COLLABORATIVE RESEARCH: Late Cretaceous Vertebrates from Madagascar: Implications for Gondwanan Biogeography

9705637哈特曼国家科学基金会资助的在马达加斯加西北部Mahajanga盆地Maevarano组的实地考察工作，使岛上先前已知的晚白垩世脊椎动物的多样性增加了四倍。动物名录现在至少包括32种鱼、青蛙、海龟、蜥蜴、蛇、鳄鱼、恐龙、鸟类和哺乳动物。几个分类群由几乎完整、保存完好的骨架代表。本提案的主要目标是通过进一步发现脊椎动物化石、对新样本进行系统发育分析、阐明地层学、沉积学和埋藏学背景，以及测试与南部超级大陆冈瓦那的碎裂有关的各种生物地理假说来扩展这项工作。冈瓦那在晚侏罗世和白垩纪分裂成许多孤立的陆块，给相关的生物群带来了戏剧性的后果。这种大陆碎裂的重建几乎完全基于地球物理证据，在古生物学上仍然没有得到充分的验证。根据最近的模型，南极洲、澳大利亚、马达加斯加和印度次大陆在晚侏罗世(150-160 Ma)的某个时候从东非分离出来，成为一个单一的构造单元。南极洲可能在整个中生代都与南美洲保持着物理联系。在白垩纪早期(125-130 Ma)，马达加斯加达到目前的位置，距离非洲东海岸约400公里，并与印度次大陆一起与南极-澳大利亚分离。马达加斯加和印度次大陆一直是一个不同的构造单元，直到约88 Ma，很好地在Maevarano FM沉积之前。与马达加斯加生物地理历史有关的几个可验证的假说源于这种碎片化模式：1)马达加斯加的晚白垩世陆生动物群将与来自非洲的同时代动物群，而不是与来自印度亚等深线的动物群，以及其次是南极洲、澳大利亚和南美洲的动物群具有更大的系统发育亲缘关系；2)相应地，来自印度次大陆的晚白垩世动物群将显示出最大的系统发育相似性，不是与亚洲，而是与马达加斯加和其他冈德瓦大陆；以及3)马达加斯加高度特有和不平衡的现存脊椎动物群的大部分主要成分(例如，猴形目灵长类动物)在晚白垩世孤立该岛后定居在该岛。本研究将通过对马达加斯加和其他冈德瓦大陆的晚白垩世脊椎动物进行严格的系统发育(分支)分析来解决这些假说。虽然最近从Mahajanga盆地Maevarano组发现的脊椎动物遗骸为这些测试提供了重要的基础，但还需要更多的材料。因此，建议继续在Maevarano组进行古生物和地质野外工作，并扩大到Mahajanga盆地的较低层位。因此，这个项目的意义是双重的。在区域层面上，我们的努力将继续为马达加斯加晚白垩世脊椎动物的解剖学、古生物学和系统发育关系提供重要的基线见解，并将它们置于详细的年代地层和古环境背景中。在全球范围内，该项目将阐明冈瓦那中生代脊椎动物之间更大规模的历史和生物地理模式，从而提供测试构造碎裂的地球物理模型的独立手段。

9705637 Hartman NSF-funded fieldwork in the Maevarano Formation of the Mahajanga Basin, northwestern Madagascar, has quadrupled the previously known diversity of Late Cretaceous vertebrates from the island. The faunal list now includes a minimum of 32 species of fishes, frogs, turtles, lizards, snakes, crocodyliforms, dinosaurs, birds, and mammals. Several taxa are represented by nearly complete, exquisitely preserved skeletons. The primary goal of the present proposal is to expand upon this work through additional discoveries of fossil vertebrates, phylogenetic analysis of the new samples, elucidation of the stratigraphic, sedimentologic, and taphonomic context, and testing of various biogeographic hypotheses related to the fragmentation of the southern supercontinent, Gondwana. Gondwana fragmented into a number of isolated landmasses during the Late Jurassic and Cretaceous, with dramatic consequences for associated biotas. Reconstructions of this continental fragmentation are based almost entirely on geophysical evidence, and remain poorly tested paleontologically. According to recent models, Antarctica, Australia, Madagascar, and the Indian subcontinent broke away as a single tectonic unit from eastern Africa sometime in the Late Jurassic (150-160 Ma). Antarctica likely retained physical connections with South America throughout the Mesozoic. During the Early Cretaceous (125-130 Ma), Madagascar attained its current position approximately 400 km off the east coast of Africa and, together with the Indian subcontinent, separated from Antarcto-Australia. Madagascar and the Indian subcontinent persisted as a distinct tectonic unit until ~88 Ma, well before deposition of the Maevarano Fm. Several testable hypotheses relating to the biogeographic history of Madagascar derive from this pattern of fragmentation: 1) Late cretaceous terrestrial faunas from Madagascar will share greater phylogenetic affinities not with coeval faunas from Africa, but with those from the Indian subcontine nt and, secondarily, Antarctica, Australia, and South America; 2) correspondingly, Late Cretaceous faunas from the Indian subcontinent will show greatest phylogenetic similarities not with Asia, but with Madagascar and other Gondwanan landmasses; and 3) Most of the major elements of the highly endemic and imbalanced extant vertebrate fauna on Madagascar (e.g., lemuriform primates) colonized the island after its isolation in the Late Cretaceous. The present study will address these hypotheses through rigorous phylogenetic (cladistic) analyses of Late Cretaceous vertebrates from Madagascar and other Gondwanan landmasses. While the vertebrate remains recently recovered from the Maevarano Formation of the Mahajanga Basin provide a crucial foundation for these tests, additional materials are required. It is therefore proposed that paleontological and geological fieldwork be continued in the Maevarano Formation, and expanded to lower horizons in the Mahajanga Basin. Thus, the significance of this project is two tiered. On a regional level, our efforts will continue to provide important, baseline insights into the anatomy, paleobiology, and phylogenetic relationships of Late Cretaceous vertebrates from Madagascar, as well as place them in a detailed chronostratigraphic and paleoenvironmental context. On a global level, this project will elucidate larger-scale historical and biogeographic patterns among Mesozoic vertebrates from Gondwana, and thereby provide independent means of testing geophysical models of tectonic fragmentation.