Topic: Beak structures in reptiles and birds
Among reptile taxa with beak structures, we find several cases of convergent evolution, for example between turtles, Uromastyx lizards, a number of herbivorous dinosaurs and the tuatara (Sphenodon) of New Zealand.
The majority of reptiles possess a fairly simple dentition composed of peg-like teeth of similar shape and size throughout the jaw (termed ‘homodont’), used to grasp prey. These teeth are continuously shed and replaced throughout the lifespan, and the upper and lower tooth rows do not contact one another; instead, the lower tooth row rests inside the upper row when the jaw is closed. However, a diversity of more specialised reptile dentition has evolved, as adaptations for efficiently capturing and subduing prey, or processing food (e.g. by grinding or shearing) prior to swallowing. The following paragraphs concern dentition types that include a pointed ‘beak’ at the front of the upper jaw, specialized for acquiring and processing, or simply grasping food material. Among reptile taxa with such beak structures, we find notable cases of convergent evolution, for example between turtles (Order Testudines) and lizards of the genus Uromastyx, and between a number of herbivorous dinosaur groups and the tuatara (Order Sphenodontia) of New Zealand, with tentative functional analogues also being found in more distant taxa such as birds, certain synapsids (‘mammal-like reptiles’ and mammals) and even fish.
Convergent beak structures in turtles, Uromastyx lizards and birds
The turtles are a major group of anapsids or ‘para’-reptiles. Their relationship to other reptiles is currently defined by skull and palate morphology, as well as molecular evidence. Primitive turtles, or testudines, appear in the Triassic (e.g. Proganochelys, Paleochersis, Australochelys), and diversify through intermediate Jurassic forms such as Condorchelys, to give the 300 or so advanced turtle species living today. Some primitive turtles had peg-like teeth arising from a bone on the upper palate called the vomer, but ‘advanced’ turtles, characterising the group Chelonia, are united by a lack of vomerine teeth. Indeed, chelonians completely lack any kind of teeth (on the plate or jaw), and instead possess a hard ‘beak’, made of bone covered in horny material (keratin) to form a structure technically termed a rhamphotheca. The upper and lower beak surfaces meet at a ridged surface termed the ‘triturating’ or processing surface. The ridges are of different forms depending on the diet; herbivorous turtles (e.g. the Kalahari tent tortoise, Psammobates oculiferus) have grooves giving a serrated-edge to the ridge, suitable for cutting through tough plant material, and carnivorous turtles (e.g. the loggerhead turtle, Caretta caretta) have knife-sharp ridges capable of slicing through prey as hard as crabs and molluscs. Effective beak function is supported by a powerful jaw closure mechanism that evolved in the Jurassic, dependent on a ‘pulley’ system of muscles in the otic (ear) region.
The agamid lizard Uromastyx is a member of the squamates (lizards, snakes and amphisbaenians), a group of reptiles with ‘diapsid’ type skulls, and thus independent from the turtles and their relatives. In spite of this great evolutionary separation, Uromastyx adults possess a beak that is remarkably convergent in form and function with that of plant-eating chelonians such as the green turtle, Chelonia mydas. Juvenile Uromastyx have two pairs of sharp incisors in the upper and lower jaw for catching small prey during this early, carnivorous phase. As the lizard grows, anterior teeth are lost and a beak structure develops, highly adapted for the herbivorous, adult diet. The most anterior bone of the upper jaw, the pre-maxilla, grows downwards to overhang the lower jaw, resulting in extensive wear of juvenile incisors and lateral teeth, with the mature pre-maxillary ‘beak’ functioning as a single chisel-like tooth. Oblique wear and tooth loss also causes the contact between upper maxillary and lower dentary bones to sharpen, providing an effective cutting surface. Tooth replacement is repressed in Uromastyx species (e.g. the Indian spiny-tailed lizard, Uromastyx hardwicki), so the beak continuously retains its integrity as an excellent structure for cropping and cutting vegetation.
Aside from chelonians and Uromastyx, the only other group to have evolved an edentulous beak are the birds, and they are evidently the most successful animal group to have done so. The bird beak, or bill, is composed of porous or hollow upper (maxillary) and lower (mandibular) jaw bones, each covered in a thin keratin sheath (or ‘rhamphotheca’). Bills of distinct shapes and sizes are adapted for specific feeding strategies, in addition to grooming and courtship. For example, insects are caught in narrow, sharp beaks; seeds are picked up and broken in short, sturdy beaks; fleshy prey are caught and torn up by sharp, raptorial beaks (as in vultures); mollusks are picked up by strong, pointed beaks and broken against hard surfaces; long, thin beaks probe flowers for nectar, and mud or bark for invertebrates; and large, scooped beaks are adapted for skimming, dipping or filtering water for aquatic prey (as in sea petrels such as prions and shearwaters). Although the toothless bills of birds are more distinct and lightly built compared to the powerful rhamphotheca of turtles or the bony beak of Uromastyx, they nevertheless represent convergent loss of dentition and adoption of a specialized beak structure for feeding. Birds (group Aves) evolved from a branch of dinosaurs (theropods) which were members of the diapsid reptile grouping Archosauria, in contrast with Uromastyx which belongs to Lepidosauria, the other major diapsid group. As such, birds, Uromastyx lizards and turtles all evolved beaks from within lineages as distantly related as three reptilian groups could ever be, thus providing a notable case of convergence.
Convergent ‘beak’ structures in archosaurs and tuatara (Sphenodon)
Two groups of herbivorous dinosaurs (ceratopsians and hadrosaurs), one primitive archosaur family (Trilophosauridae) and the lizard-like tuatara of New Zealand all have an anterior ‘beak’, and yet unlike turtles and Uromastyx, their beaks are used in concert with complex food-processing dentition. The dominant function of this type of beak is thus to simply grasp prey or pluck vegetation before it is sliced or ground up by specialised teeth located further back in the mouth.
