Fossils of the ursid Indarctos from Withlacoochee River 4A of Florida (late early Hemphillian North American Land Mammal Age, Hh2, ~ 7.5–6.5 Ma) represent the best sample of this genus in North America, including both craniodental and postcranial specimens, yet only the skull has been described. In this study, we describe the other material of this bear from the same locality and review the records of Indarctos in North America. Indarctos from Withlacoochee River 4A has dental characters in accordance with those of typical Indarctos oregonensis, but has distinctly more slender postcranial bones. Indarctos from other localities of North America contains two morphs with regards to postcranial robustness, but lacks a clear geographic pattern. Indarctos from Withlacoochee River 4A shares traits with the Old World Indarctos zdanskyi and may be its descendant in the New World. The machairodont from Withlacoochee River 4A shows typical characters (e.g., presence of distinct P4 preparastyle and mandibular flange) that allow referral to Amphimachairodus rather than to Nimravides. Its morphology also shows a stronger affinity of this population to the Old World Amphimachairodus horribilis rather than to Hh3 Amphimachairodus coloradensis. Previously, most North American records of Amphimachairodus were late Hemphillian (Hh3–4, ~ 6.5–4.5 Ma), when it co-occurred with a different ursid, “Agriotherium.” The Withlacoochee River 4A specimens provide evidence that Amphimachairodus dispersed from Asia during the Hh2. The special morphology of Indarctos and the presence of Amphimachairodus in a Hh2 fauna suggest that the environment had begun to change before the significant fauna turnover between the early and late Hemphillian.

在ursid化石Indarctos佛罗里达Withlacoochee河4A（晚早Hemphillian北美陆地哺乳动物时代，HH2，〜7.5-6.5马）代表该属在北美的最好的样本，包括craniodental和颅后的标本，但只头骨已被描述。在这项研究中，我们描述了来自同一地区的这只熊的其他材料，并回顾了Indarctos在北美的记录。来自Withlacoochee河4A的Indarctos具有与典型的Indarctos oregonensis相似的牙齿特征，但具有明显更细长的颅后骨。因达科托斯（Indarctos）北美其他地区的后代在颅后稳定性方面存在两种变体，但缺乏明确的地理格局。来自Withlacoochee River 4A的Indarctos与旧世界Indarctos zdanskyi具有相同的特征，并且可能是其在新世界中的后代。来自Withlacoochee河4A的machairodont显示出典型的特征（例如，存在明显的P4 preparastyle和下颌突缘），这些特征允许转介到Amphimachairodus而不是Nimravides。它的形态还显示该种群对旧世界的Amphimachairodus horribilis而不是对Hh3 Amphimachairodus coloradensis的亲和力强。以前，大多数北美记录Amphimachairodus迟到Hemphillian（Hh3-4，〜6.5-4.5马），当它共发生了不同ursid，“郊熊。Withlacoochee河4A标本提供了证据，证明Hmph2期间Amphimachairodus从亚洲扩散。的特殊形态Indarctos和存在Amphimachairodus在HH2动物表明环境有早期和晚期Hemphillian之间的显著动物成交前开始发生变化。