A new madtsoiid snake from the Paleogene of South America (northwestern Argentina), based on an articulated postcranial skeleton

The Madtsoiidae are an extinct lineage of snakes known from the Late Cretaceous to the Late Pleistocene, with a rich fossil record distributed mainly across Gondwanan landmasses. However, only a few taxa are represented by cranial or articulated remains, and most madtsoiids are known only by isolated vertebrae. The unambiguous record of Madtsoiidae from the Cenozoic in South America had been restricted to the genus <i>Madtsoia</i> from Eocene and Oligocene deposits of Patagonia and Brazil. Here, we describe a new madtsoiid taxon, <i>Powellophis andina</i> gen. et sp. nov., based on an articulated postcranial skeleton from the Mealla Formation (middle–late Paleocene) in northwestern Argentina. The new taxon is estimated to be around 3 meters long, with a vertebral morphology sharing similar features with other mid-to-large forms. Its inclusion in a recent analysis of madtsoiid relationships recovers <i>Powellophis</i> as an early member of a clade formed by mostly large bodied and gigantic taxa. Its presence in the Paleocene of northwestern Argentina fills the gap between the diverse Late Cretaceous and Eocene–Oligocene records of madtsoiids in South America, confirms their presence in northern Gondwana by the early Paleogene, and expands the diversity of the group.

玛德松蛇科（Madtsoiidae）是一类已灭绝的蛇类演化支，其化石记录时代跨度从晚白垩世至晚更新世，主要分布于冈瓦纳大陆各地块，化石遗存较为丰富。但目前仅有少数分类单元保留有头骨或关联完整的化石遗骸，绝大多数玛德松蛇类仅通过孤立的脊椎骨得以确认。此前南美新生代地层中玛德松蛇科的确凿记录仅局限于巴塔哥尼亚与巴西始新世-渐新世地层中的玛德松蛇属（Madtsoia）。本研究基于阿根廷西北部梅利亚组（中古新世-晚古新世）中一件关联完整的颅后骨骼标本，记述了一新的玛德松蛇科分类单元——安第斯鲍威尔蛇（*Powellophis andina*）gen. et sp. nov.。该新类群体长约3米，其脊椎形态与其他中大型玛德松蛇类具有相似特征。将其纳入最新的玛德松蛇科系统发育关系分析后，结果显示鲍威尔蛇属（Powellophis）为一支以大型至巨型分类单元为主的演化支的早期成员。该新种发现于阿根廷西北部古新世地层，填补了南美地区玛德松蛇科从多样的晚白垩世记录到始新世-渐新世记录之间的演化空白，证实了古近纪早期冈瓦纳北部已有该类群分布，并进一步拓展了该科的物种多样性。