Turtles' Head Rotation: Unveiling Their Unique Neck Flexibility

Turtles are well-known for their ability to retract their heads inside their shells, a behaviour often associated with protection from predators. However, recent research suggests that this ability may have evolved for hunting purposes rather than defence. There are two main suborders of turtles: Cryptodira, which includes most turtles and tortoises, and Pleurodira, also known as side-necked turtles. Cryptodires retract their heads by lowering them and contracting their vertebrae to allow their heads to go beneath their spines, completely hiding their necks. On the other hand, Pleurodires fold their heads sideways along their bodies and into one of their leg compartments, leaving part of their necks exposed. While the ability to retract their heads provides turtles with protection, it is important to handle them with care to avoid causing injury or stress to these fascinating creatures.

Turtles' necks: protection or food?

Turtles are well-known for their ability to retract their heads into their shells, a behaviour often associated with protection from predators. However, recent research has revealed a more complex story behind the evolution of this trait. By examining the fossil record and studying the behaviour of modern turtles, scientists are uncovering new insights into the purpose and evolution of turtles' neck retraction abilities.

The Traditional View: Protection

For years, evolutionary researchers assumed that turtles evolved the ability to retract their heads as a defence mechanism. When faced with danger, turtles can quickly pull their heads into their shells, seemingly finding safety behind their shield-like carapace. This interpretation of neck retraction as a protective measure was reinforced by the common cultural depiction of turtles, such as in the Cold War-era animated film where a turtle serves as a symbol of protection during a nuclear attack.

A New Perspective: Food

However, this traditional view has been challenged by a 2017 study published in the journal Scientific Reports. Paleontologist Jérémy Anquetin and his team examined the fossil of Platychelys oberndorferi, a turtle species that lived in central Europe around 150 million years ago. This particular fossil preserved the delicate vertebrae of the turtle, providing crucial insights into its neck retraction abilities.

The study found that P. oberndorferi, despite being classified as a pleurodire (a type of turtle that typically pulls its head into the shell sideways), had vertebrae shaped more like those of cryptodires, turtles that pull their heads straight back into their shells. Specifically, P. oberndorferi's vertebrae allowed for partial retraction of the head straight back, rather than to the side. This partial retraction would offer little protection against predators, as the vulnerable, curled neck would still be exposed.

The shape of P. oberndorferi's vertebrae suggests that neck retraction initially evolved for a purpose other than defence. One possible explanation is that it provided an advantage when hunting. Modern turtles, such as the mata mata turtles of New Zealand and North America's snapping turtles, use their neck retraction abilities not just for protection but also for hunting. They launch their necks violently forward to attack prey, gaining momentum and surprising their victims. The ability to partially retract their necks, as P. oberndorferi likely did, would enable these turtles to strike their prey with greater force.

A Complex Evolutionary Story

The evolution of neck retraction in turtles is a complex process that likely involved multiple factors and purposes. While protection from predators may have played a role, the original purpose of this trait may have been to enhance hunting abilities, as seen in modern turtles with similar neck retraction capabilities. This interpretation aligns with the concept of exapation, where features that evolved for one purpose are co-opted for a different function.

Further research is needed to fully understand the evolution of neck retraction in turtles, and well-preserved early turtle fossils are scarce. However, by studying the behaviour and anatomy of modern turtles, scientists can gain valuable insights into the different functions of neck retraction and how this trait may have evolved over time.

Pleurodires vs cryptodires

Turtles are divided into two groups based on how they retract their necks: pleurodires and cryptodires. Pleurodires, or side-necked turtles, are able to retract their heads by folding their necks sideways along their bodies under the shell. On the other hand, cryptodires, or hidden-neck turtles, lower their necks and pull their heads straight back into their shells. This difference in neck retraction is due to the morphology and arrangement of cervical vertebrae in the two groups. Pleurodires have narrow vertebrae with closely positioned zygapophyses, while cryptodires have more distended vertebrae with widely spaced zygapophyses, allowing for a condition called ginglymoidy, or the ability to bend the neck into an S shape for retraction.

The evolution of neck retraction in turtles has long been assumed to be for protection against predators. However, new research suggests that it may have originally evolved for a different reason: hunting. Platychelys oberndorferi, a species of turtle that lived in central Europe around 150 million years ago, had vertebrae shaped similarly to those of cryptodires, suggesting that it may have been an early example of neck retraction evolution for hunting rather than defence.

Cryptodires are a suborder of Testudines that include most living tortoises and turtles, such as freshwater turtles, snapping turtles, tortoises, softshell turtles, and sea turtles. They are characterised by their ability to retract their heads in the vertical plane, resulting in restricted lateral movements outside the shell. Cryptodires evolved from pleurodires during the early Jurassic period in South America and Southeast Asia. By the end of the Jurassic, cryptodires had largely replaced pleurodires in lakes and rivers and had started to develop land-based species.

Pleurodires, on the other hand, became the dominant freshwater testudines in the Cretaceous to Eocene period in Europe and also produced a family of marine species known as Bothremydidae. Pleurodires are believed to have evolved over 200 million years ago, with cryptodires branching off from them around 165 million years ago. While the primary purpose of neck retraction in cryptodires may have been for hunting, it likely provided secondary defensive benefits against predators.

