How many sabertooths? Reevaluating the number of carnivoran sabertooth lineages with total-evidence Bayesian techniques and a novel origin of the Miocene Nimravidae

Sabertooth craniodental adaptations have evolved numerous times amongst carnivorous mammals. Some of the most extreme sabertooth adaptations are found within the carnivoran subfamily Barbourofelinae. However, the evolutionary origins of this group have been uncertain for more than 170 years, with variable placement as an independent case of sabertooth acquisition, as a clade within the Nimravidae (Eocene to Oligocene ‘false sabertooth cats’), or as a member of the Machairodontinae (true sabertooth cats such as Smilodon). Here we present a novel approach to assessing the validity of three independent sabertooth clades within Carnivora. We performed a total-evidence Bayesian analysis in Beast2 across all major carnivoran families, using the fossilized birth-death (FBD) model and incorporating 223 morphological characters, nuclear and mitochondrial gene sequences, and stratigraphic occurrence data. Our results place barbourofelines as terminal members of the Nimravidae, sister to the Nimravini (0.91 posterior probability), a relationship not found in prior cladistic studies. Ancestral area estimation performed in the R package BioGeoBEARS best supports a primarily European paleobiogeographic center for the barbourofelines with multiple dispersal events to other continents, a finding in direct opposition to past hypotheses for this group. Furthermore, new patterns in convergence between nimravids and machairodontines were revealed via Bayesian ancestral state estimation in BayesTraits. Results support a hypothesis of cats copying nimravids, and nimravids cats in certain aspects of sabertooth morphology, and not total evolutionary independence of these features as typically envisioned.

剑齿状颅齿适应性（sabertooth craniodental adaptations）在食肉哺乳动物中已独立演化多次。其中最为极端的剑齿适应性特征见于食肉目（Carnivora）巴博剑齿虎亚科（Barbourofelinae）类群。然而，该类群的演化起源迄今已有170余年未有定论，其系统发育位置曾被争议为独立获得剑齿特征的类群、尼姆兽科（Nimravidae，即始新世至渐新世的‘假剑齿虎’类群）内部的一个演化支，或是剑齿虎亚科（Machairodontinae，即斯剑虎（Smilodon）等‘真剑齿虎’类群）的成员。本研究提出了一种全新方法，用以评估食肉目内三个独立剑齿虎演化支的有效性。我们基于全证据贝叶斯分析法（total-evidence Bayesian analysis），在Beast2软件中针对所有主要食肉目类群开展分析，采用化石出生-死亡（fossilized birth-death, FBD）模型，整合了223个形态学特征、核基因与线粒体基因序列，以及地层产出数据。研究结果显示，巴博剑齿虎类群属于尼姆兽科的末端类群，与尼姆虎族（Nimravini）互为姊妹群（后验概率0.91），这一系统发育关系此前的分支分类学研究从未发现。通过R语言包BioGeoBEARS开展的祖先分布区估算结果表明，巴博剑齿虎类群的古生物地理起源中心主要为欧洲，且存在多次向其他大陆的扩散事件，这一发现与该类群此前的相关假说直接相悖。此外，借助BayesTraits中的贝叶斯祖先状态估算，本研究还揭示了尼姆兽科与剑齿虎亚科之间全新的趋同演化模式。研究结果支持‘剑齿虎形态特征并非如以往所设想的那样完全独立演化，而是猫科类群借鉴了尼姆兽科、尼姆兽科也借鉴了猫科类群的部分特征’这一假说。