Användare:Sannab/Hästdjurens utveckling

Wikipedia

Hästens utveckling med rekonstruktioner av fossila arter från successiva strata. Diagrammen över fötter visar alla vänster fot framifrån. The third metacarpal is shaped throughout. The teeth are shown in longitudinal section. It should be noted that the animals shown are only selected examples—the evolution of horses was by no means a linear process.

[redigera] Eocen och oligocen: Tidiga hästdjur

[redigera] Hyracotherium

Hyracotherium, även kallad "eohippus".

Det tidigaste djuret med hästlik anatomi var Hyracotherium. Dess vetenskapliga namn kommer från den förvirring som inledningsvis rådde över hur tidiga ofullständiga fossil var relaterade till levande arter: Richard Owen beskrev fossil av Hyracotherium som att de liknade "to a hare in one passage and to something between a hog och a hyrax in another" (som en hare på ett ställe och som något mittemellan ett svin och en hyrax på ett annat).^[1] Ett senare namn för Hyracotherium - "eohippus" (gryningshäst) blev också populärt, men Hyracotherium har fått företräde då denna benämning är äldre.^[2]

Hyracotherium uppstod under tidig eocen (54–34 miljoner år sedan). Det var ett djur ungefär av en rävs storlek (250–450 millimeter hög), med ett relativt kort huvud och nacke samt med en krökt rygg. Den hade 44 tänder med låg krona, placerad på ett typiskt sätt för ett allätande betande däggdjur: tre framtänder, en hörntand, fyra premolarer (främre kindtänder) och tre molarer (bakre kindtänder) på var sida av käken. Molarerna var ojämna och knöliga och användes huvudsakligen för att mala sönder lövverk. Molarernas uddar hängde samman något.Hyracotherium åt mjukt lövverk och frukt, och slank förmodligen från buskage till buskage på ett sätt som påminner om dagens muntjak; arten hade en liten hjärna, i synnerhet framloberna var mycket små.^[3]

Hyracotherium, with left forefoot (third metacarpal colored) och tooth (a enamel; b dentin; c cement) detailed.

Benen var relativt långa i förhållande till kroppen och visar redan på en början till en anpassning till löpning. However, all of the major leg bones were unfused, leaving the legs flexible och rotatable. Its wrist och hock joints were low to the ground. På frambenen fanns fem tår, av vilka enbart fyra hade små proto-hovar; den största femte "stortån" vidrörde inte marken. På bakbenen had tre av de fem tårna hovar medan de kvarvarande resterna av första och femte tån ej rörde vid marken. Den hade trampdynor som till exempel en hund, men med små hovar på varje tå istället för klor.

Under en period av runt 20 miljoner år frodades Hyracotherium utan att några större evolutionära förändringar inträffade. Det största förändringen var tänderna som började anpassa sig till Hyracotherias förändrade diet när dessa tidiga hästdjur började beta i allt större utsträckning. Under eocen så delades en art av Hyracotherium (troligen Hyracotherium vassacciense) upp i flera olika typer av hästdjur. Tusentals kompletta fossiliserade skelett av dessa djur har frilagts i nordamerikanska strata från eocen; mestadels i dalen runt Wind River i Wyoming. Liknande fossil av hästdjur har också upptäckts i Europa, som Propalaeotherium (som inte anses vara en förfader till den moderna hästen).^[4]

[redigera] Orohippus

Approximately 50 miljoner years ago, in the early-to-middle Eocene, Hyracotherium smoothly transitioned into Orohippus over a gradual series of changes.^[4] Although its name means "mountain horse", Orohippus did not live in the mountains. It resembled Hyracotherium is size, but had a slimmer body, an elongated head, och slimmer forelimbs och longer hind legs, all of which are characteristics of a good jumper. Although Orohippus was still pad-footed, the vestigial outer toes of Hyracotherium were not present in the Orohippus; there were four toes on each forelimb, och three on each hind leg.

The most dramatic change between Hyracotherium och Orohippus was in their teeth: the first of the premolar teeth were dwarfed, the last premolar shifted in shape och function into a molar, och the crests on the teeth became more pronounced. Both of these factors gave the teeth of Orohippus greater grinding ability, suggesting that Orohippus was subsisting on tougher plant material.