Ceratopsians and hadrosaurs are both advanced plant-eating dinosaurs that were dominant during the Late Cretaceous. Ceratopsians, or horned dinosaurs, were characterized by a notable anterior beak, a dense battery of shearing ‘cheek’ teeth, a large skull, a bony neck shield or ‘frill’, and two horns (e.g. in Zuniceratops) or three horns (e.g. in Triceratops) on the brow and nasal regions. The beak was entirely edentulous (toothless) and made of thin bone covered in a keratin sheath. The tip of the upper jaw, comprising a rostral bone sutured to the premaxillary, curved downwards and over the tip of the lower, upward curving predentary bone. Opposed curvature of the two jaws, combined with the light bone structure and blunt-edged beak margins indicates that the function of the ceratopsian beak was mainly to grasp and pluck vegetation, rather than to cut it. Behind the beak the premaxillary and anterior dentary bones were also toothless, but the maxilla and posterior dentary bones housed a battery of up to 40 teeth, each tooth the exposed member of a stacked column of 3-5 replacement teeth. In addition to the capacity for rapid tooth replacement, each tooth cusp was serrated, creating a continuous, powerful shearing surface, capable of slicing fibrous plant material such as palm or cycad fronds. Hadrosaurs, or ‘duck-billed’ dinosaurs had a toothless, bony beak that was broad, low and elongated, somewhat resembling a duck’s bill (hence the common name). Edmontosaurus is a typical hadrosaur, and its very close relative Anatotitan provides one of the most extreme examples, its whole frontal skull being flattened and elongated into a ‘duck-bill’ shape. The hadrosaur bill was presumably used to clip leaves and twigs from tough plants such as gymnosperms (e.g. cycads), which were then crushed by a powerful battery of maxillary and dentary teeth. The dental battery comprised a continuous ‘pavement’ of interlocking teeth, up to five stacked near-vertically at each position for continuous tooth replacement. The mobile upper jaw allowed effective use of the beak and also effective occlusion (contact) between teeth for grinding and crushing, resulting in rapid tooth wear from grinding on huge volumes of abrasive plant material.
Trilophosaurs inhabited what is now North America and Europe during the Triassic, and are generally termed ‘archosauromorphs’, as they are ancestral to true archosaurs such as dinosaurs and crocodiles. Trilophosaurus species were distinguished by an extremely heavily built skull, sharp anterior beak and complex ‘cheek’ teeth. The premaxillary and anterior dentary bones were without teeth and covered in keratin to form a tough, sharp beak. The skull and jaws of adults were heavily strengthened to withstand the impact of the rapid and forceful vertical chomping action that enabled the beak to cut through tough vegetation. Behind the edentulous beak were teeth, many of which are notable for their complexity and resemblance to the multi-cusped teeth of primitive mammals. Tuatara male” width=”218″ height=”218″>Teeth of the upper (maxillary) and lower (mandibular or dentary) jaws were expanded, bearing up to three cusps and interlocking when biting to make an effective surface for further cutting up plant material before swallowing. Thus, as in ceratopsians and hadrosaurs, the complex ‘cheek’ teeth were remarkable tools for processing fibrous plants, but depended on use of the edentulous beak in order to first pluck or cut vegetation to be eaten.
Tuataras, of the genus Sphenodon, are unusual primitive reptiles related to squamates (lizards, snakes and amphisbaenians) but belonging to their own Order Sphenodontia. The tip of the Sphenodon upper jaw is slightly hooked or beak-like and is used to grasp food items (e.g. insects) which are then passed into the mouth. Although the Sphenodon ‘beak’ externally resembles that of herbivorous Uromastyx, Uromastyx entirely lacks teeth whereas tuatara possesses simple teeth on both upper and lower jaws. Once food is acquired, a unique double upper row and single lower row of teeth interlock and together with posterior-anterior (‘proal’) jaw movement, work to shear and crush it before swallowing.
Given the independent evolutionary descent of the trilophosaurs, ceratopsians, hadrosaurs and sphenodontids, it is clear that their beaks, although varying slightly in form, are worthy of note as convergent jaw structures. Of note, these beak structures function in concert with specialized dentition further back in the mouth, unlike the entirely toothless and remarkably convergent beaks of turtles and Uromastyx.
Incisor teeth with beak-like function in fish and mammals?
It is interesting to note the structural and functional analogy between sharp reptilian beaks adapted to bite through tough material and the sharp incisors of certain non-reptile species. For example, in extinct pycnodont fish the two front teeth are chisel-shaped, incisor-like (‘incisiform’) and used in grasping prey; similar tooth form is also known in extant fish of the families Labridae (e.g. the long, re-curved front teeth of Xyrichthys razorfish) and Sparidae (e.g. the red porgy, Pagrus pagrus). Sharp front incisors are of course typical of the true mammals, and are also known in their closest antecedants, such as the symmetrodont Chronoperates (technically a primitive mammal) and mammal-like reptiles known as tritylodonts (e.g. Bientheroides, Bocatherium, Tritylodon). These incisor, or incisiform, teeth, serve a shared function with sharp reptilian beak structures owing to their chisel-like form at the front of the mouth. As such incisors and sharp beaks could be considered to represent convergent forms of dentition, both solving the problem of grasping prey or biting vegetation through a similar mechanism, but based on the evolution of different specialised structures in distantly related groups.
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Map of Life - "Beak structures in reptiles and birds"
December 9, 2018