Why Did Turtle Shells Evolve?

News to Know


Extinct turtle Eunotosaurus was digging, not evolving a shell.

Like children assembling a jigsaw puzzle, evolutionists have long been trying to piece together the mysteries of turtle shell origins. And their various versions of “Why the turtle got its shell” sound like tales from Rudyard Kipling’s Just So Stories. According to Dr. Tyler Lyson, lead author of “Fossorial Origin of the Turtle Shell” published in Current Biology, “The answer seems pretty obvious”—the turtle evolved its shell “for protection.”1

The “protection” answer is not so obvious, however, when we remember that turtle shells vary a lot, even among living turtles. A fully developed carapace with fused flat ribs protects a turtle. But a set of flattened unfused ribs—like the “incomplete” carapace of the fossil turtle Eunotosaurus—offers no apparent protection. In fact, it seems like those wide, unfused ribs would just get in the way, making it difficult to breathe and move. If such “incomplete” shells offer no protection and clear disadvantages, how could protection be the driving force for turtle shell evolution?

Believed to be the first reptile to transition toward turtlehood, Eunotosaurus has until now presented an evolutionary mystery: How could its broad, flat, unfused ribs offer any survival advantage? A new and fairly complete Eunotosaurus fossil has allowed turtle experts to solve this mystery. Their discovery has not completed the picture of turtle shell evolution, however, but instead revealed an extinct turtle variety that was exceptionally well designed for digging.

“You’re Not a Lizard”

Eunotosaurus (pronounced like you-not’-a-saurus) looks like a lizard that swallowed a Frisbee,2 but despite the “saurus” in its name, it is not a lizard.3 Eunotosaurus is an extinct turtle, though like many turtles both living and extinct, it does not have a fused shell. Though we tend to think of a turtle’s hard shell as an essential part of its turtle-ness, there is a lot of morphological diversity among turtles.4 But all turtles—and only turtles—have the unusually shaped ribs found in Eunotosaurus.

Eunotosaurus artwork

This is an older artist’s conception of Eunotosaurus, an extinct variety of turtle that had the broad, flat ribs of a turtle but no shell. The latest fossil evidence might now prompt artists to redraw it. It had a thick, muscular neck and its front legs and feet were, we now know, thicker and more powerful than its hindlimbs. Eunotosaurus was designed to dig! Image by Materialscientist, via Wikimedia Commons.

Wide, flat ribs give Eunotosaurus’s back a rounded, turtle-like shape. These ribs were not fused together with other bony elements to form a carapace. The ribs were, however, T-shaped in cross-section, a signature feature of turtles. These ribs and the rounded shape of its back fed the illusion that scientists had discovered a transitional form, a much-sought missing link between turtles and other reptiles. As one writer put it, its back was “a kind of ‘proto-shell’ that one can easily imagine evolving (over the course of tens of millions of years) into the giant carapaces of Protostega and Meiolania,”5 two large extinct turtles with classic carapaces. The operative word here is imagining, for only evolutionary imagination supplies the connections between different kinds of animals on the tree of life.

The amazing diversity of God’s creation is evident whether we examine living animals or those only evident from the fossil record. What appear to be evolving transitional forms to those with evolutionary presuppositions are just examples of the diverse animals God designed with unusual sets of features, equipping them to inhabit a wide array of environments.6 We would like to understand the purpose for which God designed the in-between appearance, evident in the Eunotosaurus’s anatomy. That is not an evolutionary question at all! And the analysis of Eunotosaurus’s more complete fossil has now answered that question.

Turtle Tidbits

A turtle’s shell is unlike the armor of any other animal. The familiar turtle shell consists of a domed carapace on top and a flatter plastron protecting the underbelly. Such a shell encloses the pectoral and pelvic girdles, which are the bones connecting the legs to the rest of the skeleton. This is an unusual trait among vertebrates. The typical shell is made of osteoderms (ossified dermis) with an epidermal covering, and these osteoderms are fused to the turtle’s axial skeleton. A turtle’s upper shell (carapace) consists of ribs and a backbone fused to overlying osteoderms. Its lower shell (plastron) consists of the sternum fused to more overlying osteoderms.

A turtle’s shell is unlike the armor of any other animal.

Both parts of a “classic” hard turtle shell are covered with highly keratinized epidermis in the form of tough, thin, broad, plate-like scales called scutes. However, turtle shells vary a lot. Leatherback marine turtles and softshell turtles have a soft epidermis on their shells. They lack the solid protection of a hard shell enclosure. Eunotosaurus, an extinct turtle without a fused shell, also had no osteoderms. Even among hard-shelled turtles today, there are variations.

