Chapter two of TIAJ is the first attack on evolutionary theory. Predictably, Perloff goes straight for the fossil record as so many creationists before him have done. He begins by giving a very linear and simplistic view of evolution. He claims that evolutionary theory states that:
‘About three billion years ago, life began as simple cells. Eventually,
these evolved into multicellular organisms, which then became invertebrates …
These in turn evolved into vertebrates and the first fish.’
In essence this is correct, but the way in which it is stated here makes it seem that species A turned into species B which turned into species C and so on in one long line of descent – rather like a ladder. The true picture of evolution is far more ‘bush-like’, and there is also no real reason why parent and daughter species should not exist at the same time. In other words, Perloff seems to be aware of phyletic but not of cladistic evolution. If this were not true, then invertebrates would have been superseded by vertebrates and we would probably not see invertebrates anywhere in the world today.
Perloff explains that it is the process of Natural Selection that allows evolution to progress. He says:
‘Organisms best suited to their environment were able to survive, and
passed their strengths on to the next generation. Thus, over time, creatures
became more highly developed – in much the same way, we are told, that breeders
develop better fruits, flowers or racehorses. Less fit creatures became
extinct’
We are not ‘told’ that at all. Selective breeding is subtly different from natural selection, and we shall return to this in later chapters. All organisms within a species vary in different ways from one another. These variations may lead to the individual having a certain advantage over others in some way – perhaps a variation in stomach enzyme allowing it to digest slightly different materials and therefore be able to take advantage of a new food source. It is then inevitable that such individuals will lead more prosperous lives than individuals without these particular variations and therefore produce more offspring. These offspring will inherit the advantageous variation whereas the smaller number of offspring from an individual without the variation will not. Therefore, over time, the variation will become more common in the population. This is natural selection – the advantageous variant is naturally selected over time to become the norm. The source of the variations (whether advantageous or deleterious) is the mutation of the genome – a subject that Perloff devotes the next chapter to.
Artificial selection – that is by fruit, flower and racehorse breeders is slightly different. People are unable to direct variation in a species, and can only take advantage of the variation that already exists. Therefore, a racehorse breeder will observe that there is a range of speeds in racehorses. In order to produce the fastest racehorse (s)he will breed the fastest two horses available. The offspring will then get faster and faster as the generations go by. However, it should be realised that eventually the variation runs out, and new generations are not faster than their predecessors. This is because there are no new mutations in the genome to allow a faster horse. Another reason why artificial selection is different to natural selection is because artificial selection tries to push a certain characteristic further and further into an extreme. For example – the speed of the racehorse; breeders want their horses to go as fast as possible and so attempt to breed faster and faster horses. However, the natural selective process depends highly on the environment. In the example above, the new type of food is the environmental factor allowing the new stomach enzyme to be selected. If there was no new type of food, the new stomach enzyme would be useless and therefore not selected. Therefore, natural selection tends to lead to a fixed point, whereas artificial selection allows the process to run away with itself. There is a process called sexual selection – which is a type of natural selection in which a characteristic can also run away with itself. A good example of this is the peacocks tail.
But still – we were supposed to be talking about fossils. Perloff’s point is that if there is a constant gradual change in species through the process of natural selection, we should be able to see this change reflected in the fossil record – and – according to him and various other creationists – we cannot:
‘… Darwinism claims fish transformed into land creatures by evolving
little arms and legs over eons. If true, there should be innumerable fossils of
fish with rudimentary arms and legs. Yet we do not find them! In fact all
organisms appear in the fossil record fully formed, without transitional
stages.’
This statement is quite simply not true. However, the description given here does give a misleading impression – giving the reader a mental image of a goldfish with human arms and legs sticking out of the sides. It is also obvious that all organisms in the fossil record must appear fully formed – what would be the point in an animal with a half-formed leg which was functionally useless? It is important to remember that nowhere in evolution were there animals that were only half-functioning just so that modern animals could function the way we know they do. Therefore, all fossils reflect this. However, to say that there are no transitions between fossils, showing an evolutionary trend is simply untrue.
General Lineages
Before starting my discussion of transitional fossils, I must explain what we mean by a transitional fossil. There are two meanings to this phrase; the first is the general lineage – a series of fossils that connect one older group to a very different younger one. Sometimes, the fossils in these lineages are not thought to be directly related to the next fossil in the lineage, but are thought to be cousins. An example of a general lineage is the connection between the reptiles and mammals. The second is the species to species transition and this is a series of fossils formed in a geologically short space of time (for example, half a million years). The latter are rarer than the former, because exceptional conditions are required. Many organisms need to be laid down under constant and rapid sedimentation. In general, in order to see a species to species transition a fossils needs to be formed every twenty to eighty thousand years or so – a very short space of time. Any longer than this between fossils, and it becomes only possible to determine the order of the species, but not the transition between the species. However, having said this, there are many examples of species to species transition in the fossil record.
