Horse evolution

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The impetus for producing this major, gorgeously-illustrated feature was a telephone conversation with a man whom I otherwise enjoy. He’s a cowboy, and grew up on a ranch. He’s a kind fellow who has done a good job with not only a lot of colts, but also his kids, who are likewise very fine people. He’s a Christian – a commitment that I admire – and his children were not only raised in their church, but have gone on to attend Bible College. All of this certainly deserves praise. However, this fellow knows that I’m a paleontologist interested in fossil horses. As we were discussing some of this, at one point in the conversation he raised his voice and said, “I can’t understand it! Belief in evolution is simply stupid!”
Well, it’s with any such point of view that I will never be able to agree. In the first place, such an attitude invalidates my experience and beliefs in a way that I would never impose in return – not to mention that it goes against nearly every piece of advice St. Paul ever gave. What could possibly cause an otherwise reasonable man to make such a statement so vehemently to someone he knows it is going to offend? Why -- because the fellow cares about me, of course; he wants my soul to be saved, even at the cost of his own.
Why does he think my soul is in danger of damnation? Because he believes that the words he reads in the Bible are literally true – no room for metaphorical interpretations or alternative readings. To this guy, there is one translation of the Bible (I don’t know which one) that is the true and accurate recording of God’s thoughts, desires, and commands for us. Fundamentalism prevents him from being able to reconcile the time-span that geology and paleontology propose for the world, because what mention of that subject there is in Scripture indicates that the world was created less than 5,000 years ago. Fundamentalism has crippled his ability to accept any evidence that would indicate otherwise. But to deny the existence of the (as you are about to see, thousands) of fossilized skeletons of horses and other animals known to science is simply foolish. Worse, to claim as some folks have that they have been “carved out of the rock” in an effort to deceive the public, is an irresponsible lie. And yet, this guy, so schooled in looking for the truth within Scripture, is inclined to believe such lies and distortions. He even gave me a link to a website by a creationist with a Ph.D. degree (in Economics) that spouts even more of them!.
I think the news media have also helped to destroy some peoples’ faith in the reliability of science. This even applies to straightforward science reporting. Mis-teaching in the public schools hasn’t helped, either – but the fault for all of this lies squarely at the doorstep of my own profession, which has failed to make an adequate public educational effort. Thus, evolutionist vs. creationist debates are still staged; in Kansas the legislature once again wants to ban the teaching of evolution in public schools; and the people of the United States have elected a string of conservative legislators and Presidents who, responding to strong lobbying by conservative and fundamentalist Christians, have cut Museum funding to the bone.
The concept that the public has of what evolution is and how it works simply does not match that which paleontologists hold. In fact, it hasn’t matched for more than a century. When your High School biology teacher taught you that evolution means one animal changing into another animal over a long span of time – and the picture he conveyed was of morphing -- he taught you badly; for there is no known mechanism for “crossfading” among living things. Instead, change through time is iterative by nature – change usually occurs one “click” at a time, like working a lock with a roller dial. When you go to your local Museum and you see an exhibit that shows a single series of horses gradually getting bigger through time – what I call an “inflate-a-horse” exhibit – this is a story that gives the idea that one horse species has morphed into the next. It so far fails to convey the real story that it is almost a crime. This review seeks to convey a more correct idea.
Evolution means “change through time”. Charles Darwin said that it occurred as a result of predation, accident, and disease acting upon variability within a population of animals. This was sloganized by Huxley into the “survival of the fittest.” But it needs to be remembered that that what Darwin was proposing was one possible mechanism by which animals in later generations might accumulate enough changes to be recognizable as a species different from the ancestor. Other mechanisms for change through time have also been proposed – the two most famous alternatives to Darwinism coming from Ernst Mayr and Stephen Jay Gould (see the extensive bibliography at the end of this feature for citations to their writings). My personal belief is that there is no incompatibility at all between believing that animals have changed or evolved through time, and that God created the universe. For paleontology says nothing at all about how the universe began; its only interest is in what has unfolded afterward. It is not paleontologists but fundamentalist creationists who arrogantly insist that they know exactly how God has nourished, or how He continues to nourish, this unfolding.
In the horse family, change through time is very well documented, based on literally thousands of fossil skeletons that have been unearthed and studied over the past two and a half centuries. Students who wish to understand the subject will want to pay attention to two areas in particular: logical systems for classifying living things, and age determination for the remains of animals that have been found within rock strata. In this feature, I present the information on stratigraphy and age-determination primarily in pictorial form. The rules and procedures of taxonomy (the science of classifying living things) are explained below. The example used throughout is, of course, the horse – so here you will also get a complete classification of the horse along with the actual, technical reasons for that classification. I have also supplied many wonderful photos showing fossil specimens exhibited in museums around the world, so that this issue of “The Inner Horseman” functions as a virtual World Museum Tour. I certainly hope that you find studying this material fun as well as enlightening!
