"Though a possible whale-to-mammal transitional sequence has recently been unearthed, critics maintain that transitional sequences are rare, at best. For this reason, critics argue that Darwin's theory has failed an important test.Explore Evolution, p. 27
Scientists have long thought that amphibians were a transitioinal form between aquatic and land-dwelling life forms. Why? Because amphibians can live in both the water and on land. Yet, the fossil record has revealed at least two problems with this idea... land-dwelling amphibians, themselves, appear suddenly in the fossil record.Explore Evolution, p. 27
Darwin himself was well aware of the problems that the fossil record posed for his theory. … Where were the multitudes of transitional forms connecting different groups, as predicted (and expected) by his theory?Explore Evolution, p. 30
Some critics say neo-Darwinism is not consistent with fossil data. Other critics say that punctuated equilibrium is consistent with fossil evidence, but lacks and adequate mechanism. Critics of both views argue that there are still far fewer transitional forms in the fossil record than we would expect, even if new forms of life did arise quickly.Explore Evolution, p. 33
First, a note on terminology. The phrase "neo-Darwinism" is not widely used by scientists, and may reflect a desire by creationists to dismiss Darwin's ideas merely as another "-ism," rather than a robust scientific theory. A PubMed search of over 18 million scientific articles found 131 variations on "neo-Darwinism," compared to 226,476 uses of "evolution."
Explore Evolution compounds many errors in attempting to claim that fossils representing evolutionary transitions are rare. The first error is its reliance on concepts like 'missing links' and 'transitional forms'. These terms are outdated and founded in incorrect and archaic ways of categorizing life. Until relatively recently, the classification system used to group living things did not aim to represent true evolutionary relationships, and some groups contained only some descendants of a common ancestor, excluding others. For example, birds were not traditionally placed within the reptiles while the 'Sarcopterygii' (lungfish, coelocanths, etc.) classically excludes tetrapods. When we try to connect these poorly defined groups with the grade school evolutionary view that, "fish gave rise to amphibians, which gave rise to reptiles, which gave rise to…," the problems with the underlying classification system confuse the matter. Some so-called 'fish' were more closely related to amphibians than to other so-called 'fish' – and some so-called 'reptiles' were more closely related to non-reptiles (e.g., birds) than they were to other 'reptiles'. Drawing upon discontinuities produced by our misclassification, people sought to find 'transitions' or 'links' between wrongly grouped 'fish' and 'amphibians' or 'reptiles' and 'birds' – historically, and quite literally creating the concept of 'missing link' or 'transitional form.'
Fortunately, evolutionary biologists have been doing away with such artificial groups for some time now. We no longer accept that birds evolved from reptiles and that birds are not reptiles themselves - since the term Reptilia now includes birds. Viewing life's history and classification in this more realistic (i.e., evolutionary) context – where we name groups based on a common ancestor plus all of it's descendants – we come to realize that quite literally, all critters have 'transitional' features. In other words, all living things possess a combination of ancestral and derived traits. The shared derived traits (discussed further in the response to chapter 4) inform us of close relationships, while ancestral traits include ancient features retained from an early evolutionary heritage. For example, salmon – which most people wouldn't have any trouble classifying – retain paired appendages and jaws – ancestral traits shared with species like sharks – but also have derived traits like bone that forms from a cartilaginous precursor – a trait which sharks do not have, but which tetrapods do. This sort of bone is a derived feature linking salmon as closer relatives of tetrapods than sharks are, even though salmon still have fins and other 'fishy' traits that sharks share.
Does this mean salmon are transitional forms or 'links' between sharks and tetrapods? Were salmon our ancestors? Of course not. The issue is that upon properly classifying life into groups sharing a common ancestor, we see that all of life is characterized by these ancestral and derived traits. Just as salmon did not give rise to tetrapods, frogs are not links between salmon and reptiles. These traits inform us of common ancestry among groups, not a sequential movement of an entire group into another group (e.g., amphibians to reptiles).
As to the actual rarity of fossils illustrating evolutionary transitions, Explore Evolution makes additional errors. First, fossils in general are rare, relative to the actual diversity of life which once existed. The chances of a given species fossilizing are slight. Thus, the fossils referred to as transitional are not necessarily the direct ancestors of modern taxa, but may represent failed branches off of the stem which led to modern forms. Depending where they branched off, they possess some, but not all, of the traits we associate with modern groups, which provides evidence of the form the transition took, even if we lack fossils of the directly ancestral species. Knowing the age of these fossils, we can have substantial certainty about the latest date at which various evolutionary novelties must have originated. Explore Evolution fails to explain what paleontologists actually use fossils to illustrate, sowing confusion among students where it ought to bring clarity.
