The notion that dinosaurs and birds are related dates back to the 1861 discovery of a pigeon-size fossil named Archaeopteryx, which had a bird-like skull, perching feet, and was a powered flyer, with wings of the basic pattern and proportions of the modern avian wing. It also had feathers identical to modern flying birds. Evolutionary scientists believe Archaeopteryx was a transitional creature---related to dinosaurs but well along the evolutionary pathway to modern birds. Other scientists say that the fact it had claws on its wings does not necessarily indicate reptilian ancestry; they cite three birds living today---the South American hoatzin, African touraco, and ostrich---as each having claws on its wings and yet are true birds. Although Archaeopteryx had teeth, considered to be another reptilian feature, some fossil birds had teeth and some did not. That this should be true is not surprising, since this is true of all other classes of vertebrates---fish, amphibians, reptiles and mammals.

Mr. Poling:

Another misleading paragraph, through error of omission. The reptilian features of Archaeopteryx go far beyond the fact that Archaeopteryx's claws had not yet fused like those of modern birds. Archaeopteryx had a reptilian tail. The skull was constructed like that of a theropod dinosaur, not a modern bird as stated above. The hip is constructed like that of a theropod dinosaur, specifically the Dromaeosaurids. The teeth of Archaeopteryx had a ridge of bone between each tooth, a condition found only in the Dinosaur lineage. And there are more similarities ... indeed, there are so many similarities that one and possibly two Archaeopteryx skeletons were misidentified as Compsognathus for 100 years.

You also leave out characteristics of modern birds that link them to a reptilian ancestry, such as the fact that birds have scales.

Addendum:

Here is a picture of Archaeopteryx. Note the reptilian features mentioned above that are visible in the picture: teeth, no fusion of the manual digits, reptilian tail, dinosaurian skull and dromaeosaurid hip.

Mr. Buckna:

Although Archaeopteryx has some features that are similar in morphology to those of reptiles, so what? It does not necessarily follow that Archaeopteryx was a transitional creature between reptile and bird.

Mr. Poling:

You are correct, it does not necessarily follow. Convergent evolution happens. But taking the attitude of "so what" is nuts. Similar morphology is what defines relationships. Dogs have hair, three bones in the middle ear, and lactate. "So what" you say? Just because cats also have hair, three bones in the middle ear, and lactate, doesn't mean they are related in any way, you say.

Mr. Buckna:

Even some evolutionists recognize that Archaeopteryx was a bird: L. Martin (1985, p. 182) concluded that "Archaeopteryx was a true bird." Colin Brown (1987, p. 78) concluded that "the creature was a true bird and not some kind of intermediate stage between reptiles and birds." Junker and Scherer (1992, p. 199) and Feduccia (1993) state that Archaeopteryx was surely a bird because of feathers that are identical to those of modern birds.

Addendum:

Every person cited by Mr. Buckna is an Ornithologist, not a dinosaur paleontologist. From discussions with both Ornithologists and dinosaur paleontologists, it appears to me that most (but not all!) Ornithologists are unfamiliar with dinosaur anatomy, whereas most dinosaur paleontologists are familiar with bird anatomy. The reason for this is that neontologists have live examples of the animals they study from which they can learn anatomy and biomechanics, whereas paleontologists do not. Even paleontologists must learn biomechanics and anatomy, so must also work on live animals. This generally gives them a broader base of comparison with extinct animals. The reverse is typically not true: neontologists do not have to become familiar with extinct animals, such as dinosaurs.

Mr. Poling:

Anything with feathers is, BY DEFINITION, a bird. However, feathers are a weak synapomorphy for birds, because there is no absolute proof that certain creatures that are believed to be closely related to birds did not have feathers.

Additionally, and this is something that I've wanted to know since this whole thread started, what are the definitions of "reptile", "bird", and "transitional form" (or "intermediate stage") being used? That's one thing the article definitely needs, definitions of how these terms are being used. These words mean different things to different people. I can't even begin to address Colin Brown's assertion without knowing on what he considers to be an "intermediate stage," "reptile" and "bird," and on what facts he is basing his assertion.

