Understanding morphological variation in the extant koala as a framework for identification of species boundaries in extinct koalas (Phascolarctidae; Marsupialia)

We document morphological variation (both geographical and sexual) in the dentition of the extant koala, <i>Phascolarctos cinereus</i>, in order to facilitate discrimination of species boundaries in extinct phascolarctids. Considerable variation is evident in dental structures previously used to diagnose several phascolarctid fossil species. Consistent patterns of morphological variation are not evident between sexes or geographic regions, with variation as great between samples as within them. Metric variation is evident between the sexes in upper molar dimensions with Victorian (southern) males significantly larger than Victorian females, although this is not reflected in lower molar dimensions or in the Queensland (northern) sample. Male koalas from southern populations generally display significantly larger molars than their northern counterparts; however this trend is not evident in female upper molar dimensions. In both males and females, some, but not all, lower molar dimensions are larger in southern populations than northern. In light of these results, a systematic revision of species of <i>Litokoala</i> suggests <i>L.</i> ‘<i>dicktedfordi</i>’ is a junior synonym of <i>L. kutjamarpensis</i>, and the poorly known <i>L. thurmerae</i> is regarded to be a <i>nomen dubium</i>. Further, we describe a partial cranium of a new species of koala from Early Miocene sediments in the Riversleigh World Heritage Area, northern Australia. <i>Litokoala dicksmithi</i> sp. nov. is the fifth koala species recorded from the diverse rainforest assemblages of Riversleigh and the third species referred to the Oligo-Miocene genus <i>Litokoala</i>. Aspects of cranial morphology, including a shortened robust rostrum and broad, irregular nasal aperture, confirm placement of <i>Litokoala</i> as sister taxon to the modern genus <i>Phascolarctos</i>. Relatively large orbits and small body size suggest the possibility that <i>L. dicksmithi</i> was nocturnal, had enhanced visual acuity, and was a more agile arboreal species than the relatively sedentary extant koala.http://zoobank.org/urn:lsid:zoobank.org:pub:EE5D13C1-47BB-4432-B281-16398A1781E1

本研究记录现生树袋熊（*Phascolarctos cinereus*）齿列的形态变异（涵盖地理与性别两类变异），旨在为已灭绝袋熊科（Phascolarctidae）类群的物种界限界定提供依据。此前用于区分多种袋熊科化石物种的齿部结构均存在显著形态变异。不同性别与地理区域间未呈现统一的形态变异模式，样本间的变异程度与样本内部相当。上臼齿尺寸存在显著的性别二态性：维多利亚州（南部）雄性的上臼齿显著大于同区域雌性，但该差异在下臼齿尺寸以及昆士兰州（北部）的样本中未体现。南部种群的雄性树袋熊臼齿普遍显著大于北部种群雄性，但该趋势在雌性上臼齿尺寸中并不明显。无论雌雄，南部种群的部分（而非全部）下臼齿尺寸均大于北部种群。基于上述结果，对*Litokoala*属物种的系统学修订表明，*L.* ‘*dicktedfordi*’为*L. kutjamarpensis*的同物异名，而研究程度较低的*L. thurmerae*被视为疑名（nomen dubium）。此外，本研究描述了采自澳大利亚北部瑞弗斯利世界遗产地早中新世地层的一种树袋熊新物种的部分颅骨。*Litokoala dicksmithi* sp. nov.是瑞弗斯利地区多样雨林组合中已记录的第五种树袋熊物种，同时也是归入渐新世-中新世*Litokoala*属的第三个物种。颅骨形态特征（包括短粗的吻部以及宽阔不规则的鼻腔开口）证实*Litokoala*属与现生*Phascolarctos*属为姊妹群。相对较大的眼眶与较小的体型表明，*L. dicksmithi*可能为夜行性物种，具备更强的视觉敏锐度，且相较于行动相对迟缓的现生树袋熊，是更为敏捷的树栖物种。http://zoobank.org/urn:lsid:zoobank.org:pub:EE5D13C1-47BB-4432-B281-16398A1781E1