Given the general rarity of fossils, fossils showing evolutionary transitions are not at all rare. This can be illustrated with a range of examples, of which the record of hominid fossils is especially striking. The skulls shown below display a clear, smooth transition from the early ancestors of modern humans to the modern form of the human skull. A wag might suggest that there is a gap between each of those fossils, and demand a transitional fossil to fill each such gap. Because this evolutionary sequence is relatively recent, there are enough fossils that we can only show the full transition with graphs like the one below.
That graph illustrates one particular aspect of human evolution, the growth of the brain over the last 3.5 million years of human evolution. At no point could anyone credibly point to a discontinuity between our australopithecine ancestors and modern humans which is not filled by some ancestral fossil form.As we look at events further back in time, the chances of a fossil surviving decrease, so forms of life from the more distant past tend to show larger gaps - greater morphological variation - between fossils. Nonetheless, new fossils are constantly being found which shrink the gaps between ancient species, such that cases once presented by creationists as insurmountable problems for evolution are now textbook examples of fossil transitions. Indeed, of the two sources Explore Evolution cites to support the claim that "Paleontologists have identified many gaps that remain to be filled in the fossil record" (p. 20), only one actually addresses the quality of the fossil record, and it is from 1981. Even that paper does not support the claim that such gaps reflect an absence of transitional forms. Everett C. Olson wrote:
The problem of the existence of linkages and phylogenies at the species and generic levels has been much reduced during the last one hundred and twenty years. How this reduction supports or denies Darwin's concepts of phyletic gradualism is still a matter of interpretation of the evidence. At familial and higher levels, the establishment of linkages between categories has been much less successful, and decreasingly so at each successive higher level. Under the very best circumstances, however, morphological and stratigraphically graded transitions between classes and subclasses have been found.Everett C. Olson (1980) "The Problem of Missing Links: Today and Yesterday," The Quarterly Review of Biology 56(4):405-442.
Even twenty-seven years ago, the record of species-level transitions was considered quite good, and at higher taxonomic levels, the situation was improving and quite strong in situations where preservation of fossils had been favorable. Since that time, the state of transitional fossils has only improved. Explore Evolution uses a 1982 reference in an attempt to discredit these recent fossil discoveries (without actually mentioning what those discoveries are). Staying up to date with research in science is critical for students and for textbook authors, and Explore Evolution's reliance on an outdated, non-applicable, 25 year old reference is unacceptable.
A recent example from the news is the discovery of the fossil species Tiktaalik roseae.
Tiktaalik is a transitional form in the evolution of vertebrates on four legs. Ahlberg and Clack (2006) describe the importance of the discovery:
It demonstrates the predictive capacity of palaeontology. The Nunavut field project had the express aim of finding an intermediate between Panderichthys and tetrapods, by searching in sediments from the most probable environment (rivers) and time (early Late Devonian). Second, Tiktaalik adds enormously to our understanding of the fish–tetrapod transition because of its position on the tree and the combination of characters it displays.Tiktaalik roseae was predicted before it was discovered. As Neil Shubin describes in his 2008 book Your Inner Fish:Per Erik Ahlberg and Jennifer A. Clack (2006) "Palaeontology: A firm step from water to land," Nature 440:747-749
My colleague Jenny Clack at Cambridge University and others have uncovered amphibians from rocks in Greenland that are about 365 million years old. WIth their necks, their ears, and their four legs, they do not look like fish. But in rocks that are about 385 million years old, we find whole fish that look like, well, fish. They have fins, conical heads, and scales; and they have no necks. Given this, it is probably no great surprise that we should focus on rocks about 375 million years old to find evidence of the transition between fish and land-living animals.Neil Shubin, 2008. Your Inner Fish. Pantheon Books, New York, 0375424472. P. 10.
And when Shubin started investigating sedimentary rocks laid down in shallow water about 375 million years old, he found Tiktaalik roseae.
Subsequent investigation confirmed that Tiktaalik roseae's transitional morphology.