[redigera] Epihippus

In the mid-Eocene, about 47 miljoner years ago, Epihippus, a species which continued the evolutionary trend of increasingly efficient grinding teeth, evolved from Orohippus. Epihippus had five grinding, low-crowned cheek teeth with well-formed crests. A late form of Epihippus, sometimes called Duchesnehippus, had teeth similar to oligocen equids, although slightly less developed. Whether Duchesnehippus was a subspecies of Epihippus or a single species is disputed.

[redigera] Mesohippus

Mesohippus, an equid more adapted to the changing climate of the early Oligocene.

In the late eocen och the early stages of the oligocen epoch (32–24 miljoner years ago), the climate of North America became drier, och the earliest grasses began to evolve. The forests were yielding to flatlands, home to grasses och various kinds of brush. In a few areas these plains were covered in sand, creating the type of environment resembling the present-day prairies.

In response to the changing environment, equids, too, began to change. In the late Eocene, they began developing tougher teeth och becoming slightly larger och leggier, allowing for faster running speeds in open areas, och thus for evading predators in non-wooded areas. About 40 miljoner years ago, the Mesohippus ("middle horse") suddenly developed in response to strong new selective pressures to adapt, beginning with the species Mesohippus celer och soon followed by Mesohippus westoni.

Mesohippus diagram.

In the early Oligocene, Mesohippus was one of the more widespread mammals in North America. It walked on three toes on each of its front och hind feet (the first och fifth toes remained, but were small och not used in walking). The third toe was stronger than the outer ones, och thus more weighted; the fourth front toe was diminished to a vestigial nub. Judging by its longer och slimmer limbs, Mesohippus was an agile animal.

Mesohippus was slightly larger than Epihippus, about 610 mm (24") at the shoulder. Its back was less arched, och its face, snout, och neck were somewhat longer. It had significantly larger cerebral hemispheres, och had a small, shallow depression on its skull called a fossa, which in later horses became quite detailed, och serves as a useful marker for identifying an equine fossil's species. Mesohippus had six grinding "cheek teeth", with a single premolar in front—a trait all later equid species would retain. Mesohippus also had the sharp tooth crests of Epihippus, improving its ability to grind down tough vegetation.^[5]

[redigera] Miohippus

Around 36 miljoner years ago, Soon after the development of the Mesohippus, the Miohippus ("lesser horse") emerged, the earliest species being Miohippus assiniboiensis. Like Mesohippus, Miohippus's evolution was relatively abrupt, though a few transitional fossils linking the two genera have been found. It was once believed that the Mesohippus had anagenetically evolved into the Miohippus by a gradual series of progressions, but new evidence has shown that Miohippus's evolution was cladogenetic: a Miohippus population split off from the main Mesohippus genus, coexisted with Mesohippus for around 4 miljoner years, och then over time came to replace Mesohippus.^[6]

The Miohippus was significantly larger than its predecessors, och its ankle joints had subtly changed. Its facial fossa was larger och deeper, och it also began to show a variable extra crest in its upper cheek teeth, a trait that became a characteristic feature of later equid teeth.

The Miohippus ushered in a major new period of equid diversification.^[2] While Mesohippus died out in the mid-Oligocene, Miohippus continued to thrive, och in the early miocen (24–5.3 miljoner years ago), it began to rapidly diversify och speciate. It branched out into two major groups, one of which adjusted to the life in forests once again, while the other remained suited to life on the prairies.^{[källa behövs]}

[redigera] Miocen och pliocen: egentliga hästdjur

[redigera] Kalobatippus

The forest-suited form was Kalobatippus (or Miohippus intermedius, depending on whether it was a new genus or species), whose second och fourth front toes were long, well-suited travel on the soft forest floors. Kalobatippus travelled to Asien via the Bering Strait land bridge, och from there to Europa, where its fossils were formerly described under the name Anchitherium. Kalobatippus is believed to be ancestral to another Europaan species known as Hyohippus, which became extinct near the beginning of the Pliocene.Mall:Citeneeded

[redigera] Parahippus

The Miohippus population that remained on the steppes is believed to be ancestral to Parahippus, a North American animal about the size of a small pony, with a prolonged skull och a facial structure resembling the horses of today. Its third toe was stronger och larger, och carried the main weight of the body. Its four premolars resembled the molar teeth och the first were small och almost nonexistent. The incisive teeth of Parahippus, like those of its predecessors, had a crown as humans do; however, the top incisors had a trace of a shallow crease marking the beginning of the core/cup.

