|
| |
HORSES.
Term Paper ID:30800
|
|
|
Essay Subject:
Describes the evolutionary development of horses.... More...
|
9 Pages / 2025 Words
8 sources, 43 Citations,
APA Format
$36.00
Return to List of Papers
|
Paper Abstract:
Describes the evolutionary development of horses. Evidence obtained from fossil records that supports the anagenetic view of horse phylogeny as well as cladogenetic pattern. Fossil record as a clasic example of macroevolution. Branching speciation. Support of the view that a gradual microevolutionary process occurred within the species. Focuses on the North American Eocene and Oligocene fossil records.
Paper Introduction:
A Cladogenetic View of the Early Evolutionary History of Horses
Abstract
Drawing upon the theoretical and empirical literature, this report describes the evidence obtained from the fossil record that supports 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. It is also associated with 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 heyday of horses. The latter two were much moreadvanced than the Haplohippus and are among the best known of the fossilhorses. Foot andtoe evolution in different branches of the family was not revealed as anorthogenetic process in the fossil record. The skull muzzlewas lengthened and a larger gap between the anterior nipping teeth orincisors and the posterior chewing teeth had developed. MacFadden (1992) identified thekey morphological characters recognized within the evolution of the familyEquidae. There was, saysSimpson (1951), no constant and overall increase in size or a definitivepattern of steady change from four to three and finally, one toe. Fossil Horses. 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. 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. New York: Benjamin Blom. MacFadden, B.J. Thus, the following discussion will reflect 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. Cladogenesis, orbranching as opposed to phyletic speciation, is the dominant mode ofspeciation in rapidly evolving clades, particularly during an adaptiveradiation (MacFadden, 1992). References Gould, S.J. 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. Simpson (1951) also rejectedthe idea that horse evolution was a totally random process. 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. Life's little joke. New York: Oxford University Press. The Origin and Influence of the ThoroughbredHorse. When one compares different fossil recordsor equid clades found across North America, one finds enormous simultaneousvariations in tooth-crown height. 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. Dietary data straight from the horse's mouth.Science, 283 (54 3), 773. Natural History,1 3 (4), 63 -66. Between 1973 and 1987,paleontologists named an average of three new species of fossil horses peryear. MacFadden (1992) moves forward from the work of Simpson (1951) anddifferentiates between the two major modes of species evolution and thespeciation process generally interpreted from the fossil record. HorsesThrough Time. Simpson, G.G. These species varied significantly in size, butdisappeared from America at the time of the Spanish conquest. 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. Evidence obtainedfrom the fossil record at Costillo Pocket in south-central Coloradoprovides evidence of two disparate species, one larger and one smaller. 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. However, MacFadden (1992) has successfully 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. 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. This suggests that fairly significantcladogenetic branching was in fact taking place during this evolutionaryperiod. 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. (1999). It was during the Miocene that horses diversified rapidly to include largespecies and even a few dwarf lineages. MacFadden B.J. Natural History,96 (4), 16 - 24. The fossil record, says MacFadden (1992), is an excellent mediumfor examining both anagenesis and cladogenesis. A Cladogenetic View of the Early Evolutionary History of Horses Abstract Drawing upon the theoretical and empirical literature, this reportdescribes the evidence obtained from the fossil record that supports theanagenetic view of the horse phylogeny and its early evolutionarydevelopment. Cambridge: CambridgeUniversity Press. Most significantly, in facial and bodyproportions, Mesohippus and Miohippus had much more classic horse-likefeatures than older genera. New York: Roberts Rinehart, 11 -24. Morell, V. Such an examination will demonstrate that from the Oligoceneperiod on, cladogenetic evolution of the horse was occurring. The anagenetic perspective advances the notion that a gradualmicroevolutionary process has occurred within a species. 182). Originating from the late Oligocene Miohippus, two main lines ofdescent are recognized from the early Miocene of North America. These are the generaHaplohippus, Mesohippus, and Miohippus. 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. Ridgeway, W. A distinctdepression or pit known as a facial fossa were present on the side of theskull in front of the opening for the eye. Both periods are considered to be atime of low equine diversity, depicted as a simple anagenetic progressionof genera from Mesohippus to Miohippus representing a single trunk of thephylogenetic tree. 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. (1951). Of these groups, it wasfrom Anchitherium that the ancestors of Equus were identified. The fifth digit of the forefoot was lost andthere were three toes on both the forefoot and hindfoot. Fossil horses underwent an explosiveadaptive radiation resulting in numerous clades with high-crowned teeth forgrazing on abrasive plants, particularly grasses. (1987). He argued thata distinct process known as transformation leading to specialization can beidentified in the fossil record. (1999). 