[Tiktaalik roseae shows] a marked reorganization of the cranial endoskeleton ... [with] morphology intermediate between the condition observed in more primitive fish and that observed in tetrapods.Downs, J.P., Daescher, E.B., Jenkins, F.A., and Shubin, N.H., 2008. "The cranial endoskeleton of Tiktaalik roseae." Nature, vol. 455, no. 16, 16 Oct 2008, pp. 925-929.
Based on known fossils, scientists could estimate what time period the transitional form had to have existed in. Based on the known locations of fossil beds, they could select a bed known to be from the right time and to have possessed the right environment 375 million years ago to contain a transitional form. They knew what sorts of fossils to look for at that site by considering the known fossils from before and after the era in question. And after selecting that site, they found exactly the fossils they sought, a transitional form which allowed a detailed examination of the evolution of critical structures in the transition from aquatic fish to terrestrial tetrapods.
This process is exactly how science works, and a textbook interested in encouraging students to explore the way evolutionary biology is practiced would do well to help students see how paleontologists actually deal with gaps in our knowledge of the fossil record.
The approach Explore Evolution takes does not present any such understanding of the inquiry-based process of science. Gaps in our current knowledge are treated as insurmountable barriers. If scientists truly took that approach, we would never have achieved the sorts of advances seen in paleontology over the last 20 years, let alone the last 150.
Some scientists say the absence of transitional forms should dramatically change the story we tell about life's history. They point out that when we study the fossil we have actually found, the evidence does not lead us to connect the major lines of descent through a single, branching tree.Explore Evolution, p. 35
It would be very interesting to know who "some scientists" actually are. The "they" Explore Evolution discusses have not published in the peer-reviewed literature, because there is in fact substantial agreement that all the major phyla lines do indeed come from a single, branching tree.
Look through any college-level biology textbook (Campbell, p. 470-71; Raven, p. 654-55; Starr, p. 318-321), and you will see diagrams showing only the single, branching tree model. A multiple-tree view is not shown simply because the evidence for it is so weak, and the evidence for a single-tree so strong, that the multiple-tree model can be discarded in the same way a flat earth model can be discarded from a geography textbook.
This straw man logic—set up a false claim, knock it down easily, declare victory—is itself a lesson on how not to teach logic and rhetoric to children.
Some advocates of punctuated equilibrium do acknowledge that the absence of transitions between major groups of organisms is an unsolved problem for evolutionary theory as whole.Explore Evolution, p. 35
These unnamed, uncited "advocates" strike again. While certainly paleontologists would enjoy having more "transitional" fossils—whatever that vague terms actually means—there are many examples of fossils that bridge the gap between species. Thus, the phrase "absence of transitions" is wrong to imply that there are no transitions.
The fossil record provides many examples of living organisms that have remained stable in their form and structure over many millions of years--sometimes over hundreds of millions of years.Explore Evolution, p. 25
Summary of problems with claim: This is not evidence against evolution.
Full discussion: Explore Evolution brings this up to suggest:
The long-lived, unchanged existence of an organism in the fossil only poses a problem to evolution is one makes the (false) assumption that change occurs at a steady background pace. In fact, a better analogy is Newton's First Law: "Objects at rest stay at rest unless acted upon." If an organism lives in a stable environment and is able to reproduce in sufficient numbers to pass on its genes, then there is no impetus to change.
Other examples of long-lived, relatively unchanged species include sharks, the coelacanth, the oppossum, crocodiles, and the horseshoe crab.
Darwin himself was well aware of the problems that the fossil record posed for his theory...Where were the multitudes of transitional forms connecting different groups, as predicted (and expected) by his theory?Explore Evolution, p. 30
What Darwin actually wrote:
Why then is not every geological formation and every stratum full of such intermediate links? Geology assuredly does not reveal any such finely graduated organic chain; and this, perhaps, is the most obvious and gravest objection which can be urged against my theory. The explanation lies, as I believe, in the extreme imperfection of the geological record.
Charles Darwin (1859), The Origin of Species, p. 280.
Darwin's statement acknowledges an incomplete fossil record rather than a problem with his theory.
Summary of problems with claim:
Paleontologists employ methods to test whether their results are better than would be expected by chance. Evolutionary biologists draw data from multiple lines of evidence, and each of those lines of evidence reveals the same pattern — the same branching tree of life.