Darwin famously noted that there was a lack of transitional forms in the fossil record, and creationists are fond of quoting him. His explanation for the observation was that the fossil record was extremely imperfect and that not many organisms were sampled in the course of evolutionary history. As Perloff says, many critics of Darwin’s theory at the time of its publication were scientists and one of their objections was the same observation. The reason for this is really quite simple. At the time of Darwin, and for many decades afterward, the number of fossils found and studied was really quite small. It was not that the fossil record was as imperfect as Darwin had hypothesised, but that people had not yet dug them up. In more recent decades the science of palaeontology has grown and the fossil record is now sampled to a far greater degree than at the time of Darwin. It should be noted, however that even now – at the beginning of the twenty-first century – it is estimated that the fossil record remains vastly undiscovered, with the only continents subjected to a relatively systematic study being Europe and North America. However, there is no need for me to use this as an excuse for intermediate forms, as there are many examples – of both general lineages and species to species transitions.
Most of the complaints that Perloff has with the fossil record concern general lineages. He talks about the evolution of vertebrates from invertebrates, amphibians from fish, reptiles from amphibians, and mammals and birds from reptiles. Somewhat annoyingly, Perloff also states, near the end of this chapter that:
‘Die-hard evolutionists continue to assert that intermediates exist,
but since they must continually cite the same examples – the horse,
Archaeopteryx, mammal-like reptiles, and scant others – we see just how rare
they are.’
The reason that Perloff has only heard of the listed examples of intermediates is that these are the only ones that creationists ever cite (Perloff providing a prime example), and then evolutionists write in their defence. A little research into the subject shows that there is a wealth of knowledge about many other intermediates and I shall discuss the transition between fish and amphibians, amphibians and reptiles and reptiles and mammals here. I will start with the evolution of amphibians from fish. Many more intermediate fossils are discussed by Kathleen Hunt on the talk.origins FAQ page – where most of the following information was obtained.
The evolution of amphibians from fish was essentially the evolution of feet. This did not mean a change from water to land, but just the development of rudimentary feet which allowed the creature to manoeuvre itself about at the bottom of the water. This meant that the change did not need to occur in a short space of time, and further developments of the limbs and skeleton were not required immediately for weight-bearing purposes. Our starting point for this general lineage are the Paleoniscoids (eg, Cheirolepis). These were ray-finned fish that are the ancestors of most of the fish living today.
The next in the sequence is the Osteolepis, which was a lobe-finned fish. This had paired fins and an internal arrangement of limb bones resembling the leg. For example, it was able to flex a rudimentary elbow joint. Its skull and teeth also resembled an early amphibian.
Eusthenopteron followed on from Osteolepis and was another lobe-finned fish of which there are remarkably complete fossils. The skull resembles an amphibian (although it is still fish-like in its proportions) and there is a strong amphibian-like spine. The feet are visibly more amphibian-like, with the bones and muscle attachments visible, although there are no toes – just a set of fins. The knee and elbow joints are more like those found in tetrapods. Overall, the body is quite fish-like in proportion.
Next were the panderichthyids, which closely resembled the tetrapods in their overall proportion – unlike the Eusthenopteron. Their bodies were much more flattened and their eyes were placed more dorsally. They also had straight tails and foot-like feet.
The earliest known tetrapod is called Obruchevichthys and only fragments of limbs and teeth have been found. The humerus is largely tetrapod-like, but does retain some fish characteristics. Ahlberg, who discovered these remains said:
‘[the humerus] is more
tetrapod-like than any fish humerus, but lacks the characteristic ‘L-shape’ …
this seems to be a primative, fish-like character … although the tibia clearly
belongs to a leg, the humerus differs enough from the early tetrapod pattern to
make it uncertain whether the appendage carried digits or a fin. At first sight
the combination of two such extremities in the same animal seems highly
unlikely on functional grounds. If, however, tetrapod limbs evolved for equatic
rather than terrestrial locomotion … such a morphology might be perfectly
workable.’
There is now technically a gap in the record, since ideally it would be nice to have an intact skeleton or fossil of Obruchevichthys, but nobody has found one yet. The fragments of the limbs will suffice for now.
Ichthyostega shows a complete tetrapod foot, but also retains some fish-like features such as the arrangement of their teeth and bones in the skull, the presence of pieces of gill-like structures between the cheek and shoulder and the structure of the vertebrae. The Labyrinthodonts such as Pholidogaster also have these features, but have lost the fin rays in their tail, have strong interlocking vertebrae and have a nasal passage adapted for air intake.
It is also interesting to note that Acanthostega (an early amphibian around at about the same time as Ichthyostega) still had internal gills and an opercular chamber - both pieces of anatomy necessary for fish to breath. This suggests that the earliest tetrapods were not fully terestrial, but spent a lot of time in the water as well – so much so that they may not have been able to survive on land for long periods of time. It should also be noted that as well as internal gills, these animals had lungs. Lungs did not develop in the transition between water and land as many may think, but while the creature was still fully aquatic. One of the ancestors of all the animals described above was Parasemionotus and this creature (clearly resembling a fish) already had lungs.