The shape of a turtle’s rib is fairly consistent, however. Each rib has a broad upper surface with a ridge projecting below, giving it a T-shape in cross section. Other terrestrial vertebrates have intercostal muscles between their ribs to assist with breathing, but turtles do not have these muscles between their closely spaced ribs. T-shaped ribs lacking intercostal muscles are a unique turtle trait, whether the wide ribs are completely fused into a solid carapace or not.

Primordial Turtle Soup

In trying to piece together the tale of turtle-shell evolution, many evolutionists, like the authors of the current study, have turned to turtle embryology for answers. They argue that evolution followed the pattern laid out in the embryo.7 The shell’s bony components form separately in the turtle embryo. As a turtle embryo develops, its ribs widen and ultimately form the characteristic T-shape. Then projections on the vertebrae broaden. Step by step the collarbone, broadened vertebrae, sternum, wide ribs, and shoulder blades fuse together.8 Osteoderms form within the skin and fuse with the other bones. Epidermal scutes develop atop these bony elements.

Embryology reveals a continuous sequence of steps that successfully produce a fully developed organism. Naturally evolutionists trying to devise a story to construct a step-by-step evolutionary just-so story9 for an animal’s origins look to embryology for guidance. Embryology, however, is observable; the evolution of increasingly complex new kinds of animals is not. (The fossil record does not record millions of years of evolutionary transitions, just the catastrophic burial of a lot of organisms.) The developing turtle embryo does not provide a record of evolution. The embryo already possesses the genetic assembly instructions for all the steps to build a turtle’s body. A non-turtle evolutionary ancestor would not possess any of this complex, specific information necessary to build an additional feature like a turtle shell. Mutations do not generate these complex instructions for new anatomical structures.

The Trouble With Flat Ribs

Believing that the first step in unobservable turtle evolution was Eunotosaurus’s development of wide, flattened ribs—because it is the observable first step toward shell-making in the turtle embryo—evolutionary scientists have been hard-pressed to explain away their apparent evolutionary disadvantages. Let’s consider for a moment the downside of those broad ribs.

For this specialized morphology to have evolved via natural selection, an adaptive advantage that outweighs these costs was required.

When a four-footed non-turtle walks, its rib cage stabilizes its chest wall while still allowing the body to twist and bend from side to side. This allows it to take long, smooth strides in a sprawling gait. But the wide ribs of a turtle, fused or not, stiffen the torso and make lateral bending difficult, resulting in decreased stride length and speed. Since Eunotosaurus’s wide overlapping ribs would have made its body wall fairly rigid, Lyson and colleagues deduced, “For this specialized morphology to have evolved via natural selection, an adaptive advantage that outweighs these costs was required.”10

Speed wouldn’t be the only cost of such a transition. When you inhale, your ribs, lifted by the intercostal muscles that link them, rise like a stack of bucket handles to expand your chest, drawing in a nice, deep breath. And when you exhale, the ribs collapse back on each other to empty air from your lungs. A deeper set of intercostal muscles can pull the ribs down to force even more air out. (Try it. Breathe in deeply; breathe out as much as you can. Notice how the size of your rib cage changes. Isn’t that a wonderful design?) Turtles can’t do that. Even turtles with unfused shells, like Eunotosaurus, can’t do that. Their ribs are too wide, and they don’t have the intercostal muscles that cause your rib cage to work like a bellows.

We need not feel sorry for turtles. The muscles that help turtles breathe are attached in different places and therefore work a different way.11 But remember, evolutionists are assuming that a pre-turtle had the usual arrangement of ribs and muscles, breathing and walking like other four-footed vertebrates, and then replaced these features via random genetic changes with new features that must offer survival advantages. It is that survival advantage that Lyson and colleagues report they have found.

The Design Is in the Details

Thanks to the discovery of new Eunotosaurus fossils, especially a virtually complete one donated by an eight-year-old South African, Lyson’s international team of scientists has discovered several clues to this unusual turtle’s lifestyle. Everything points to burrowing. Eunotosaurus it seems was a fine-tuned digging machine!

Picture for a moment the real problem with being a burrower.

Picture for a moment the real problem with being a burrower. No matter how powerfully you dig, you have to move dirt, instead of your body, out of the hole. As Archimedes said “Give me a firm spot on which to stand, and I shall move the earth.”12 Like Archimedes, a burrower needs more than a shovel; it needs good leverage, a way to brace itself while it shovels. To see if Eunotosaurus was equipped to solve this problem, Lyson’s team compared Eunotosaurus to the giant anteater, a burrowing mammal with comparatively broad ribs. In both animals, broad ribs provide “a stable base on which to operate a powerful shoulder and forelimb digging mechanism, as well as bestowing additional stability to the vertebral column, which joins the digging forelimbs to the bracing, supporting hindlimbs.”13

