Why Evolution is True is a blog written by Jerry Coyne, centered on evolution and biology but also dealing with diverse topics like politics, culture, and cats.
For the midpoint of Amphibian Week, Chris Petersen of Department of Defense Partners in Amphibian and Reptile Conservation sent me these amphibian facts.
Characteristics of Amphibians:
· Include frogs and toads, salamanders and caecilians (approximately 8,300 species worldwide)
· All are vertebrates (have a backbone)
· Are ectothermic (meaning they rely on external sources from the surrounding environment to maintain their body temperature)
· Most live part of their life in water and part on land (although there are many exceptions)
· Most have moist glandular skin through which they can respire (breathe) to various extents (some exclusively so, but most also through lungs or gills)
· Lay unshelled (jelly-like) eggs in moist to wet environments
· Most go through a process called metamorphosis to develop from a water-living life stage to a land-living stage
I then headed out to Greenquist Pond here at UW-Parkside to see what amphibians were about. You’ll recall that Chorus Frogs and American Toads have been calling on campus, but I hadn’t seen them at this pond. Here’s what I found.
Bullfrog, Rana catesbeiana, Greenquist Pond, Somers, Wisconsin, 5 May 2021.
I walked around three sides of the pond, and heard or briefly saw several Rana jump into the water, many emitting a little “yelp” as they dove in. I think both Green Frogs (Rana clamitans) and Bullfrogs (Rana catesbeiana) make that noise, so I wasn’t sure of the species. All were smallish, except for one that was bigger, but could have been either a large Green or a medium Bullfrog in size. I was heading back, reconciled to failure, when I spotted this medium-sized Bullfrog on the bank, which didn’t spook. I was able to get pretty close to get this shot, and even was using sticks to bend shadowing leaves out of the way, but it stayed put.
The Green Frogs and American Toads I showed in earlier Amphibian Week 2021 posts were also from this pond, but I’ve not seen them at the pond yet this year. (Some of the frogs today may have been Green Frogs.)
There were also turtles, so I’ll cheat a bit (they are reptiles, of course) and throw them in here. There were four five Painted Turtles (Chrysemys picta),
plus this Red-eared Slider (Trachemys scripta elegans), with another two Painted Turtles behind.
Red-eared Slider, Trachemys scripta elegans, and , in back, two Painted Turtles, Chrysemys picta, Greenquist Pond, Somers, WI, 5 May
There were a total of four five Painted Turtles, all with the slider in this corner of the pond. The slider is the most popular turtle in the pet trade, and is not native to Wisconsin. Although we find them not infrequently, they all seem to be released or escaped– they don’t seem to breed up here, even though they can survive the winters. (I had my own “Spot the …” moment– I didn’t see the further back Painted Turtle in the above photo until I’d posted it here!)
Midland painted turtles (Chrysemy picta marginata) are the most abundant turtles in southeastern Wisconsin, and they live in and nest around Greenquist Pond, the pond right outside the building where my office is. Hatchlings don’t always walk the right way from their wintering abodes, and need help if they are to survive.
On May 9, 2019 (almost exactly five years since a similar reptile rescue), a group of students found this hatchling on its back on the concrete plaza between the lower levels of Wyllie and Comm Arts, 100 or more yards from the Pond.
Knowing who to go to for reptile advice, they brought the little fellow to me. Given its size, and its incompletely healed umbilical scar, this turtle hatched last summer (2018), and may have overwintered in the nest, emerging this spring. The turtles often nest in the area to the south and southeast of the Pond, and this one must have headed the wrong way after its winter nap.
Hatchling midland painted turtle (Chrysemys picta marginata), UW-Parkside, Somers, WI, May 9, 2019. Note the incompletely healed umbilical scar.
At my suggestion, the students took the turtle to the Pond, and released it in the shallows at its NE corner, where it could make its home with the other painted turtles, including, in all probability, its parents.
The students who rescued the turtle, and released it in Greenquist Pond. The turtle is cupped in one of the rescuers’ hands.
