. The PBE is most apparent at transverse enameloid ridges of Ptychodus teeth. Columns of dentine penetrate into the tooth enameloid, and the crystallites near the dentine are randomly oriented. These observations bolster the argument that Ptychodus is a genus of highly specialized selachimorph shark, rather than a hybodont or batoid.

Micro-computed tomography imaging reveals the development of a unique tooth mineralization pattern in mackerel sharks (Chondrichthyes; Lamniformes) in deep time

“…Many sharks, rays and skates (Elasmobranchii) previously have been considered to have the osteodont histotype according to the traditional definition of histotypes (pulp cavity filled with osteodentine) 20,21,25 : Myliobatiformes 22 , Ptychodus 22,47,48 , Hexanchiformes 41 , Squatiniformes 49 , Heterodontiformes 22 , Orectolobiformes 49 , Hemipristis spp. (Carcharhiniformes) 25,26,40 , Lamniformes 26,31,34,43 .…”

Enameloid‐bound δ15N reveals large trophic separation among Late Cretaceous sharks in the northern Gulf of Mexico

“…From a single fossil site, we measured δ 15 N EB of Squalicorax kaupi, Scapanorhynchus texanus (both Lamniformes, here on referred to as lamniforms), Meristodonoides sp. (Hybodontiformes, from here on, hybodontiforms), and Ptychodus mortoni (order incertae sedis; e.g., Hoffman et al, 2016;Jambura & Kriwet, 2020). All four taxa have distinct tooth morphologies (including molariform and cuspidate forms) and, thus, different inferred tooth functions (e.g., crushing, clutching, piercing, cutting) and predicted diets.…”

“…Estimated trophiclevel offsets between lamniforms, Meristodonoides and Ptychodus, based on a TDF of 4.1 ± 0.8‰. dentin inclusion during drilling due to thinner enameloid(Hoffman et al, 2016). However, fossil dentin retains so little organic N(Lüdecke et al, 2022) that, if present, it would have a minimal effect on measured δ 15 N EB .…”

A new cuspidate ptychodontid shark (Chondrichthyes; Elasmobranchii), from the Upper Cretaceous of Morocco with comments on tooth functionalities and replacement patterns

“…The marked interspecific and intraspecific morphological variabilities displayed by the dentitions of † Ptychodus often complicate its taxonomic identification and systematic position to and within elasmobranchs ( Nicholls, 2010 ). Nevertheless, a recent histological-based study considers this enigmatic predator as closely related to elasmobranchs rather than extinct hybodontiform sharks (see Hoffman et al, 2016 ).…”