We will continue, our next brief study being of the transitional fossils between amphibians and reptiles. Amphibians have paired aortas (the main blood vessel taking blood from the heart) called systemic arches – one on either side of their body. Reptiles (and mammals and birds for that matter) only have one. However, reptiles are divided into two groups depending on whether they have a right systemic arch or a left systemic arch. Both groups have descended from the original paired arch pattern that was and is present in amphibians.
The amphibian skull is also much weaker than the reptilian version, and the differences in skull anatomy began to evolve after the beginning of the evolutionary road to a one-sided aorta. Therefore, all reptiles that have a right sided aorta have one sort of skull structure while those reptiles with a left sided aorta have a different skull structure.
One set of reptiles developed a right sided aorta and strengthened the skull by swinging their quadrate bone inferiorly and anteriorly, thereby creating space for the otic notch and allowed hearing to develop without much further modification of the skull. This group of reptiles can then be divided into three more groups, depending on the number of holes (fenestrae) in the skull. The first are the anapsids with no fenestrae, which are related to the turtle family. The second are the diapsids that have two fenestrae and are the group from which dinosaurs and birds descended. The final group has two fenestrae that are fused together and these are called the eurapsids. The ichthyosaurs were descended from this group.
The second set of reptiles developed a left-sided aorta and strengthened their skull in quite a different way. Their quadrate bone moved superiorly and posteriorly through evolution and ended up wiping out the otic notch altogether. The result of this was that hearing had to develop using a different mechanism and involved the jaw. They had a single fenestra on each side and also developed many other features that the first set of reptiles did not, such as homeothermy, a bigger brain size and more efficient teeth. A particular group of these reptiles called the therapsids developed these features to a greater extent, and it is thought that these eventually became the mammals. I shall discuss these later in this chapter.
So, what are the examples of the amphibian to reptilian change? I will start this lineage from an amphibian similar to the labyrinthodont that I ended the previous lineage with. It is an anthracosaur called Proterogyrinus. It has a labyrinthodont skull and teeth, but has a more reptilian pelvis, humerus and verterbrae. The phalanges are the bones that make up the fingers (and thumb in humans). There are two phalanges in the thumb and three in each of the other four fingers (“23333”). In reptiles this number is 23454. The phalangeal bones in the Proterogyrinus are 23453, which is intermediate between amphibian and reptilian.
Limnoscelis and Tseajaia follow on from the anthracosaurs and have developed several other reptilian features such as the structure of the jaw and neural arches.
Solenodonsaurus is an incomplete fossil showing some features of the fossils mentioned above. It shows loss of the lateral line on the head, which was present in amphibians, but still has the single sacral vertebra of the amphibian.
Hylonomus and Paleothyris are very primitive reptiles, which still had amphibian skulls, shoulder joints, pelvis and limbs. The teeth and vertbrae are intermediate in structure between amphibian and reptilian. The rest of the skeleton is reptilian.
Perhaps the best example of a general lineage is the transition from reptiles to mammals. This will be discussed next. Firstly, consider the actual differences between reptiles and mammals. There are many:
|
Reptile feature |
Mammal comparison |
|
No fenestrae in skull |
Massive fenestrae expose all of
braincase |
|
Braincase attached loosely |
Braincase attached firmly to
skull |
|
No secondary palate |
Complete bony secondary palate |
|
Undifferentiated dentition |
Incisors, canines, premolars,
molars |
|
Cheek teeth uncrowned points |
Premolars and molars crowned
& cusped |
|
Teeth replaced continuously |
Teeth replaced once at most |
|
Teeth single rooted |
Molars double rooted |
|
Lower jaw of several bones |
Lower jaw of one bone |
|
Single ear bone (stapes) |
Three ear bones (stapes, incus,
malleus) |
|
Joined external nares |
Separate external nares |
|
Single occipital condyle |
Double occipital condyle |
|
Long cervical ribs |
Tiny cervical ribs, fused to
vertebrae |
|
Lumbar region with ribs |
No lumbar ribs |
|
No diaphragm |
Diaphragm |
|
Limbs sprawled out from body |
Limbs under body |
|
Scapula simple |
Scapula with spines for
attaching muscles |
|
Pelvic bones unfused |
Pelvis fused |
|
Two sacral vertebrae |
Three or more sacral vertebrae |
|
Toe phalanges 23454 |
Toe phalanges 23333 |
|
Body temperature variable |
Body temperature constant |
It is useful to keep these differences in mind when discussing the changes in the fossils between reptiles and mammals. Anybody questioning whether these fossils are truly ‘intermediate’ should think about their mix of characteristics. If a fossil shows evidence of lumbar ribs and a toe phalangeal count of 23333 – is that reptilian or mammalian? The truth is that taxologically it may have been classed as one or the other, but its actual morphology shows that it is halfway between the two. It is therefore an intermediate.
I will begin with Paleothyris and describe the changes seen in the fossils through the lineage until the first ‘gap’. I have then given a table of further fossils in the lineage along with some very basic details of their anatomy.
