De Wikipedia, la enciclopedia libre
Saltar a navegación Saltar a búsqueda

Los amniotes son un clado de vertebrados tetrápodos que comprenden saurópsidos (incluidos reptiles y aves ), sinápsidos (incluidos mamíferos ) y sus antepasados, que se remontan (pero no incluyen) a los anfibios. Se distinguen por una membrana ( amnios ) que protege al embrión y la falta de un estadio larvario . [5] Gracias a esto, los amniotes ponen huevos en tierra o los retienen dentro de la madre, a diferencia de los anamniotes ( peces y anfibios), que normalmente pone huevos en el agua. Fuentes más antiguas, particularmente antes del siglo XX, pueden referirse a los amniotas como "vertebrados superiores" y a los anamniotas como "vertebrados inferiores", basándose en la idea desacreditada de la gran cadena evolutiva del ser . El término amniote viene del griego ἀμνίον amnios , "membrana que rodea al feto", y "tazón en el que la sangre de los animales sacrificados fue capturado" anteriormente, desde ἀμνός Amnos , "cordero". [6]

El amnios comprende varias membranas extensas. En los mamíferos euterios (como los humanos), estos incluyen el saco amniótico que rodea al feto . El amnios es una divergencia crítica dentro de los vertebrados que permite que los embriones sobrevivan fuera del agua. Esto permitió que los amniotas se reprodujeran en tierra y, por lo tanto, se trasladaran a ambientes más secos, sin la necesidad de volver al agua para reproducirse como anfibios. Los huevos también pueden "respirar" y hacer frente a los desechos, lo que permite que los huevos y los individuos evolucionen hacia formas más grandes.

Los primeros amniotas, denominados "amniotas basales", se parecían a los pequeños lagartos y evolucionaron a partir de los reptilomorfos anfibios hace unos 312 millones de años, [7] en el período geológico Carbonífero . Los amniotes se extendieron por la tierra de la Tierra y se convirtieron en los vertebrados terrestres dominantes. [7] Pronto se separaron en sinápsidos y saurópsidos , que persisten en la actualidad. El sinápsido fósil más antiguo conocido es Protoclepsydrops de hace unos 312 millones de años, [7] mientras que el saurópsido más antiguo conocido es probablemente Paleothyris , en el orden Captorhinida , delÉpoca de Pensilvania Media (c. 306-312 millones de años).

Descripción [ editar ]

Anatomía de un huevo amniótico:
  1. Cáscara de huevo
  2. Membrana externa
  3. Membrana interna
  4. Chalaza
  5. Exteriores albúmina (albúmina fina exterior)
  6. Albúmina media (albúmina gruesa interna)
  7. Membrana vitelina
  8. Núcleo de Pander
  9. Disco germinal ( blastodermo )
  10. Yema amarilla
  11. Yema blanca
  12. Albúmina interna
  13. Chalaza
  14. Celda de aire
  15. Cutícula
Diagrama de huevo de cocodrilo:
  1. cáscara de huevo
  2. saco vitelino
  3. yema (nutrientes)
  4. vasos
  5. amnios
  6. corion
  7. espacio aéreo
  8. alantoides
  9. albúmina (clara de huevo)
  10. Saco amniótico
  11. embrión de cocodrilo
  12. líquido amniótico

Los zoólogos caracterizan los amniotas en parte por el desarrollo embrionario que incluye la formación de varias membranas extensas, el amnios , corion y alantoides . Los amniotas se desarrollan directamente en una forma (típicamente) terrestre con extremidades y un epitelio estratificado grueso (en lugar de entrar primero en una etapa de larva de renacuajo de alimentación seguida de metamorfosis , como lo hacen los anfibios ). En los amniotas, la transición de un peridermo de dos capas a un epitelio cornificado es provocada por la hormona tiroidea durante el desarrollo embrionario, más que por la metamorfosis.[8] Las características embrionarias únicas de los amniotas pueden reflejar especializaciones para que los huevos sobrevivan a ambientes más secos; o el aumento en el tamaño y el contenido de yema de los huevos puede haber permitido y coevolucionado con el desarrollo directo del embrión a un tamaño grande.

