Comparisons of extant and extinct biodiversity are often dependent on objective morphology-based identifications of fossils and assume a well-established and comparable taxonomy for both fossil and modern taxa. However, since many modern (cryptic) species are delimitated mainly via external morphology and / or molecular data, it is often unclear to what degree fossilized (osteological) remains allow classification to a similar level. When intraspecific morphological variation in extant taxa is poorly known, the definition of extinct species as well as the referral of fossils to extant species can be heavily biased, particularly if fossils are represented by incomplete isolated skeletal elements. This problem is especially pronounced in squamates (lizards and snakes) owing to a lack of osteological comparative knowledge for many lower taxonomic groups, concomitant with a recent increase of molecular studies revealing great cryptic diversity. Here, we apply a quantitative approach using 3D geometric morphometrics on 238 individuals of 14 genera of extant Australian and Papua New Guinean agamid lizards to test the value of two isolated skull bones (frontals and maxillae) for inferring taxonomic and ecological affinities. We further test for the consistency of intra- and interspecific morphological variability of these elements as a proxy for extinct taxonomic richness. We show that both bones are diagnostic at the generic level, and both can infer microhabitat and are of palaeoecological utility. However, species-level diversity is likely underestimated by both elements, with ~30-40% of species pairs showing no significant differences in shape. Mean intraspecific morphological variability is largely consistent across species and bones and thus a useful proxy for extinct species diversity. Reducing sample size and landmark completeness to approximate fossil specimens led to decreased classification accuracy and increased variance of morphological disparity, raising further doubts on the transferability of modern species borders to the fossil record of agamids. Our results highlight the need to establish appropriate levels of morphology-based taxonomic or ecological groupings prior to comparing extant and extinct biodiversity.

中文翻译：

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在化石记录中可以检测到现代神秘物种吗？阿加米德蜥蜴的案例研究


现存和已灭绝生物多样性的比较通常取决于基于形态学的客观化石鉴定，并假设化石和现代分类群都有成熟且可比较的分类法。然而，由于许多现代（神秘的）物种主要通过外部形态学和/或分子数据来界定，因此通常不清楚化石（骨学）遗骸在多大程度上允许分类到类似的水平。当现存分类群的种内形态变异知之甚少时，已灭绝物种的定义以及将化石归类到现存物种的引用可能会有很大的偏差，特别是如果化石由不完整的孤立骨骼元素表示。这个问题在鳞状动物（蜥蜴和蛇）中尤为明显，因为缺乏对许多低等分类群的骨学比较知识，同时最近分子研究的增加揭示了巨大的隐蔽多样性。在这里，我们对现存的澳大利亚和巴布亚新几内亚阿加米蜥蜴 14 属的 238 个个体应用了 3D 几何形态计量学的定量方法，以测试两个孤立的颅骨（额骨和上颌骨）在推断分类学和生态亲和力方面的价值。我们进一步测试了这些元素的种内和种间形态变异性的一致性，作为已灭绝分类学丰富度的代表。我们表明，这两块骨头在通用水平上都具有诊断意义，并且都可以推断微生境并且具有古生态学效用。然而，这两个元素可能低估了物种水平的多样性，~30-40% 的物种对在形状上没有显着差异。 平均种内形态变异性在物种和骨骼之间基本一致，因此是已灭绝物种多样性的有用代表。将样本量和地标完整性减小到近似化石标本导致分类准确性降低和形态差异方差增加，从而进一步质疑现代物种边界是否可转移到 agamids 化石记录。我们的结果强调，在比较现存和已灭绝的生物多样性之前，需要建立适当水平的基于形态学的分类学或生态分组。