Strata containing the Iniyoo Local Fauna (LF) crop out in the outskirts of Santiago Yolomécatl in the Yolomécatl Subbasin of the southeastern Tlaxiaco Basin in northwestern Oaxaca, southern Mexico. The assemblage is from a 325-m-thick fluviolacustrine unit previously assigned to the Yanhuitlán Formation and then the Yolomécatl Formation, which was restricted to the Yolomécatl Subbasin and its immediate vicinity. However, recent field investigations and laboratory analyses indicate the unit is, instead, a marginal facies of the dominantly lacustrine Chilapa Formation, which unconformably overlies the Yanhuitlán Formation where intervening volcanic units are absent. Consequently, Yolomécatl Formation is abandoned in favor of Chilapa Formation on the bases of synonymy and priority. Similarly, San Marcos Andesite and Nicananduta “Group” were applied to the same exposures of an andesitic unit interfingering with the Chilapa Formation. Accordingly, Nicananduta “Group” is abandoned and replaced by San Marcos Andesite for the same reasons.

The Iniyoo LF is earliest Arikareean (Ar1) (i.e., later early Oligocene) in age, not latest Uintan to late Chadronian (i.e., later early middle to latest Eocene), based on the first appearance datum for the amphicyonid Mammacyon and the last appearance datums of the tayassuid Perchoerus probus and the rhinocerotid Subhyracodon. That age assignment is supported by (1) corrected potassium-argon (K–Ar) dates of 28.939 ± 0.6 and 30.376 ± 0.6 Ma for the Yucudaac Andesite's lowermost flow or lower tongue, which underlies the Chilapa Formation north of the Yolomécatl Subbasin, (2) a corrected ⁴⁰argon/³⁹argon (Ar/Ar) date of 27.829 ± 0.716 Ma for the correlative Nicananduta “Group” (= San Marcos Andesite), which overlies the Chilapa Formation in the Yolomécatl Subbasin, and (3) uranium-lead (U–Pb) maximum depositional ages of 30.6 ± 0.77 and 30.62 ± 0.67 Ma for detrital zircon grains from sandstone beds occurring above the bottom and top, respectively, of the nearly 88.4-m-thick vertebrate fossil-bearing interval. The K–Ar and Ar/Ar dates further constrain the age of the Iniyoo LF to an interval spanning about 27.83–28.94 Ma ago.

A corrected Ar/Ar date of 40.561 ± 1.009 Ma for potassium-feldspar crystals in a bed originally identified as a silicified tuff sheet and later determined during the current investigation to be a sandstone bed was bracketed by the two sandstone beds subjected to U–Pb dating analyses. However, the last Ar/Ar age is much too old when compared with the two U–Pb dates for the Chilapa Formation and the three K–Ar and Ar/Ar for volcanic units bracketing the formation. Presumably, the dated material represented volcanic debris reworked from a substantially older unit. Consequently, the Ar/Ar date and any land mammal age assignment based on it are considered highly unreliable.

墨西哥南部瓦哈卡州西北部Tlaxiaco盆地东南部Yolomécatl盆地的SantiagoYolomécatl郊区，含有Iniyoo本地动物区系（LF）的地层。该组合来自一个之前分配给Yanhuitlán组的325米厚的氟湖相单元，然后分配给Yolomécatl组，该组仅限于Yolomécatl盆地及其附近。但是，最近的野外调查和实验室分析表明，该单元是占主导地位的湖相Chilapa组的边缘相，它不整合地覆盖了不存在火山单元的Yanhuitlán组。因此，在同义和优先权的基础上，放弃了Yolomécatl编组，转而使用Chilapa编组。相似地，San Marcos安山岩和Nicananduta“组”被应用于与Chilapa组互穿的安山岩单元的相同暴露。因此，出于同样的原因，Nicananduta“集团”被放弃，由圣马科斯·安山岩取代。

Iniyoo LF是最早的Arikareean（Ar1）（即晚渐新世晚期），而不是从Uintan到Chadronian晚期（即，晚始于中新世晚期），这是基于两栖类Ma头哺乳动物的首次出现数据和最后一次出现tayassuid perchoerus probus和犀头蛛网膜下孔的基准。该年龄分配得到以下因素的支持：（1）尤卡达克安山岩最低流动的或下部的舌头校正后的钾-氩（K-Ar）日期为28.939±0.6和30.376±0.6 Ma，它位于Yolomécatl子盆地以北的Chilapa地层之下，（ 2）校正后的⁴⁰氩/ ³⁹相关的Nicananduta“群”（= San Marcos Andesite）的氩（Ar / Ar）日期为27.829±0.716 Ma，其覆盖在约洛梅卡特次盆地的Chilapa组，以及（3）最大的铀铅（U–Pb）沉积在近88.4 m厚的脊椎动物化石承载层的底部和顶部上方的砂岩层中，碎屑锆石的年龄分别为30.6±0.77和30.62±0.67 Ma。K–Ar和Ar / Ar日期进一步将Iniyoo LF的年龄限制在大约Ma。27.83–28.94 Ma的间隔内。

在最初被鉴定为硅化凝灰岩层的床上，钾长石晶体的校正Ar / Ar日期为40.561±1.009 Ma，后来在本次调查中确定为砂岩床，并在经过U-Pb处理的两个砂岩床之间进行了固定。约会分析。但是，与Chilapa地层的两个U–Pb日期和包围该地层的火山单元的三个K–Ar和Ar / Ar相比，最后的Ar / Ar年龄太老了。据推测，标有日期的材料是从一个较旧的单元中重新加工而成的火山残骸。因此，Ar / Ar日期和基于该日期的任何陆地哺乳动物年龄分配都被认为是高度不可靠的。