For more information
Call 1-800-351-0222
Call 1-800-351-0222
EVOLUTION OF HORSES.
Term Paper ID:28254
Essay Subject:
Analysis of fossil records & new investigative technologies with differing findings on evolutionary development of horses.... More...
12 Pages / 2700 Words
8 sources, 46 Citations,
APA Format
$48.00
More Papers on This Topic
Paper Abstract:
Analysis of fossil records & new investigative technologies with differing findings on evolutionary development of horses.
Paper Introduction:
A Cladogenetic View of the Early Evolutionary History of Horses
Abstract
Drawing upon the theoretical and empirical literature, this report describes evidence obtained from the fossil record supporting the anagenetic view of the horse phylogeny and its early evolutionary development. The anagenetic perspective advances the notion that a gradual microevolutionary process has occurred within a species. This view is also associated with a phyletic transformation from ancestral to descendant species. The fossil record for horses presents, in general, a progressive replacement of one genus with another without a strong indication of temporary overlapping. However, the literature also suggests that anagenetic development was dominant within the Eocene and Oligocene, while genetic diversity (branching) suggesting a
Text of the Paper:
The entire text of the paper is shown below. However, the text is somewhat scrambled. We want to give you as much information as we possibly can about our papers and essays, but we cannot give them away for free. In the text below you will find that while disordered, many of the phrases are essentially intact. From this text you will be able to get a solid sense of the writing style, the concepts addressed, and the sources used in the research paper.
The following table presents these characters. Fewsatisfactory explanations for this disappearance have been offered, thoughRidgeway (1972) makes reference to the possibility that after coming to theend of their evolutionary tether in the attainment of speed, these horsesfell prey to one or more predators. MacFadden, B.J. Additionally, competition with cud-chewing, hoofed herbivores(e.g., deer and bison) may also have affected horse diversity. Onthe one hand, paleontologists including George Gaylord Simpson (1951)argued that the evolution of the horse family was definitely notorthogenetic, or straight and linear in terms of evolutionary pattern.There was, says Simpson (1951), no constant and overall increase in size ora definitive pattern of steady change from four toes to three and finally,one toe. Natural History,1 3 (4), 63 -66. Ancient diets, ecology, and extinction of 5-million-year-old horses from Florida. As of twomillion years ago, only the single horse genus Equus, consisting of a fewspecies, remained in the Northern Hemisphere. Life's little joke. For many years, paleontologists held that theevolution of horses, as evident in the fossil record, was anagenetic, orprogressing in a virtually straight, single line of evolution. Gould (1987) claims that Simpson held a life-long commitment to thepredominant role of evolution by transformational change within populationsrather than by accumulation across numerous events of discrete, branchingspeciation. (1992). It is the purpose of thisreport to draw upon the literature to examine the fossil record from thelate Eocene and the Oligocene with specific reference to Miohippus andMesohippus. MacFadden (1992) moved forward from the work of Simpson (1951) anddifferentiated between the two major modes of species evolution and thespeciation process generally interpreted from the fossil record. Cambridge: CambridgeUniversity Press. Theanchitheres retained the low-crowned, relatively simple teeth and paddedfeet of Miohippus, but reached a considerable size, comparable to modernEquus and achieving a body weight of 2 to 4 kilograms. The fossil record, says MacFadden (1992), is an excellent mediumfor examining both anagenesis and cladogenesis. The fossil record for horses presents, in general, a progressivereplacement of one genus with another without a strong indication oftemporary overlapping. The skull muzzlewas lengthened and a larger gap between the anterior nipping teeth orincisors and the posterior chewing teeth had developed. New York: Benjamin Blom. However, when the species switchedback to eating browse, the teeth did not change. (1994). This view is alsoassociated with a phyletic transformation from ancestral to descendantspecies. Originating from the late Oligocene Miohippus, two main lines ofdescent are recognized from the early Miocene of North America. Such an examination will demonstrate that from the Oligoceneperiod on, cladogenetic evolution of the horse was occurring - a processthat is rooted in the Eocene, but most readily observable in the latterperiod. These species varied significantly in size, butdisappeared from America at the time of the Spanish conquest. Hulbert (1996) speculates thatthe increase in size of the horse's skull may have been associated with acorresponding increase in "intelligence," though perhaps