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Evolución Morfológica Y Ontogenia: Un Enfoque Paleobiológico

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Evolución Morfológica Y Ontogenia: Un Enfoque Paleobiológico UNIVERSIDAD DE BUENOS AIRES Facultad de Ciencias Exactas y Naturales Departamento de Ciencias Geológicas Evolución morfológica y ontogenia: un enfoque paleobiológico Tesis presentada para optar por el título de Doctor de la Universidad de Buenos Aires en el área de Ciencias Geológicas Celeste Marina Pérez Ben Directores de Tesis: Dra. Ana María Báez Dr. Rainer Schoch Consejero de Estudios: Dr. Ricardo Palma Lugares de Trabajo: Laboratorio de Paleonotología Evolutiva de Vertebrados, Departamento de Ciencias Geológicas, FCEN -UBA Staatliches Museum für Naturkunde, Stuttgart, Alemania Buenos Aires, 20 de Marzo de 2018 RESUMEN Evolución morfológica y ontogenia: un enfoque paleobiológico Entender la evolución del desarrollo ontogenético es esencial para comprender cómo la evolución morfológica se mueve en el espacio fenotípico y el origen de la disparidad morfológica resultante. No todas las variantes morfológicas son igualmente posibles de ocurrir porque las propiedades variacionales del desarrollo influyen en qué tan propensos son los rasgos a variar, limitando la respuesta evolutiva a factores externos de selección a ciertas direcciones del espacio fenotípico. Como aporte a esta problemática desde un punto de vista paleobiológico, presento en esta tesis el análisis de tres patrones morfológicos que son relevantes para discutir distintos mecanismos ontogenéticos de evolución morfológica: alometría, integración morfológica y miniaturización. Estos aspectos fueron analizados utilizando como modelo a los temnospóndilos, un clado de anfibios particularmente especioso durante el Paleozoico tardío y el Triásico. La elección de este grupo se debe a que su abundante y exquisito registro fósil permite el análisis cuantitativo de patrones ontogenéticos y de formas adultas a gran escala taxonómica y, ademas, hipótesis actuales proponen que miembros de un clado de temnospóndilos paleozoicos, los dissorophoideos, fueron ancestrales a los batracios (anuros y salamandras). Por lo tanto, el estudio de ese grupo también contribuye a comprender la evolución de uno de los grandes grupos de vertebrados vivientes. Primeramente, utilizando herramientas de morfometría geométrica, muestro que los patrones alométricos del desarrollo del dermatocráneo son altamente conservados en temnospóndilos y que dichos patrones también se observan a nivel macroevolutivo, lo cual indica que la alometría ontogenética pudo haber actuado como una línea de menor resistencia para la evolución morfológica del clado. Paralelamente, comparo los patrones de integración morfológica craneana intraespecífica y evolutiva en distintas especies y clados de temnospóndilos, respectivamente. A diferencia de la alometría ontogenética, la integración intraespecífica resulta ser muy variable en temnospóndilos y los patrones a nivel I evolutivo no se correlacionan con los correspondientes a nivel de especie. Esto sugiere que la integración craneana podría haberse visto más condicionada por factores extrínsecos como la selección natural que por mecanismos intrínsecos del desarrollo. Por último, pongo a prueba hipótesis previas que proponen que la miniaturización (i.e., la evolución de un tamaño corporal extremadamente pequeño) en dissorophoideos fue la principal causa de cambios morfológicos drásticos, jugando un rol central en la evolución morfológica del clado y en el origen de los batracios a partir del mismo. En base a observaciones cualitativas y datos morfométricos lineales y geométricos, expongo que no hay evidencia que sostenga que la morfología de los dissorophoideos más pequeños sea radicalmente distinta a la de taxones emparentados de mayor tamaño corporal. En cambio, la similitud entre los patrones alométricos craneanos a nivel evolutivo en dissorophoideos y los alométricos ontogenéticos de un dissorophoideo basal indica que muchos rasgos del cráneo de los taxones pequeños son pedomórficos. A partir de estos resultados, concluyo que el truncamiento de la ontogenia, y no la miniaturización, pudo haber jugado un rol predominante en la evolución morfológica del clado. Teniendo esto en cuenta, discuto el supuesto rol de la miniaturización en el origen de los batracios dentro de los dissorophoideos. Palabras clave: Evolución morfológica - Ontogenia - Temnospóndilos - Alometría - Integración morfológica – Miniaturización II ABSTRACT Morphological evolution and ontogeny: a paleobiological approach Understanding the evolution of development is essential to unravel how morphological evolution moves in phenotypic space and how the resulting morphological disparity develops. Not every morphological variant is equally likely to occur because the variational properties of development influence