Explore Evolution asserts:
Given the millions of different fossil forms in the fossil record, critics argue that we would expect to find, if only by pure chance, at least a few fossil forms that could be arranged in plausible evolutionary sequences. To understand what they mean, imagine that a representative of every organism that has ever lived on earth was randomly pasted to an enormous wall representing the geologic column. Most of the fossils would bear no relationship to the other fossils stuck closest to them on the wall. Nevertheless, by chance a few of them might end up next to forms that do have some resemblance. These forms might then appear to be related as ancestors and descendants, even if they were not. Is it possible that the mammal-like reptile sequence is a statistical anomaly rather than a legitimate sequence of ancestors and descendants.Explore Evolution, p. 27
Paleontology doesn't work in a vacuum. In other words, paleontology is only one branch (or subset) of evolutionary biology. Paleontological contributions consist of morphology and stratigraphic sequence data, data that is then integrated with morphological data from living critters, their DNA (and other molecules), as well as developmental data that all work together to support common descent.
In other words, as DNA changes, that affects the proteins produced, the developmental trajectory, and the subsequent morphology of traits in individuals and lineages through time. All of these fields – molecular biology, developmental biology, and comparative anatomy – are intertwined at the heart of evolutionary theory. These fields each work from independent data sets, yet still return to converge on similar answers. For example, molecular sequence data tells us that humans and rats share more DNA in common than either do with a bird; that birds, humans, and rats are more genetically similar than any are to salamanders; and that salamanders, birds, rats, and humans share more DNA with each other than any do with salmon. In turn, developmental and morphological features are shared uniquely with this same pattern of similarity and dissimilarity. For example, humans and rats have placentas (as embryos) and hair as adults while birds do not. Birds, rats, and humans possess a chorion and allantois (extra-embryonic membranes – modified in mammals, though still present) during development, and a fully developed atlas/axis (vertebral arches) that allow up/down and side-to-side movement of the head – features which salamanders do not have. Finally, salamanders, birds, humans, and rats all share unique modes of digit development and adult hips that are fused to the vertebral column – features lacking in salmon. Thus, evolutionary biologists do not solely rely on fossils to understand evolutionary relationships; instead, fossils fall into a much larger picture of other data that returns to corroborate our understanding of history.
The statistical argument presented by Explore Evolution not only fails from a strict paleontological perspective – since paleontologists test for randomness in their data sets – but moreover, paleontologists work within the bracket of extant diversity, and data from living critters also contributes to our understanding of relationships and ancestral/derived traits.
A recent paper addressing this evidence explained:
It is clear that the fossil record cannot be read literally (Darwin 1859). There are many gaps, and many organisms, and indeed whole groups of poorly preservable organisms that have never been preserved and are doubtless lost for ever (Raup 1972). Some have even gone so far as to suggest that the fossil record is almost entirely an artifact of the rock record, with appearances and disappearances of fossil taxa controlled by the occurrence of suitable rock units for their preservation (Peters and Foote 2001, 2002), or the matching rock and fossil records controlled by a third common cause (Peters 2005). However, the widespread congruence between the order of fossils in the rocks and the order of nodes in cladograms (Norell and Novacek 1992; Benton et al. 2000) indicates that the order of appearance of lineages within the fossil record is not a random pattern.Michael J. Benton and Philip C. J. Donoghue (2007) "Paleontological Evidence to Date the Tree of Life,"Molecular Biology and Evolution, 24(1): 26-53
The Norell and Novacek (1992) citation is the same source Explore Evolution cites to justify the claim that the fossil record is statistically problematic. How can this conflict exist? The problem comes from two sources. First, that the authors of Explore Evolution do not understand (and count on their audience not understanding) how fossils are actually used. As Benton and Donoghue observe, "Fossils can provide good 'minimum' age estimates for branches in the tree, but 'maximum' constraints on those ages are poorer." In order to find a fossil possessing a transitional feature, it is necessary for that feature to have evolved, for the population in which it evolved to diversify, and for some descendant of the first individual with that trait to have died, been fossilized, and that a paleontologist have discovered that fossil. More intense sampling will never move the oldest date of a feature closer to the present, but will move that earliest occurrence back further, approaching the time of its first occurrence.
A second error derives from the authors' fundamental misunderstanding of evolutionary processes. Lineages with traits characteristic of a transitional form may persist long after another lineage has evolved novel traits, and which lineage will have the oldest fossil will depend on where and how fossils from each group formed, and where paleontologists have looked for those fossils. Complaints that such fossils are not in sequence are equivalent to claiming that my grandmother could not be ancestor because she and I lived at the same time. The figure illustrating mammalian evolution in the section below demonstrates how the overlapping histories of different lineages could produce fossils which appear to be out of order if the branching evolutionary process were not clear.