Addendum:

Actually, there is no formal taxonomic definition of "birds." The closest formal taxon is the clade Aves: the most recent common ancestor of Neornithes and Archaeopteryx, and all its descendants (Luis Chiappe). Usually, Aves is used synonymously with "birds," but "birds" is not a proper taxon as it has no explicit definition. Feathers are used as a diagnosis of clade Aves, but should non-avian dinosaurs be found to have been feathered, the diagnosis of feathers would move to another clade. The formal definitions and names of the clades would remain the same, but whether the informal name "birds" would move with the diagnosis of feathers to its new clade would be up to the whims of the public.

In general, Mr. Buckna is using obsolete concepts of evolution and taxonomy, the science of naming and grouping animals, to further many of his arguments. Because the changes in these sciences occurred mainly over the last decade or so, most people are not aware of them. As a result, Mr. Buckna's arguments based on these obsolete concepts sound reasonable to them, and they do not realize that certain concepts have been rendered nonsensical. These concepts are discussed on page 4.

Mr. Buckna:

Views on what creatures were the ancestors of Archaeopteryx have changed over the years, with the prevailing view being that it descended from small running dinosaurs (coelurosaurian theropods). See Ostrom (1985) and Padian (1989).

Mr. Poling:

That is by no means universal, and I wonder if it is even the most prevalent these days (your refs are 6 and 10 years old). I am familiar with Ostrom's hypotheses (I've actually briefly discussed them with the man in person, one-on-one), and the fact is that he is, or was, of the old school that did not take into account arboreal dinosaurs due to their (probable) lack of representation in the fossil record. These days, many paleontologists, and, possibly, finally, Ostrom himself, take into account the slim chances of arboreal forms fossilizing, and therefore are more likely to buy the notion that Archaeopteryx descended not from cursorial forms, but arboreal forms. I, personally, am of the "arboreal dinosaur" school of avian ancestry.

Addendum:

The most prevalent theory of avian ancestry these days seems to be that birds and dromaeosaurs both descended from a small, arboreal, non-avian dinosaur that looked very much like Deinonychus or Velociraptor. This common ancestry of birds and dromaeosaurs is supported by an incredible number of physical characteristics, including the famous semi-lunate (half-moon shaped) bone in the wrist and slashing-claw on the second toe. Although Archaeopteryx lacked the actual hypertrophied claw, the second toe was still hyper-extendible and therefore capable of the same range of motion as the slashing second toe of dromaeosaurs. Several years ago, a fossil was found in Madagascar of a bird, Rahonavis ostromi, that either still retained, or had re-expressed, the slashing claw.1 At right is a link to a drawing of Rahonavis ostromi. Click on the thumbnail image to view the picture. Click on your browser's "back" button to return.

Mr. Buckna:

But there are significant differences between Archaeopteryx and coelurosaurian theropods:

1. Archaeopteryx had feathers identical to those of modern birds, whereas theropods had none.

Mr. Poling:

How do you know that theropods had no feathers? Soft body parts such as feathers and hair do NOT fossilize except under very specific conditions. There are very few skin impressions of dinosaurs known today. It is not inconceivable that feathers, if they existed, would have rotted or been washed away. Regardless, all but one impression is from a large form. Today, large mammals such as elephants, hippos and whales have little or no hair. Given the warmer climate and larger size of dinosaurs, I see little reason why large dinosaurs would need feathers (for insulation ... I don't see them doing much flying).

Further, all but two of the skin impressions are from Ornithischians. Ornithischians are only related to birds at the very base of the dinosaurian family tree. If feathers were present in the first dinosaur, it is not inconceivable that Ornithischians would have lost them as they evolved. If feathers evolved after the split, Ornithischians wouldn't have them at all.

Of the two skin impressions from theropods, one is from a large ceratosaur. Again, it is a large animal and only related to birds at a point slightly above the base of the dinosaurian family tree.

What of the other theropod skin impression? It is of a small, basal ornithomimosaur called Pelicanimimus related to birds at the base of the clade of bird-like dinosaurs. Does this skin impression show feathers? Possibly ... the cast shows featherlike structures growing out of Pelicanimimus's skin. I don't know if anybody has come right out and said "these are absolutely feathers", but there was SOMETHING growing out of Pelicanimimus's skin that was very featherlike.

Addendum:

At right is a link to a picture of a dinosaur skin impression. It is most likely from a hadrosaur, as are most of the others. Click on the thumbnail image to view the picture. Click on your browser's "back" button to return.