A couple of Sundays ago, June 17, 2018, my wife and I took a paddle along the Root River, in Racine, WI. Starting out at the Root River Environmental Center (REC), we went upstream, around the island in Island Park, and back down to the REC. Along the way we saw quite a few turtles– 15-20, although at least a few were the same turtles seen going both up and back.
Here, a shelled reptile and a glorified reptile share a tree trunk in mid stream.
Female mallard and map turtle in Root River, Racine, WI, 17 June 2018.
On the next picture, it’s a bit of “spot the turtle”– the smaller one is inconspicuous. Both these two and the one in the previous picture appear to be map turtles (Graptemys). These turtles are typically more riverine than lacustrine, and thus might be expected in the river, except that the Root River is outside the range of map turtles, which occur in Illinois to the south and along the larger rivers of western Wisconsin. The map turtles of southeast Wisconsin are almost certainly introduced. What species they are is not clear to me. The species-level taxonomy of map turtles is not completely worked out, especially down South, where each river that drains into the Gulf of Mexico seems to have a more or less distinctive population of map turtles.
Two map turtles in Root River, Racine, WI, 17 June 2018.
Although it might be natural to think that one of the midwestern species was introduced into southeast Wisconsin, southern turtles can be found in the pet trade, and there may be more than one species present in the Root River. (In Kenosha, just south of Racine, I’ve seen at least two map turtle species.) It’s not known if they are breeding, and if so, whether different forms are crossing. I did find a hatchling in Kenosha, but I can’t rule out– in fact I lean toward– the possibility that it was released, rather than bred, there.
This next turtle is definitely a map turtle. Note the hint of serration or knobs on the shell along the midline, and the white neck markings.
Map turtle in Root River, Racine, WI, 17 June 2018.
The next turtle is a snapping turtle (Chelydra serpentina), hauled out on the island in Island Park. It’s not a very good picture– that’s its tail you’re looking at– as the turtle slipped into the water as we maneuvered for a better shot, but snapping turtles so rarely bask on land that I though it worth showing. (They often float right at the surface, which is their usual way of ‘basking’.)
Snapping turtle on island in Island Park, Root River, Racine, WI, 17 June 2018.
The species we saw the most of were midland painted turtles (Chrysemys picta marginata), which, like snapping turtles, are more of a pond than river species. The Root River is shallow and slow-moving, though, so the conditions are fairly pond-like. You can tell it’s the midland subspecies because the seams between the costal (‘rib’) scutes don’t line up with the seams between the vertebral scutes
Painted turtle in Root River, Racine, WI, 17 June 2018.
We saw a bunch, but the gal above let us get the closest, so she gets a closeup. (You can tell it’s a she by the large size and the short ‘fingernails’– males are smaller, and have longer front claws.)
Painted turtle in Root River, Racine, WI, 17 June 2018.
We did see two or three of southeast Wisconsin’s classic native river turtle, the smooth softshell (Apalone mutica). They are baskers, but very skittish, and thus hard to approach. I was using a 55-200 zoom lens on this trip, and got a decent picture of one. Notice that the ‘log’ it is on is actually an old piling or dock piece– note the bolt, nut, and metal plate.
Smooth softshell turtle in Root River, Racine, WI, 17 June 2018.
Finally, towards the end of our two-hour paddle, we encountered what I believe to be the same two map turtles we saw at the start of the trip, who are in the first picture above– it is the same log. Sexual size dimorphism is stronger in map turtles than painted turtles, so this could be a female and a male.
Two map turtles on the Root River, Racine, WI, 17 June 2018.
The possible male dove first, but we got close enough to see the neck markings and hint of dorsal serration in the probable female. Of the two native map turtles in western Wisconsin, the plain old map turtle, Graptemys geographica, is less serrated than the false map turtle, Graptemys pseudogeographica, so this would be a geographica, except that the two Wisconsin species aren’t the only possibilities. (One of the two map turtle species I’ve seen in Kenosha is definitely a ‘white-eyed’ southern form.)