Living things are classified on the basis of similarity in structure. This means that you must first notice the structure of whatever living thing you mean to study, noting particularly any features (taxonomists call them “characters”) unique to that creature. Similarities shared by a range of organisms, or features unique to only one or a few of them, can then be tabulated.
Characters common to a large number of organisms are said to be primitive. “Primitive” in this sense does not imply crudeness, for all organisms living at all times must have a body plan that works very well in order to stay alive at all. Nor does the term “primitive” necessarily imply that the feature occurs early in time, for in fact most of the characters of most organisms now alive have been inherited unchanged through untold generations, over the span of millennia.
Because of this, characters which are unique to a given organism – characters which of necessity must be modifications, great or small, of earlier designs – have special significance in classification, for only such “derived” features can define the boundaries of taxonomic units -- groups of related species. The logical system of classification, called cladistics, makes use of derived features shared by all members of a taxonomic unit to define that unit. Because derived features can only arise from pre-existing designs (those which are comparatively more widespread or more primitive), a cladistic logic-diagram also shows us the most likely order in which the descendants of any given lineage acquired derived features.
The science of classification formally began in the mid-18th century with the work of Swedish scientist Carl Linné. Linné (whose name is usually Latinized to Carolus Linnaeus) proposed the system of binomial nomenclature which is still used today. In this naming system, all species receive two names – the first signifying the genus to which the species belongs. When the second term, called the trivial, is paired with the genus name, the two terms taken together uniquely name and identify the species. The example most relevant to our interests would be:
Equus – the genus for horses, asses, onagers, quaggas, and zebras. The Latin word “equus” means “weighting all four feet equally”.
caballus – the trivial term is a Latin word meaning “nag”. Note that by itself, the trivial term does not identify the horse -- or any other animal. There is, for example, at least one beetle and also one plant whose genus names are followed by the trivial “caballus”. Thus, in order to be sure that we’re discussing the living horse, we need to write:
Equus caballus – this binomial term is the scientific name, used in taxonomic classification, that uniquely identifies the horse.
Please note the importance of capitalization and italicization. In scientific writing, it is mandatory to capitalize the first letter of the genus name, and to not capitalize the first letter of the trivial. The genus name, which is OK to use alone, or the full binomial when it is used, must be italicized or in some other manner set off from the surrounding text (i.e., boldface or underline are OK too). While these rules are mandatory in any technical journal, good usage demands that they be followed in all publications (so for example, the better-edited newspapers and monthlies such as Equus Magazine scrupulously follow these rules).
Naming Species: Importance of the Type Specimen
There are also important rules concerning the naming of species. The person who is the first to describe and publish, in an encyclopedia or peer-reviewed journal, any new species gets permanent credit for doing so. The most important rule of taxonomy is that the trivial term is permanently associated with a “type specimen.” Ideally this would be a complete skeleton; practically in horse paleontology it is often a skull, a partial dentition, or even a single tooth. The type specimen must be housed in a Museum of Natural History or other permanent and professionally curated collection, it must bear a unique specimen number, and it must remain accessible in perpetuity for any scientist who wishes to examine it.
Once the type specimen has been designated and named, the real work concerning it begins. This consists of observation, discussion, and even controversy among all the paleontologists who have examined it. Because the trivial can never appear, and has no scientific meaning, unless it is assigned to and paired with a genus name, the original namer must assign it to some genus. But that by no means guarantees that’s where it’s going to stay! After examining the type, another scientist may feel justified in re-assigning the specimen to a different genus. These re-assignments may go on for years – often because other discoveries, whether in the area of the geology and stratigraphy or as a result of further study of the skeletal material itself, help to put the type specimen into better perspective. There so many examples of this in the paleontological literature that I can only say, “let the student be forewarned”. Here’s a table containing a few examples; boldface type indicates instances where the current expert opinion is that the original namer assigned the species to the correct genus:
Original Date & Namer Original Assignment Present Assignment
Erwin Hinckley Barbour, 1914 Hypohippus matthewi Megahippus matthewi
Edward Drinker Cope, 1873 Anchitherium cuneatum Mesohippus cuneatus

