The old shell game
Raymond Ramirez | Friday, February 22, 2019
Let me get right to the point: turtles are awesome. The basic turtle body plan is remarkable, most notably for the shell that surrounds and protects the turtle’s body. The shell consists of an upper section, the carapace, which ranges from relatively flat to domed, and a bottom half, the plastron. Depending on the species, the shell may be hard and rigid or soft and able to flex in a limited manner.
Turtles were long a favorite of creationists, who pointed to the turtle shell as evidence of a structure so complicated that it could not possible have been arrived at in stages. Indeed, no current “half turtles” could be found, and persons who believed in an “intelligent designer” saw turtles as proof that evolution could not account for the turtle’s shell or other physiological features unique to turtles.
As is often the case with the creationism “debate,” the proof for intelligent design typically is little more than a lack of understanding of how the usually slow, deliberate process of evolution works, along with a general lack of knowledge regarding the fossil record, which paleontologists are constantly supplementing with new finds that fill in gaps in the story of how species evolve. The classic go-to example for creationists used to be whales. Whale fossils were once scarce, but the lineage of whales has been filled in nicely over the past few decades, and whale evolution from four-legged carnivores to ocean-going giants is now well-documented.
Creationists would also point to specific organic structures that seemed so complicated that they could not possibly have evolved, but must have been created in their present forms; foremost among these was the eye. Nature provides numerous examples of eyes evolving independently in a number of species, sometime producing similar designs — as with the eyes of mammals and cephalopods such as squid — or even elaborate structures capable of sight in 360 degrees and in portions of the spectrum beyond our ability, as with mantis shrimp.
One missing concept for persons who fail to grasp the workings of evolution is that of “deep time.” An individual human life will span at best little more than a century. In that time, a person may have seen communication change from operator-assisted wired telephones to mobile devices, but change in individual species may be hardly noticed. Given enough time, the slow accretion of changes, most typically manifested through variations or mutations that better fit the environment, will accumulate over time. Whales had more than 60 million years to evolve. Turtles had even more time, with the earliest turtle ancestor dating from 260 million years ago.
Admittedly, there have been notable gaps in the turtle-fossil record, but a fossil gathered in 2006 and more fully described four years ago provided invaluable insight into turtle development. Pappochelys was a relatively small turtle species, about eight inches long, that lived about 240 million years ago during the Middle Triassic era, shortly after the first dinosaurs appeared. The skull of Pappochelys provided evidence turtles are most closely related to other modern reptiles, such as lizards and snakes. Pappochelys had a turtle-like head with short peg-like teeth, a long tail and was relatively shell-free, resembling the cartoon image of a turtle without a shell (think Cecil Turtle and Bugs Bunny).
Pappochelys has helped answer the mystery of how the turtle got its shell. Its belly was protected by an array of rod-like bones, some of which had fused to form a solid protective shield. Apparently, this proto-plastron protected Pappochelys from below as it swam near the surface looking for food. The carapace would come later as turtles continued to evolve. As if all this insight gained from one extinct turtle species wasn’t enough, Pappochelys recently found itself in the news again.
Recent examination of a Pappochelys fossil found in 2008 revealed that one of the hind legs included a malignancy the researchers identified as a type of bone cancer. Evidence of cancer is rarely preserved in fossils because it typically affects soft tissues. The scarring on the bone was so clear researchers identified the fossilized cancer as periosteal osteosarcoma, noting it was almost identical to osteosarcomas afflicting about 800 Americans each year. This 240-million-year-old cancer is the earliest case ever recorded in fossils of reptiles, birds and mammals. Note that modern sea turtles are regularly found with cancerous skin growths, attributed to increased ocean pollution.
The mechanisms of cancer have evolved right along with the creatures they victimize, and these cancers have also had millions of years to confound defenses raised to limit or destroy them. Paleopathology, the study of ancient disease, is important to understand the evolution of pathogens, immune systems, healing physiology and ultimately, the environment. Creatures such as Pappochelys evolved in response to changing environments, not unlike the environmental challenges of pollution and climate change that humans endure. Humans have evolved fairly rapidly over the last few hundred-thousand years; as a consequence, we are more susceptible to cancer than most animals. Perhaps an appreciation for Pappochelys and other messengers from deep time can help us find solutions to the health challenges facing our young and fragile species.
The views expressed in this column are those of the author and not necessarily those of The Observer.