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. 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. 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. He also suggests that parallel andconvergent evolution took place simultaneously and that selected quantumshifts involving adaptive relationships also influenced this evolutionaryprocess (Simpson, 1951). Clearly, this supports the idea that a cladogeneticpattern of evolution was beginning to occur. The ancestry of the horse. (1972). MacFadden (1996) departsfrom Simpson (1951) by emphasizing cladogenetic evolution. MacFadden (1999) has suggested that one of the critical explanationsfor the evolutionary pattern observed in horses in the North Americanfossil record relates to diet. The connection betweenobserved tooth crown height and interpreted diet of extinct horses is awidely accepted model of long-term adaptation and evolution. (1996). MacFadden (1994) states that 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. (1994). A combination ofnatural climate changes leading to increased global aridity and lessproductive land ecosystems is generally associated with the end ofbranching. 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. Hulbert, R.C. 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. Simpson's (1951) seminal work in this field is thereforeaugmented and expanded by that of contemporary researchers such as Hulbertand MacFadden. 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. MacFadden (1992) is convinced that the advent of hypsodonty -- high-crowned dentition -- in Miocene horses allowed them to invade a newadaptive zone. It also demonstrates that as the fossil recordyields more and more of its "secrets" to paleontologists and newinvestigative technologies, established theories of necessity requirerevision. It is alsoassociated with phyletic transformation from ancestral to descendantspecies. 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. 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. 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. 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, 1992, p. The following table presents these characters. (1992). 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. 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. Hedefines "anagenesis" as associated with macroevolution or phyletictransformation from ancestral to descendant species. Additionally, competition with cud-chewing, hoofed herbivores(e.g., deer and bison) may also have affected horse diversity. Nevertheless, the great diversity of species in the fossil record hasconvinced paleontologists that cladogenetic branching was going on apace inthis particular era. 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. Horses. 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). The report focuses, therefore, on the North AmericanEocene and Oligocene fossil records. 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. A multitude of evolutionary changes through branching anddiversification ultimately resulted in the emergence of Equus. As of twomillion years ago, only the single horse genus Equus, consisting of a fewspecies, remained in the Northern Hemisphere. Ultimately, itwas the Equinae in which significant cladogenetic branching took place(Hulbert, 1996). Science, 283 (54 3), 824 - 828. Introduction The fossil record of horses (family Equidae) over the past 55 millionyears is a classic example of macroevolution (MacFadden, 1999). However, the literature also suggests thatanagenetic development was dominant within the Eocene and Oligocene, whilegenetic diversity (branching) suggesting a cladogenetic developmentalpattern can be observed from the fossil record from the second half ofhorse development. 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. Ancient diets, ecology, and extinction of 5-million-year-old horses from Florida. The fossil record for horses presents, in general, a progressivereplacement of one genus with another without a strong indication oftemporary overlapping. Table I Key Morphological Characters, Family Equidae Age Key CharacterTaxonHyracotherium 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(1999) also believes that the fossil record supports varied patterns infood consumption, ranging from grazing to browsers with correspondingshifts in tooth-crown height. Their numerical superiority in the fossil record notwithstanding,it was Equus which would eventually assume primacy among all horse species. Earlier, MacFadden(1994) and Simpson (1951) agree that an anagenetic macroevolutionarytransformation took place, leading to limited diversity and numbers ofspecies. What occurred was precisely the kind of branching thatis associated with cladogenetic evolution. This discussion not only serves to explicate the thesis advanced inthe outset of the report with respect to the appropriateness of thecladogenetic interpretation. MacFadden, B.J. DiscussionAnagenesis and Cladogenesis Hulbert (1996) states that paleontologists have been analyzing theequid fossil record for well over 15 years. However, when the species switchedback to eating browse, the teeth did not change. However, from about 2 -1 million years ago, the fossilrecord suggests that horses evolved rapidly, adapting to variousenvironments and ways of life. Onthe one hand, paleontologists like George Gaylord Simpson (1951) arguedthat the evolution of the horse family was definitely not orthogenetic, orstraight and linear in terms of evolutionary pattern. 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. Miocene horses, says MacFadden (1994) appear to haveminimized competition for available food and space by occupying severaldifferent niches.
If this paper is not what you are looking for, you can search again:
or
Click here to request an essay written just for you.
|
|
|
