Topic: Teeth in aquatic reptiles
Aquatic reptiles tend to display one of three dentition types, well adapted to either seize and slice large vertebrate prey, pierce and gouge slippery fish, or entrap small prey such as crustaceans.
Several groups of reptiles have independently evolved adaptations to life in aquatic environments, from freshwater rivers to near-shore and open marine habitats. One of the most important of these adaptations is dentition, as distinctly specialised teeth and jaws are critical to capturing and processing prey underwater. In spite of the diversity of reptile taxa that have taken to feeding in water, a limited range of tooth types have been described, presenting us with a clear example of convergent evolution. Distinct aquatic reptile groups tend to display one of three dentition types, well adapted to either seize and slice large vertebrate prey, pierce and gouge slippery fish, or entrap small prey such as crustaceans. The following paragraphs describe each of the prey-specific dentition types in turn, detailing instances of convergent evolution between relevant reptiles (both extinct and living), and indeed between reptiles and even more distantly related groups, such as mammals and birds.
Seizing large prey
Carnivorous aquatic reptiles that feed on large vertebrates (e.g. large fish, or tetrapods that come to water to drink or breed) must be able to manage the powerful struggle and potentially slippery skin of their prey. The teeth of these predators tend to be broad, conical and recurved (‘caniniform’), as well as very sharp, providing an optimal means to capture, subdue and slice up prey into edible pieces. In many cases the teeth are all of a similar form (termed ‘homodonty’), sometimes with particularly large ‘caniniforms’ at a middle-anterior position in the jaw to ensure a secure and lethal bite. Robust dentition is enhanced in these and most other aquatic reptiles by association with a dorso-ventrally compressed skull, allowing fast, lateral head movements for swiftly grabbing prey without excess water resistance. In addition, the teeth are well rooted within a strong, reinforced jaw, enabling resilient prey to be successfully subdued.
Well known examples of marine reptiles with robust, sharp teeth for seizing large prey include Mesozoic predators such as ichthyosaurs (e.g. Ichthyosaurus, Stenopterygius) and mosasaurs (e.g. Tylosaurus, Goronyosaurus), both of which were fish-eaters (technically termed ‘piscivores’) with powerful skull structure and carnivorous dentition. Ichthyosaurs lived from the Early Triassic (around 245 – 230Ma) to the end of the Cretaceous period (65Ma), acquiring an increasingly ‘homodont’ dentition of uniformally conical, recurved teeth: Early Triassic ichthyosaurs such as Mixosaurus, and Grippia were succeeded by voracious, fully homodont taxa such as Ichthyosaurus. Mosasaurs were dominant marine predators of the Late Cretaceous that evolved from primitive Early Cretaceous piscivores known as aigalosaurs (e.g. Carsosaurus). The dentition, strong jaw structure and slender, dorsoventrally compressed skull of both ichthyosaurs and mosasaurs are clearly convergent adaptations, as they evolved independently within the large reptile group ‘Lepidosauromorpha’, which encompasses all Mesozoic reptiles, lizards, snakes and their allies.
Key characteristics of ichthyosaur and mososaur dentition are also to be found in specialised aquatic predators that are widely divergent from marine reptiles in evolutionary terms, including an impressive number of archosaurs (a major reptile group distinct from the lepidosaurs), lamnid sharks and even certain mammals. These diverse aquatic vertebrates provide us with rich window into convergent evolution, as similar solutions to being large marine predators was achieved in distantly related creatures.
Within the archosaurs, several crocodile-like lineages evolved skull and tooth features convergent upon aspects of ichthyosaur-mosasaur design. These lineages included short-snouted members of the Late Triassic alligator-like Parasuchidae (e.g. Nicrosaurus kapfii, Smilosuchus), robust-skulled crocodylomorphs of the Dyrosauridae (e.g. Phosphatosaurus, Hyposaurus, Guarinisuchus), primitive crocodiles from the family Proterosuchidae (e.g. Chasmatosaurus), and members of the more familiar Crocodylidae (crocodiles) and Alligatoridae (alligators). One of the most specialised Late Cretaceous mosasaur dentitions is seen in Goronyosaurus nigerienses, which had similar feeding habits to modern crocodiles and displays enlarged, forward-pointing premaxillary ‘caniniform’ teeth. Goronyosaurus shares important features of tooth and skull morphology with a dyrosaurid called Sokotosuchus ianwilsoni that lived alongside it, as well as modern crocodiles (e.g. Crocodilus niloticus). Fossil evidence suggests that when mosasaurs went extinct at the end of the Cretaceous (65Ma) their niche was initially filled, at least in some parts of the world, by dyrosaurids such as Guarinisuchus and lamnid sharks that had similarly powerful dentition and recurved teeth. Elsewhere among the archosaurs, a number of pterosaurs (e.g. Cearadactylus, Dorygnathus) had teeth adapted for seizing large fish from the water during flight. Furthermore, from within a very derived lineage of dinosaurs, the Cretaceous diving bird Hesperornis regalis also evolved broad, sharp-edged and recurved teeth as adaptations to piscivory.
Following the extinction of marine reptiles such as mosasaurs (65Ma) and dyrosaurs (around 50Ma), several marine mammals from within the derived order Cetacea independently evolved sharp, robust teeth for grasping and slicing large prey. Most well known among these are the killer whale (Orcinus orca), which attacks creatures such as sharks, fish, seals, dolphins, penguins and even other whales, and the false killer whale (Pseudorca crassidens), which has a similar diet.
