Mammals are the only surviving group of Cynodontia, a synapsid clade that first appeared in the fossil record in the late Permian, ~260 million years ago. Using three metrics that capture skeletal completeness, we quantify the quality of the early cynodont fossil record in time and space to evaluate the impact of sampling and preservational biases on our understanding of the group's evolutionary history. There is no consistent global sampling signal for early cynodonts. Completeness of the cynodont fossil record increases across the Permian–Triassic boundary, peaking in the Early to early Late Triassic. This peak is dominated by specimens from southern Africa and South America, where a highly seasonal climate probably favoured preservation. Completeness is generally lower thereafter, correlating with a shift from a Gondwanan to a predominantly Laurasian fossil record. Phylogenetic and stratigraphic congruence in early cynodonts is high, although their fossil record exhibits less skeletal completeness overall than other tetrapod clades, including the contemporaneous anomodont synapsids. This discrepancy could be due to differences in the diagnosability of their fossils, especially for small‐bodied species. Establishing the timing and assembly of derived (‘mammalian’) anatomical features in Cynodontia is obscured by sampling. Two of the major nodes at which acquisition of mammalian features is concentrated (Cynodontia and Mammaliamorpha) suffer from lengthy intervals of poor sampling before becoming abundant parts of tetrapod faunas. Low completeness in these intervals limits our ability to determine when certain ‘key’ mammalian characteristics evolved, or to identify the selective pressures that might have driven their origins.

中文翻译：

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早期犬齿化石记录完整性的时空变化及其对哺乳动物进化史的影响

哺乳动物是唯一幸存的犬齿齿科，这是一个突触进化枝，最早出现在约2.6亿年前的二叠纪晚期的化石记录中。使用捕获骨骼完整性的三个指标，我们在时间和空间上量化了早期犬牙龙化石记录的质量，以评估采样和保存偏见对我们对该小组进化史的理解的影响。对于早期的犬齿，没有一致的全局采样信号。犬齿龙化石记录的完整性在二叠纪-三叠纪边界处增加，在三叠纪早期至早期达到顶峰。该峰主要来自南部非洲和南美的标本，那里的季节性气候很可能有利于保存。此后，完整性通常较低 与从冈瓦南（Gonwanan）转变为主要的Laurasian化石记录有关。早期犬齿类的系统发育和地层一致性很高，尽管它们的化石记录显示出其骨骼完整度比其他四足类进化枝（包括同期的突齿突触）少。这种差异可能是由于其化石的可诊断性差异而引起的，特别是对于小物种而言。通过采样无法确定犬齿畸形中派生（“哺乳动物”）解剖特征的时间安排和组装。采集哺乳动物特征的两个主要节点（犬齿目和哺乳动物目）在成为四足动物群的丰富部分之前遭受了很长的采样间隔。