北美发现的新种恐龙Moros intrepidus的复原图。图源：Jorge Gonzalez；版权： Lindsay Zanno

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2月底，《通讯-生物学》在线发表的一篇论文Diminutive fleet-footed tyrannosauroid narrows the 70-million-year gap in the North American fossil record报道了一个暴龙超科的新种恐龙。该恐龙属于白垩纪时期同类中体型最小的一种，它的发现帮助填补了北美化石记录中长到7000万年的空白。

著名的君王暴龙（也称霸王龙，Tyrannosaurus rex）是一种大型的顶级掠食者。但是，其它已知的暴龙超科动物在1亿年的演化历史中，大部分时候都是小型的次级掠食者。虽然亚洲的化石记录较为完整，但是北美的化石记录却相对匮乏，这限制了我们对君王暴龙以及其它大型暴龙超科动物的主要演化事件的理解。

Moros intrepidus化石及其发现地。图源：Zanno等

美国北卡罗莱纳州自然科学博物馆的Lindsay Zanno及其同事在美国犹他州埃默里县发现了属于一个体型异常之小的暴龙超科的化石。这个被命名为Moros intrepidus的新物种，将北美的化石记录延长了1500万年，填补了此前跨越侏罗-白垩纪之交和白垩纪大部分时期的7000万年的化石记录空白。

研究人员估计成年M. Intrepdius 的体重仅为78千克，肢体长1.2米。作者对M. intrepidus及其亚洲近亲的分析结果显示，暴龙超科在约1500万年间一直体型较小，之后在不到1600万年的时间内演化形成像君王暴龙这样的大型恐龙。

摘要

To date, eco-evolutionary dynamics in the ascent of tyrannosauroids to top predator roles have been obscured by a 70-million-year gap in the North American (NA) record. Here we report discovery of the oldest Cretaceous NA tyrannosauroid, extending the lineage by ~15 million years. The new taxon—Moros intrepidus gen. et sp. nov.—is represented by a hind limb from an individual nearing skeletal maturity at 6–7 years. With a ~1.2-m limb length and 78-kg mass, M. intrepidus ranks among the smallest Cretaceous tyrannosauroids, restricting the window for rapid mass increases preceding the appearance of colossal eutyrannosaurs. Phylogenetic affinity with Asian taxa supports transcontinental interchange as the means by which iconic biotas of the terminal Cretaceous were established in NA. The unexpectedly diminutive and highly cursorial bauplan of NA’s earliest Cretaceous tyrannosauroids reveals an evolutionary strategy reliant on speed and small size during their prolonged stint as marginal predators.

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期刊介绍：Communications Biology(https://www.nature.com/commsbio/) is an open access journal from Nature Research publishing high-quality research, reviews and commentary in all areas of the biological sciences. Research papers published by the journal represent significant advances bringing new biological insight to a specialized area of research.