Data from: Evolution of early Eocene Palaeosinopa (Mammalia, Pantolestidae) in the Willwood Formation of the Bighorn Basin, Wyoming

Species-level diversity and evolution of Palaeosinopa from the Willwood Formation of the Bighorn Basin is reassessed based on substantial new material from the Bighorn, Powder River, and Wind River basins. We recognize three species of Palaeosinopa in the Willwood Formation of the Bighorn Basin: P. lutreola, P. incerta, and P. veterrima. The late Wasatchian species P. didelphoides is not present in the Bighorn Basin. The Willwood species can be differentiated based only on size. P. veterrima is the most common and wide-ranging species and is the most variable in size and morphology: the stratigraphically lowest individuals are smaller, with narrower, more crestiform lower molars; whereas the highest are larger, with wider, more bunodont teeth. Although it could be argued that these represent distinct species, we demonstrate that this morphological evolution occurred as the gradual and mosaic accumulation of features, suggesting in situ anagenetic evolution. The two smaller species are present only low in the section (biochrons Wa0–Wa4) and show no discernable evolution in size or morphology. A new skeleton of Palaeosinopa veterrima from the Willwood Formation is described, and other new postcrania are reported. The skeleton is the oldest associated skeleton of Palaeosinopa known, yet it is remarkably similar to those of younger, more derived pantolestids, the primary disparities being minor differences in proportions of the innominate, femur, and tibia, and co-ossification of the distal tibia and fibula. Either P. incerta or P. lutreola was likely the ancestral population that gave rise to the other Wasatchian Palaeosinopa. Alternatively, P. veterrima may have migrated into the Bighorn Basin from the Powder River Basin.

基于采自大角盆地（Bighorn Basin）、粉河盆地（Powder River Basin）与风河盆地（Wind River Basin）的大量新标本，本研究针对大角盆地威尔伍德组（Willwood Formation）产出的古新兽属（Palaeosinopa）的物种级多样性与演化历程展开重新评估。研究团队在大角盆地的威尔伍德组中识别出古新兽属的3个物种：水獭古新兽（P. lutreola）、未定古新兽（P. incerta）以及古老古新兽（P. veterrima）。晚瓦萨奇期（Wasatchian）物种双齿古新兽（P. didelphoides）并未见于大角盆地。本研究中的威尔伍德组物种仅可通过体型进行区分。其中古老古新兽是最为常见、分布最广的类群，同时也是体型与形态变异程度最高的物种：地层位置最低的个体体型更小，下臼齿更窄且呈脊形；而地层位置最高的个体体型更大，牙齿更宽且呈丘形。尽管有观点认为这些类群应被视为独立物种，但本研究证实，该形态演化是特征逐步、镶嵌式积累的结果，暗示了原地线系前进演化（anagenetic evolution）的发生。另外两个体型较小的物种仅见于剖面下部（生物带biochrons Wa0–Wa4），未表现出可识别的体型或形态演化。本文描述了一件产自威尔伍德组的古老古新兽全新骨架标本，并报道了其他新发现的颅后骨骼（postcrania）材料。该骨架是目前已知最古老的关联保存的古新兽属骨架，但其与更年轻、特化程度更高的潘托兽类（pantolestids）骨骼形态极为相似，二者的主要差异在于无名骨（innominate）、股骨（femur）与胫骨（tibia）的比例存在细微差别，以及胫骨远端与腓骨发生了联合骨化。未定古新兽与水獭古新兽二者之一大概率为祖先类群，演化出了其余瓦萨奇期古新兽属类群；此外，古老古新兽也有可能从粉河盆地迁徙进入大角盆地。