How Did the Turtle Get Its Shell?
The cover story of Science this week is about turtle evolution. The caption on the cover illustration, which compares the skeleton of a turtle, chicken and mouse, reads, “The turtle body plan is unusual in that the ribs are transformed into a carapace, and the scapula, situated outside the ribs in other animals, is found inside the carapace. A report on page 193 explains the evolutionary origin of this inside-out skeletal morphology.” So let’s walk outside-in to this issue and see if the promised explanation can be found.
The title of our entry is the same as Olivier Rieppel (Field Museum, Chicago): “How Did the Turtle Get Its Shell?” The first thing we learn from Rieppel is that there are two opposing camps among evolutionary biologists: the transformationists and the emergentists. The first group sounds like old-style Darwinians: “The classic transformationist approach sees morphological evolution as a result of natural selection working on variation manifest in reproducing organisms.” The emergentists, by contrast, look for variations in embryonic development. This difference determines what members of either paradigm are looking for to explain the unique skeletons and shells of turtles. Transformationists look for adaptations in the adult form that might have been passed on to the progeny. They might look for incipient plates in the skin, for instance, that could have ossified over the generations, then fused into a shell. Emergentists, instead, would observe the developmental stages of turtles to look for clues about their evolutionary history. That’s the approach members of the Laboratory for Evolutionary Morphology at the RIKEN Center for Developmental Biology in Japan took in their scientific paper in same issue of Science.2
A key player in the story was the fossil turtle Odontochelys announced last year (see 11/29/2008), which had a plastron (front shell) but no carapace (back shell). Scientists back then were debating whether the fossil was a missing link or a specialized turtle derived from pre-existing fully-formed turtles. This team acknowledged the debate: “It cannot be ruled out that the carapace of this animal merely underwent a secondary degeneration,” they said; “however, if it really possessed the precarapacial dorsal ribs as reconstructed (Fig. 4), the evolution of the turtle body plan would be consistent with the embryonic development of the modern turtle.” This means that their hypothesis about turtle evolution depends on accepting one side of the debate.
As for how the skeleton of a pre-turtle vertebrate could have undergone the spectacular modifications required, in which the scapula bones dived inside the rib cage (instead of remaining outside as in all other vertebrates), and the ribs fused to the carapace, forming a complete circle and ridge connected to the plastron, the authors looked to turtle embryos for evidence. Rieppel summarized their research:
Nagashima et al. observed that during early development of the Chinese soft-shelled turtle Pelodiscus sinensis (see the figure), translocation of the ribs to a position outside the shoulder blade involves folding of the lateral body wall along a line that defines the later formation of the carapacial ridge. This folding restricts rib growth to the horizontal plane of the carapacial disk and also maintains the shoulder blade in its superficial position relative to the folded body wall. This organization is thought to characterize ancestral turtles. Some muscles that develop from the muscle plate that is associated with the folding body wall even retain their “ancestral connectivities” in the adult.
Since there are no ancestral turtle embryos to observe, how can they think about what characterized them? Here’s where they tied in their story with Odontochelys. Rieppel continues:
Nagashima et al. hypothesize that in this ancestral turtle, the carapacial ridge was differentiated only along the side of the trunk, remaining incomplete anteriorly and posteriorly. Only later during the evolution of turtles would the carapacial ridge be completed, causing the anteriormost trunk rib to grow across the shoulder blade and localizing the latter inside the ribcage.
So the researchers would not only have to take the emergentist view from the start, they would also have to assume that Odontochelys was a missing link instead of a specialized form. This stacks two assumptions on top of each other. It even sounds a bit like Haeckel’s discredited “Biogenetic Law” (also called the Recapitulation Theory) that asserted, “Ontogeny recapitulates phylogeny.” The authors almost said that, in fact. Watch for that word recapitulate and see how they used it:
Odontochelys reconstructed by Li et al. resembles the embryonic modern turtles in some respects (Fig. 2, A and E, and Fig. 4), and this animal may represent an ancestral state. The Odontochelys-like, ancestral pattern is still retained in the first rib in modern turtles (Fig. 4, right). Although it remains to be seen whether latissimus dorsi of Odontochelys was shifted rostrally (Fig. 4, middle), its pectoralis would have established a new attachment to the dorsal aspect of the plastron (Fig. 4, middle). Thus, the developmental sequence of P. sinensis may not wholly recapitulate the suggested evolutionary sequence of turtles. Nevertheless, the above suggests that the dorsal arrest of ribs can now be assumed to have taken place by the common ancestor of Odontochelys and modern turtles, and in the latter, the completed CR would have allowed for emergence of the carapace (Fig. 4, bottom). The modern turtles have acquired their unique body plan by passing through an Odontochelys-like ancestral state during embryonic development. Our embryological study may help to explain the developmental changes involved in both the pre- and post-Odontochelys steps of turtle evolution, from an evolutionary developmental perspective.
