Tooth mineralization and histology patterns in extinct and extant snaggletooth sharks, Hemipristis (Carcharhiniformes, Hemigaleidae)—Evolutionary significance or ecological adaptation?

Abstract

Shark jaws exhibit teeth that are arranged into distinct series and files and display great diversities in shapes and structures, which not only is related to their function (grasping, cutting, crushing) during feeding, but also bear a strong phylogenetic signal. So far, most research on the relationship between shark teeth and feeding ecology and systematics focused on the external tooth morphology only. Although the tooth histology of sharks has been examined since the early 19th century, its functional and systematic implications are still ambiguous. Shark teeth normally consist of either a porous, cellular dentine, osteodentine (in lamniform sharks and some batoids) or a dense layer of orthodentine (known from different sharks). Sharks of the order Carcharhiniformes, comprising ca. 60% of all extant shark species, are known to have orthodont teeth, with a single exception—the snaggletooth shark, Hemipristis elongata. High resolution micro-CT images of jaws and teeth from selected carcharhiniform sharks (including extant and fossil snaggletooth sharks) and tooth sections of teeth of Hemipristis, other carcharhiniform and lamniform sharks, have revealed that (1) Hemipristis is indeed the only carcharhiniform shark filling its pulp cavity with osteodentine in addition to orthodentine, (2) the tooth histology of Hemipristis elongata differs from the osteodont histotype, which evolved in lamniform sharks and conversely represents a modified orthodonty, and (3) this modified orthodonty was already present in extinct Hemipristis species but the mineralization sequence has changed over time. Our results clearly show the presence of a third tooth histotype—the pseudoosteodont histotype, which is present in Hemipristis. The unique tooth histology of lamniform sharks might provide a phylogenetic signal for this group, but more research is necessary to understand the phylogenetic importance of tooth histology in sharks in general.

Evolutionary trajectories of tooth histology patterns in modern sharks (Chondrichthyes, Elasmobranchii)

“…mayumbensis, but not in †Physogaleus sp., which could be caused by diagenetic processes that altered the chemical constitution (and density) of the enameloid (see also Jambura et al 2019). Both species display the orthodont histotype of most carcharhiniformn sharks except for Hemiprists (Jambura et al 2018).…”

“…Kriwet et al ; Cappetta, ). The role of tooth composition in feeding performance or as a taxonomic criterion has been tested for distinct taxa in the past, but remains ambiguous (Radinsky, ; Glickman, ; Whitenack et al ; Moyer et al ; Jambura et al ; Martínez‐Pérez et al ; Jambura et al ).…”

“…The dentinal core can consist of orthodentine, osteodentine, or both. Orthodentine exhibits parallel, branching tubules that were originally described to surround a hollow pulp cavity (Radinsky, ; Smith & Sansom, ), but which can also surround a dentinal core of osteodentine in shark teeth (Jambura et al )). Originally, two histological patterns were distinguished in shark teeth, the orthodont histotype (presence of a hollow pulp cavity) and the osteodont histotype [the pulp cavity is replaced by osteodentine that extends from the root (Tomes, ; Glickman, ; Compagno, ; Moyer et al ; Schnetz et al ; Martínez‐Pérez et al )].…”

“…Originally, two histological patterns were distinguished in shark teeth, the orthodont histotype (presence of a hollow pulp cavity) and the osteodont histotype [the pulp cavity is replaced by osteodentine that extends from the root (Tomes, ; Glickman, ; Compagno, ; Moyer et al ; Schnetz et al ; Martínez‐Pérez et al )]. More recent studies, however, defined three histotypes, depending on the presence or absence of orthodentine and osteodentine in the crown: (i) orthodont teeth, which have a central hollow pulp cavity, encapsulated by a substantial layer of orthodentine; (ii) pseudoosteodont teeth, in which osteodentine intrudes from the root into the hollow pulp cavity, which in fully mineralized teeth is replaced by an osteodont core that is surrounded by orthodentine; and (iii) osteodont teeth, in which no orthodentine is developed, but the complete dentinal core of the crown consists of osteodentine and replaces the hollow pulp cavity (Jambura et al ; Jambura et al ).…”

“…Two well‐examined shark groups in regard to their tooth histology are carcharhiniform and lamniform sharks. With the exception of the snaggletooth shark Hemipristis elongata , which has pseudoosteodont teeth, carcharhiniform sharks display the orthodont histotype (Compagno, ; Compagno, ; Herman et al ; Herman et al ; Jambura et al ). In contrast, the sister group, lamniform sharks, exhibit the osteodont tooth histology, except for the basking shark Cetorhinus maximus , which also has pseudoosteodont teeth (Moyer et al ; Schnetz et al ; Jambura et al ; Jambura et al ).…”