Piercing slippery prey
Aquatic carnivorous reptiles that feed exclusively on fish are equipped with dentition adapted to pierce and gouge the flesh of fast, slippery prey. Teeth are slender, pointed and conical, providing a piercing grip. To reduce water resistance during swift, lateral head movements when grabbing prey, the snout is generally long and narrow, and the head dorso-ventrally flattened. The skull and jaws are not very strongly reinforced, as fish prey do not usually show great physical resistance once caught.
A number of widely divergent piscivorous reptiles display specialised piercing dentition; these include extinct taxa such as the well-known plesiosaurs, some dyrosaurids (e.g. Dyrosaurus), long-snouted Triassic parasuchids (e.g. Paleorhinus, Rutiodon, Mystriosuchus), and the living gharial (Gavialis gangeticus) of India, a large crocodile-like reptile of the family Gavialidae. Plesiosaurs (e.g. Plesiosaurus, Thalassomedon) and their close relatives such as pachypleurosaurs, pistosaurs and nothosaurs are all part of a large group of Mesozoic lepidosaurs (called ‘euryapsids’), whereas dyrosaurids, parasuchids and gavialids are all archosaurs.
The wide evolutionary separation between plesiosaurs, dyrosaurids, parasuchids and gavialids makes them an illuminating instance of convergence, and yet remarkably similar adaptations to an exclusively piscivorous diet are also known in a group of marine creatures entirely unrelated to any kind of reptile at all. Namely, among marine mammals known as cetaceans, we find the dolphins (family Odontoceti), which have converged on plesiosaur-like dentition to an extreme degree. For example, the Pacific White-sided dolphin (Lagenorhynchus obliquidens) possesses many slender, pointed teeth, ideal for piercing fast-moving fish, and the Long-snouted Spinner dolphin (Stenella longirostris) has – as its name suggests – an elongated narrow jaw containing up to 252 long, thin, pointed teeth, similarly equipping it for a diet of fish.
Trapping small prey
A group of Early Permian (320 – 280Ma) marine reptiles known as mesosaurs had remarkable dentition, comprising a large number of long, slender, pointed teeth shallowly implanted in the upper jaw (premaxilla and maxillary bone) and lower jaw (dentary bone), leaving very little space between teeth when the jaws were closed. Teeth were all similar in form (homodont), but tooth orientation varied from vertical at the back of the mouth to more outward-pointing at the front, and the tip of each tooth curved inwards towards the palate. Mesosaurs (e.g. Mesosaurus tenuidens, Stereosternum tumidum) were gharial-like in having a long, narrow snout and plesiosaur-like in having a compressed, lightly-built skull and a long neck – optimally designed for reducing drag and pitching of the head when in motion. The mysterious dentition of Mesosaurus was initially proposed to be an adaptation to filter-feeding, but closer examination of how teeth interlocked in life has shown this to be untrue. The very long, slender teeth served to entrap very small nektonic animals (e.g. crustaceans), effectively ‘caging’ them in the mouth before swallowing them whole. This interpretation is further supported by the fact that mesosaur teeth show little evidence of physical wear, as they were not used for grasping or chewing, and each tooth was thin, long (approximately 3cm) and yet only weakly attached to the jaw bone, indicating lack of a mechanical role in prey acquisition. Mesosaurs are primitive, Palaeozoic members of the ‘anapsid’ reptiles (or ‘para-reptiles’), inhabiting the so-called ‘Mesosaurus Sea’ prior to the Mesozoic radiation of ‘diapsid’ reptiles (archosaurs and lepidosaurs) and ‘synapsids’ (mammals and their relatives). The Mesosaurus Sea was rich in notocaridid crustaceans, a Palaeozoic version of ‘krill’ that abound in Antarctic seas today. These small (a few cm long) crustaceans were very probably the primary, if not exclusive, component of the mesosaur diet.
Mesosaur dentition appears to be unique in both form and function within the reptiles, and yet it arguably has a convergent counterpart in the highly complex post-canine teeth of the crabeater seal Lobodon carcinophagus. This unusual marine mammal is a cetacean that lives in the pack-ice zone of Antarctica, feeding not on crabs (as its name would suggest) but on Antarctic krill (Euphausia superba). Behind two pairs of canine teeth, Lobodon possesses five pairs of strangely shaped post-canines; each cusp is covered in multiple deep lobes, and the teeth of upper and lower jaws closely interlock save for the gaps between lobes on opposed teeth. This dentition provides an ideal prey entrapment mechanism, as water and krill (few cm long crustaceans) are sucked into the mouth, and then water expelled with the multilobed teeth firmly close, retaining Euphausia as they are too large to fit through the gaps.
The multi-lobed post-canines of Lobodon carcinophagus are a functional analogue to the long, thin cage-like teeth of Mesosaurus, as both cage and prevent the escape of small crustacean prey. These two animals have not shared a common ancestor since the divergence of anapsid and synapsid reptiles, and yet they have converged upon an astonishingly similar general mechanism of using dentition to trap the only prey widely available in their respective marine ecosystems.
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Map of Life - "Teeth in aquatic reptiles"
January 23, 2018