Furthermore, like the modern gopher tortoise, which uses its head and neck to brace itself against a burrow while digging, Eunotosaurus had a short spade-shaped skull. The broad back part of the skull provided attachment for strong neck muscles, and correspondingly massive neck vertebrae support the idea that this extinct turtle used its head when digging a burrow.14 Furthermore, the size of the eyes—not tiny like a mole’s but fairly large—matched that of animals that typically burrow for shelter but spend a lot of time above ground.15

Like many burrowing animals alive today, Eunotosaurus was well-equipped to dig, but is there more evidence that this extinct turtle really was a digger? Yes indeed! Eunotosaurus’s shoulders and forelimbs seemed to be designed to dig, and their apparent muscularity suggests that in life this animal put its digging design to work. The bones of the shoulders and forelimbs were thicker than the hind limbs. Thicker bones with prominent attachments for muscles suggest the animal had particularly powerful muscles in those places. The front feet and claws were a lot larger than those on the hind limbs too, and the ends of digits on the front feet were shaped like little shovels.

Turtle Pectoral Girdle

This is a photograph of a snapping turtle’s pectoral girdle, the shoulder bones attaching the front legs to the skeleton. All turtles, including Eunotosaurus, have a “triradiate pectoral girdle.” This convergence of three bones—numbered 1,2, and 3 here—form an attachment for the upper arm bone (the humerus, upper right). Necessitated by the enclosure of its limb attachments inside its shell—whether a fused bony one, a soft one, or a set of broadened, T-shaped ribs—the triradiate shoulder girdle is another unique feature of turtles. Image by Fama Clamosa, via Wikimedia Commons.

The pectoral girdle (shoulder bones) in Eunotosaurus had the unique shape of a turtle’s—a convergence of three bones forming a connection for the humerus. (See the photograph of a turtle’s triradiate pectoral girdle above.) Not only is this additional confirmation of its identity as a turtle variety, but in Eunotosaurus these pectoral bones were unusually robust. And it had front legs to match! The point of shoulder attachment for the triceps muscle was particularly well developed. The bony attachment for the chest muscles to the humerus as well as the turtle’s elbow, to which arm muscles attach, were quite prominent. Microscopic examination confirmed that the surfaces where muscles attached to the shoulder and forelimb were “exceptionally thick”16—about 40% thicker than the corresponding hind limb bones, and that larger muscles had been attached to them.17 Clearly this animal had upper body strength that outstripped that of its hindquarters.

Different Starting Points—Different Conclusions

The evolutionary authors of this study have made some great observations and drawn very reasonable conclusions about the probable lifestyle of Eunotosaurus. But they deduce that the lifestyle advantages conferred by its anatomy explain why the turtle shell evolved. They expect that if similar anatomy is found in other extinct turtles with unfused shells (like Odontochelys and Pappochelys), then they will have confirmed that the need to dig was the driving force behind turtle shell evolution and that broad flat ribs paved the way.

They maintain that the advantage conferred by the broad ribs—the ability to hold itself steady while digging—“provided the initial impetus for the origin of the turtle shell and represents a crucial stage in the evolutionary history of turtles.”18 But they present no mechanism by which a pre-turtle could obtain the genetic information to change its ribs into the characteristic T-shape of a turtle’s, nor any mechanism to explain how the turtle obtained other unique features like its unusual pectoral girdle.

The authors interpret their observations this way because they have an evolutionary starting point: they already believe that all living things share a simple common ancestor. They assume that the evolution of new and more complex kinds of animals from simpler ones explains the existence of all living things. They believe this despite the lack of any observational evidence that living things can acquire the novel genetic information coding for new complex anatomical structures from mindless natural processes. The basis for their belief is the evolutionary notion that only materialistic natural processes can be invoked to explain the origin of all things.

The basis for their belief is the evolutionary notion that only materialistic natural processes can be invoked to explain the origin of all things.

Bible-believing scientists have a different starting point. No scientist was present to observe the origin of life or all living things. But we know from the Word of the Creator, the God of the Bible, that God created all kinds of living things in the space of 6 days about 6,000 years ago, without evolution. And He has told us of a global Flood that would have catastrophically buried many animals like these turtles now preserved as fossils. All living things reproduce and vary only within their created kinds—His Divine design affirmed by our scientific observations.

Turtles come from turtles. How many kinds of turtles did God create in the beginning? A reasonable estimate is around 14, including 2 marine kinds.19 Reshuffling of the genetic information in the original kinds of turtles and the loss of genetic information through mutations, natural selection, and other ordinary genetic processes—not evolutionary gain—explains the enormous biodiversity we see in turtles today and in the fossil record. And in a turtle like Eunotosaurus, we see not a half-formed shell that would evolve more completely over millions of years, but a variety of turtle perfectly designed by God to dig.

