Abstract
The shapes of vertebrate teeth are often used as hallmarks of diet. Here, however, we demonstrate evidence of frequent piscivory by cartilaginous fishes with pebble-like teeth that are typically associated with durophagy, the eating of hard-shelled prey. High-resolution micro-computed tomography observation of a jaw specimen from one batoid species and visual investigation of those of two additional species reveal large numbers of embedded stingray spines, arguing that stingray predation of a scale rivalling that of the largest carnivorous sharks may not be uncommon for large, predatory batoids with rounded, non-cutting dentition. Our observations demonstrate that tooth morphology is not always a reliable indicator of diet and that stingray spines are not as potent a deterrent to predation as normally believed. In addition, we show that several spines in close contact with the jaw skeleton of a wedgefish (Rhynchobatus) have become encased in a disorganized mineralized tissue with a distinctive ultrastructure, the first natural and unequivocal evidence of a callus-building response in the tessellated cartilage unique to elasmobranch skeletons. Our findings reveal sampling and analysis biases in vertebrate ecology, especially with regard to the role of large, predatory species, while also illustrating that large body size may provide an escape from anatomical constraints on diet (e.g. gape size, specialist dentition). Our observations inform our concepts of skeletal biology and evolution in showing that tessellated cartilage—an ancient alternative to bone—is incapable of foreign tissue resorption or of restoring damaged skeletal tissue to its original state, and attest to the value of museum and skeletal specimens as records of important aspects of animal life history.
Highlights
A long-standing paradigm in vertebrate functional morphology dictates that the gross form of a tooth attests to its function and can be used to delineate broad dietary differences [1,2,3,4]
There is no information available on the animal’s size and maturity; using computed tomography (CT) scans of a R. djiddensis specimen (LACM 38116-24) from a previous study [31], we measured the gape and jaw widths as approximately 7 and approximately 11%, respectively, of the animal’s 76 cm total length (TL). This was similar to the gape/TL ratio for the R. australiae shown in figure 1
The presence of so many stingray spines in the jaw tissue of this Rhynchobatus specimen, the location of the spines in tissue on both the labial and lingual sides of the jaw and around the jaw joint, the fact that some spines appear to have been embedded for a long period of time, and the similar appearance of jaw specimens from sharks known to feed on stingrays (e.g. [23]) all argue that this wedgefish was preying on stingrays with some regularity
Summary
A long-standing paradigm in vertebrate functional morphology dictates that the gross form of a tooth attests to its function and can be used to delineate broad dietary differences [1,2,3,4]. The elasmobranch fishes (sharks, rays and relatives) have confounded the notion of an invariant, simplistic link between tooth form and diet They are far less speciose, elasmobranchs possess a dental diversity that rivals that of mammals [13,14], with teeth often described by their inferred function (e.g. as ‘clutching’ or ‘tearing’ or ‘crushing’ teeth [7,15]). Pronounced differences in cusp shape or curvature do not necessarily translate to differences in puncturing, cutting or crushing performance [14,16,17,18], some species eat foods that are far harder or softer than their dentitions suggest [13,19], and two species have been shown to be able to reorient their teeth to perform multiple roles not predictable by their tooth morphology [13,20]
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