This explains why Norrell and Novacek, after observing that the fossil record of primates is spotty because the sequence of the earliest representatives (as of 1992) of two groups is not as predicted (because the fossil record is limited or absent for those groups), nonetheless state, "Despite these discrepancies, there is a noteworthy correspondence between the fossil record and the independently constructed phylogeny for many vertebrate groups. Statistically significant correlations were found in 18 of 24 cases examined" (Mark A. Norrell and Michael J. Novacek (1992) "The fossil record and evolution: comparing cladistic and paleontological evidence for vertebrate history," Science 255(5052):1690-1693). It is worth noting that new fossil discoveries and improved phylogenetic reconstructions in the 15 years since they wrote that paper, have resulted in a much improved fit between hypothesis and the hominid fossil record. This illustrates the danger in basing an argument on what we don't know, the core of the argument of Explore Evolution.
Full discussion: This is another example of the authors of Explore Evolution exploiting the vagueness of the phrase "some scientists." Here, they make it appear as if a creationist position (no fossils illustrating the transition between walking mammals and whales) has significant scientific support.
Recently, some scientists think they have discovered a transitional fossil sequence connecting land dwelling mammals to whales.Explore Evolution, p. 20
The authors neglect to mention that the terrestrial forebears of whales were correctly hypothesized in the 1800's. In the 1980's, a compelling fossil sequence for whale evolution was put forth and since then, the fossil sequence has grown to dozens of intermediates. Anyone familiar with scientific literature on this topic knows that the fossils of "whales with legs" are famous throughout evolutionary biology, are the subject of dozens of papers in top journals like Science, used in many textbooks, and have been covered by numerous science journalists. There is no scientific opposition to the idea that these fossils show transitional morphology.
For a review of the walking-mammal to whale transition, see:
It is interesting to compare the treatment of whale fossils in Explore Evolution with the treatment of whale fossils in its creationist ancestors. Creationist Duane Gish wrote:
The marine mammals abruptly appear in the fossil record as whales, dolphins, sea-cows, etc. … There simply are no transitional forms in the fossil record between the marine mammals and their supposed land mammal ancestors.Duane Gish (1992)
Evolution: The Challenge of the Fossil Record. Creation-Life Publishers: El Cajon, CA. p. 79
In Evolution: A Theory in Crisis Michael Denton spends several pages commenting on what he believed to be the unfortunate necessity of having:
…to postulate a large number of entirely extinct hypothetical species starting from a small, relatively unspecialized land mammal … and leading successively through an otter-like state, seal-like stage, sirenian-like stage and finally to a putative organism which could serve as the ancestor of the modern whales. Even from the hypothetical whale ancestor stage we need to postulate many hypothetical primitive whales to bridge the not inconsiderable gaps which separate the modern filter feeders (baleen whales) and the toothed whales.Denton (1985) Evolution: A Theory in Crisis
Adler & Adler Publishers:Chevy Chase, MD. p. 174
In his next book, published in 1998 (after the fossils described above where discovered), whale fossils were no longer a subject of discussion. Likewise, the authors of Explore Evolution, rather than celebrating the growth of scientific knowledge, stir up confusion around it. Needless to say, this approach is neither inquiry-based nor scientific, and sows confusion where a textbook should educate and inspire.
Summary of problems with claim: The point of presenting fossils at the same size is to illustrate the appearance of novel anatomical structures. Size is a feature that changes with age, diet and changes relatively easily in response to evolutionary pressures. The shift from three bones on each side of the lower jaw to a single dentary bone is far rarer and more informative about evolutionary history.
Full discussion: Explore Evolution takes umbrage at a diagram from T.S. Kemp's 2005 book The Origin and Evolution of Mammals. Page 21 of Explore Evolution shows a series of skulls (Figure 1:6), each the same size, and then compares this on page 29 (Figure 1:8) to the same skulls at relative sizes, where some are much larger than others.
Altering the scale is done for clarity, not deception, as the authors well know. Explore Evolution does the very same scaling in its Figures 2:1 and 2:4, on pages 41 and 43, where the arm of a bat, a porpoise, a horse, and a human (2:1) and cricket and a mole (2:4) are all drawn at similar scales.
