Science in Christian Perspective
THE PALEONTOLOGY OF THE HORSE
Instructor in Geology
From: JASA, 2, (December 1950): 32-36.
In summary, Eohippus, Orohippus, Epihippus and Equus are stratigraphically
isolated either in basins or otherwise, and therefore
their relative ages cannot be determined without resorting to paleontologic methods of
correlation, a procedure which we have ruled out a priori in this case. The remaining horses which have been
mentioned all have representatives in South Dakota-Nebraska strata whose order of
deposition can be determined fairly easily. Therefore at least this portion of our
originally cited sequence appears valid:
Mesohippus is succeeded by Miohippus; Miohippus is partly contemporaneous with Parahippus but is eventually replaced by it; Parahippus subsequently is joined by Merychippus which then takes the ascendency while Parahippus continues. Pliohippus is found in beds above Merychippus and above those in which the final forms of Parahippus occur.
Obviously it cannot be proved definitely that any one of these forms %as not contemporaneous with any or all of the others, This is a negative argument, albeit an important one, If.9 however, all these forms were contemporaneous, it is an almost unbelievable coincidence that they just happened to become fossilized in a deceptive sequence, for as Scott says, "Between Hyracotherium and Equus there is an immense difference in all parts of the dentition and skeleton, so great, in fact, that without the intermediate steps of modification.9 hardly anyone would be so bold as to assert that Equus had descended from Hyracotherium" (Scott, 1941, p- 911).
The trends represented by these successive forms are unmistakable: the increase in size, the development of a monodactyl conditions the appearance of hypsodonto curved teeth, the lengthening of the face. Surely there is reason to fit the Eocene horses into the picture, since they readily fall into line with these trends, and it appears equally valid to regard Equus as the culmination of the processes of change.
Our discussion thus far has dealt with the postulated main line of equid evolution, showing that certain genera which probably descended from one another have manifested trends of development, This same phenomenon can be observed on the level of the species. The Sheep Creek beds of western Nebraska have already been mentioned. Here in different horizons of a layer of sandy shale Osborn found fragments of five individuals of the species Merychippus isonesus. Each of these had distinguishing characteristics of sufficient magnitude that born considered them five separate varieties. The bottom-most individual he assigned to the variety M.isonesus primus and used secundus, tertius, quartus, and quintus to designate the other in the order of their superposition. Each one was larger than the one which occurred just below it, although the limb bones in M. isonesus Suartus were the same weight as those of M. isonesus quartius. The limbs of the second, third and fifth varieties form a series of increasingly robust limb shafts. In M. isonesus secundus the u3na and radius are compressed but not fused, In the overlying variety, these bones are actually fused for a short distance, These and other successive changes In this series vividly demonstrate that horses were constantly in a state of flux and that their changes exhibited a significant amount of pattern even in minor trendm6 or minor portions of major trends.
The whole question of pattern In the development of successive genera is fascinating. Granted that our horse series is a phylogeny, we have then admitted that a certain amount of evolution has taken place, and further,, that this evolution has not been haphazard but has proceeded along well-defined patway, including increase of body size, development of one-toed feet, and so forth. Such "straight-line" development has been called orthogenesis, and according to Romer "has played a prominent part in paleontological thought," being "usually coupled with the supposition that some mysterious principle lies behind the observed phenomena," "a deity, or 'Nature'" (Romer, 1949, p. 105t 107). Romer continues by pointing out that the history of the horse was long thought to be undeviatingly orthogeneties but that now the family tree is believed to have many branches and that the succession which me have presented is only the unbranched portion of that family tree. Then, since the definite trends which we have cited in this succession cannot be ignored, he offers this summary of current thought upon the subjects following closely after Simpson (Simpson, 1949).
Orthogenetic: phenomena, then, are probably much less common than they were thought to be. But even in cases where straight-line phenomena are present, there is no need to postulate any teleological principle to explain them. Phyletic lines of this sort are reasonably to be considered as due to orthoselection, process of increasingly improved adaptation to a relatively stable environment. Under such conditions any deviation from the normal line would be negative as to survival values and would tend to be eliminated; the potential branches of the 'tree? would tend to be pruned by selection before they became marked enough to become apparent in the fossil record" (Romer, 1949, p. 107).
Thus the horses have given us strong indication that directional change has occurred. Incidentally, to call this change "progress" is only valid insofar as it can be demonstrated that Equs, is better off in his environment than Eohoppus was in his, and this, of course, cannot be demonstrated, Beyond this point the paleontologist cannot go, and he must turn to genetics for an explanation of how such changes may have arisen initially.
Dunbar, Carl 0. 1945L Historical Geology. John Wiley & Sons, Inc.