improved"instinct" may be a more apt description of this phenomenon. Hulbert (1996) further suggests that the sensorydevelopment of Equinae may well have been superior to that of otherspecies, thus facilitating its survival and advancement. Morell (1999) has summarized recent evidence from the fossil recordand stated that the teeth of ancient equines may have evolved in partbecause of the existence of new food. MacFadden (1994) also says that horsediversity increased so dramatically that at some fossil sites from 15million years ago, as many as a dozen species have been found. Earlier, MacFadden(1994) and Simpson (1951) agree that an anagenetic macroevolutionarytransformation took place, leading to limited diversity and numbers ofspecies. (1987). Ridgeway, W. Fossil Horses. Dietary data straight from the horse's mouth.Science, 283 (54 3), 773. It was during the Miocene that horses diversified rapidly to include largespecies and even a few dwarf lineages. Most significantly, in facial and bodyproportions, Mesohippus and Miohippus had much more classic horse-likefeatures than older genera. These are the generaHaplohippus, Mesohippus, and Miohippus. Each of these events speaks further to thequestion of what kind of evolutionary process was at work and supports theassertion that a cladogenetic process of evolution was occurring during theera under discussion herein.Summary and Conclusions The purpose of this report was to examine a specific evolutionarytopic and to draw upon the literature to identify current thinking andresearch on the topic. With respect to dietary variations, Morell (1999) states that grazinghorses typically ate grasses, which in many regions use the so-called C4photosynthetic pathway to turn carbon dioxide into sugars and starches.Such plants incorporate different amounts of the isotopes carbon-12 andcarbon-13 than do C3 plants, which are primarily trees and shrubs.Evidence from fossil teeth suggests that some of the six horse species of 5million years ago ate solely C4 grasses, but others chewed a mix of C4grasses and C3 shrubs and twigs, while a couple fed on mostly C3 plants. The ancestry of the horse. Simpson, G.G. Atthe end of the early Eocene, the continental drift severed the directconnection between North America and Europe and major climatic changesbegan which continued throughout the Oligocene and Miocene periods.Hulbert (1996) says that at this juncture, North American horses took adifferent course in their evolutionary history and became increasinglyadapted for living in open habitats and for running. From this time on, a cladogenetic developmental processis seen to be at work. The anagenetic perspective advances the notion that a gradualmicroevolutionary process has occurred within a species. Miocene horses, says MacFadden (1994) appear to haveminimized competition for available food and space by occupying severaldifferent niches. MacFadden, B.J. Introduction The fossil record of horses (family Equidae) over the past 55 millionyears is a classic example of macroevolution (MacFadden, 1999). The heyday of horses. Ultimately, itwas the Equinae in which significant cladogenetic branching took place(Hulbert, 1996). Hulbert (1996) states that evolving from a common ancestor, three-toed hipparionine and equine horses diversified rapidly between 18 and 15million years ago with hipparionine horses predominating in North Americanfaunas. Hulbert, R.C. A multitude of evolutionary changes through branching anddiversification ultimately resulted in the emergence of Equus. Clearly, this supports the idea that a cladogeneticpattern of evolution was beginning to occur. Morerecently, as the work of Hulbert (1996) and MacFadden (1992; 1994; 1999)demonstrates, paleontologists have found evidence supporting a cladogeneticevolutionary pattern in the Miocene which created a multiplicity of horsespecies or "branches" on the evolutionary trunk. Table I Key Morphological Characters, Family EquidaeTaxon Age Key CharacterHyracotherium Eocene Increased ratio of brain size to body size and expanded neocortex.Mesohippus/Miohippus Oligocene Molarized premolars, tridactyl feetEquinae Miocene- Cement-covered, high-crowned cheek Recent teeth; springing foot.Equus Pliocene- Evolution of the passive stay Recent apparatus.(MacFadden, 1992, p. Simpson's (1951) seminal work in this field is thereforeaugmented and expanded by that of contemporary researchers such as Hulbertand MacFadden. Of these groups, it wasfrom Anchitherium that the ancestors of Equus were identified. He argued that a distinct process known astransformation leading to specialization can be identified in the fossilrecord. NorthAmerica is the ancestral home of horses and many fossil sites across thecontinent have been identified as containing abundant remains of ancientmembers of the family. Both periods are considered to be a time oflow equine diversity, depicted as a simple anagenetic progression of generafrom Mesohippus to Miohippus representing a single trunk of thephylogenetic tree. (1999). However, from about 2 -1 million years ago, the fossilrecord indicates that horses evolved rapidly, adapting