Where The Turtle Got Its Shell?
Our planet has a rather intriguing trait in that it's the only planet that has life on it. Throughout evolution, the rise in organisms and their complexity has reached unprecedented heights never heard of before. Throughout the years, researchers have extensively worked on deducing the secrets of the past but one of the theories I find intriguing is about one of my favourite organisms in Turtles. They are one of the most beautiful and complex creatures that have mostly been unchanged for almost 210 million years now. They are also abundant in our world's oceans from the Atlantic to the Pacific except for the Antarctic and Arctic waters. In this article, I'll be talking about this interesting evolutionary theory on how turtles got their shell. Even Crush from Finding Nemo is excited about this!!
Who are Turtles?
Turtles are organisms that are found in fresh and seawater around the world except for Antarctica and the Arctic. They belong to the order of reptiles called Testudines. There are around 360 living species of turtles, including freshwater turtles and terrapins. The largest of them is the Leatherback Sea Turtle (Dermochelys coriacea) is the largest and heaviest of the currently existing turtles weighing around 500 kgs and reaching lengths of 6 feet. The most minor however is a native to the African continent in the Speckled tortoise (Chersobius signatures) measuring less than 10 cm and weighing around 172 gm.
The turtles and tortoises share something common we all know by now in their distinct shell they possess. Still, unlike tortoises except for the Galapagos tortoises other land-dwelling tortoises can retract their head and limbs inside their body, this isn't the case with turtles. Turtles can't retract their flippers and head inside their shells, therefore, making them easy targets for predators and marine debris. The shell in turtles and tortoises are made by the fusion of Carapace (top) and Plastron (bottom) forming a bony covering over these organisms from their ribs. But there are ancestors or closely related species that have shown either no shells or something in terms of only flattened lower surface. To learn more about these animals we'll have to dive deeper towards the Permian period.
The Origins
During the Permian period, the abundance of organisms was observed with reptiles displacing amphibians as the dominant land-dwelling organism found on Earth at the time. This was the start of the most successful organisms that thrived for more than 260 million years. During this time, one such organism in Eunotosaurs Africanis roamed around Southern Africa during the middle Permian. Eunotosaurs have been the subject of being an early ancestor of modern-day turtles. Eunotosaurs lived in the swamps of Southern Africa and had a unique morphology, they possessed 9 flat elongated trunk ribs in their midsection and also possessed teeth something not present in modern-day tortoises and turtles.
These elongated ribs were the base of the theory of whether the eunotosaurus africanus was an early ancestor of modern-day turtles and tortoises we see today.
The Triassic
The Triassic was the beginning of the Archosaurs becoming the dominant group of organisms that branched into many organisms such as Crocodiles and one of the most successful groups of organisms in dinosaurs till the end of the late Cretaceous period. But we’re talking about turtles here, Odontocheyls semitestacea is another species of stem turtle that lived in this period around 220 million years ago in the waters of China. They also possessed similar morphology to that of Eunotosaurus but they had a well-developed plastron but an underdeveloped carapace along with a set of neural plates, it is now believed that the carapace of turtles and tortoises are developed from these neural plates.
The Cretaceous
Now, in the period with the major dominance of dinosaurs, one of the largest turtles swam the waters of North America. Archelon was a prehistoric turtle that grew up to 4 metres in length from head to tail and weighed around 2 to 3 tonnes in weight. These turtles had a perfectly formed carapace and plastron which is commonly found in modern-day turtles today. Their shell had a leathery outer cover eerily similar to the Leatherback Sea turtle (Dermochelys coriacea) we see today. Another similarity includes the hook-shaped beak of these turtles. This beak allowed Archelon to hunt on small crustaceans, cephalopods, and other such creatures that roamed at the time.
Archelon went extinct around 66 million years ago along with the dinosaurs in the late Cretaceous but most researchers predict they went extinct due to climate change and rising sea temperatures.
Modern-Day Turtles
As mentioned above, there are around 360 species of turtles and tortoises but most of them are generally divided into 2 orders Pleurodira and Cryptodira. Pleurodira is also known as side-necked turtles as they retract their head sideways within the space between their legs, this order of turtles has cervical vertebrae that are spool-shaped and narrow consisting of biconcave centra that act as a double joint allowing sideways retraction of their heads Ex: Yellow-spotted River Turtle ( Podocnemis unifilis). Whereas, Cryptodira is called hidden-neck turtles where they lower their neck and retract their heads in their shell, this order consists of living turtles and tortoises ex: the Galapagos Giant Tortoise (Chelonoidis niger).
Another unique trait that turtles possess is that they make their nests by digging into the sand and laying their eggs but this is something very similar to not only their ancestors but also to some other groups of reptiles particularly crocodiles and other lizards. Their locomotion may be restricted due to a rigid spine but their movement gives another perspective on how evolution works. Despite being armed with flippers instead of claws, turtles are good at the art of digging which they use to dig burrows to lay their eggs.
Hawksbill turtles are known to lay the most eggs at around 110 to 200 eggs per clutch highest amount of eggs ever recorded was at about 238 eggs. Compared to the Leatherback Sea turtle who lay around 80 fertilized eggs. While turtles are solitary reptiles that only come in contact with each other during courtship and mating, turtles like the Kemps Olive Riddley (Lepidochelys kempii) come in tens of thousands and lay their eggs. This massive nesting event is known as an arribada.
So how did the turtle get their shell?
Turtles have existed for more than 100 million years and are one of the most intriguing creatures to ever exist on the planet. Their shell development is one of the most crucial elements to research based on how sis they develop such this adaptation and what role has it played throughout evolution towards the survival of their specific species and families. One of the most important points to consider is the formation of elongated ribs, these ribs grew on the outer skin forming a strong bony armour along the ventral side coinciding with the binding of neural plates on the dorsal side. These neural plates bind together and form lateral foldings in return forming an upper layer of osteoderms combing with the neural plates and fusing with the bony plastron and voila! The hard armoured fortresses of turtles we see today.
This is my take on how the turtles got their shells, there are so many mysteries that we have yet to shed light on this topic and we still have a lot to learn and unearth the secrets of evolutionary history that our world possesses.
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