You've been sucked straight in. All of those fossil horses were running around at more or less the same time and there's no way to claim any one of them is older than or ancestral to any other. You have made a very fanciful misinterpretation of a dialogue that happened in mainstream science starting about 130 years ago and ending with George Gaylord Simpson back in the 1940s. Horse evolution, like just about everything else, is a tree structure and not a straight-line progression. For all that, it is indeed a progression through time and lots of it.
Just for instance:
Eohippus/Hyracotherium, 50 million years ago.
Merychippus, 10 million years ago.
Just one tiny part of the picture is formed by the changes in the toe bones.
Figure 6. Stages in horse evolution showing the reduction in the number of toes and foot bones. Forefeet above, hind feet below. (A) Hyracotherium, a primitive early Eocene horse with four toes in front and three behind, (B) Miohippus, an Oligocene three-toed horse, (C) Merychippus, a late Miocene form with reduced lateral toes, and (D) Equus. (From Vertebrate Paleontology by Alfred Sherwood Romer published by The University of Chicago Press, copyright © 1945, 1966 by The University of Chicago. All rights reserved. This material may be used and shared with the fair-use provisions of US copyright law, and it may be archived and redistributed in electronic form, provided that this entire notice, including copyright information, is carried and provided that the University of Chicago Press is notified and no fee is charged for access. Archiving, redistribution, or republication of this text on other terms, in any medium, requires both the consent of the authors and the University of Chicago Press.)
From
Taxonomy, Transitional Forms, and the Fossil Record.
I'm a little late arriving at this thread, but I couldn't ignore your comment regarding horse evolution.
Excerpted from http://www.talkorigins.org/faqs/horses/horse_evol.html
his is a companion file for the Transitional Fossils FAQ and is part of the Fossil Horses FAQs. In this post I will try to describe the modern view of evolution within the horse family. I apologize in advance for the length; I didn't want to cut it down any more than this, because horse evolution has been oversimplified too many times already. I wanted people to see some of the detail and complexity of the fossil record of a fairly well known vertebrate group. (In fact, even at this length, this post is still only a summary!) People who are in a hurry may just want to read the intro and summary and look at the tree.
Outline
- Historical background -- why fossil horses are famous
- Timescale and horse family tree
- Small equids of the Eocene
- Medium-sized browsing equids, late Eocene and Oligocene
- The Miohippus radiation of browsing equids (24 My)
- Horses move onto the plains: spring-foot & high-crowned teeth (18 My)
- The merychippine radiation of the late Miocene (15 My)
- One-toed grazing horses of the Pliocene & Pleistocene
- Modern equines
- Summary
- References
I. Historical Background
In the 1870's, the paleontologist O.C. Marsh published a description of newly discovered horse fossils from North America. At the time, very few transitional fossils were known, apart from Archeopteryx. The sequence of horse fossils that Marsh described (and that T.H. Huxley popularized) was a striking example of evolution taking place in a single lineage. Here, one could see the fossil species "Eohippus" transformed into an almost totally different-looking (and very familiar) descendent, Equus, through a series of clear intermediates. Biologists and interested laypeople were justifiably excited. Some years later, the American Museum of Natural History assembled a famous exhibit of these fossil horses, designed to show gradual evolution from "Eohippus" (now called Hyracotherium) to modern Equus. Such exhibits focussed attention on the horse family not only as evidence for evolution per se, but also specifically as a model of gradual, straight-line evolution, with Equus being the "goal" of equine evolution. This story of the horse family was soon included in all biology textbooks.
As new fossils were discovered, though, it became clear that the old model of horse evolution was a serious oversimplification. The ancestors of the modern horse were roughly what that series showed, and were clear evidence that evolution had occurred. But it was misleading to portray horse evolution in that smooth straight line, for two reasons:
- First, horse evolution didn't proceed in a straight line. We now know of many other branches of horse evolution. Our familiar Equus is merely one twig on a once-flourishing bush of equine species. We only have the illusion of straight-line evolution because Equus is the only twig that survived. (See Gould's essay "Life's Little Joke" in Bully for Brontosaurus for more on this topic.)
- Second, horse evolution was not smooth and gradual. Different traits evolved at different rates, didn't always evolve together, and occasionally reversed "direction". Also, horse species did not always come into being by gradual transformation ("anagenesis") of their ancestors; instead, sometimes new species "split off" from ancestors ("cladogenesis") and then co-existed with those ancestors for some time. Some species arose gradually, others suddenly.
Overall, the horse family demonstrates the diversity of evolutionary mechanisms, and it would be misleading -- and would be a real pity -- to reduce it to an oversimplified straight-line diagram.
With this in mind, I'll take you through a tour of the major genera of the horse family, Equidae. CAUTION: I will place emphasis on those genera that led to the modern Equus. Do not be misled into thinking that Equus was the target of evolution! Bear in mind that there are other major branches of the horse tree that I will mention only in passing. (See the horse tree for a lovely ASCII depiction.)
