Rhynchocephalia

Rhynchocephalia is an order of lizard-like reptiles that includes only one living species today, the tuatara (Sphenodon punctatus) of New Zealand. Although there are very few rhynchocephalians left, they were once a large and diverse group during the Mesozoic era.

The oldest fossils of rhynchocephalians date back to the Middle Triassic around 244 million years ago, and they were found all over the world by the Early Jurassic. Most rhynchocephalians belong to the suborder called Sphenodontia, which means 'wedge-teeth'. They are most closely related to modern lizards and snakes, and together they are part of the group called Lepidosauria.

Rhynchocephalians have special traits that set them apart from other reptiles, like rib-like gastralia bones in their belly and a proatlas bone in their neck. Many also have teeth that are fused to the tops of their jaws, a feature also seen in some modern lizards like agamids. During the time of the Cretaceous period, rhynchocephalians declined greatly and almost disappeared by the start of the Cenozoic era. The tuatara today mainly eats insects and small animals, but in the past, some rhynchocephalians were plant-eaters, while others had special ways of living, including some that lived in water.

Research history

Tuatara were first described in 1831 and were thought to be a type of lizard. It wasn’t until 1867 that a scientist named Albert Günther noticed they looked more like birds, turtles, and crocodiles. He created a new group called Rhynchocephalia, meaning “beak head,” for the tuatara and its ancient relatives. Later, in 1925, another scientist named Samuel Wendell Williston suggested a more specific name, Sphenodontia, just for tuatara and their closest ancient relatives. Over time, many different species were incorrectly added to this group, making it messy. Later studies showed that many of these species didn’t belong and helped scientists better understand this group of animals.

Anatomy

Rhynchocephalia and their close relatives, the Squamata, which include lizards, snakes, and amphisbaenians, belong to the group Lepidosauria. They share some special traits, like being able to lose their tails when threatened and having certain bones in their skulls and bodies.

One special feature of rhynchocephalians, like the tuatara, is a small "third eye" on top of their heads. This eye can't see details but helps them sense light and dark, which is important for knowing day from night. Most modern animals have lost this eye, but it was common in ancient times.

Rhynchocephalians also have some different traits from other lizards. For example, they often have extra bones in their bellies and lack certain bones in their lower jaws. These differences help scientists understand how these special reptiles evolved over millions of years.

The size of rhynchocephalians can vary a lot, from very small to quite large. The tuatara, the only living type today, can live a very long time, sometimes over 70 years in the wild.

Classification

The group Rhynchocephalia includes mostly extinct reptiles, but today we have one living species called the tuatara, found only in New Zealand. Scientists study how these reptiles are related to each other, and they have found several groups or "clades" within Rhynchocephalia.

One major group is called Sphenodontia, which includes many extinct species and the tuatara. Some scientists have divided Sphenodontia into smaller groups. For example, Eusphenodontia includes species like Polysphenodon and Clevosaurus hudsoni, while Neosphenodontia includes the tuatara and its closest relatives. These groups help scientists understand how these ancient reptiles evolved and changed over millions of years.

Ecology

The fossil record shows that rhynchocephalians were a diverse group that lived in many different places and had many different ways of getting food. Early rhynchocephalians had small teeth and probably ate insects. Some rhynchocephalians, called pleurosaurs, lived in the ocean and had long, snake-like bodies. They likely ate fish. Other rhynchocephalians, such as Kallimodon and Leptosaurus, might have spent time in water and also ate fish.

Eilenodontines are thought to have eaten plants, using wide teeth to grind them up. The sapheosaurids, such as Oenosaurus and Sapheosaurus, had special broad teeth that they used to crush hard-shelled animals. Some scientists think that Sphenovipera might have had venom, but this idea is debated. Pamizinsaurus had bony plates on its body, possibly to protect it from predators. Sphenodraco likely climbed trees, unlike the mostly land-based tuatara. Other extinct rhynchocephalians with longer limbs, like Navajosphenodon and Homoeosaurus, might have also been good climbers.

Evolutionary history

Rhynchocephalia, meaning 'beak-heads,' are a group of lizard-like reptiles. Today, only one species, the tuatara of New Zealand, is alive. But in the past, especially during a time called the Mesozoic, they were much more common and varied.

Scientists debate when these reptiles first appeared. Some think it was around 270 to 252 million years ago, while others suggest around 242 million years ago. The oldest known rhynchocephalian is called Agriodontosaurus, found in the UK and dating to about 244 to 241.5 million years ago. Another early one is Wirtembergia from Germany, about 238-240 million years old.

During a time called the Late Triassic, rhynchocephalians spread worldwide. By the end of this period, they lived across a supercontinent called Pangaea. They came in different sizes, from small to larger, and some were specialized for eating plants or living in water. In the Jurassic period, they were the most common small reptiles globally.

Their numbers began to fall in the Early Cretaceous period. They disappeared from places like North America and Europe, possibly because newer lizards and mammals competed with them. But they stayed in parts of South America, where they were much more common than lizards. The youngest known rhynchocephalians outside of New Zealand lived in Patagonia about 64-63 million years ago. The tuatara likely lived in New Zealand when it separated from Antarctica between 80 and 66 million years ago.