Skin impressions thought to belong to tyrannosaurs show a pattern similar to the one pictured above. However, it is not at all clear whether these similar skin patterns were characteristic of all dinosaurs, large dinosaurs, or just these two dinosaurs. Further complicating the matter is the fact that none of these skin patterns were preserved in matrix that would have preserved hair or feathers if they were present. Very, very fine sediments like those of Solnhofen (Bavaria) and Liaoning (China) are needed. Regardless, paleontologists would love to know the chemical makeup of the structures causing the "bumps" in the skin impressions (why they should be so interested is discussed on page 5).

The structures present on the fossil of Pelicanimimus turned out not to be feathers nor hair, but the preserved impressions of muscle tissue. However, another non-avian dinosaur called Sinosauropteryx has been found with fibrous, featherlike impressions around the fossil skeleton. All four known specimens show these impressions. Four western scientists have examined the specimens and declared that the structures are not modern avian feathers, but that was all they concluded. They do not know what the structures are. They could be some sort of collagen support structure, true feathers of a morphology different from modern avian feathers, or some sort of protofeather.²

If these are, indeed, feathers or protofeathers, then not only is it possible that everything that descended from Sinosauropteryx, or one if its ancestors, had feathers, protofeathers, or some other featherlike structure, it is possible that all dinosaurs had them.

The fact that the sister-group of the dinosaurs, pterosaurs, had hair that has been likened to the shafts of feathers supports the argument that featherlike structures are an archosaurian character trait, making them an inherited, primitive trait in dinosaurs. A recent experiment, discussed on page 5, also supports the hypothesis that feathers are an archosaurian trait.

The recent discoveries of Caudipteryx and Protarchaeopteryx in Liaoning may strengthen the dinosaurian, if not archosaurian, affinities of feathers. The fossils of these two animals have impressions of symmetrical, avian type feathers, but the animals are taxonomically placed outside the clade Aves. Should these two animals remain among the non-avian theropods after further study, it would be the strongest evidence yet that theropods were feathered.

Unequivocal evidence may be soon to come. Rumor has it that the Chinese have found a feathered therizinosauroid in the Liaoning beds. Therizinosaurs are an oddball group and where they belong within the Dinosauria is hotly debated. However, there is no question in any researcher's mind that therizinosaurs are not birds. Look for publication in Nature within the next several months.

Mr. Buckna:

2. Archaeopteryx had a hypertrophied furcula (fused clavicles); theropods do not have one.

Mr. Poling:

In fact, Oviraptors are confirmed to have had furculae. There's even a picture of one in The Dinosauria. Recent fossil finds, and reexamination of old specimens, have shown that Allosaurids had furculae. I do believe Mononykus, which you stated was a dinosaur, had a furcula. A recently discovered gracile theropod has a confirmed furcula.

Addendum:

The types of non-avian theropods with confirmed furculae are dromaeosaurids, oviraptorids, tyrannosaurids, troodontids coelophysids and allosauroids.³

Here's an illustration of the furcula of Oviraptor philoceratops.

Mr. Buckna:

3. Manus claws of Archaeopteryx differ markedly from those of predatory dinosaurs (Feduccia, 1993)

Mr. Poling:

In fact, the manus of Archaeopteryx is almost an exact scaled-down version of the manus of the predatory Dinosaur, Deinonychus (Ostrom and others). This includes the claws. Feduccia's hypotheses have been called "interesting," but could not stand up to analysis and review by other paleontologists. Dr. Tom Holtz presented an analysis at a Society of Vertebrate Paleontologists meeting one or two years ago showing exactly where Feduccia was in error.

Addendum:

The figures below are illustrations of the mani of Archaeopteryx and the non-avian theropod Deinonychus (figures are not to scale). Note whether the mani are remarkably similar, as I claim, or differ markedly as Mr. Buckna claims.

Fig. 1: Archaeopteryx	Fig. 2: Deinonychus

Mr. Buckna:

4. Archaeopteryx had a fully reversed hallux, the large rear toe, with a strongly curved claw on the ungual phalanx, which is typical of modern perching birds and unlike any known theropod dinosaur (Feduccia, 1993).

Mr. Poling:

Recent discussion on the Dinosaur Mailing List indicates that the orientation and function of the hallux has, until recently, been an assumption of the preparator/exhibitor. Apparently the articulation of the hallux with the ankle and other toes allows for preparators to position the hallux in both a forward and retroverted position. Without a precisely and perfectly articulated skeleton, placement of the hallux in certain theropods is speculative. Whether the hallux of Archaeopteryx was unlike any other theropod has become, recently, a "bone" of contention. That Archaeopteryx may have a feature that is derived compared to its ancestors is a non-issue: this is what evolution is all about.