Edinger, Tilly. 1948. Evolution of the Horse Brain. Memoirs of the Geological Society of America, Memoir 25.
Osborn, Henry F. 1890. Equidae of the Oligocene. Memoirs of the American Museum of Natural, History, n. s.s vol, 2, pp. 87-100.
Romer, Alfred S.
University of Chicago Press.
______1941. Man and the Vertebrates. University of Chicago Press, 3rd ed.
_____ 1949. Time Series and Trends in Animal Evolution, in Genetics Paleont and Evolution. Princeton University Presso
Scott, William Berryman. 194l. Part V. Perissodactyla, in the Mammalian Fauna of the White River Oligocene by Scott and Jepsen. Transactions of the American Philosophical Society, n. s. vol. 28.
Schuchert$ Charles. 1923. Stratigraphy of the Eastern and Central United States. John Wiley and Sons Inc.
Dr. mixter: Eohippus is North American. There was an equivalent specimen in Europe
called acoTheriam; which Miss Erdman referred to. That apparently became extinct
rather ea-rTy-. TS sequence is all North American, and this portion Of it, as she
emphasized, is found in superimposed strata so we're pretty sure that these specimens
were descendents of one another.
Dr. Saarnivaara-. Professor Frank Lo Morse, says in his book, Creaticn, Evolution, Ed 'S"cience, which was published in Washington, D. C, in l944-,'TTk-f7he hoof ol mode-r-n-Fo-rse has been found in Colorado deeper in conformable strata than Eohippus. Do you know anything about that? And how to explain it correctly?.
Miss Erdman: I have heard rumors of Equus, (that is the modern horse), occuring in strata e=ar ter or deeper in the earth-'=an Eohippus, but in none of the literature with which I was dealing did I come across in-y r6f-eirence, to this. And I have never had any specific documentation for that* I would be very glad to know about it, I'd like to know who found if, if he was a competent geologist, if he knew his stratigraphy, and what the stratigraphic conditions were at the site of the discovery* And before we have those facts I don't believe we can make any comment; at least we can't make any definite commitment as to the occurence of Equus in much older strata* If me did find Equus in that much older strata it would ceRMly indicate that this horse sequence was merely a coincidental sequence,, which as I tried to point out in my paper I think would be a very large coincidencee I am afraid that I can1t give you any positive answer on that becauseof not ever having found the documentation for it.
Dr* Monsma, Chmor. Probably you have the documentation for it Dr. Saarnivaara,
Dro Saarnivaara: I have read it in the book I mentioned.
Miss Erdman: You have read it in a book but, you see, if the book was quoting some(We-eTs'e,you would really have to know who he was quoting.
Dr. Kul : This is quite trivial, but I wonder if Dr. Hartzler or Dr. Bender or some -one Frod Goshen couldnft go in the library and get that book of Morsels and see if there is a reference. I certainly would say that if this is'true, it is a very significant thing. But we have also read in some of the literature labelled "apologetic"and written by people who think that an geology can be interpreted in six thousand years, the statement that there was a shoe found in Mesozoic sandstone out in Nevada. There was no reference given there either, and that sort of thing you have to treat like flying saucers,, I'm afraid, We should be very diligent to find any kind of available data like that,, but the chances are that the specimen was taken out of a cave or some loose slump material. We must be very careful, but we certainly should go after it. If someone thinks they have some information, let's tract it all the way down and find out where it is.
Dr. Tinkle: I came across that same statement but it was several years ago and in a WO-n--sclMnific newspaper; therefore, it requires much more search, But I am very much interested in it, But what I rose to ask about is that we have more of a discussion on size. Now there are some three toed horses that have been found but have considerable size, But again I have nof. read this recently and I cannot give the names or just where they are found, but I should like to have more discussion on that matter.
I have some reasons for wanting to know more about size. In the first place the loss of toes can readily be explained as has been explained through mutation. But if there was a gradual increase in size at the same time that there was a gradual
decrease in the number of toes through mutation, thatts something that we don't find in our modem genetics, It usually doesn't work that way. It isn't often that mutation gives us added size and for a series of mutations, each one to give us an added size, would be something that is almost not found at all. And then, it may be that environment has something to do with size. Perhaps some of you saw the small horses that were taken from a canyon in California several years ago. It seemed that those horses had been trapped there a number of years ago, at least they could not get out. They could see them from the heights above. Finally they rigged up an expedition, let some men down in a cable basket and after considerable difficulty they caught three of these horses and brought them to the surface and exhibited them rather widely. How many of you saw them? I see two hands, I believe. Their backs were about the height of these tables. I saw two of them and I asked them about the third. They said the third one was taking care of a colt and,, therefore, they did not care to exhibit it at that time. And then I asked them how the colt vas getting along. And they said, very wen, and they could see that it, would be considerably larger than its parents,, which indicates clearly that at least part of the small size of those horses was due to environment: they just had not gotten enough to eat. N&Tj, it may be that the size of some of these fossil specimens visa determined to a certain extent by their environment. That is something that is a little hard for us to check on at the present time. However, in cases where there are a large number of fossils we perhaps would not think that the small size was due so much to lack of food, I'd like to have a little more discussion on size.
