Hsiang Allison Y, Field Daniel J, Webster Timothy H, Behlke Adam D B, Davis Matthew B, Racicot Rachel A, Gauthier Jacques A
Department of Geology and Geophysics, Yale University, New Haven, Connecticut, 06520, USA.
Department of Vertebrate Zoology, National Museum of Natural History, Smithsonian Institution, Washington, DC, 20560, USA.
BMC Evol Biol. 2015 May 20;15:87. doi: 10.1186/s12862-015-0358-5.
The highly derived morphology and astounding diversity of snakes has long inspired debate regarding the ecological and evolutionary origin of both the snake total-group (Pan-Serpentes) and crown snakes (Serpentes). Although speculation abounds on the ecology, behavior, and provenance of the earliest snakes, a rigorous, clade-wide analysis of snake origins has yet to be attempted, in part due to a dearth of adequate paleontological data on early stem snakes. Here, we present the first comprehensive analytical reconstruction of the ancestor of crown snakes and the ancestor of the snake total-group, as inferred using multiple methods of ancestral state reconstruction. We use a combined-data approach that includes new information from the fossil record on extinct crown snakes, new data on the anatomy of the stem snakes Najash rionegrina, Dinilysia patagonica, and Coniophis precedens, and a deeper understanding of the distribution of phenotypic apomorphies among the major clades of fossil and Recent snakes. Additionally, we infer time-calibrated phylogenies using both new 'tip-dating' and traditional node-based approaches, providing new insights on temporal patterns in the early evolutionary history of snakes.
Comprehensive ancestral state reconstructions reveal that both the ancestor of crown snakes and the ancestor of total-group snakes were nocturnal, widely foraging, non-constricting stealth hunters. They likely consumed soft-bodied vertebrate and invertebrate prey that was subequal to head size, and occupied terrestrial settings in warm, well-watered, and well-vegetated environments. The snake total-group - approximated by the Coniophis node - is inferred to have originated on land during the middle Early Cretaceous (~128.5 Ma), with the crown-group following about 20 million years later, during the Albian stage. Our inferred divergence dates provide strong evidence for a major radiation of henophidian snake diversity in the wake of the Cretaceous-Paleogene (K-Pg) mass extinction, clarifying the pattern and timing of the extant snake radiation. Although the snake crown-group most likely arose on the supercontinent of Gondwana, our results suggest the possibility that the snake total-group originated on Laurasia.
Our study provides new insights into when, where, and how snakes originated, and presents the most complete picture of the early evolution of snakes to date. More broadly, we demonstrate the striking influence of including fossils and phenotypic data in combined analyses aimed at both phylogenetic topology inference and ancestral state reconstruction.
蛇类高度特化的形态和惊人的多样性长期以来引发了关于蛇类总群（泛蛇类）和冠群蛇类（真蛇类）的生态及进化起源的争论。尽管关于最早蛇类的生态、行为和起源地的猜测众多，但尚未有人尝试对蛇类起源进行严格的、全分支范围的分析，部分原因是早期基干蛇类的古生物学数据不足。在此，我们展示了首次使用多种祖先状态重建方法对冠群蛇类的祖先和蛇类总群的祖先进行的全面分析性重建。我们采用了一种综合数据方法。该方法纳入了来自已灭绝冠群蛇类化石记录的新信息、基干蛇类里奥内格罗纳贾沙蛇、巴塔哥尼亚迪尼利西亚蛇和先前列蛇解剖结构的新数据，以及对化石和现生蛇类主要分支间表型近裔性状分布的更深入理解。此外，我们使用新的“枝端定年”方法和传统的基于节点的方法推断时间校准的系统发育，为蛇类早期进化历史中的时间模式提供了新见解。
全面的祖先状态重建表明，冠群蛇类的祖先和总群蛇类的祖先都是夜行性、广泛觅食、非缢缩性的潜行猎手。它们可能以与头部大小相当的软体脊椎动物和无脊椎动物为食，并栖息于温暖、水源充足且植被茂盛的陆地环境。由列蛇节点近似代表的蛇类总群被推断在早白垩世中期（约1.285亿年前）起源于陆地，冠群则在约2000万年后的阿尔布阶出现。我们推断的分歧时间为白垩纪 - 古近纪（K - Pg）大灭绝后盲蛇类蛇类多样性的主要辐射提供了有力证据，阐明了现存蛇类辐射的模式和时间。尽管蛇类冠群很可能起源于冈瓦纳超大陆，但我们的结果表明蛇类总群有可能起源于劳亚大陆。
我们的研究为蛇类何时何地起源以及如何起源提供了新见解，并呈现了迄今为止蛇类早期进化的最完整图景。更广泛地说，我们证明了在旨在进行系统发育拓扑推断和祖先状态重建的综合分析中纳入化石和表型数据的显著影响。
