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 <i>Smilodon</i>). 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.

食肉哺乳动物中，剑齿颅齿适应性特征已独立演化多次。其中最为极端的剑齿适应性特征见于食肉目巴博剑齿虎亚科（Barbourofelinae）。然而，该类群的演化起源至今已有170余年悬而未决，其系统发育位置长期存在争议：曾被视为剑齿性状独立获得的独立支系，或是归入尼姆罗瓦猫科（Nimravidae，即始新世至渐新世的“假剑齿虎类”）内部的一个演化支，亦或是作为剑齿虎亚科（Machairodontinae，如刃齿虎<Smilodon>这类“真剑齿虎”）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BEAST2软件，采用化石出生-死亡（FBD）模型，整合223项形态学特征、核基因与线粒体基因序列以及地层出现数据，对所有主要食肉目类群开展了总证据贝叶斯分析。
分析结果显示，巴博剑齿虎类属于尼姆罗瓦猫科的末端类群，与尼姆罗瓦族（Nimravini）互为姊妹群（后验概率为0.91），这一系统发育关系此前的分支分类学研究从未报道。
借助R包BioGeoBEARS完成的祖先区域重建分析表明，巴博剑齿虎类的古生物地理起源核心区域主要为欧洲，且存在多次向其他大陆的扩散事件，这一发现与该类群既往的演化假说完全相悖。
此外，通过BayesTraits的贝叶斯祖先状态重建分析，本研究还揭示了尼姆罗瓦猫科与剑齿虎亚科之间全新的趋同演化模式。研究结果支持如下假说：剑齿虎类与尼姆罗瓦猫科在部分剑齿形态特征上存在相互模仿的演化关系，而非此前普遍认为的这类特征完全独立演化。