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Nyasasaurus Parringtoni Nesbitt Et Al., 2012 El Dinosaurio Que Estábamos Esperando

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Nyasasaurus Parringtoni Nesbitt Et Al., 2012 El Dinosaurio Que Estábamos Esperando Nyasasaurus parringtoni (Nesbitt et al ., 2012) ¿El dinosaurio que estábamos esperando? Roberto Díaz Aros Centro de Estudios Paleontológicos de Chile [email protected] RESUMEN : Encontrar un representante del grupo de dinosaurios basales que dio origen a todos los dinosaurios posteriores y más derivados, (saurisquios y ornitisquios), es una tarea que la paleontología ha tenido pendiente desde hace mucho tiempo y desde que comenzaron a aparecer en el registro de fósiles los representantes de los grupos más afines a Dinosauria dentro de Archosauria. PALABRAS CLAVES : Dinosauria, dinosaurio basal. Restos fósiles de Nyasasaurus parringtoni. (Tomado de: Nesbitt et al ., 2012) INTRODUCCIÓN afines a Dinosauria (Owen, 1842) dentro de Archosauria (Cope, 1869). Encontrar un representante del grupo de dinosaurios basales que dio origen a En relación a esta búsqueda del todos los dinosaurios posteriores y más dinosaurio ancestral, el reciente derivados, (saurisquios y ornitisquios), reporte de un dinosaurio muy antiguo es una tarea que la paleontología ha de África, nombrado como tenido pendiente desde hace mucho Nyasasaurus parringtoni (Nesbitt et al ., tiempo y desde que comenzaron a 2012), llama nuevamente la atención aparecer en el registro de fósiles los respecto al origen, evolución y representantes de los grupos más dispersión temprana de los dinosaurios y nos transporta hacia el tiempo en que 1 aparecieron las primeras poblaciones según Brusate et al ., 2010, una de las de dinosaurios en la Tierra como parte diagnosis más aceptadas para el grupo. de una radiación muy amplia del taxón 1.- Uno de estos rasgos es la cresta Dinosauriformes (Novas, 1992) durante deltopectoral que se observa en la zona el Triásico Medio, aspecto que a pesar anteroventral del tercio proximal del de todo el esfuerzo de investigación húmero de Nyasasarus y que permite desplegado hasta el momento, concluir además que el animal permanece como un pasaje evidenciaba un gran desarrollo de los pobremente conocido de la evolución músculos pectorales, comprometidos de los vertebrados. en la movilidad de la extremidad Los restos de Nyasasaurus, que anterior, con efectos probablemente comprenden un húmero derecho, tres distintos a la locomoción. vértebras presacras parcialmente Lo anterior, permite además asumir conservadas y tres vértebras sacras, con un cierto nivel de certeza que permiten concluir, de acuerdo a la data Nyasasaurus era un animal bípedo y de antiguedad asignada a estos restos que evidenciaba una alometría negativa fósiles (Anisiano, Triásico Medio), que para la extremidad anterior, rasgo que estamos ante el dinosaurio más antiguo también se deja ver de manera clara en del cual se tiene conocimiento, dinosaurios basales, reconociéndose pudiendo eventualmente corresponder que la bipedia es el rasgo plesiomórfico a un representante de las poblaciones para Dinosauria y que la cuadrupedia, de dinosaurios basales de las que es un rasgo derivado que se desarrolla derivaron todos los dinosaurios en dinosaurios posteriores a partir de posteriores (saurisquios y ornitisquios). bípedos basales en grupos como ¿NYASASAURUS ES UN DINOSAURIO? Sauropodomorpha (Huene, 1932) y Ornithischia (Seely, 1888). Los rasgos anatómicos de Nyasasaurus, que se desprenden de los pocos restos 2.- Igualmente, la presencia de tres fósiles conservados, permiten concluir vértebras en el sacro de Nyasasaurus , con un rango aceptable de certeza que constituye un rasgo diagnóstico de estamos en presencia de un verdadero Dinosauria, y que se establece de dinosaurio o de un grupo filogenético manera definitiva en el grupo a partir muy estrechamente relacionado a los de saurisquios y ornitisquios, aunque dinosaurios y mucho más cercano a algunas formas más muy antiguas Dinosauria que cualquier otro dentro de dinosaurios, presentan sólo dinosauriforme hasta ahora conocido, dos vértebras sacras, dejando ver que en el entendido que dichos rasgos aún retenían un rasgo plesiomórfico y anatómicos forman parte de los característico de Archosauria. elementos diagnósticos de Dinosauria 2 La presencia de dos vértebras en el La presencia de tres vértebras en el sacro se deja ver a lo largo de todo el sacro, a su vez obliga la expansión grupo Archosauria, aunque formas más anteroposterior del ilión, rasgo también afines a Dinosauria como Silesaurus diagnóstico de Dinosauria, y que según opolensis (Dzik, 2003) evidencian tres se desprende de las tres vértebras vértebras en el sacro. Sin embargo, sacras de Nyasasaurus, también estaba, Herrerasaurus ischigualastensis (Reig, como rasgo diagnóstico, presente en 1963) y Staurikosaurus pricei (Colbert, este taxón. 1970), dos dinosaurios muy antiguos, 3.