Prologue
If the previous post in this blog were any guide, this one should be the second of a two-parter on cetacean ear bones, and, overall, the third on specimens in a recently acquired collection of Miocene fossils from Plum Point, Maryland. But I am skipping that second installment on ear bones for the moment because of fundamental gaps in my understanding of the periotic bone, the intended topic of the post. Much as I’ve tried, I cannot resolve this issue. Such are the challenges inherent in being self-taught in most aspects of natural history.
Given that I abandoned the original topic intended for this post, I went in search of another that might be drawn from the Plum Point collection. It helped that I’d already been struck by the abundance of fossils in the collection that appeared to be from turtles. Later, while I was on a walk through a local park, the gods of coincidence arranged for this chalk drawing to be on my route:
So, Plum Point turtles fossils became the launching off point for this post. At the outset, I assembled a small selection of fossil specimens from the collection that I can with some confidence attribute to turtle shells.
Although intact turtle shells are readily identifiable, disarticulated shell bones, which is how turtle fossils are typically found by amateurs in the field, can pose identification challenges. Those pictured above are fairly easily classified as shell bones from their morphology: relatively thin, often slightly curved, generally smooth on at least one side, sometimes marked on a side with shallow curved grooves. Which specific bones these might be is another issue altogether and is touched on below.
It’s the shallow grooves often adorning turtle shell bones that ended up being the focus of this post. They are somewhat visible in the top center and bottom left specimens in the picture above, and most clearly apparent in the top left specimen. They are something I thought I understood but, as is often the case, there’s more to it than I realized. These grooves are known as sulci in the plural and sulcus in the singular. The name comes from the Latin word sulcus meaning “a furrow, groove, trench.” (Borror, p. 98; full citations of all references are given in the Sources section at the end of this post.) I mentioned sulci in a previous post (A Pastiche of Turtles, August 21, 2013) and briefly described their origin.
Before delving into that, I think a bit of basic shell “architecture” is in order. The turtle shell is a remarkable feature of these animals and “stands out as one of the most distinctive and morphologically novel structures in all of Vertebrata.” (Tyler R. Lyson, et al., 2013, p. 317.) Its evolution has long been the source of fascination, speculation, and debate. The shell consists of two main bony elements: the dorsal carapace and the ventral plastron. Both are covered in keratinous scutes which are analogous to scales in other reptiles. They give the turtle shell color and patterning, and they serve to retain water and body heat. Scutes are lacking in softshell and leatherback turtles which also have significantly diminished carapace bones. (Descriptions of the morphology of the turtle shell can be found in many different sources. For this post I drew in particular Ernst and Lovich, 2009; Sathe, no date; Dodd, 2001; and Romer, 1966.)
Sulci are the impressions on shell bones left by the scutes. Though I could recite that origin story, I did not understand its deeper meaning. That only registered for me after some interesting but seemingly unfocused floundering, a bit of which I, of course, describe below.
My initial foray into the research literature was just intended to get a feel for the Plum Point specimens pictured above with no particular attention to the sulci since I didn’t know where this journey would end up. This consisted primarily of a quick scan of many pictured fossils to see if any stood out. Quite early on, one specimen did. I was struck by pictures (shown below) of the dorsal and ventral sides of the nuchal bone from a newly named Pliocene turtle fossil from the Lee Creek Mine (Zug, taken from figure 5, page 211)
The fossil shown in these images appears similar to the uppermost fossil in the photograph above of the Plum Point turtle fossils. I assume both fossils are not complete bones. Here’s a closer look at the Plum Point fossil which is somewhat smaller than the Pliocene specimen.
Nuchal bone? I hadn’t a clue what that was. Turns out this is a significant bone in all turtles’ carapaces. It is the major plate at the anterior end of the carapace and in most turtles is the meeting place for three lines of bones. Its name comes from the Middle Latin nucha meaning nape (back of the neck). (Borror, p. 64.) These other bones of the carapace are the neurals covering the top of the shell (the animal’s vertebrae are attached to them ventrally), the costals encircling the middle of the sides of the shell (these are “modified ribs”), and the peripherals forming the skirts that ring the bottom of the carapace. The nuchal bone “is recognizable for the distinctive pattern of lines from the overlying epidermal scutes, and for its bilateral symmetry.” (Sathe et al.) In the illustration below of the bones of a box turtle carapace, I shaded the nuchal black and the neural immediately behind the nuchal gray, inserted a crosshatched pattern in the two anterior-most costals, and highlighted the two anterior-most peripherals with a diamond pattern. This illustration is based on Figure 1-3a in herpetologist C. Kenneth Dodd’s North American Box Turtles (p. 8).