The Platychelys oberndorferi species

The Platychelys oberndorferi had a shell made up of sharp knobs and a wide skull with eyes close to the nose. It is classified as a pleurodire, which means it turns its neck to pull its head into its shell sideways. However, the Platychelys oberndorferi's vertebrae were shaped to allow the turtle to partially pull its head straight back into its shell, not to the side like other pleurodires. This partial retraction of the head provides no protective benefits as a predator could easily reach and attack the turtle's curled, vulnerable head. Therefore, it is hypothesised that this trait evolved for hunting rather than protection.

How to handle a turtle

Turtles are becoming increasingly popular as pets, but they are not like cats or dogs and require different handling. Turtles prefer living in complete solitude and do not enjoy being handled or petted. However, they can get accustomed to human presence, and most species can identify their caregiver as the source of food.

How to Pick Up a Turtle

If you need to pick up a turtle, the method will depend on its size and type. For a baby turtle, use your thumb and fingers to encircle its shell and lift it onto your palm. Ensure the turtle is secure and cannot jump off. For larger turtles, use both hands, placing each hand on each side of the shell between the front and back legs. Be cautious of aquatic and semi-aquatic species, as they can kick very strongly and have sharp claws.

What Not to Do

• Never pick up a turtle by its tail, as this can cause painful bone injuries.
• Avoid twisting or turning the turtle quickly, as this can cause pain to its connecting tissue and organs.
• Do not keep the turtle on its back for long, as this position is very stressful for the animal.
• Avoid sudden hand movements, as these can stress the turtle.
• Do not put your fingers between the top and bottom shell of a box turtle, as they can clamp down tightly and pinch your fingers.
• Be cautious of long-necked turtles, such as common snappers, soft-shelled turtles, and snake-necked turtles, as they can reach around their shell and bite.
• Avoid handling large softshell turtles without expert help, as they are difficult to pick up safely and can easily kick or bite.
• Never scratch the shell, as this can cause fungal infections.
• Do not drop the turtle, as this can cause severe injuries, including a broken shell or torn limbs.

Turtle Behaviour

Turtles will often try to communicate their discomfort when being handled. If a turtle withdraws into its shell, makes a hissing sound, tries to scratch or kick, attempts to bite, or empties its cloaca on your hand, it is expressing its fear and concern for its safety.

Health and Safety

Turtles can carry Salmonella bacteria on their skin, which can be harmful to humans. Always wash your hands before and after handling a turtle and disinfect any surfaces you touch. Supervise children closely when they interact with turtles, and do not allow them to put their hands in their mouths. Pregnant women should avoid handling turtles altogether.

The evolution of the turtle shell

While it is not known exactly how far a turtle can rotate its head, we do know that there are two broad groups of turtles based on how they retract their necks: pleurodires, which turn their necks to pull their heads into their shells sideways, and cryptodires, which pull their heads straight back into their shells.

Now, onto the evolution of the turtle shell. Turtles are a unique group of vertebrates, and their shells are their most striking feature. The shell is what makes them easily recognisable as turtles, but how they evolved to have shells is still an area of great research interest and is strongly debated in the scientific community.

Learning from fossils

Fossils are the best indicators of what ancient fauna looked like. For a long time, the oldest turtle fossils we knew of were dated to about 210 million years old and belonged to the species Proganochelys quenstedtii. This turtle was roughly 70 cm long and lived in the area of today's Germany, Greenland, and Thailand, suggesting it was found all over the ancient continent of Laurasia. The species was first discovered and described in 1887 by German palaeontologist Georg Baur. Proganochelys had a full shell covering the upper and lower parts of the body, just like modern turtles.

The evolution of the shell

In 2008, a novel 40 cm fossil was described from China, consisting of a turtle-like reptile with only half a shell. The species, named Odontochelys semitestacea, or 'toothed turtle with half a shell', had a hard shell on the underside of the body, like the plastron of modern turtles, but the upper part, the carapace, was missing. Instead, Odontochelys only had enlarged ribs, indicating that the bottom part of the turtle shell evolved before the top. Since the fossil was found in marine deposits, one hypothesis describes the evolution of the plastron as a defence mechanism against predators coming from beneath in a marine environment. The Odontochelys fossil predates Proganochelys by roughly 10 million years, thus moving turtle origins back in time to about 220 million years ago.

The 'grandfather turtle'

In 2015, another fossil further improved our knowledge of the connection between turtles and other reptiles. Pappochelys rosinae, the 20 cm 'grandfather turtle', is a small reptile with significantly enlarged and flattened ribs. In contrast to other fossil turtles, Pappochelys did not have a shell and thus most likely represents a transitional form between lizard-like reptiles and turtles, what is colloquially referred to as a 'missing link'. Pappochelys rosinae is similar to other lizards, such as Eunotosaurus africanus, which also had enlarged and T-shaped ribs, elongated vertebra, and a generally rather round body shape. Both species were terrestrial animals capable of digging. Eunotosaurus was first speculated to be an ancestor of turtles in 1892. Scientists currently argue that the enlarged ribs are a result of the fossorial lifestyle of both, granting higher stability during digging activity.

Older than dinosaurs

Pappochelys dates back to the Triassic period, about 240 million years ago, and Eunotosaurus is even older, at 260 million years old. Thus, the origin of turtles can be traced back to a time before the dinosaurs became the dominant vertebrates on land. By the time Tyrannosaurus rex set foot on what is now North America (68 million years ago), Proganochelys, as one of the first fully shelled turtles, was already extinct for 140 million years.

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