Mr. Buckna:

5. Although Archaeopteryx had teeth, the crowns of its teeth were unserrated, the waist present, the root expanded, and the tooth replacement resorption pit oval to circular. On the other hand, in the reptiles Pseudosuchia and Coelurosauria, the crowns were serrated, the waist absent, the root straight and unexpended, and tooth replacement resorption pit elongate (Brown 1987, p. 78)

Mr. Poling:

Dinosaurs have an incredible diversity in tooth morphology, especially among coelurosaurs. Except for size, the teeth of Ornitholestes and Compsognathus are identical to those of Archaeopteryx. The jaw/toothroot morphology of Archaeopteryx bavarica is seen only in theropods and a few basal archosaurs. Lastly, the teeth of Archaeopteryx were millimeters tall, apparently making them too small to have effective serrations.

Addendum:

The recent discovery of Protarchaeopteryx adds an interesting twist to these arguments. The tiny teeth of Protarchaeopteryx were serrated.

Mr. Buckna:

Not only are there problems linking Archaeopteryx to theropods, there is no link from it to any modern birds. Martin (1985, p. 182) states: "Archaeopteryx is not ancestral to any group of modern birds. It has specializations in its tarsometatarsus and skull which show conclusively that it is on a side branch of avian evolution."

Mr. Poling:

I can't speak to that last statement without knowing exactly what those specializations are.

As for the statement that there is NO link to modern birds ... studies of Archaeopteryx have produced at least 80 derived characteristics shared by modern birds and Archaeopteryx. I have a file on my homepage listing more than 20 of these characteristics.

Mr. Buckna:

If this is so, where then are the alleged intermediates lying on the main branch?

Mr. Poling:

Confuciusornis, Hesperornis and Icthyornis show a pretty good progression.

Additionally, you have an outright factual error. According to Ronald Orenstein of the International Wildlife Coalition, Ostriches and Touracos do NOT have claws on their wings. The Hoatzin only has wing claws as a chick: once it reaches adulthood, the claws are fused together as one unit in the typical avian condition.

Mr. Buckna:

You say that I made "a factual error" when I stated that ostriches and Touracos have claws on their wings. The fact that the hoatzin only has wing claws as a chick does not make my statement incorrect. Perhaps it would be useful to add in my article that the claws only appear on the young, but it doesn't change the fact that they DO have them.

Mr. Poling:

The statements that Ostriches and Touracos do not have claws and Hoatzin young do are totally separate statements. One is not used in any way to prove or disprove the other.

Also, a distinction should be made between DIGITS and claws, as I explore below.

Mr. Buckna:

The South American hoatzin also has claws on its wings when young. And, contrary to what you stated, the ostrich also has claws on its wings; in fact it has THREE claws. I suggest that you and Ronald Orenstein (International Wildlife Coalition) check your facts.

Mr. Poling:

Technically, ALL birds have three DIGITS. They have simply been fused into one mass. Most do not have CLAWS, in terms of having a horn covering of the ungual phalanx of the digits, but a few do. I pored through a stack of books on avian anatomy and there was no mention whatsoever of these claws being external to the wing, with the exception of the Hoatzin. The Hoatzin chick is also the only bird with external digits, which are absorbed into the wing in adulthood in the typical avian manner.

Nothing I read, nor any of the quotes from avian anatomy books mentioned claws on Ostriches. If they do have them, they're not on external digits in the same way that they are on the Hoatzin chick.

Mr. Buckna:

So, are claws on the wings evidence of a transition between reptiles and birds?

Mr. Poling:

The issue isn't claws per se, but the entire manus of birds and whether, at some time in its ancestry, this manus had separate digits, and whether they were similar to those of an animal that could be considered ancestral. The fact that the Hoatzin chick has external digits is good evidence that they were, indeed, free and independent at one time. Whether they are similar to a possible ancestral form is explored [above].

Addendum:

Here are two pictures of the Hoatzin manus, one as a chick and one as an adult (figures are not to scale). The mani of Archaeopteryx and Deinonychus are again presented, for comparison purposes. As before, note should be taken of any similarities.

