In the Midwest, where I grew up, April is when box turtles emerge from hibernation deep in the woods and crawl to less secluded areas to perform their summer activities. This transition often requires crossing country roads, and the turtles occasionally cause car accidents as drivers mercifully try to avoid hitting these armored critters.

What’s the nature of their armor? It comes in two major parts. The carapace is the domed upper shell and represents transformed ribs and vertebrae that have spread out and fused together. The plastron forms the flat “belly” and likely developed from an expanded breastbone and possibly from abdominal ribs. Abdominal ribs, known as gastralia, are present in modern-day crocodiles, alligators, and some lizards. Gastralia also strengthened the abdominal walls in many dinosaurs, including Tyrannosaurus rex.

Bony plates comprising the carapace (left) and plastron (right) of the desert tortoise (Gopherus agassizii). (Nevada State Museum.)
Apalachiosaurus with gastral basket (abdominal ribs). (Denver Museum of Nature and Science)

Covering the tightly interdigitated bony plates that constitute the plastron and carapace are shield-like osteoderms—bony deposits in the skin, from which turtles derive their patterned coloration. Osteoderms are not part of the skeleton but, nonetheless, make themselves available for fossilization and examination millions of years later. Consequently, we not only know about prehistoric turtles and their predecessors but also about armadillos (mammals), past and present, that also tote osteoderms.

Upper left: Osteoderms on Gargoyleosaurus parkinorum, 145 million years old. (Denver Museum of Nature and Science)
Lower left: Osteoderms (and skull) on a modern-day crocodile. (Museum and Art Gallery of the Northern Territory, Australia)Above:
Upper Right: The giant armadillo, Glyptodon, was 9 feet long and ranged over South American and lower North America, became extinct 11,500 years ago. (Tellus Museum of Science)
Lower Right: The extant nine-banded armadillo, Dasypus novemcinctus, is 30 inches long from tip to tail and is found in South, Central, and North America. (Museum of Osteology)

As beautiful and amazing as turtle shells are, they get really interesting when examined from the inside. Only the vertebrae in the neck and tail move on one other, while most of the spine and all the ribs are fused to the inner surface of the shell—a stiff back indeed. What astounds me and until recently has also confounded biologists is that a turtle’s shoulder blades are inside its rib cage rather than outside as they are on all amphibians, other reptiles (including birds), and mammals. Investigators have recently compared the embryologic development of mice, chicks, and turtles. All of these animals start out the same way, but then turtles experience an enfolding of the body wall, and the shoulder blades end up inside.

Cut-away specimens show shoulder blades (arrows) inside the rib cage and the mobile neck and tail bones.  (Above: French National Museum of Natural History. Below: Nevada State Museum.)

Despite the shell’s strength and its unique skeletal anatomy, a turtle typically comes out on the losing end of an encounter with an automobile, but not always. There are two reports of turtles being flipped into the air by a passing car and crashing through the windshield of a following vehicle. In both instances, the startled occupants and the turtles sustained only minor injuries. Way to go, turtle shells. This is yet one more reason to drive carefully.

Here are two more.

Galapagos Tortoise (Los Angeles County Museum of Natural History)
The largest turtle ever, Archelon ischyros, measured 11.5 feet head to tail and became extinct about 66 million years ago. Its spiky plastron is visible through the fenestrated carapace. (Yale Peabody Museum)

Leave a Reply

Your email address will not be published.