So although they couched their Biogenetic-Law explanation with the disclaimer that the developmental sequence (ontogeny) of modern turtle embryos “may not wholly recapitulate” the ancestral evolutionary sequence (phylogeny), they turned right around and depended on Recapitulation Theory to explain turtle evolution. They said, “The modern turtles have acquired their unique body plan by passing through an Odontochelys-like ancestral state during embryonic development.” This would only make sense, of course, “from an evolutionary developmental perspective” – i.e., the emergentist view of evolution, which may itself be a recapitulation of Haeckel’s view.
1. Olivier Rieppel, “Evolution: How Did the Turtle Get Its Shell?”, Science, 10 July 2009: Vol. 325. no. 5937, pp. 154-155, DOI: 10.1126/science.1177446.
2. Nagashima, Sugahara, Takechi, Ericcson, Kawashima-Ohya, Narita and Kuratani, “Evolution of the Turtle Body Plan by the Folding and Creation of New Muscle Connections,” Science, 10 July 2009: Vol. 325. no. 5937, pp. 193-196, DOI: 10.1126/science.1173826.
This entry should not be entitled, “How did the turtle get its shell?” but rather, “How did the evolutionist get its tall tale about how the turtle got its shell?” The BBC News called this a “spectacular insight into turtle evolution.” National Geographic contorted this story with the line, “Turtles Have Shells Due to Embryo Origami,” and said “The findings shed light on turtle evolution.” *Sigh.*
It is really quite shocking to see slipshod Haeckelian logic employed by today’s evolutionists, and for Science to publish it, knowing that the popular media will gobble it whole and barf it out for the public (see next entry). Stephen Jay Gould would have been appalled. Recapitulation was tossed into the dustbin of Darwinism decades ago. There is no reason even from an “evolutionary perspective” to expect modern embryos to retain any memory of their assumed evolutionary past, or to think that adult forms are somehow more evolved than the embryo is. Stephen Jay Gould argued that the adult is actually a degenerate form of the embryo (neoteny), not a more advanced stage. That’s the reverse of what the Recapitulation Theory paradigm teaches. Besides, one can’t explain that modern turtle embryos are recapitulating their evolutionary past without assuming the very thing one needs to prove. Yet here it is: Haeckel Recapitulation Theory Biogenetic Law Nonsense popping up again in Science.
Worse yet, the emergentist view of evolution is little more than a restatement of the Stuff Happens Law (09/15/2008 commentary). Something weird happened in a pre-turtle vertebrate embryo, things got shuffled around, and presto! the turtle was born. Why? Stuff happens. If you need more convincing that the evolutionary just-so story “How the Turtle Got Its Shell” is summarized by “Stuff Happens,” look at prior attempts: 11/22/2008 piece, “Turtle Vaults Over 65 Million Year Evolutionary Hurdle,” where the explanation amounted to, “We have no idea.” In the 10/09/2008 entry, the scientists said, “Exactly why turtles evolved their shell remains a mystery.” Check out the 07/03/2002 entry, where some evolutionists tried to convince readers that the chickens and turtles are sisters despite their radically different skeletons. Coming up with that idea required contorted attempts at card stacking. Conclusion: evolutionists are clueless about why these amazingly-adapted, completely-formed animals are the way they are.
The observational facts do not allow for stories about turtle evolution. There are no fossil pre-turtles. If scientists want to stick to empiricism, they cannot appeal to unobservable entities like some mythical common ancestor of turtles. The evidence only permits them to state scientifically that “turtles have always been turtles.” Why not leave it at that? Answer: evolutionary religion requires them to insert turtles into the great chain of being known as Turtle Cosmology.