to variousenvironments and ways of life. DiscussionAnagenesis and Cladogenesis Hulbert (1996) states that paleontologists have been analyzing theequid fossil record for well over 15 years. The Origin and Influence of the ThoroughbredHorse. MacFadden (1992) believes that recent work has elucidated the species-level systematics of middle-Miocene horse phylogeny and been responsiblefor a revised interpretation of the evolutionary patterns of an importantadaptive radiation. MacFadden (1999)suggested that the emergence of high-crowned teeth in horses represented anirreversible evolutionary change, though Morell (1999) points out that thisis a somewhat controversial conclusion. Morell, V. MacFadden (1994) believes that the coexistence of so many species ofsimilar ancestry and general adaptive traits in the same ecosystemssuggests that horses divided up the niches and resources available to them. What this suggests, in the view of many professionals, is that acladogenetic or branching speciation took place during the Eocene andcontinued into the Oligocene, with rapid diversification becomingcharacteristic of the early Miocene (Hulbert, 1996). Foot and toe evolution in different branches of the family werenot revealed as an orthogenetic process in the fossil record. Nevertheless, the great diversity of species in the fossil record hasconvinced paleontologists that cladogenetic branching was going on apace inthis particular era. References Gould, S.J. (1996). MacFadden (1994) states that at any given fossillocality in North America from about 5 -55 to 2 -25 million years ago, itis usually possible to find 2-4 species of horses that seem to have livedside-by-side. MacFadden (1999) has suggested that one of the critical explanationsfor the evolutionary pattern observed in horses in the North Americanfossil record relates to diet. Simpson (1951) also rejected the idea that horse evolution was atotally random process. New York: Roberts Rinehart, 11 -24. 182). Contrary to Cope's Rule - in whichan increase in body size over time results in descendant species beinglarger on average than their ancestors - the fossil record of the Miocenesuggests that species size was not a linear progression in terms ofdevelopment. Hulbert (1996) also points out that Miohippus was the genera fromwhich the most striking example of cladogenetic branching can be observed.By the early Miocene, Kalobatippus, Anchitherium, and Desmatippus, alongwith Parahippus and Archaeohippus had all emerged. Thus, the following discussion will identify a cladogeneticbranching pattern in the North American equine fossil record, beginning inthe late Eocene and continuing into and beyond the Oligocene.The Emergence of Speciated Branching Ridgeway (1972) has claimed that it was only at a comparatively lateepoch in the history of mammals that the ancestors of the horse made theirfirst appearance. MacFadden (1992) is convinced that the advent of hypsodonty - high-crowned dentition - in Miocene horses allowed them to invade a new adaptivezone. The connection betweenobserved tooth crown height and interpreted diet of extinct horses is awidely accepted model of long-term adaptation and evolution. Additionally,improved eye placement is believed to have been linked to improvements inthe vision of these early horses. This suggests that fairly significantcladogenetic branching was in fact taking place during this evolutionaryperiod. Fossil remains ofhorses are found abundantly in deposits of the most recent geological agein almost every part of America and Ridgeway (1972) believes that in pre-glacial times, North America possessed at least nine perfectly distinctwild species of Equidae. While Simpson (1951) and other early paleontologists did discusscladogenesis, it is only recently that cladogenesis has achieved prominencebecause of what MacFadden (1996) characterizes as its importance insystematic methodologies and evolutionary models. In both the New World and the Old World, the advent of a new climaticphase in the late Miocene period is said to have been responsible for theextinction of four genera and ten species of horses in the first event andfour more genera and six more species in the second event (Hulbert, 1996).Only three horse species survived, eliminating the maximum diversityobserved in the late Miocene. When one compares different fossil recordsor equid clades found across North America, one finds enormous simultaneousvariations in tooth-crown height. Their numerical superiority in the fossil record notwithstanding,it was Equus which would eventually assume primacy among all horse species. (1951). MacFadden(1999) also believes that the fossil record supports varied patterns infood consumption, ranging from grazing to browsers with correspondingshifts in tooth-crown height. The latter two were much moreadvanced than the Haplohippus and are among the best known of the fossilhorses. Evidence obtainedfrom the fossil record at Costillo Pocket in south-central Coloradoprovides evidence of two disparate species, one larger and one smaller. He also suggested that parallel and convergent evolution