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

由Hartman nsf资助的对马达加斯加西北部Mahajanga盆地Maevarano组的实地考察发现，该岛白垩纪晚期脊椎动物的多样性是之前已知的四倍。动物名单现在包括至少32种鱼类、青蛙、海龟、蜥蜴、蛇、鳄鱼、恐龙、鸟类和哺乳动物。有几个分类群的骨骼几乎完整，保存得很好。本提案的主要目标是通过对脊椎动物化石的额外发现、对新样本的系统发育分析、对地层学、沉积学和地语学背景的阐明，以及对与南部超大陆冈瓦纳破碎有关的各种生物地理学假设的测试，来扩展这项工作。冈瓦纳在晚侏罗世和白垩纪分裂成许多孤立的大陆，对相关的生物群造成了巨大的影响。这种大陆分裂的重建几乎完全基于地球物理证据，并且在古生物学上仍然缺乏验证。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）的某个时候作为一个单一的构造单元从东非分离出来。在整个中生代，南极洲可能与南美洲保持着物理联系。早白垩纪（125-130 Ma）期间，马达加斯加在离非洲东海岸约400公里处形成了现在的位置，并与印度次大陆一起从南极洲-澳大利亚分离出来。马达加斯加和印度次大陆作为一个独立的构造单元一直持续到~88 Ma，远早于Maevarano组沉积。关于马达加斯加生物地理历史的几个可检验的假设来源于这种破碎模式：1)马达加斯加晚白垩世陆生动物在系统发育上与来自印度次大陆、其次是南极洲、澳大利亚和南美洲的动物有更大的亲缘关系，而不是来自非洲的同时期动物；2)相应地，来自印度次大陆的晚白垩世动物群在系统发育上与马达加斯加和其他冈瓦纳大陆的相似性最大，而不是与亚洲；3)马达加斯加现存的高度地方性和不平衡的脊椎动物动物群（如狐猴类灵长类动物）的大部分主要元素在晚白垩纪孤立之后移居该岛。本研究将通过对马达加斯加和其他冈瓦纳大陆的晚白垩纪脊椎动物进行严格的系统发育（分支）分析来解决这些假设。虽然最近从Mahajanga盆地的Maevarano地层中发现的脊椎动物遗骸为这些测试提供了重要的基础，但还需要额外的材料。因此，建议继续在Maevarano组开展古生物和地质野外工作，并扩大到Mahajanga盆地的下位层。因此，这个项目的意义有两个层面。在区域层面上，我们的工作将继续为马达加斯加晚白垩纪脊椎动物的解剖学、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，该项目将阐明冈瓦纳中生代脊椎动物的历史和生物地理格局，从而为检验构造破碎的地球物理模型提供独立的手段。

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.