Ornitorrinco monotrema

¿Qué son las monotremas y los marsupiales?

¿Qué animales son las monotremas?

Son animales paradoxales. Son indiscutiblemente mamíferos, porque tienen el cuerpo cubierto de pelo y glándulas mamarias, pero son más primitivos ya que, como los reptiles, ponen huevos y tienen una única abertura terminal para el canal alimenticio y el aparato urogenital (del griego, monotrema significa «con una sola abertura»).

¿Dónde viven las monotremas?

Actualmente viven en Australia, Tasmania y Nueva Guinea, y comprenden seis especies: el ornitorrinco (Ornitorhynchus anatinus) y los equidnas, pertenecientes al género Tachyglossus y Zaoglossus.

¿Qué animales son los marsupiales?

Tras las monotremas, los marsupiales representan la segunda etapa en la historia evolutiva de los mamíferos: noo ponen huevos pero conservan en el cuerpo los embriones en desarrollo; después de un cierto período, cuando los embriones son todavía pequeños, se transfieren a un bolsillo cutájneo situado sobre el abdomen (algunas veces sobre el dorso) de la madre, en el que son amamantados y finalizan su desarrollo. Este bolsiollo cutáneo es llamado marsupio, nombre del que deriva el de marsupiales, único orden de la subcvlase de los metaterios.

¿Cómo han evolucionado los marsupiales?

Hacia el final de la Era Secundaria los marsupiales se extendían por los dos enromes «supercontinentes» que comprendían entonces gran parte de las tierras emergidas, Laurasia y Gondwana, mientras que los placentados, más recientes, estaban todavía poco difundidos. Cuando Austrealia se separó del gran bloque de Gondwana solo vivían los marsupiales, que se adaptaron a todos los ambientes y ocuparon todos los «nichos ecológicos» disponibles: así han aparecido marsupiales con aspecto de carnívoros, de roedores, de insectívoros, de osos hirmigueros; y especies capaces de correr, de saltar, de nadar, de trepar por los árboles, de excavar e incluso de realizar vuelos planeados.

marsupial canguro

¿Qué longitud tiene un canguro al nacer?

Sólo pocos milímetros y presenta únicamente un esbozo de las extremidades posteriores. Las patas anteriores están más desarrolladas, con ellas los pequeños trepan por la piel materna hasta alcanzar un pezón.

zarigüella

¿Exixten marsupiales fuera de Oceanía?

En los otros contintentes los marsupiales han sido «aplastados» por la competencia, excepto en la América meridional, donde los placentados avanzaban menos que en otros lugares. En ese continente se encuentran todavía algunas especies de marsupiales, la mas difundida de las cuales es la zarigüella.

 

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