E.D. Cope, 1873 Protohippus sejunctus Protohippus sejunctus

E.D. Cope, 1879 Anchitherium praestans Kalobatippus praestans

E.D. Cope, 1889 Hippotherium isonesum Calippus isonesus

E.D. Cope, 1889 Hippotherium sphenodus Griphippus sphenodus

E.D. Cope, 1889 Hippotherium retrusum Calippus retrusus

E.D. Cope, 1892 Equus simplicidens Equus simplicidens

E.D. Cope, 1893 Hippidium interpolatum Pliohippus interpolatus

E.D. Cope, 1893 Protohippus lenticularis Nannippus lenticulare

E.D. Cope, 1893 Equus eurystylus Neohipparion eurystyle

J.W. Gidley, 1903 Neohipparion whitneyi Neohipparion whitneyi

J.W. Gidley, 1907 Merychippus campestris Pliohippus campestris

Joseph Leidy, 1850 Palaeotherium baridii Mesohippus bairdii

J. Leidy, 1856 Hipparion occidentale Cormohipparion occidentale

J. Leidy, 1858 Eohippus perditus Pseudhipparion perditus

J. Leidy, 1869 Protohippus placidus Calippus placidus

J. Leidy, 1869 Protohippus supremus Pliohippus supremus

J. Leidy, 1869 Hipparion gratum Griphippus gratus

Othniel C. Marsh, 1874 Pliohippus pernix Pliohippus pernix
Henry Fairfield Osborn, 1918 Miohippus gidleyi Miohippus gidleyi

H.F. Osborn, 1918 Merychippus republicanus Pseudhipparion republicanum

H.F. Osborn, 1918 Pliohippus leidyanus Dinohippus leidyanus
O.A. Peterson, 1907 Parahippus nebrascensis Parahippus nebrascensis
E.H. Sellards, 1916 Hipparion minor Nannippus minor