Paleothyris, the primitive reptilian that has already been mentioned. This animal had no fenestrae in its skull. Protoclepsydrops haplous was a very early synapsid reptile (the group of reptiles to which the therapsids belong). It had a small fenestra in its skull and had amphibian vertebrae with very small neural processes.
Clepsydrops is another early synapsid, but not quite as old as Protoclepsydrops. Archaeothyris belongs to the same family as Clepsydrops but is slightly later still. It had a small fenestra, and the braincase was still loosely attached to the skull. The teeth are very slightly differentiated, but still has primitive jaw, feet and skull. The limbs are still splayed out from the body, but the hip bone was slightly enlarged.
Varanops has a bigger fenestra, and the inferior aspect of the braincase was beginning to attach more firmly to the skull. The lower jaw shows evidence of altered musculature. The overall body shape is longer and deeper than previously and the vertebral column is more strongly constructed. The hip bone is enlarged further and there is evidence of changes in the musculature of the lower limb. Dating of Varanops shows that it is too late to be a direct ancestor of the next species in the lineage, but it is thought to be a cousin.
Haptodus belongs to a group known as the sphenacodonts. The teeth were differentiated by size (but not so much by shape) and were biggest in the canine region. The overall number of teeth was reduced. The jaw muscles were stronger. The vertebrae show more mammalian features, and the sacrum was made up of three fused vertebrae rather than two.
A later sphenacodont, Dimetrodon is obviously related to the first therapsids, which are discussed next. It is represented by some very complete fossils, and is thought to be another cousin. It had a medium sized fenestra, and more differentiated teeth with incisors and canines followed by smaller cheek teeth. It had no premolars or molars and all the teeth were replaced continuously. The jaw hinge was still fully reptilian, and the jaw bone itself was made of several bones. It also had a process which was later involved in the formation of the eardrum, but Dimetrodon itself had no eardrum. It could therefore only hear vibrations through the ground, rather than airborne ones, since it did have a reptilian middle ear.
Biarmosuchia is one of the most primitive therapsids. It has some sphenacodont features and some new ones. The sphenacodont features include the jaw structure, the muscles of the skull, and palatal teeth. The new features included a further enlargement of the skull fenestra, which now took up nearly all of the cheek. The braincase was attached more firmly to the skull and there were mammalian changes in the bones that make up the back of the skull. The maxillary bone (that which makes up the upper jaw) had expanded to separate the nasal bone from the lacrimnal bone to bring it to a position clearly intermediate between the reptilian and mammalian morphology. There was no secondary palate but the vomer bone had moved slightly backwards – the first step towards such a structure. The canine teeth were now quite large, and the teeth were replaced in a variable pattern – not constantly replaced. Some of the animals in the same group as Biarmosuchia such as Scylacosaurus had a single canine which was no longer replaced after the animal reached adulthood. The jaw hinge was more mammalian and the jaw muscles had become stronger still. The amphibian-like hinged upper jaw became fixed and immobile. The animal moved in a much different way to its ancestors, being able to move the forelimb to a greater extent. The hindlimb was more upright and the pelvis and femur (upper leg bone) were more mammalian. However, in the upper limb, the humerus still resembled that of a sphenacodont. The toes were beginning to become equal in length – the pattern seen in mammals and the phalangeal count was variable between the reptilian and mammalian number. The cervical and tail vertebrae had differentiated from the others and had obviously different anatomy. It is also thought that this animal had a primitive ear drum that was positioned near the jaw hinge.
Following on from Biarmosuchia was Procynosuchus. This is the first of a group of mammal-like therapsid reptiles known as the cynodonts and they are sometimes considered the first mammals. Perloff mentions these ‘mammal-like reptiles’ in his second chapter. He quotes Michael Denton, who says:
‘The
possibility that the mammal-like reptiles were completely reptilian in terms of
their anatomy and physiology cannot be excluded. The only evidence we have
regarding their soft biology [those tissues other than bone] is their
cranial endocasts and these suggest that, as far as their central nervous
systems were concerned, they were entirely reptilian’
But Perloff completely ignores the information that can be gained from examining the hard tissues of these animals, and it is this information that again suggests that they were of intermediate form between reptiles and mammals (although at this stage, they are much closer to mammals than reptiles). Whether Denton is right in saying that the central nervous system of these animals was entirely reptilian is by-the-by, since the anatomy of the hard tissues suggests that they were quite mammalian. Therefore, if Denton is right and the interpretation of the hard tissues found in the fossils is also right, then what can we say about the animal overall? That it is intermediate.
