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Utente:Kalumet Sioux/Mammalia: differenze tra le versioni

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===Hadrocodium===
TheRispetto consensusai family[[Symmetrodonta|Simmetrodonti]] treeed aboveai shows[[[[kuehneotheriidae|Kueneoteriidi]], l'''[[Hadrocodium]]'' asè anconsiderato "aunt"un ofpiù truelontano mammals,parente whiledei [[Symmetrodonta|symmetrodonts]]veri andmammiferi. [[kuehneotheriidae|kuehneotheriids]]Ma arei morefossili closelydei relatedprimi todue truegruppi mammals.sono Butcosì fossilspochi ofe symmetrodontsframmentari andche kuehneotheriidsrisulta aredifficile socapirne fewle andrelazioni fragmentarysistematiche, thatcon they arel'incertezza poorlyche understoodpotrebbero andessere maydei betaxa [[paraphyleticparafiletico|parafiletici]]. OnD'altra theparte othervi handsono therebuoni arefossili good fossils ofdi ''Hadrocodium'' (aboutdatati 195Mcirca years195 agoM.d.a. infa, theall'inizio verydel early [[Jurassic]]Giurassico) and they haveche somerecano importantimportanti featurescaratteristiche:<ref>{{cite web | url=http://www.palaeos.com/Vertebrates/Units/Unit420/420.300.html | title=Symmetrodonta - Palaeos}}</ref>
*l'articolazione mascellare avviene tra lo squamoso ed il dentale, il quale, a differenza dei terapsidi, è l'unico osso della mandibola.
*The jaw joint consists only of the squamosal and dentary bones, and the jaw contains no smaller bones to the rear of the dentary, unlike the therapsid design.
*Nei terapsidi ed in molti mammaliformi la [[membrana timpanica]] si tende al di sopra di un'avvallamento della parte posteriore della mandibola. Nell'''Hadrocodium'' non si ha una simile cavità, il che suggerisce che l'[[orecchio]] era parte integrante del cranio (come nei mammiferi), con l'articolare e quadrato migrati nell'orecchio medio a formare il martello e l'incudine. Tuttavia il retro del dentale presenta un'insenatura che manca nei mammiferi. Ciò suggerisce che il suo osso dentale abbia conservato la stessa forma che avrebbe avuto se l'articolare ed il quadrato fossero rimasti a far parte dell'articolazione, e quindi che l'''Hadrocodium'' - od un suo stretto antenato - potrebbero essere stati i primi a possedere un orecchio medio di tipo mammaliano.
*In [[therapsids]] and most [[mammaliformes]] the [[eardrum]] stretched over a trough at the rear of the lower jaw. But ''Hadrocodium'' had no such trough, which suggests its ear was part of the [[cranium]], as it is in mammals - and hence that the former [[articular]] and [[quadrate]] had migrated to the middle ear and become the [[malleus]] and [[incus]]. On the other hand the dentary has a "bay" at the rear which mammals lack. This suggests that ''Hadrocodium's'' dentary bone retained the same shape that it would have had if the articular and quadrate had remained part of the jaw joint, and therefore that ''Hadroconium'' or a very close ancestor may have been the first to have a fully mammalian middle ear.
*Therapsids and earlier mammaliforms had their jaw joints very far back in the skull, partly because the ear was at the rear end of the jaw but also had to be close to the brain. This arrangement limited the size of the braincase, because it forced the jaw muscles to run round and over it. ''Hadrocodium's'' braincase and jaws were no longer bound to each other by the need to support the ear, and its jaw joint was further forward. In its descendants or those of animals with a similar arrangement, the brain case was free to expand without being constrained by the jaw and the jaw was free to change without being constrained by the need to keep the ear near the brain - in other words it now became possible for mammal-like animals both to develop large brains and to adapt their jaws and teeth in ways that were purely specialized for eating.
 
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