William Berryman Scott, 1893 Anchitherium equinum Hypohippus equinus

In all cases, however, even where later research has proven the generic assignment incorrect, the original namer continues in perpetuity to receive credit for naming and describing the type – his name is forever associated with it. So, for example, even though it was Morris Skinner and Bruce MacFadden who did the work and had the insight to re-assign the type specimen of Hipparion occidentale to Cormohipparion, the full correct citation for “Cormohipparion occidentale” is Cormohipparion occidentale (Leidy, 1856).
The Classification Hierarchy
Linnaeus’ original system of classification was hierarchical. A hierarchical classification functions like bowls that stack one inside the other: into the biggest bowl go many organisms, which are themselves bundled into groups. Each of these groups, in turn, functions like a bowl which holds still smaller bowls, and so on until we reach the level of the genus, species, and subspecies.
Figs. XXX – XXX graphically illustrate the whole hierarchical classification of the living horse, Equus caballus. Students of the horse need to be familiar with the meaning of such terms as Perissodactyl (vs. Artiodactyl), and Equid vs. Equine (capital “E”) vs. equine (little “e”). All taxonomic terms are defined in terms of structure. In other words, a member of the Order Perissodactyla is identified as such because it possesses certain unique, derived features that are visible on the skeleton. This, at root, is what makes a horse a horse – and not a deer, a goat, a dog, or a primate. Each of the accompanying “learning units” is designed to familiarize you with the particular characteristics that define and characterize each taxonomic unit, whether Kingdom, Phylum, Class, Order, Family, Genus, or species.
How Species are Defined
The smallest unit recognized by the science of taxonomy is the subspecies. However, where horses are concerned, it is obvious that we often benefit from looking at still smaller units – for example, herds within a subspecies, and even the individual within the herd. The reason that taxonomic classification stops at the level of the subspecies is that this is the level where panmixia occurs.
“Panmixia” is a term relating to the genetics of populations. Remember that, in classifying animals, we do not look at their genes; instead, we look at the results of gene action which are manifested as the structural peculiarities of the visible body. Panmixia is the condition which occurs within a population of animals in which every breeding individual has an equal chance of mating with every other individual. When this is the case, any given gene has its best shot at showing up in the maximum number of individuals. “Gene flow” through a panmictic population is free of restrictions or bottlenecks. Subspecies are, by definition, panmictic populations.
A species, which may contain one or more than one subspecies, is a group of populations in which any individual is capable of breeding with any other individual to produce viable offspring that can themselves breed to produce viable offspring. In other words, a species is defined by reproduction and gene flow (which in turn creates a certain limited range of skeletal structures within that population). When a species is spread out over a lot of territory – and the horse at one time represented one of the best examples of such – inevitably there will be barriers to reproduction. For example, a mountain range may intervene between the easternmost and westernmost populations of a species. The difficulty in crossing the mountains will make it less likely that individuals from the eastern vs. western populations will meet and interbreed.
When there are barriers to panmixia, and when environmental conditions differ from one range to another, populations or herds inhabiting different regions will tend to become adapted to the particular conditions found in that region. This is, of course, not only because they tend to breed only with each other, but also because individuals less physically suited to a given regime of terrain, forage, and climate tend to reproduce less often and produce fewer offspring. They may also be more susceptible to predation, accident, and disease. Over time, the physical type they represent becomes rare and may become altogether extinct. This is how the gene pool of a population becomes characteristic of that population. We detect the range of different gene combinations within a given gene pool through its expression in the physical bodies of individuals.
When dealing with the fossil record, we are, of course, lucky to find preserved even one member of any population. It has been estimated that somewhere between 1% and 15% of the skeletons of all the mammals that ever lived were fossilized. Of this small number, only a tiny fraction are actually found, collected, studied, and reported. These statistics are often used as a kind of apology for any mistakes that paleontologists might make in interpreting the fossil record – we have to work from very partial data. However, the same statistic should also be used in reverse -- as a measure of the number and diversity of animals that have lived on Earth. For example, the student may consider the fossil record of horses: in the five largest collections of fossil mammals in North America (the American Museum of Natural History in New York City, the National Museum of Natural History/Smithsonian Institution in Washington D.C., the University of Nebraska at Lincoln, the University of California at Berkeley, and the University of Florida at Gainesville) there are tens of thousands of horse bones and teeth sufficiently complete to permit identification at least to the level of genus. There are also numerous other, smaller collections: The University of Kansas at Lawrence, the University of Michigan at Ann Arbor, the Page LaBrea Tar Pit Museum in Los Angeles, the Los Angeles County Museum of Natural History, and the University of Colorado Museum at Denver, not to mention the National Museum of Mexico in Mexico City, the National Museum of Canada in Ottawa, and the Drumheller Museum in Alberta. If this is the case, it is simply awesome to contmplate the number of animals belonging to the horse lineage that have lived on Earth.
