Diversity through time and space in the Upper Jurassic Morrison Formation, western U.S.A.

Understanding how biodiversity has changed through time and space is a central aim of paleobiology. To elucidate accurate biodiversity patterns in deep time, regional case studies, where sampling biases can be minimized, are needed. The Upper Jurassic Morrison Formation of the western U.S.A. crops out over 1.2 million km2 and covers 12° of latitude. It was deposited over a ∼9-million-year time period, and was home to some of the most iconic dinosaurs. Utilizing a new, high resolution chronostratigraphic framework for the formation, tetrapod occurrences from the Paleobiology Database were temporally and spatially mapped to examine patterns of diversity change through time and space, and the geographic ranges of taxa were examined to shed light on niche partitioning. Latitudinally, diversity was found to peak in the center of the basin, perhaps due to the availability of water resources. Diversity increased over time in the Morrison Formation, and there is no evidence to indicate a decline in diversity prior to the extinction of the fauna at the end of the Jurassic. There appears to be some degree of geographic separation of faunas in the Morrison basin, with a southeastern and northwestern fauna, albeit with a number of overlapping taxa. High resolution climate models paired with detailed sedimentological analysis could help to elucidate the drivers of the patterns observed here.探究生物多样性随时间与空间的演化规律，是古生物学（paleobiology）的核心研究目标之一。为精准阐明地史时期的生物多样性分布格局，亟需开展可最大程度降低采样偏差的区域案例研究。美国西部的上侏罗统莫里逊组（Upper Jurassic Morrison Formation）出露面积逾120万平方千米，纬度跨度达12度。该地层沉积时长约900万年，孕育了诸多极具标志性的恐龙类群。研究团队依托该地层全新的高分辨率年代地层框架，对古生物学数据库（Paleobiology Database）中的四足动物化石产出记录开展时空展布制图，以解析其多样性随时间与空间的变化格局；同时对各分类单元的地理分布范围进行分析，以揭示类群间的生态位分化（niche partitioning）机制。纬度分布层面，研究发现多样性峰值出现在盆地中部，这或与水资源的可获得性密切相关。莫里逊组的生物多样性随时间推移整体呈上升趋势，目前尚无证据表明侏罗纪末期该动物群灭绝前存在多样性下降现象。莫里逊盆地内的动物群存在一定程度的地理分隔，可划分为东南与西北两个动物群组合，尽管二者存在不少共有分类单元。结合高分辨率气候模型与精细的沉积学分析（sedimentological analysis），将有助于阐明本次研究中观测到的分布格局的形成驱动机制。