Small preface: All equids (members of the family Equidae) are perissodactyls -- members of the order of hoofed animals that bear their weight on the central 3rd toe. (Other perissodactyls are tapirs and rhinos, and possibly hyraxes.) The most modern equids (descendents of Parahippus) are called "equines". Strictly speaking, only the very modern genus Equus contains "horses", but I will call all equids "horses" rather indiscriminately.
Most horse species, including all the ancestors of Equus, arose in North America.
II. Timescale and Horse Family Tree
|
| Recent |
10,000 years ago to present |
| Pleistocene |
2.5-0.01 My (million years ago) |
| Pliocene |
5.3-2.5 My |
| Miocene |
24-5.3 My |
| Oligocene |
34-24 My |
| Eocene |
54-34 My |
|
And here's the tree...note that the timescale is a bit weird (e.g. the Oligocene is compressed almost to nothing) to keep it from being too long. All the names on the tree are genus names, so recall that each genus encompasses a cluster of closely related species.
| The is a brief description of the tree for those who are visually impaired. Hyracotherium is shown giving rise to three lineages. Two lineages quickly go extinct. The third branches many times. There are many branches alive during most times until two million years ago when only the various species of Equus remain. The tree itself is unreadable to those who are visually impaired so skip the tree graphic. |
2My Old & New World Equus
\ | /
\ | /
4My Hippidion Equus Stylohipparion
| | Neohipparion Hipparion Cormohipparion
| | Astrohippus | | |
| | Pliohippus ---------------------------
12My Dinohippus Calippus \ | /
| | Pseudhipparion \ | /
| | | |
------------------------------------------- Sinohippus
15My \ | / |
\ | / Megahippus |
17My Merychippus | |
| Anchitherium Hypohippus
| | |
23My Parahippus Anchitherium Archeohippus
| | |
(Kalobatippus?)-----------------------------------------
25My \ | /
\ | /
|
35My |
Miohippus Mesohippus
| |
40My Mesohippus
|
|
|
45My Paleotherium |
| Epihippus
| |
Propalaeotherium | Haplohippus
| | |
50My Pachynolophus | Orohippus
| | |
| | |
------------------------------
\ | /
\ | /
55My Hyracotherium
|
X. SUMMARY
For many people, the horse family remains the classic example of evolution. As more and more horse fossils have been found, some ideas about horse evolution have changed, but the horse family remains a good example of evolution. In fact, we now have enough fossils of enough species in enough genera to examine subtle details of evolutionary change, such as modes of speciation.
In addition to showing that evolution has occurred, the fossil Equidae also show the following characteristics of evolution:
- Evolution does not occur in a straight line toward a goal, like a ladder; rather, evolution is like a branching bush, with no predetermined goal.
Horse species were constantly branching off the "evolutionary tree" and evolving along various unrelated routes. There's no discernable "straight line" of horse evolution. Many horse species were usually present at the same time, with various numbers of toes, adapted to various different diets. In other words, horse evolution had no inherent direction. We only have the impression of straight-line evolution because only one genus happens to still be alive, which deceives some people into thinking that that one genus was somehow the "target" of all the evolution. Instead, that one genus is merely the last surviving branch of a once mighty and sprawling "bush".
The view of equine evolution as a complex bush with many contemporary species has been around for several decades, and is commonly recounted in modern biology and evolution textbooks.
- There are no truly consistent "trends".
Tracing a line of descent from Hyracotherium to Equus reveals several apparant trends: reduction of toe number, increase in size of cheek teeth, lengthening of the face, increase in body size. But these trends are not seen in all of the horse lines. On the whole, horses got larger, but some horses (Archeohippus, Calippus) then got smaller again. Many recent horses evolved complex facial pits, and then some of their descendants lost them again. Most of the recent (5-10 My) horses were three-toed, not one-toed, and we see a "trend" to one toe only because all the three-toed lines have recently become extinct.
Additionally, these traits do not necessarily evolve together, or at a steady rate. The various morphological characters each evolved in fits and starts, and did not evolve as a suite of characters. For example, throughout the Eocene, the feet changed little, and only the teeth evolved. Throughout the Miocene, both feet and teeth evolved rapidly. Rates of evolution depend on the ecological pressures facing the species.
The "direction" of evolution depends on the ecological challenges facing the individuals of a species and on the variation in that species, not on an inherent "evolutionary trend".
- New species can arise through several different evolutionary mechanisms.
Sometimes, new species split off suddenly from their ancestors (e.g., Miohippus from Mesohippus) and then co-existed with those ancestors. Other species came into being through anagenetic transformation of the ancestor, until the ancestor had changed appearance enough to be given a new name (e.g. Equus from Dinohippus). Sometimes only one or a few species arose; sometimes there were long periods of stasis (e.g. Hyracotherium throughout the early Eocene); and sometimes there were enormous bursts of evolution, when new ecological opportunities arose (the merychippine radiation). Again, evolution proceeds according to the ecological pressures facing the individuals of a species and on the variation present within that species. Evolution takes place in the real world, with diverse rates and modes, and cannot be reduced to a single, simple process.