Extinct Turtles: From Research to Paleoecology

The study of extinct turtles is a fascinating field that requires a multidisciplinary approach, combining research from paleontology, ecology, and geology.

By analyzing fossil records, scientists can reconstruct the habitats and ecosystems of ancient turtles, providing valuable insights into their evolution and extinction.

The discovery of a 230 million-year-old turtle fossil in South Africa, for example, revealed a turtle species that lived in a coastal environment with abundant vegetation and marine life.

This fossil find highlights the importance of considering the paleoecological context when studying extinct turtles, as it helps to understand their adaptations and interactions with their environment.

The research history of Archelon is a fascinating story. The holotype specimen, YPM 3000, was collected from the Late Campanian-age Pierre Shale of South Dakota in 1895 by American paleontologist George Reber Wieland.

George Wieland described the specimen the following year, naming it Archelon ischyros, which translates to "mighty turtle" from Ancient Greek. The genus name comes from the words "first" and "turtle".

A second specimen, a skull, was discovered in 1897 in the same region. This find helped to solidify the species' identity.

In 1900, Wieland described a second species, A. marshii, which was later reclassified as Protostega marshii in 1909. This reclassification was a significant shift in the understanding of the species.

A third, mostly complete specimen was collected along the Cheyenne River in 1902. This find added to the growing body of knowledge about Archelon.

The largest specimen to date, nicknamed "Brigitta", was discovered in 1992 in Oglala Lakota County, South Dakota. It resides in the Natural History Museum Vienna.

A fresh viewpoint: Common Snapping Turtle Species

Archelon's elongated and narrow head was a distinctive feature, measuring up to 100 cm in length. The head was also hooked, with a beak that was probably covered in a sheath in life. The premaxillae, the front part of the beak, were elongated, contributing to the head's length.

The nostrils were elongated and unusually horizontal compared to sea turtles. The jugal bones, or cheekbones, were rounded, unlike those of sea turtles. The jaw moved in a hammering motion, likely due to the heavy cartilage that encased the articular bone.

Archelon's neck vertebrae were X-shaped, procoelous, and had a thick frame, indicating strong neck muscles. The carapace, or shell, had eight neuralia and nine pleuralia, with the plates forming highly irregular and finger-like sutures. The neuralia and pleuralia were particularly thin, while the ribs and shoulder girdle were unusually heavy.

Archelon's head was distinctly elongated and narrow, measuring 60 cm (2 ft) in length. This unique shape is due to the elongated premaxillae and maxillae, which make up the front part of the beak.

The beak was hooked and probably covered in a sheath, similar to the beaks of birds of prey. However, the cutting edge of the beak was dull compared to such animals.

The nostrils were elongated and rested on the top of the skull, slightly positioned forward, and were unusually horizontal compared to sea turtles. The jugal bones, or cheek bones, were rounded and didn't project as far as they do in other turtles.

Archelon's neck vertebrae were X-shaped, procoelous, and had a thick frame, indicating strong neck muscles. Five neck vertebrae were recovered from the holotype, and it probably had eight in total in life.

The thoracic vertebrae found were ten in number, increasing in size until the sixth and then rapidly decreasing. The humeri in the upper arms were proportionally massive, and the radii and ulnae of the forearms were short and compact.

The flippers of Archelon would have had a spread of between 490 and 610 cm (16 and 20 ft), with the more conservative estimate being the more likely one.

The carapace of Archelon is a remarkable feature, comprising eight neuralia plates closest to the midline and nine pleuralia plates connecting the midline to the ribs. The plates are mostly uniform in dimensions, with the exception of the two pairs of plates corresponding to the eighth thoracic vertebra, which are smaller.

Each of the second to fifth ribs projects at a right angle from the midline, measuring 100 cm (3.3 ft) in length. The ribs increase in thickness in the vertical direction distally, and are relatively larger and more well-developed than those of sea turtles.

The neuralia and pleuralia form highly irregular and finger-like sutures where they meet, and one plate may have lain over the other plate while the bone was still developing and malleable. This unique feature is likely an adaptation to compensate for the extra weight of the ribs and shoulder girdle.

The carapace is also characterized by osteosclerotic structures, where the bone is dense and heavy, serving as ballasts in life similar to the limb bones of whales and other open-ocean animals. This feature is also seen in open-ocean going vertebrates such as dolphins or ichthyosaurs.

The plastron, the underside of a turtle's shell, is a fascinating feature that's often overlooked. It's thick and measures up to 7 feet in length, as seen in a specimen described in 1898.

The plastron is made up of several bones, including the epiplastron, entoplastron, hyoplastron, hypoplastron, and xiphiplastron. In some turtles, like protostegids, the epiplastron and entoplastron are fused together into a single unit called an entepiplastron.

This entepiplastron is T-shaped, with a rounded top edge and a small projection at the center. The outward side is slightly convex and bends away from the body.

A thick ridge connects the hyoplastron, hypoplastron, and xiphiplastron, forming a solid structure. The hyoplastron features a large number of spines, with the first seven to nine spines being short and triangular.

The middle six spines are long and thin, while the last 19 spines are flat. There are no marks indicating contact with the entepiplastron.

The giant turtle, Stupendemys, was a massive creature that weighed in at 1,145 kg (~2,500 pounds). This is almost 100 times more than its closest living relative, the big-headed Amazon river turtle.