Dr. Mixter: There is quite a discussion on size in Simpson Is Tempo and Mode in 2-vol-vEffon. I'll give it to Dr, Tinkle.
Don Earl Boatman: I'd like to know on what basis they call the Eohippus a horse. IT-i-say EnTeMsTanding that there were 300 major differences andT7m'Ju-st a layman and trying to exercise some horse sense, but it seems to me that the perscn who would call that a horse is a jackass. Three hundred differences seems to me would disqualify the creature to be a horse. Now I'm hoping that someone can explain why they call this one a horse. Thatts for information.
Dr. Mixter: There were 300 minor differences as mentioned by Simpson, You see, he Taid-M-there was no appreciable difference between the two. This is the reason for thinking itis a horse. Itts a backboned animal; it has hair and nursed its young by milk; it was attached to its mother by a placenta--part of the after-birth; it was a hoofed animal--it had hoofs on each of the three toes and four toes; it was an uneven-number-of-toes animal; it was a hippemorph, that is, horse-shaped in general curvature of the back, and so on; it was an equid,, which means in many details of structure it was horse-like, "which is a classificatory way of saying that the vast majority of its multitude ef morphological characters was already the same as those preserved in the modern horse." You can find statements in these books that if we only had Equus, the top one and Eoh .
not connect the*, that is, we would think they are-entirely different genera or
even different families. But it is because we have the gradual stages in between
that we connect the two, and it's that sort of thing that has been found in a number
of instances between species,, bet , ween genera, that makes it look as though descent
with modification the simplest explanation of their origin* And yet I maintain
that the great.gaps between the orders indicate that the first members of each of
the orders was a creation.
Irvin Wills: I'd like to address a question to Miss Erdman concerning how complete fossifforms of these horses have been found. In other words) have you found complete fossilizations intact or in situ of say EohipLus, EpEihippusp or some of those?
Vs Erdmant I believe that I partly answered this question in my paper in referring t6 the abundance of horse material which is actually found, and there I stated that we do not to my knowledge have many if any complete skeletons omf horses but that we have many partially complete skeletons, that is, certainly the significant parts of the horse, such as the limb bones and the skulls and the teeth which are probably the most important of all in dealing with vertebrates and particularly with horses. We have one specimen here which has a certain part of his anatomy preserved and here we'll have another specimen with a complementary part of the anatomy preservedt so that through comparing the types we are very readily able to compile a complete horse. In other words, if you have overlapping parts of two different specimens sufficiently complete to assure you that you are dealing with similar or the same animal, then it is perfectly valid to supplement one by means of the other~ so that although we do not have any one complete series of intact complete skeletons we have enough material to make up such a series.
Dr. Mixter; Simpson's figures show 397 specimens of members of the family from lower Mwe_ne7_57~ from middle Eocene, 11 from upper Eocene. In the next stratum, Oligocene, 30 from the lower, 125 from the middle and 39 from the upper*
Dr. Monsma- It is always true however, isn't it, that when these specimens are 7sudTe=ere are two things that the investigators have in mind. One thing is the fact of actually finding the specimen and then the evolutionary ideal how does it fit in with it* I think that always plays a part and I suppose it should, in some way or other if we are studying the subject of evolution.
Robert Fischer: I'd like to ask a rather non-technical question. It's been suggested that Th-e-C=hrstian in his own mind accepts creative acts of various types and yet in view of a quotation which was given about some of the previously considered missing-, links being found, an appreciable number apparently, it seems as though we are in a rather weak position and almost approaching a position of accepting creation merely to cover up our present ignorance. Is our position thus weak or are those really significant missing links definitely not being found.
Dr,'Mixte r: This is a very significant question, You see the evolutionist believes
Me ja-pswill be filled, We have said that the absence of specimens is in favor of
the revelation me have which says that essentially there were no specimens filling
the gaps. So itts a matter of believing whether they will be filled or not. Simpson
said in 1944, that not only is there a gap between Eohippus and previous ancestral
types, but this is true of all the thirty-two order-s-7 _iaammals~ and in most cases
the break in the record is still more striking than in the case of the horse record
for which a known earlier group does at least provide a good structural ancestory.
