STUDY GUIDE
BI 120: BIOLOGICAL PRINCIPLES
FALL 1996
Page 20
REPTILE-MAMMAL TRANSITION
In his book Evolution: the fossils say no! (1978, Creation-Life Publishers, San Diego, CA)
the biochemist Duane Gish discusses the evidence for transitional forms between reptiles
and mammals. He first notes that the two classes are difficult to distinguish on the basis
of bones:
It is at the amphibian-reptilian and the reptilian-mammalian boundaries
that strongest claims have been advanced for transitional types bridging
classes. But these are just those classes that are most closely similar in
skeletal features, parts that are preserved in the fossil record.
Gish here notes that transitions between invertebrate and vertebrate, fish and tetrapod, and
nonflying and flying animals should be more readily preserved, then goes on to discuss the
reptile-mammal evidence:
Many of the diagnostic features of mammals, of course, reside in their
soft anatomy or physiology. These include their mode of reproduction,
warm-bloodedness, mode of breathing due to possession of a diaphragm,
suckling of the young, and possession of hair.
The two most easily distinguishable osteological differences between
reptiles and mammals, however, have never been bridged by transitional
series. All mammals, living or fossils, have a single bone, the dentary, on
each side of the lower jaw, and all mammals, living or fossil, have tree
auditory ossicles or ear bones, the malleus, incus, and stapes, In some fossil
reptiles the number and size of the bones of the lower jaw are reduced
compared to living reptiles. Every reptile, living or fossil, however, has at
least four bones in the lower jaw and only one auditory ossicle, the stapes.
There are no transitional forms showing, for instance, three or two
jaw bones, or two ear bones. No one has explained yet, for that matter, how
the transitional form would have managed to chew while his jaw was being
unhinged and rearticulated, or how he would hear while dragging two of his
jaw bones up into his ear.
Note: Gish does not cite references in these passages. The only reference he cites in
dealing with the reptile-mammal transition elsewhere in the same chapter is A.S. Romer,
1966, Vertebrate Paleontology, University of Chicago Press, Chicago, Ill.
Read The transition between reptiles and mammals, by R.E. Sloan (in Evolution vs
Creationism: the public education controversy, ed. J.P. Zetterberg, 1983, Oryx Press,
Phoenix, AZ.)
Do Gish and Sloan agree about the diagnostic skeletal features separating reptiles
and mammals? List them.
According to Sloan, are there transitional forms with three or two jaw bones?
How could we check to see which author is correct?
For each of the hypotheses, does the fossil evidence falsify, support, weaken,
prove, require it to be modified, or give no conclusive information about it? Explain each
answer.
From E. H. Colbert, 1980. Evolution of the Vertebrates, John Wiley & Sons, New York,
NY
Among the earliest of the mammals are docodonts known as morganucodonts,
represented by the genus Morganucodon (which is probably the same as an earlier-named
genus, Eozostrodon) from the Upper Triassic of Europe and by the genus Megazostrodon
from the Upper Triassic of South Africa. Originally, these forms were known from
scattered teeth and a few jaw fragments but, within recent years, fossil bones in large
numbers have been recovered from Triassic fissure fillings within Carboniferous limestones
in South Wales. Skulls with associated skeletons are known from South Africa. In
addition, some closely related materials have been found in southwestern china.
The morganucodonts were tiny mammals with slender lower jaws. In these early
mammals the lower jaw was of mammalian form, and functionally was composed of a
single bone., the dentary, at the back of which there was a large and high coronoid process
for the attachment of strong temporal muscles, and a well-formed condyle for articulation
with the squamosal bone of the skull. But, significantly, on the inner side of the jaw was a
groove within which was preserved the remnant of the articular bone -- a sort of
paleontological reminder of the old reptilian jaw joint, still preserved in this ancient
mammal. Since the quadrate bone of the skull was also preserved, obviously these animals
possessed both jaw joints, as did the mammal-like reptile, Diarthrognathus, described in
Chapter 9.
Figure 93. The lower jaw of Morganucodon, a Triassic mammal. Note the reptilian bones still retained. Abbreviations. a, articular; an, angular; sa, surangular. About four times natural size.
Here we see examples of the gradual transition from reptile to mammal. Diarthrognathus,
from the Upper Triassic of South Africa, is on the reptilian side of the line because,
although it had the double jaw joint, the quadrate-articular articulation was still dominant.
Morganucodon, from the Upper Triassic of Europe and Asia is on the mammalian side of the
line because, although it too had the elements of both articulations, the squamosal-dentary
joint was the dominant one. Notice that the two genera are not related to each other.
Figure 71. A comparison of the skeletons in the earliest known bird, Archaeopteryx of Jurassic age, and in a modern pigeon. Comparable regions
of the skeleton (brain case, hand, sternum, rib, pelvis, tail) are shaded black. Not to scale. In the modern bird, the brain case is expanded, the bones
of the wing are coalesced, the pelvis is fused into a single, solid structure, the bony tail is reduced, the ribs are expanded, and the sternum or breast
bone is greatly enlarged for the attachment of strong wing muscles. All these adaptations, and many others, make the modern bird an efficient
flying animal.
Figure 26. A comparison of the pelvic region and the hind limb in the Devonian crossopterygian fish, Eusthenopteron (left), and the Permian
labyrinthodont amphibian, Trematops (right). (A) Pelvis, (B) Femur, (C) Tibia-fibula, (D) Pes.
From: Science as a Way of Knowing, the Foundations of Modern Biology, John A. Moore, 1993, Harvard University Press
Figure 26. The jaw articulation and ear ossicles of four vertebrates. Although novelty does arise in evolution, an exceedingly common pattern is the alteration of an existing structure. Not one of the three ear bones of mammals -- stapes, incus, and mallus -- is really new. the stapes, which began as a part of the hyomandibular in the primitive fish, became the single ear bone in amphibians, reptiles, and birds. The incus was a prominent upper jawbone (pterygoquadrate) in primitive fishes and became reduced to the quadrate in amphibians, reptiles, and birds. It formed a jaw joint with the articular in the lower jaw, the later a modification of Meckel’s cartilage. In the mammal-like reptiles the dentary of the lower jaw begins to enlarge: and finally, in the mammals it forms a new jaw joint with squamosal of the upper part of the skull. Of the two bones of the old joint, the quadrate becomes the incus and the articular becomes the malleus.
Maintained by
Dr. David Starrett... email: dstarret@biology.semo.edu