June 20th was the first day of summer! The weather here in Pittsburgh is already beautiful. It’s enough to make one dream of a socially distant beach! Summer, of course, is sea turtle nesting season: during the next several weeks, female sea turtles all across our planet’s Northern Hemisphere will return to the beach where they hatched, drag themselves onto land, and lay their eggs in the sand. It would have been an incredible sight to see Protostega gigas, one of the largest sea turtles of all time, hauling itself onto the beach to lay its eggs! For June’s Mesozoic Monthly, we’re going to “dive in” to the paleontology of this giant reptile.
Carnegie Museum of Natural History’s spectacular skeleton of Protostega gigas is a composite made from the fossilized bones of two different individuals. Come see it on display in our Dinosaurs in Their Time exhibition when the museum reopens at the end of this month. But don’t forget to purchase your timed ticket in advance!
All turtles, including sea turtles like Protostega and tortoises like the Galápagos giant tortoise, belong to the group Testudines. This group originated during the Triassic Period, the first of the three time periods of the Mesozoic Era (aka the Age of Dinosaurs). Turtles split from other reptiles to form their own group before crocodiles and dinosaurs evolved! This means that turtles are not descended from dinosaurs, no matter how primordial some tortoises may look. Turtles differ from other reptiles in many ways, the most noticeable being their iconic shells.
A turtle shell is formed of two main parts: the carapace, or top shell, and the plastron, or bottom shell. The shell is made of bone fused directly to the spine and ribcage, so a turtle cannot crawl out of its shell without leaving its skeleton behind! Another major difference between turtles and other modern reptiles involves skull anatomy. Turtles have anapsid skulls: the bony case that protects their brain lacks any external openings behind their eyes (known as temporal openings). All other extant reptiles plus birds are diapsids, meaning their skulls have two holes behind their eyes. Mammals differ from both conditions because we have only one temporal opening, making us synapsids. Traditionally, the anapsid condition of turtle skulls has been taken to indicate that they are the most primitive of living reptiles. More recently, however, many paleontologists and biologists have uncovered evidence that turtles are in fact diapsids whose evolutionary course led, for some reason, to a secondary closure of their temporal openings. According to these scientists, the closest relatives of turtles among today’s diapsids are either lepidosaurs (lizards, snakes, and kin) or archosaurs (crocodilians and birds).
A bird’s (or pterosaur’s!) eye view of Protostega gigas (left) swimming past two long-necked elasmosaurid plesiosaurs in shallow waters of North America’s Western Interior Seaway roughly 85 million years ago. (This scene is set in what’s now Kansas!) Art by Julio Lacerda; see more of his beautiful work here.
Reptiles, mammals, and birds all belong to a group called Amniota, and the key defining feature of amniotes is a protective layer around their eggs that allows this vulnerable life stage to survive on land. Having eggs that did not have to be laid in water meant that animals could move to less-wet habitats, a significant step in evolution! Unfortunately for sea turtles, which spend most of their lives at sea, this means they must return to land to lay their eggs. An amniotic egg would “drown” in water because the embryo still needs access to air. As a sea turtle, Protostega would have faced these same reproductive challenges, plus one more: it was huge!The largest modern turtle, the leatherback sea turtle, can grow over seven feet (2.1 meters) long; Protostega dwarfs it at 9.8 feet (3 meters)! If you’ve ever seen video of a sea turtle crawling onto the beach to nest, you know that it’s an awkward process. Imagine seeing a turtle that weighs at least a ton try to do the same! Although surely clumsy on land, Protostega was a graceful swimmer, using its four rigid flippers like wings to “fly” through the water.
Protostega lived in the Western Interior Seaway, an inland sea that stretched across much of North America during the Cretaceous Period (the third and final period of the Mesozoic Era). The seaway was warm, shallow, and teeming with all kinds of aquatic life: the perfect habitat for an omnivorous sea turtle. Because sea turtles are ectothermic (sometimes erroneously called “cold-blooded”), they cannot regulate their own body temperature. Instead, Protostega relied on warm water temperatures and sunlight hitting its back to keep warm. Although we don’t have a fossil record of the coloration of Protostega, we know that today’s large sea turtles are counter-shaded, with heat-absorbing, dark-colored backs and pale undersides. In an ocean environment where both predator and prey shift positions in the water column, this combination aids concealment. From below, a light-colored underside blends with light-saturated water. From above, a dark back blends with dark water. Camouflage in the water was an important feature when living alongside so many sizable predators. Protostega fossils have been found with bite marks from the large shark Cretoxyrhina mantelli, and it almost certainly was also on the menu for the mighty mosasaurs as well. Fortunately for us, we humans can enjoy the ocean knowing that few creatures are interested in eating us!
Lindsay Kastroll is a volunteer and paleontology student working in the Section of Vertebrate Paleontology at Carnegie Museum of Natural History. Museum staff, volunteers, and interns are encouraged to blog about their unique experiences and knowledge gained from working at the museum.