Turtles, turtles, turtles. It’s been a glorious summer for encounters with turtles . . . but, despite a lifelong love affair with them, it’s also been a time for discovering how little I know about these reptiles. There is no “clever” hook for this post, no point larger than the fact that familiarity does not lead to understanding. It is simply a pastiche of turtle encounters and lessons begun to be learned. (Not much for the 200th post on this blog.)
In July, when an Eastern box turtle (Terrapene carolina carolina) crossed the yard of my summer cottage, it prompted a post about the scientific debate over the origin of turtles (land or sea). A question I’d never considered.
Later this summer, while hiking in the Quogue Wildlife Refuge on the South Fork of Long Island, New York, I chanced upon more chelonians (turtles constitute the order Chelonia). The refuge offers a wonderful 300 acres or so of diverse environments, from a large pond (the Old Ice Pond where ice was harvested in the 19th and early 20th centuries) to marshes and wetlands to very dry, deeply scented Pine Barrens. As I crossed a footbridge over one extension of the pond, there in the water below I spotted several Eastern painted turtles (Chrysemys picta picta) lazily swimming and drifting amid the vegetation.
On that particular day, I felt I had met with an abundance of turtles. But abundance certainly is not the collective noun, not the term of venery, for turtles. I turned to James Lipton’s marvelous book An Exaltation of Larks, and learned that the word I wanted was bale – it’s a bale of turtles. Lipton, when discussing the possible source for this term, concluded that the “origin remains as well concealed as the turtle.” (p. 62)
The purchase of a small turtle fossil this summer taught me something else about turtles. A fundamental element of the shell anatomy of most chelonians – made clear by this fossil – had escaped me all these years: the shell is composed of layers, a layer of bony plates overlain by a layer of scutes. Scutes are pieces of keratin that, in the species that do not shed them, build up over time. The preferred adjective to describe turtle scutes in the scientific literature is horny (Oliver Perry Hay, The Fossil Turtles of North America, 1908, p. 4; Alfred Sherwood Romer, Vertebrate Paleontology, 1966, p.112), which makes sense since keratin is the stuff of animal horns and hoofs (and our fingernails and hair). Scutes are arranged atop the series of bony plates of both the carapace (the top or dorsal element of the shell) and the plastron (the bottom or ventral element). The so-called bridge, which joins the carapace and plastron together, is the third major component of the shell. The pattern of intersections of the scutes only marginally overlaps that of the underlying bony plates; impressions (sulci) of the scute pattern are left on the bony plates. (Scott F. Gilbert, et al., How the Turtle Gets Its Shell, in Jeanette Wyneken, et al., eds., Biology of Turtles: From Structures to Strategies of Life, 2008, p. 2; Richard E. Nicholls, The Running Press Book of Turtles, 1977, p. 18-19.)
Shown below are the fossil remains of a tortoise, Stylemys nebrascensis, which were found in the Nebraska badlands, specifically in the Brule Formation on the Shalimar Ranch in Sioux County, Nebraska. This fossil is from the Oligocene epoch, early in that epoch, making it roughly 32 million years old. The anterior of the shell is facing up in this photo.
Trying to comprehend what’s missing and what’s present in this fossil shell helped me gain a slight sense of the turtle’s shell. Most of the carapace is missing, with a few bony plates clinging to the periphery; all, or nearly all, of the bony plates that make up the plastron are present. What of the scutes? They’re gone, separated in death. As paleontologist Alfred Sherwood Romer noted about turtle fossils, “The scutes are not, of course, preserved in fossils, although the outlines are often indicated by grooves in the bones which lay beneath them.” (p. 112) Yet, I suspect there may be fossil turtle shells with the scutes still present. One study of what happens to turtle shells post-mortem showed that, though a significant percentage of scutes loosen and detach relatively early in the decay process, that still takes, on average, more than a month. I would think that specimens quickly “entombed” should still retain scutes and possibly fossilize with them. (C. Kenneth Dodd, Jr., Disarticulation of Turtle Shells in North-central Florida: How Long Does a Shell Remain in the Woods?, The American Midland Naturalist, Volume 134, Number 2, 1995.) I’m also beginning to think that there is some confusion on the web, outside of the scientific literature, about the distinction between turtles’ bony plates and scutes.
One thing that isn’t evident from casual encounters with turtle, but which is a fundamental aspect of the carapace, is that the ribs and vertebrae that lie beneath certain of the bony plates are fused to them. Other skeletal features of the reptile are contained within the animal’s ribcage.
The turtle’s shell is truly an amazing structure with long term consequences for its possessor. But, I am struck by the fact that some of the scientists who study them assume that the rest of us have become blasé about them. Biologists Carl H. Ernst and Jeffrey E. Lovich (Turtles of the United States and Canada, 2009) put it this way, “The single feature that most defines living turtles is their bony shell. . . . Our familiarity with turtles trivializes this fantastic architecture.” (p. 3) In this regard, they quote Romer who concluded, “The chelonians are the most bizarre, and yet in many respects the most conservative, of reptilian groups. Because they are still living, turtles are commonplace objects to us; were they entirely extinct, their shells – the most remarkable defensive armor ever assumed by a tetrapod – would be a cause for wonder.” (p. 112)
I think they have it wrong. Familiarity, in this case, doesn’t lead us to accept turtles as commonplace, or make us experience a loss of wonder over their presence. On the contrary, we are deeply attracted to turtles, approaching them with a mixture of wonder and affection, tempered with a touch of disbelief. Familiarity also, at least in my case, hasn't led to understanding.
Posts
No comments:
Post a Comment