tookplace simultaneously and that selected quantum shifts involving adaptiverelationships also influenced this evolutionary process (Simpson, 1951).Simpson (1951) also argued that the more progressive horses of the middleOligocene and all the horses of the late Oligocene are placed by conventionin a single genus, Miohippus, but that Miohippus and Mesohippus intergradeso perfectly and the differences between them are very slight. MacFadden B.J. Fossil horses underwent an explosiveadaptive radiation resulting in numerous clades with high-crowned teeth forgrazing on abrasive plants, particularly grasses. However, MacFadden (1992) has convincingly argued that despitethe inherent limitations of the fossil record, it is becoming increasinglyobvious that during the second half of horse evolution, represented by theadaptive radiation of hypsodont forms, the pattern was decidedlycladogenetic. Another theory is that an infestationof parasites of various types may have led to the extinction of the NorthAmerican Equidae, which may most probably have crossed into Asia prior tosuch an infestation. Hedefines "anagenesis" as associated with macroevolution or phyletictransformation from ancestral to descendant species. A distinctdepression or pit known as a facial fossa were present on the side of theskull in front of the opening for the eye. A combination ofnatural climate changes leading to increased global aridity and lessproductive land ecosystems is generally associated with the end ofbranching. The report focuses, therefore, on the North American Eoceneand Oligocene fossil records. (1972). New York: Oxford University Press.----------------------- 14 Cladogenesis, orbranching as opposed to phyletic speciation, is the dominant mode ofspeciation in rapidly evolving clades, particularly during an adaptiveradiation (MacFadden, 1992). Hoofed animals began to occur in the Tertiary period.With two extinct families of the Perissodactyles, the earliest ancestralforms of the horses and tapirs of today can be observed. Natural History,96 (4), 16 - 24. (1999). A Cladogenetic View of the Early Evolutionary History of Horses Abstract Drawing upon the theoretical and empirical literature, this reportdescribes evidence obtained from the fossil record supporting theanagenetic view of the horse phylogeny and its early evolutionarydevelopment. HorsesThrough Time. Startingabout eight million years ago, horse diversity dropped dramatically.MacFadden (1994) identifies the fossil record from this time presenting aportrait of a return to pre-Miocene levels of only three to five species atany given fossil locality in the Northern Hemisphere. Between 1973 and 1987,paleontologists named an average of three new species of fossil horses peryear. MacFadden (1992) identified thekey morphological characters recognized within the evolution of the familyEquidae. Hulbert (1996) states that at the beginning of the Eocene epoch,about 57 million years ago, tiny "dawn horses" known as hyracotheresappeared in the faunas of North America and Eurasia. It also demonstrates that as the fossil recordyields more and more of its "secrets" to paleontologists and newinvestigative technologies, established theories of necessity requirerevision. D.Mexicanus, for example, had high tooth crowns but teeth marked as would beanticipated in a browser rather than a grazer. What occurred was precisely the kind of branching thatis associated with cladogenetic evolution. Many palentological interpretations are controversial, withcontending and alternative hypotheses and theories coming into vogue andthen falling by the wayside as new fossil evidence is uncovered.Throughout the course of the twentieth century, the trail of horseevolution has followed what Hulbert (1996) calls a two-track system. However, the literature also suggests thatanagenetic development was dominant within the Eocene and Oligocene, whilegenetic diversity (branching) suggesting a cladogenetic developmentalpattern is observed in the fossil record from the second half of horsedevelopment. The fifth digit of the forefoot was lost andthere were three toes on both the forefoot and hindfoot. At the beginning of the late Eocene, contemporary paleontologists,including Hulbert (1996) and MacFadden (1992; 1999) state that three newtypes of horses appeared in North America. Sensory developments of this kind - alongwith improvements in the olfactory and auditory senses - took place asbranching occurred. MacFadden (1996) departsfrom Simpson (1951) by emphasizing cladogenetic evolution, and thisdeparture has become a standardized interpretation of horse evolution. This discussion not only serves to explicate the thesis advanced inthe outset of the report with respect to the appropriateness of thecladogenetic interpretation. Horses. Science, 283 (54 3), 824 - 828. Hulbert (1996) claims that we have a wealth of evidence from theincredibly fossilferous rocks of late-Eocene and Oligocene age in SouthDakota and adjacent states.
If this paper is not what you are looking for, you can search again:
or
We can write a Custom Essay just for you.
Browse Essays by Subject