Procynosuchus had a very large fossa in its temporal bone, which allowed the attachment of very strong mammalian-like muscles, and overlying this was the zygomatic arch (cheekbone). The secondary palate was now made up of the palatine bones – in a mammalian arrangement – rather than the vomers in the reptilian arrangement, and was still only partially bony. The rest was thought to be made up of soft tissues, like the soft palate in mammals today. The teeth of the lower jaw were also more mammalian; there were four incisors on each side. Previously, there were six, and early mammals had three. 75% of the lower jaw is now made up of one bone, while the other bones are clustered around the jaw hinge, which is still reptilian. The vertebral column was beginning to look more mammalian – the first two vertebrae were modified for head movements and the lumbar vertebrae had begun to lose their ribs. The vertebrae had also begun to differentiate between the thoracic and lumbar regions. The scapula (shoulder blade) had begun to change shape, and the hip bone was much bigger. The pubic bone had reduced in size. It is also thought that a diaphragm may have been present in Procynosuchus.
Another cynodont called Dvinia also illustrates the transitional status of the group. The teeth have changed shape so that they are no longer just stabbing points, and the fenestra had increased in size again. The brain was much bigger, and the lower jaw was made up of predominantly one bone with the other bones clustered by the jaw hinge. The single occipital condyle had begun to split into two. Unfortunately nothing is known about the skeleton of Dvinia outside the skull, and so it is not known whether changes described later had already begun in this animal. It is thought that while Dvinia did not have a constant body temperature yet, its metabolic rate was increased to a state of homeothermy.
Thrinaxodon is a more advanced cynodont. The fenestra was larger again, as were the attachments for the jaw muscles. The secondary palate was almost at the stage of mammals today, and the teeth had differentiated into incisors (four on the upper jaw and three on the lower), canines, and about eight cheek teeth with cusps – a specialisation for chewing. There was still no differentiation of the cheek teeth into premolars and molars yet, and were all single-rooted. They were also replaced throughout life, although not ‘one-at-a-time’ but in sets. The lower jaw bone was now larger still and the other jaw bones began to articulate with the stapes – the bone used for hearing in reptiles. In fact, hearing had developed to the extent where Thrinaxodon could hear low-frequency airborne vibrations. The eardrum was located near the jaw hinge on the lower jaw and the sound was transmitted via the small jaw bones (which were at this stage called the articular and the quadrate) to the stapes and then the cochlear. The articular and the quadrate were loosely attached, allowing them to vibrate in response to sound and also function as a jaw joint. The reptilian jaw hinge had finally begun to change into a more mammalian form, and there was a ligament attaching the lower jaw to the skull, which stablised the structure so that the articular and quadrate could function in both ways. The occipital condyle had now separated into two, but these were positioned closer together than they are in the mammal. The vertebrae were connected in a more mammalian fashion, and the lumbar ribs were reduced again – suggesting the presence of a diaphragm and the need for a higher oxygen intake. The scapula displays evidence of the attachment of a new mammalian-like muscle. The legs are now under the body rather than sprawling from the sides, the hip bone is larger again, and the tail is short. The phalanges in the toes are 23443 in their pattern, and the fourth phalanx in the third and fourth toes was very small.
The mechanism of hearing described above is seen in all early mammals from the Lower Jurassic period, and slightly later, in the mid-Jurassic period mammals had lost the reptilian jaw-joint altogether, the bones used in the hearing mechanism having reduced in size further, and migrated to the middle ear by this time – making the structure more sensitive to higher-frequency sounds. The reptilian jaw morphology can still be seen for a brief period of time during the development of the mammalian embryo.
A number of Thrinaxodon fossils have been found in a relatively perculiar position for reptiles – curled up into a ball – possibly because they were attempting to conserve heat, and had therefore developed a more ‘warm-blooded’ metabolism (known as ‘endothermy’) rather than the ‘cold-blooded’ pattern seen in reptiles. Some groups of these fossils have been found in ‘families’ with adults and juveniles together – suggesting that parents may have looked after their offspring. This is not seen in reptilian behaviour.
Cynognathus was an advanced cynodont. Once again, its fenestra had enlarged further, and the teeth were more differentiated. The cheek teeth began to occlude between the upper and lower jaw so there was efficient slicing of food, and the rate at which they were replaced was much reduced. The teeth also had mammalian-style roots – although no double roots were present yet. Over 90% of the lower jaw was now composed of a single bone, and there were two jaw joints present – one clearly reptilian and the other clearly mammalian. There were also changes in the ribs, making them more mammalian, the scapula was further adapted for strong shoulder muscles and the legs were carried under the body rather than splayed out from the sides. There is some evidence from fossilised foot prints that this animal had a furry coat.
Diademodon is a fossil found alongside Cynognathus, but thought to descend from it. The fenestra were still larger, and the secondary palate was exactly the same morphology as it is in mammals, although did not stretch as far dorsally. The nose had developed turbinate bones – another indication that this animal may have been warm blooded. The rate of cheek teeth replacement was reduced further, and they had also developed more efficient cusps and adaptions for occlusion. The lower jaw was now entirely made up of a single bone, except for the very small articular near the hinge, but there was still a double (half-reptilian, half-mammalian) jaw joint. The ribs were now much shorter in the lumbar region, which allowed the diaphragm to move more freely and also allowed the animal more agile locomotion. The phalanges in the toes had now reached the mammalian count of 23333 – however, some species closely related to Diademodon still had a variable number of tow phalanges.