A probable plain old map turtle on the Root River, Racine, WI, 17 June 2018.
Given that it’s a small river hemmed in by human development on all sides, with a past history of industrial usage, four species of turtle, all reasonably abundant– all with multiple sightings during the trip, except for the snapper, which, as a non-basker, is often not seen– is actually a decent amount of biodiversity.
I was not aware that yesterday was World Turtle Day. My only excuse is that I had spent the previous nine days in Costa Rica, and thus was largely out of touch with the Internet. (More on Costa Rica later.) So, a day late, here are my turtles.
First, here’s Slidey, a Red-eared Slider (Trachemys scripta elegans).
Slidey a Red-eared Slider.
This is a southern U.S. subspecies of a species complex widespread from the southeastern U.S. down into South America, and also found widely in the West Indies. This is one of the most popular turtles in the pet trade, and has become invasive in places. Even outside areas it can successfully reproduce, released individuals can survive. I have seen released/escaped individuals in New York, Maryland, and Wisconsin.
Here is me with Slidey (and also Toady, my Giant Toad [Bufo marinus]), at an eco-fair at Gateway Technical College in Racine, Wisconsin, in March, 2016. The theme of my exhibit was invasive species. (I also had a preserved lamprey, just barely visible in the jar below Slidey, and a small buckthorn which I had uprooted and brought in whole, whose branches can be seen sticking up above the Dell monitor. All these species are invasives which, at least in some places, have had negative consequences.)
Me, with Toady (a Bufo marinus) and Slidey.
Here is Snappy, a Snapping Turtle, (Chelydra serpentina), which is not an invasive species. It (or close relatives) is native from Canada down to northern South America.
Snappy, a Snapping Turtle.
And finally, not one of my turtles, but a Galapagos Tortoise from Charles Island.
It’s a bit late in the day, but I must announce that today is, at least in the USA, National Reptile Awareness Day. Happy Reptile Day to all! I must admit, as a herpetologist who joined all the major American herpetological societies while still in high school (1975), I had never heard of National Reptile Awareness Day before today. Reptiles Magazine, a fanciers outlet, is the only group I can find who are promoting it, although even they admit not to know how or when it started. Despite its obscurity, we’ll celebrate with a few reptile pictures.
First, a wild red-eared slider, a southern US turtle popular in the pet trade, and often released, but less often established, in places outside its native range, like Wisconsin.
Red-eared slider (Trachemys scripta elegans), Greenquist Pond, Somers, Wisconsin, 14 September 2016.
Next, my ball python Vivian, whom I’ve had for about 18 years.
Ball python (Python regius), captive, at alumni event at UW-Parkside, September, 2016.
A snapping turtle from UW-Parkside, at the same alumni event as Vivian.
And we’ll finish up with a series of eastern massasauga (Sistrurus catenatus catenatus) pictures; I believe all the pictures were taken in Cass, Michigan. They were taken by my former student Eric Hileman, who did his Ph.D. at Northern Illinois University with Rich King. Eric successfully defended his dissertation on the population ecology of massasaugas just this past Wednesday, and I was privileged to be able to attend. So we can all take this National Reptile Awareness Day as a day to send this joyous message to Eric: “Congratulations. Now get back to work.”