Adaptaciones para la vida terrestre [ editar ]

Las características de los amniotas desarrollados para sobrevivir en la tierra incluyen una cáscara de huevo dura, coriácea o dura, resistente pero porosa, y un alantoides que facilita la respiración al mismo tiempo que proporciona un depósito para la eliminación de desechos. Sus riñones e intestino grueso también son adecuados para la retención de agua. La mayoría de los mamíferos no ponen huevos, pero las estructuras correspondientes se desarrollan dentro de la placenta .

The ancestors of true amniotes, such as Casineria kiddi, which lived about 340 million years ago, evolved from amphibian reptiliomorphs and resembled small lizards. At the late Devonian mass extinction (360 million years ago), all known tetrapods were essentially aquatic and fish-like. Because the reptiliomorphs were already established 20 million years later when all their fishlike relatives were extinct, it appears they separated from the other tetrapods somewhere during Romer's gap, when the adult tetrapods became fully terrestrial (some forms would later become secondarily aquatic).[9] The modest-sized ancestors of the amniotes laid their eggs in moist places, such as depressions under fallen logs or other suitable places in the Carboniferous swamps and forests; and dry conditions probably do not account for the emergence of the soft shell.[10] Indeed, many modern-day amniotes require moisture to keep their eggs from desiccating.[11] Although some modern amphibians lay eggs on land, all amphibians lack advanced traits like an amnion. The amniotic egg formed through a series of evolutionary steps. After internal fertilization and the habit of laying eggs in terrestrial environments became a reproduction strategy amongst the amniote ancestors, the next major breakthrough appears to have involved a gradual replacement of the gelatinous coating covering the amphibian egg with a fibrous shell membrane. This allowed the egg to increase both its size and in the rate of gas exchange, permitting a larger, metabolically more active embryo to reach full development before hatching. Further developments, like extraembryonic membranes (amnion, chorion, and allantois) and a calcified shell, were not essential and probably evolved later.[12] It has been suggested that shelled terrestrial eggs without extraembryonic membranes could still not have been more than about 1 cm (0.4 inch) in diameter because of diffusion problems, like the inability to get rid of carbon dioxide if the egg was larger. The combination of small eggs and the absence of a larval stage, where posthatching growth occurs in anamniotic tetrapods before turning into juveniles, would limit the size of the adults. This is supported by the fact that extant squamate species that lay eggs less than 1 cm in diameter have adults whose snout-vent length is less than 10 cm. The only way for the eggs to increase in size would be to develop new internal structures specialized for respiration and for waste products. As this happened, it would also affect how much the juveniles could grow before they reached adulthood.[13]

The egg membranes[edit]

Fish and amphibian eggs have only one inner membrane, the embryonic membrane. Evolution of the amniote egg required increased exchange of gases and wastes between the embryo and the atmosphere. Structures to permit these traits allowed further adaption that increased the feasible size of amniote eggs and enabled breeding in progressively drier habitats. The increased size of eggs permitted increase in size of offspring and consequently of adults. Further growth for the latter, however, was limited by their position in the terrestrial food-chain, which was restricted to level three and below, with only invertebrates occupying level two. Amniotes would eventually experience adaptive radiations when some species evolved the ability to digest plants and new ecological niches opened up, permitting larger body-size for herbivores, omnivores and predators.[citation needed]

Amniote traits[edit]

While the early amniotes resembled their amphibian ancestors in many respects, a key difference was the lack of an otic notch at the back margin of the skull roof. In their ancestors, this notch held a spiracle, an unnecessary structure in an animal without an aquatic larval stage.[14] There are three main lines of amniotes, which may be distinguished by the structure of the skull and in particular the number of temporal fenestrae (openings) behind each eye. In anapsids, the ancestral condition, there are none, in synapsids (mammals and their extinct relatives) there is one, and most diapsids (including birds, crocodilians, squamates, and tuataras), have two. Turtles were traditionally classified as anapsids because they lack fenestrae, but molecular testing firmly places them in the diapsid line of descent - they therefore secondarily lost their fenestrae.

Post-cranial remains of amniotes can be identified from their Labyrinthodont ancestors by their having at least two pairs of sacral ribs, a sternum in the pectoral girdle (some amniotes have lost it) and an astragalus bone in the ankle.[15]

Definition and classification[edit]

Amniota was first formally described by the embryologist Ernst Haeckel in 1866 on the presence of the amnion, hence the name. A problem with this definition is that the trait (apomorphy) in question does not fossilize, and the status of fossil forms has to be inferred from other traits.