Probelesodon had a very large fenestra, and a secondary palate that was longer than before, but still not quite the same as modern mammals. Some of the cheek teeth were now double rooted, the nares had separated, and the jaw joint was stronger. The lumbar ribs were now completely absent, and the thoracic ribs showed a more mammalian appearance. The vertebral column was now very similar to the mammalian, as were the hip and femur.
Probainognathus had a larger brain, with some other skull changes, making it more mammalian. The cheek teeth had changed shape a little more, giving them more cusps. There were still two jaw joints, and there were still lumbar and cervical ribs – but these were tiny. The thoracic ribs had lost some of their reptilian characteristics, and the toe phalanges had the mammalian pattern.
Exaeretodon had a more mammalian jaw shape, in order to support the eardrum more efficiently. Like mammals, there were only three incisors and the thoracic ribs had lost all their reptilian characteristics. It is thought that this animal was another cousin, since it has some new dental changes that are neither reptilian nor mammalian.
There is now another gap in the record, which corresponds to about thirty million years. Following this, there are more species which show intermediate characteristics, and these are listed in the following table – in order to save some space!
|
Name of Species |
Relevant Adaptions /
Anatomy |
|
Oligokyphus, Kayentatherium |
Double-rooted teeth. Skeleton
like egg-laying mammals (monotremes). Double jaw joint. Atlas & axis
present. Simple tail vertebrae – more mammalian. |
|
Pachygenelus, Diarthrognathus |
Eustachian tube present.
Reptilian jaw joint still present, but now able to pick up higher frequencies.
Mammalian neck vertebrae, hip bones, and femur. Double occipital condyles. |
|
Adelobasileus cromptoni |
Specialisation of bony case for
cochlear. Considered first true mammal. |
|
Sinoconodon |
Permanent cheek teeth. Tiny
reptilian jaw joint still present. |
|
Kuehneotherium |
|
|
Eozostrodon, Morganucodon |
Mammalian teeth – replaced only
once, and differentiation of premolars and molars. |
|
Peramus |
Related to placentals and
marsupials. Lost reptilian jaw joint. Has three-boned middle ear. |
|
Endotherium |
|
|
Kielantherium, Aegialodon |
|
|
Steropodon galmani |
First definite monotreme. |
|
Vincelestes neuquenianus |
Shows characteristics of both
placental and marsupial mammals. |
|
Pariadens kirklandi |
First definite marsupial. |
|
Kennalestes, Asioryctes |
Possibly ancestor of all
placental mammals. |
|
Cimolestes, Procerberus |
|
Now, what was it that Perloff said?
‘By
citing so many authorities, I haven’t meant to beat a dead horse – only to
demonstrate beyond question that the fossil record does not support evolution.
This is true for every class of animal’
I dare say that by beating a dead horse, I have bored most of my readers to death by now, but I hope I have shown that this statement is plainly ridiculous. We have followed some of the fossil evidence from fish through to amphibians through to reptiles through to mammals in the context of the general lineage, and I will discuss some species-to-species transitions presently. I must also stress that this list of fossils is by no means exhaustive, and certainly represents far less than 1% of all transitional fossils known.
I will also repeat my previous point here: One wonders whether Perloff and other creationists are aware of any of the above examples and dismiss them because although they fall within the general lineage, they still appear in the fossil record fully formed. To an evolutionist, this seems like a strange thing to say because it is impossible for an animal to have lived without being fully formed. Evolutionary theory does not say that once, many millions of years ago, there were a bunch of animals with half-formed, non-functional adaptations walking around so that they could continue to adapt and eventually become functional today. Each adaptation was always fully functional at every stage, even if at one stage it was slightly less advanced and/or slightly less efficient than at the stage following it. If the above fossils are not thought of as intermediate by Perloff and other creationists, then I am – as are many other evolutionists – bewildered by what a fossil has to do to be thought of as intermediate.
Another point should be made about transitional fossils – particularly those of the general lineages. Perloff says:
‘Supposedly,
invertebrates evolved into vertebrates – surely a very long process. Yet
despite countless fossils from both groups, there is not one specimen
intermediate between them!’
Lets think about why this may be. Firstly, do we believe – having read Perloff’s other statements and then considering the list of intermediates above – that all the fossils referred to here show no characteristics intermediate between invertebrates and vertebrates? Well, if you’re like me, then no. However, how many of these fossils are classed as intermediate between invertebrates and vertebrates? Answer: None. Why? Whenever a new fossil is found, a group of people – known in the trade as taxologists – decide which group the new fossil should be put into. These people decide as best they can, even if the fossil does not fall clearly into one group of the other. Because, as scientists, we love to classify and to order things, we never leave a fossil out or say that it is in between the groups. Lets face it, the border between one group and the next is purely arbitery, and put where it is deemed reasonable. We could, for example, put every human being in a different class from every other human being because of a different physical appearance.