Update: Yesterday afternoon on the way home, at about 5:30 PM, just hours after posting this, I came across a large snapping turtle (Chelydra serpentina) crossing in the middle of the road. It was a windy 2-lane road with a mix of homes, woodland, and fields (Wood Road, between A and KR in Somers, WI). The car ahead of me maneuvered around the turtle, and pulled into a long driveway. I pulled off just behind the turtle (though my car was still half on the road– there’s really no shoulder there), put on my flashers, got out and picked it up. It was big, 11.5 inches carapace length (measured by marking off on a stiff map I had in the car– I didn’t have a meter stick with me), a smooth shell with lots of algae growing on it, and very snappy– much more so than a captive turtle that’s used to people: a magnificent specimen. I tried one-handing it by the back of the shell, but couldn’t do it, so I grabbed one thigh in each hand and held it with its plastron toward my leg (so it couldn’t reach out and bite me, which it wanted to do). The driveway that the car went in led up hill into the woods, and I didn’t see anyone. There were no obvious bodies of water to bring it to, so I rang the bell at the house where I parked, but it looked dark. Looking back down the road, I saw a guardrail (which are often placed by ditches/water), walked down to it, and found a small creek going under the road coming out of a fairly dense woods, so I let it go on the edge. (A big frog jumped out of the way as I let it go!) As I walked back to my car, the neighbor whose bell I had rung came to the side of the road. As I explained, he said there were five ponds back in the woods where I’d let it go, and that he saw big snappers wandering around occasionally, although not one that big recently. He was a good observer, since he knew details about snappers that most people don’t. We both thought it would be just fine back there, which was in fact probably where it had come from. Unfortunately, I did not have camera to take a picture. This morning on the way to campus, I checked the road carefully– no carcass or blood stains. It either stayed in the creek/woods/ponds, or if it persisted in trying to cross, it made it across safely.
I am occasionally contacted by local or state authorities to assist in identifying, capturing, and/or taking care of reptiles that have, for various reasons, come to their attention. Sometimes this involves animals that are suspected of being inadequately cared for; such cases have come to be called “rescues”, a term that I am not entirely happy with, but which I use since everyone else does. There are cases though where the animal needs rescue, even though no direct human action has caused its predicament. Here are two such cases.
Early in May, I went out one evening with a friend and his son to look for frogs and toads, to help his son work on his Reptile and Amphibian Study merit badge. We were heading to Greenquist Pond, a small, artificial farm pond, now surrounded on two sides by woodland, on my campus. I decided to stop at an outside stairwell to the basement of a building near the pond, knowing that small animals can go down the stairs, but not up. In the leaf litter at the bottom of the stairs, my friend found a hatchling midland painted turtle (Chrysemys picta marginata).
Chrysemys picta marginata, UW-Parkside, Kenosha, WI, 6 May 2015.
When first found, the turtle was entirely immobile– I feared it was dead. A few minutes of being held in the hand, though, warmed it up, and it became active. We brought it back to the lab to be measured (26 mm carapace length) and photographed. Left in the stairwell, it would have undoubtedly died, as the steps were much too high for it to climb up, and in the stairwell it would have starved, died of cold, or been eaten (by a visiting bird or raccoon, or perhaps by a shrew, which also get trapped in stairwells and starve, but will eat anything down there with them before they do). We went on to see and/or hear at one or more of Greenquist Pond, Desch Pond, and the Willow Swamp bull frogs (Rana catesbeiana), green frogs (Rana clamitans), American toads (Bufo americanus) and chorus frogs (Pseudacris triseriata).
The hint of red you can see on the marginal scutes in the photo above is well developed on the plastron.
Plastron of hatchling painted turtle.
The turtle was a hatchling, but was from 2014’s egglaying. It had hibernated over the winter (perhaps in the nest), had emerged in spring 2015, but made a wrong turn on the way to the pond, and found itself in the stairwell. Once over the first step, it was ratcheted down to the bottom– it can fall down a step, but cannot go up a step. I was very glad to see it, since I had not found any painted turtle nests near the pond for some years, and feared that changes in land use around the pond might have turned the resident painted turtles into a non-reproducing population. The hatchling proves they are reproducing, and the location of the stairwell relative to the pond indicates they are nesting at least close to where they used to, although I’ve not found the nests.
After a few days in the lab, it was time to release the little fellow in the pond. Here he is on a patch of moss near the edge of the pond.
Hatchling painted turtle about to enter Greenquist Pond, UW-Parkside, Kenosha, WI.
And here he is entering the water.