In Figure 1:8 the miniscule size of Thrinaxodon or Probainognathus makes it impossible to identify bones and structures. In Figure 1:6, critical features which distinguish mammals from their amniote ancestors (structures like the opening in the bones behind the eyes and the locations of bones in the lower jaw) can be seen quite clearly. Mammals range in size from a few grams (e.g., the Bumblebee Bat) to several tons (e.g., a Blue Whale), but nevertheless, all of them have a single bone (the dentary) that makes up their lower jaw, hair, mammary glands, and numerous other features that diagnoses them as "mammals." Indeed, the range of sizes seen in domestic dogs is greater than the range shown in figure 1:8 (see the discussion of dog size and morphology in the critique of chapter 7), and that size range does not interfere with our understanding of the "close genealogical relationship" (Explore Evolution, p. 29) between dogs. The illustration in figure 1:6 (from Kemp's The Origin and Evolution of Mammals) is meant to illustrate the transition of a particular set of structures, and not (as Explore Evolution suggests) to make a point about the size of the organisms. Explore Evolution’s point about size is ultimately a semantic and silly argument which misrepresents (or misunderstands) what scientists look for in assessing fossil transitions. Size is not regarded as a factor which signifies "close genealogical relationship," while the arrangement of post-orbital and jaw bones is significant.
Proper scaling in figures is a great pedagogical tool that helps students and researchers in their comparative anatomy - and at least in professional publications, scale bars are commonly included so viewers and critics can ascertain specimen size. Explore Evolution’s figure 1.8 hides information, obscuring evidence of the evolution of evolutionarily significant features like postdentary bones whose modification is coupled to the origin of the mammalian middle ear.
In evaluating the evolution of modern mammals from the amniote ancestors of reptiles and mammals, there are several important traits that scientists examine. Mammalogy, by Vaughan, et al. (2000) lists 25 major features, selected from a much longer list of traits distinguishing mammals. The traits that do not fossilize from that list include skin glands (mammary glands, sweat glands, sebaceous glands), hair, specialized muscles in the skin, the epiglottis, details of the soft anatomy of the lung and diaphragm, brain structures, facial muscles, red blood cells lacking nuclei, and the anatomy of the heart. We will discuss some of them in more detail in the critique of chapter 12.
Skeletal characteristics are easier to identify in fossils. These include a change in the bones of the jaw with a shift of three bones out of the structure of the jaw and their reuse in the ear, a trait paleontologists regard as the dividing line between mammals and their non-mammalian ancestors. Vaughan, et al. explain "By this definition, Mammalia does not include the extinct near-mammals, the Mammaliaformes" (p. 11). Paleontologists debate which fossils represent the earliest mammals because of differing criteria, and the increasingly fine differences between the fossils make it harder to draw a clear line. Vaughan, et al. explain:
…when mammals first appeared in the Triassic period, they represented no radical structural departure from the therapsid plan but had attained a level of development … that is interpreted by most vertebrate paleontologists as a key indication that the animals had crossed the non-mammalian-mammalian boundary. … Many of the mammalian characters discussed in this chapter resulted from evolutionary trends clearly characteristic of therapsids.Terry A. Vaughan, James M. Ryan and Nicholas J. Czaplewski (2000) Mammalogy 4th ed., Saunders College Publishing: Orlando, FL. p. 10 of 565.
The "mammal-like reptiles" that Explore Evolution refers to are these therapsids, as well as other members of the Clade Synapsida. As mentioned above, scientists do not refer to the group as "mammal-like reptiles." The University of California Museum of Paleontology explains "This term is now discouraged because although many had characteristics in common with mammals, none of them were actually reptiles." Reptiles are a lineage which shares a common ancestor with mammals and other synapsids, not a group ancestral to mammals. Understanding that relationship can help clarify much of the confusion that laces the treatment of transitions in Explore Evolution.
The ancestors of modern reptiles, mammals and birds are known as amniotes. That name refers to a feature of the eggs of all those groups, one of several shared, derived characteristics which suggests that those groups share a common ancestor. A common challenge in talking about the transitional forms between mammals and reptiles is attempting to imagine a form intermediate to modern reptiles and modern mammals. In evolutionary terms, a transitional form is a common ancestor of two groups, one which shares traits with earlier forms and possesses a few of the traits which uniquely identify later lineages. We have such fossils illustrating the transition of early amniotes to the several lineages which led to modern mammals and to reptiles.The first skull in the figure at the right shows the basic anatomy of the ancestral condition of the amniote lineage. Of particular importance, there is no hole in the skull behind the eye socket. The lower jaw in all of these fossil species consists of three bones, one of which is on the inside of the jaw, not visible in the illustration.