Species to species transitions
For the sake of easy-reading, I will confine myself to listing some examples of known species to species transitions in the following table:
|
Starting species |
Finishing species |
Where is transition? |
|
Styracosaurus |
Pachyrhinosaurus |
Dinosaurs |
|
Lambeosaurus |
Hypacrosaurus |
Dinosaurs |
|
Stegoceras |
Pachycephalosaurus |
Dinosaurs |
|
Tyrannosaurus |
Daspletosaurus |
Dinosaurs |
|
Cantius ralstoni |
Cantius trigonodus |
Early primates |
|
Teilhardina americana |
Tetonoides tenuiculus |
Early primates |
|
Tetonius homunculus |
Pseudotetonius ambiguus |
Early primates |
|
Theropithecus darti |
Theropithecus oswaldi |
Early primates |
|
Macaca florentina |
Macaca sylvana |
Early primates |
|
Haplogale media |
Proailurus lemansis |
Cats |
|
Pseudaelurus transitorius |
Ps. quadridentatus |
Cats |
|
Felis issiodorensis |
Felis pardina |
Cats (leopards) |
|
Gulo schlosseri |
Gulo gulo |
Wolves |
|
Cuon majori |
Cuon alpinus |
Wolves |
|
Nekrolagus |
Sylvilagus |
Lagomorph |
|
Pratilepus |
Aluralagus |
Lagomorph |
|
Hyracotherium grangeri |
Hyracothermium aemulor |
Horse |
|
Orohippus |
Mesohippus celer |
Horse |
|
Dinohippus |
Equus |
Horse |
|
Mammuthus meridionalis |
Mammuthus primigenius |
Mammoth |
Having shown that Perloff’s sweeping statements about transitional fossils are untrue, I will now move on to discussing some of the specific examples of transitional fossils that he mentions in his second chapter. Some are more famous than others – but most are examples that appear in standard creationist arguments.
Whales
I find it almost amusing that Perloff devotes about half a page to the evolution of sea-going mammals; it is an obvious attempt to include as many examples as he can without doing the slightest research into the subject. The reasons for this will become clear. Perloff himself devotes just four lines to the subject – the rest being comprised of a quote from Douglas Dewar:
‘Both whales and sea cows swim by the up and down movement of the great flattened tail. Such movement is impossible in a land animal that has a pelvis, but a well-developed pelvis is essential to every land animal which uses its hind limbs for walking … I have repeatedly asked evolutionists to describe or draw the skeleton of a creature of which the pelvis and hind legs are anatomically midway between the state that prevails in whales and sea cows on the one hand, and a land quadruped on the other. No one has accepted the challenge, and of course a fossil of such a creature has not been found, and never will be.’
When I read this quote I was very confused. It seems to be implying that the great flattened tail of the sea-going mammal is derived from the pelvis and hind limbs. It is not – their tail is a tail like any other, and is derived from the lower extreme of the spine. The hind limbs and pelvis are not involved in its function (except – one might add – that some muscles attach to the pelvis, allowing tail movement). If the skeleton of the whale is inspected, the small pelvis and vestigial femurs are still present, located approximately two thirds down the body from the head – with the huge tail vertebrae dorsal to them.
My next task was to do some research on the evolution of the whale, and evidence from the fossil record. It became obvious fairly quickly that transitional forms between land-going quadrupeds and marine-going whales and sea cows were well known. So why had Dewar claimed that no such fossils had been found? I ventured into the depths of Perloff’s bibliography and found the answer. The above quote comes from a book published in 1968 – before any transitional fossils in this field were discovered. TIAJ was published in 1999. So much for Dewar’s prediction! Incidentally, this tactic is one used by creationists with alarming frequency – quoting people who appear to support their argument, without pointing out that the evidence offered is hopelessly out of date.
The sea-going mammals are known collectively as the cetaceans. The above quote from Dewar was accurate until just a few years ago (although fossils were found years before the publication of TIAJ!) and there are still no species to species transitions known. However, evolutionists had predicted creatures that were half land-dwelling and half sea-going. This apparent gap in the fossil record was filled by specimens found in India and Pakistan along the shores of the ancient Tethys sea in the late 1980s and early 1990s.
At the beginning of the lineage is Eoconodon which was a quadruped that showed some signs of meat eating – having strong canine teeth, but also crushing cheek teeth and claws on its feet. Microclaenodon follows on from this and shows more signs in the teeth of meat eating, and Dissacus shows further development in this direction. There were still five toes in the foot.
Hapalodectes was a creature following on from Dissacus and could probably swim by paddling its feet as well as run on land. It may be a cousin rather than a true ancestor, since it is found in the record slightly too late. The skull and teeth resemble those of the early whales, in that they have differentiated incisors, canines, premolars and molars with multiple roots.