I deliberately used the illustration of a box turtle carapace because it became crucial to my understanding of the sulci. Among the various specimens in my natural history collection is the carapace of what I believe to be an eastern box turtle, Terrapene carolina carolina, found in West Virginia woods many years ago. It is approximately 13 cm in length from anterior to posterior. Although wood turtles have also been found in those particular woods, the high dome of this carapace suggests it's from an eastern box turtle.
When I drew it out of the drawer where I store it, I was quite flummoxed. There was no apparent nuchal bone and, even more disturbing, the pattern of the pieces on the dorsal side of the carapace bore no resemblance to the diagram of carapace bones shown above. It took me awhile to realize my error; someone with experience and training in herpetology or the fossils of turtles wouldn’t have made it. My carapace is indeed most likely that of an eastern box turtle, and it actually has a nuchal bone and the other carapace bones which are organized as the earlier illustration would have them. What I failed to realize was that, though this carapace had weathered for some time in the woods and had been stripped in the process of any other feature of the animal (such as skull bones or leg bones), it had not lost its scutes. The illustration below of the scutes that cover the carapace of the eastern box turtle makes this fact painfully obvious (illustration based on Dodd, Figure 1-2a, p. 8).
As I already knew, though scutes do form an interesting pattern on a turtle’s carapace, that pattern does not coincide with that of the underlying carapace bones, and this disconnect is the source of the sulci. That said, I’d never realized the sheer extent of this disconnect. And that led me to ask the question I should have posed much earlier: What is gained for the turtle shell by this dissimilarity in layouts between bones and scutes?
I am embarrassed to admit that the answer had been given in some of what I’d already read but which I’d glossed over, never absorbing it. The “aha” (and “I’m an idiot”) moment came when I reread paleontologist Alfred Sherwood Romer’s description of the turtle shell. In it, he wrote:
The arrangement of the outlines of scutes and bones does not coincide; in general, there is an alternating arrangement, which gives greater strength to the combined structure. (Romer, 1966, p. 112)
Later, I came across Dodd’s instructive analogy. He posited that “overlapping scutes provide additional strength and protection, much as shingles do on a roof.” (Dodd, p. 138) Though the sulci are not the source of this strengthening of the shell, they are the evidence that remains on shell bones of this beneficial arrangement.
So, that’s it. No big reveal at the end of this journey, only a mild “aha” moment and a better understanding of shell bones, scutes, and sulci.
I’ve already admitted that I started this post with no particular destination in mind. I was buoyed by the hope that something interesting might turn up if I pondered some of the Plum Point turtle fossils, and I think it did. This brings to mind a wonderful, new song by J.J. Shiplett which has the refrain:
You never really know just where you’re going to go until you’re already there. (Already There, 2023.)
Sources
Donald J. Borrow, Dictionary of Word Roots and Combining Forms, 1960.
C. Kenneth Dodd, North American Box Turtles: A Natural History, 2001.
Carl H. Ernst and Jeffrey E. Lovich, Turtles of the United States and Canada, Second Edition, 2009.
Tyler R. Lyson, et al., Homology of the Enigmatic Nuchal Bones Reveals Novel Reorganization of the Shoulder Girdle in the Evolution of the Turtle Shell, Evolution & Development, Volume 15, Number 5, 2013.
Erik Sathe, et al., Morphology of the Vertebrate Skeleton: A Manual for Lab and Field, Department of Integrative Biology, University California, Berkeley, no date.
Alfred Sherwood Romer, Vertebrate Paleontology, Third Edition, 1966.
George R. Zug, Turtles of the Lee Creek Mine (Pliocene: North Carolina), appearing in Geology and Paleontology of the Lee Creek Mine, North Carolina III, edited by Clayton E. Ray and David J. Bohaska, Smithsonian Contributions to Paleobiology, Number 90, 2001.
Posts
