Abstract:

Pantodonta was one of the first groups of eutherians to evolve at the beginning of Cenozoic era, including the largest herbivores at that time. Pantodonta shows considerable diversity during the Paleocene and Eocene, with most of the species having been discovered in Asia and North America. Here, we report on a new pantodont, represented by lower jaws with well-preserved dentition, discovered from the Middle Paleocene Nongshan Formation of the Nanxiong Basin, Guangdong Province, China. Its unique dental and mandibular characteristics distinguish it well from any known Asian pantodont, but are quite consistent with North American taxa, especially Pantolambda and Titanoides. The new specimen is identified as Nanxiongilambda yei gen. et sp. nov., characterized by the combination of the following morphological features: thick and robust mandible with a conspicuous anteroexternal flange, high positioned condyloid process, posteriorly protruding angular process, robust but not elongated lower canines, double-rooted p1, small but distinct talonids on posterior lower premolars, talonids nearly as wide as trigonids on lower molars, and m3 with a well-developed hypoconulid and a distinct entoconid. The new discovery marks the first record of a pantolambdid pantodont outside of North America, suggesting a broader geographical distribution and intercontinental dispersal of this clade during the Paleocene. Considering the more primitive Pantolambda only found from Torrejonian to early Tiffanian NALMA (equivalent to middle-late Shanghuan to early Nongshanian ALMA), which is slightly earlier than Nanxiongilambda (early Nongshanian ALMA), pantolambdids have likely undergone a particular wave of migration from North America to Asia during the Early-Middle Paleocene. Previous researches have suggested that pantodonts had dispersed only from Asia to North America during the Early Paleocene, but our new specimen indicates the biotic dispersal may have occurred in the opposite direction. The new specimen also contributes to the renewal of the evolutionary history of pantodonts and provides further insights into the migration and dispersal of Paleocene mammals.

Key words: Nanxiong Basin, Paleocene, Nongshan Formation, Pantodonta, Pantolambdidae, Nanxiongilambda

摘要：

全齿目是新生代早期首批出现的真兽类哺乳动物之一，其中不乏当时体型最大的一些植食性动物。全齿目在古新世和始新世期间相当多样化，在亚洲和北美洲具有较丰富的化石记录。报道并描述了一件采自广东南雄盆地中古新统浓山组竹桂坑段的全齿类下颌骨化石材料。其独特的下颌和牙齿特征明显区别于任何已知的亚洲全齿类，而与北美的全齿类，尤其是全棱兽属(Pantolambda)和巨脊齿兽属(Titanoides)较为相似。新标本被命名为叶氏南雄脊齿兽(Nanxiongilambda yei gen. et sp. nov.), 其鉴定特征包括：下颌骨较粗壮且具前外凸缘，髁突位置显著高于颊齿齿列，角突最后端向后延伸至略超过髁突后缘的位置，下犬齿粗壮但并不前后向延长，下前臼齿具双齿根，下臼齿下跟座发育，m3下次小尖发育且向后伸展，m3下内尖独立且清晰可辨。形态特征的综合对比显示，南雄脊齿兽与北美洲的全棱兽类有着明确的亲缘关系。这是全棱兽类在北美以外的首次发现，表明古新世期间全棱兽类具有比此前认为更加广泛的地理分布。鉴于更加原始的早期全棱兽类仅存在于北美洲(Torrejonian-early Tiffanian, 相当于上湖期中晚期-浓山期早期), 略早于南雄脊齿兽(浓山期早期), 全棱兽类很可能在早古新世晚期-中古新世早期经历了一次从北美到东亚的迁徙。以前认为全齿类的扩散仅发生在早古新世从亚洲至北美的方向上，新材料的发现表明在早-中古新世期间，全齿类仍有可能从北美扩散到亚洲。南雄脊齿兽的发现不仅为全齿类的演化历史提供了新的信息，也为古新世期间哺乳动物的洲际迁徙与扩散提供了新的证据。

关键词: 南雄盆地, 古新世, 浓山组, 全齿目, 全棱兽科, 南雄脊齿兽