Pakicetus has the same skull anatomy as Hapalodectes – including the eardrum, which is specialised for working in low pressure environments – such as air. Pakicetus could therefore not dive very deep. The molar teeth still resemble the early land-going creatures, but the rest of the dentition resembles that of whales. The nostrils were still separate and there was therefore no blowhole. The limbs are unfortunately unknown, but the fossil was found with fossils of land-going animals. The length of the creature was about two and a half metres, and may have resembled the modern-day hippopotamus.
Ambulocetus natans was a creature clearly intermediate between land quadrupeds and the cetaceans. It had four legs, which were all short – but the front legs were particularly stump-like. The hind limbs were well developed despite their length, and the feet were specialised with large broaded areas. Therefore, A. natans could manoeuvre on land or in the water. It was the size of a modern day sea lion and still had no blowhole. Walking on land would have been achieved in much the same way a modern sea lion does, whereas swimming would have been a combination of propulsion with the hind limbs and steering with the front.
Rodhocetus was discovered in 1993 and had hind legs considerably smaller than A. natans, but could probably still function on land to some degree. It had a fairly large tail – seen in the large tail vertebrae and could probably stay out at sea for long periods of time. The nostrils had moved back from the end of the snout, but were still separate.
Basilosaurus isis, Protocetus, and Indocetus ramani still had small legs, but they were functionally useless for walking – the creature could not longer come out of the water. The hind limbs were still able to swing forward into a straddle position as an aid to copulation.
Prozeuglodon had now vestigial hind legs, which were about six inches long, and found attached to the side of the fifteen foot long body.
Eocetus had now lost their hind limbs altogether, but still have a primitive whale set of teeth. Also, the nostrils – although slowly moving up the head – had still not fused. Eocetus was also a much bigger animal, with specimens reaching twenty five metres. The body was elongated, and the body streamlined. A cartilaginous tail fluke was present, and the ear was modified for hearing under higher pressures such as those found deeper underwater.
The whale lineage then splits into toothed whales and toothless whales, each group having several species in the lineage – fossil examples of which have been found.
|
Toothed whales |
Toothless whales (‘Baleen’ whales) |
|
Agorophius |
Aetiocetus |
|
Prosqualodon |
Mesocetus |
|
Kentriodon |
Modern
baleen whales |
Archaeopteryx
This is possibly the most famous transitional fossil in the world of the evolution – creation debate. Although, according to the creationists Archaeopteryx is nothing more than a bird – the same as birds found today. Perloff opens his attack on this intermediate by saying:
‘… Archaeopteryx was noted to have feathers
like a bird – but, like a reptile, teeth, claws and a rather long tail.
‘That exemplifies something evolutionists often do: equate similarities to relationships. Thoughtful consideration and new discoveries have demonstrated that Archaeopteryx was a true bird. Only birds have feathers…’
So it seems we’re back to the old classification problem: If a creature has feathers then it is a bird. What if we applied that sort of logic to (say) scales? If a creature has scales then it is a fish. But if we do this, aren’t we leaving out all the reptiles? Or should we reclassify reptiles as fish? The point is that it is not possible to classify (albeit arbitarily, as discussed earlier) an organism on a single characteristic alone. It is true to say that in the world today the only creatures with feathers are clearly birds – but why should this have always been the case? It is perhaps the essence of evolution that life is not static, but fluid.
Archaeopteryx has many anatomical features that, if used alone to classify the creature, would put it into either the bird category or the reptilian category. For example, as pointed out by Perloff if we used feathers to classify this creature then we would say it was a bird. However, if we used the presence or absence of gastralia (‘anterior / ventral ribs’ found in the anterior abdominal wall) to classify it, then we would say it was a reptile, since only reptiles have this feature – and birds do not.
It is therefore clear that we must use more than one characteristic in order to classify an animal properly. The reason that Archaeopteryx is so famous is that it is something of a problem to classify. It has many characteristics that would allow it to be classified as a bird, but also many that would allow it to be classified as a reptile. It is therefore an intermediate.
The following table lists characteristics of Archaeopteryx that are considered either avian (bird-like) or reptilian. If anything, it seems that Archaeopteryx has more features normally considered to be reptilian than avian:
|
Characteristic |
Birds? |
Reptiles? |
Archaeopteryx? |
|
Feathers |
Yes |
No |
Yes |
|
Opposable hallux (“thumb”) |
Yes |
No |
Yes |
|
Furcula (“wishbone”) |
Yes |
No |
Yes |
|
Elongated, backward pointing pubic bone |
Yes |
No |
Yes |
|
Bill |
Yes |
No |
No |
|
Fused trunk vertebrae |
Yes |
No |
No |
|
Pneumatic bones |
No |
Yes |
Yes |
|
Plate-like pubic shafts |
No |
Yes |
No |
|
Cerebellum behind midbrain |
No |
Yes |
Yes |
|
Neck attaches to skull from below |
Yes |
No |
No |
|
Concave articular facets on cervical vertebrae |
No |
Yes |
Yes |
|
Long tail with unfused vertebrae |
No |
Yes |
Yes |
|
Teeth |
No |
Yes |
Yes |
|
Ribs articulate with sternum |
Yes |
No |
No |
|
N |