[redigera] Merychippus

Merychippus, an effective grazer och runner.

In the middle of the miocen epoch, an animal called Merychippus was alive. Merychippus had wider molars than its predecessors, which are believed to have been used for crunching the hard grasses of the steppes. The hind legs, which were relatively short, had side toes equipped with small hooves, but they probably only touched the ground when running.^[2]

[redigera] Hipparion

Three new equids are believed to be descended from the numerous varieties of Merychippus: Hipparion, Protohippus och Pliohippus. The most different from Merychippus was Hipparion. The main difference was in the structure of tooth enamel: in comparison with other equids, the inside, or tongue side, had a completely isolated parapet. A complete och well-preserved skeleton of the North American Hipparion shows an animal the size of a small pony. They were very slim, rather like antelopes, och were adapted to life on dry prairies. On its slim legs, Hipparion had three toes equipped with small hooves, but the side toes did not touch the ground.

An American form of Hipparion, also known as Neohipparion, proliferated into many kinds of equids several of which managed to migrate to Asien och Europa during the pliocen epoch. (The Europaan Hipperia differs from the American Hipparion in the smaller body size – the best-known discovery of these fossils was near Athens.)

Recent research suggests that Hipparion is an ancestor of the zebra och the donkey, rather than the horse.

[redigera] Pliohippus

Pliohippus uppstod ur Calippus under mellersta miocen för omkring 15 miljoner år sedan. Pliohippus påminde i hög grad om Equus, men den hade två långa extratår på båda sidor om hoven, som på utsidan knappt kunde ses som valkiga stumpar. De långa och slanka benen hos Pliohippus skvallrar om ett snabbfotat stäppdjur.

Tills helt nyligen troddes Pliohippus vara den moderna hästens förfader eftersom de anatomiska likheterna är så många. Även om Pliohippus helt klart är nära släkt med Equus så antyder vissa drag på skallen (till exempel en djup fossa - ränna) och de krökta tänderna (den moderna hästen har mycket raka tänder) att den inte är en förfader till Equus. Troligen är Pliohippus istället ursprunget till Astrohippus.^[7]

[redigera] Dinohippus

Dinohippus var det vanligaste hästdjuret in Nordamerika under yngre pliocen. Från början trodde man att Dinohippus var ett entåigt hästdjur men en fossil som återfanns 1981 visar att den var tretåig.

[redigera] Plesippus

Plesippus anses ofta vara en övergångsfas mellan Dinohippus och den moderna hästen Equus.

I slutet av pliocen blev klimatet i Nordamerika betydligt svalare och många djur tvingades söderöver. En grupp av Plesippus tog sig över till Sydamerika och en annan över Berings sund till Asien och Europa. Några blev kvar i södra Nordamerika.

I Sydamerika utvecklades Hippidium ur Plesippus. Hippidium hade relativt korta ben och en djupt nedsjunken nässkåra , mycket tunna och känsliga näsborrar och långa ectoflexider i underkäkens premolarer. Hippidum levde kvar i Sydamerika en mycket lång tid, men dog så småningom ut.

[redigera] Moderna hästar

[redigera] Equus

The oldest species of "true" horse, Equus stenonis, was discovered in Italy, och is believed to have evolved from Plesippus-like animals at the end of the Tertiary or beginning of the Quaternary periods. Equus stenonis proliferated into two branches, one lighter in body mass och one heavier.

Equus stenonis crossed into North America, where similar forms known as Equus scotti are common; some types (Equus scotti var. giganteus) exceeded the modern horse in size. However, all the horses in North America ultimately became extinct, approximately 11,000 years ago, perhaps due to climate change or some pandemic. It has also been suggested that humans hunted horses to extinction, as the appearance of humans in the Americas occurred at about the same time as the extinction of most large mammals in the Americas. However, there are no known kill sites of pleistocen horses in North America, och so this scenario remains unsupported.^[8]

Recent studies by a team of geneticists headed by C. Vila indicate that the horse line split from the zebra/donkey line between 4 och 2 miljoner years ago. Equus ferus, ancestor species to Equus caballus, appeared 630 000 to 320 000 years bp. Equus caballus was formed from several subspecies of Equus ferus by selective breeding widely over Eurasia for an extended time. The details of this process are currently a target of research by archaeologists och geneticists.