A Classification of Horses
Animals are multi-cellular animals characterized by the power to move. Although as adults some become rooted by a stalk or pedicle (example: clams, crinoids, anemones), for at least part of their lives they are motile. The cells of animals are thin-walled, lacking stiff cellulose reinforcement.
PHYLUM Chordates
Animals that have a stiff rod of cartilage extending along the dorsal part of the body. This rod, called a notochord, functions to protect the central nerve cord, which is also located dorsally. In most, the notochord is present only at an early embryonic stage, but in a few (the tunicates) it is present in early life, or (in Amphioxus) it persists throughout life. Chordates possess the full compliment of body systems, i.e. they have a complete digestive tube with a front opening (mouth) and rear opening (anus); central and peripheral nervous systems; segmented muscles; a heart that recirculates blood through a closed circulatory system. There is a brain and sense organs concentrated at the front end of the body, and the body itself is bilaterally symmetrical. The creature breathes by means of a long series of gills, and although there is a mouth opening, there are no jaws. The sense of “hearing” is diffuse, the whole body being sensitive to vibrations in the water; there are no ears.
SUBPHYLUM Vertebrates
Chordates in which the notochord becomes replaced during embryonic life by a chain of vertebrae which enclose and protect the dorsal nerve cord. Generally, there are additional parts as well – limbs -- to compose an internal skeleton (endo-skeleton). In most vertebrates, the skeleton in the adult creature is bony, but in some specialized forms, such as sharks, the skeleton is cartilagenous. A skull, or at least the lower part of the skull (the palate area and basicranium) is present, and part of the gill series is modified to become jaws that articulate with the skull. A specialized, “concentrated” organ of hearing appears on the side of the head in the area of the jaw joint. The mouth possesses rows of hard teeth.
CLASS Mammals
Vertebrates which have fur, secrete milk for their young, and whose jaw joint is specifically formed by the articulation of the dentary element of the jaw with the squamosal part of the temporal bone of the skull. This latter is, of course, the criterion by which paleontologists distinguish mammals from near-mammals and non-mammals.
Generally speaking, mammals possess teeth of several different shapes – i.e., incisors, canines, premolars and molars; but some mammals lose all teeth or reduce them to vestiges (armadillos, pangolins, aardvarks, sloths), others modify them to fantastic new structures (baleen whales), and still others return to having teeth of uniform morphology (dolphins, killer whales). No matter how modified, however, mammals have only one row of teeth along the margins of the jaws, and they replace only the anterior part of the dental arcade only one time. The teeth develop in, and erupt through the gums out of, sockets called alveoli.
Primitively, the limbs of mammals terminate in five digits of about equal length arranged in a fan shape. From this original design many modifications have been tried, including loss of some digits, lengthening or shortening of one or more digits, fusion of digits to form a single stout digit, change from claws to either fingernails or hoofs, or change of the whole limb from a paw into a flipper or a wing.
With few exceptions, mammals have exactly 7 cervical vertebrae (whales and dolphins, and also giant ground sloths, have less than 7 vertebrae).
Each half of the mandible (the jawbone) in mammals is made up of but one, single element, the dentary bone (in reptiles, amphibians, fish, and birds the mandible is formed either of more than one bony element, or is formed from a different bone than the dentary).
The pelvis of mammals is likewise relatively simple, each half being composed of only three elements, the ischium, ilium, and pubis (the pelvis in other vertebrates tends to have additional elements). Importantly, mammals possess a sacrum composed of five or more vertebrae which fuse to make a rod above the pelvis. Birds and frogs also show fusions in this area, but they are far more extensive and serve to prevent flexion of the pelvis on the lumbar vertebrae. In mammals, up-and-down flexion of the spine is a crucial and characteristic element of locomotion, in contrast to the characteristically side-to-side, sinusoidal motions of the vertebral chain in reptiles, amphibians, and fishes (you can tell a fish from a whale or dolphin by the orientation of its tail fins: the fish’s tail fin is vertical, because in order to swim, he “wags his tail” from side to side. Marine mammals, by contrast, have their flukes oriented horizontally, because the main swimming motion is “humping” or up-and-down action of the tail).
Every student of the horse should be able to name and know the characteristics of all five of the classes of vertebrates: mammals, birds, reptiles, amphibians, and fishes. By contrasting mammals with the other four, we derive a richer picture of the unique nature of all mammals, and of the horse in particular.
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