The origin of dinosaurs could arguably be traced back as far as the origin of life itself, which was about 3.8 billion years ago, but for the purposes of this book the evolution of amniotes is a more reasonable starting point. The development of an amniotic egg (one with an amnion, or fluid-filled sac surrounding the embryo: Fig. 6.4), from amphibian ancestors for reproduction of offspring outside of aquatic environments, is often heralded as one of the major adaptations in vertebrate evolution. Unfortunately, the first appearance of this defining characteristic of Clade Amniota, which is inferred to have happened during the Carboniferous Period, is currently unknown. No definitive fossil eggs or nests have been interpreted from rocks older than the Late Triassic. This gap in the fossil record, for such a well-established behavior, is likely an artifact of the non-mineralization of eggs before the Triassic (causing a preservation bias), or paleontologists not recognizing nest structures, or a combination of these two factors. However, similarities in the eggs
FIGURE 6.4 Components of an amniotic egg, including the eggshell, allantois, yolk sac, amnion, and embryo. Such eggs are a defining character of the clade Amniota, and by extension of dinosaurs. After Cowen (2000), History of Life, 3e, Blackwell Science, Inc., Malden, MA, p. 147, fig. 9.12.
of all living amniotes and their close resemblance to fossil eggs argue that this trait is a synapomorphy of amniotes, and it is currently inferred to have evolved just before the skeletal record for amniotes begins.
Despite this lack of evidence, three early species of amniotes in the fossil record are recognized from the Carboniferous Period: Westlothiana, Hylonomus, and Paleothyris. The interpretation of these three small vertebrates as amniotes is based on some anatomical traits distinctive from their amphibian ancestors:
■ Dermal bones on the ventral surface of the skull (such as parietals, frontals, and nasals) overlying a bony braincase.
■ Reduced head size relative to the overall body size and lightening of the skull.
■ Highly modified pelvis consisting of a reinforced pubis and ischium.
■ Astragalus and calcaneum in the ankle.
Defining whether some fossils were reptile-like amphibians or amphibian-like reptiles is problematic because of their shared features. Traits of an amniote that differ from that of an amphibian are also more numerous than those listed previously and they summarily reflect adaptations to a terrestrial lifestyle that was increasingly independent of nearby water bodies. As long as aquatic environments were abundant and widespread, amphibians probably did not undergo natural selection that would have favored inheritance of genotypes for sturdier skeletal parts adapted for moving long distances on land away from water.
So as long as aquatic environments were abundant, the buoyancy of water, which helps to relieve gravitational stresses in a vertebrate skeleton, would have negated selection for a heavier skeleton reinforced for extended periods of locomotion out of water. But with changes in environmental conditions to drier climates or the creation of niches apart from water (such as forests), exaptations or other evolutionary factors favored adaptations of pre-amniotic ancestors toward amniotes. The ability of these non-amniotic ancestors to move about freely on dry land required modifications to their skeletons that supported their weight (that is, a lighter skull, stronger hips, flexible ankles), thus natural selection may have already resulted in amphibians that were divorcing themselves from their dependency on aquatic environments.
The development of an enclosed egg among the descendants of pre-amniotic ancestors was probably the result of natural selection, as only a few eggs (from originally large numbers) had rudimentary membranes enclosing aqueous solutions and prototypes of a yolk sac and allantois (respiratory organ for the embryo). Only then would the embryos have survived. Another major evolutionary requirement for the development of amniotic eggs would have been internal fertilization, so sex had to have become more up-close and personal than was previously experienced by amphibians. A few examples of modern amphibians show such a reproductive mode, which means that the same inheritable behavior and anatomical attributes could have been selected in favor of increasing the chances of fertilization. Also, embryos would have been retained within the reproductive tract of the female until a sufficiently protective membrane had developed around them.
The next step in amniotic egg evolution would have been an embryo that underwent growth within the protective membrane to form a miniature version of the adult animal, in contrast to the incompletely developed and intermediate larval (tadpole) stage seen in most amphibians. Although fossil evidence for a sequence
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