That first skull dates to roughly 315 million years ago, and represents one of the earliest known amniotes. Fossils of this and other early amniotes are found in the fossilized stumps of a species of tree which grew in floodplains. These early amniotes took refuge in the hollow stumps of those trees, and without the discovery of those stumps, our knowledge of the base of the amniote tree of life would be much poorer. Those sorts of historical contingencies are common in paleontology, and help explain the unevenness of the fossil record.
The next skull dates from around 300 million years ago, and belongs to an ancestor of mammals. The main difference between it and the first species is that there is a gap between two bones behind the eye socket. This gap may have allowed greater freedom for jaw muscles, or may have carried neither adaptive benefit nor harm. The size of that gap varies between fossils, as does the size of that gap, but its existence marks descendants of a common ancestor. Complaints about the size of the skull miss a critical point about the shared derived characters that united that lineage, named the synapsids (to which all mammals belong). Another major branch of the amniotes evolved two holes in the skull, as shown in the third part of the figure. This group, the diapsids, includes birds and most reptiles. The fourth skull represents a derived lineage of diapsids, in which one of the gaps expanded, secondarily producing a skull with a single hole but which shares other traits with diapsids that demonstrate its common ancestry.
By examining the traits that these skulls share, it is possible to trace the origin of several separate lineages as they originated. The many similarities between these skulls demonstrate their close relatedness, and suggest that they all would have looked more similar to a large iguana or salamander than to any living mammal or bird. Nonetheless, certain novel traits in the skulls indicate that they represent very different lineages, the ancestors of modern groups that differ widely.
The evolution of early mammals, mentioned briefly, then criticized trivially in Explore Evolution helps demonstrate other important aspects of the scientific evaluation of fossils. As mentioned previously, the major feature that distinguishes the earliest mammals from their ancestors is the presence of a single bone in the lower jaw, rather than the four bones seen in amniotes and in amniotes other than mammals. PICTURE 2 As the figure above shows, the transition in the jaw bones can be traced through fossils. Figure 1:6 in Explore Evolution shows additional transitional fossils, and there are even more species which fill in the gaps between those species. The figure above highlights the bones that transitioned into the ear in different colors, making it easier to see how the relative sizes and locations of those bones changed over a hundred million years, allowing them to serve a greater role in transmitting sound, while the jaw hinge shifted from one of those bones (the articulate) to the dentary. In pelycosaurs, the first major lineage of synapsids, the four bones of the lower jaw are firmly joined together. One bone, the articulate, has structures which may have helped transmit sound, but it is unclear how effectively that would have worked.
The pelycosaurs differentiated into several major lineages, and a branch from one of those lineages further diversified into the therapsids. In therapsids, the sutures joining the post-dentary bones became looser, allowing the bones to vibrate in response to sound, and making them less useful as structural components of the jaw. While the major hinge in the jaw remained on the articulate in the therapsids, members of a group of therapsids known as cynodonts developed a second hinge on the dentary bone. This transition was probably driven partly by the increasing strength of jaw musculature, and the growing role of the postdentary bones in the ears. The formation of the second joint can be found in later cynodonts, and the older joint is much reduced in early mammals like Morganucodon, disappearing entirely in modern mammals. The jaws of embryonic marsupials go through similar transitions, indicating that the ancestral developmental processes are still at work in the formation of the jaw.
The shift in the jaw hinge and the change in size, shape, and location of earbones/jawbones is powerful evidence linking modern mammals with therapsids, pelycosaurs and the ancestors of all amniotes. Other transitions in the shape of the teeth and other details of the skeleton confirm this pattern, revealing the nested hierarchy of traits that is predicted by evolution and common descent.
Explore Evolution invites to consider this and other transitions, but without actually presenting any actual evidence for them to consider. Indeed, it is not clear whether the authors of Explore Evolution themselves understand this transition, since their main objection to calling these fossils "transitional" seems to be that the fossils are of different sizes. They ignore the actual morphological transitions that scientists study, instead focusing on size differences which carry little evolutionary significance. Far from establishing any problem with the fossil record of the transition from amniotes to mammals (not, as Explore Evolution puts it, from reptiles to mammals), the discussion in Explore Evolution yet again demonstrates the authors' own problems.