The Evolution of Sea Turtles
Mike Salmon, Ph.D.
Department of Biological Sciences
Florida Atlantic University at Boca Raton
We’re all lucky to be alive at this time, and in this place, because of so many advances in science and medicine. For example, many illnesses that 50 years ago couldn’t be cured and caused endless suffering or even death can today be prevented by an injection, suppressed and eventually eliminated by an antibiotic, or removed by a surgical procedure that requires nothing more than a brief, in-house visit to your personal physician. One can only wonder what miracles await those born 50 years from now, assuming that we manage to find the right solutions to the myriad of environmental problems that we now face on this planet.

These are also exciting times for biologists, especially those who work in the fields of evolutionary biology, because new techniques now allow us to gain an understanding of both events that transpired in the distant past, as well as those occurring now. What I hope to do here is provide you with a glimpse of how that understanding has been applied to our favorite animals – the marine turtles. But first, a few definitions and details…
Recall that biological evolution is defined as descent (from a common ancestor) accompanied by modification. That definition incorporates two key concepts. The first is that all life evolved once on this planet and so can be traced back to ancestral forms. The second is that over time, newer kinds of living creatures have been produced by changes (“modifications”) of the older kinds. Modification occurs because, as Darwin pointed out some years ago, (i) all kinds of organisms (We call them “species”.) produce more offspring than can possibly survive, (ii) those offspring vary in their appearance, physiology and behavior, and (iii) some of those variants are better able to survive than others. Thus, over succeeding generations, the appearance of organisms changes as the competitively superior individuals replace those that are less able to compete. Darwin called this process “natural selection”. Repeat this generation after generation, over many thousands or even millions of years, and the result is the creation of new species. Each of these can be considered an experiment that is either successful (as witnessed by the proliferation of other, similar species)
Figure 1. This phylogeny depicts the evolution of the modern marine turtles, and should be read from left to right. See the text for how to interpret the details. The red arrow, bottom right, shows when in time the modern humans (genus Homo) arose. That should impress readers with how relatively “young” our lineage is, compared to the marine turtles! Another way of interpreting that difference: the marine turtles have proven their success as an evolutionary experiment by hanging in there for so long a time period. We have yet to do so! (Diagram from J. Spotila, Sea Turtles: a Complete Guide to their Biology, Behavior and Conservation)
or unsuccessful (lasting only a few million years or so, then disappearing). But there is no arguing about one consequence: all species eventually go extinct. Museum drawers are full of the fossils of plants and animals that had “their day in the sun”, and no longer exist. In fact, well over 95 % of all the species that existed in the past no longer exist, and have been replaced by new species.

It’s also important to realize that natural selection isn’t the only force that influences which species persist and which disappear. There is good evidence that chance events also may play a role. For example, we know that “mass
extinctions” in which ~ 75% of all species suddenly go extinct, have occurred at least 5 times at roughly 26 million year intervals back in time. What caused these rather sudden events? Current thinking is that meteor strikes and large scale volcanic eruptions may have been involved and temporarily changed the physical and chemical environment on earth so radically that most organisms were unable to survive. Regardless of the cause(s), however, the consequences were much the same. The survivors suddenly found themselves in a world without competitors, and so what followed was an explosion of new species that exploited the habitats formally occupied by the extinct forms.
Figure 2. The size and appearance of the extinct, and ancient, marine turtles. Top diagram (A) shows their size relative to humans and the largest of the present marine turtles, the leatherback. The lower diagrams are reconstructions of the appearance of Archelon (B, a Toxochelid), the largest marine turtle that ever lived, and (C) a Protostegid that was almost as large.
The mammals, for example, diversified to occupy many of the habitats formally occupied by the reptiles, and so did the extant birds, which fossil and anatomical evidence indicate are dinosaurs that survived the extinction event that doomed their close relatives. One final detail should be mentioned. Because all organisms living today are descended from those that lived in the past, biologists have created a naming system that reflects not only their relationships to a common ancestor and to one another, both also the degree to which they have changed. Three of those categories, reflecting those increasing differences, are genera (composed of closely related species), families (composed of closely related genera), and orders (composed of closely related families). Biologists show these relationships in diagrams called a Phylogeny, and each such diagram is presumed to be an historical account
of the significant evolutionary events that transpired over time.

Armed with this background, let’s explore the history of the marine turtles by examining the details of their phylogeny as shown in Figure 1. Note that the diagram itself starts to the left, with a short horizontal line labelled “modern sea turtles”. The time scale below depicts when those forms arose, based upon fossil evidence. The modern sea turtles arose from a common ancestor about 110 million years ago and gave rise to 4 families of marine turtles. What followed was a proliferation of a number of truly huge marine turtle species in two of these families, the Toxochelydae and Protostegidae; both are now extinct (see Figure 2). The remaining two families (the Cheloniidae, or hard-shelled turtles, and the Dermochelidae, or leathery turtles) are the remnants of the many species of marine turtles that descended from a common ancestor about 95 million years ago. While there are 6 species of hard-shelled turtle survivors, there is only one surviving leathery turtle in existence today, and it is both large and strange! Over its evolutionary history it developed a number of unique characteristics such as an ability to dive deeper than any other marine turtle in its search for prey (jellyfishes and salps), its ability to retain heat so it can remain active and feed in waters too cold for other marine turtles, and an unusually rapid growth rate, especially given that its prey are nutritionally poor as a food source.

But how do biologists make these determinations, what problems do they face, and how have modern developments improved our confidence in these assessments? When the species being compared exist only as fossils, determinations of relationships are based primarily upon two criteria: their resemblances to one another and their existence as fossils in successive layers of sediment or rock, indicating how their structural modifications changed sequentially over time. For marine turtles, which are heavy-boned and large animals, that fossil record is excellent rendering the interpretations of evolutionary pathways by paleontologists on comparatively firm footing.

Ironically, determining the relationships among the species currently alive posed real problems, at least until recently, because those relationships were based in the past primarily upon resemblance. Resemblance, alone, turns out not to be a very reliable criterion because the conclusions finally reached depends upon which structural characters are selected and how they are weighed in terms of importance. For example, some years ago the green turtle was
considered to be represented by two species: the common green turtle found in our waters and the black turtle, found in eastern Pacific waters. These turtles differed not only in color but also in body shape and so the question became “Are these differences sufficiently important to justify the separation of those populations into two species”? The answer was a matter of opinion, with those saying “yes!” characterized among biologists as “splitters” and those who disagreed characterized by their colleagues as “lumpers”.

Here’s where modern techniques can be applied to settle the question. Instead of examining appearance alone, the genes of the two groups were compared to see to what extent they differed. Using genetics as a criterion for distinguishing species is much more objective than using appearance because modifications among species generally reflect genetic change. It turned out that the genetic differences were negligible compared to the changes in appearance, and so the green turtles are presently considered one species. Green turtles are also now characterized as a species that (for unknown reasons) varies widely in its appearance across the oceans of the world where it is found. Similar kinds of genetic comparisons among the other species of hard-shelled turtles were used to establish their relationships, as indicated by the red circles shown in Figure 1. Four of these species (loggerhead, hawksbill and the two ridley turtles) are more closely related to one another than to the green turtle and flatback, which are close relatives.

As a result of these determinations we can now compare and contrast differences between these species in where they live, what they eat, and how they behave to better understand the evolutionary pathways each have taken to enable their existence today. Those kinds of discoveries are ongoing and will be described in more detail in future issues of Environmental Outreach magazine, so stay tuned.
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