The previous September, I had found five American toads (Bufo americanus) in that same stairwell, and seven more in a deep (ca. 20 feet) window well outside my building (which is also quite close to Greenquist Pond and adjacent patch of woods). I typically go down the ladder into the window well when out with my vertebrate zoology class, because we often find trapped vertebrates in there (frogs, toads, small rodents). We brought the toads back to the lab for identification and measurement. Some of them were quite emaciated, and I decided to hold on to these for a bit to try to fatten them up before releasing them. Another was one of the largest American toads I’ve ever found around here, and I kept her to get some photos. But after releasing the others, the weather got cold before they fattened up or I got a chance to take photos, so I wound up keeping the big female and one of the emaciated toads over the winter (the other emaciated toad was eaten by the big female!). I released them in Greenquist Woods the same day this spring that I released the turtle.
American toads, Greenquist Woods, UW-Parkside, Kenosha, WI.
Here they are (above), just after release, the formerly emaciated male on the left, the big female on the right. At the time of capture, the female was 75 mm snout-vent length. When released, she’d grown to 84 mm, and weighed 84 g. I’m not sure exactly how big the emaciated male was when captured, because I didn’t segregate the measurements of the emaciated ones I kept, but the set of smaller ones caught had sizes ranging from 20-32 mm. By the time of release he’d grown to 66 mm and weighed 31 g, and gotten kind of chubby even. He had also sexually matured, having keratinous nuptial thumb pads, grabbing onto the female’s back and holding tight (as males do during amplexus), and giving the “release” call when squeezed at the waist (as males and unreceptive females will do). In the wild, of course, he would not have grown over the winter, and thus may have sexually matured a year earlier relative to his cohort.
Here are portraits of them, taken just before or just after release.
Male Bufo americanus, UW-Parkside.Female Bufo americanus, UW-Parkside.
I’d mentioned that these animals’ predicaments were not due to direct human action, but their need for rescue did arise from human action– essentially we’ve dotted the landscape with inescapable pits– just not from actions directed at the animals.
(And of course, toads are amphibians, but ‘reptile rescue’ is more terse and euphonious.)
[JAC: In response to my own and other readers’ questions about turtle morphology and evolution, Greg kindly put up another post to clarify matters.]
by Greg Mayer
To really appreciate what turtles have done with their shells, it helps to see into one. So here’s a view into a turtle’s shell. The shell has been cut parasagitally, to the right of the midline, so we can see the vertebrae, but the rest of the skeleton– head, neck, limbs, girdles, and tail– are left intact so we can see their relation to the shell. (As Turtle Dundee once said, “That’s not an evolutionary novelty– that’san evolutionary novelty.”)
Inside a turtle.
Note that the shoulder and pelvic girdles are both within the dome of the carapace. And in turtles, the shoulder girdle is a bigger affair than it is in mammals. Turtles not only have a scapula, but also a coracoid as a major bony element, plus turtle scapulas are two pronged affairs with an acromial process nearly as big as the main part of the scapula itself, so that the whole girdle is a tri-radiate structure.
The shell itself is composed of epidermal, dermal, and deeper skeletal elements, and all three can be seen in this view. The horny scutes on the exterior of the shell, made of the same material as scales and fingernails, are epidermal, with a layer of live cells between the scutes and the bone. The bony part composes parts of the deeper axial skeleton: the vertebrae and ribs, which are preformed in cartilage during development; and the more superficial dermal bones, that ossify directly in the dermis without being preformed in cartilage. The plastron (bottom shell) consists of just epidermal and dermal elements. The plastral bones may be homologous to some of the dermal bones of the shoulder girdle (clavicle and interclavicle) and the gastralia of other reptiles.
As mentioned in the previous post, what the new paper by Lyson and colleagues (see previous turtle post for references) has especially done is to try to interpret the disputed turtle precursor Eunotosaurus in terms of the developmental processes proposed to underlie the evolution of the shell as seen in the undoubted turtle precursor Odontochelys and more derived turtles. Much of that recent developmental work has come from the Laboratory of Evolutionary Morphology at the RIKEN Center for Developmental Biology in Japan, especially this 2009 paper by Hiroshi Nagashima and colleagues (see additional figures in RIKEN’s press release; and a nice review by Shigeru Kuratani and colleagues from 2011).