Condylarthra look like horses in some respects so they could have been the ancestors.
The earliest and most primitive members of every order already have the basic
ordinal character, and in no case is an approximately continuous sequence from one
order to another known." "In no case:" says this leading specialist in the subject.
"In most cases the break is so sharp and the gaps so large that the origin of the
order is speculative and much disputed# Of course, the orders all converge back
ward in time to different degrees. The earliest known members are much more alike
than the latest known members
and there is little doubt,, for instance, but that all the highly diverse ungulates did have a common ancestroy, but the line making actual eonnection with such an ancestory is not known in even one instance." He says there is a systematic deficiency of record and it looks to me as if this systematic deficiency is significant.
Dr. Monzma: It is interesting that you read this word speculation here. I recall !-course I once took in the evolution of algae in which the teacher repeatedly stated, "Well, these are speculation&--these are evolutionary speculations," and I think-that these men will admit that too. ften we are dealing with this evolution
we have what we called the other day "a great deal of imputation," and it falls within the realm of speculation. Now how much speculation is that? Is there too much speculation to warrant our adherence to the theory or is there an expected amount of speculation here? That does make a great deal of difference, Of course, ouryiewpoint does enter into the answer to that question, it seems to me, - our fundamental outlook on the.thing.
Mr. Brenem4ni Regarding that matter of the gaps in the recordj,- I would just like to make an o=sorvation which is advanced by those who believe must emphatically in evolution. You can take it for what you think it's worth. Experiments have been performed on some of the lower animals at least which show that when you put the animal under severe stress, such as extreme temperature, starvation or dehydration and lack of water, sufficiently severe to produce death in a very considerable percentage of the individuals,, that this greatly increases the rate of mutation of the genes. Now, as to the missing record, one of the things that we always notice that whenever there is a non-conformity of the geological record that we see a sudden occurence of a new species on the two sides of non-conformity. Now, what has produced this non-conformity? There is probably no universal answer over the worldJ, but a very common cne, of course, is a rise in the elevation of the conti~ent which' produces wide erosion, a condition which does not make it very favorable for the preservation of specimens during that period of erosionp and such specimens as are washed down in the streams are deposited out in the ocean which later, when the continent submerges, becomes deeply buried in sea deposits that me cannot inspect. I just wonder to what extent those things have taken place--that these changes have been produced under severe stress of living conditions and the severe stress occurs at the time of elevation of continents during which the changes and mutations are taking place and when the conditions are not favorable for preserving of the record.
Dr. Tinkle: Whenever we get to talking about change through mutation there tends to U9 a-U=of duscussion on another type of change which I think we should not overlook, and that is change through segregation an4~recombination of genes. It brings about considerable change. Not if the animals * plants are members of the pure line, but we do not find pure lines in nature. Pure Ipmes have been formed by man. Our strains of inbred corn are examples of practical~.y pure lines. They are not entirely pure lines. But where animals or plants are heterozygotes then in future generations we can expect segregation and recombination of genes and it results in considerable change. We dontt have to be believers in evolution to accept that* Genetics teaches us that* ~oetve known that for a number of years. I mustn't take more time on it. I have an article on this subject that will be coming out some of these times in the Journal. I have expressed my ideas more at length there.
Dr. Monsma: I think we can perhaps close this discussion by calling attention to Tgis-fa=cthat Dr, Mixter has pointed out3 that changes can occur, and again Dre Tinkle has pointed it out, but that these changes, according to these men are definitely limited to certain possibilities that have been put in the organism by Ahe Creator and that they do not go beyond certain limits.
Dr.- Kulp: I'd like to have a general question answered from biology. I td like to Sow=there is any modern experiment to prove definitely that stress of temperature or pressure may increase the mutation rate. It has been shown that increase in radioactivity does.
Dro Monsma: Are mutation rates changed due to pressure and temperature stress? WT127s-omeone answer that question for us before we leave this noon?
Dr. Kulp: Not temperature and pressure. I said,, pressure or temperature.
Dr, Monsmat That's a logical change, isn't it? Any pressure of any kind--do they Mn_F-RuTation rates?
Dr,. Tinkles That is true in some cases. It's not only x-r-ays and ultra-violet ITght-B'uralso heat will increase mutation rate in certain organisms --certain seeds-and also the age of the seed has something to do with it, In Datura or the Jimson weed in which so many mutations have occurred they find that wE-e-F-Me seed is old the mutation rate is higher, They have gotten this seed to live for as much as eight years, anct seeds that are seven or eight years old will have more mutations than fresh seed.