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  1. Laura Geggel, “8-Year-Old’s Fossil Discovery Explains Why Turtles Have Shells,” Live Science, July 18, 2016, http://www.livescience.com/55436-why-turtles-have-shells.html.

  2. “First described as a strange, gluttonous lizard that swallowed a small Frisbee, Eunotosaurus, a shell-less lizard-like reptile, is now recognized to be the earliest known ancestor of turtles.” From “Eunotosaurus: Bloated Lizard-like Reptile Recognized as World’s Earliest Turtle,” Headlines and Global News, September 3, 2015, https://www.hngn.com/articles/125919/20150903/eunotosaurus-bloated-lizard-reptile-recognized-world-s-earliest-turtle.htm.

  3. The suffix –saurus means “lizard,” but Eunotosaurus is neither a lizard nor a transitional form between lizards and turtles. Neither is Pappochelys, another extinct turtle with a similar unusual, almost lizard-like skull.

    Most reptiles, including lizards, have a diapsid skull with two window-like openings in the temporal region on each side. Most sorts of turtles, living and extinct, have a comparatively “windowless” anapsid skull, the openings being covered by bone. The extinct turtle Pappochelys had a diapsid skull. In 2015 researchers confirmed that, while the juvenile Eunotosaurus was also diapsid, the adult Eunotosaurus had a single window on each side of its skull. The other window was detectable but covered by bony overgrowth. Thus both of these animals had skulls differing from the skulls of typical turtles.

    Evolutionists impose their evolutionary worldview on this observable diversity among turtles by concluding that Eunotosaurus was evolving away from the diapsid skull of its supposed non-turtle ancestors and toward the typical anapsid turtle skull 260 million years ago. Bible-believing creation scientists understand that the observable variations in these skulls demonstrate the biodiversity among turtles and turtle-like reptiles that populated the pre-Flood world and were buried about 4,350 years ago during the global Flood. (Eunotosaurus’s interesting skull morphology is described in Gabe S. Bever et al., “Evolutionary Origin of the Turtle Skull,” Nature 525 (2015): 239, doi:10:1038/nature14900.

  4. Turtle shells clearly exhibit a high degree of morphological diversity. For instance, the pig-nosed turtle, softshell turtles, and leatherback turtles have a leathery covering instead of scutes. Softshell turtles have only a small amount of bone in the carapace, which is not fused to the plastron even in those species that have a bony plastron. And the carapace of leatherback turtles contains no bone at all.

  5. “Eunotosaurus,” About Education, March 4, 2016, http://dinosaurs.about.com/od/predinosaurreptiles/p/Eunotosaurus.htm.

  6. For more examples of this read “Pinnipeds—Blurring the Boundaries.”

  7. Other evolutionists believe the shell evolved as external osteoderms fused with internal ribs. The two camps have been debating these two ideas for many years. The authors of the research discussed in this article believe turtle embryology recapitulates its evolutionary history.

  8. The ontogeny (embryonic development) of the turtle is reviewed in comparison with the evolutionary conclusions inferred from it in T. Lyson et al., “Evolutionary Origin of the Turtle Shell,” Current Biology (17 Jun 2013) doi:10.1016/j.cub.2013.05.003.

  9. Derived from the title of Rudyard Kipling’s famous and highly entertaining book Just So Stories for Little Children, “just-so story” refers to an unverifiable and sometimes fanciful narrative detailing the origin of a biological feature or cultural practice. Stories from Kipling’s 1902 book of “why’s” include “How the Camel Got His Hump” and “How the Elephant Got His Trunk.”

  10. Tyler Lyson et al., “Fossorial Origin of the Turtle Shell,” Current Biology 26, no. 14 (2016): 3, doi:10.1016/j.cub.2016.05.020.

  11. As Lyson explains, turtles have “a unique abdominal muscular sling that wraps around their lungs and organs to help them breathe.” See Melissa Hogenboom, “How the Turtle Got Its Unique Hard Shell,” BBC, May 31, 2013, http://www.bbc.com/news/science-environment-22715018.

  12. Translations vary.

  13. Lyson et al., “Fossorial Origin of the Turtle Shell.”

  14. Ibid.

  15. Ibid., 6.

  16. Ibid., 3.

  17. That the forelimb muscles were larger and stronger than the hind limb muscles is indicated by the abundance of Sharpey’s fibers in those areas of muscle attachment. Sharpey’s fibers attach the periosteum to the underlying bone and also attach muscle to the periosteum and underlying bone. Therefore, an abundance of Sharpey’s fibers indicates there was once an abundance of muscle tissue attached.

  18. Lyson et al., “Fossorial Origin of the Turtle Shell,” 7.

  19. Estimates vary. This estimate is based on the enumeration of 14 families in Laurie Vitt and Janalee Caldwell, Herpetology 4th ed. (London: Academic Press), 523.


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