Amniotes
By the Mesozoic, 150 million years ago, sauropsids included the largest animals anywhere. Shown are some late Jurassic dinosaurs, including the early bird Archaeopteryx perched on a tree stump.

Traditional classification[edit]

Classifications of the amniotes have traditionally recognised three classes based on major traits and physiology:[17][18][19][20]

  • Class Reptilia (reptiles)
    • Subclass Anapsida ("proto-reptiles", possibly including turtles)
    • Subclass Diapsida (majority of reptiles,[21] progenitors of birds)
    • Subclass Euryapsida (plesiosaurs, placodonts, and ichthyosaurs)
    • Subclass Synapsida (mammal-like reptiles, progenitors of mammals)
  • Class Aves (birds)
    • Subclass Archaeornithes (reptile-like birds, progenitors of all other birds)
    • Subclass Enantiornithes (early birds with an alternative shoulder joint)[22]
    • Subclass Hesperornithes (toothed aquatic flightless birds)
    • Subclass Ichthyornithes (toothed, but otherwise modern birds)
    • Subclass Neornithes (all living birds)
  • Class Mammalia (mammals)
    • Subclass Prototheria (Monotremata, egg-laying mammals)
    • Subclass Theria (metatheria (such as marsupials) and eutheria (such as placental mammals))

This rather orderly scheme is the one most commonly found in popular and basic scientific works. It has come under critique from cladistics, as the class Reptilia is paraphyletic—it has given rise to two other classes not included in Reptilia.

Classification into monophyletic taxa[edit]

A different approach is adopted by writers who reject paraphyletic groupings. One such classification, by Michael Benton, is presented in simplified form below.[23]

  • Series Amniota
    • Class Synapsida
      • A series of unassigned families, corresponding to Pelycosauria
      • Order Therapsida
        • Class Mammalia – mammals
    • Class Reptilia
      • Subclass Parareptilia
        • Family Mesosauridae †
        • Family Millerettidae †
        • Family Bolosauridae †
        • Family Procolophonidae †
        • Order Pareiasauromorpha
          • Family Nycteroleteridae †
          • Family Pareiasauridae †
      • Subclass Eureptilia
        • Family Captorhinidae †
        • Infraclass Diapsida
          • Family Araeoscelididae †
          • Family Weigeltisauridae †
          • Order Younginiformes †
          • Infraclass Neodiapsida
            • Order Testudinata
              • Suborder Testudines – turtles
            • Infraclass Lepidosauromorpha
              • Unnamed infrasubclass
                • Infraclass Ichthyosauria †
                • Order Thalattosauria †
                • Superorder Lepidosauriformes
                  • Order Sphenodontida – tuatara
                  • Order Squamata – lizards & snakes
              • Infrasubclass Sauropterygia †
                • Order Placodontia †
                • Order Eosauropterygia †
                  • Suborder Pachypleurosauria †
                  • Suborder Nothosauria †
                • Order Plesiosauria †
            • Infraclass Archosauromorpha
              • Family Trilophosauridae †
              • Order Rhynchosauria †
              • Order Protorosauria †
              • Division Archosauriformes
                • Subdivision Archosauria
                  • Infradivision Crurotarsi
                    • Order Phytosauria†
                    • Family Ornithosuchidae †
                    • Family Stagonolepididae †
                    • Family Rauisuchidae †
                    • Superfamily Poposauroidea †
                    • Superorder Crocodylomorpha
                      • Order Crocodylia – crocodilians
                  • Infradivision Avemetatarsalia
                    • Infrasubdivision Ornithodira
                      • Order Pterosauria †
                      • Family Lagerpetidae †
                      • Family Silesauridae †
                      • Superorder Dinosauria – dinosaurs
                        • Order Ornithischia †
                        • Order Saurischia
                          • Suborder Theropoda - theropods
                            • Class Aves – birds

Phylogenetic classification[edit]

With the advent of cladistics, other researchers have attempted to establish new classes, based on phylogeny, but disregarding the physiological and anatomical unity of the groups. Unlike Benton, for example, Jacques Gauthier and colleagues forwarded a definition of Amniota in 1988 as "the most recent common ancestor of extant mammals and reptiles, and all its descendants".[15] As Gauthier makes use of a crown group definition, Amniota has a slightly different content than the biological amniotes as defined by an apomorphy.[24]

Cladogram[edit]

The cladogram presented here illustrates the phylogeny (family tree) of amniotes, and follows a simplified version of the relationships found by Laurin & Reisz (1995),[25] with the exception of turtles, which more recent morphological and molecular phylogenetic studies placed firmly within diapsids.[26][27][28][29][30][31] The cladogram covers the group as defined under Gauthier's definition.