The carapace, or shell, of the Stupendemys turtle reached lengths of almost three meters, making it one of the largest turtles to have ever existed.

Archelon, a prehistoric sea turtle, was initially placed in the family Protostegidae in 1896. This family included smaller turtles like Protostega and Protosphargis.

Protostegidae was later split into two subfamilies: Chelospharginae and Protosteginae. Chelosphargis and Calcarichelys were assigned to the former, while Archelon and Protostega were placed in the latter.

Phylogenetic studies have revealed that protostegids represent a separate, ancient lineage that originated in the Late Jurassic. This means that Archelon does not share a marine ancestor with any sea turtle.

Here's a breakdown of the Protostegidae family:

In fact, protostegids are so distinct that they have been removed from the superfamily Chelonioidea, which includes all sea turtles.

Archelon was a turtle that lived on the soft, muddy seafloor and likely spent its time as a slow-moving bottom feeder.

The turtle's thick plastron suggests it probably spent a lot of time on the ocean floor. The jaws were adapted for crushing, implying the turtle ate large mollusks and crustaceans.

Archelon may have occasionally scavenged off the surface water, but it preferred shallower, calmer waters due to its weaker arms and less swimming power. It may have been a moderately-good swimmer, capable of open-ocean travel.

Archelon was an obligate carnivore, which means it only ate meat. This is supported by its thick plastron, which suggests it spent a lot of time on the soft, muddy seafloor.

The animal likely spent a lot of time on the seafloor because its jaws were adapted for crushing, implying it ate large mollusks and crustaceans.

Archelon probably ate thin-shelled, bottom-dwelling Cretaceous bivalves, some of which exceeded 120 cm in diameter. However, these bivalves were likely absent in the central Western Interior Seaway by the Early Campanian.

The beak of Archelon may have been adapted for shearing flesh, allowing it to target larger fish and reptiles, as well as soft-bodied creatures like squid and jellyfish.

Archelon may have also occasionally scavenged off the surface water. It probably had weaker arms than the leatherback sea turtle, which limited its swimming power and preferred shallower, calmer waters.

The similarity of the humerus/arm and hand/arm ratios of Archelon and cheloniids suggests they had poor development of the limbs into flippers and a preference for shallow water.

Archelon probably dug out a hollow in the sand to lay its eggs, like other turtles, and took no part in child rearing.

The paleoecology of the time period in which Archelon lived is fascinating. The Western Interior Seaway, where Archelon inhabited, was a shallow body of water with a muddy, oxygen-depleted seafloor, averaging around 180 meters below the surface.

Average water temperatures were around 17°C (63°F) in the Campanian. This was a relatively warm climate, and the area was teeming with life. Plesiosaurs, hesperornithiform seabirds, and mosasaurs, like Platecarpus, were all present in the Northern Inland Subprovince.

The seafloor was home to a diverse array of invertebrates, including ammonites, bivalves, belemnites, and nautilus. Sharks, like Squalus, Squalicorax, and Cretolamna, were also common in the area.

As the seaway migrated southward, Archelon may have been unable to migrate with it. This could have led to increased predation on its eggs and hatchlings, ultimately contributing to its extinction.

Here's a breakdown of the different turtle families that existed during the time period:

The extinction of Archelon and the Protostegidae family may have been linked to a cooling trend in the climate, which other turtles were able to survive due to their thermoregulatory capabilities.

Several extinct turtle species of the genus Chelonoidis were huge herbivorous tortoises. These species include the Bahamian giant tortoise, Cuban giant tortoise, and Lutz's giant tortoise.

The Floreana Island Tortoise, or C. Niger, was one of the lesser-known species. Unfortunately, very little is known about these turtles.

The strong powerful jaws of C. indica were strongly serrated, and males were bigger than females.

The Mauritius Giant Tortoise was a species that went extinct shortly after the discovery of the Mascarene Islands. It was endemic to Mauritius and was closely related to the Saddle-backed Mauritius giant tortoise.

These tortoises were relatively small, measuring between 23.6 to 27.5 inches in length. They fed on fallen leaves, grass, and fruits on the forest floors.

The C. triserrata was another extinct giant of the genus Cylindraspis. It was easy to hunt and collect, which led to its demise.

The main threats to the C. triserrata included overcollection for food, fat, and oil, as well as the introduction of invasive species that competed with the tortoises for food and hunted them.

Some of the invasive species that contributed to the decline of the C. triserrata included cats, rats, and pigs, which preyed on the hatchlings and eggs of the species. Goats also competed with the tortoises for food.

Fernanda, a female giant tortoise, was discovered on the Galapagos' Fernandina Island in 2019. She was thought to be the last of her species, the Fernandina Giant Tortoise (Chelonoidis phantasticus), which was believed to have gone extinct for 112 years.

The team was confident that Fernanda was the "lost" species, but to confirm, they sent blood samples to geneticists at Yale University. The results confirmed that she was indeed the Fernandina Giant Tortoise.

Researchers are now urgently searching the island to find other tortoises. They hope to avoid what happened to the famous Lonesome George, a tortoise that was the last of another species called the Pinta Island tortoise (Chelonoidis abingdoni).

If a male giant tortoise of the same species is found, the team will attempt to unite him with Fernanda at the Galapagos National Park's Giant Tortoise Breeding Center in Santa Cruz. This could potentially lead to breeding and the conservation of the species.

Additional reading: Are Galapagos Turtles Extinct