Fig. 1: Archaeopteryx	Fig. 2: Deinonychus
Fig. 3: Hoatzin chick	Fig. 4: Hoatzin adult

Also, there is no such thing as a "transition" between reptile and bird (see page 4).

Mr. Buckna:

Obviously not, for these birds are very much alive today.

Mr. Poling:

I'm frankly confused as to why you think wing claws in extant birds disproves reptilian ancestry. The only thing I can figure is you're laboring under the false notion that descendant forms cannot share characteristics with ancestral forms, especially if those descendant forms are extant. I don't think any evolutionist has believed that for a century.

Addendum:

Presumably, when an animal evolves, it becomes better suited to its environment than the species it evolved from. This new species has a competitive edge over the old, and drives the old to extinction. This is not, however, usually the case.

Populations of species can become isolated from each other. Each population is subjected to different environmental pressures, and therefore different natural selection. One or more populations can evolve into new species, while a different population remains in its original form. These species, "parent" and "children," continue to exist as the populations are isolated and not in competition with each other.

It is not required that the populations remain isolated. During their isolation, one population could evolve mechanisms to eat nuts while the original population continues to eat insects. Then, some environmental change brings the two populations back into direct contact with each other. Since the new nut eating species is not in direct competition for food with the old insect eating species, the two populations continue to exist side by side.

The fossil record shows that sometimes the parent species can even outlive child species, especially if the child species are highly specialized while the parent species is generalist. Should the environment shift to conditions unfavorable to the nut eating species in the example above, the child species could go extinct while the parent species continues to thrive. The parent species outlives the child species, and can produce even more child species.

Complicating the argument is the fact that some researchers use a concept called the phylogenetic species concept.⁴ This concept states that the ancestral species goes extinct upon speciation, even if one of the daughter lineages is indistinguishable from the ancestral lineage. Many researchers do not utilize this concept as it does not fully reflect our understanding of evolution. Indeed, the concept seems to be a concession to the idea that direct ancestors can only be rarely identified in the fossil record (for example, some extant species of warbler are distinguishable only by their plumage, something that would not be preserved in fossils) rather than an attempt to explain "reality."

The notion put forward by Mr. Buckna that extant species cannot share features with its extinct progenitors is a strange one indeed. Just as parents pass on certain traits to their children, parent species pass on certain traits to their child species. Just because an animal has evolved doesn't mean it is completely dissimilar to what it evolved from. The false notion that descendant species are completely different from ancestral species is, unfortunately, a common one. This notion is often used by those who are semi-evolutionists in forming their arguments.

During the Rush Limbaugh radio show, the Conservative talk-show host mentioned that he believed in evolution within species, but not between species ("cross-species evolution"). What this means is that he believes animals can evolve new characteristics, such as different coloration, or an extra digit on the hand, or somesuch, but does not believe that a Dimetrodon can "become" a mammal or a dinosaur can "become" a bird. For this to happen, an otherworldly "creator" must intervene.

There are two basic tenets to this argument. The first is the idea that one species cannot become another. A simple thought exercise should illustrate the fallacy of this idea.

Imagine a single species of animal. It lives in an environment that is constantly changing. A new anatomic feature is selected for in every generation. Anatomical features do not simply appear by magic. They are controlled by DNA. Both changed and new anatomical features require new coding in the DNA. As the DNA of the original population and offspring become more disparate, the less compatible the genetic codes become.

As each generation is born, it is able to breed with members identical to the original, unchanged population. However, each generation is less able to produce viable offspring, when breeding with members of the original population, than the previous one. Eventually, the new population becomes so genetically different that it cannot breed at all with members of the old. Ability to breed is one of the basic hallmarks of speciation; therefore, the new population has become a new species. Taken to its extreme, after millions of generations, the offspring can be very different from the original population. There is no reason to believe, then, that with so many generations of evolution between the original species and subsequent species that a dinosaur cannot "become" a bird.

The second basic tenet to the "no cross-species evolution" argument is the idea that Dimetrodon is something completely different from a mammal, and that a non-avian dinosaur is something completely different from a bird.

At right is a picture of Dimetrodon (left, with fin). Dimetrodon was a pelycosaur. Pelycosaurs were the dominant land animals in the early Permian period of the Paleozoic.

