A new baenid turtle, <i>Palatobaena knellerorum</i> sp. nov., from the lower Paleocene (Danian) Denver Formation of south-central Colorado, U.S.A.

Baenidae are the most diverse clade of turtles from the latest Cretaceous and earliest Paleogene of North America. <i>Palatobaena</i> species have a distinctive cranial bauplan within baenid turtles and we herein describe a new species, <i>P. knellerorum</i>, based on a complete cranium from the lower Paleocene Denver Formation in the Denver Basin, Colorado. <i>Palatobaena knellerorum</i> differs from other <i>Palatobaena</i> species in the extreme degree of its dorsally oriented orbits, as well as the presence of a broad pentagonal midline crest formed by the supraoccipital and parietals. It can be further diagnosed from other species of <i>Palatobaena</i> by the following unique combination of characters: having a broadly rounded cranium, presence of upper temporal emargination that extends well anterior to the otic chamber, a broad exposure of the supraoccipital on the dorsal skull roof, a broadly rounded ‘tongue groove’ between the maxillae and premaxillae, and a deeply emarginated nasal region that exposes the underlying premaxillae in dorsal view. <i>Palatobaena knellerorum</i> most closely resembles the Maastrichtian/Danian taxon <i>Palatobaena cohen</i> and the Danian taxon <i>P. bairdi.</i> A phylogenetic analysis places <i>P. knellerorum</i> as sister to <i>P. bairdi</i>, and these taxa are sister to <i>P. cohen</i>. <i>Palatobaena knellerorum</i> overlaps in time with <i>P. cohen</i>, but the two are separated geographically with <i>P. knellerorum</i> restricted to the Denver Basin and <i>P. cohen</i> restricted to the Williston Basin. Cranial anatomical data and sedimentological data suggest <i>Palatobaena</i> spp. were bottom dwelling turtles living in shallow water environments that used a putative proboscis to prey upon freshwater-shelled invertebrates.巴恩鳖科（Baenidae）是北美晚白垩世至古新世早期物种多样性最高的龟类支系。腭鳖属（Palatobaena）物种在巴恩鳖科龟类中具备独特的颅骨体型构型（bauplan），本文基于采自美国科罗拉多州丹佛盆地下古新统丹佛组的一具完整颅骨，描述一新种——克氏腭鳖（*Palatobaena knellerorum*）。 克氏腭鳖与其他腭鳖属物种的区别在于：其眼眶极度背向延伸，且存在由上枕骨（supraoccipital）与顶骨（parietals）共同构成的宽阔五边形中脊。此外，还可通过以下独有特征组合与其他腭鳖属物种区分：颅骨整体呈宽圆形；颞上凹陷（upper temporal emargination）大幅向前延伸至听室（otic chamber）前方；颅顶背侧广泛暴露上枕骨；上颌骨（maxillae）与前上颌骨（premaxillae）之间存在宽圆形的“舌沟”；背视可见深陷的鼻区，暴露下方的前上颌骨。 克氏腭鳖与马斯特里赫特期/达宁期的科恩腭鳖（*Palatobaena cohen*）以及达宁期的贝尔德腭鳖（*Palatobaena bairdi*）形态最为相近。系统发育分析显示，克氏腭鳖与贝尔德腭鳖构成姐妹群，二者又与科恩腭鳖互为姐妹群。 克氏腭鳖的生存时代与科恩腭鳖存在重叠，但二者地理分布相互隔离：克氏腭鳖仅分布于丹佛盆地，科恩腭鳖则局限于威利斯顿盆地。 颅骨解剖学与沉积学数据表明，腭鳖属物种均为底栖龟类，栖息于浅水环境，推测通过特化吻突捕食淡水具壳无脊椎动物。