References[edit]

  1. ^ Paton, R. L.; Smithson, T. R.; Clack, J. A. (8 April 1999). "An amniote-like skeleton from the Early Carboniferous of Scotland". Nature. 398 (6727): 508–513. Bibcode:1999Natur.398..508P. doi:10.1038/19071. ISSN 0028-0836.
  2. ^ Irmis, R. B.; Parker, W. G. (2005). "Unusual tetrapod teeth from the Upper Triassic Chinle Formation, Arizona, USA" (PDF). Canadian Journal of Earth Sciences. 42 (7): 1339–1345. doi:10.1139/e05-031.
  3. ^ Arjan Mann; Jason D. Pardo; Hillary C. Maddin (2019). "Infernovenator steenae, a new serpentine recumbirostran from the 'Mazon Creek' Lagertätte further clarifies lysorophian origins". Zoological Journal of the Linnean Society. 187 (2): 506–517. doi:10.1093/zoolinnean/zlz026.
  4. ^ Jason D. Pardo; Matt Szostakiwskyj; Per E. Ahlberg; Jason S. Anderson (2017). "Hidden morphological diversity among early tetrapods". Nature. 546 (7660): 642–645. Bibcode:2017Natur.546..642P. doi:10.1038/nature22966. PMID 28636600. S2CID 2478132.
  5. ^ Benton, Michael J. (1997). Vertebrate Palaeontology. London: Chapman & Hall. pp. 105–109. ISBN 978-0-412-73810-4.
  6. ^ Oxford English Dictionary
  7. ^ a b c Benton, M.J.; Donoghue, P.C.J. (2006). "Palaeontological evidence to date the tree of life". Molecular Biology and Evolution. 24 (1): 26–53. doi:10.1093/molbev/msl150. PMID 17047029.
  8. ^ Alexander M. Schreiber; Donald D. Brown (2003). "Tadpole skin dies autonomously in response to thyroid hormone at metamorphosis". Proceedings of the National Academy of Sciences. 100 (4): 1769–1774. doi:10.1073/pnas.252774999. PMC 149908. PMID 12560472.
  9. ^ "the_mid_palaeozoic_biotic_crisis - Ocean and Earth Science, National Oceanography Centre Southampton - University of Southampton".
  10. ^ Stewart J. R. (1997): Morphology and evolution of the egg of oviparous amniotes. In: S. Sumida and K. Martin (ed.) Amniote Origins-Completing the Transition to Land (1): 291–326. London: Academic Press.
  11. ^ Cunningham, B.; Huene, E. (Jul–Aug 1938). "Further Studies on Water Absorption by Reptile Eggs". The American Naturalist. 72 (741): 380–385. doi:10.1086/280791. JSTOR 2457547. S2CID 84258651.
  12. ^ Shell Game » American Scientist
  13. ^ Michel Laurin (2004). "The evolution of body size, Cope's rule and the origin of amniotes". Systematic Biology. 53 (4): 594–622. doi:10.1080/10635150490445706. PMID 15371249.
  14. ^ Lombard, R. E. & Bolt, J. R. (1979): Evolution of the tetrapod ear: an analysis and reinterpretation. Biological Journal of the Linnean Society No 11: pp 19–76 Abstract
  15. ^ a b Gauthier, J., Kluge, A.G. and Rowe, T. (1988). "The early evolution of the Amniota." Pp. 103–155 in Benton, M.J. (ed.), The phylogeny and classification of the tetrapods, Volume 1: amphibians, reptiles, birds. Oxford: Clarendon Press.
  16. ^ Falcon-Lang, H J; Benton, M J; Stimson, M (2007). "Ecology of early reptiles inferred from Lower Pennsylvanian trackways". Journal of the Geological Society. 164 (6): 1113–1118. doi:10.1016/j.palaeo.2010.06.020.
  17. ^ Romer A S and Parsons T S (1985) The Vertebrate Body. (6th ed.) Saunders, Philadelphia.
  18. ^ Carroll, R. L. (1988), Vertebrate Paleontology and Evolution, WH Freeman & Co.