“[T]he amniotes’ ribs and muscle plate grow together ventrally and make a single layer in body, outside of which the scapula is situated. In turtles, ribs grow laterally and are confined dorsally. However muscle plate is folded at the tip of ribs and runs inside the scapula as in other amniotes, showing basic topology between the elements is not changed both in turtles and other amniotes.” From RIKEN.Lyson and colleagues integrate Eunotosaurus into Nagashima and colleagues’ developmental scenario by proposing that the rib broadening seen in Eunotosaurus is homologous with that in Odontochelys and Proganochelys. One problem that I see with this attempt is that, if I’m interpreting Eunotosaurus correctly, the distal ends of its ribs curve ventrally and tuck in around the lateral edge of the body, while in turtles the ribs grow out straight to the sides towards a feature in the embryo called the carapacial ridge; this is how, in fact, the scapula gets inside the ribs. It could be argued, though, that this straightness is a later evolved feature, although it’s the straight lateral growth that is associated with getting the cartilaginous ribs associated withe bone-producing dermis that produces the broad plates of bone.
Figure 4 from Lyson et al. 2013 (click to see enlarged view).
To stress again how profound are turtles’ morphological changes in skeletal and soft tissues, and how they ramify throughout its physiology, ecology and behavior, let me quote from the famous morphologist and paleontologist Rainer Zangerl (1969; and who, in the quoted paper, also refers to the development of the turtle shell as “astounding”):
This shell did not merely cover the pre-existing anatomical structures of the body, but it modified them profoundly. The drastic alteration was probably the consequence of an intimate involvement of the dermal shield with parts of the axial skeleton (vertebrae and ribs) and with dermal portions of the primary skeleton (clavicles, interclavicle and gastralia). The restructuring had far reaching morphological, physiological, evolutionary, and ecological consequences . Anatomically, the presence of a rigid shell led to extensive modification of the structure of the body wall, and to the differentiation of the locomotor apparatus, of the neck region and of the copulatory mechanism, to mention just a few . Functionally it necessitated changes in the mode of respiration ; it impaired locomotion, especially on the ground, and restricted aquatic locomotion to the “paddle types” ; it delimited the storage capacity of the body for air, food, water, fats (oil in the shell bones of sea turtles), and waste materials . It probably affected the permeability of the body wall and hence modified the capacity of the animal to retain water . It drastically modified the behavioral pattern of the animal.
Kuratani, S., S. Kuraku, and H. Nagashima. 2011. Evolutionary developmental perspective for the origin of turtles: the folding theory for the shell based on the developmental nature of the carapacial ridge Evolution & Development 13:1-14. abstract
Nagashima, H. et al. 2009. Evolution of the turtle body plan by the folding and creation of new muscle connections Science 325: 193-196. pdf
Zangerl, R. 1969. The turtle shell. Biology of the Reptilia 1: 311-339. pdf
In a paper soon to be published in Current Biology (abstract), Tyler Lyson and colleagues propose a model for the origin of turtles, using the Permian (ca. 260 mya) fossil Eunotosuarus as a transitional form.
The origin of turtles is a fascinating and important area of study, although one that is perhaps generally underappreciated (Burke 2009). The reason for its importance is that the turtle shell is probably the most novel and highly derived of all skeletal structures in the vertebrates. Nothing in human evolution, for example, compares. What do we have that’s distinctive? A really short tail? It’s been done to death, and just involves some reduction in size and number of bones. Bipedality? Also done many times. A large cranium? Just a few bones getting bigger.
What have turtles done? They’ve moved their shoulder and pelvic girdles inside their rib cages! If you want to appreciate what this means, reach behind your back and touch your shoulder blades, then reach down and touch your hip bones. Now feel your ribcage, front and back. And now imagine getting your shoulder blades and hips inside your rib cage– that’s what turtles have done.
