中国晚侏罗世多瘤齿兽类哺乳动物的发现及其意义

多瘤齿兽是一类已绝灭的哺乳动物,其以后部牙齿上有很多小瘤(小的隆起,或者小尖)来适于咀嚼植物为特征。总的来说,它们的门齿和臼齿与那些啮齿类相似,但在演化时间上多瘤齿兽比新生代的啮齿类早的多。多瘤齿兽是杂食者,或者说是成功开拓了其它脊椎动物无法开拓的植食生境的植食者(可以树叶、种子、蕨类、裸子植物,再加上蠕虫和昆虫为食)。由于它们多种食性和运动方式,多瘤齿兽成为中生代数量最多的哺乳动物,几乎占侏罗纪和白垩纪时期所有哺乳动物种类的一半。它们的支系是哺乳动物历史上生存时间最长的支系,出现于170 Ma前,大约绝灭于35 Ma。但是,因为多瘤齿兽具有很多独特的、高度特化的牙齿和头骨特征,古生物学家们长期困惑于它们的进化起源。最近,中国辽西发现了一件产自晚侏罗世髫髻山组的多瘤齿兽类哺乳动物。由于其牙齿上发育明显的纹饰,如很多小的脊、沟、凹槽等,新发现的哺乳动物被命名为皱纹齿兽(Rugosodon)。另一方面,由于与皱纹齿兽亲缘关系最近的哺乳动物产自西欧的侏罗纪地层,因此其种名被命名为欧亚皱纹齿兽(Rugosodon eurasiaticus)。新发现为晚侏罗世时期欧洲和亚洲的哺乳动物具有高度的相似性提供了最新的证据。皱纹齿兽是一种夜行的哺乳动物,生活于气候温和的湖滨,与长羽毛的恐龙近鸟龙、翼龙类达尔文翼龙、大量节肢动物、其它几种哺乳动物共同生活于一起。根据与现生哺乳动物的比较,并依据欧亚皱纹齿兽手部比例、末端指节的形态等,推论欧亚皱纹齿兽很可能是地栖哺乳动物,或者说地栖比攀爬的可能性更大。这一发现对于研究多瘤齿兽早期的演化、食性分异、运动适应起源等具有重要的科学意义。

The Discovery of A Late Jurassic Multituberculate (Mammalia: Allotheria) from China and Its Significance

Multituberculate mammals are characterized by numerous tubercles (tiny bumps, or cusps) on their back teeth for chewing on plants. Overall their incisor and molar teeth are similar to those of rodents, but multituberculates evolved long before rodents of the Cenozoic Era. Multituberculates are either omnivores that could feed on almost anything and everything, or efficient plant eaters that are successful in exploiting herbivorous niches not accessible to other vertebrates. Thanks to their versatile feeding and locomotor adaptations, multituberculates became the most abundant mammals of the Mesozoic Era and constitute almost half of all mammal species that lived in the Jurassic and Cretaceous. Their lineage has the distinction as the most long-lived lineage in the mammalian history, starting from 170 million years ago, and went extinct around 35 million years. However, because multituberculates have many unique and highly specialized tooth and skull features, paleontologists have long been puzzled about the evolutionary origins of multituberculates. Recently, a new fossil mammal was unearthed from beds of 160 million years in Jianchang County of Liaoning Province. This fossil helps to shed the light on the earliest evolution of multituberculates, a major group of extinct mammals that lived in the Mesozoic times of dinosaurs and ultimately survived the mass extinction that wiped out dinosaurs 65 million years ago. The new mammal is named Rugosodon eurasiaticus after the rugose teeth ornamented by numerous tiny ridges and grooves and pits, indicating that it was an omnivore that fed on leaves and seeds of ferns and gymnosperm plants, plus worms and insects. The closest relative of Rugosodon is from the Jurassic beds of Western Europe, so the new species was named Rugosodon eurasiaticus because this fossil and its mammalian family have provided the newest evidence that mammalian faunas of Europe and Asia were very similar during the Late Jurassic. Its ankle bones are surprisingly mobile and flexible, suggesting that Rugosodon eurasiaticus was a fast-running and agile mammal. Also very important is that Rugosodon eurasiaticus is the earliest-known skeletal fossil of multituberculates, by studying it paleontologists can trace the evolutionary origins of the versatile and diverse locomotor adaptations of the later multituberculates that would include tree climbers, ground runners, so as to dig mammals that lived underground. Rugosodon eurasiaticus is an nocturnal mammal (see the life reconstruction) and lived in a temperate climate on lakeshores in what is now northeastern China, and it shared the land with the feathered dinosaur Anchiornis, the pterosaur Darwinipterus, and abundant arthropods, as well as several other mammals.