  19. ^ Hildebrand, M. & G. E. Goslow, Jr. Principal ill. Viola Hildebrand. (2001). Analysis of vertebrate structure. New York: Wiley. p. 429. ISBN 978-0-471-29505-1.CS1 maint: uses authors parameter (link)
  20. ^ Colbert, E.H. & Morales, M. (2001): Colbert's Evolution of the Vertebrates: A History of the Backboned Animals Through Time. 4th edition. John Wiley & Sons, Inc, New York — ISBN 978-0-471-38461-8.
  21. ^ Reeder, Tod W.; Townsend, Ted M.; Mulcahy, Daniel G.; Noonan, Brice P.; Wood, Perry L.; Sites, Jack W.; Wiens, John J. (2015). "Integrated Analyses Resolve Conflicts over Squamate Reptile Phylogeny and Reveal Unexpected Placements for Fossil Taxa". PLOS ONE. 10 (3): e0118199. Bibcode:2015PLoSO..1018199R. doi:10.1371/journal.pone.0118199. PMC 4372529. PMID 25803280.
  22. ^ *Hope, S. (2002) The Mesozoic record of Neornithes (modern birds). In: Chiappe, L.M. and Witmer, L.M. (eds.): Mesozoic Birds: Above the Heads of Dinosaurs: 339–388. University of California Press, Berkeley. ISBN 0-520-20094-2
  23. ^ Benton, M.J. (2015). "Appendix: Classification of the Vertebrates". Vertebrate Paleontology (4th ed.). Wiley Blackwell. 433–447. ISBN 978-1-118-40684-7.
  24. ^ Lee, M.S.Y. & Spencer, P.S. (1997): Crown clades, key characters and taxonomic stability: when is an amniote not an amniote? In: Sumida S.S. & Martin K.L.M. (eds.) Amniote Origins: completing the transition to land. Academic Press, pp 61–84. Google books
  25. ^ Laurin, M.; Reisz, R.R. (1995). "A reevaluation of early amniote phylogeny" (PDF). Zoological Journal of the Linnean Society. 113 (2): 165–223. doi:10.1111/j.1096-3642.1995.tb00932.x. Archived from the original (PDF) on 2019-06-08. Retrieved 2017-11-02.
  26. ^ Rieppel, O.; DeBraga, M. (1996). "Turtles as diapsid reptiles". Nature. 384 (6608): 453–5. Bibcode:1996Natur.384..453R. doi:10.1038/384453a0. S2CID 4264378.
  27. ^ Müller, Johannes (2004). "The relationships among diapsid reptiles and the influence of taxon selection". In Arratia, G; Wilson, M.V.H.; Cloutier, R. (eds.). Recent Advances in the Origin and Early Radiation of Vertebrates. Verlag Dr. Friedrich Pfeil. pp. 379–408. ISBN 978-3-89937-052-2.
  28. ^ Tyler R. Lyson; Erik A. Sperling; Alysha M. Heimberg; Jacques A. Gauthier; Benjamin L. King; Kevin J. Peterson (2012-02-23). "MicroRNAs support a turtle + lizard clade". Biology Letters. 8 (1): 104–107. doi:10.1098/rsbl.2011.0477. PMC 3259949. PMID 21775315.
  29. ^ Iwabe, N.; Hara, Y.; Kumazawa, Y.; Shibamoto, K.; Saito, Y.; Miyata, T.; Katoh, K. (2004-12-29). "Sister group relationship of turtles to the bird-crocodilian clade revealed by nuclear DNA-coded proteins". Molecular Biology and Evolution. 22 (4): 810–813. doi:10.1093/molbev/msi075. PMID 15625185.
  30. ^ Roos, Jonas; Aggarwal, Ramesh K.; Janke, Axel (Nov 2007). "Extended mitogenomic phylogenetic analyses yield new insight into crocodylian evolution and their survival of the Cretaceous–Tertiary boundary". Molecular Phylogenetics and Evolution. 45 (2): 663–673. doi:10.1016/j.ympev.2007.06.018. PMID 17719245.
  31. ^ Katsu, Y.; Braun, E. L.; Guillette, L. J. Jr.; Iguchi, T. (2010-03-17). "From reptilian phylogenomics to reptilian genomes: analyses of c-Jun and DJ-1 proto-oncogenes". Cytogenetic and Genome Research. 127 (2–4): 79–93. doi:10.1159/000297715. PMID 20234127. S2CID 12116018.