- La presencia de articulaciones que evidencian una data de antigüedad accesorias (hiposfeno – hipantro) en las de ± 230 millones de años, presentan vértebras dorsales de Nyasasaurus , sólo dos vértebras en el sacro, aunque deja ver además una adaptación es necesario decir que en anatómica que tiende a dar mayor Herreresaurus , se observa la última estabilidad a la columna vertebral y que vértebra dorsal en proceso de se deja ver ya en dinosaurios más sacralización, mientras que en derivados como Herrerasaurus por Staurikosaurus se observa la primera ejemplo. vértebra caudal en proceso de sacralización, detalle anatómico que Según la diagnosis aportada para deja ver claramente que el grupo Dinosauria por Brusatte et al., 2010 y Dinosauria estaba en vías de incorporar considerando los rasgos anatómicos definitivamente una tercera vértebra descritos para Nyasasaurus , podemos en el sacro, proceso que claramente anticipar que estamos en presencia de responde a una necesidad biomecánica un verdadero dinosaurio, cuya como resultado de la exigencia antigüedad excede a cualquier otro impuesta por una tendencia creciente a dinosaurio anteriormente conocido por adquirir un sistema de locomoción la paleontología. bípedo más definido que otros arcosaurios más basales y aparte de Desgraciadamente y dado que los Dinosauria. restos fósiles de Nyasasaurus son incompletos, faltando material La presencia de tres vértebras en el diagnóstico como el pubis, ilión e sacro se observa sin embargo, de isquión, no se puede documentar por manera muy definida en Eoraptor ejemplo la presencia de fossa brevis , lunensis (Sereno et. al., 1993), un aquella cavidad que se deja ver en la dinosaurio del Carniano de Argentina y parte posteroventral del ilión y que cuya data de antigüedad es en mucho aloja el músculo caudifemoralis brevis , semejante a la de Herrerasaurus y un poderoso retractor femoral y que es Staurikosaurus . un rasgo diagnóstico de dinosaurios. Tampoco se puede documentar de 3 manera fehaciente un pubis elongado, inmediatamente posterior a ese o en el la presencia de pie púbico o un ilión límite más temprano de aquel período, expandido anteroposteriormente o la debió aparecer la población de presencia de un acetábulo perforado, dinosaurios que dio origen a todos los rasgo este último que es exclusivo de dinosaurios posteriores, que aunque dinosaurios y que no se registra en muy antiguos, ya eran dinosaurios taxones de dinosauriformes fuera de derivados, como Herrerasaurus Dinosauria. ischigualestensis (Reig, 1963), Pisanosaurus mertii (Casamiquela, No obstante, del poco material fósil 1967), Eoraptor lunensis (Sereno et. al. , que se dispone de Nyasasaurus , se 1993) y Staurikosaurus pricei (Colbert, permite asumir con cierto grado de 1970), todos con una antigüedad certeza, que estamos en presencia de aproximada de ± 230 millones de años un dinosaurio y cuya data de (fines del Ladiniano – inicio del antigüedad nos remonta a un período Carniano). del cual no se poseía registros de este grupo dentro de Archosauria. Se hace claro a partir de lo anterior que ya en una media de tiempo de ± 230 La evidencia que surge a partir de milllones de años, ya existían Nyasasaurus y lo que se plantea en el dinosaurios derivados, algunos que documento de su publicación, incluso han sido clasificados como relacionado con el surgimiento de saurisquios basales, como grupos Dinosauria en el contexto evolutivo de cercanos a terópodos o bien como los arcosaurios, hace necesario una terópodos basales por algunos autores, diligente consideración del fósil, a fin lo cual deja ver que estos grupos de no sólo de asignarle una acertada dinosaurios no constituyen a los ubicación en la sistemática de dinosaurios basales, de los cuales se Dinosauria, sino además para darle la derivaron todos los dinosaurios importancia que requiere en relación al posteriores (Novas, 1992, 1993; conocimiento que se tiene sobre el Sereno, 1993; Sereno & Novas, 1992, origen de los dinosaurios. 1993). CONCLUSIÓN En este contexto, algunos autores han Puesto que las formas más afines a propuesto en los últimos años que el Dinosauria se ubican dentro de la grupo basal de dinosaurios y del cual Familia Lagosuchidae (Bonaparte, 1975) derivan todos los dinosaurios y Silesauridae (Langer et al. , 2010) posteriores, debió aparecer con toda dentro de Dinosauriformes (Novas, seguridad en un tiempo anterior al 1992), todos pertenecientes a rocas del Carniano, probablemente en el Triásico Inferior (± 251 – 245 millones Ladiniano, a saber a partir de ± 237 - de años), es claro que en un tiempo 228 millones de años (Colbert, 1970; 4 Martínez & Alcober, 2009; Langer et al ., dinosaurios es muy anterior al 2010; Díaz, 2012). Carniano, aspecto que se ve confirmado con el reporte de No obstante, el reporte de Nyasasaurus , cuyos restos fósiles Nyasasaurus , como un verdadero corresponden
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