Understanding of the origin of turtles has been hampered by the lack of a good fossil record. For many years the earliest known turtle was Proganochelys from the Late Triassic (ca. 210 mya), which, while primitive in various respects, had a full shell, both top (carapace) and bottom (plastron). The first fossil breakthrough came in 2008, when Li and colleagues described Odontochelys, a Late Triassic turtle a bit older than Proganochelys, but in which the plastron was well formed, but the carapace (top) consisted of only neural bones above the vertebrae, and enlarged ribs.
Lyson and colleagues extend the model of shell origins, incorporating further data on shell ontogeny in modern turtles, and, most significantly adding Eunotosaurus as an early ancestral turtle, which has a proto-carapace of expanded ribs. They also point to Milleretta, another Permian reptile, as a possible very first step in the direction of turtles. (At this point, we pause for Snappy to say hello, so readers don’t forget what the object of our discussion is.)
“Snappy”, Chelydra serpentina, Somers, WI
There are two phylogenetic controversies that are involved in this scenario of turtle origin. First, although Eunotosaurus has long been bandied about as a turtle precursor, the consensus has long been that it is not, although Lyson and colleagues (2010) have argued it is. Second, there is great debate about where turtles fit in amongst reptiles in general. Because of the generally primitive nature of their skulls (lacking any openings or fenestrations), turtles have often been linked with one or another of various early reptile groups, and Lyson and colleagues favor this view. There is an almost equally old hypothesis, however, that turtles, despite lacking any skull openings, are nonetheless members of the Diapsida, the great group of two-fenestra reptiles (and their modern feathery descendants) that includes Archosaurs (crocodiles and birds among living taxa, dinosaurs and pterosaurs among the extinct) and Lepidosaurs (tuatara, lizards and snakes among living taxa, mosasaurs among the extinct).
Recent molecular data, including two new genome studies (Wang et al. 2013, Shaffer et al. 2013; see Gilbert and Corfe 2013), have supported earlier molecular studies (including one of my own) in placing turtles among the Diapsida, and indeed, well within the Diapsida, as the sister group of the extant archosaurs. Although the resolution of these latter debates will have bearing on the full turtle origin story, one thing that I think is now clear is the stepwise origin of the turtle shell, with the various components having been assembled sequentially.
Burke, A.C. 2009. Turtles… …again. Evolution & Development 11:622-624.
Gilbert, S.F. and I. Corfe. 2013. Turtle origins: picking up speed. Developmental Cell 25:326-328.
Kirsch, J.A.W. and G.C. Mayer. 1998. The platypus is not a rodent: DNA hybridization, amniote phylogeny and the palimpsest theory. Philosophical Transactions of the Royal Society B 353:1221-1237. pdf
Li, C, X.-C. Wu, O. Rieppel, L.-T. Wang and L.-J. Zhao. 2008. An ancestral turtle from the Late Triassic of southwestern China. Nature 456:497-501. abstract
Lyson, T.R., G.S. Bever, B.S. Bhullar, W.G. Joyce, and J.A. Gauthier. 2010. Transitional fossils and the origin of turtles. Biology Letters 6:830-833. pdf
Lyson, T.R., G.S. Bever, T. M. Scheyer, A.Y. Hsiang, and J.A. Gauthier. 2013. Evolutionary origin of the turtle shell. Current Biology in press. abstract
Shaffer, H.B. et al. 2013. The western painted turtle genome, a model for the evolution of extreme physiological adaptations in a slowly evolving lineage. Genome Biology in press. pdf
Wang, Z., et al. 2013. The draft genomes of soft-shell turtle and green sea turtle yield insights into the development and evolution of the turtle-specific body plan. Nature Genetics 45 701-706. pdf
!["[T]he amniotes' ribs and muscle plate grow together ventrally and make a single layer in body, outside of which the scapula is situated. In turtles, ribs grow laterally and are confined dorsally. However muscle plate is folded at the tip of ribs and runs inside the scapula as in other amniotes, showing basic topology between the elements is not changed both in turtles and other amniotes." From RIKEN.](https://i0.wp.com/whyevolutionistrue.com/wp-content/uploads/2013/06/turtle-shell-muscles.gif?resize=350%2C575)
