ABSTRACT

North American soft-shelled turtles, including trionychines and plastomenids, are incredibly abundant in latest Cretaceous through earliest Paleocene sediments. Here we describe a new species of plastomenid turtle, Plastomenus joycei, based on a nearly complete early Danian skeleton from the Denver Formation in the Denver Basin, Colorado. Plastomenus joycei is differentiated from all other plastomenid turtles based on the presence of large eighth costals that are much longer than wide, sinusoidal raised ridges on the carapace, flat posterior edge of the carapace, spike-like epiplastra that lack callosities, a broad entoplastron that lacks a callosity, hyoplastra with two lateral processes, presence of metaplastically ossified hyoplastral shoulders (i.e., anteriorly protruding lappets), and a broad midline contact between the hypo- and xiphiplastra. Plastomenus joycei broadly resembles the Cretaceous Gilmoremys spp. and the Eocene Plastomenus thomasii and exhibits intermediate morphology between the two, most notably in the degree of ossification of the plastron. The increase in plastral ossification, as well as a decrease in overall size and an increase in the doming of the shell, co-occurs with the Cretaceous–Paleogene mass extinction and these morphological changes may be in response to an increase in mammalian predators during the early Paleogene. Plastomenus joycei is most commonly found in riverine sandstone sediments and is hypothesized to be a riverine turtle.

http://zoobank.org/urn:lsid:zoobank.org:pub:F4FE56A9-3E63-4BF0-A5CB-0CF0F9B6B045

摘要

北美软壳龟，包括三甲龟和质体龟，在晚白垩世到最早的古新世沉积物中极其丰富。在这里，我们根据科罗拉多州丹佛盆地丹佛地层的几乎完整的早期大年骨骼骨骼描述了一种新的质体龟类，即Plastomenus joycei。疟原虫区别于所有其他质体龟的基础是存在比宽长得多的大第八肋，甲壳上的正弦隆起脊，甲壳的平坦后缘，没有胼胝体的尖刺状上皮，一个没有胼胝体的宽阔内质体胼胝体，具有两个侧突的hyoplastra，存在化生骨化的hyoplastral 肩部（即向前突出的lappets），以及hyoplastra 和xiphiplastra 之间广泛的中线接触。Plastomenus joycei大致类似于白垩纪的Gilmoremys spp。和始新世Plastomenus thomasii并表现出两者之间的中间形态，最显着的是腹甲的骨化程度。质体骨化的增加，以及整体尺寸的减小和贝壳圆顶的增加，与白垩纪 - 古近纪大灭绝同时发生，这些形态变化可能是对哺乳动物捕食者在古近纪早期。Plastomenus joycei最常见于河流砂岩沉积物中，据推测是一种河流龟。

http://zoobank.org/urn:lsid:zoobank.org:pub:F4FE56A9-3E63-4BF0-A5CB-0CF0F9B6B045