The Spanish began to import Iberian horses to breeding ranches on Cuba, Haiti, och other large islands offshore of the Americas beginning with Columbus' second voyage in 1493. The first Conquistador horses to land on the main continent were most likely spare stallions och expendable(infertile) mares from these island ranches. Later, as Spanish missions were founded on the main land, horses och cattle would eventually be lost, och would proliferate into large feral herds.

At the end of the 15th century, when the first Europaans came to America, no native horses were reported observed by them in the regions where they landed, or as they began to explore the interior. The natives of post-Colombian Mexico och Peru did not have a specific name for the animal, calling it in their language a "sun dog" or "deer". In one incident, a lame horse was left behind by a Spanish expedition to be cared for by a local tribe. It was reported that the indigenous people of that area attempted to feed it meat, och were surprised that it preferred to graze on vegetation instead. For these reasons, it has long been assumed that no one, anywhere, on the continent had ever seen a horse before. Though, to this day, some Native Americans (in particular those of the northwestern United States, och southwestern Canada) by way of their oral traditions, disagree.

[redigera] Detaljer

[redigera] Tår

Hästens förfäder utvecklades till att enbart gå på spetsen av den tredje tån och båda sidotårna. Skelettrester visar uppenbart slitage på baksidan av båda sidorna av mellanhandsbenen på både främre och bakre extremiteter. Dessa är rester av den andra och fjärde tån. Dessa rester finns kvar ännu hos moderna hästar och i motsats till en vanlig missuppfattning så spelar de stor roll för att stödja knälederna med mera.

It is not unheard of that foals are occasionally born with three toes equipped with hooves. This is called a phylogenetic atavism caused by arrested development at a certain embryonic stage.

[redigera] Tänder

Throughout the phylogenetic development, the teeth of the horse underwent significant changes. The type of the original omnivorous teeth with short, "bumpy" molars, with which the prime members of the evolutionary line distinguished themselves, gradually changed into the teeth common to herbivorous mammals. They became long (as much as 100 mm), roughly cubical molars equipped with a flat grinding surface. In conjunction with the teeth, during the horse’s evolution the elongation of the facial part of the skull is apparent, och can also be observed in the backward set eyeholes. In addition, the relatively short neck of the equine ancestors became longer with equal elongation of the legs. Finally, the size of the body grew as well.

[redigera] Källor

• Delar av denna artikel är baserad på en översättning från engelska Wikipedias artikel Evolution of the horse

[redigera] Noter

1. ^ Engelska Wikipedia anger följande som källa:Gould, Stephen Jay (1991). "The Case of the Creeping Fox Terrier Clone." Bully for Brontosaurus: Reflections in Natural History (pp. 155–167). New York: W.W. Norton & Co.
2. ^ [a b c] Engelska Wikipedia anger följande som källa:Fossil Horses In Cyberspace. Florida Museum of Natural History och National Science Foundation.
3. ^
4. ^ [a b] Engelska Wikipedia anger följande som källa:MacFadden, B. J. (1976). "Cladistic analysis of primitive equids with notes on other perissodactyls." Syst. Zool. 25(1):1-14.
5. ^ Engelska Wikipedia anger följande som källa:Däggdjurens tid: Hästar
6. ^ Engelska Wikipedia anger följande som källa:Prothero, D.R. och Shubin, N. (1989). "The evolution of oligocen horses." The Evolution of Perissodactyls (pp. 142–175). New York: Clarendon Press.
7. ^ Engelska Wikipedia anger följande som källa:MacFadden, B. J. (1984). "Astrohippus and Dinohippus". J. Vert. Paleon. 4(2):273-283.
8. ^ Engelska Wikipedia anger följande som källa:Vilà, Carles; Leonard, Jennifer A. Leonard; Götherström, Anders; Marklund, Stefan; Sandberg, Kaj; Lidén, Kerstin; Wayne, Robert K.; Ellegren, Hans (2001). "Widespread Origins of Domestic Horse Lineages". Nature Vol. 291 (pp. 474–477).