What, then, was a pelycosaur? It looked like a reptile, yes? It looked like a huge lizard with a fin on its back. But it was not a reptile. It was a mammal ancestor, one of the first animals known as synapsids. From the synapsids evolved the therapsids, also known as proto-mammals. And from the proto-mammals arose the true mammals.

A quasi-evolutionist like Mr. Limbaugh would say "impossible." Dimetrodon looks completely dissimilar to a modern mammal. But is it completely dissimilar? The reason paleontologists know that Dimetrodon is not a reptile but a grandfather of the mammals is the fact that modern mammals, including humans, still posses aspects of Dimetrodon's anatomy. For example, the hinge joints of the jaw of Dimetrodon are made up of the quadrate and articular bones.⁵ In true mammals, the hinge joints are made up of the squamosal and dentary bones. The makeup of the mammalian joint is one of the characteristics that uniquely identify, or diagnose, mammals -- or, put another way, it is the absence of the quadrate and articular bones that uniquely identifies mammals. But are these bones truly missing? No. The fossil record shows these bones slowly moving their position, changing their form, and changing their use in the descendants of Dimetrodon. Paleontologists have even found fossils of animals in the mammal family tree with double jaw joints, with joints at the quadrate and articular bones and the squamosal and dentary bones.⁶ This fossil progression has shown that today's mammals still posses the quadrate and articular bones. They are more commonly called the anvil and hammer, two of the bones of the middle ear. The fact that mammals posses bones that are shown, by the progression of the fossils of both clear Dimetrodon descendants and clear mammal ancestors, to be derived aspects of Dimetrodon's anatomy, is an excellent illustration of evolution and that mammals are descended from pelycosaurs. The most compelling evidence, though, is that all of today's mammals, at some point in their development, exhibit the character that defines what a synapsid, like Dimetrodon, is: a single temporal fenestra.

In addition to the evidence of the fossil record, modern DNA analyses provide strong evidence that this is indeed the case. In its infancy, DNA theories stated that as animals evolved, the DNA sequences that controlled the evolving characteristics changed as well. Recent research has shown that in most cases this is not what occurs, that instead new DNA sequences are added that control the expression of the new characteristics. The old sequences remain, and are either switched off by, or the results of their actions modified by, the new sequences. This can be seen in the animal world as occasionally a snake will be born with vestigial legs or a whale will be found with a complete pelvis and upper leg, including the knee. Experimentation has also demonstrated this: scientists managed to shut off the different sequences suppressing tooth and tail growth in birds, resulting in teeth⁷ and long, bony tails in embryonic chicks.

Snakes are still tetrapods, four-limbed animals, even though they have no limbs. This is considered a modification of the four limbed condition, rather than a lack. This condition is used to diagnose snakes from other tetrapods. Snakes still carry the original DNA for limbs and evolved new DNA to modify the actions of the old. As mentioned above, snakes are occasionally found today with tiny, vestigial legs due to the newer DNA sequences malfunctioning and not suppressing the old.

Despite appearances, no animal is 100% dissimilar from its ancestors. No matter how far an animal evolves from a certain point in their evolution, they still carry in their DNA, unless it has been damaged by mutation, everything that came before. In our nut/insect eater example, the original species has four toes, a trait which uniquely identifies it. The branch species evolved to eat a different food. Since the evolutionary pressure was on food gathering, rather than locomotion, there was no reason for the new species to have a different number of toes. Therefore, the four toed trait remains unchanged, and clearly shows its child relationship with the original species. It was shown above that mammals are not completely dissimilar from Dimetrodon. It was shown on earlier pages that birds are not completely dissimilar from theropod dinosaurs; indeed, most of the physical characteristics of birds, including, it seems, feathers, are modifications of the basic theropod dinosaur body plan. When humans evolved, they did not stop being mammals. As the ancestors of birds, lizards, turtles, snakes, non-avian dinosaurs, pterosaurs, mammals and other animals evolved, they did not stop being amniotes. Indeed, all of those modern animals are still amniotes today, even after millions of years of disparate evolutionary tracts.

So, evolving animals can become new species, yet are not something completely different from their ancestors. Neither are they exact, and it is easy to see how, after millions of generations of evolution, an animal can be so changed that it appears to have become something else. It should now be easy for you to see how Dimetrodon can "become" a mammal, and dinosaurs can "become" birds. Indeed, mammals are synapsids and birds are dinosaurs, just as both are, unquestionably, amniotes, as shown by their physical characteristics and retention of "primitive" DNA.