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