Herbivorous Reptiles

Lepidosaur herbivory describes herbivorous lepidosaurs. Living non-avian reptiles form a paraphyletic group that consists of over 9, 000 species of crocodiles, turtles, and lepidosaurs. The most diverse group, Lepidosauria, is first known from the Middle Triassic (240 million years ago) fossils, but likely originated in the Permian (approximately 300-250 million years ago). Living lepidosaurs, which include snakes, lizards, and rhynchocephalians, occupy a wide range of environments and niches. Though widely viewed as obligate carnivores, a small number of lepidosaurs are known to consume plant material. For example, there are roughly 3, 300 species of living lizards and approximately 3% of them eat at least some plants. Though the exact definition of herbivory varies significantly between scientists, most define herbivorous lepidosaurs as those that consume plants for approximately 70-90% of its diet.

Herbivory in fossil lepidosaurs[edit]

The fossil record of herbivorous lepidosaurs extends into the Late Triassic period, nearly 227 million years ago. A recently described sphenodontian reptile is one of the oldest known herbivorous lepidosaurs. This animal possesses a number of features that demonstrate an herbivorous lifestyle, such as a specialized jaw movement (propalinal) and closely packed teeth that appear to be useful in the shearing of plant material.Phylogenetic analysis shows that herbivory likely evolved in the sphenodontian clade once, specifically within in a group called the Opisthodontia. This group of herbivorous sphenodonts possessed wide, shredding teeth and went extinct during the Late Cretaceous (approximately 65 million years ago). These ancient herbivorous animals clearly demonstrate that the ability to consume plant material evolved very early in the history of lepidosaurs.

The evolution of herbivory in squamates (the group that includes lizards and snakes) is less well known, though herbivorous species are found in the fossil record. A large bodied lizard, Polyglyphanodon sternbergi, from the Late Cretaceous of Utah is likely an herbivore. The diet is reconstructed based on the tooth shape; a thin transverse blade-like tooth, with fine serrations, similar to living iguanas. Others, such as Barbaturex morrisoni, illuminate important ecological niches that living squamates do not fill in modern environments.B. morrisoni is an herbivorous acrodontan lizard found in the middle Eocene of Myanmar and is notable for its large body size (approximately 1 meter). The large body size demonstrates that competition with herbivorous mammals did not limit the mass of this genus.B. morrisoni also provides evidence that large lizards can occupy ecological niches that are commonly thought to belong solely to mammals, such as a large bodied terrestrial herbivore.

Modern herbivory[edit]

In their 2002 study, William Cooper Jr. and Laurie Vitt investigated the dietary patterns of over a hundred lizards and found that many groups have representatives that consume plant material. Omnivory has evolved independently several times within lepidosaurs, whereas the evolution of obligate herbivory has proved to be more rare. For example, in the Iguania clade, omnivory and herbivory have independently evolved at least nine times. When other lepidosaur clades are included, Cooper and Vitt (2002) demonstrate that omnivory evolved at least 32 separate times. Obligate herbivory, on the other hand, has only evolved 10 times in living, non-liolaemid lizards (nearly 7, 700 species). Though, one clade seems to be an exception to this pattern of infrequently developed herbivory. A 2004 study investigated a unique group of South American lizards called the Liolaemidae. The authors of this study found that this clade evolved herbivory approximately 65 times faster than other squamates. Specifically, herbivory has evolved 18 times in liolaemids (approximately 170 species). It has been clearly demonstrated that these herbivorous animals have evolved from omnivorous ancestors and not directly from carnivores. Unfortunately, many biological studies have not investigated the possible timing that key herbivorous characteristics evolved; leaving the amount of time these organisms have possessed these traits unknown. The distinctions in dietary ecology that are observed today are likely a result of early ecological differences between clades. The separation of the Iguania and Scleroglossa (a group including geckos, snakes, skinks, and varanids) clades in the late Triassic best demonstrates this. It is hypothesized that the early Scleroglossa possessed traits that were better suited for hunting, such as the ability to distinguish prey by means of chemical processes, whereas the Iguania did not possess these traits. Therefore, the current dietary diversity within the Iguania (including a large number of herbivores) is due to certain traits held by early representatives of this clade.