the variation of traits, limiting the evolutionary response to extrinsic selection factors to certain directions of phenotypic space. To shed light on this issue from a paleobiological perspective, in this dissertation I present the analyses of three morphological patterns that are relevant to discuss different ontogenetic mechanisms of morphological evolution: allometry, morphological integration, and miniaturization. These analyses were addressed using temnospondyls, an amphibian clade particularly specious (speciose) during the late Paleozoic and Triassic, as model. This group was selected because its large and exquisite fossil record enables quantitative analyses of ontogenetic and adult morphological patterns in a broad taxonomic scale and, in addition, current hypotheses propose that members of a Paleozoic temnospondyl clade, the dissorophoids, are ancestral to batrachians (anurans and salamanders). Thus, the study of this latter group also helps us to understand the evolution of one of the major living clades of vertebrates. Firstly, using geometric morphometric methods, I show that allometric patterns of dermatocranium development are highly conserved in temnospondyls and that the same patterns occur at the macroevolutionary level, indicating that the ontogenetic allometry might have acted as a line of least resistance for the morphological evolution of the clade. I also compare the patterns of morphological integration of the skull at the intraspecific and macoroevolutionary levels in different species and clades of temnospondyls, respectively. In contrast to the ontogenetic allometry, intraspecific III integration is retrieved as highly variable in temnospondyls and the patterns at the macroevolutionary level do not correlate with those at the species level. This suggests that cranial integration might have been more conditioned by extrinsic factors such as natural selection than by intrinsic developmental mechanisms. Lastly, I test previous hypotheses that propose that miniaturization (i.e., the evolution of extremely small body size) in dissorophoids was the main cause of drastic morphological changes, having played a major role in the morphological evolution of small dissorophoids and the origin of batrachians within this clade. Based on qualitative observations and linear and geometric morphometric data, I argue that there is no evidence to support the idea that the morphology of the smallest dissorophoids is significantly different from that of larger related species. Instead, the similarity between cranial allometric patterns at the macroevolutionary level in dissorophoids and allometric patterns at the ontogenetic level in a basal dissorophoid indicates that many features of the skull of small-bodied taxa are paedomorphic. Based on these results, I conclude that the truncation of the ancestral ontogeny, and not miniaturization, might have been the factor that played a major role in the morphological evolution of small dissorophoids. Taking this into account, I further discuss the putative role of miniaturization in the origin of batrachians within Dissorophoidea. Keywords: Morphological evolution - Ontogeny - Temnospondyls - Allometry - Morphological integration – Miniaturization IV AGRADECIMIENTOS Primero quiero agradecer a quienes hicieron que esta tesis fuera posible en lo que respecta a lo académico. Gracias a mis directores Ana Báez y Rainer Schoch por haberme brindado un lugar de trabajo y haberme orientado y ayudado desinteresadamente en estos años, así como a mi consejero de estudios Ricardo Palma. Agradezco también a los que me abrieron las puertas de las colecciones que visité para llevar a cabo este trabajo: Adriana López-Arbarallo y Oliver Rauhut (Múnich), Boris Ekrt (Praga), Carl Mehling y Mark Norell (Nueva York), David Wake (Berkeley, MVZ), Davit Vasilyan (Tübingen), Florian Witzmann y Thomas Schlossleitner (Berlín), Hans Hagdorn (Ingelfingen), Hebert Lutz y Thomas Engel (Mainz), Jessica Cundiff y Stephanie Pierce (Harvard), Ursula Göhlich (Viena), Josef Klembara (Bratislava), Kevin Padian (Berkeley, UCMP), Rainer Schoch (Stuttgat). Esta tesis no habría sido posible sin el financiamiento público de la educación y la ciencia. Este doctorado es el resultado directo de doce años, entre el secundario y la universidad, de educación gratuita y de calidad seguidos de seis años de financiamiento mediante becas del CONICET y del Ministerio de Educación de la Nación junto con el DAAD. Esto fue complementado por una beca del KLI que me permitió realizar una estancia en dicho instituto que marcó un punto de inflexión en mi carrera. Es por ello que quiero agradecer a los responsables más directos, Yogi Jäger y Gerd Müller, por haberme dejado ser parte de un espacio de discusión y crecimiento intelectual único en el cual se valoran las ideas de doctorandos e investigadores jóvenes.
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